Differing tire diameters with LSD

[tbm5690]

Hi all,

I just had one tire replaced on the GLI due to picking up a screw in the sidewall. Good news is that the set only had about 5K miles on them, so they are practically new. The newest tire is on the front and I'm measuring it's depth to be about 1/32" deeper than the rest. I want to keep it up front for a couple of rotation patterns to wear it down to match (vehicle is FWD).

I'm not too concerned given how close they are, but given that I have the VAQ differential on the front (basically a Haldex system only on the front axle) is that to be treated similarly to most AWD setups where circumference should be pretty close? I read somewhere that Haldex requires all tires to be within 4% on their AWD setups.

 

[Jetronic]

no, not the same. AWD requires similar axle speeds, that's not a concern here with just 1 driven axle. At least 1/32nd isn't any concern. Wildly different diameter tyres could mess with the electronics

 

[twX]

AWD systems often use viscous LSDs for the center differential, which are very sensitive to differences in shaft speed, usually requiring around a 1% difference or less.

LSDs used for front or rear axles are less sensitive because they need to allow for differences in shaft speeds between the inner and outer wheels when taking normal turns, which will be much higher than 1% on a regular basis. A few 1/32" difference in tire diameter is not a reliability concern. Larger differences may noticeably affect the way the LSD performs in left vs right turns.

 

[CKN]

AWD systems often use viscous LSDs for the center differential, which are very sensitive to differences in shaft speed, usually requiring around a 1% difference or less.

LSDs used for front or rear axles are less sensitive because they need to allow for differences in shaft speeds between the inner and outer wheels when taking normal turns, which will be much higher than 1% on a regular basis. A few 1/32" difference in tire diameter is not a reliability concern. Larger differences may noticeably affect the way the LSD performs in left vs right turns.

Some on here have said this is nonsense and haven't adhered to it when replacing less than all 4. I don't agree-just stating what others have said on here.

 

[Jetronic]

I've replaced many a haldex like system (clutch packs) after replacing just 2 tyres: those don't allow for any different diameters if the AWD is actually in use. 90% of the time the owner doesn't know the system is dead until he gets stuck or notices the front wheels spinning. The other 10% the failure is more dramatic

 

[Astro14]

Some on here have said this is nonsense and haven't adhered to it when replacing less than all 4. I don't agree-just stating what others have said on here.

Every bit of service information on my 2002 AWD Volvo, equipped with a viscous clutch, says maximum difference of 2/32” in tire diameter to prevent damage. So, I’ve adhered to it. The clutch still works. At 300,000 miles.

Haldex is a different matter. It will slip/freewheel until the pump engages the system.

Still, I do tires in sets of 4 on that car, too.

 

[CKN]

Every bit of service information on my 2002 AWD Volvo, equipped with a viscous clutch, says maximum difference of 2/32” in tire diameter to prevent damage. So, I’ve adhered to it. The clutch still works. At 300,000 miles.

Haldex is a different matter. It will slip/freewheel until the pump engages the system.

Still, I do tires in sets of 4 on that car, too.

Yes-Subarus spec 2/32nds as well.

 

[krzyss]

Mechanical LSD on the front axle will be very similar in sensitivity to AWD.
If it detects different rotation speeds it will lock the diff and may overheat or malfunction in other ways.

Could you put the new tire in the back?

Krzyś

 

[tbm5690]

Mechanical LSD on the front axle will be very similar in sensitivity to AWD.
If it detects different rotation speeds it will lock the diff and may overheat or malfunction in other ways.

Could you put the new tire in the back?

Krzyś

I could, but I'd like to keep it on the front until it wears down the match the others. Sounds like from what I gather just being 1/32" off shouldn't be an issue.

 

[930.engineering]

no, not the same. AWD requires similar axle speeds, that's not a concern here with just 1 driven axle. At least 1/32nd isn't any concern. Wildly different diameter tyres could mess with the electronics

I agree. 1/32" (0.79 mm) seems ok for your GLI. The VAQ isn't the main issue when exceeding 1 mm difference. The car's stability system may reveal more often, with earlier interventions when driving more spirited at some (higher speed) corners. Annoying. I had the same on my former GTI when I had to replace one tire.
Because the circumference is important. That circumference is also dependent on the tire pressures as well as tread depth. That's why you could run slightly different pressures on your front wheels. Slightly lower pressure on your new tire. Slightly, eg 34 and 36 psi. No more. It works anyway.

On a sidenote: I'd recommend changing your VAQ's Haldex oil (probably VW G 060 175) frequently. Every one or two years instead of three. Ravenol AWD-H, Liqui Moly Lamellenkupplungsoel or BW/Haldex' own offering (#2000884). Remove and clean that rudimentary 'filter screen' at every 2nd OC.