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 ?

 

[4xWriter]

I don't know if the Volvo you refer to uses a viscous coupling. If it does, you may want to be more attentive to the tire thing. Over the years, Jeep has had numerous problems with viscous couplings in the Quadra-Trac systems, many of them due to mismatched tires. With mismatched tires the viscous couple "sees" a difference in speed front to rear, even when the vehicle is going straight. The unit then is working full time, gets extremely hot and, at least the New Venture units used by Jeeps, can fails over long periods. I don't know what standards they have set for tire radius differences front to rear, but it is mentioned in various places. I have read reports that ordinary wear (fresh tires at one end, baldies at the other... perhaps a difference of 3/8-inch) has resulted in a failure. Usually announced well in advance by driveability issues in turns (steering wheel kickback, etc.).

My opinion is that you can compensate for wear with tire pressure. Air the "thin" tires up until their radius (or the profile) is even with the better tires. You may be running a little more air in those than you'd prefer (though not by a lot in my experience) but that would be better than buying a viscous coupling.

Some thoughts, anyway.

Jim Allen
Keeping the Good Old Days of Four-Wheeling Alive

 

[jetmaker]

Greg,

In the off-road venue, tread depths >16/32" is not uncommon. I've seen tires with 25/32" tread depths, which translates to 50/32" on the diamter.

Also, because of the cost and weight of carrying a full size spare on off-road trucks, using a "standard" tire size in combination with some of the oversized tires results in several inches in difference on the diameter.

However, in more practical real-life senarios, picture a "donut" spare being used. Again, the diameter difference can be significant.

jetmaker

 

[FoMoCoMoFo]

Please tell me there are no manufacturers out there demanding that you change all 4 tires if one needs replacement while providing you with a donut spare that's much shorter than the regular tires.
they wouldn't dare would they?

 

[vims]

I had an old Audi 4x4 from 1987. I seem to recall that they stated a maximum distance and speed you were allowed to drive on the spare tire, because of the risk of damaging the diffs.

 

[NickE]

On my subaru the same warning was given, however the donut spare is not significantly smaller than the regular tire.

I did snap a retaining ring in the center diff, however that is due to many many high torque starts in slippery conditions. (148,000miles)

nick

 

[crysta1c1ear]

Volvos use Haldex for 4WD I believe.

http://www.haldex-traction.com/technical_information/six_dimensions/six_dimensions.htm

No functional problems with tyres having uneven wear,
- pressure or size (mini spare).

I checked their site because by train of thought was as follows ... .

Put the new tyres on the front, and they are larger than the rear. Thus they will spin faster. The Haldex is for FWD cars and activates drive to the rear when the front wheels spin. Spinning is detected by the front wheels going too fast.

So putting the new tyre combination on the front of car is basically arranging that the rear drive is activated when required. If the front tyres are smaller, I presume they would spin faster and power would be diverted to the rear in an attempt to make them keep up.

Since Haldex claim no functional problems with a mini-spare which could be put on the back, my logic is either flawed or they expect the mini-spare to be replaced before the Haldex suffers.

With a Haldex, does the car learn the gear ratio between front and rear wheels when four wheel drive is active, and thus adapt by itself?

 

[crysta1c1ear]

Erratum:
Thus they will spin slower.

 

[DanEE]

Excellent point Jetmaker....

My Bridgestone Dueler tires which are a mild off road tire by comparison has a tread depth of 5/8".

We regularly take typically 6000 mile long road trips about every 2 years and several times have ended up running long distances on mixed diameter tires due to punctures or tire damage while off road.

No specific problems have been brought up on a Toyota off road forum I moderate including vehicles with the AWD torsen center diffs (mine is only part time 4x4 drive). We have pretty good sample size are aware of the problems that have come up on these specific vehicles.

So, guess it boils down to the quality of design and materials used in the specific vehicle.

http://home.4x4wire.com/deddleman/