different diameter tyres on same axle

Submitted: Saturday, May 09, 2009 at 19:01
ThreadID: 68677 Views:12237 Replies:13 FollowUps:14
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Hi everyone, I tried a quick search but couldn't find what I was after. I'm running standard 31x10.5R15's on a GQ Patrol and the tyres on my Camper Trailer are 6cm smaller in diameter. Would a 6cm difference be a problem if I used the trailer spare on the GQ? In other words, would it cause problems for the LSD if I put it on the rear or would it make steering difficult if I put it on the front? Thanks. Rob.
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Reply By: Wayne (NSW) - Saturday, May 09, 2009 at 19:20

Saturday, May 09, 2009 at 19:20


AnswerID: 364049

Reply By: Madfisher - Saturday, May 09, 2009 at 19:22

Saturday, May 09, 2009 at 19:22
Yes you can not put diff diameter tyres on the same axle with an lsd
You would get away weith it on the front as long as free wheeling hubs are not in the lock position.
Cheers Pete
AnswerID: 364050

Reply By: Member - Fred B (NT) - Saturday, May 09, 2009 at 19:22

Saturday, May 09, 2009 at 19:22
Absolutely; you will do a lot of damage to your vehicle by running tyres with that much difference. The rolling diameter on tyres fitted to same axle should be very close. The difference in rolling diameter between your tyres will be huge.
Fred B
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AnswerID: 364051

Reply By: Member - Old/new Girl (QLD) - Saturday, May 09, 2009 at 19:41

Saturday, May 09, 2009 at 19:41
Few years ago on a off road rally in the 80 we had 31's blew two tyres and used a 33 on the front. Got us out of trouble to the over night stop. Hense the two extra wheels we carry all over the country side. Also with our camper we ran the same tyres on it as the car.
AnswerID: 364055

Reply By: disco driver - Saturday, May 09, 2009 at 19:49

Saturday, May 09, 2009 at 19:49
Hi Rob,
Definitely not a good idea, the diameter difference is too great.

Even the difference between brand new and just legal tread can cause problems in some constant 4wd's. Some people, including me, always fit 4 new tyres each time to avoid this.

The only time I would even consider doing what you suggest would be in a life or death situation where there is absolutely no alternative.

AnswerID: 364058

Follow Up By: Chev-Patrol 6.5 V8 D - Sunday, May 10, 2009 at 13:36

Sunday, May 10, 2009 at 13:36
I agree............. and in which case I would try to get the rolling diameter at close, by deflating the larger tyre to a point whereby the distance between the ground and the centre of the axle is approximately equal on both sides.... and then drive at an appropriately reduced speed to avoid the deflated tyre suffering a blow-out from sidewall heat build up.
FollowupID: 631839

Reply By: Horacehighroller - Saturday, May 09, 2009 at 20:00

Saturday, May 09, 2009 at 20:00
No problems on the front of a Patrol (if necessary for a short distance) so long as you are not in 4 x 4 and with hubs preferably UN-locked.

However, different tyres on the same axle is illegal.

AnswerID: 364063

Follow Up By: disco driver - Sunday, May 10, 2009 at 14:19

Sunday, May 10, 2009 at 14:19
Please explain, my understanding is that the tyres must be of the same construction, ie; both radial (steel or rag) or conventional cross plies and of a similar rolling diameter, NOT a mixture of both.

Similarly, how does a temporary emergency type tyre comply, or doesn't it?

FollowupID: 631846

Reply By: Member - Shane D (QLD) - Saturday, May 09, 2009 at 20:01

Saturday, May 09, 2009 at 20:01
Hi Rob.
If you really needed to use the smaller spare on a LSD, what you would HAVE to do is put the same size wheel/tyre on the other side (Off the trailer) and put the "odd" sizes on the trailer.
Your patrol will look silly with bigger wheel/tyres at front but looks will not stuff your diff and you won't be able to drive in 4WD, as it will wind-up like a bastard potentionly doing damage to driveline.

There are friction plates (like clutchs) inside your LSD and they will burn out VERY quickly if driven with uneven size wheels/tyres

If faced with putting in on the front, I would (make sure hubs are UNLOCKED), strickly to get out of trouble, I wouldn't be cruising down the freeway at 110.
AnswerID: 364065

Reply By: Outbackogre - Saturday, May 09, 2009 at 20:47

Saturday, May 09, 2009 at 20:47
Thanks everyone, I get the message. Actually, the CT is on order and I've only just found out it comes with 225/75R15's (72cm), which are 6cm smaller in dia that the GQ's (78cm). I'll check out if they (Cub) can fit bigger rubber. Cheers. Rob.
AnswerID: 364075

Reply By: StormyKnight - Saturday, May 09, 2009 at 21:31

Saturday, May 09, 2009 at 21:31
A few years ago I did some research into a very similar problem.

Between standard 265/70R16s on my Prado & 265/75R16s.

Now for one thing not all tyres specified the same size are in fact the same size so the calculations we worked with where purely theorectical. I.E we went to a web site that told us the size of the tyres, rather than actually measure them.

Now te Prao is a constant 4WD vehicle so what we wanted to know was if I fit 4 BFG MT's to the car, but my spare was an original 265/70R16, what effect would this have on the rear diff if we fitted thge spare to the rear & also what would be the effect upon the centre differential.

Now to get 'the effect' into something we could relate to, I worked out what radius a roundabout would be to simulate the same difference in the rotation speed of the two tyres.

So the answers were, the rear diff would have a difference in speed between the two tyres of 1.4%. The centre diff would have a difference between the fron axel & rear half of this, so it is 0.7%.

This worked out to be an equivalent roundabout radius of around 46m.

So if this tyre was fitted, & I was driving in a straight line, the wear & tear would be the equivalent of driving around a 50 odd meter radius roundabout. 50 meters BTW is the length of an olympic swimming pool so it is quite a large circle.

Those who drive in Canberra probably do more curves than straight anyway just in there drive to work!

Also there are many European cars that have an undersized spare since they don't have the space for a full size. They probably suggest to limit your maximum speed & to get the original tyre fixed ASAP, so with consideration of that & the information above re the equivalent curve radius, I wouldn't expect any damage to the car drivetrain if you drove with reasonable caution.

AnswerID: 364084

Follow Up By: Member - Kiwi Kia - Sunday, May 10, 2009 at 07:27

Sunday, May 10, 2009 at 07:27
Well done StormyKnight, some actual facts and informed thinking. I wonder how many people have actually gone out and measured the diameter of various tyres that claim to be the same size ? different brands of 31" tyres may vary between 30" and 32", a new mud tyre and a well worn mud tyre may have up to 1" difference in diameter ! Like your example of going around round-abouts people should think about driving around and around in high rise parking buildings. Yes they do wear out tyres but some of this is caused by the vehicle cornering to fast, just listen to the tyre squeal.

FollowupID: 631785

Follow Up By: Member - Oldplodder (QLD) - Monday, May 11, 2009 at 08:37

Monday, May 11, 2009 at 08:37
A friend got caught a few years ago with a falcon.
Bought two new tyres, same make, same pattern, same size (185/70/14).

Couldn't get the wheel alignment to work.

Finally thought about checking air pressures one side to the other the same, yep the same.
Checked height to the stub axle of the ground. Different at the same pressure.
Measured the OD of the tyres, different by about 10mm.

Tyres made in different countries, in different factories, using different moulds, but same manufacturer, same pattern!

Took them back and got a matching tyres out of stock and all OK.

FollowupID: 631940

Reply By: Robin Miller - Sunday, May 10, 2009 at 08:12

Sunday, May 10, 2009 at 08:12
If you do , do that Outback gone, pump the smaller tyre up hard and let the taller one down until both are about the same diameter measured from the road surface to rim.

The bigger one usually has to go down to about 15psi , and at this pressure you could only do about 50 kph else in overheats.
Don't go below about 15 , better to leave the bigger one a little higher and let lsd take some wear.

However care and common sense and this will get you to next town - worked in practise when a friend of ours got is 3rd puncture on Birdsville after not getting the original slow down message.

I always ways see this as an advantage of locker diffs actually.

Robin Miller

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AnswerID: 364113

Follow Up By: Member - Kiwi Kia - Sunday, May 10, 2009 at 17:01

Sunday, May 10, 2009 at 17:01
Hi Robin, lowering the tyre pressure will NOT reduce the circumference of the tyre.
FollowupID: 631867

Follow Up By: Robin Miller - Monday, May 11, 2009 at 07:53

Monday, May 11, 2009 at 07:53
Agree Kiwi , I am using this in context of reducing the load & wear on diff - mind you - you get more tyre wear , so its only a temporay work around.
Robin Miller

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FollowupID: 631932

Follow Up By: Member - Kiwi Kia - Monday, May 11, 2009 at 08:47

Monday, May 11, 2009 at 08:47
I do agree with that Robin - more drag :-))
FollowupID: 631941

Reply By: briann532 - Sunday, May 10, 2009 at 12:17

Sunday, May 10, 2009 at 12:17
A quick question about the replies???

How does a patrol with a LSD go around corners???
Isn't that what a diff is for?

Surely if you were driving with different size tyres the only effect would be the axles turning at different speeds.
Maybe there is something wrong with my patrol, but it seems to travel in directions other than straight lines with no problems.

Sure it will produce extra wear and tear of the lsd, but so does turning corners. As does tyre wear, different pressures, and 4wd'ing.

I assume outbackogre's question is in relation to using the camper spare if he had allready used the vehicles spare. Ie as a backup spare.
I don't see a problem using it as a spare, short term to get you to the next town.
It would be preferable to use it as a rear than a front so it doesn't affect your steering. As far as using it as a rear, I'm sure the lsd would just think you were turning corners.

Obviously I understand long term usage of different tyres will cause abnormal wear, but as a back-up spare, I can't see why not.
AnswerID: 364145

Follow Up By: Member - Tony & Julie (FNQ) - Sunday, May 10, 2009 at 13:02

Sunday, May 10, 2009 at 13:02
The following may help. From day dot I have always been told not to run different size tyres because of the wind up effect. Never really gave it much thought, just never have :-).

Worth A Read

Cheers Tony
FollowupID: 631829

Reply By: Outbackogre - Sunday, May 10, 2009 at 12:59

Sunday, May 10, 2009 at 12:59
Thanks for the further helpful responses. Yes Brian, my question relates to using the trailer spare on the GQ if I've already holed the GQ spare. I note the comments about making the LSD work hard. The LSD's on the GQ's are notoriously tight and I can hear mine chirp when I turn a tight corner on bitumin. I don't think I'd want to make it work like this for mile afer mile on the flat at high speed. I intend to check my tyre options with Cub. Cheers.
AnswerID: 364153

Follow Up By: Member - Tony & Julie (FNQ) - Sunday, May 10, 2009 at 13:05

Sunday, May 10, 2009 at 13:05
Look at what windup can do
FollowupID: 631830

Follow Up By: Outbackogre - Sunday, May 10, 2009 at 13:26

Sunday, May 10, 2009 at 13:26
Amazing, if you'd asked me which component would fail because of wind up, I reckon I would have nominated everthing except the transfer case. Cheers. Rob.
FollowupID: 631836

Follow Up By: briann532 - Sunday, May 10, 2009 at 21:46

Sunday, May 10, 2009 at 21:46
Sorry guys, I'm obviously missing something.
I'm not trying to be a smarta#$e, I'm genuinely intrigued.

I thought that transmission windup was a problem that occured in the transfer case due to the front and rear wheels turning and or travelling at different speeds. this windup is caused because there is no differential gearing between the front and rear drive shafts in the transfer case.
How does this affect the cross axle differential?

Also in response to making the lsd work hard, I would assume that it is not working any harder then when driving round bends?

As I said, I'm not trying to be smart, throw bait, incite an argument etc etc.
I am a club trainer and am genuinely perplexed at these responses.
Perhaps someone with a better knowledge could please educate me before I pass on my "lack" of knowledge.
I also think a good explanation would help outbackogre's original question.

If I am offending anyone please accept my apologies it is not my intent.

Brian N
FollowupID: 631904

Follow Up By: Outbackogre - Sunday, May 10, 2009 at 22:13

Sunday, May 10, 2009 at 22:13
Brian, I'd suggest that the link that Tony provided shows an extreme example of what happenes when a transfer case is locked in 4WD and the vehicle is driven on a twisty bitumin surface. Transmission windup is unlikely to be a problem on loose surfaces. As you suggest, my original problem was more about cross axle differential, but all comments have been welcome. Cheers. Rob.
FollowupID: 631911

Reply By: Member - Oldplodder (QLD) - Monday, May 11, 2009 at 08:47

Monday, May 11, 2009 at 08:47
Came across a bloke with Jackaroo about 150km out of Birdsville.
Done in three tyres, only had two spares.
Lent him one of my spares and followed him back to Birdsville.
He had 245/70/16 and I had 235/85/16.
About 2" difference.
Let tyre pressures down to minimise rolling diameter difference, don't know what it did to his diff, seemed to be OK, but it got him out of trouble.

Wouldn't want to go much further without expecting damage to the diff, or serious diff wear. In this case, it could have been really difficult for him, only had one car pass us that day.

Can you find you fit 31/10.5/15 on the CT rims, even if they are 15x6? Or get some cheap 15x7 rims and fit 31/10.5/15s?

I just picked up three cheap 16x6 nissan rims for the paj. i.e. cost nothing, since the tyre place was throwing them out. Needed a sand and a coat of paint. See what you can find at the wreckers.
AnswerID: 364247

Follow Up By: Outbackogre - Monday, May 11, 2009 at 09:14

Monday, May 11, 2009 at 09:14
John, it never occured to me that Cub may use 6inch rims. I'm not sure if 31/10.5R15's will fit on 6 inch rims. If Cub can't provide what I'm after I may have to do what you suggest and purchase some aftermarket 15x7's and fit the same tyres on trailer and vehicle (the Patrol's tyres are well worn). Thanks. Rob.
FollowupID: 631944

Follow Up By: Member - Matt M (ACT) - Monday, May 11, 2009 at 09:39

Monday, May 11, 2009 at 09:39

I can't see how letting a tyre down has any impact. It will reduce the diameter (axle to ground), but the circumference will not change will it? In other words, the distance that a wheel covers to complete one rotation will be the same, and if this is different either side of the axle, then wind up would still occur.

I would have thought that, if you had to fit different diameter (hence circumference) tyres to either side of the same axle, you would actually be better over-inflating the larger circumference tyre. This way it would have less surface contact, less grip and a better chance of slipping to reduce pressure on the LSD.

FollowupID: 631949

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