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      02-03-2009, 12:44 PM   #1
ILLCOMM
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Front/Rear staggered ratio? Exceeding throws off DSC?

A rep at TireRack mentioned that I need to keep the front/rear tire size within a range to maintain proper DSC function. I know that wider rears and narrower fronts can lead to understeer so I am inclined to believe the rep.

His max recommendation for the M3 was:

255/30/20
275/30/20

What are your thoughts? I was hoping to fit a 285 or a 295 on the rear, but that was purely to make the stance look better. I don't want to sacrifice function of the DSC.

Thanks!
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      02-03-2009, 12:56 PM   #2
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I have heard of that for the xi but this is a first for other models...
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      02-03-2009, 01:29 PM   #3
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others on this forum are running as wide as 305 in the back and 265 in the front with varying ratios. They have not mentioned any issues
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      02-03-2009, 02:10 PM   #4
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Quote:
Originally Posted by JuniorDee View Post
others on this forum are running as wide as 305 in the back and 265 in the front with varying ratios. They have not mentioned any issues
Or they aren't aware of the consequences. Which may not come up with a warning light on the dash.
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      02-03-2009, 02:13 PM   #5
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Originally Posted by MDCTFTW View Post
Or they aren't aware of the consequences. Which may not come up with a warning light on the dash.
+1

there might me more of play space but not as big as going 305...
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      02-03-2009, 02:29 PM   #6
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Width has nothing to do with it. It's the change in tire height or overall diameter (rim + tire) of the new setup that you have to pay attention to. It's the ratio of overall diameter from front to rear that you need to maintain. VDC will intervene when it sees a change in wheel speed condition, but only after it sees a change above a given threshold, like 5% or something. If the OEM setup has the same overall diamter front to rear, then you should try to maintain an equal diameter no matter what rim size, profile, etc... that you chose. Same thing for an OEM setup that doesn't run equal diameter wheel/tire combos. If its a 10% difference in height, then you should try your best to keep that 10% difference, or you're VDC is going to need to be disabled to drive your car. I want to say the rule of thumb, is don't try to change your ratio (front to back) more than 3% from OEM settings
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      02-03-2009, 03:55 PM   #7
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Irb - I don't dispute your comments about diameter.

However, my question is more about whether BMW DSC has some parameters that are tuned to the stock over/understeer propensity. The fact is that putting wider tires on the rear will increase understeer. Tires won't be spinning (thus diameter issue not pertinent), instead you'll just be pushing the car through a corner after the front tires lose grip. Can the car detect this dynamically or is it making assumptions? If it's making assumptions I am worried that changing the OEM staggered ratio will break the assumptions which will be problematic if the car ever needs to rely on DSC. If it's dynamic then there's no issue (aside from the fact that you're dialing in more understeer).
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      02-04-2009, 02:42 AM   #8
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I seriously doubt the DSC system is matched or tuned to the tire sizes. The system works by sensing wheel spin and under or over steer conditions and applies the appropriate combination of brakes to counter the situation. If your new set up caused more over or understeer DSC would still work, it would just kick in more often. The right thing to do is to scale up the front and rear widths in proportion and making small adjustments away from a true proportion to slightly dial in your desired balance. Unless the car will be driven exclusively on the track it is probably best to leave just a bit of understeer. And like stock you will obtain this with a staggered set up, wider in the rear. There is a reason almost all cars have this from the factory - safety!

One other subtlety is that you can actually correct (or move toward a correction) of the OEM speedo which typically runs about 3-4% fast by altering the revs per mile away from an exact match of stock. I'm not sure if there is a 20 size that helps out here but there are 19" combo that help (based on a quick ideal calculation, not the more accurate revs/mile). 295/30-19 or 285/30-19 will both help. These could be paired with a 275/30-19 or 265/30-19. Again perhaps the speedo thing is the last thing on your priority list, but it is a possibility.

Last but not least and JMHO. Go with 19s. I'm sure you are set on 20s but they are too heavy and on the side of looks only/poseur. 18s offer the worst looks and best performance (least weight and more importantly lowest interia), 19s are a decent compromise and 20s are on the other end of the spectrum.

Last edited by swamp2; 02-04-2009 at 03:01 AM.
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      02-04-2009, 04:10 AM   #9
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I'm going to do 255 front and 285 rear. I actually find the M3 a bit too tail happy, so if I induce a bit more understeer I wouldn't mind
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      02-04-2009, 04:40 AM   #10
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Quote:
Originally Posted by swamp2 View Post
I seriously doubt the DSC system is matched or tuned to the tire sizes. The system works by sensing wheel spin and under or over steer conditions and applies the appropriate combination of brakes to counter the situation. If your new set up caused more over or understeer DSC would still work, it would just kick in more often. The right thing to do is to scale up the front and rear widths in proportion and making small adjustments away from a true proportion to slightly dial in your desired balance. Unless the car will be driven exclusively on the track it is probably best to leave just a bit of understeer. And like stock you will obtain this with a staggered set up, wider in the rear. There is a reason almost all cars have this from the factory - safety!

One other subtlety is that you can actually correct (or move toward a correction) of the OEM speedo which typically runs about 3-4% fast by altering the revs per mile away from an exact match of stock. I'm not sure if there is a 20 size that helps out here but there are 19" combo that help (based on a quick ideal calculation, not the more accurate revs/mile). 295/30-19 or 285/30-19 will both help. These could be paired with a 275/30-19 or 265/30-19. Again perhaps the speedo thing is the last thing on your priority list, but it is a possibility.

Last but not least and JMHO. Go with 19s. I'm sure you are set on 20s but they are too heavy and on the side of looks only/poseur. 18s offer the worst looks and best performance (least weight and more importantly lowest interia), 19s are a decent compromise and 20s are on the other end of the spectrum.
+1, I was trying to say this, but I'm not sure if it came out that way. Basically, tire width should have very little effect. If the yaw sensors sense an understeer or oversteer condition they are going to intervene. But this will only be under extreme turning conditions where you've exceeded the grip of the front or rear tires. Tire height doesn't matter either, only the ratio of height from front to back is what you need to pay close attention to. Put some 26 inch wheels on in the back, with the OEM fronts, and I guarantee your DSC is going to go crazy.
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      02-06-2009, 01:30 PM   #11
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For optimal track results, it sounds like 265/35 18 in front and 295/30 18 in back would be best -- as long as it would fit without rubbing. With Pilot Sport Cups, this would give loads of grip vis a vis stock sizes.
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      02-06-2009, 05:05 PM   #12
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If you can fit a 305/30R19 in there, the rolling diameter will be almost 100% the same as stock. You can pair a 265/35R19 up front. I also find the M3 to be a little bit too tail happy so a little extra understeer will help.
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      02-06-2009, 05:07 PM   #13
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What do you guys think about doing a 19x10 all around with possibly 265/30 tires? It will be a street setup mostly, rarely seeing the track
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      02-06-2009, 05:48 PM   #14
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Quote:
Originally Posted by ///M PWR 00 View Post
What do you guys think about doing a 19x10 all around with possibly 265/30 tires? It will be a street setup mostly, rarely seeing the track
For a street setup I wouldn't go square. If I were to though, I'd do like 9.5 all around with 275's.
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