Question for Frog & Subaru Guru's.

Subyroo · Post by **Subyroo** » Tue Jan 17, 2012 11:25 am

Frog, in ref. to the rolling diameter of tyres on a Subaru do you know of any official written information (ie. from Subaru) on the difference/s and what the results maybe, other than what is written in the Owners Hanbook?

I understand that the rolling diameter of the tyres are not supposed to be more than 1/4 inch or 6.3500 mm, or you may suffer damage to the center differential (torque binding), I am just curious as to whether Subaru ever released a Memo or a paper on the effects of unequal rolling diameter tyres?

The reason I ask is because a couple of Subaru owners on a 4x4 Forum I visit, did not know of the possible effects that can be caused by the difference in the rolling diameter of the tyres. One owner has over 200,000 kms up on a forester and has handed it down to his daughter and has replaced a single tyre at various times and has never suffered torque binding.
This guy is quite knowledgeable on 4x4's too so it surprises me he never knew.

NachaLuva · Post by **NachaLuva** » Tue Jan 17, 2012 1:22 pm

Exc question, cant wait to hear the discussion.

Is the 6.3500 mm you mention from Subaru or another source? 6.3500 mm is not a lot for a rolling diameter of say 700mm!

YouTube · Post by **El_Freddo** » Tue Jan 17, 2012 1:47 pm

nachaluva wrote:Exc question, cant wait to hear the discussion.

Is the 6.3500 mm you mention from Subaru or another source? 6.3500 mm is not a lot for a rolling diameter of say 700mm!

I've found this discussion about a fella's centre diff problems on ORS, and I remember a discussion about it a while ago but I can't remember which forum it's on a search hasn't turned up anything, plus I should be back in the shed!

Cheers

Bennie

MTB92 · Post by **MTB92** » Tue Jan 17, 2012 5:41 pm

cant say i am an expert on the matter, but i am pretty sure it would be a %, not a distance. there would be a huge difference in rotataion of each wheel on a low profile 14" tyre with that difference, but reletively small on a 15" with big muddy tyres on.

FROG · Post by **FROG** » Tue Jan 17, 2012 8:45 pm

Yeah Im sure Ive been quoted 1/4 inch = cant remember if that was official or not
We do the chalk test if centre diff is suspect and it generally confirms it

Ive only seen 5 or 6 dead centres in the 9 years Ive been there but would imagine more would have got away with it that I dont get to see

Will have a better dig around tomorrow, there wasnt enough hours in the day today!

My engineering background warns me it would make that diff work hard continuously with odd rolling diameters and eventally fail
I guess they can take some punishment and failure rates would vary between each unit due to conditions and manufacturing tolerences

THIS IS MY OPINION AND NO WAY OFFICIAL

FROG · Post by **FROG** » Tue Jan 17, 2012 8:47 pm

1/4 inch was diameter not circumference

circumference difference is multiplied by 3.1416

shows up well by parking on level ground and marking the bottom of the tyre with chalk at 6 oclock
then roll the car forward a couple of turns and note the chalk mark positions on all the tyres
perfect world they will be in the same place

anything more than 3/4 inch difference over one rotation would be a worry

Subyroo · Post by **Subyroo** » Tue Jan 17, 2012 9:06 pm

nachaluva wrote:Exc question, cant wait to hear the discussion.

Is the 6.3500 mm you mention from Subaru or another source? 6.3500 mm is not a lot for a rolling diameter of say 700mm!

The 6.3500 mm is the Google conversion of the ¼ inch that the USA guys quote on the subaruforester.org Forum threads relating to tyres and the rolling diameter.

Subyroo · Post by **Subyroo** » Tue Jan 17, 2012 9:47 pm

FROG wrote:1/4 inch was diameter not circumference

circumference difference is multiplied by 3.1416

shows up well by parking on level ground and marking the bottom of the tyre with chalk at 6 oclock
then roll the car forward a couple of turns and note the chalk mark positions on all the tyres
perfect world they will be in the same place

anything more than 3/4 inch difference over one rotation would be a worry

I edited the circumference/diameter part Frog.

FROG · Post by **FROG** » Tue Jan 17, 2012 9:59 pm

I was under the impression it was diameter but I have known to be wrong on the odd accasion

Subyroo · Post by **Subyroo** » Tue Jan 17, 2012 10:28 pm

FROG wrote:I was under the impression it was diameter but I have known to be wrong on the odd accasion

That makes 2 of us.

steptoe · Post by **steptoe** » Wed Jan 18, 2012 7:34 am

You've also got the tyre sales dudes who apparently insist four tyres replaced at same time on AWD to avoid them being responsible for diff failure - must be somethjing in it than profit

Phizinza · Post by **Phizinza** » Wed Jan 18, 2012 10:51 am

I forgot exactly where I read it but I did read it on an official Subaru source. 1/4 inch on the circumference is the limit.

Nachaluva, 700mm is the diameter, multiply that by pi to get the circumference. It's close enough to 1mm tread depth difference.

I measured two 215/60R16 Yokahama tyres, both H/T's but different models (g900 and g035?) they were different by 30mm on the circumference! Same brand, same quoted size. I know from previous experience that different brands can be even further different on actual size.

I would imagine most of the centre vLSDs would not bind or lock up but would just wear down and become less effective. Subaru vLSDs have a preload on them which is a constant pressure stopping slip as well as the oil friction that is implied at higher slipping speeds. This preload fuction would be under constant slipping pressure if different size tyres were used. After seeing the massive amount of metalic sludge from the 89 Liberty box I pulled apart and the remarkable similarity of colour to that sludge and the centre diffs limited slip part I would say that is what happens. It just wears down and because less effective.

Something else I've noticed is with the Liberty I had I could jack up one corner and spin a wheel with not much effort (still obvious that there was friction there to slip that vLSD but not much). When I jacked up a corner of my Outback and tried to spin a wheel it was quite stiff and it actually rolled off the jack.

FROG · Post by **FROG** » Wed Jan 18, 2012 4:38 pm

Spoke to "someone"

today an the official word is 4% of rolling circumference
which to me makes more sense than millimetres and inches if you think about it

pezimm · Post by **pezimm** » Wed Jan 18, 2012 7:29 pm

For what it's worth:

I used to work for a company that assembles and supplies drive-axles to heavy vehicles. I did a quick search on their online literature on demand website and found the following on a "Parts Failure Analysis" handbook:

Now, that's for heavy vehicles (eg: trucks and buses), but we can sort of get the drift that the 1/4in circumference seems reasonable for a Subie...

For the curious:
https://s3.amazonaws.com/helm-arm-lod/tp0445.pdf

Note: 15MB download.

Cheers,
Pedro.

Phizinza · Post by **Phizinza** » Thu Jan 19, 2012 9:16 am

https://www.subaru.com/my-subaru/faq-technical.html

That is where I read it.

MTB92 · Post by **MTB92** » Thu Jan 19, 2012 6:49 pm

FROG wrote:Spoke to "someone" today an the official word is 4% of rolling circumference
which to me makes more sense than millimetres and inches if you think about it

thats what i said

makes heaps more sense than a distance!

FROG · Post by **FROG** » Thu Jan 19, 2012 7:08 pm

MTB92 wrote:cant say i am an expert on the matter, but i am pretty sure it would be a %, not a distance. there would be a huge difference in rotataion of each wheel on a low profile 14" tyre with that difference, but reletively small on a 15" with big muddy tyres on.

yerp

straight to the top of the class

as soon as "he" said percent I went straight to the bottom of the class

Phizinza · Post by **Phizinza** » Fri Jan 20, 2012 9:13 am

Thing is, the standard Australian or US citizen wouldn't know how to measure and then convert to %. But measuring a distance difference is not so hard.

It isn't like if you go 0.3mm over that limit you will explode your gearbox. It is just a recommendation to keep you centre vLSD in good order.
Obviously if you run a different ratio diff in the rear or different sized wheels you will have damage happen!

RSR 555 · Post by **RSR 555** » Fri Jan 20, 2012 9:08 pm

FROG wrote:My engineering background warns me it would make that diff work hard continuously with odd rolling diameters and eventally fail
I guess they can take some punishment and failure rates would vary between each unit due to conditions and manufacturing tolerences

This would be my first thought too but having seen many Forester and Outbacks with failed Centre Diffs, that all so far have caused the diff to bind up and make it harder to turn sharp corners. This is definately from them having different size tyres, in turn causing the diff to be over worked and get extremely hot, then semi-like welding the plates together inside the viscous coupling.

As for tyres, never assume that all manufactures have the same size rolling diameter on the claimed same size markings. Always good practice to buy 5 of all the same size and rotate at sevice intervals.

Phizinza · Post by **Phizinza** » Sat Jan 21, 2012 9:03 pm

I suppose warping of the vLSD parts could occur... This would give symptoms like you are talking about RSR.