irregular profile difference = gearbox damage?

[Guest]

What is the tolerance of the AWD gearbox if the profiles are different?

I'm looking at staggered rims, 8/9inch, and I have to replace the fronts, from 235 to 225/45/17. I would prefer to keep the rears the same for a while to see what 235/45/17 Maxtreads are like and see if it gets sideways, without having to put a rear sway-bar on. Will get 255/40s in future.

The diameter difference between 225 and 235 45s is 1.4% / the rear wheels will be spinning 1.4km/h slower at 100km/h. Can the gearbox handle it?

Nissans seem to handle it: http://www.infinitiscene.com/forum/showthread.php?t=861&highlight=nismo

Cheers

[DRFVDR]

They suggest a 10% variance maximum,BUT saying that any variance will effect the VCD over time.

[gazzy2000]

on my 97 STI one time i accidentally ran 50 series on the front and 45 on the front, it never felt right around corners and at 50kph cruising speed created slight surging as it bound up.

Around the same time i did some "gravel work" and heard a hua of a noise in the centre diff going from gravel (where the wheels cud spin at the same speed) back on to tarmac, thats when i investigated further and did a big "hurr durr" to myself when i foun the missmatch.

I fitted the right profile tyres and the difference was huge, it felt wayy nicer cornering and the slight surging was gone.. lesson learnt!

[Guest]

 Spaz said:

What is the tolerance of the AWD gearbox if the profiles are different?

I'm looking at staggered rims, 8/9inch, and I have to replace the fronts, from 235 to 225/45/17. I would prefer to keep the rears the same for a while to see what 235/45/17 Maxtreads are like and see if it gets sideways, without having to put a rear sway-bar on. Will get 255/40s in future.

The diameter difference between 225 and 235 45s is 1.4% / the rear wheels will be spinning 1.4km/h slower at 100km/h. Can the gearbox handle it?

Nissans seem to handle it: http://www.infinitiscene.com/forum/showthread.php?t=861&highlight=nismo

Cheers

even tyres with the same size on the them can varry up to 70mm between manufacturers and models of tyres (from the test I did ages ago with 215/45x17's)

Service limit is 13mm front to rear difference for M/T and 18mm for A/T

Anything more than this WILL cause damage

Have seem many burnout Subarus at the top of big hills, (fire starts at the rear of the car and goes forward)

Reasoning ....As the gearbox gets hotter from the loading the centre diff is tighter and this causes the rear diff to take extra loading (from the tyre size difference) which it cant handle, the oil gets that hot that it can catch fire !! Have also seen heaps with burnt out rear diff subframe mount bushes - so very close to buring the car !!

[Guest]

 gazzy2000']

on my 97 STI one time i accidentally ran 50 series on the front and 45 on the front, it never felt right around corners and at 50kph cruising speed created slight surging as it bound up.

Around the same time i did some "gravel work" and heard a hua of a noise in the centre diff going from gravel (where the wheels cud spin at the same speed) back on to tarmac, thats when i investigated further and did a big "hurr durr" to myself when i foun the missmatch.

I fitted the right profile tyres and the difference was huge, it felt wayy nicer cornering and the slight surging was gone.. lesson learnt!

http://www.1010tires.com/tiresizecalculator.asp

Just ran up an estimate of tire diameter difference on what you've told me. 3.4% difference between 45 and 50 and it gives a warning that that's too much.

[quote name='Ichi Ban said:

even tyres with the same size on the them can varry up to 70mm between manufacturers and models of tyres (from the test I did ages ago with 215/45x17's)

Service limit is 13mm front to rear difference for M/T and 18mm for A/T

Anything more than this WILL cause damage

Have seem many burnout Subarus at the top of big hills, (fire starts at the rear of the car and goes forward)

Reasoning ....As the gearbox gets hotter from the loading the centre diff is tighter and this causes the rear diff to take extra loading (from the tyre size difference) which it cant handle, the oil gets that hot that it can catch fire !! Have also seen heaps with burnt out rear diff subframe mount bushes - so very close to buring the car !!

I'll pull out a tape measure when I get there and see what the diameter of the half worn maxgrade rears are and see what I can find out there that'll suit.

Should I go mixed brands and/or grades of tires at all? My RSB under-steers hard whenever I push it out of corners. Thought this could be a quick fix before I get a sway-bar.

Thanks for your advice so far

[GC8E2DD]

Hrm, I'm not sure how you are going to reduce understeer by putting narrower tyres on the front than the rear. What is your thinking regarding seeing 'if it gets sideways'. Are these Maxtread tyres bald? 255 also sounds hugely wide for a Legacy.

[Guest]

 GC8E2DD said:

Hrm, I'm not sure how you are going to reduce understeer by putting narrower tyres on the front than the rear. What is your thinking regarding seeing 'if it gets sideways'. Are these Maxtread tyres bald? 255 also sounds hugely wide for a Legacy.

Was planning on using really bad thai or chinese tires on the rear and maybe some pirellis up front. I wanted the wide rear to give me a bit more feel if it got sideways. 255 is apparently what best suits a 9" wide rear rim and from my calculations it should sit just inside the guard.

From what Ichi Ban has told me, I could have my work cut out for me finding the right diameter between euro and chinese brands

I'm going to try them on at midday.

[B_Man]

The 225/45R17 does 502 Revs/Km 1993 mm Circumference

The 235/45R17 does 495 Revs/Km 2021 mm Circumference

Over time this will do damage

[666SubSpeed]

Are you sure your not talking about Tyre profile (height) not tire width (inside to outside )???

As I cant see how a 255/45/18 would travel any further than a 195/45/18

the problem would come when u mix say 225/35/18 up front and 225/45/18 out the back

The overall radius of the wheel is what would change and cause issues not the tire width.

Not counting rolling resistance.

As I have been told the diffs in Subaru are not the best and that I should not even change 2 tires, that you should all ways do all 4 at once. but this could be the tire shops just wanting more money.

I dont know its just my 2c worth

[DRFVDR]

You have been informed correctly,And it is not just Subaru any AWD vehicle with a Viscous Centre Diff should have tires changed as set of 4,I have had a Toyota Hiace AWD blow a front diff/centre diff/front drive shaft because the Leasee decided to put 2 new tires on the rear was an expensive lesson....

The tire Series ie -----/45/------ is a percentage of the width which denotes the sidewall height SO using your examples

255/45/18 has a side wall height of about 115 mm where as the 195/45/18 has a sidewall height of about 88 mm

[GC8E2DD]

 666SubSpeed']

As I cant see how a 255/45/18 would travel any further than a 195/45/18

[quote name='DRFVDR said:

255/45/18 has a side wall height of about 115 mm where as the 195/45/18 has a sidewall height of about 88 mm

18"+(2*115mm) = 687.2mm diameter vs 18"+(2*88) = 633.2mm diameter

= ~2157.808mm traveled per rotation vs ~1988.248mm traveled per rotation

or ~8% further if my maths is even vaguely right.

[Guest]

getting too complicated now !!

stick with same tyres , change all at once = no probs

[666SubSpeed]

 DRFVDR said:

The tire Series ie -----/45/------ is a percentage of the width which denotes the sidewall height SO using your examples

255/45/18 has a side wall height of about 115 mm where as the 195/45/18 has a sidewall height of about 88 mm

Ahhhh thats what was confusing me I assumed all 45 profile tires were the same regardless of width.

I didn't know profile was measured as a percentage of the tires width.

Well you learn something new everyday..