[JABCAT]

Originally Posted by Habber

I see some people have gone 21” rear and 20” front. Why wouldn’t they have just gone 20” all around?

They do it for appearance purposes, there's no performance benefit. If you look at owners that race/track their high performance cars, they often go with a smaller diameter wheel with a slightly taller tire sidewall. My Z06 has 19/20 factory (same as the ZR1) and many owners go down to 19s all around. Even owners of the C8 Z06 that comes with 20/21 are swapping for 20s or trying to find 19s for track use.

[MineralGreyMetallic]

Regarding 21 in the rear, 100% aesthetics.

There is so little sidewall back there.

[D.Yooras]

Originally Posted by TallGibbs

I do have a question here about correcting understeer for those who track their cars: my experience with car handling/dynamics changes is only from FWD setup changes and then iRacing sim racing setups. My experience has shown that a STIFFER front sway bar (anti-roll bar) is going to ADD understeer, so I'm a bit skeptical on this advice from "the alignment guy."

Can anyone with track experience in the G87 M2 suspension verify setup changes/affect. How would go about "correcting" understeer in low to mid-speed corners (ignoring aero affects)? Stiffen the rear, or stiffen the front?

The "alignment guy" sounds like he knows what he is talking about. Good discussion about this in an older thread.

https://f87.bimmerpost.com/forums/sh....php?t=2030646

I'll repost my two major responses here:

"For a proper racing chassis that is generally true. However, because this chassis is compromise setup due to front struts, it doesn't play by the standard rules.

The most important aspect of optimizing this chassis is front camber and roll stiffness. Since strut suspension doesn't gain much negative camber in compression (and at lowered height it's basically none) the most important aspect of setting it up is ensuring the outside front wheel has sufficient camber when the car is cornering. The easiest way to achieve this is with camber plates and/or a stiff front sway bar.

The camber plates do two things, one they give more static camber but they also change the camber curve through changing the SAI (steering axis inclination). The front sway bar limits body roll to minimize camber loss during cornering AND also assists in lateral weight transfer to the outside wheels (mainly front but also the outside rear). This is especially beneficial since the front inside wheel isn't doing much in terms of grip anyway. The main drawback to tuning this way, though, it does require more diligence on turn in to ensure the lateral load transfer happens properly (too fast and outside front will lose traction before the transfer is achieved). Other than that aspect, the rest of is generally beneficial.

Overall, that is the easiest, most cost effective way to get front grip out of a chassis like this. Once front grip is maximized, then fine tune with minor changes in the rear, like springs and sway bars."

"There are different ways to approach tuning that can yield similar results but require completely different styles of driving and conditions to get there. The approach you're taking could produce results similar to the method I'm talking about in one instance/track/configuration but not work as well in others due to the nature of the compromise. Again, since we're dealing with strut front suspension there isn't a one size fits all approach.

That said, there has been a lot of development put into these cars over the decades and the best all around, all condition, compromise setup always involves a larger front sway bar. This started with the E46 M3 CSL which was a huge improvement over the regular M3, especially when it comes to front grip. This front grip improvement was the culmination of a couple aspects but the one with the most affect was the front sway bar. The regular M3 had a 26mm bar while the CSL had a 30.8mm that (although hollow) was significantly stiffer than the regular M3 bar. This greatly improved front roll stiffness which, in turn, retained front camber better in heavy cornering situations in addition to the advantageous weight transfer.

Second example, the F8X chassis. BMW made one sway bar for all F8X chassis which, educated guess, was the max size BMW would ever want to put on a factory spec/trim. This allowed BMW to make one bushing and bracket set for all F8X chassis while at the same time allowing them to tune roll stiffness anywhere from 28mm down. On the F87 the front sway bar is 28mm bar but, to de-stiffen it, BMW milled the center down to 24mm (plus paint). Sway bars are like chains, the weak link establishes the total strength of the chain. In this case, the smallest point (especially if it's in the middle) is the torsional stiffness of the bar which, in this case, is 24mm.

Third example, the M235i Racing which comes stock with H&R sway bars 28mm front and 20mm rear (among other things as well). Example four, Spec E46 where nearly everyone is running the Hotchkis front sway bar (30.2mm hollow) if not the whole set (though most people keep the OE rear sway bar because they claim it's too stiff relative to the front).

Front roll stiffness on this chassis is critical. In fact, it wouldn't surprise me if the reason the springs are stiffer in the front relative to the rear on Ohlins and MCS, is to deal with the extra weight being transferred to the outside front wheel of a properly setup track car. Since the stiff front bar allows for more grip, there is more load transfer which means there will be more weight being transferred to the outside wheels.

The last thing you want in a car like this is to have too much weight transfer to the front axle on the whole through excessively stiff rear springs and/or rear sway bar. Going back to the Spec E46 example, most of those cars are setup to where the front axle under heavy cornering has 100% lateral load transfer, meaning the inside front wheel is off the ground. Obviously having wheels off the ground isn't ideal, but again, compromise setup means compromise tuning. If you run too stiff of a rear bar, the inside rear wheel will try to lift off the ground shortly after the front one and you'll lose grip on the rear axle, possibly even cause a snap oversteer situation.

To the original point, stiffer front sway is the most important part of this equation. Put a bigger front bar on first, maximize front grip, THEN fine tune rear axle handling characteristics through aspects like springs and sway bar. It’s possible to get the weight where you want it through very stiff rear springs and/or rear bar, but you could have a car that’s a handful on the street, especially in emergency situations."

[BroDoze]

Best thing I did to my 135i for handling and reducing understeer was putting a fat hollow Hotchkis front bar on. I left the rear stock. That was the thing to do to fix that car. Completely bass-ackwards from my old STI/WRX days of a fatter, stiffer rear bar. But it worked.

[comp]

Still wanna see some pics of someone running 20 inch 826/930 all round like the G80 guys do. Would look amazing on the G87.

[TallGibbs]

D.Yooras that’s supremely helpful and great education thank you! BroDoze good to have another point of confirmation on stiffening the help improving understeer.