TECHNICAL INFO: For all those that advocate or want a staggered wheel set up, please read on...
05-18-2006, 07:19 PM
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TECHNICAL INFO: For all those that advocate or want a staggered wheel set up, please read on...
First let me say that it is not my intentions to start a heated debate here but I just want to pass on some info on the theory of the staggered concept with the use of a TorSen diff.
We have all heard there is a certain amount of play the TorSen can take although it as never been completely confirmed. I heard of a 2% difference in rolling diameter between the front wheels and the rear ones. Some say 3% and some say it does not even matter. Aside from the TorSen problem you may incur, don't forget a staggered set up will most likely have a negative impact on your handling.
The tuner who posted these pictures initially will remain nameless and so will the owner of the car. However since they used a public forum to pass a message in a very bad way, I've decided to use the pictures they provided to show you the consequences and the most likely reason(s) it happened.
I will use STaSIS's analysis and comments because they were involved in this case because the TorSen was modified by them. I'm sure they don't mind me using their name and analysis because of the educational/informative nature of the post.
The car in question is a highly modified, read powerful A4. It was thought by the tuner and owner who posted this, that the power and one road track event the car attended were the reasons for the damaged TorSen. For those who know the TorSen and as Stasis commented, <b>it is not the acceleration side</b> of the TorSen that is damaged but the <b>deceleration side</b> where the problem occurred. So the power output theory is out the window.
According to STaSIS there are two ways to damage the TorSen;
<b>One</b>, run a staggered set up that exceeds the 2% or 3% rule.
<b>Two</b>, If work was done on the tranny or the front or rear diffs and the gearing was changed in a way that the diffs were not matched anymore this could happen.
<b>NOTE:</b> A staggered set up will damage a regular or non modified TorSen too but it seems it will accelerate the process on a modified TorSen.
The car in question does run a staggered set up but is within the 3% range. Did the car ever get work done on its tranny changing the specs, that is unknown. In any case the TorSen was damaged and several things could have lead to this failure. BTW, STaSIS stepped up to the plate big time in this case. Obviously it was not their fault that the TorSen failed and they ended up paying shipping both ways to get the customer's car back on the road and fixing the TorSen free of charge to the customer! That's what I call customer service.
<b>STaSIS Quote:</b>
"We have a large number of these units in use without any failure mode of this type ever without fault of the drivetrain in the vehicle as Jason mentions. You need to understand how the TorSen center differential works in order to understand what causes a failure such as this. I would be very certain that the front and rear diff are matched and the staggered wheel setup is not more than 3% different in rolling overall diameter so this doesn't happen yet again."
<b>Pictures:</b>
Damaged TorSen side 1.
<img src="http://pictureposter.audiworld.com/46874/side1.jpg">
Damaged TorSen side 2.
<img src="http://pictureposter.audiworld.com/46874/side2.jpg">
Damaged TorSen side 3.
<img src="http://pictureposter.audiworld.com/46874/side3.jpg">
We have all heard there is a certain amount of play the TorSen can take although it as never been completely confirmed. I heard of a 2% difference in rolling diameter between the front wheels and the rear ones. Some say 3% and some say it does not even matter. Aside from the TorSen problem you may incur, don't forget a staggered set up will most likely have a negative impact on your handling.
The tuner who posted these pictures initially will remain nameless and so will the owner of the car. However since they used a public forum to pass a message in a very bad way, I've decided to use the pictures they provided to show you the consequences and the most likely reason(s) it happened.
I will use STaSIS's analysis and comments because they were involved in this case because the TorSen was modified by them. I'm sure they don't mind me using their name and analysis because of the educational/informative nature of the post.
The car in question is a highly modified, read powerful A4. It was thought by the tuner and owner who posted this, that the power and one road track event the car attended were the reasons for the damaged TorSen. For those who know the TorSen and as Stasis commented, <b>it is not the acceleration side</b> of the TorSen that is damaged but the <b>deceleration side</b> where the problem occurred. So the power output theory is out the window.
According to STaSIS there are two ways to damage the TorSen;
<b>One</b>, run a staggered set up that exceeds the 2% or 3% rule.
<b>Two</b>, If work was done on the tranny or the front or rear diffs and the gearing was changed in a way that the diffs were not matched anymore this could happen.
<b>NOTE:</b> A staggered set up will damage a regular or non modified TorSen too but it seems it will accelerate the process on a modified TorSen.
The car in question does run a staggered set up but is within the 3% range. Did the car ever get work done on its tranny changing the specs, that is unknown. In any case the TorSen was damaged and several things could have lead to this failure. BTW, STaSIS stepped up to the plate big time in this case. Obviously it was not their fault that the TorSen failed and they ended up paying shipping both ways to get the customer's car back on the road and fixing the TorSen free of charge to the customer! That's what I call customer service.
<b>STaSIS Quote:</b>
"We have a large number of these units in use without any failure mode of this type ever without fault of the drivetrain in the vehicle as Jason mentions. You need to understand how the TorSen center differential works in order to understand what causes a failure such as this. I would be very certain that the front and rear diff are matched and the staggered wheel setup is not more than 3% different in rolling overall diameter so this doesn't happen yet again."
<b>Pictures:</b>
Damaged TorSen side 1.
<img src="http://pictureposter.audiworld.com/46874/side1.jpg">
Damaged TorSen side 2.
<img src="http://pictureposter.audiworld.com/46874/side2.jpg">
Damaged TorSen side 3.
<img src="http://pictureposter.audiworld.com/46874/side3.jpg">
05-18-2006, 07:27 PM
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Excellent read. I'd just like to clarify that in your context, "staggered" means in diameter, not
in wheel width.
Yes, you mentioned it several times in the post, but to most of us that considered the "Staggered setup" at times, we usually think of wider rears, which would thereby diminish our handling characteristics since we're a nose heavy, awd car.
(before anyone says what about the 996tt for instance, remember their engine is in the rear. i.e..on the S, you'd want wider fronts ;-D)
Woot. That was great Jet. Please confirm my clarification. TIA
Yes, you mentioned it several times in the post, but to most of us that considered the "Staggered setup" at times, we usually think of wider rears, which would thereby diminish our handling characteristics since we're a nose heavy, awd car.
(before anyone says what about the 996tt for instance, remember their engine is in the rear. i.e..on the S, you'd want wider fronts ;-D)
Woot. That was great Jet. Please confirm my clarification. TIA
05-18-2006, 07:57 PM
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I'm assuming since the wheels were staggered the rear tires were also wider then the fronts
This would make the rear diameter larger because a 255/40/18 tire has a larger diameter then a 235/40/18 (I have no idea if these are the sizes in question - this is just an example).
05-18-2006, 08:06 PM
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Well, you don't just change the 235 vs 255 in that equation without adjusting the ratio (40)
unless, or course, you're tying to have off diameters.
Most people who run wider tires, the 255 vs 235 dimension in mm's (approx 9.9 inches vs 9.2, respectively) adjust the ratio for the overall diameter.
Know what I mean?
However, I see the problem in this. You're not going to get the same diameter even with the ratio correction that people will try; the options aren't specific enough in terms of tire sizes...however, with enough effort, it should be less than a 2% difference; i.e. my new 245/35/19 vs my old 235/40/18, less than 1% diff (still, negligible in terms of torsen since I have the same size all around.)
What I find interesting about people getting the same sized wheels, but having varying offsets to give it the "Staggered look" is that handling wise, it's still worsened on our S4's. Yes, torsen isn't affected, but the rears sit out wider than the fronts; looks hot, but still not great for performance.
Most people who run wider tires, the 255 vs 235 dimension in mm's (approx 9.9 inches vs 9.2, respectively) adjust the ratio for the overall diameter.
Know what I mean?
However, I see the problem in this. You're not going to get the same diameter even with the ratio correction that people will try; the options aren't specific enough in terms of tire sizes...however, with enough effort, it should be less than a 2% difference; i.e. my new 245/35/19 vs my old 235/40/18, less than 1% diff (still, negligible in terms of torsen since I have the same size all around.)
What I find interesting about people getting the same sized wheels, but having varying offsets to give it the "Staggered look" is that handling wise, it's still worsened on our S4's. Yes, torsen isn't affected, but the rears sit out wider than the fronts; looks hot, but still not great for performance.
05-18-2006, 08:18 PM
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What I mean by "rolling diameter"...
The front wheels and rear wheels should be within the 2% or 3% of each other. So if the front as a rolling diameter of 25.0" the rear should also be 25.0".
2% of 25" is only 0.50 of an inch (1/2").
3% of 25" is only 0.75 of an inch (3/4").
Just tire inflation can bump you up and down at least an eight (1/8") of an inch.
Here is where it gets tricky and I'm not even sure myself if there is a problem.
If you run a 245/40/19 on a 8.5 inch wide wheel in the front and decide to run the same tire size on a 9.5 inch wide wheel in the back, technically the tire are the same but because the rear ones get an extra inch to stretch on, they are now lower in rolling diameter, by how much, I don't know. You would have to use a string and measure both to see if they remain within the specs.
Now the other scenario I can think of is what if you run a 235/40/19 in the front on a wheel 8.5 inches wide and run a 245/40/19 in the rear on a 9.5 inch wheel, well I think you just screwed the pooch.
Another thing I just thought about and I'm not sure if I'm going too far here but besides the rolling diameter what about the side load(s) factor imposed on the axles and perhaps that even affects the TorSen somewhat. If you have a narrower tire in the front with a much wider rear as the car turns, won't the wider tire exercise more pressure in the turn and force the wheels to turn even slower or slide sideways, not that you would see it. Would the fact that there is more resistance to turn because the wider tire is "scrubbing" the surface, wouldn't that affect the whole drivetrain?
Oh well more questions than answers I guess.
BTW, alot of cars run a staggered set up from the factory because they were designed that way for handling reasons.
2% of 25" is only 0.50 of an inch (1/2").
3% of 25" is only 0.75 of an inch (3/4").
Just tire inflation can bump you up and down at least an eight (1/8") of an inch.
Here is where it gets tricky and I'm not even sure myself if there is a problem.
If you run a 245/40/19 on a 8.5 inch wide wheel in the front and decide to run the same tire size on a 9.5 inch wide wheel in the back, technically the tire are the same but because the rear ones get an extra inch to stretch on, they are now lower in rolling diameter, by how much, I don't know. You would have to use a string and measure both to see if they remain within the specs.
Now the other scenario I can think of is what if you run a 235/40/19 in the front on a wheel 8.5 inches wide and run a 245/40/19 in the rear on a 9.5 inch wheel, well I think you just screwed the pooch.
Another thing I just thought about and I'm not sure if I'm going too far here but besides the rolling diameter what about the side load(s) factor imposed on the axles and perhaps that even affects the TorSen somewhat. If you have a narrower tire in the front with a much wider rear as the car turns, won't the wider tire exercise more pressure in the turn and force the wheels to turn even slower or slide sideways, not that you would see it. Would the fact that there is more resistance to turn because the wider tire is "scrubbing" the surface, wouldn't that affect the whole drivetrain?
Oh well more questions than answers I guess.
BTW, alot of cars run a staggered set up from the factory because they were designed that way for handling reasons.
