Mixed wheel sizes - anyone done this?

[Mr. Maboomba]

@HB, the BMWs with mixed wheels use a taller sidewall in the front so the overall OD is the same. My best guess is that they do this for less sidewall flex in the rear and more in the front for a balance of handling and ride compliance.

While the Corvette engineers have said they started using mixed sizes for aesthetics, I believe the Germans are seeing benefits on their Nurburgring testing times.

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[Jaymar]

You’re saying that Porsche did this exclusively on the GT3RS for aesthetics? And BMW did it on the M3 CS and M4 GTS for aesthetics? I seriously doubt that.

Yes, that's exactly what I said. Are those road going vehicles that have to be sold to the public to fund and homogenize the race cars or are those purpose built race cars built with one function in mind? The latter typically don't have air conditioning or license plates.

 

[Mr. Maboomba]

Here's what I read on the BMW forums: the staggered diameter rim is mainly to have different tire sidewall height front and rear. The larger front sidewall improves front grip to reduce understeer and the shorter rear sidewall provides more stability from improved response in transitions.

Host: "When I look at the side of the car I can see actually that the rear wheels are bigger than the front, and that's something I don't usually see on a BMW?"
BMW: "We actually started introducing the split wheels on special models like the m4 gts and it's basically because the engineers worked out that you can have a sharper turn in with a slightly smaller wheel on the front. It's actually a smaller rim but a wider tire so really sharpens the turn in of the car. It's just so neutral it does exactly what you want but without ever overstepping the mark. It's properly dialed in."

Porsche uses different wheel sizes with similar sidewalls, resulting in different ODs. They reportedly claim that is to provide a larger contact patch in the rear without having to further increase tire width.

https://www.motorauthority.com/news/...r-the-2020-911

In relation to this thread:
August Achleitner, vice president of the 911 and 718 product lines

2) Enlarge the diameter of the rear wheels.

Calculations and simulations showed that enlarging the diameter of the rear wheels would improve dynamics even though it would also add weight. The 2020 Porsche 911 S and 4S now ride on 21-inch wheels at the rear and 20s up front, though the tire sizes haven't changed at 305/30 rear and 245/35 up front. While this influenced all of the surrounding areas of the suspension and the packaging of the car, Porsche felt it was worth the effort because it would make the handling more neutral and controllable. The company had experience with the practice as the 918 Spider and GT2 and GT3 cars already use staggered wheel sizes, and there are no better handling Porsches than those. Handling becomes more neutral because the bigger rear wheels provide greater rear end stability and grip without having to go with a wider tire.


Here’s the other one and it’s actually Achleitner again but a different outlet:

https://www.roadandtrack.com/car-sho...specs-details/

So what's the effect of the increase in wheel size and track width.

I'll let Achleitner explain:

- When you widen the track in the front and in the rear...you can transmit rolling forces a little bit more with just the spring and damper, and then you can soften the stabilizers. When you soften the stabilizers, you get better traction out of the car, [which] makes the car faster.

-We have a bigger rear wheel, and that's the first time in the history of the 911 Carrera, that the rear wheel is a diameter bigger than the front one...From this, we get a better, bigger [contact patch] not only by width, but also by length. And with the enlargement of the tire, we were also able to reduce the tire pressure a little bit.

These different approaches appear to be tuning for the individual chassis: more rear grip for the Porsche 992, more front grip for the BMW F80/F82 and G80/G82.

 

[honeybadger]

Here's what I read on the BMW forums: the staggered diameter rim is mainly to have different tire sidewall height front and rear. The larger front sidewall improves front grip to reduce understeer and the shorter rear sidewall provides more stability from improved response in transitions.



Porsche uses different wheel sizes with similar sidewalls, resulting in different ODs. They reportedly claim that is to provide a larger contact patch in the rear without having to further increase tire width.



These different approaches appear to be tuning for the individual chassis: more rear grip for the Porsche 992, more front grip for the BMW F80/F82 and G80/G82.

I could be wrong, but I think they’re still balancing styling vs. performance. As far as I know, there’s no real benefit to running larger wheels than needed to clear the brake package. But we all know the masses love larger wheels to “fill the fender gap.” While I doubt there is no simple answer here, I’ll bet the engineers would tell you it’s a compromise based on the other constraints they have.

I’d be curious to hear of any legit reason to run a larger wheel on the rear than front other than to compensate for some other variable like the above. From a physics standpoint, you typically want to run the smallest wheel that clears the front brake package.

 

[Jaymar]

So they used it as a minor tuning tool where they were probably constrained on shock, bar, etc. much like the rim width on the PP2. If that had resulted in some goofy looking bigger wheels on the front you can be sure they would have found a different way. The point is it's a very small knob to turn, you should get the large knobs dialed before you worry about that.

 

[Mr. Maboomba]

One of the reasons I doubt the aesthetics argument, HB, is that BMW and Porsche only do this on a small number of pinnacle models. BMW did it on the 2018 M3/M4 CS and 2016 M4 GTS. The M4 GTS was a low-volume, $133K model in 2016 versus the $66K MSRP of the mainstream M4. Porsche did it only on the 911 GT3 RS, a $189K model, but not on the 911 GT3 ($143K) or mainstream 911s (which started at $91K in 2019, the last year the GT3 RS was sold). If they were doing this for looks it seems like they would do it on the mainstream models where sales volume matters.

I promise I didn't start this thread with an answer in mind, but I've done some reading since posting, and "it's just for looks" is supremely unsatisfying.

So they used it as a minor tuning tool where they were probably constrained on shock, bar, etc. much like the rim width on the PP2. If that had resulted in some goofy looking bigger wheels on the front you can be sure they would have found a different way. The point is it's a very small knob to turn, you should get the large knobs dialed before you worry about that.

Yes, I agree with that. Since the right answer seems to depend on the needs of a particular chassis, finding the right answer for the S550 would require more experimentation with wheel/tire combos than I have the appetite for.

 

[Firsttexan]

It could also be related to final final drive/gearing. Different tire height in the rear will make a larger difference in acceleration and top speed than most people think.

That said, most purpose-built race cars use a smaller diameter wheel and/or larger sidewall tire in the rear than the front. So while having a larger wheel in the rear isn't abnormal, using the wheel size isn't the best way to do it. Makes the sidewall smaller and adds weight.

Increased diameter, puts more rubber on the ground in a straight line. More than added width does.

 

[honeybadger]

One of the reasons I doubt the aesthetics argument, HB, is that BMW and Porsche only do this on a small number of pinnacle models. BMW did it on the 2018 M3/M4 CS and 2016 M4 GTS. The M4 GTS was a low-volume, $133K model in 2016 versus the $66K MSRP of the mainstream M4. Porsche did it only on the 911 GT3 RS, a $189K model, but not on the 911 GT3 ($143K) or mainstream 911s (which started at $91K in 2019, the last year the GT3 RS was sold). If they were doing this for looks it seems like they would do it on the mainstream models where sales volume matters.

I promise I didn't start this thread with an answer in mind, but I've done some reading since posting, and "it's just for looks" is supremely unsatisfying.

I think you might be thinking about this too black and white. It can easily be both because of aesthetics and performance. It's all about managing the compromises for the OEM - customer wants, regulatory requirements, performance goals, costs, etc.

I'm not personally saying it's only of aesthetics, but I do think they come into play for at least some of these cars. Maybe Porsche wanted a larger diameter tire in rear to maximize contact path, but they didn't like the aesthetics of the larger sidewall. Or maybe the chassis isn't tuned for a larger sidewall tire, so they decided it would be less effort to increase wheel size instead. I suspect only the engineers know the real answer.

Back to your original question - ironically I just saw a Mustang GT trollin' around town with 19s on front and 20s on the rear. Looked like it gave the car some rake. Personally, I'd start with what problem/deficiency/challenge you're trying to solve--which I haven't seen yet (unless I missed it). That'll help people give you better guidance.

Increased diameter, puts more rubber on the ground in a straight line. More than added width does.

Agreed (within reasons--air pressure/weight both affect the contact patch). Plus, you can also increase diameter with the tire sidewall (like they do on race cars). You don't need a 1in larger wheel size to do that.

 

[Mr. Maboomba]

I think you might be thinking about this too black and white. It can easily be both because of aesthetics and performance.

It makes sense to me that it can be a combination of things. I was reacting to the posts that were waving it off as “just for looks.” That seems to be an incomplete answer to me. I don’t deny looks could be part of the equation.

Personally, I'd start with what problem/deficiency/challenge you're trying to solve--which I haven't seen yet (unless I missed it). That'll help people give you better guidance.

I am planning to invest in track wheels and tires. I’d like to do that once in the best possible way. I know 19x11 rotatable square setups are very popular and that’s my plan, but with the increasing number of “double staggered” setups appearing in the market I wondered if we might be overlooking an opportunity as a community.

 

[Demonic]

If I recall correctly from the Carroll Smith Tune to Win and Engineer to Win books, which were some of the old car design classics, part of the original reason for the smaller front wheels was for less drag on formula cars and in some cases suspension geometry relative to where the center of the wheel axis was located. I could be wrong though, I am all the time. Then in later years I read another car design book talking about certain companies like Ferrari doing it for looks on some of their 80's cars, but the suspensions was still designed to accommodate the rake. So as I understand, a car could be designed with the larger rear diameter from the start for the sake of the raked look, and then the suspension designed around the different diameters, but I've never seen anyone advocate for any increased performance in different front and rear diameters on a car that was originally designed to use the same diameter wheels.

 

[JAJ]

Has anyone mixed 19s on the front and 20s on the rear? What was the result?

BMW does this some of their models (e.g., 2018 M3 CS, 2021+ M3). Mercedes does this with the C63S. Chevrolet does this with the Corvette. Porsche does this with the 991 GT3 RS. There must be a benefit to it.

@50 Deep @APEX Race Parts have you sold any custom wheel sets that are 19” front 20” rear?

As @Firsttexan said earlier, the 2013/14 GT500's had staggered front/rear rim sizes. The tires differed 1/2" in overall diameter and the fronts were 765 turns per mile, and the rears were 750. No visible difference to speak of in terms of rake - it was only 1/4" front to rear. For context, an S550 GT PP has a larger difference at 0.3" front to rear.

The 13/14 GT500's had massive torque, so I wouldn't be surprised if the reason Ford took 1/2" out of the sidewall height at the rear was to improve grip while accelerating. The fronts had taller sidewalls to improve lateral grip, the rears were shorter to improve acceleration, something the car did remarkably well, by the way.

 

[Firsttexan]

As @Firsttexan said earlier, the 2013/14 GT500's had staggered front/rear rim sizes. The tires differed 1/2" in overall diameter and the fronts were 765 turns per mile, and the rears were 750. No visible difference to speak of in terms of rake - it was only 1/4" front to rear. For context, an S550 GT PP has a larger difference at 0.3" front to rear.

The 13/14 GT500's had massive torque, so I wouldn't be surprised if the reason Ford took 1/2" out of the sidewall height at the rear was to improve grip while accelerating. The fronts had taller sidewalls to improve lateral grip, the rears were shorter to improve acceleration, something the car did remarkably well, by the way.

Yes, 662hp pulls hard as a fothermucker.

 

[JK XLR8]

When I bought my 2014 Gt vert (used) it came with AMR 19/20 setup and the tire sizes made the front and rear out of balance to my eye....drove me nuts. Researched and found tire sizes for the GT500 and bought new tires to make it look better. Somehow, if you have OCD like me, it still didnt look right. Now I run 20s with matching sidewall height to get that balanced look....I know, lord help me!

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