tire question: anyone running 255s in front, 275s in back?
09-07-2008, 11:17 PM
#4
AudiWorld Super User
He's talking about rolling diameter, not widths... also, R8 is a completely different animal...
Can't really compare the two except or sharing essentially the same engine. The RS4 has a 40:60 torque bias to the rear, while the R8's version of quattro has a 15:85 split, variable with no more than 35% going to the front and no less than 15%. I'm pretty sure Audi has the quattro system setup for the R8's rear-biased weight distribution (56%) which necessitated fatter rear tires.
09-08-2008, 04:04 AM
#5
This question raises a lot of issues......
1. The 275/30/19 tires that people are putting on their cars actually have a rolling diameter that is 13.5mm less than the stock 255/35/19. This is a 2% decrease in rolling diameter and is bordering on significant in terms of the ABS and electronic stability functions of the vehicle.
2. The tire that would come closest to the stock rolling diameter would be a 295/30/19. This would yield a rolling diameter of only 1.5mm less than stock which is for all practical purposes, identical. A 295 tire, however, won't fit this car.
3. With a properly designed custom 9.5 wide wheel, such as the HRE M40, you can fit a 285/30/19 tire. Such a tire would yield a rolling diameter which is 7.5mm less than stock and, as such, is an acceptable diameter to substitute for the stock 255/35/19 tire.
That's the tire fitment issue....now you have the issue of staggered width.
1. Yes you can run a staggered width with quattro contrary to popular belief, but it would not be the best for handling. The contact patch of the tire is related in large part to the weight distribution of the car. Given the same sized tire on both F/R, on the RS4, the front contact patch is slightly longer due to the front biased weight distribution. Now if you make the rear tire wider (thus shortening the contact patch length), you will have a short, wide contact patch in the rear and a narrow, long contact patch in front. This is exactly what you don't want in a car that has a tendency to understeer.
2. This can be offset, however, by using spacers and widening the front track, or by calibrating you aftermarket suspension spring rates to compensate.
3. Yet another issue is the torque distribution. One could argue that at baseline a 60:40 rear biased torque split might actually mate well with a slightly increased rear tire width, but since the quattro system is able to adjust power delivery based on traction, it doesn't really matter. And also remember, since a narrower tire will have a longer contact patch, within a small range of width differences, the forward traction won't be much different, since the contact AREA will be the same as a wider shorter contact patch.
4. Lateral movements/handling (as it relates to staggered tire width) is a different story altogether and since I've already written an essay on this subject, I won't go into it.
In conclusion:
You can run staggered. You want 245/35/19 front on 19x8.5 wheels and 275/30/19 rear on 19x9.5 wheels
OR:
255/35/19 front on 19x9 wheels with 285/30/19 rear on 19x9.5 wheels.
In both cases, use a spacer in FRONT ONLY. 10mm if using a 9" front wheel and 20mm if using an 8.5" front wheel.
If using an aftermarket suspension, soften the front spring rates by 50lbs or so, so that the front tires will bite more during turns .
Personally, I would not run staggered without BOTH an aftermarket suspension and front spacers, and in the end, on the RS4, there probably isn't an advantage when you add up all the issues. As pointed out, R8 is a totally different vehicle in terms of weight distribution and torque split, so you really can't compare.
2. The tire that would come closest to the stock rolling diameter would be a 295/30/19. This would yield a rolling diameter of only 1.5mm less than stock which is for all practical purposes, identical. A 295 tire, however, won't fit this car.
3. With a properly designed custom 9.5 wide wheel, such as the HRE M40, you can fit a 285/30/19 tire. Such a tire would yield a rolling diameter which is 7.5mm less than stock and, as such, is an acceptable diameter to substitute for the stock 255/35/19 tire.
That's the tire fitment issue....now you have the issue of staggered width.
1. Yes you can run a staggered width with quattro contrary to popular belief, but it would not be the best for handling. The contact patch of the tire is related in large part to the weight distribution of the car. Given the same sized tire on both F/R, on the RS4, the front contact patch is slightly longer due to the front biased weight distribution. Now if you make the rear tire wider (thus shortening the contact patch length), you will have a short, wide contact patch in the rear and a narrow, long contact patch in front. This is exactly what you don't want in a car that has a tendency to understeer.
2. This can be offset, however, by using spacers and widening the front track, or by calibrating you aftermarket suspension spring rates to compensate.
3. Yet another issue is the torque distribution. One could argue that at baseline a 60:40 rear biased torque split might actually mate well with a slightly increased rear tire width, but since the quattro system is able to adjust power delivery based on traction, it doesn't really matter. And also remember, since a narrower tire will have a longer contact patch, within a small range of width differences, the forward traction won't be much different, since the contact AREA will be the same as a wider shorter contact patch.
4. Lateral movements/handling (as it relates to staggered tire width) is a different story altogether and since I've already written an essay on this subject, I won't go into it.
In conclusion:
You can run staggered. You want 245/35/19 front on 19x8.5 wheels and 275/30/19 rear on 19x9.5 wheels
OR:
255/35/19 front on 19x9 wheels with 285/30/19 rear on 19x9.5 wheels.
In both cases, use a spacer in FRONT ONLY. 10mm if using a 9" front wheel and 20mm if using an 8.5" front wheel.
If using an aftermarket suspension, soften the front spring rates by 50lbs or so, so that the front tires will bite more during turns .
Personally, I would not run staggered without BOTH an aftermarket suspension and front spacers, and in the end, on the RS4, there probably isn't an advantage when you add up all the issues. As pointed out, R8 is a totally different vehicle in terms of weight distribution and torque split, so you really can't compare.
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09-08-2008, 06:38 AM
#8
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Yes the R8 is a different animal...as is the 911TT, yet my point.was...
as Vijay put so well below that different width wheels can be used on AWD cars...and yes, they can be used on the RS4...nothing to do with different rolling diameters, or with comparing the R8 to the RS4 other than an audi AWD system that comes stock with different width wheels...
09-08-2008, 06:53 AM
#10
Doing this will increase oversteer
Increasing traction on the rear wheels will increase oversteer in the corners.
Rotational sensors in each wheel hub will allow ABS and traction control to compensate for the difference in tire diameter
Rotational sensors in each wheel hub will allow ABS and traction control to compensate for the difference in tire diameter