Replacing tires on all wheel drive vehicles 3

[RWF7437]

" In the last few years many vehicle manufacturers that offer all-wheel-drive recommend replacing all four tires if one tire is unserviceable. Although Toyota doesn’t seem to have a specific recommendation, here is what Volvo recommends for their all-wheel-drive vehicles. “When tire replacement is necessary, Volvo strongly recommends replacing all four tires at the same time with identical tires. Failure to do so can result in damage to the transmission and all wheel drive system. If only one or two tires are replaced, the new tire(s) must be identical to the tires with which the car was built, and must be mounted on the front axle only. Failure to do this may damage the transmission and all-wheel-drive system.” Based on this information I would replace two tires and put them on the front of your RAV4."

My questions are:

Why is this necessary or desirable ?

What dire consequences can or will result if Volvo's advice is ignored ?

If this is "dangerous" why does Volvo have a specific recommendation and Toyota does not?

Is this just a scam to sell more tires ?

 

[MacGyverS2000]

If the tires are fairly new, complete replacement is not necessary. If the tires are significantly worn and one is replaced, it will result in an uneven load to the diff.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

To limit motion in the 3 sets of differential gears.

I guess Volvo think their differential gears are not durable enough to give adequate service under these conditions.

I guess Toyota have more confidence in their differential gears, or less concern for short life.

What do Volvo say about tyre pressures

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Bear in mind that the rolling radius of the tire varies quite significantly with load (typical stiffness is 250 N/mm) so the centre diff already has to cope with several mm difference, on most cars.

Now, I need to go and look at the wheel speed transducers to see if the revs per mile are affected, I have a sneaking suspicion the answer is no.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RWF7437]

Thanks Mcgyvers2000 and Patprimer,

Toyota says "keep tires ( tyres ) properly inflated" Big help. Don't own a Volvo so I don't know what they say.

Don't know how much is "significantly worn" 50% tread life left? 20% tread life left? 1% tread life left?

If differential gears must have their "motion limited" , in what direction? Rotational motion ? Longitudinal motion? Lateral motion ?

If, as I have done, I replace two tires on the front are the risks less? What can happen ? This car already has 125,000 miles in it.

Sounds like a scam or an attempt to avoid lawsuits.

Thanks again.

 

[GregLocock]

They want you to put new tires on the front because that way you'll get better braking in the wet.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Further to all the above, it is all about variations in rolling diameter.

This can occur due to different loads, different wear, different pressure, different tyre structure and different tyre size.

The differential is there to compensate for these as well as different distances travelled when turning oe even when traversing very uneven ground.

Differential gears normally cope very well with this unless abused, like hooligans doing burnouts with one tyre only spinning, or a 4 wheel drive with one wheel or one axle with good grip and others with poor, then wheel spin being maintained for long periods.

Many modern 4 wheel drives have locking devices in the diff to limit wheel spin. These locking devices can be fluid couplings, clutches, locking tapers or dog clutches.

I expect in this day and age, some might be computerised traction control devices. If they are set very sensitive, they may attempt to lock out the corrections due to different rolling diameters. As said earlier, different rolling diameters can result from quite a few variables, only one of which is tyre wear.

If a manufacturer is so careless or irresponsible so as to produce a system that is damaged by relatively normal variations in rolling diameter, all I can suggest is to avoid rewarding that manufacturer with business. If enough do that, they will change as they are driven by their view of the market.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Peter7307]

Some years ago VOLVO UK refused a warranty cliam to repair / replace the centre differential on one of their AWD cars when an owner changed a worn (but still legal) tyre for the unused spare after a flat.

A bunfight ensued and Which Car? magazine took up the cause on behalf of the owner.

VOLVO eventually replaced the centre diff at no charge to the owner.

This may be part of the reason for VOLVO making their strong statement.

Pete.

 

[RWF7437]

Thanks to all, especially Patprimer. I think I understand the "risks" better and they seem pretty slight. The guy at the tire store said the diameters had to be "within 7% of each other for all four tires". Where he got this number I don't know and neither did he. For a tire with a diameter of 30-inches this would be 2.1 inches. I suspect I'd notice such a large difference even in normal road conditions.

By the way, I did Google this and searched this website before posting. Which one of the other 21 admonitions in the FAQ might have I transgressed ?

Thank you all, again.

 

[GregLocock]

That's a sig, not a comment aimed at you! I must change mine, you are the second person to read it as a admonishment.

Your question is a perfectly good one. As you say 7% sounds far too much.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Like Greg said

But since you ask, the question was not work related but if the question is reasonable, we tend to be a bit slack with red flags in this forum.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[FoMoCoMoFo]

see also

http://www.eng-tips.com/viewthread.cfm?qid=204990&page=1

 

[FEMdude]

"If only one or two tires are replaced, the new tire(s) must be identical to the tires with which the car was built, and must be mounted on the front axle only"

This is a good advise only from transmission point of view. It however changes your car behaviour dangerously on snowy/icy surfaces (even with abs, it turns the car sideways if you drive a corner too fast and/or brake in the corner). Frontal crash is always preferred to side crash.

 

[MintJulep]

My Subaru has a similar recommendation.

However it doesn't really have a center "differential" at all. It has a clutch pack.

Fully-engaged = 50/50 F/R torque split.

"Minimum" engaged - 90/10 F/R torque split.

The 90/10 split is what is in effect almost all of the time in dry weather.

So if the center clutch is designed to slip under the most prevalent conditions, how much damage could actually be caused by mis-matched tire diameters?

 

[MacGyverS2000]

Greg, put a few lines just above your sig to separate... even after a year+ I still read your sig as part of the discussion from time to time.

Just do this:
------------------------------------

Dan - Owner

http://www.Hi-TecDesigns.com

 

[MikeHalloran]

The issue arises because of the bearings within a typical diff gear set... there aren't any.

Stated more accurately, there aren't any rolling element bearings in a diff gear set, because there isn't space for them, and because even properly sized rolling element bearings would fail from false brinelling because of limited motion.

What has been shown to work well enough, in axle diffs, is hardened gears with hardened backs running on iron housings and hardened bores running on hardened pins, with all bearing surfaces intentionally relieved (with tool scars, etc) to allow lubricant to occasionally find its way to the sliding surfaces.

What happens when you run such a gearset at a higher than normal duty cycle is that it gets hot, and eventually the lubricant fails, and things progress badly from there. That's why 'stock' cars on short tracks have pumps and big coolers for the axle lube.

The 4WD's center diff gets stressed in the same way when you run different tires f/r, and is not typically equipped with a pump and a cooler.

If the FWD has a planetary diff (as opposed to a bevel gear diff), the torque split is typically not 50/50, and there may or may not be rolling element bearings or clutch packs present, but they all are designed to deal with only so much heat, and if you operate them too far off design condition, bad things happen.




Mike Halloran
Pembroke Pines, FL, USA

 

[oab]

My experience with AWD is confined to the Subaru types. Both the Transfer clutch type and the Varied Torque Differential type all detect a difference in wheel speed of more than 20% as wheel spin, and apply the clutch to reduce it. This has a pronounced effect on low speed tight turning, as the clutch is applied and has to slip under load, causing binding that accelerates the clutch wear.

As most AWD cars don’t have a Viscous diff in the front, but may have one in the rear, it should have the uneven wheels on the front, to prevent heating of the Viscous fluid.

Harvey.

 

[IceStationZebra]

I think this may lead back to Greg's answer "Bear in mind that the rolling radius of the tire varies quite significantly with load" The computer is programmed to handle a smaller radius rear tire as a matter of course. But is does not not expect to see a significantly smaller radius front tire. It probably gets confused and tries doing some silly thing like braking some of the tires or going into full-time-4WD.

I also totally agree with the "excessive differential spinning" answers above. My expectations for a Volvo/Subaru in this regard are much different than a Jeep or Toyota.

ISZ

 

[jetmaker]

As an owner of 4WD vehicles for over the past 20 years, the policy has always to be replace tires in axle pairs unless there is limited wear on the other tire.

The reason for this was overheating of the differentials. Even a small change in tire diameter, say 1", puts the differential to work full-time under normal driving conditions. The extra rotation at highway driving speeds and long distances results in early breakdown of the diff fluid.

Now with AWD, it is similar for the center diff. which is why some mfgs recommend replacing all 4 tires. BTW... a few years ago, I saw a sign posted in Wal-Mart that said they would only replace all 4 tires on the AWD vehicles.

jetmaker

 

[GregLocock]

1"????????? There's only 9/32" of tread there in the first place.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.