By on January 19, 2015

Yum. (photo courtesy: www.crownvic.net)

 

Long-time TTAC Commentator 86er writes:

Hi Sajeev,

Could Piston Slap furnish me with a be-all/end-all explanation about wheel offsets? The more I try to read up on it on the web, the more confused I get. I’m pretty clear that RWD (at least traditionally) went with the low-offset while the FWD revolution made high positive offsets the industry standard, at least in passenger cars.

A few years back, I had purchased a set of winter tires on rims for my trusty ol’ 92 Vic and later after research found out that the rims were medium-offset that went on a 4×4 Ranger of similar years. I’ve heard that putting on a different-offset wheel can hurt steering/suspension parts like ball joints, but I’ve never seen it in black-and-white, so to speak.

Sajeev answers:

Let’s cover the basics of both wheel offset and backspacing: offset is the location of the mounting hub in relation to the center of the wheel’s barrel.  This mounting hub goes to flat surface where car’s suspension holds the wheel (i.e. the hub on the spindle).

http://www.fastcar.co.uk/

Image Courtesy: www.fastcar.co.uk

 

A positive offset pushes the wheel’s hub away from center, closer to the outside of the car. Negative offset is the opposite: sucking the wheel’s hub closer to the inside of the car. Zero offset means it’s smack dab in the center.

I question if the traditional FWD/RWD offset difference still holds water.  While FWD wheels often have a more positive offset than their RWD counterparts, all (most?) modern vehicles have flat faced wheels (for aerodynamics and countless suspension needs?) stemming from a more positive offset wheel. Need proof? Look at your own platform: peep the redesigned front clip and the mandated wheel redesign of the 2003+ Crown Vic.

CrownVicFrontSusp05_06_edited

(photo courtesy: http://www.ridetech.com)

Oh wait, the Crown Vic barely changed at all from 1979 to 2011.  It was such an antiquated pile: must remember to toe the autojourno line, never speak of Panther Love! But I digress…

In theory you should keep a close-to-factory offset to optimize steering geometry and wheel bearing health.  In practice, it might not matter: especially for a set of winter tires. You probably can’t drive aggressive/fast enough to care.  Probably…

There’s also the matter of torque steer on FWD machines, mostly for those with unequal length half-shafts. But most modern vehicles use equal length shafts?  (Have at that, B&B!)

You also need to consider backspacing. This ensures the width and offset of wheel you chose will clear your body or suspension, especially on cars with strut suspensions.  Instead of my usual ramble, I think this video really nails it.

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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23 Comments on “Piston Slap: A Primer on Wheel Offset and Backspacing...”


  • avatar
    rpol35

    “A positive offset pushes the hub away from center, closer to the outside of the car. Negative offset is the opposite: sucking the hub closer to the inside of the car. Zero offset is smack dab in the center.”

    Maybe I’m slow this morning Sajeev, but the accompanying pictorial looks like it is showing the reverse; positive offset moving in closer to the hub and negative pushing further out away from the hub. Have I missed something?

    • 0 avatar
      sportyaccordy

      Sajeev’s got it right- he’s talking about the hub’s location relative to the center of the car. Positive offset pushes the hubs to the corners away from the center, negative offset does the opposite. Relativity is tricky.

      • 0 avatar
        Skink

        The hubs are fixed – there’s no pushing the hubs in or out. Changing the offset pushes the contact patch in or out. Accordingly, pushing the contact patches outboard relative to the hub results in them being offset more – if offset is understood to mean ‘away’. The more I look at what various sources say about it, they disagree with my literal-minded analysis. Oh well.

        • 0 avatar
          Skink

          By hub, the better term would be mounting plate.

          Backspace seems to equal offset + half the wheel width. With “negative” offset being expressed as a negative number!

          • 0 avatar
            Scoutdude

            Close you have the right idea however the advertized wheel width is the tire bead mounting surface which is less than the overall width. On most wheels if you measure from outside to outside you’ll find that it is approximately 1″ wider than the advertised width.

            So backspace can be roughtly calculated by advertised wheel width + 1″ /2 + offset (not forgetting the – on those wheels that are negative offset).

          • 0 avatar
            Skink

            You misquoted me. I didn’t write “advertised wheel width”, did I? As for the formula, you merely restated what I already wrote(except for imprecision in using advertised wheel width instead of actual wheel width). Your method, though, is just fine and close enough for country music. ;)

          • 0 avatar
            Scoutdude

            You are correct that you did not say advertised width I just added that because the majority of people I’ve talked with think that the advertised width is the overall width.

    • 0 avatar
      ghills

      the diagram is right but the narrative is ballsed up.

      Deep dish rims have negative offset

  • avatar
    TR4

    most modern vehicles use equal length shafts?

    The manual transaxle on the regular Chevrolet HHR (and probably other Delta platform vehicles?) has grossly unequal shaft lengths and yes, the torque steer is awful. They fixed this on the SS version by adding an external bearing.

  • avatar
    cdotson

    There’s another dynamic that has pushed almost all platforms to high(er) positive offset and that is wheel bearings. This probably had more to do with converting RWD platforms than aerodynamic wheels. As the drive to both improve NVH and increase service intervals progressed automakers wholesale dropped the use of spindle-mounted tapered roller bearings and switched to sealed hub/bearing units using double row angular/annular contact radial ball bearings. As ball bearings cannot handle moment loads imparted by offset radial forces, especially since the dual rows in a unitized package aren’t spaced as far apart as the inner/outer tapered rollers typically were, the bearing portion of the hub must be placed on or much closer to the wheel center line to maximize bearing life.

    In addition to compromising bearing life with incorrect wheel offset steering geometry can be negatively impacted as well. The point at which the axis between the upper and lower wheel end pivots intersects the ground and it’s relationship to the contact patch center plays key roles in the vehicle’s steering response. Both wheel offset and wheel diameter can impact this relationship. Too much deviation from the factory settings will increase steering effort and torque steer in FWD. Not enough positive offset, or too small of a tire even with correct offset and your steering will not return to center on its own and will feel as if it wanders.

    • 0 avatar
      heavy handle

      Great answer. The only thing I would add is that extra offset in the negative direction (pushing the tires out) creates a greater lever effect which amplifies the force of tire impacts on the suspension and steering. That means more wear and less control.

  • avatar
    86er

    (OP)

    Some more details for clarification:

    The winter wheels in question are 5.5 inch, while stock on those years of Crown Vic are 6.5. This may or may not affect the scrub radius or what-have-you of the tire vis a vis the suspension. As of this writing I am not sure what the exact backspacing and offset is for the Ranger wheel I’m running currently and the OEM wheels I use in the dry months.

    When comparing offerings for the Ranger and Crown Vic, websites like Tire Rack are no help either because they offer aftermarket wheels in a variety of offsets and backspacing, especially for the Ranger.

    I’ve read different sources saying the stock wheels on a ’92 Crown Vic are slightly positive (+13 if I recall) while offerings for aftermarket wheels on Tire Rack seem to all be -6.

    I’ve also read that too much offset is where the real problems begin, such as with the tuner crowd that like the wheels sticking way out, that way lies wheel bearing premature wear.

    I’ve assumed that the difference in offsets between the Ranger wheel and the Crown Vic wheel aren’t enormous, but I stand to be corrected. I have run this setup for 4-5 years now and have yet to require wheel bearing work. In fact the car has needed ridiculously little steering/suspension work in the 7 years I’ve owned it. A tie-rod early on in late 2007, a few sway bar endlinks and most recently one idler arm, and that’s it. The vehicle now has 277,000 km, 125,000 of which has been under my ownership.

    • 0 avatar
      Scoutdude

      It depends on the actual wheel in question. Since you say that the OE wheels were 6.5″ that would mean that you have the aluminum wheels. Those are +6mm offset though there are some +8mm that were used too. It is marked on the inside of the wheel. The steel wheels on that era were 0 offset. The 16″ “lacy spoke” wheels mainly found on the HPP cars used 0 offset.

      Are the Ranger wheels factory units and what year are they from? the early Ranger used a smaller wheel center bore than the Panther so they will not bolt fit even though the bolt pattern is the same. They also used a 0 offset. I put a set of my old 92 CV wheels on my mother in law’s Ranger because tires that fit the 14″ wheels it came with are not that common any more are there isn’t much selection.

      • 0 avatar
        86er

        I don’t know what year the Ranger wheels are from but they’re steel wheels like so: (http://www.wheelsandcaps.com/ford-ranger-1985_p-23395-steel-wheel-rim-15×55-1314.aspx)

        I know clearing the hub was no problem; these fit the hub just fine.

  • avatar
    Slowtege

    Great article. It can be confusing at times, which is why I’ll dive into “Show Us Your (insert model of car here) Lowered/On Aftermarket Wheels” threads on forums to try and get a better reference for real world looks.

    I also came here to comment on THAT VIC. Sitting really pretty on those wheels–probably SN95-New Edge-era Mustang GT/Cobra variants–with that offset and drop. I like the deeper dished pre-’03 wheels, but naturally the ’03+ upgrades to everything else. Gah, this is making me want one (again)!

    • 0 avatar
      Scoutdude

      The rears are definitely not OE Mustang wheels. That era Panther uses zero or minimal offset wheels. To put the new edge Mustang wheels on them you need to use spacers. 1″ is what is commonly used but if you want to really tuck tire like that one a little smaller spacer is good. On my 92 CV I was able to tuck some 255/45-18 front 285/45-18 rear on 18×9 AFS Mach 1 replicas that come with a 25mm offset, instead of the 30mm that comes on the stock 17″ versions. For the front I use 1″ spacers and 3/4″ spacers in the rear. My car sits about as low. The featured car looks like it is running the OE 17″ Bullit wheels up front and the aftermarket 10.5″ wheels. I ran the OE 17″ Bullits on mine for many years.

      • 0 avatar
        86er

        Yeah, that’s certainly not my 92 Vic pictured!

        It sounds like you got away with a cons1derable offset difference w/o any undue strain on bearings or other components.

        For the record I haven’t changed wheel bearings in 7 1/2 years and I can’t find record of the previous owner (the only previous owner) doing it either. No grinding or popping to be heard either.

        The folks at grandmarq.net were fairly helpful, none of them thought that having these rims on the Vic would be unduly harmful, especially as the tires/rims sit a little inboard perhaps on account of the 5.5 inch-wide rims.

        They say that wheels that stick out a lot tend to do more damage to wheel bearings.

        • 0 avatar
          Scoutdude

          No I did not get away with a big difference in offset I used spacers to keep the offset the vehicle sees within the range that it saw with factory wheels. A +25mm wheel with a 1″ spacer means that on the front the offset the vehicle “sees” is -.4mm or close enough to the 0 offset used on the steel and 16″ wheels in those years. On the back the combination of the 25mm offset and 3/4″ spacer means the vehicle “sees” a 6mm offset or what the factory 15″ aluminum wheels were in 92.

          • 0 avatar
            86er

            It just keeps getting more confusing!

          • 0 avatar
            Scoutdude

            OK I’ll see if I can clear up some of the confusion. The thing that matters is the actual wheel center line to wheel mounting surface, ie rotor or drum. A given wheel bearing, suspension and steering system is designed with the location of the wheel center line in mind.

            Another way to think about is to mentally weld the spacer to the wheel and then measure the new effective offset.

      • 0 avatar
        Slowtege

        I should have clarified my ‘GT/Cobra variant’ phrase as I had, in my head, meant that it included aftermarket replicas that took the spoke design but modified the barrel/rim width. I agree the wheels on that white Vic look like New Edge Bullitts (and not GT or Cobra wheels), but with a super deep dish for the rears.

        Not really many or any CVs showing up on an image search of your era car with Mach 1 reps. How did you like the look of them compared to the Bullitts? Did the ride suffer going from 17 to 18 or was there still enough sidewall to mitigate that change, or at least make it less drastic?

        I think my favorite wheels on a newer Vic are the GT500 wheels from around 2010–the narrow split five spoke ones. There’s a guy with a black CV, lowered just right, with those wheels (with spacers) and it just looks right.

        • 0 avatar
          Scoutdude

          It was not a huge change going from the 17’s to 18’s but I was running 235/50-17 F and 255/50 R to initially 255/45-18 all the way around.

          I really like the look of both the Bullit and the Mach 1 on the CVs.

          For the 03 up Panthers the 05 up Mustangs are direct fit no need for spacers since they both use 45 or 50 mm offset. In fact the Marauder wheel was optional on V6 Mustangs in a couple of years. They are slightly different in the fact that they changed the depth of the machining for the center caps. That means the Marauder center cap sticks out about 3/16″ on the Mustang version.

          AFS now makes an 18″ Mach 1 style wheel with the 50mm offset so those are tied with the Marauder wheel for my choice for an 03 up Panther. Of course the 8″ Marauder wheel could use widening and it wouldn’t hurt to widen the 9″ AFS wheels too so that you can use 295 or 305 on the rear. Once you widen them since it is done on the rear portion of the barrel spacers do become necessary.


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