Altering stock tire sizes, Messing with DSC - Real or placebo?

[JoeFromPA]

All,

I'm starting this thread to hopefully begin a reasonable dialogue to help set expectations on what tire sizing can be used, reliably, without increasing DSC intervention/MDM intervention in any meaningful manner:

I've read and imbibed the conventional wisdom/kool aid on here that altering the stock tire size differences front to back - both in width and height - will mess with DSC and create undesirable side effects in DSC intervention. What this means is that you want to keep the tire width difference front to rear to about 20mm (i.e. 255 front and 275 rear, 275 front and 295 rear). You also want to maintain height differences - so a 255/40/18 front and 275/40/18 rear on the stock m3 18" wheels would desire to keep very similar height characteristics as size changes).

This being said, we see expert shops like TMS putting 255 wide front and 295 wide rear tires on their demonstration car (see latest issue of roundel - beautiful forgelines on tanzanite blue car). They are lending out this car - so I would assume they want DSC/MDM on as much as possible...

Now, on the one hand, it makes sense that DSC is going to be programmed to a specific set of parameters and that changing those parameters could result in more DSC intervention.

On the other hand, tire sizes widely vary in height and width even among the same specification. Our 275 wide rear stock PSS appears to be about 1" narrower than most other 275 wide rear tires. And of course you can put a brand new set of tires on the front at 10/32nds tread depth and drive on a rear set with 3/32nds tread depth, creating a 7mm differential, and not expect to create DSC intervention issues.

Lastly, on a personal note, my car has alot of rear end wiggle and traction loss at WOT in MDM mode (i.e. DSC intervention). I want to reduce/eliminate that while maintaining tire sizing and tire compounds that do well in the wet, standing water, have ok tread life, and down to 40 degree fahrenheit. This means I really want to increase rear tire size while keeping front tire size static. So a 255 front and 295 rear would be ideal for me....if it doesn't paradoxically increase DSC intervention.

Joe

[JoeFromPA]

P.s. as an example, if 295 wide rear is fine without changing front, we could do 255/40/18 and 295/35/18 in Michelin PSS (standard) for $1100 in tires.

Or Toyo Proxes 1 in 265/40/18 and 295/35/18.

It opens up very interesting options to not feel constrained by a given size differential front and rear.

[CanAutM3]

Regarding DSC/MDM calibration, I would say it is mostly keeping the front-to-rear rolling radius stagger ratio as close to stock that is important.

While changing the front-to-rear width ratio might affect the overall handling balance of the car itself, I seriously doubt it would have an effect on the DSC system calibration per-se.

With the stock setup, the rear tires are 2.1% (675mm/661mm) taller than the fronts. IMO, the system has sufficient margin to operate well enough even if that ratio is not maintained. However, straying from that ratio would have the DSC/MDM operating less optimally. Since BMW recommend a square setup for winter, we can only assume that the DSC system can cope with this. Running a smaller rear-to-front ratio means more interventions, hence a "safer" system, but also a less "fun" one. This is very obvious to me when running a square setup, where there are much more useless interventions from DSC/MDM (even with Euro-MDM). And I observe this either with my grippier track tires or my less grippy winter tires. Sometimes, when flooring it, the power completely dies; quite frustrating .

As for tire wear, 1/32" of an inch does not equate to 1 mm. 7/32" wear on the tread equates to 5.5mm on the radius, hence 11mm on the diameter. So running completely worn rear tires with brand new front would yield almost the same front and rear diameters (661mm vs 664mm). Still perfectly safe, but likely with more DSC interventions. Further, that would be an unlikely scenario, as who would put brand new tires on one axle while keeping completely worn tires on the other one.

Going with a larger rear-to-front diameter ratio, such as a 255-295 combo, would yield the opposite effect. Where there would be less intervention as the system is tricked in thinking that there is less slip in the rear than there actually is.

On a last note, don't confuse DSC intervention with loss of traction. DSC can do interventions before an actual loss of traction occurs. Tires are constantly slipping, even in a straight line. DSC monitors that slip level and compares the slip between the different wheels. If it detects more slip than the programmed threshold, that is when it will intervene. I actually find that my car has less "twitchiness" or "rear-end wiggle" with DSC off. It almost seems as if it is DSC that cause the wiggles by applying the left/right brakes when it anticipates a potential loss of traction. Naturally, this become worse when running a square setup.

[Pak317]

Do you think a 285/35/18 would be ok with the 255/40/18 fronts?

[JamesGames]

Originally Posted by CanAutM3

Regarding DSC/MDM calibration, I would say it is mostly keeping the front-to-rear rolling radius stagger ratio as close to stock that is important.

While changing the front-to-rear width ratio might affect the overall handling balance of the car itself, I seriously doubt it would have an effect on the DSC system calibration per-se.

With the stock setup, the rear tires are 2.1% (675mm/661mm) taller than the fronts. IMO, the system has sufficient margin to operate well enough even if that ratio is not maintained. However, straying from that ratio would have the DSC/MDM operating less optimally. Since BMW recommend a square setup for winter, we can only assume that the DSC system can cope with this. Running a smaller rear-to-front ratio means more interventions, hence a "safer" system, but also a less "fun" one. This is very obvious to me when running a square setup, where there are much more useless interventions from DSC/MDM (even with Euro-MDM). And I observe this either with my grippier track tires or my less grippy winter tires. Sometimes, when flooring it, the power completely dies; quite frustrating .

As for tire wear, 1/32" of an inch does not equate to 1 mm. 7/32" wear on the tread equates to 5.5mm on the radius, hence 11mm on the diameter. So running completely worn rear tires with brand new front would yield almost the same front and rear diameters (661mm vs 664mm). Still perfectly safe, but likely with more DSC interventions. Further, that would be an unlikely scenario, as who would put brand new tires on one axle while keeping completely worn tires on the other one.

Going with a larger rear-to-front diameter ratio, such as a 255-295 combo, would yield the opposite effect. Where there would be less intervention as the system is tricked in thinking that there is less slip in the rear than there actually is.

On a last note, don't confuse DSC intervention with loss of traction. DSC can do interventions before an actual loss of traction occurs. Tires are constantly slipping, even in a straight line. DSC monitors that slip level and compares the slip between the different wheels. If it detects more slip than the programmed threshold, that is when it will intervene. I actually find that my car has less "twitchiness" or "rear-end wiggle" with DSC off. It almost seems as if it is DSC that cause the wiggles by applying the left/right brakes when it anticipates a potential loss of traction. Naturally, this become worse when running a square setup.

Couldn't agree with you more with everything you said. I was taking a freeway transition last night and my lights on the dash (I believe it was DSC light since I was in MDM mode) were flashing all over the place.. I was thinking in my head that MDM / DCS intervenes way too early. I have US MDM. I didn't feel I was understeering / oversteering but I could feel that I was near the limit of the tires grip before it started slipping a bit, or it maybe have been slipping a little bit. It was a decreasing radius turn (as in the turn got tighter near the end) and it was flashing the entire way. The higher the speed I am going, the more DSC intervenes it seems.. low speed I can almost get the tires to make noises before DSC intervenes on a turn.

I then tried 2nd gear WOT pulls afterwards using MDM and full TCS off, and even without TCS I felt the power was putting down just fine, maybe MINOR slipping (I am a 6speed though) but I'm running hankook ventus v12s. With MDM mode, the TCS light is always flashing on a WOT 2nd gear.