By on January 10, 2009

This subject came up in the comments underneath my review of the Bugatti T40. [If you haven't come across dogboxes before, they're explained (or at least chewed over) here.] Googling to shed some light on the debate, I came across this hair-raising video of BMW supercar with a straightcut dogbox eating up the competition, sounding like something out of Star Wars. My take is that while straight cut spur gear are weaker (in principle), they can be constructed to be enough bigger in the same space that they end up stronger (in practice). Perhaps TTAC’s Best and Brightest would care to comment?

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17 Comments on “Ask the Best and Brightest: Dog Engagement vs. Synchromesh...”


  • avatar
    osnofla

    dog if i can have that nasty bmw and lessons, otherwise i’ll take synchro.

    were those other cars lister storms?

  • avatar
    snafu

    In the late 60′s and early 70′s GM introduced a heavy duty 4- speed manual transmission that earned the nickname rock crusher because it sounded like it was crushing rocks.

    That transmission came with straight cut gears and was the strongest out of the family of manual transmissions for GM passenger vehicles. The synchro was still part of the shifting arrangement for the rock crusher.

    If you look at the way the teeth are cut in the photo above, input shaft cluster on the left, the aggressive tapered angle at which they are cut, in theory, serve to function in a similar manner as a synchro with added noise and perhaps a bit of added effort.

  • avatar
    degrouch

    Isn’t what they call a “dog box” also called a “sequential gear box” like most motorcycles and IndyCars use? Advantage–very fast shifts, (clutching optional, if you’re good.) Disadvantage–gears must be selected in order. (ie: you can’t go from 5th to 1st, w/o shifting through 4, 3, 2 on the way down.) And every time you miss a shift, you grind/chip off a little of the dogs, until the tranny is shot.

    Regarding the whine, I have read that is the sound of the straight-cut teeth on the meshing gears “hitting” against each other as they engage. They are stronger than helical cut gears, since the entire face of each tooth is engaged, where helical gears have a smaller point of contact. But helical cut gears are mucn quieter, since helical cut gear teeth “slide” together, instead of slapping together. Most non-sport motorcycle trannies use helical gears for quietness. So any racing/performance application of a sequential gear box will have straight-cut gear teeth, and the associated whine.

  • avatar

    I think degrouch is right about sequential boxes…in the video you can see the BMW driver shifting in just the manner he describes, with multiple flicking shifts…

  • avatar
    chuckR

    The only thing I can conclude from that video is that profound hearing loss isn’t an issue for race car drivers. Or that they wear high performance ear protection. The sound was wince inducing even thru tiny laptop internal speakers.

  • avatar
    Mericet

    Straight cut gears also provide less drive train losses.

  • avatar
    porschespeed

    Dog boxes can utilize either straight or helical cut gears. Depends on your application.

    As to penultimate strength, the highest rated apps from all manufacturers I know of (Jericho, Lenco, etc) all use straight cut gears.

    In fact Lencos (drag only) rated 2000+ HP, are straight cut planetary sets.

  • avatar
    i6

    I haven’t read the article that lead to this subject but wow, is there a lot of confusion in the terms being used here! The distinctions are as follows…

    - Dog ring vs. synchromesh gearboxes
    - Straight-cut vs. helical-cut gears
    - Sequential vs. h-pattern shifters

    These can be mixed and matched in any combination.

    In the first case, dogs vs. synchros refers to the way gears are engaged/disengaged inside the gearbox. ‘Dog boxes’ have gears that spin freely on their shafts, they are engaged by sliding the dog ring (which spins with the shaft) against the side of the gear where it locks into matching grooves, making the gear spin with the shaft. In ‘synchro boxes’, the gears themselves slide on the shaft on splines and the teeth of the gears physically lock and unlock from each other. This can easily damage the gears, so synchros are used to help match the speed of the gears as they engage. Kind of like a clutch. Dog boxes are used in racing because they allow faster shifts but need more maintenance.

    Straight-cut gears are more efficient than helical gears but create that loud whining noise as heard in the video. They are used exclusively on race cars. Awesome video, BTW. I never get tired of seeing it.

    Sequential vs. h-pattern simply refers to the way the gearbox is linked to the shift mechanism and how the driver operates it. Sequential shifters are like dog boxes in that they allow faster shifts but also require more maintenance, which is why the two are often used in combination.

  • avatar

    We have a similar debate in motorcycling around clutches. Dry clutches are hallmarks of racing machines and some road bikes (mainly Ducatis, but also some other marques, and BMW with its car-type diaphram clutch). The advantage is ease of maintenance and replacement, not filling your oil with clutch material, and supposedly reducing friction losses associated with a wet clutch running in the engine oil (a point that is often debated and as yet unproven). The disadvantage is high wear (clutches on my particular Duc last around 10 000 miles, if I go easy on them), changing engagement points as the temperature goes up, and the most incredible racket you will ever hear. A Ducati style dry clutch sounds, to quote someone else, like a couple of skeletons having sex in a metal trash can. Personally I love the noise, it’s part of the raw appeal of the machine, one of many brutal elements in the design and performance of the thing. I constantly have people ask what that noise is, or what is wrong with my bike. I shrug it off and enjoy the mechanical symphony, which punctuates shifts with a “ka-chink” noise and low speed running with a deafening clatter.

    http://it.youtube.com/watch?v=UsnNgU8pKoc

  • avatar
    dolo54

    If it sounds like that, as cool as it would be for a track toy, there’s no way I could drive that on a regular basis. I think you would go deaf in a matter of weeks. I have always wanted to try a car with a sequential gear box however.

  • avatar
    davis

    sraigt cut vs. helical gears:
    straigt gears/bull cut gears are noisier than helical typically they also typically have bigger teeth and less of them which makes them a little more durable in a heavy service environment.
    helical gears are cut with an offset opposing tooth pattern which typically runs quieter than strait cut gears.The real advantage to helical gears is the offset pattern which will pull the gears together when in rotation/under load which removes stress from the bearings on the shaft.
    I think that the real question here is not about how the gears are cut but about manual transmission types.
    There are two common types.
    1st there vis the old style referred cto as a crashbox/non synchro shift transmission.This style of trans would typically shift tre actual gear to mate with another that was not allready in motion these tansmissions took patience and practice to operate without grinding gears.
    The second style is referred to as synchromesh
    transmission.synchromesh transmissions typically are a constant mesh type. constant mesh means that all gears are moving and engaged at the same time. This style shifts ranges/gears by moving a splined coupler onto matching splines on the gear to engage with the output shaft.The synchro is a guide or sometimes a clutch between the spline on the coupler and the splines on the gear which will help line up the coupler with the gear which makes for much smoother/faster shifts.
    both trans types will typically have both types of gear contained within them helical gears for foreward engagement and strait cut gears for reverse.[reverse gear in a constant mesh transmission is typically not constantly meshed and is actually sfed the old fashioned way]

  • avatar

    Another gear and the windows would shatter…

    Thanks for the educational responses, guys. I learned quite a bit from them.

  • avatar
    bolhuijo

    Perhaps the largest factor in gear noise is the contact ratio. This is the number of gear teeth that are in contact at any given time. More teeth in contact means less stress on each tooth for a given load. Gear teeth are cut to mesh “just right” for quiet operation, but a gear tooth will deflect just a little under heavy load causing the next two teeth that mesh to hit together just a bit. At speed, this produces a whine proportional to the frequency of tooth engagement.
    Helical cut gears have a much higher contact ratio, spreading the load across more teeth simultaneously, reducing gear noise. These gears also produce axial thrust in operation, which must be dealt with.

  • avatar
    rpn453

    Cool video. I’ll keep my synchronized h-pattern transmission with helical gears for street use though.

  • avatar

    Thanks all, the Best and Brightest have it covered. It seems that in any modern, moderate load application helical synchro is the no-brainer choice, that in a few high-stress, high-load situations, dump trucks and wild-ass race cars, straight-cut can be built stronger and are the ultimate solutions. The Bugattis’ gear noise was never in the dentist-drill frequency range exhibited by that Dutch Uber-Bimmer. Unless that driver is Old Nick or has some incredible hearing protection, his hearing is going to last as long as the gearbox: one season at best.

  • avatar
    Eric Bryant

    I think there are a few misconceptions in the above posts.

    Straight-cut gears vs. helical – The whole point of helical gears is to intentionally create a thrust load (along the axis of the shaft’s rotation) to force the gears into engagement and thus decrease noise. The drawback is that this thrust force puts a hell of a load on the case, and can actually lead to failure of the case or endplates. Thus, straight-cut gears are stronger, but the amount of noise is pretty much unacceptable for street use.

    What you’ll see in heavy-duty transmissions (like the later versions of the Tremec T56) is that the lower gear ratios have a lower engagement angle (not quite straight-cut, but getting close) to get some more strength where it’s usually needed. The upper ratios go with a larger angle to decrease noise when cruising.

    Dog-cut vs. synchros – Both types of gearbox use the constant-mesh principle, so we’re not banging the actual gear teeth in and out of engagement. Rather, the difference is in how we lock those gears to the shaft. A synchro uses some friction material to match the speed between the shaft and the gear that’s being selected. This friction material may be organic in lower-perf boxes, and carbon in higher-end boxes – it’s a trade-off between performance when the box is dead-cold on a winter morning, and after a few hard laps at the track or passes down the strip. Once the friction material gets everything spinning at the same speed, a series of small teeth are aligned and mate to transmit the torque. Dog boxes don’t use any friction material, and depend on the mechanical engagement of large blocks of material (the “dogs”).

    The “dogs” in a dogbox are typically much bigger and stronger than the teeth in a synchro box, so the dogbox is usually more durable. These larger teeth are also typically spaced as to allow greater misalignment prior to engagement, reducing the chance that the transmission prevents the driver from executing a gear change. The dogbox also demands a rather forceful touch (some also require the driver to properly match revs), and thus can be wiped out quickly by someone who doesn’t know what he is doing.

    It’s possible to mix-and-match the above technologies, but in practice this is rarely done. If you’re going after such an all-out effort that you choose dog engagement or straightcut gears, then you’ll probably pick both. This isn’t a hard-and-fast rule, though; for example, you can find aftermarket vendors that will dog-cut your production gearbox to eliminate the synchros but leave the helical-cut gears in place.

    Note that most motorcycle transmissions are dog-cut and use straight (or very nearly straight) teeth.

  • avatar
    SonyAD

    All modern automotive manual and robotised manual gearboxes are constant mesh with only Reverse being the exception. Premium cars with manual trannies may offer a constant mesh reverse as well, complete with synchronizer.

    Most, however, employ a sliding idle pinion solution. Which is why the gears involved in reversing also have to have a straight cut tooth profile leading to that characteristic whine. To permit the sliding in and out of engagement of the idler gear without lateral thrust on it.

    Except for lateral thrust, helical profile gears are superior in every respect to spur gears, including torque limits.

    The proper, elegant engineering solution to the side thrust issue is to employ double helical/chevron gears to cancel it out. Unsurprisingly, it was the hallmark of French auto maker Citroën. Because the French have always been in the bad habit of actually engineering solutions instead of hacking stuff together like the Germans. But I digress.

    The drawback, there was bound to be one, is that chevron gears are very unsuitable to mass production. They are damn near impossible to machine without welding two helical gears together or leaving a flat ring round the middle of the gear. Leaving the flat ring in the middle makes the gears wider than they could be.

    They are used in very high power/industrial applications.

    Of course, the corner cutting approach is to just use spur gears and rebuild/replace the box each season.


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