rolling diameter/finger pointing !
 

as some of you may be aware on my red SVX I run 18x8s front 235/40 and 18x9s rear with 265/35 tyres same brand, in recent weeks the planetary gear section of the AT has had a major phart and to use Subarus description .."destroyed"..

I have been scanning several threads on rolling diameters as the AT specialist I have who is planning AT recovery is stating that they will not warrant their repairs as I'm running different sized tyres, and that this was the most likely cause of the AT failure.

Before my wheel/tyre purchase I looked extensively thru the threads here and concluded that as long as rolling diameters were as close to equal that all should be good... The only thing I didn't pick up on was tyre pressures, in that i didn't use a similar diff to that of std 33 fr and 29 rear, my tyres were 40psi at both ends, mind you I also only drove the car for perhaps 2500kms with new rim/tyres before AT failure..

Also in one of the many posts/responses Trevor in NZ spoke of JDM ATs being different to the US models ATs, JDMs are VTD - viscous couple [these were also in our AUD models as well] which I understood from his points made that these boxes could better handle differences in rolling diameters ...

Anyway I'd hate to think that incorrect trye pressures have brought me unstuck..and if so within 2500kms of driving..

Thoughts pls, I haven't yet but I will when its back on the road do the rolling diameter 'white out' thing and then fiddle with air pressures.

Any feedback would be appreciated
David

I run different tire pressure to make the rolling diameter the same on wheels and tires of the same size. It has been said that if you replace tires, you should do all 4 at the same time because the difference can cause this problem.

I don't know what would happen (maybe I do now) if you have different sizes front and back. I would not try it though.

Sorry you had problems. :(

tyre diameters more
 

thats why I was particular with the wheels I chose..here are the dimensions that come from wheel/tyre calculators

Tire size results
Specification Sidewall Radius Diameter Circumference Revolutions Difference
225/50-16 4.4 in 12.4 in 24.9 in 78.1 in 811 / mile N/A
235/40-18 3.7 in 12.7 in 25.4 in 79.8 in 794 / mile 2.2% front
265/35-18 3.7 in 12.7 in 25.3 in 79.5 in 797 / mile 1.8% back

as you can see there is 0.3 inch diameter difference or 3 revolutions difference over a mile between the new front to rear tyres.

I would have thought the difference between these 2 would have negligible etc, and in hindsight juggling tyre pressures as per OEM 33/29psi .~ 10% more in the front tyres in OEM, may have fixed the problem.

So then with my new setup as my fronts are slightly larger than the new rears then % difference front to back perhaps should be also be slightly less...recommended pressures for the new tyres because of their lower profile is higher psi, so if they suggest 40psi then using the ~10% lower rear assumption [OEM] I will should then try 40 front 37/8 rears..

I've done this today on my green SVX but it has same tyre sizes fr to rear 235/40x18, so pressures I set were 40fr and 36 rear psi and I'll do the white out test, when i get some time to get actuals...and then when the red car gets back on the ground I'll check current tyre pressures to see what they were just before the event, and also do the actuals vs computer calculated..

Any other thoughts..incidently what pressures are you running front to back...


:confused:

What I have done in the past is measure the distance from the ground to the top of the wheel on each tire and adjusted the air pressure to make each the same. Some of us have found up to 8 psi difference between the front and back tires. Both front and both rears should be the same. :)

Re: tyre diameters more
 

David,

Here's another way to look at what happened to your trans. The rear tires had a rolling circumference greater than the front tires by ~0.5%. For argument's sake, if the rear tires revolved 800 times each mile, the fronts would have revolved 804 times.

These extra 4 revolutions wouldn't have just vanished in the drivetrain, but would have caused a slow grinding of the transmission and transfer clutch gears. Over enough distance (in your case 2500 kilometers), the gears would have been grinded away leading to failure of your trans.

Sorry bud, but this is probably why the AT specialist has voided your warranty. No trans no matter how bullet-proof would be able to hold up to this 'abuse' indefinitely... ;)

-Chike

foot pump ?
 

ensteele and chiketd thanks for your input

With tyre pressures upto 40psi would a foot pump do the trick..my local garage with the latest air line seems to struggle with tyre pressures above 35psi, it doesn't fix on well so and you need several attempts before you get it right, whereas with my own foot pump I can keep fiddling with air pressures etc

interestingly my wifes car a BMW X5 4.6is has its std 20 in rims with different sized tyres front to back - 275/40x20 fr and 315/55x20 rears Michelins they recommend 220kpa both ends..running these thru tyre calc computers these are way out, so they obviously must balance each other out with the weight of the engine over the front..

Tire size results
Specification Sidewall Radius Diameter Circumference Revolutions Difference
275/40-20 4.3 in 14.3 in 28.7 in 90.0 in 704 / mile N/A
315/55-20 6.8 in 16.8 in 33.6 in 105.7 in 599 / mile 17.4%

Anyway a Foot pump ..good idea:rolleyes:

Re: foot pump ?
 

I assume that your wife's BMW is the model with 4wd/awd (not too familiar with the X5 model line)? What I can say is that it'll be a different system to Subaru's and it was designed from the factory with that discrepancy in tire size front-to-back.

-Chike

Re: Re: tyre diameters more
 

Since our friend David lives in Australia, he doesn't have a transfer clutch. It's true that on US-market SVXs, a difference between the front and rear rolling diameter will wear on the transfer clutch, but I don't know how it would affect a mechanical center diff.

I would actually think that such a system would be better able to handle the difference, but make no mistake - it will wear on it more.

I don't see how it would affect the planetary set, but you gave them an out with the mismatched tires. :)

Re: foot pump ?
 

I think it's more likely that BMW has geared the car to handle the difference in wheel and tire sizes in the first place. I don't know the X5's AWD system, so I don't know how much better or worse it can handle the difference.

Miss Adventure.
 

Hi Dave, sorry I was not around when you had this trouble.

I run 35 front 33 rear, this measures ok using Earls method. The VTD center diff is still effected by differences in rotation. The diff has a clutch that operates across the diff to lock it up, if the rotational difference is too much, like when front or rear wheels spin. Under normal conditions the sun and planet gears rotate as a solid unit, it is only when there is a difference, that the planetary gears run.

A small difference front to back won't worry it, but to have the trouble that you had, the car would have to have the front wheels lifted and be towed on the back wheels.

When the transmission is in park the front wheels are locked, the back wheels are not, so when it is towed on the back wheels, they drive the transmission output shaft, through the planetary gears. Thats why it was destroyed, it wasn't miss matched tyres that did this.
I might be going out on a limb here, but there can be no doubt that this was done between WA and Q,LAND. Somewhere they did a front lift, towed it at 60kph for long enough, to destroy the gearset.

Harvey.;)

mate !!
 

Harvey

You are the second person who's described this event, subaru said after they had pulled the planetaries apart for an inspection that the only other Subaru they had seen like this was one other SVX, and that had been towed also, and in their case for over 30km...there assessment was that it would take an enormous force to sheer the teeth off the planetary gears as it has on mine... I'll get some pics in the next day or too so the damage can be viewed by all, the car was insured whilst it was in transit ...I'll give them a call:( but I wont hold my breath.

Thanks Harvey again, will stay in touch
Currently the AT guy has ordered in a planetary set from NZ, and will then go thru the rest of the box to check clutch plates, and so forth, he also showed me from JATCO one of their own TSB on this box that suggested some mods [from memeory 3 small holes to be drilled] to improve fluid flow..I'll see if I can get more info on this if you like...

Thanks all for the input, even after she's up and going I will do the wheel diameter vs PSI exercise so that rolling diameters are kept as close to each other as poss.

Thanks Again

Dave :)

wheel radius measures
 

I've had a few minutes to run a ruler spirit level and tape measure over my green cars wheels front to rear, and I was pleasently surprised to discover that the front was 625mm and the rears was also 625 ..maybe 624 depending on which way my tongue was hanging out or according to my tape 24.5"..[tyres all cold] - method - ruler spirit across top of tyre horizontal then measured perpendicular thru centre line of tyre/rim to the concrete floor below .. that = 625mm

So then just now checking my earlier posts for 235/40x18 the radius was 12.7" and diameter 25.4" [~ 645mm] so why the difference ..call it 20mm or roughly 0.75inches...

i am pleased that they were at least same front to back...

Re: wheel radius measures
 

If you do the rolling circumference measurement, I think you'll find that even though the tires are the same height, they will not have the same rolling circumference.

I have wondered about that too. This is the system that some of the members had a long discussion about and came up with this. I wondered about this then and now really wonder. If you look at a tire, and measure the height, and do the same thing to another, they may not look the same. To exaggerate, it could look more like a tank track. Real wide, but the same height. This would not have the same rolling diameter. :(

back on the road..but !!
 

OK after I think nearly after 2 months my red car was back on the road today...new set of planetary gears and completely rebuilt transmission..even AT oil cooler fitted ...BUT...like it used to before when you tap the brakes at over 60kph you get a thumping sensation [like a sticking caliper or driveshaft loosness ??]...before I removed the car from Subaru to have the AT rebuilt..the tech said that he checked the brakes they were all good..and that the sensation was the car going in/out of AWD ??...so tonite I found the spare 10amp fuse and placed into the "diff lock' spot ..took for a test ... NO clunking thummping sensation..smooth braking ...SO WHAT IS IT ..I'm confounded by this ...and perhaps this AWD sensation [thumping] may have caused the planetaries to fail in the first place...and I just spent a small fortune on it ...I don't want the same again

Thoughts... Harvey... anyone
Remember our Aussie box is the VTD box from Japan

david :confused: