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LSD Issues and Tire Size

yomamma219

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So in the mustang manual there is a part in the "Rear Axle" chapter where it says:

"Extended use of tire sizes other than the manufacturer’s specified tire size on these axles could result in a permanent reduction in effectiveness. This loss does not affect your normal driving and should not be noticeable."

Now I was just wondering if the "permanent reduction in effectiveness" is actually something I should be concern with after going to up a 315 tire in the rear?

I have't tracked my car yet but I hope to get into that soonish, so I think I fall in the unaffected category. The whole statement seems contradictory and unnecessarily ominous though, but being in the manual the validity of it gives me some concern.

Does anyone know why this is an issue? What would the permanent reduction be from? I thought I heard the Torsen was a mechanical (non-clutch) LSD so it didn't have to worry about clutch packs wearing out. Is it just due to the LSD not being able to function as well when the tire has more grip than it was originally intended?

PS: I have an EB so not the Torsen LSD (Traction-Lok I think?). I don't really know what the specific engineering difference is.
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BmacIL

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So in the mustang manual there is a part in the "Rear Axle" chapter where it says:

"Extended use of tire sizes other than the manufacturer’s specified tire size on these axles could result in a permanent reduction in effectiveness. This loss does not affect your normal driving and should not be noticeable."

Now I was just wondering if the "permanent reduction in effectiveness" is actually something I should be concern with after going to up a 315 tire in the rear?

I have't tracked my car yet but I hope to get into that soonish, so I think I fall in the unaffected category. The whole statement seems contradictory and unnecessarily ominous though, but being in the manual the validity of it gives me some concern.

Does anyone know why this is an issue? What would the permanent reduction be from? I thought I heard the Torsen was a mechanical (non-clutch) LSD so it didn't have to worry about clutch packs wearing out. Is it just due to the LSD not being able to function as well when the tire has more grip than it was originally intended?

PS: I have an EB so not the Torsen LSD (Traction-Lok I think?). I don't really know what the specific engineering difference is.
The only time it would be an issue is if you were to put different diameter tires left and right on the rear. It will work just fine with more grip/larger tires.
 
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yomamma219

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The only time it would be an issue is if you were to put different diameter tires left and right on the rear. It will work just fine with more grip/larger tires.
ohhh what if I need to run a donut in the back? :p The manual seems to be speaking to the effect of width/grip or larger diameter not so much that type of insanity.

I can sort of imagine there being increase wear from increase grip or larger diameter putting a larger torque load on the LSD. I was mostly curious if this was an actual issue, and if it could cause Ford to void a differential warranty claim or something. Seems like a no to me though.

Just having a slow day so figured I'd bring up something on my mind I hadn't seen discussed on the forum before.
 

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What kills clutch-pack LSDs is forcing them to differentiate fast for a period of time.

So, where this issue comes into play is, as BmacIL said, is if you have a different diameter tire left to right. If they're both the same size, but bigger or wider, the diff won't really know or care. A wider, grippier, tire will make it differentiate more often, but it isn't as significant.

OTOH, if you go a long distance at high speed with two different diameter tires, you can burn up the clutches. So, if you do use a mini spare at some point, it is important to observe the rated speed and distance limitations that come with it.
 

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jasonstang

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ohhh what if I need to run a donut in the back? :p The manual seems to be speaking to the effect of width/grip or larger diameter not so much that type of insanity.

I can sort of imagine there being increase wear from increase grip or larger diameter putting a larger torque load on the LSD. I was mostly curious if this was an actual issue, and if it could cause Ford to void a differential warranty claim or something. Seems like a no to me though.

Just having a slow day so figured I'd bring up something on my mind I hadn't seen discussed on the forum before.
You don't. That's why if you have a flat on the driving axle, you swap a wheel from the non driving axle and put the spare on the non driving axle.
This is why most 4x4's come with full size spare.
 

EXP Jawa

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The issue with 4x4s can be more complicated then that, but at the end of the day, if your car's comes with a mini spare, follow the manual's guidelines for usage. Putting the spare on the front is probably safer from a driveline point of view, but is necessarily necessary, depending on circumstances. Again, check your manual and follow its advice.
 
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yomamma219

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Well the problem I initially see with changing two tires (front to back, donut to front) is that I now have a staggered setup so that seems less practical than just getting a donut on the back and getting to the nearest shop.

I really so mentioned the spare as a joke mostly tbh as I don't even have one for this car. I have the fix-a-flat kit. I have been thinking about getting the dount spare kit as I have already puncture one of my old tires that I had to use the F-A-F kit on a couple months ago, and then got it patched. Then had a screw in one of my brand new tires last week but fortunately it didn't puncture it just was embedded in the tread. Thank god for the Nitto NT555's much thicker tread than the OEM Pir-crappis
 

BmacIL

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Well the problem I initially see with changing two tires (front to back, donut to front) is that I now have a staggered setup so that seems less practical than just getting a donut on the back and getting to the nearest shop.

I really so mentioned the spare as a joke mostly tbh as I don't even have one for this car. I have the fix-a-flat kit. I have been thinking about getting the dount spare kit as I have already puncture one of my old tires that I had to use the F-A-F kit on a couple months ago, and then got it patched. Then had a screw in one of my brand new tires last week but fortunately it didn't puncture it just was embedded in the tread. Thank god for the Nitto NT555's much thicker tread than the OEM Pir-crappis
And this is one reason staggered setups suck.
 

jasonstang

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Well the problem I initially see with changing two tires (front to back, donut to front) is that I now have a staggered setup so that seems less practical than just getting a donut on the back and getting to the nearest shop.

I really so mentioned the spare as a joke mostly tbh as I don't even have one for this car. I have the fix-a-flat kit. I have been thinking about getting the dount spare kit as I have already puncture one of my old tires that I had to use the F-A-F kit on a couple months ago, and then got it patched. Then had a screw in one of my brand new tires last week but fortunately it didn't puncture it just was embedded in the tread. Thank god for the Nitto NT555's much thicker tread than the OEM Pir-crappis
Staggered is not that big of a problem as long as the tires have roughly same circumference.
The donut is much smaller.
 

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EXP Jawa

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We've qualified differentials with minispare tires, sometimes as much as 25% smaller than the normal tires. That will build heat in the axle, as the differential is forced to differentiate under load and is generating friction all the time. But that's part why the tires have speed and distance caps.
 
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yomamma219

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And this is one reason staggered setups suck.
Yes unfortunately was one of the downsides I was well aware of when deciding to go with the wheel/tire set I did.

What doesn't suck though is my lovely fitment on stock suspension without spacers. :cool:
 

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Would there be any issue with running significantly smaller rolling diameter on all four corners (provided they are all the same diameter)? I will be running 295/35/18 r-compounds on my track wheels which are quite a bit smaller than the 275/40/19 PP rears.
 
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yomamma219

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We've qualified differentials with minispare tires, sometimes as much as 25% smaller than the normal tires. That will build heat in the axle, as the differential is forced to differentiate under load and is generating friction all the time. But that's part why the tires have speed and distance caps.
That makes sense. I had always thought the speed/distance rating was more so for wear/stress limits on the donut itself. Never really contemplated that it was for other components sake

[MENTION=8992]EXP Jawa[/MENTION] Out of curiosity would you mind sharing what company you work for? Just wondering as I am interested in getting into the auto industry and am always learning about new companies/ micro industries involved with it that are beyond parts, manufacturing or racing.
 

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Would there be any issue with running significantly smaller rolling diameter on all four corners (provided they are all the same diameter)? I will be running 295/35/18 r-compounds on my track wheels which are quite a bit smaller than the 275/40/19 PP rears.
Apart from your speedometer will be off, you should be fine.
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