[G30M]

i see a few cars that BMW spec with either
- larger rear wheel vs front
- 10mm wider rear tire vs front
- or both

what is the reasoning for this? i presume it has something to do with performance?

[jkoral]

Originally Posted by g21

what is the reasoning for this? i presume it has something to do with performance?

General consensus is it has more to do with safety -- to induce understeer near the limits, which is considered safer for the general driving population.

[F32Fleet]

Originally Posted by g21

i see a few cars that BMW spec with either
- larger rear wheel vs front
- 10mm wider rear tire vs front
- or both

what is the reasoning for this? i presume it has something to do with performance?

Grip and understeer (counter the oversteer which is inherent w/RWD).

[Llarry]

What they said -- but also for appearance.

[M3SQRD]

A 20” dia rear with a 285/30 tire and a 19” dia rear with a 285/35 tire have the same tire rolling dia but the 20” setup has a longer contact length (circumferential direction, same contact patch width of 285 mm). This would increase the rear’s ability to put the power/torque to the ground.

Smaller front dia wheel will reduce the rotating inertia compared to a 20” front dia wheel so turn-in/change of direction is a little easier to initiate.

Staggered F & R tire widths help dial in understeer, which is safer, and allows the car to more easily get the power/torque to the ground.

[grannyknot]

It's for when you are at the drag strip, you can pop a wheelie easier with a lighter tire in the front.

[tracer bullet]

Originally Posted by M3SQRD

have the same tire rolling dia but the 20” setup has a longer contact length (circumferential direction, same contact patch width of 285 mm).

If they are the same diameter, and have the same width, the only way I see the contact patch stretching farther front to back is if 1) the back end is heavier or 2) the rear tire has a lower psi in it. Neither of which is an inherent property of the tire.

I'm not saying you're wrong, just not seeing it. Care to explain further?

Usually fatter rear tires shorten the front / rear contact patch length and can help limit sliding sideways, but are worse for actually putting power to the ground. The size of the contact patch is psi / weight dependent and the dimensions of the tire change the shape of the patch sideways vs front to back, but the overall size remains the same. Shorter in the front to back direction makes spinning the tires more likely, not less, when hitting the gas.

[M3SQRD]

Originally Posted by tracer bullet

If they are the same diameter, and have the same width, the only way I see the contact patch stretching farther front to back is if 1) the back end is heavier or 2) the rear tire has a lower psi in it. Neither of which is an inherent property of the tire.

I'm not saying you're wrong, just not seeing it. Care to explain further?

Usually fatter rear tires shorten the front / rear contact patch length and can help limit sliding sideways, but are worse for actually putting power to the ground. The size of the contact patch is psi / weight dependent and the dimensions of the tire change the shape of the patch sideways vs front to back, but the overall size remains the same. Shorter in the front to back direction makes spinning the tires more likely, not less, when hitting the gas.

I’ll have to dig up the sources where I read this. It’s a topic that comes up quite often in Porsche, Lamborghini, etc. discussions. I have the same mental struggle with it. It was in multiple sources, not car forum discussions. I also agree that the static contact patch is based on car corner weight and tire pressure. A shorter sidewall also goes against increasing traction - just look at drag tires that produce the most grip, taller sidewalls.

[///MPhatic]

Originally Posted by tracer bullet

If they are the same diameter, and have the same width, the only way I see the contact patch stretching farther front to back is if 1) the back end is heavier or 2) the rear tire has a lower psi in it. Neither of which is an inherent property of the tire.

I'm not saying you're wrong, just not seeing it. Care to explain further?

Usually fatter rear tires shorten the front / rear contact patch length and can help limit sliding sideways, but are worse for actually putting power to the ground. The size of the contact patch is psi / weight dependent and the dimensions of the tire change the shape of the patch sideways vs front to back, but the overall size remains the same. Shorter in the front to back direction makes spinning the tires more likely, not less, when hitting the gas.

This is absolutely spot on. And adding to that, 20's would have worse acceleration than 19's because the sidewall is going to be shorter, and therefore harder and less-compliant (with the same OD). You're also adding unsprung weight further from the hub with a 20" wheel, so acceleration would degrade even further.

[tracer bullet]

Originally Posted by M3SQRD

I also agree that the static contact patch is based on car corner weight and tire pressure. A shorter sidewall also goes against increasing traction - just look at drag tires that produce the most grip, taller sidewalls.

Yeah so if you see the contact patch thing, you can logically follow that the tire width will keep it the same just change its size. As you go wider it gets more narrow and vice-versa. Traction benefits in whichever direction it grows, and vice-versa.

Shorter sidewalls tend to stiffen the tires, sure, but in that direction, the bump absorbing direction. The actual tread of the tire which touches the road doesn't change much if at all. There's sort of a certain amount of tread "blocks" on the ground and they don't change unless of course the tire's like actually flat.

True drag tires are sort of a different animal. They go wide of course, but they're also really low on pressure (15psi???) so that they get that front to back grip as well. Ever seen the slow motion take-offs and watch the wheel turn before the tire does and how the entire sidewall twists and buckles?

I'd love to see some resources showing otherwise, if you find them. Not to prove any points or right / wrongs just because I think it's pretty interesting.

I kind of wish cars would have square setups, I know big rear wheels look cool but as I get older I'm less interested in expensive tires due to their sizes and more happy to actually rotate them and make them last longer. Maybe my front lefts from the on / off ramps wouldn't wear so fast if I could rotate them, LOL. I know a lot has to do with balance, over and understeer, etc. and I know a little understeer is safer, but I wish they'd have square setups and then do the rest with suspension tuning. It's certainly far easier to fit big tires int he rear, there's no engine or steering just eating up trunk space.

[DrFerry]

Originally Posted by F32Fleet

Grip and underwater (counter the oversteer which is inherent w/RWD).

Oh yeah the underwater part. I forgot about that.

[///MPhatic]

Originally Posted by tracer bullet

I'd love to see some resources showing otherwise, if you find them. Not to prove any points or right / wrongs just because I think it's pretty interesting.

A friend (he works for a time attack team) and I have gone around (re: argued) this ad nauseam because sometimes real life application results don't mirror paper results.

Adding to that is the fact that direct comparisons are impossible. Like if you're testing different widths of the same make/model of tire it's going to be impossible to keep the weight the same because it increases as the tire gets larger. Even the construction of the tire can differ, depending on the size.

He and I agree that to increase acceleration grip the psi in the tire should be lowered first, then the compound, then the size, and finally the weight-transfer, but that's just our agreement, not set in stone.

There is some sort of inarticulate law of diminishing returns too, like if you're running a 255 at perfect grip, a 285 might offer better acceleration than a 245 would, but it's so application dependent that it's not worth noting.

But in general, when guys add wider tires and get better acceleration it's usually down to the newer tire or one/multiple of the myriad of parameters has changed.

[M3SQRD]

Originally Posted by tracer bullet

Yeah so if you see the contact patch thing, you can logically follow that the tire width will keep it the same just change its size. As you go wider it gets more narrow and vice-versa. Traction benefits in whichever direction it grows, and vice-versa.

Shorter sidewalls tend to stiffen the tires, sure, but in that direction, the bump absorbing direction. The actual tread of the tire which touches the road doesn't change much if at all. There's sort of a certain amount of tread "blocks" on the ground and they don't change unless of course the tire's like actually flat.

True drag tires are sort of a different animal. They go wide of course, but they're also really low on pressure (15psi???) so that they get that front to back grip as well. Ever seen the slow motion take-offs and watch the wheel turn before the tire does and how the entire sidewall twists and buckles?

I'd love to see some resources showing otherwise, if you find them. Not to prove any points or right / wrongs just because I think it's pretty interesting.

I kind of wish cars would have square setups, I know big rear wheels look cool but as I get older I'm less interested in expensive tires due to their sizes and more happy to actually rotate them and make them last longer. Maybe my front lefts from the on / off ramps wouldn't wear so fast if I could rotate them, LOL. I know a lot has to do with balance, over and understeer, etc. and I know a little understeer is safer, but I wish they'd have square setups and then do the rest with suspension tuning. It's certainly far easier to fit big tires int he rear, there's no engine or steering just eating up trunk space.

Like I said, I’ll look for the sources.

Tire contact area is not as simple as corner weight divided by inflation pressure. They are the primary variables but not the only two variables that affect contact area. Tire construction/stiffness of tread and sidewall do alter the static, and dynamic, contact area. Inflation pressure contact length and width do NOT change linearly with varying pressure. As the pressure is increased, there’s a pressure above which the circumferential length no longer decreases as the pressure continually increased. Similarly, as you reduce tire pressure, the contact width will stop decreasing at a certain pressure. This has been demonstrated by many tests and these data/results are out there. Contact patch area and shape vary with pressure stiffness and structural tread/sidewall stiffness. Data shows very highly non-linear relationships in circumferential contact patch length vs. inflation pressure. For example, a 20” tire will have a higher structural stiffness than a 19” tire of the same width but they’ll have different contact patch length-pressure relationships as well as a different transition pressure where the length stops decreasing with increasing pressure. So for a given pressure it’s possible for a 20” tire to have a longer or shorter contact length than a 19” tire. Tire design is very complex and tires have highly nonlinear relationships between lateral grip and slip angle, grip and normal load, grip and temperature, etc. Nonlinear relationships also exist for contact patch length and width (bilinear) vs. pressure.

So two 285 mm tires, on the same or different wheel diameters, from two different tire manufacturers will have different contact areas, shapes, widths and lengths.

I’m talking about drag tires used on street cars - people put on 17” wheels (along with smaller calipers and diameter brake rotors!) so the can maximize sidewall height and flexibility to “wind up” the sidewall at launch. Yes, lower pressures than street tire pressures are used as well to maximize grip.

In the E generation M3s I ran square tire setups but with the F and G generations I find a staggered setup works better because of the massive torque these cars have. I still run a 285-295 mm front tire but the rear is at least a 305 mm tire. It sucks not being able to rotate but the staggered setup yields better times.

[tracer bullet]

Originally Posted by M3SQRD

Tire contact area is not as simple as corner weight divided by inflation pressure.

Yeah, we are totally on the same page I think. I agree with everything you wrote. I guess on the internet you never really know at first how much to get into or how much someone already knows.

[RugbyBro]

Square

[M3SQRD]

Originally Posted by RugbyBro

Square

I agree with you but my driving style likes the wider rear tire. Thinking of switching to the GTmore F camber plates and MCS 3WR eye-to-eye struts. Then I’d be able to run a 305 mm square setup. However, it feels like selling 1/2 of a complete MCS 2WR setup will be painful finding a buyer and on price.

[RugbyBro]

Originally Posted by M3SQRD

I agree with you but my driving style likes the wider rear tire. Thinking of switching to the GTmore F camber plates and MCS 3WR eye-to-eye struts. Then I’d be able to run a 305 mm square setup. However, it feels like selling 1/2 of a complete MCS 2WR setup will be painful finding a buyer and on price.

tbh when I made the decision on which MCS setup to run, I concluded that I wanted a more fuss free setup and I was unlikely to ever run more than 305 square. So I went with regular 2WR which is a little more user friendly. 275 - 295 is more than enough for me as an amateur driver. Especially since the fastest TA/TT F8X lap times i've seen personally run tires in that range.

[F32Fleet]

Originally Posted by M3SQRD

I’ll have to dig up the sources where I read this. It’s a topic that comes up quite often in Porsche, Lamborghini, etc. discussions. I have the same mental struggle with it. It was in multiple sources, not car forum discussions. I also agree that the static contact patch is based on car corner weight and tire pressure. A shorter sidewall also goes against increasing traction - just look at drag tires that produce the most grip, taller sidewalls.

But they turn like shit

[M3SQRD]

Originally Posted by F32Fleet

But they turn like shit

What “turns like shit”? A 992 GT3RS definitely doesn’t “turns like shit”. There are quite a few super/hyper cars that also corner extremely well.

[///MPhatic]

Originally Posted by M3SQRD

Tire contact area is not as simple as corner weight divided by inflation pressure. They are the primary variables but not the only two variables that affect contact area.

It'd help if most every manufacturer didn't set their cars up to be 4x4 height and understeer like pigs. If the suspension came correct from the factory it'd be easy to select tires, but since it doesn't, and most everyone's first priority is to rid themselves of that front gap, any mods become a bad band-aid, including reasonable tire choices. To do it right is to consider the application, then the correct suspension/settings, then everything else, it just doesn't end up getting done that way by most users.

[RugbyBro]

Originally Posted by M3SQRD

What “turns like shit”? A 992 GT3RS definitely doesn’t “turns like shit”. There are quite a few super/hyper cars that also corner extremely well.

this^

Also, it's hard to compare different engine + drivetrain layouts. A major reason why BMWs like square (or have a smaller 10-20mm stagger) is because of the front engine, rear drive dynamics. Front needs grip due to weight, rear needs grip due to drive wheels.

Mid and rear engine cars typically have a much more extreme stagger as they don't need the extra grip up front to support the engine weight.

[crashoverride]

Uhm cus it looks as sick as hell.... Duh...!!