Rolling radius mismatch; your opinions on possible harm
 

My Jersey Girl came with very poor tyres, perished rubber and generally in very poor condition.

These I have replaced with the tyres from Black Betty, which are Dunlop Sport 2000E standard fitment 225/50 ZR 16. Betty had recently passed an MOT and two new tyres were mandatory, so effectively I have four tyres the exact same fitment, two are brand new, two are part worn, possibly half worn I would say.

The way they are fitted is with both new tyres on the right side, and both worn tyres on the left side. I did not want the front and rear axles having different rolling radii.

This is my question; does anybody think this difference in rolling radius left to right will cause detrimental effects at the gearbox?

The actual measured radii in cms is 31.6 on the left side and 31.8 on the right side. [The rolling radius figures will be lower than these once weight is applied, but there should be the same ratio proportions]

My own opinion is it should be OK, as it will be the same to the gearbox as if the car is turning right all the time [the left hand wheels make more revolutions that the right side do, just as when turning, and the diff should be able to cope with the difference]

I would like your opinions. Am I being naive? There are two solutions;shave off half the rubber on the new tyres; or buy two new ones and throw away the half worn ones. Either solution involves throwing out about €160 worth of rubber [$240 in your money] and I would rather not waste if there are no ill effects.

Joe

Will the side/side mismatch cause an issue with the rear LSD???

I don't know Dave. That's one of the reasons I asked the question and quoted the actual radii. I thought perhaps one of the engineers on board here that understood rotational forces would venture an opinion.

Joe

I think it would be better to fit the two new to the front, the worn to the rear. The VTD won't be as worried about the difference as the rear diff that will heat up due to the difference. We have had a few rear end fires.

Harvey.

Would you think so Harvey?

It's doing very little miles at present, so I have not managed to check temperatures at the LSD.

It would mean swapping the tyres around on the wheels, but that would not be too difficult, I suppose.

Thanks

Joe

Harvey is right. I have heard fire stories from the rear diff but it is really only possible to get the rear that hot by overworking the LSD w/ mismatched tires... So no diff fires please:eek: Always keep equal diameter tires on the axles

Tom

I'm no gearbox expert, but I reckon mismatched tyres on the same axle is asking for trouble.

I recently had two punctures on my green SVX. I swapped on two wheels from another SVX. But when driving it I could hear an unusual whirring noise from either the trans or the front diff. :confused: So I put the undamaged originals on the back and bought two new ones for the front and the noise went away.

As Harvey said, you should swap them around and put the new ones on the front. Then we can tease you about two of your wheels being on backwards.

Thanks for the advice guys.

Based on Harvey's reply I took an urgent view on the move. I swapped around two of the tyres. I now have the two new tyres on front, plus the two possibly half worn ones on the back. Each axle even.

In addition I have had the 4 wheel alignment done by laser. Car drives very well now.

Phil, no chance of teasing me for now, I have all wheels rotating backwards as they are supposed to do. :rolleyes: :p

However, I am thinking of changing them all around so they all appear to be rotating forwards, as this seems to drive the pedants and the nerds into apoplectic fits, for some reason. ;) That'll give you a chance to tease me and see if I give a flying f.. er emmm, ehhh see if I care.

:D

Joe - your wheels are on backwards!!!!! :rolleyes: ;) :lol:

Not just yet Earl, I'm thinking about it!

;) :D

.2cm difference in rolling radius is minimal, and if you put the tires that measured 31.8cm on the front, the extra weight will bring them down some (more so then the 31.6cm tires on the back due to more weight in front).

So I would say running mismatched front to back will be better than left to right, in your case.




Dustin

That's actually the way they are now fitted Dustin.

I'll keep you posted on how they go.

Thanks for the advice, much appreciated.

Joe

The front tires are going to wear out faster than the rear tires...

After few, few thousands of miles, they'd be getting closer...:cool:

So if mismatched tires cause diff fires, why did Subaru give us a donut as a spare:eek:. That being said, if I get a flat, where would be the best place to put the donut? ( and don't say in my mouth:lol:)

always put the donut on the front axle;)

Tom