# Ur-Turn: Saturation Dive Into The ZF 9-Speed

By on February 25, 2014

A TTAC reader is an engineer with a major powertrain company, and offered his extremely detailed analysis of the ZF 9-speed. Consider this an AP level course in powertrain engineering.

Before we dive right in to the 9-speed gearbox, let’s take a quick refresher on the basics of gears. The simplest gear set consists of 2 parallel gears mounted on 2 parallel shafts. Shown in Fig.1 is a gear set with a 20 tooth drive gear on the right and a 30 tooth driven gear on the left. For this gear set the speed of the driven gear is 1.5 times lower than the drive gear, and assuming no frictional losses anywhere, the torque on the driven gear is 1.5 times higher. This gear set has a ratio of 1.5:1. This type of a gear set is usually not favorable for packaging since it requires 2 parallel shafts, and there are largest separating forces that push the 2 gears apart which means that the bearings supporting the shafts have significant radial loads on them, in addition to an axial load if the gears are helical.

A simple planetary arrangement is shown in Fig.2 – this is the basis of most modern automatic transmissions. A simple planetary gear set has 3 members mounted on concentric shafts, the innermost gear is called a sun gear, the outermost gear is called the ring gear, and there are evenly spaced planetary pinions that mesh with both the sun gear and the ring gear. These pinions are free to spin around their own axes, and ride on the planetary carrier, which is the third concentric member. The radial forces in a planetary gear arrangement cancel out due to symmetry, and therefore the bearings supporting these shafts do not see much, if any radial loads. Since the 3 shafts are concentric, there are significant packaging advantages as well. This particular planetary arrangement has a 30 tooth sun gear, 72 tooth ring gear, and 21 tooth pinions. For this gear arrangement to go together, the difference between the number of teeth on the ring gear and the number of teeth on the sun gear has to be an even number, and the sum of teeth on the ring gear and the sun gear has to be divisible by the number of planetary pinions. In this case, the sum of the number of teeth on the ring gear and the sun gear is 102, which is divisible by the number of planets (3), hence this is a feasible gear arrangement.

Since there are 3 members in a planetary gear set, one member has to be grounded (i.e. forced to stand still) for there to be a ratio. There are 3 possible ground members (the sun gear, the ring gear, or the carrier), and 2 possible input and output combinations possible for each ground member, therefore this arrangement can provide 6 different speeds. If the number of teeth on the ring gear is denoted by R and the number of teeth on the sun gear is denoted by S

1. If the ring gear is grounded by the use of a brake, the sun gear is the input and the carrier is the output, the
ratio of this arrangement is S+R/S or 3.4, i.e. the carrier is rotating 3.4 times slower
than the sun gear. Therefore in this configuration output is underdriven with respect to the input. If the carrier is
the input, and the sun gear is the output, then the output is overdriven by the same ratio.
2. If the sun gear is grounded, the ring gear is the input and the carrier is the output, the
ratio of this arrangement is S+R/R or 1.417, i.e. the carrier is rotating 1.417 times
slower than the ring gear. If the carrier is
the input, and the ring gear is the output, then the output is overdriven by the same ratio.
3. If the planetary carrier is grounded, the sun gear is the input and the ring gear is the
output, the ratio of this arrangement is R/S or -2.1875, i.e. the ring gear is rotating
2.1875 times slower than the sun gear and in the opposite direction. Therefore this arrangement
provides a reverse underdrive gear. If the ring gear is the input and the sun gear is the output,
this arrangement becomes a reverse overdrive, and who needs a reverse overdrive?

If 2 of the members are tied together, then the ratio of planetary becomes 1:1, as all members turn at the same speed. A single planetary gear with the right set of clutches and brakes to change the ground member, the input, and the output can provide 5 forward ratios and 2 reverse ratios with 9 shifting elements (6 clutches and 3 brakes). Of course it is not be possible to package the all 9 of the shifting
elements in a practical manner, and the 5 forward ratios are 3.4, 1.417, 1.0, 0.7, and 0.29 – not very useful even if it were possible to achieve them. In engineering literature, a stick diagram”is often used as short-hand to describe planetary gear sets, for the planetary shown in Fig.2 the stick diagram is shown in Fig.3.

Planetary gears can also carry a lot more torque in the same packaging envelope because the load is distributed between multiple gear meshes. Need more torque capacity than the 3 planet gears can provide? You can nearly double that by putting in 6 planets on the planetary carrier.

Now on to the ZF 9 speed – there is a high level presentation available from the ZF website. This presentation has some detailed CAD renders in it, but not a whole lot of detail on the exact function of the transmission. The 2 CAD renders are shown tell us that there are 4 planetary gear sets in this transmission.

From these CAD renders the patient among the B&B can see that the one ring gear visible in Fig.4 has 86 teeth, and the 4 planets have 22 teeth, which means the sun gear that is not fully visible in the view is a 42 tooth gear. Fig.6 shows the stick diagram representation of the ZF 9 speed transmission, with 4 planetary gear sets numbered 1 through 4 from left to right. If one were to spend 10 minutes gawking at the cut-away transmissions that ZF does bring to trade shows, the following gear specifications can be established quite easily

1. Gear set 1 has a 42 tooth sun gear and a 110 tooth ring gear
2. Gear set 2 has a 42 tooth sun gear and a 110 tooth ring gear
3. Gear set 3 has a 91 tooth sun gear and a 133 tooth ring gear 1
4. Gear set 4 has a 42 tooth sun gear and a 86 tooth ring gear

The input is the output shaft of the torque converter, which is not shown in Fig.6. The torque converter is obviously driven by the engine. The planetary carrier of gear set 1 is the output to the final drive of the transmission. The following elements are rigidly linked

1. The 2 sun gears for gear sets 1 and 2 are connected together, in fact it is one wide gear
2. The ring gear for gear set 1 is linked to the planetary carrier of gear set 2.
3. The ring gear for gear set 2 is connected to the planetary carrier for gear set 3, which is also the
planetary carrier for gear set 4
4. The sun gear for gear set 3 is linked to the sun gear of gear set 4

Additionally, the 6 shifting elements work as follows

1. Dog clutch A in connected state connects the sun gear of gear set 3 and the ring gear of gear set 4 to the
input shaft
2. Friction clutch B couples the sun gear of gear set 4 to the input shaft
3. Friction brake C ties the sun gear of gear set 4 to ground, i.e. stops it from turning
4. Friction brake D ties the ring gear of gear set 3 to ground
5. Friction clutch E couples the planetary carrier of gear set 2 and the ring gear of gear set 1
to the input shaft
6. Dog brake F ties the sun gears of gear sets 1 and 2 to the ground

Now on to the gory calculations

### First gear

First gear is achieved by engaging shift elements A, F, and D. In this configuration gear sets 1, 2, and 3 are used in series as underdrives, gear set 4 is just along for the ride. The sun gear of gear set 3 is connected to the input, and the ring gear is grounded, which leads to the carrier going slower than the input. The carrier is in turn connected to the ring gear of gear set 2, while the sun gear for gear set 2 is
grounded, causing the carrier of gear set 2 to be further under driven. Since the planetary carrier of gear set 2 is connected to the ring gear of gear set 1, and the sun gear of gear set 1 is connected to ground as well, the transmission output is underdriven even more. The overall ratio is

1st=
 S3+R3 S3
 S2+R2 R2
 S1+R1 R1
=
 224 91
 152 110
 152 110
=4.700
(1)

### Second gear

An upshift to second gear is achieved by turning friction brake D off and engaging friction brake C, i.e. shift elements A, F, and C are engaged. Now gears sets 1, 2, and 4 are used as a cascaded series of underdrives. Operation of gears sets 1 and 2 is identical to the first gear, gear set 4 acts as an underdrive, while gear set 3 is along for the ride now. Gear set 4 acts as an underdrive because the transmission input is connected to the ring gear, the sun gear is held stationary by brake C, and the planetary carrier is the output. This leads to a ratio of

2ndgear=
 S4+R4 R4
 S2+R2 R2
 S1+R1 R1
=
 128 86
 152 110
 152 110
=2.8419
(2)

### Third gear

The upshift to third hear is accomplished by releasing brake C and engaging clutch B. This operation causes both the ring gear and the sun gear of gear set 4 to be connected to the input, therefore the planetary carrier also turns at the same speed as the input. Since the planetary carrier of gear set 4 is connected to the ring gear of gear set 2, they both turn at the same speed, i.e. the input speed. The operation of gear sets 1 and 2 is unchanged, and they act as cascaded underdrives, yielding a gear ratio of

3rdgear=
 S2+R2 R2
 S1+R1 R1
=
 152 110
 152 110
=1.9094
(3)

### Fourth gear

By releasing clutch B and engaging clutch E, the transmission up shifts to 4th gear, i.e. shift elements A, F, and E are engaged. This action connects the ring gear of gear set 1 to the input, while the sun gear is connected to ground, setting up a straightforward underdrive ratio of

3rd=
 S1+R1 R1
=
 152 110
=1.3818
(4)

At this point, the 4 gear ratios have been achieved by leaving the 2 dog” elements engaged, and
cycling through the 4 friction elements – and the shifts between these gears are therefore expected
to feel normal”. At this point, the vehicle is going at 30-35 mph and the shift to gear 5 is coming
up, and things get a little interesting.

### Fifth gear

To achieve fifth gear, dog brake F needs to be disengaged. This now leads to a brief torque interruption because as brake F is disengaged, the transmission is in Neutral and engaging the frictional element B prematurely would just lead to wear and tear on the transmission for no reason. At this point the transmission ECU and engine ECU are working in tandem to get this upshift done as quickly as possible. The ratio calculation is fairly trivial though, all 4 gear sets are turning at the speed of the input. Why? Because engaging elements A, B, and E means that

• The ring gear and the sun gear of gear set 4 are connected to the input, i.e. the planetary carrier
spins at the same speed as the input
• Consequently, the sun gear and the planetary carrier for gear set 3 are spinning at the same
speed as the input, i.e. the ring gear gears for gear sets 2 and 3 are turning at the same speed as
the input
• Through shift element E, the carrier of gear set 2 is turning at the same speed as the input,
therefore the sun gear of gear set 2 (which is also the sun gear for gear set 1) is spinning at
input speed
• Since the ring gear and the sun gear of gear set 1 is turning at the same speed as the input,
the planetary carrier which is the transmission output is turning at input speed

The ratio therefore is quite simply

 5th=1.000 (5)

### Sixth gear

So far, things have been pretty simple but now the magic begins where planetary gear sets are going to act as mixer” modules, i.e. the input and output turn at different speeds, but the reacting or grounding member is also turning at some speed. The upshift to sixth gear is achieved by releasing clutch B and engaging brake C. This causes gear set 4 to act as an underdrive just like second gear, therefore ring gear of gear set 2 is turning at a speed which is approximately 1.5 times slower than the input speed. The difference between second gear and sixth gear is that brake F is disengaged andclutch E is engaged, which means that the common sun gear for gear sets 1 and 2 is spinning at approximately 1.85 times faster than the input. This sets up a kinematic state for gear set 1 where the ring gear is turning at the same speed as the input but the sun gear is turning at 1.85 times the speed of the input, therefore the carrier has to spin at approximately 1.25 times faster than the input speed – overdrive!. Since the B&B do not deal in approximations, the exact ratio is

6th=
1
1+
 S4 S4+R4
 R2 S2
 S1 R1+S1
=0.8081
(6)

### Seventh gear

As sixth gear shows us, an underdriven ring gear of gear set 2 sets up an overdrive, seventh gear kicks it up a notch by underdriving the ring gear of gear set 2 even further. This is accomplished by releasing brake C and engaging brake D. The sun gear of gear set 3 through clutch A is connected to the input, while the ring gear is connected to the ground via brake D, which means that the carrier spins 2.46
times slower than the ring input, and the carrier is connected to ring gear of gear set 2. Therefore gear set 3 is in the same kinematic state as it is in first gear. This sets up a kinematic state for gear set 1 where the ring gear turns at the same speed as the input, but
the sun spins at a speed that is 2.85 times higher. Therefore at 2000 rpm engine speed, the sun gear of gear set 1 is spinning at 5700 rpm. A dog brake has essentially 0 parasitic losses, while a friction brake would have cost about a 0.2 horsepower drag. The ratio is

7th=
1
1+
 R3 S3+R3
 R2 S2
 S1 R1+S1
=0.6995
(7)

### Eighth gear

Eighth gear is achieved by closing brakes C and D at the same time, while disconnecting clutch A. At this point in time, the torque levels are low enough that in my humble opinion only the most discerning driver would be able to feel the torque interruption. This causes gear sets 3 and 4 to stop turning entirely, therefore the ring gear of gear set 2 is grounded. This causes the sun gear for gear sets 1 and 2 to spin faster – 3.65 times the input speed and sets up another over drive ratio

8th=
1
1+
 R3 S3+R3
 R2 S2
 S1 R1+S1
=0.5802
(8)

Therefore at 2000 rpm engine the sun gear for gear sets 1 and 2 is turning at 7300 rpm. But we are
not done yet – things turn faster.

### Ninth gear

If underdriving the ring gear of gear set 2 set up 2 overdrives, and grounding it set up another one, there is only one thing left to do, spin it backwards. Ninth gear does exactly that – by connecting clutch B and by the virtue of the fact that the ring gear for gear set 4 and the sun gear for gear set 3 are linked together, we have a very interesting kinematic state. Sun gear of gear set 4 spins at the input speed, the carrier for gear set 4 spins backwards at approximately half the input speed, and the ring gear turns backwards at 1.2 times the input speed. This means that the ring gear for gear set 2 is now spinning backwards at approximately half the input speed. The sun
gear of gear sets 1 and 2 is now turning at 4.95 times the input speed. If the car is going at 85 mph in the 4 cylinder engine variant at an engine speed of 2000 rpm, this little gear is going at any eye watering 9500 rpm. The use of a dog brake at F instead of a friction brake is saving 0.4 hp or about 0.4 miles per gallon, the ratio is

9th=
1
1+
 R4R3 R4R3S4S3
 R2 S2
 S1 R1+S1
=0.4792
(9)

The interesting thing about the Ninth gear is that there are parts in the transmission spinning
backwards to send you forward.

### Reverse gear

Reverse and 9th have the same kinematic states for gear sets 3 and 4, i.e. the ring gear for gear set 2 turns backwards at approximately half in the input speed. But gear sets 1 and 2 are switched over to an underdrive configuration which is identical to the configuration in First gear, i.e. brake F is engaged. The ratio is therefore

Rev=
 S4S3R4R3 S4S3
 S2+R2 R2
 S1+R1 R1
=3.8049
(10)

### Downshifts

The only real kink when shifting up through the gears is that the 4 to 5 shift might have an objectionable torque interruption, but otherwise this transmission is going to be well behaved. Downshifting from say 7th to 5th is no problem as well, but a downshift from 8th or 9th to 5th is hard work for this design. As an example if the engine speed is 1700 rpm and a shift from 8th to 5th
is required (passing on 2 lane highways), clutch A has 1700 rpm of slip that needs to be reduced to 0 before it can be engaged. So as a first order of business brakes D needs to be disengaged (100 milliseconds), then engine ECU needs to blip the throttle” to increase speed to approximately 2350 rpm (another 400 milliseconds), then engine power needs to be cut and dog clutch A needs to be
engaged (another 100 milliseconds), and now you are in sixth after a half a second of no torque at all at the wheels, then another 200 milliseconds of low torque as the transmission finds fifth with a more conventional frictional clutch to frictional clutch shift and your engine speed is finally at the 2900 rpm.

From eighth gear to fourth gear is going to be even more of a contortion, with a torque interruption that is about a second long as the transmission ECU and the engine ECU do this delicate dance required to get both the dog shifting elements to engage. When you are looking to pass on a 2 lane road at 60 mph, a second can feel like an eternity, especially to a driver who has to use those paddle shifters to get into the right gear before executing the pass.

### Gear spacing

Another issue that the reviewers (including our own Alex L. Dykes) tend to take note of is the wide spacing between first and second gears. Unfortunately this is a direct result of the transmission lay out. Gear set 3 is pushed to the limit with the ratio, trying to make first gear ratio any lower than 4.7 would make the planetary gear pinion speed unreasonably high. So the first gear ratio is more or less
a given. It would be possible to change the tooth count on gear set 4 to numerically increase the second gear ratio. As a thought experiment, we could change the sun gear of gear set 4 to 46 or 50 teeth instead of the 42 it has, the result?

 Gear Ratios with S4 = 42 Ratios with S4 = 50 Ratios with S4=46 1st 4.700 4.700 4.700 2nd 2.842 3.020 2.931 3rd 1.909 1.909 1.909 4th 1.382 1.382 1.382 5th 1.000 1.000 1.000 6th 0.808 0.790 0.799 7th 0.699 0.699 0.699 8th 0.580 0.580 0.580 9th 0.479 0.454 0.467 Rev -3.805 -2.891 -3.308

So a 50 tooth sun gets better spacing between first and second, somewhat worse spacing between second and third, all other gears are largely unchanged, expect that reverse gets screwed up – it is perhaps not low enough for vehicles with off road ambitions. So perhaps a happier middle ground could have been a 46 tooth sun of gear set 4, that leads to a reverse of 3.308 with a second gear ratio of
2.9307. It also gives the transmission an overall ratio spread of 10.065, which sounds better for marketing purposes than the 9.81 that exists currently. This alternate reverse ratio is almost identical to the reverse in the ZF 8 speed RWD transmissions. So this 42 tooth sun gear is a bit of a head scratcher – perhaps keeping the tooth count at 42 saves some money because there is manufacturing tools can be shared between this sun gear and the sun gear for gear sets 1 and 2.

### Conclusions

I give a Colbert Tip of the Hat” to the engineers at ZF for this design. It is obviously a clever design but one that could cause some drivability surprises to an average driver, though durability-wise, I see nothing that causes major concerns. Design and development of a transmission concept like this ranges from 20 million dollars
to 50 million dollars, so the ZF management had some serious cojones to OK this design, it is a risk that has paid off to an extent. I know of many a management teams that would have said no, but time will tell just how compact other 8/9/10 speed transmissions and just how good a decision this is..

1
Some CAD renders
available on line show a 78 tooth sun with 114 tooth ring gear, this is a kinematic
equivalent of 91 tooth sun gear and 133 tooth ring gear

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## 119 Comments on “Ur-Turn: Saturation Dive Into The ZF 9-Speed...”

• LALoser

Wow! Good stuff!

• highdesertcat

Amen to that ^.

Sure makes my days of rebuilding B&M Hydramatics, C6s, C4s, TorqueFlites and THM350s seem downright antiquated.

• zbnutcase

No kidding. Rebuilt many old school automatics myself, but these will be viewed in the same light as hydrostatic trans axle’s on cheap Home Depot garden tractors…not worth fixing, scrap said vehicle…sad

• carguy

Thanks for that thorough analysis!

It’s a welcome break from the too many opinion pieces which seem to have crept into these pages lately.

• Travis

Always good to learn something. I liked this a lot, too!

• Nick 2012

+1. Pieces like this separate TTAC from other auto sites. Keep it up.

EDIT: If the author went to Purdue please disregard my praise.

• udham

The author did not go to Purdue, but he is guilty of applying to Purdue a long time back. In his defence, Purdue did not accept him.

• qa

+1 even though my head is aching this is really good stuff. How about the sequential gearbox fitted in rally cars? Would someone be kind enough to describe how it works?

• hgrunt

I believe sequential rally car transmissions are far simpler than a traditional automatic like this one. The gist of it is, the setup is nearly the same as a traditional manual transmission, except instead of an H-pattern shifter, it’s connected to a linkage that will toggle through the gears sequentially when the driver pushes/pulls on a lever or paddle.

BMW SMG, Ferrari F1, and other single-clutch automated manuals also work this way.

• qa

Thanks bud. Do they rally drivers really skip using the clutch pedal or are their feet faster than the eye?

• Power6

Think of the sequential gearboxes in a rally car like a motorcycle trans, a true sequential gearbox is what they are. These gearboxes absolutely do not require the clutch to shift, just ease up on the throttle slightly.

• > Do they rally drivers really skip using the clutch pedal or are their feet faster than the eye?

It depends on the racing series. Specifically for WRC the clutch is only used on launches and the computer will blip the throttle automatically for downshifts. GT racing the throttle needs to be controlled by driver so they still toe-heel on downshifts. It’s arbitrary rules of the sanctioning body.

F1 & most consumer/non-racing cars will get rid of the clutch pedal altogether & the computer does the rev-matching, and neutral’s activated by some other arbitrary action.

• > These gearboxes absolutely do not require the clutch to shift, just ease up on the throttle slightly.

Just because you *can* doesn’t mean you want to, esp on downshifts.

• zbnutcase

+1 Need more tech articles here. If this site was the automotive equivalent to what CYCLE magazine was in the ’70’s, I would probably never leave home. The person who wrote this article is truly the second coming of Gordon Jennings…

• calgarytek

Excellent. Thank you for taking the time to put this together for TTAC!

(And that’s why engineers are the backbone of a prosperous country)…

• highdesertcat

What country was this tranny developed in?

• Chicago Dude

US Patent #8,657,717:

Inventors: Gumpoltsberger; Gerhard (Friedrichsafen, DE), Sibla; Christian (Friedrichshafen, DE), Beck; Stefan (Eriskirch, DE), Haupt; Josef (Tettnang, DE), Bauknecht; Gert (Friedrichshafen, DE), Ziemer; Peter (Tettnang, DE)

Assignee: ZF Friedrichshafen AG (Friedrichshafen, DE)

SUMMARY OF THE INVENTION

The objective of the present invention is to propose a multi step transmission of the initially described type, which has nine forward gears and at least one reverse gear having a sufficient transmission ratio, in which the design complexity and the overall size, in particular the overall length and the weight, are optimized, and in which efficiency is improved with respect to drag losses and gearing losses. In addition, in the multi step transmission according to the invention, minimal support moments should act on the shift elements. The transmission, according to the invention, should be particularly suited for a front transverse installation.

By the way, learning how to search the online patent database and then learning how to read a patent is an extremely useful skill, especially for automotive stuff. The patent is supposed to describe exactly how something is supposed to work, why it is useful and what sort of problem it solves. The only downside is that the automakers and suppliers patent a ton of stuff, so you don’t always know which patent applies to which real-world object you want to understand – unless you have access to that real-world object and it lists the patent number on it.

• highdesertcat

Hey thanks Chicago Dude for posting the above.

I already knew the answer and posed a (satirical) question to calgarytek’s “(And that’s why engineers are the backbone of a prosperous country)…” since this tranny was not developed in North America.

• thegamper

My head exploded after figure 2. Its magic as far as I am concerned.

• RogerB34

If CVT is an advancement in efficiency over a gear transmission why would a more complex gear transmission be the choice?

• segfault

Consumer preference and responsiveness of the gear transmission versus the CVT, for one; also, I don’t think there is a truck application of the CVT yet.

• skitter

A CVT can optimize the theoretical engine efficiency. But the gains there can be offset by poor driveline efficiency. DAF/snomobile style belt CVTs are generally in the 60-70% range. Meaning they send 60-70% of the engine power to the wheels and turn the rest into heat. Modern automotive CVTs can be much better, but to my knowledge have a hard time matching the driveline efficiency of transmissions with actual gears, which are commonly deep into the mid 90s efficiency wise. Locking torque converters are critical to keep the efficiency of the entire driveline high.

• th009

Modern CVTs have better than 95% efficiency, reaching 96-97% for most of the operating range.

• highdesertcat

Yes, but the sacrificial part is still the steel belt since the cones rarely wear.

Simplicity itself, why does it cost so much more to rebuild a CVT?

• TTACFanatic

Simplicity itself, why does it cost so much more to rebuild a CVT?”

Because as far as I know nobody rebuilds (passenger vehicle) CVT’s. It is common practice to pull and replace with a new unit.

This is my primary dislike of CVT’s. Problems are hardly rare, and are fantastically expensive when they do crop up. Though with the trend of nine and ten speed automatics the price difference for repairs may be non-existent.

• highdesertcat

Having had more than one Altima as a rental, I have decided that I don’t like the way a CVT accelerates.

I am not an aggressive driver but I do like response from my gas pedal when I need it.

OTOH, there may be a Lexus LS460 8, 9, or 10-speed in our future when we can no longer climb in and out of our SUVs and Truck, and this article sure casts a nice light on these cutting edge automatic transmissions.

• 28-Cars-Later

Depending on what you bring, the Cottman shop I deal with might recommend a reman or junkyard standard automatic transmission swap as opposed to a rebuild.

• I think nobody uses cones in CVT anymore, or at least it looked that way from cutout pictures. Instead it’s a bit of a system like iris that changes the diameter of pulleys.

• highdesertcat

Pete Zaitcev, I think you’re right. The cones were old school even though the power transfer using a steel belt concept remains pretty much the same, these days assisted by a computer and better, more precise hydraulics and actuators.

• danio3834

With the addition of more gear ratios like this 9 speed, the ratio spread of a stepped automatic can exceed that of existing CVTs. As stated in the article, ZF unit has and adveritised spread of 9.8:1 where the latest Jatco units are about 7:1, similar to that of the ZF 8 speed.

The inherent design of locking components together rather than relying on the friction of a belt is more efficient as noted by skitter.

Also, many CVTs have had some durability and noise issues in service, the Jatco models in particular.

• wmba

The big loss in CVTs is the hydraulic oil pump required to force the two halves of each pulley together to avoid slip in the steel belt. Since both the stepped automatic and CVT generally have a pump to supply the torque converter each has, the continuing pumping requirement for the CVT after the converter locks up is the main loss. It varies by rpm and torque demand required of the engine. The pump and associated parts are the main areas being addressed for efficiency in CVTs.

• udham

Chrysler used the Jatco CVT in the Caliber/Compass vehicles. Then they dropped it and went with ZF for this generation, what does that tell you?

• danio3834

The CVT is still used in those vehicles but only on those equipped with the Trail Rated off road package. The 6 speed auto that largely replaced the CVT is a Hyundai powertech unit, and is far more pleasant.

• link3721

The CVT used in those shouldn’t be compared to modern CVTs. At the time, they provided improved fuel economy compared to the 4spd automatics of the day. Caliber and Compass we’re introduced 8 years ago, making the CVT pretty old tech. If they used an updated CVT for their new models, they would be tainted due to the current CVTs reputation.

• pragmatist

CVTs have their own issues. They are rolling friction driven with parasitic losses as well as a tricky job of keeping friction element (such as a chain in typical designs) with JUST enough friction to carry the load but not so much as to cause wear and friction. Because they utilize dynamic friction, they are more vulnerable to loading, wearing faster under heavier loads (as long as design load is not exceeded, conventional automatics are not seriously affected) .

Also most CVTs have a quite limited range between low and high gears.

• Alfisti

Moments like these make me realise how dumb I am.

• RS

More articles like this please.

• AMC_CJ

Good job, we need more content like this. This kind of stuff is what perpetrates TTAC from the rest of the drivel blogs out there.

• dtremit

This is really fascinating, and I look forward to reading it in more detail when I have more time.

That said, you should think about putting some of it behind a break — it is taking over the front page.

• raresleeper

All specs aside, it’s going to be intriguing seeing how the magical ZF9 wonder-box holds up for the long haul.

My auto trannied car (SUV rather)- a GLK350- has seven gears under it’s belt, which seems to more than enough for a slushbox.

But nine?

That’s a big number for a complicated mass of gearing we call the automatic transmission.

• gmichaelj

A couple of points:

1. Finally, thank you for the TRUTH about the downshift issue:

“From eighth gear to fourth gear is going to be even more of a contortion, with a torque interruption that is about a second long as the transmission ECU and the engine ECU do this delicate dance required to get both the dog shifting elements to engage. When you are looking to pass on a 2 lane road at 60 mph, a second can feel like an eternity, especially to a driver who has to use those paddle shifters to get into the right gear before executing the pass.”

The issue didn’t bother Dykes, so he left comments about the downshifting lag on “the cutting room floor”
http://www.thetruthaboutcars.com/2014/02/review-2014-jeep-cherokee-limited-v6-4×4-with-video/#postcomments

It would bother me to distraction.

2. So based on the detail of the article and the apparent knowledge of the author, I’ll amend my earlier comment on not reading “Anonymous” articles (it was the subject matter that reeled me in).

3. Nevertheless, if a reader has an issue with something in the article, how will “Anonymous” respond? Will he get a temporary Appellation from the editors? Like answering the above question from RogerB34 on using a CVT instead

• We can post his answers for him.

• Alex L. Dykes

I commented on it over here in the separate transmission article: http://www.thetruthaboutcars.com/2014/02/zfs-9-speed-9hp-transmission-puts-dog-clutches-on-the-leash/

While the downshift time may seem excessive, the actual shift time from 8th to 4th quoted by ZF is 400 milliseconds which *is* slow by modern standards but it’s far faster than a second. It’s also faster than it would take many 1990s era 4-speads to drop from 4th to 2nd. It’s something that I think most people would get used to, just like most drivers would get used to a CVT, or a hybrid, or an EV, etc.

• fredtal

Yet I’d still rather have a old fahion manual box and clutch.

• heavy handle

This article answers the question “why didn’t they just make it 8 speed, wouldn’t that be cheaper/more reliable?”

It’s a 9 speed transmission because that’s what you get from four planetary gear sets with those gear ratios. You can’t build it without 9th, so there’s no point in not including 9th, even if it engages above the legal limit in most states. In other words, all of the gear sets are used in all of the ratios, unlike a traditional two-shaft manual where only one gear set is transmitting torque at any given time.

• Zackman

Like carburetors, these things along with differentials baffle me. I just can’t get through the first two or three steps, but this is good stuff just the same.

Now, since this is a Chrysler transmission, are the parts of equal quality to the design? Will the dipsticks be marked correctly, and will the fluid be specified right?

Yeah, yeah, I know – that was years ago and I’m teasing, but even I can’t get Ultradrive and 2.7L out of my head, though I haven’t bought a Chrysler vehicle since 1999 and have not been affected by those problems.

True – I haven’t heard anything recent concerning Chrysler trannys grenading and engines seizing, so that’s good.

But the question remains: For some reason Chrysler vehicles are always near the bottom of CR’s quality ratings, and should we expect improvement now, though positive steps have been forthcoming?

This transmission, while amazing, scares the daylights out of me!

• imag

It’s not just a Chrysler transmission. It’s being used in Jags and Land Rovers (not the best quality benchmarks, I know), to – it is rumored – some soon-to-arrive Hondas and Acuras:

http://www.automobilemag.com/features/news/deep-dive-honda-acura-could-adopt-zf-9-speed-automatic-by-2014-118443/

Insert Honda transmission joke here, I guess.

• Timothy

Wow. Does my head hurt.

• Kinosh

This is awesome. Keep more of this coming. I’ll have to spend some time with my gear design book before I’ll be able to fully digest it.

Would there be any interest in an article explaining the concepts behind thermodynamic efficiency and what automakers are doing with engine tech to increase it?

• Luke

Excellent article and explanation!

• johnhowington

“this little gear is going at any eye watering 9500 rpm”

“durability-wise, I see nothing that causes major concerns”

its always the minor ones that add up to a major one.

• Brian P

If you look at the diagram, that small-diameter sun gear is just spinning freely on bearings, centered between the two planetary gear sets. With the forces in those planetary sets balanced, there is next to no load on the bearings, and with the dog clutch disengaged, there are no clutch plates spinning at high speed relative to each other.

Remember also that with 9th gear being that tall, 2000 engine rpm is about 140 km/h road speed or maybe even more. If one were to attempt a top-speed test on a vehicle with this transmission, the engine probably won’t pull 9th gear up to top speed. It’s pretty likely that top speed would be in 7th, and then that sun gear wouldn’t be spinning quite so fast. In fact, it wouldn’t surprise me one bit if the programming for the vehicle disallows 9th above a certain (high) road speed, and maybe even disallows 8th above some even higher road speed, to contain the speed of that sun gear.

This transmission is an ingenious design, and this is article does a great explanation of it.

• Joebaldheadedgranny

Thank you for this excellent piece. I’m reassured by your confidence in the long term viability of the ZF 9 Speed. I’ve always wondered why CVT wouldn’t get the upper hand but now I’d willingly place my bet on the ZF.

• jettaGL

I think we overlooked the best part, it will come with “lifetime” fluid.

• Matt Foley

Excellent and informative article. Not only is this author brilliant, he even knows when to use “i.e.” as opposed to “e.g.”

For more fun, imagine the article being read by Desmond Llewelyn as Q. “Now pay attention, 007! Before we dive right into the 9-speed gearbox, let’s take a quick refresher on the basics of gears…”

• cRacK hEaD aLLeY

Great article, thank you for taking the time to write all this for us.

• 71 MKIV

Hey Zackman. this u tube video explains differentials better than anything else I have found.

http://youtu.be/yYAw79386WI

71 MKIV

• Autobraz

This is such a good video. It was the one that really made me understand the principles behind the differential. Thanks for reminding me of it, 71MKIV.

• Lie2me

This is a great video 77 years old but the principles are the same, now how does it translate to FWD based AWD system? (scratches head)

• Brian P

The diff in a front-drive works exactly the same way.

The tricky bit with AWD systems is how the torque gets split front to rear. It COULD be done using a diff (thus 3 total in the system – first the split front to rear, then each of those splits left to right), but nowadays, it’s usually not done that way. It’s usually just a clutch, that can either be completely disengaged (operating only with two drive wheels), or locked together, or modulated somewhere in between.

• > The diff in a front-drive works exactly the same way.

To be more specific the diff in fwd is in practice integrated into the transmission aka transaxle. The more interesting part in a fwd driveline is the constant velocity joint which allows for steering on the driven wheels.

• tedward

Great article. It’s not just the content it’s the perspective.

• walleyeman57

Great article.

I am old enough to remember getting transmissions rebuilt at local shops for \$300-\$500. I was under the impression that these were taken apart and fixed on site. Of course the durability of both the OEM and the rebuild was maybe 100K miles.

Our ’04 Sienna has over 360,000 miles on the original tranny and it does not show any obvious signs of wear.

Can we expect similar life span from the ZF? Time-and miles- will tell.

• I once saw a 3-speed transmission like that in a museum. It was a manual from 1920s. I am wondering, why is it that we don’t see manual transmissions with planetary gear? Would it be possible to have something like a 6-speed where the shift lever engages suitable brakes and dog or friction clutches? Seems way more robust than traditional synchromesh, but obviously it’s not the case. But why?

• jpolicke

Wasn’t that how the trans in the Model T worked?

• wmba

+1

The same question has bugged me ever since VW brought out their DSG. Why add yet another parallel shaft with gears attempting to spread themselves apart under load, when a mechanical/electric solenoid gear selection of a planetary transmission would do the trick with much lower friction loss and stress.

I have an SAE book on the design of transmissions up to about 1990, both manual, preselector, fluid drivr and automatics. I cannot fathom the Hobbs Mechamatic transmission that David Hobbs’ father designed back in the ’50s and ’60s, and fitted to a racing Lotus Elite. Hobbsy decimated the opposition with it when it worked. Wonder if it were something like this. Anyone know?

• > The same question has bugged me ever since VW brought out their DSG. Why add yet another parallel shaft with gears attempting to spread themselves apart under load, when a mechanical/electric solenoid gear selection of a planetary transmission would do the trick with much lower friction loss and stress.

Generally speaking, it’s my assumption that you can make any auto a “manual” by replacing the converter w/ a clutch, and using the same control logic to shift. This would have the downside of less tactile controls and the usual added expense.

On the other hand DSG have the benefit of *much* faster shifts. So the question should be if planetaries should be used on each shaft of a DSG. I assume it has the same downside of greater cost for questionable gain.

• jmo

“But why?”

Cost, durability, feel, drive ability, etc.?

If it worked perfectly other then the need to double clutch from 3-4 that would likely be a deal killer in terms of consumer adoption, I’d think.

It would be interesting to know.

• heavy handle

There’s no point trying to control the shifts mechanically. Much cheaper to make the gear shift sequential and control everything with actuators. Of course, at this point you’ve basically got an automatic transmission without the shift logic and the PRNDL trim on the console.

I think you see where I’m going with this: you’ve described every planetary gear automatic that has paddle shifters or a +/- gate in the shifter. Minus the “D” mode that most customers want. At a total saving of \$nothing (because it costs more to stock two nearly identical parts).

• udham

Because doing a shift system that would allow for a conventional H-pattern shifter to control such a transmission with any sort of reasonable shift effort and shifter travel is impossible.

• > Would it be possible to have something like a 6-speed where the shift lever engages suitable brakes and dog or friction clutches?

If you can look up a pic of the mechanical hydraulic “logic” controller of an older auto, it will likely be less trivial than the manual’s forks, and it’ll still be more expensive yet less gears.

• raph

Check out a Lenco ( http://www.lencoracing.com/ ) although I don’t know what complexity would be required to prevent having an individual shifter for each element.

• DC Bruce

As I put my exploded head back in one piece, thanks for taking them time to explain this.

This will take several reads to digest . . . but that’s o.k.

• sportyaccordy

This is not a piece mere mortals can read in one sitting. I am going to have to come back to this. This is exactly the kind of stuff we need here though. Great work

• johnhowington

as everyone has declared this to be such an excellent article, i will go against the grain and accuse it of being unintelligible as “The intrinsic Hodge theory of p-adic hyperbolic curves.”

• wmba

Great article. The contortions required to obtain each ratio flow from the design – if anything goes wrong in the sequencing when you floor it at 55mph and it dithers around going from say 7th down to third, a rather large amount of finely machined parts will meet the countryside if something doesn’t work perfectly.

Call me an old Luddite of an engineer, but I was never a proponent of doing something just because you can and thereby appear brilliant. So I hope some risk scenarios were thoroughly explored to see whether something this complex in control strategy using mere commercial grade parts in the electronics is a wise thing to sell.

But that’s me. This guy knows his transmissions, and he seems relatively happy. If it were me, I’d have the big Overdrive on sixth gear, with the first five ratios closer together for urban/suburban use. Six gears is enough, and a bit of a clunk engaging OD sixth is positive reinforcement to the customer that yup, now I’m in cruise mode. You then select whatever ratio a vehicle needs for economical cruise.

Today, we now have so-called 4K TVs. But nobody changed the bandwidth of the broadcast, which is heavily compressed anyway with 1080i. So it’s all paper wars and big numbers to impress the proles. Kind of the way this 9 speeder strikes me.

• Long term, broadcast is dead. More and more simply watch everything through Youtbes and Netflixes, which can support the bandwidth, especially if pre-buffered.

• wmba

You’d need more than the 20 mbits/sec we have now by a huge amount to get true 4K tv, surely. Maybe not by a factor of 16 perhaps, but significant. Does your ISP have true consistent 100mbits/sec feed for each customer? I just wonder.

• > You’d need more than the 20 mbits/sec we have now by a huge amount to get true 4K tv, surely.

Online streaming can use arbitrarily complex compression. Netflix is already going to stream 4k at under those rates.

• indi500fan

Once the engine guys gave us access for “full authority control” this transmission stuff got a whole lot easier. Back in the old days when all you had was a vacuum modulator, life was hard, LOL.

• 28-Cars-Later

Kudos to our anonymous author, this is an excellent article.

• koshchei

Amazing article! Thanks for this!

• frozenman

Mind bending but insightful article, makes me think I need to buy the last model V6 Accord with 6 speed auto and keep it going until I’m dead!

• mfgreen40

Jpoliceke yup the model T had a band that stopped the drum for low gear,another band for reverse and a spring loaded disc to engage direct drive. These are great articles, it makes the 9 speed trans. seem a bit more reasonable.

• brenschluss

Chewy stuff, thank you for posting.

• JKC

Great article. More like this, please.

• IndianaDriver

Great explanation of this transmission. I guess we’ll see how this pays off for ZF after their \$400 million investment to build a factory in South Carolina to make these along with their 8 speed transmissions. It’s common to see the major automakers lay out money like that, but not the suppliers.

• amca

I was thinking I can’t grasp this without a visual aid. Naturally, there’s one on YouTube:

http://www.youtube.com/watch?v=r1BYOOJKyaQ

Cool? Yeah! Feel less stupid? Definitely.

• For those enthusiasts interested in insights into how a car works, I highly recommend reading an auto mechanics school text/manual. The best in brevity is probably Auto Fundamentals – Stockel, although they’re all largely interchangeable. The level beyond this is auto eng texts that show why/how to do calcs like those above, which is probably excessive for hobbyists.

• Big Al from Oz

Very well presented.

I saw a question regarding the servicing schedule for this transmission. You should be able to find out in a Jeep owner’s manual.

It’s a very complex transmission. I wonder when and if unscheduled maintenance is required will the car dealers R&I the transmission, send it off to a contractor for repair.

If this is the case will the manufacturer carry enough stock so the turnaround time is shortened during transmission maintenance.

Aviation doesn’t have transmissions this complex, though we do frequently use planetary gear arrangements for flight controls and wingfold mechanisms with high torque loads. These are simple single speed transmissions, with a brake to lock the flight control from a transmission failure.

• Brian P

I think the capabilities of local transmission shops end with 4-speed rear-drive automatics and *maybe* some front-drive 4-speed automatics. If it has 5 speeds or more or is a CVT, it’s not locally rebuildable. If anyone knows differently, post up.

If there is anything beyond a minor externally-serviceable problem (fluid level, valve body, sensors and actuators), the normal approach nowadays is for the local shop to only remove and install. The transmission they put in can be anything between factory-fresh, or rebuilt by a central authorized facility who are the only ones who can get parts for them through official channels, or (more commonly, if you’re footing the bill) one from a junkyard.

• NN

just wanted to also share my appreciation for such an informative and interesting write-up. Thanks!

• Domestic Hearse

My English major brain just locked into a Big Gulp sized Brain Freeze. Owwwwwwww.

Okay, better.

Having just purchased the 2014 Cherokee, it’s neat to know a bit (understanding is still somewhat a stretch) about the transmission in our new ride.

We got the Limited with the V6. The more we drive it, the more it becomes an acquired talent to get the vehicle to launch properly.

From what I can tell, 1st and 2nd are so dang close in ratio, the car lunges a bit, just a few feet from its start-point. One expects a smooth acceleration up to about 15 mph, but the Jeep grabs second right away. The sensation is a bit like when I taught my daughter (then 15) to drive a manual clutch. Revvvvvv, clutch dump, head/neck snap. Only the Jeep keeps going while my daughter would stall, again and again, in a cloud of clutch smoke.

The key, I think, to launching the 2014 Cherokee is to be somewhat aggressive, if possible. The instinct is to feather the throttle, as its tip-in is pretty sensitive, and that only causes the 1st-to-2nd lunge to be more noticeable. Get into the throttle and it steams away much smoother. However, traffic conditions do not always allow this. Since the vehicle allows for manual shifting, I think in heavy traffic I’ll experiment with holding the car in 2nd, perhaps 3rd, just to see if it’ll stop the upshift lunge.

If you have any thoughts, as an engineer, about how the 14 Cherokee shifts, and ways to smooth out performance, I sure welcome them.

• A simply amazing article…this article is a perfect example of why I follow publications such as TTAC.

• Neat article. The author noted some oddity with the wide 1-2 ratio spread. I don’t know if this illuminates things or not, but in the LR Evoque, this transmission is cited as almost always starting off in 2nd, with 1st only used when towing or extracting yourself from a ditch, or some such.

With that consideration, it might be quite deliberate that 1st is an extra-low gear, and who cares if the 1-2 shift is weird because most owners will hardly ever experience it,

What is true of the Evoque may also be true in other applications.

• WildcatMatt

More like this, yes please.

• wmba

And if you got this far, you’d like to know how it works, right?

Not well.

The Chrysler 200 V6 programming isn’t much good. Constant hunting on city speed uphill grades as you feather the gas up and down to maintain speed and distance to the vehicle in front of you. Sudden acceleration at 60 mph takes a second or more while the trans scratches its head to deliver a gear. Trying to gear down using the paddle going down a steep grade leaves the car freewheeling in some neutral limbo. Highly disappointing.

The Acura TLX V6 has better city tuning with its version. The other two objections remain.

Enough to put me off both cars. Cannot imagine why Honda uses this thing.

• wbdeford

Very interesting thing about the gear-sets is they essentially combine the rear and reverse units of GM’s old 4 speed Hydra-Matic of the 1940s to early 1960s, connecting their output with a (3 speed) Simpson gear-set (GM switched to a Simpson setup in the mid-1960s)

Hydramatic Rear Unit corresponds to set 4.
Hydramatic Reverse Unit correspons to set 3.
Simpson corresponds to sets 1 and 2.

Sets 3 and 4 allow things the original Hydramatics did not–namely, being able to lock S4, and being able to drive R4 forward while R3 is locked.

Amazing the similarities…thanks for posting this!

• niranjan007

HI can anyone please explain me how in sixth gear sun gear in set 1 and set 2 is said to rotate 1.85 times the input speed, I am new to automatic transmission trying to figure out the 9 speed gear ratio calculation if somebody give a quick reply it would be immensely helpful thanks….

• niranjan007

Can somebody explain the calculation of 9th gear Gear ratio.I could not figure out how the speed of carrier of gear set 4 is rotating backwards at approximately half the input speed.
Can anyone give a quick reply it would be immensely helpful.

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