LR3 Blew a Tire. Can I Get 2?

spikemd

Explorer
The LR3 blew a tire yesterday, thankfully with me driving and not just my wife. Required jacking up vehicle, removing hitch, lowering spare which is oversized and deflated ion order to fit under the car, inflating spare on the vehicle. The tire is not repairable with a major slice under a tread. Wondering if it was damaged in Moab.

They are Michelin AT2s with 33k miles on them with lots of life left in size 265/65 r18. Tread depth is currently 9/32s with new at 14/32s and the last set of AT2s on the 4Runner lasted more than 80k miles.

I am wondering if I can get 2 tires and then rotate them side to side for the remaining life of the tires, or do I have to buy 4 new ones. I don't want to screw up the center diff.
 

jymmiejamz

Adventurer
You won't have any problems doing just two. Its common for the front tires to wear much more than the rears in a stock setup depending on driving style.
 

Ian_Barry

Observer
Not So Fast

Its common for the front tires to wear much more than the rears in a stock setup depending on driving style.
That's certainly true, and why you rotate tires - to make the wear more even. I will let someone with mechanical engineering chops (not necessarily engineering, but more information available in post #7 by cmb6s assuaging concerns and explaining function of center differential) tackle the exact why if necessary but here's the basics of why this isn't the best call (and at the bottom a viable solution).

You will cause differential wind up (center differential). The drive train of the truck is 4wd - all of the time. The differentials allow the wheels to spin at slightly different speeds to allow the truck to make turns. The differential is not capable of changing the division of power between the fore/aft. What will happen is that you will be requiring the center differential to account for different rotating speeds fore/aft constantly, which is not really what it's good at - but it will work in the short term. I believe the consequence will be overheating and damaging the differential.

The simple solution is have a tire shop "shave" down the tread to match.

Hope that helps,

Ian
 
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jymmiejamz

Adventurer
That's certainly true, and why you rotate tires - to make the wear more even. I will let someone with mechanical engineering chops tackle the exact why if necessary but here's the basics of why this isn't the best call (and at the bottom a viable solution).

The simple solution is have a tire shop "shave" down the tread to match.
I've never heard of any shop that 'shaves' tires. If the OP decides to ask a shop to do this, please film the shops reaction.

The fact of the matter is that in practice, there is no issue with mismatched treads on tires. I have personal experience in 100's upon 100's on Land Rovers with like new tires in the rear and worn tires in the front. From the owners manual...

Ideally, tires should be replaced as sets of four, but if this is not possible, replace the tires as axle sets.
 

spikemd

Explorer
Thanks for the replies. It's a moot point now since Michelin discontinued the AT2s I bought. But Michelin and America's Tire are stepping up and taking care of me. The tires are at 9/32s from 14/32s so they are giving me 60% return on the 3 good tires, 100% on the blown one and I am getting what I think is the best all terrain tire, the new BFG AT KO2 in 265/65 r18s. I am also getting 5 this time to rotate all around.

America's Tire also set me up when I blew out 2 sidewalls on my Revo2s and I stepped up to BFG KM2s on the range rover. Highly recommend them.
 

cmb6s

Adventurer
You will cause differential wind up (center differential). The drive train of the truck is 4wd - all of the time. The differentials allow the wheels to spin at slightly different speeds to allow the truck to make turns. The differential is not capable of changing the division of power between the fore/aft. What will happen is that you will be requiring the center differential to account for different rotating speeds fore/aft constantly, which is not really what it's good at - but it will work in the short term. I believe the consequence will be overheating and damaging the differential.
While the DD295 transfer box that the LR3 uses is certainly in 4wd all the time with a 50/50 torque bias as the default, you will not "cause differential wind up" by running slightly different sized rear tires. The transfer box has a bevel gear center differential with an electronically controlled multi-plate clutch that will allow slip and also vary the torque distribution between the front and rear axles based on a number of different inputs that the ECU monitors continuously. I have yet to see numbers on much torque can be sent to each axle via use of the clutch plates. However, if you got into a situation where the ECU told the transfer case to lock, that could theoretically cause problems because it is a mechanical lock (not through the use of clutch plates). However, you would have to be experiencing significant slippage on one axle to ever get into that situation in the first place, so I think you'd be ok.
 

Ian_Barry

Observer
Thanks for the extra information cmb6, very good to know.

The same is true for the front and rear differentials?
 
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Ian_Barry

Observer
RE: JymmieJamz

I suppose the center differential and the axle differentials operate differently. In your experience have you seen running mismatched treads on the same axle? What did that do to the vehicle's drivetrain?

Also, on less mechanically advanced 4wd systems have you found different results?
 

cmb6s

Adventurer
Well, somebody please correct me if I'm wrong here, but the front and rear (non-locking) differentials are simple open differentials using a hypoid gear layout. They have no ability to transfer torque to one wheel or the other. All torque transfer is done reactively vs. proactively via the ABS system (brake one wheel to prevent it from spinning and it inherently transfers torque to the other wheel with traction). Land Rover has a fantastically smart system in this respect, but it's still reactive. I don't believe this is considered a limited slip differential in any way, although it does allow for very slight wheel speed variations by allowing the two planet gears (the small ones inside the cage that join the axles) to rotate on their axes. The electronic locking rear differential (with the HD packing) is much more like the transfer case. It has a multi-clutch pack that provides torque biasing as well as a locking feature which provides a 50/50 power split between the rear wheels. It can do this more proactively than the ABS system. I say "more proactively" because it is still computer controlled and the computer must sense the need for it's use before it's actuated. That being said, once the system locks the differential, it stays locked until the computer senses that it's not needed anymore. The ABS system, on the other hand, only responds to whatever is happening at that particular moment... if a wheel isn't slipping, the brakes don't stay engaged on any particular wheel. It's only after a wheel is already slipping that anything happens.

In reality, the purpose of the multi-plate clutch is to PREVENT differential slip, so I don't think I'd really want to run different size tires regardless. I guess my point was just that you shouldn't cause any sort of gear binding on those diffs with the clutch packs. :)
 

Ian_Barry

Observer
Thank you for replying.

I'm not sure I fully understand why you couldn't cause the multi plate clutch to bind, but I do understand why it's not likely.

Do you know what the maximum torque split fore/aft is in the LR3?
 

jymmiejamz

Adventurer
RE: JymmieJamz

I suppose the center differential and the axle differentials operate differently. In your experience have you seen running mismatched treads on the same axle? What did that do to the vehicle's drivetrain?

Also, on less mechanically advanced 4wd systems have you found different results?
I've seen Land Rovers come in to the shop with tires that a completely different diameters and none of them have had any driveline issues that I know. Everyone seems to be overthinking this IMO.
 

mowerman

Adventurer
I've seen Land Rovers come in to the shop with tires that a completely different diameters and none of them have had any driveline issues that I know. Everyone seems to be overthinking this IMO.
You are correct there, there is a tolerance in the drivetrain to allow for a 3% variance in tyre diameter bother forward/ aft and across the axles, this is to allow for production variance within the tyre (manufactured tyres are rarely the same diameter as each other) and to allow for different wear rates as described in the OP, and of course to prevent differential damage should a tyre be under inflated.
This comes from working for Pirelli for 10 years supplying JLR with fitted units.
 

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