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      11-04-2020, 09:55 PM   #1
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Lightbulb ZCP Tire Specs vs. GTS Questions

So on doing research about a new wheel, tires, and suspension setup I did some research and found out that the BMW M4 GTS unsurprisingly runs a different setup then the BMW M4 ZCP. My question is the GTS runs 19 x 9.5 fronts on 265/35/19s and 20 x 10.5 fronts on 285/30/20s, while the standard ZCP runs 20 x 9 265/35/20s and 20 x 10 285/30/20s, why would they make the wheel wider? Isn't the tire just stretched more with the wider wheel and the wheel is heavier? I also know that the GTS runs Michelin Sport Cup 2's which obviously don't run super stretched like the MPSS. The only possible reasons for this I can think of is so they could camber the wheels for cornering? I'm no expert but the other conclusion I've come to is that 19 in the front will allow better cornering while 20 rear will extend the length of the contact patch.

Furthermore, I'm currently debating a new wheel and tire setup, where I took inspiration from this setup and planned on getting the V-FF 103's from Vorsteiner in 19 x 9.5 with 275/35/19R MPS4S and 20 x 10.5 305/30/20R MPS4S, I'm by no means an expert and have a lot of learning to do but this should give better straight line performance while still breaking sideways when I want it to and cornering better, additionally the wheels would be lighter. I'm currently on stock 20" ZCP setup. I'm lowering on Swift Spec-R's all though KW HAS is alluring as I'd like that rake look the GTS has and I don't think the wheels will provide it.

Source > https://bimmertips.com/oem-bmw-f82-m...l-style-specs/
P.S the CS also runs 19 front 20 rear but the same width as ZCP

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      11-04-2020, 11:07 PM   #2
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I have 763M wheels with PS Cup2s that came on my M3 ZCP as a factory option. CS and 763M set up is the same tire sizes as GTS, but narrower wheels by 0.5 inch.

I think the CS setup works amazing. I feel that smaller front wheels allow for improved ride and feel, while the large/wide wheels in the rear help reduce oversteer and increase grip (maybe also improve handling/rear feel). I feel the CS/763M has the ideal set up but many may not like 19 front and 20 back. I love it though and wheels look great.

And to finally answer your question... I think they are cambering the tire sidewalls for cornering to improve turn in response. I remember the old E46 M3 ZCP had really aggressive sidewall setup with PS2 (stock) and I felt it was for improved turn in at the price of harshness. It might help to slightly reduce tire walls from moving during cornering (just a guess, not an engineer anymore).
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      11-05-2020, 07:02 AM   #3
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In general terms, for a given tire, a wider wheel will offer crisper handling response at the expense of ultimate grip, while a narrower wheel will offer more grip at the expense of handling crispness.
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      11-05-2020, 07:10 AM   #4
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Quote:
Originally Posted by CanAutM3 View Post
In general terms, for a given tire, a wider wheel will offer crisper handling response at the expense of ultimate grip, while a narrower wheel will offer more grip at the expense of handling crispness.
Did you get that backwards?
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      11-05-2020, 07:30 AM   #5
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Quote:
Originally Posted by exmowner View Post
Did you get that backwards?
Nope.
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      11-05-2020, 09:56 AM   #6
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Quote:
Originally Posted by exmowner View Post
Quote:
Originally Posted by CanAutM3 View Post
In general terms, for a given tire, a wider wheel will offer crisper handling response at the expense of ultimate grip, while a narrower wheel will offer more grip at the expense of handling crispness.
Did you get that backwards?
Oh man...

Back in the day me and my friend went back and forth with this for days....

There is the law of diminishing returns and and optimal set up.

All things being equal.
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      11-05-2020, 03:51 PM   #7
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So would you guys personally recommend a 19 front 20 rear setup then? I appreciate all the wisdom.
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      11-05-2020, 03:55 PM   #8
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Quote:
Originally Posted by TickleMePink View Post
So would you guys personally recommend a 19 front 20 rear setup then? I appreciate all the wisdom.
This setup seems to be working great on my M4cs. Note that the new G8X M3/4 are also equipped with this type of staggered setup.
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      11-05-2020, 04:07 PM   #9
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Quote:
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So would you guys personally recommend a 19 front 20 rear setup then? I appreciate all the wisdom.
Yep. I love it on my ZCP as I mentioned earlier. CS-like wheels.
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      11-10-2020, 05:51 AM   #10
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Quote:
Originally Posted by CanAutM3 View Post
In general terms, for a given tire, a wider wheel will offer crisper handling response at the expense of ultimate grip, while a narrower wheel will offer more grip at the expense of handling crispness.
Do you have data for that? Clearly an increase in wheel width (holding section width constant) increases the spring rate of the sidewall and steepens the cornering force vs slip angle slope; i.e. makes the tire more responsive to transient inputs.

However, do you have empirical data on identical tires showing a decrease in maximum cornering force with increase in rim width (staying within the TRA rim width window for the tire size of course)? I've only seen the opposite of that, so I'm curious what data you're referring to.
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      11-10-2020, 07:27 AM   #11
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Quote:
Originally Posted by CSBM5 View Post
Do you have data for that? Clearly an increase in wheel width (holding section width constant) increases the spring rate of the sidewall and steepens the cornering force vs slip angle slope; i.e. makes the tire more responsive to transient inputs.

However, do you have empirical data on identical tires showing a decrease in maximum cornering force with increase in rim width (staying within the TRA rim width window for the tire size of course)? I've only seen the opposite of that, so I'm curious what data you're referring to.
I don't have any empirical data on hand, but it is what I've read in quite a few specialized literature on the science of tires. It is essentially the same general principle as with suspension tuning, a stiffer suspension will yield improved response at the expense of ultimate grip while a softer suspension will yield better ultimate grip at the expense of handling response. A softer tuning allows the tire to stay more in contact with the tarmac as it faces irregularities which improves its ultimate grip.

If there were only performance advantages to stretched tires (wider wheels for a given tire size), we would see all sports, super and hyper cars on super stretched tires, which often is not the case. Like everything else in life, it is all about the desired compromise.
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      11-10-2020, 07:39 AM   #12
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Quote:
Originally Posted by CanAutM3 View Post
I don't have any empirical data on hand, but it is what I've read in quite a few specialized literature on the science of tires. It is essentially the same general principle as with suspension tuning, a stiffer suspension will yield improved response at the expense of ultimate grip while a softer suspension will yield better ultimate grip at the expense of handling response. A softer tuning allows the tire to stay more in contact with the tarmac as it faces irregularities which improves its ultimate grip.

If there were only performance advantages to stretched tires (wider wheels for a given tire size), we would see all sports, super and hyper cars on super stretched tires, which often is not the case. Like everything else in life, it is all about the desired compromise.
Oh no, not talking about "stretched" tires at all here; only tire/wheel combinations within the official TRA window on rim width. In rapid transient conditions and/or rough pavement where sidewall spring rate increases may yield slightly less max Cf, perhaps there is a tiny reduction in maximum Cf available momentarily. However, from all data I've seen over the years, there is not a drop off in max Cf with a wider rim (within the TRA width specification for that tire size) in steady state conditions; in most cases it is the opposite...especially if you're comparing a tire mounted to the min TRA width spec versus on the other end (i.e. a 275/35-18 on a 9" rim versus a 10.5" rim; note max TRA spec in this case is 11").
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      11-10-2020, 08:31 AM   #13
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Quote:
Originally Posted by CSBM5 View Post
Oh no, not talking about "stretched" tires at all here; only tire/wheel combinations within the official TRA window on rim width. In rapid transient conditions and/or rough pavement where sidewall spring rate increases may yield slightly less max Cf, perhaps there is a tiny reduction in maximum Cf available momentarily. However, from all data I've seen over the years, there is not a drop off in max Cf with a wider rim (within the TRA width specification for that tire size) in steady state conditions; in most cases it is the opposite...especially if you're comparing a tire mounted to the min TRA width spec versus on the other end (i.e. a 275/35-18 on a 9" rim versus a 10.5" rim; note max TRA spec in this case is 11").
When I said "stretched", I implied with the approved range. My point still holds, if there were only performance advantages to going to the maximum allowed wheel width for a given tire, all sport/super/hyper cars would be going that route, and it is not the case.

A tire that is more stretched over a wider wheel will be more likely to "skip" over minute bumps on the road surface and lose grip in the process.
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      11-10-2020, 08:50 AM   #14
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Quote:
Originally Posted by CanAutM3 View Post
When I said "stretched", I implied with the approved range. My point still holds, if there were only performance advantages to going to the maximum allowed wheel width for a given tire, all sport/super/hyper cars would be going that route, and it is not the case.

A tire that is more stretched over a wider wheel will be more likely to "skip" over minute bumps on the road surface and lose grip in the process.
I think we're in somewhat agreement except for the fact that max steady state cornering force does increase with additional wheel width with TRA range. I wouldn't make a binary statement about any given manufacturer's chosen spot within TRA spec since a whole range of metrics figures into that choice other than purely maximum cornering force.
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      11-10-2020, 09:04 AM   #15
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Quote:
Originally Posted by CSBM5 View Post
I think we're in somewhat agreement except for the fact that max steady state cornering force does increase with additional wheel width with TRA range. I wouldn't make a binary statement about any given manufacturer's chosen spot within TRA spec since a whole range of metrics figures into that choice other than purely maximum cornering force.
OK, so in your view, what is the performance drawback of using a wider wheel?
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      11-10-2020, 09:20 AM   #16
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Quote:
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OK, so in your view, what is the performance drawback of using a wider wheel?
On track? None essentially, depending on the specific tire though however there may be different peak rim widths, but we're talking very small incremental changes here in terms of peak cornering force if we're only discussing say a 10" wheel versus a 10.5" wheel for a 275/35. Transient response is the biggest improvement, and the control-ability of the tire is improved with increasing width; i.e. a 9" wheel in this case feels sloppy and imprecise in transients while a 10 or 10.5 feels nailed down (especially in the rear) with nice communication.

On the street, there a huge range of OE considerations from ride comfort, rim protection, fender clearance (especially in the EU), etc. Wider wheels reduce all of the above.
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      11-10-2020, 11:56 AM   #17
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the wider tire will give you more cornering grip. I would go with the 10.5 or 11.
As another example the 997.1 GT3 ran a 305 rear section tire on a 12" wheel. The 997.1 Turbo ran a same sized 305 rear section tire on an 11" rear wheel. It made a huge difference in the shoulder shape and how it could take camber on hard cornering etc.
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