AWD and tire size what is the limit

[yoshimitsuspeed]

Any tire shop will tell you if you mess up one tire on your AWD you automatically need to replace all of them. I know that's not necessarily true but I am trying to find some parameters of what would be acceptable variance and what could be bad for the drivetrain.
It occurred to me one big factor would be if the car has 0,1 or two LSDs.
If there is variance between left and right tires but the average is about the same front to rear then it seems to me it wouldn't be much of an issue with a car with open diffs.
If you had a rear LSD it seems to me you could match the rear tires as closely as possible then put the two most opposing tires up front. The open diff would take up that variance and the center diff would see very little.

I am also assuming realistically you should be able to measure tire circumference and get close enough to make a solid assessment on whether the variance would be within a safe range or not.

So what are peoples thoughts on what would be acceptable range and how would that range be affected by a car with 0,1 or 2 LSDs?

If I did my math right if you have a tire 25" in dia and one that's 25.25" which would be a pretty noticeable difference in size that's a 1% difference in circumference right? And should equate to the diff slipping about 14 RPM at 80 mph right?
An open diff would have no problem. The only issue would be the center diff but say three tires were 25" and one was 25.25 then there would only be a half percent difference on the average of front tires and rear tires. The RPM and slippage of the center diff would be dependent on the final drive ratio but whatever the case the center diff would be seeing a pretty minimal amount of slippage.
Even if those odd sized tires had an LSD it seems to me 14 RPM slippage wouldn't be catastrophic. Sure it might lead to slightly faster wear but would it be significant?

So what's a safe range for tire variance in an AWD?

 

[IceStationZebra]

I don't know the answer, but I think another thing to consider is at what point do the stability control and anti-lock brake systems kick in? Is 1% difference enough? My assumption is no, because one could see more difference than that if the vehicle was loaded only loaded on one side. But I know someone with indepth experience will answer this post.

ISZ

 

[GregLocock]

Another data point is the temporary space saver (money saver) spare. Check the circumference of one of those compared with the largest standard tire.

Cheers

Greg Locock


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[izzmus]

It's a solid "it depends".

Audi claims that front vs. rear tires on their Torsen equipped models can have up to a 4% difference in size.

Don't try that on, say, an early Grand Cherokee with the viscous limited slip center diff.

 

[yoshimitsuspeed]

I don't think the spare will really tell much. For one thing all space savers I have seen warn to drive under 50 mph or so. This would keep heat and friction considerably lower than say interstate speeds.
They also tell you not to drive further than something like 50 miles. I am sure the manufacturer would be comfortable getting away with a greater variance in that situation than with a tire that may run 10-20k miles.

Izzmus you bring up a good point on different style LSDs having different limits.

 

[izzmus]

They also are figuring that the average driver is probably not going to be prepared to deal with the handling curiosities resulting from having a 145mm tire on one corner and 225mm tires on the other three, and that he's also not going to be getting the lug nuts tight enough.

At least one car stipulated that the space saver spare was only to be mounted to the front, so save the rear diff. In the event of a rear flat, one of the front tires was to be moved to that position.

I have never seen a production AWD vehicle that didn't have either a full size spare, or a space saver that was the same nominal outside diameter as the standard tires.

 

[Warpspeed]

A great deal depends on the surface and the type of 4WD system.
Full time 4WD with three true torque splitting differentials, will be very different to a solidly locked up system.

What you can get away with might depend on the type of LSDs fitted, most will quite happily creep a bit without causing any undue problem.
I have heard a figure of 3% diameter mentioned.
I know my own 4WD (with the centre diff locked) can only be pushed by hand in neutral a few feet before it becomes immovable, and that is with supposedly identical tires.