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RWF7437 (Civil/Environmental) (OP)
18 Jan 08 19:13
" 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 (Electrical)
18 Jan 08 20:12
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

patprimmer (Publican)
18 Jan 08 20:16
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


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Helpful Member!  GregLocock (Automotive)
18 Jan 08 20:27
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.


Greg Locock

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RWF7437 (Civil/Environmental) (OP)
18 Jan 08 20:29
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 (Automotive)
18 Jan 08 20:37
They want you to put new tires on the front because that way you'll get better braking in the wet.


Greg Locock

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patprimmer (Publican)
18 Jan 08 20:47
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.


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Peter7307 (Structural)
18 Jan 08 20:47
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.

RWF7437 (Civil/Environmental) (OP)
18 Jan 08 20:58
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 (Automotive)
18 Jan 08 21:07
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.


Greg Locock

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patprimmer (Publican)
18 Jan 08 21:27
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.


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FEMdude (Structural)
19 Jan 08 6:54
"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 (Mechanical)
19 Jan 08 10:33
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 (Electrical)
19 Jan 08 11:13
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

MikeHalloran (Mechanical)
19 Jan 08 13:06
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 (Automotive)
20 Jan 08 19:48

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.

IceStationZebra (Mechanical)
21 Jan 08 0:56
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.

Helpful Member!  jetmaker (Aeronautics)
23 Jan 08 9:02
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.

GregLocock (Automotive)
23 Jan 08 18:44
1"????????? There's only 9/32" of tread there in the first place.


Greg Locock

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4xWriter (Automotive)
26 Jan 08 12:50
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 (Aeronautics)
28 Jan 08 19:28

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.

FoMoCoMoFo (Automotive)
28 Jan 08 21:23
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 (Mechanical)
29 Jan 08 6:58
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 (Materials)
30 Jan 08 10:04
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)

crysta1c1ear (Automotive)
30 Jan 08 12:04
Volvos use Haldex for 4WD I believe.
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 (Automotive)
30 Jan 08 12:16
Thus they will spin slower.
DanEE (Electrical)
1 Feb 08 17:03
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.

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