Risk of overwide/over-diameter wheels on differential? (225/45 on GC8)

[ztikmaen]

Hey guys!

 

I have a GC8 STI (Type RA with DCCD) and recently bought some wheels. I intend to change the tyres, but gave it a drive around the block as is.

 

However, I noticed the tyres made a pretty obvious scrubbing noise going around even moderate corners. So now I'm wondering if this is a product of excessive rolling diameter, or if it's due to wider width/track?

 

Factory size: R16 205/50 - 6.5" ET55

New size: R16 225/50 - 7" ET48

Based on this conversion website, this results in:

Rolling Diameter: +4%

Width: +10%

https://www.tyredispatch.co.nz/pages/tyre-calculator?srsltid=AfmBOoqtUZ6fsM_eu2AA1Xsh2g4gkPVGXXBk9BQcidescbPzxH2ayKe2

 

I was going to change to R16 225/45 (same rolling diameter as factory) - But would like to sense check just in case the width/track increase could cause issues with the diff?

 

Thanks for your help!

[Gripless]

Subaru wants only 5-6mm difference.

Wheels and tires are cheaper than a diff rebuild kit.

 

Did you change front or back wheels, and how low is the car?

 

 

Look at something like this where you’ll see more data…

https://www.wheel-size.com/calc/?wheel1=205-50-16X6.5ET55&wheel2=225-50-16X7ET48&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm

 

look at

circumference … 64mm per revolution difference in travel 

revolutions per mile … 27 additional rotations per mile or 17 per km

scrub radius .. 4mm per side off the factory rotational centre when turning front wheels

None of the calculators take into account.

stretch so look at tire specs to see their rim size and if you are in that range there shouldn’t be any.

Tire pressure also running higher or lower pressure changes rolling diameter 

[ztikmaen]

Thanks Gripless, 

 

I changed all 4 wheels, so no mismatch between them. The car is factory height. 

 

I guess I'm trying to ensure that 225/45 R16 won't cause issues (since it's the same rolling diameter as factory), aside from being wider. Intuitively I don't see this being a problem unless a widened track affects the diff?

 

Thanks

Reuben

 

 

[ztikmaen]

If I look at a 22B for reference, they run 8.5" ET48 wheels, so surely 7" ET48 would be fine with 225-wide tyres. I guess the issue must've just been the difference in overall rolling diameter

[Gripless]

If all 4 are the same diameter there is no diff issue. Only the speedo will read out by the increase or decrease in diameter.

 

wider wheels when turned travel inwards towards the frame more, so can rub. Or the rear bumper mounts where it connects to the wheel arch  can rub too. 

[boon]

Its the tyres rubbing on the body/guard liner, not some diff issue....

[ginganinja]

It will be the width. GC8's don't like 225 wide tyres without rolling gaurds. Even though rolling diameter is the same due to offset the wheel has moved 7mm further out and then the tyre has gained another 10mm in width to the outside. 

 

Go to a 215 tyre and it will likely be fine. 

 

If you have lowered suspension this will make gaurd rubbing even worse.

 

I am running 215/45R17 tyres on ET53 wheels and no rubbing at all. 

On 30/10/2024 at 2:34 PM, ztikmaen said:

If I look at a 22B for reference, they run 8.5" ET48 wheels, so surely 7" ET48 would be fine with 225-wide tyres. I guess the issue must've just been the difference in overall rolling diameter

The 22B also has HUGE gaurds compared to a stock GC8, making room for the extra width. 

[newsuba]

When I got my BBS wheels they came with 225/45-17 Goodyear F1's - an absolutely awful tyre, in a totally wrong size for a GC8.

Rub-dub-dub and raised the overall gearing enough to take the edge off performance.

Switched to 215/45 tyres - much better response and grip.

215/40-17 was even better, lowered the gearing enough to make the car feel like it had an RA

[GC8E2DD]

And conversely if you have an R/RA gearbox then going to 215/40 can be annoyingly short if you ever have to drive long distances on a highway. Lots of people here have run 225s on GC/GF platforms in competition so it wearing your diffs is probably the last thing you need to worry about. Rubbing on the guards gets old quickly though!

[newsuba]

True.

Though a 215/40 is closer to stock rolling diameter than a 215/45

The biggest problem is finding quality 215/40 tyres.

Back in the day one of the white GC8 wagon mob was running 205/40s on the track.

 

Edited November 8, 2024 by newsuba

[Gripless]

Some newer models have 18x7 or 18x7.5 so maybe tire option for those .

 

 

 

[Loren]

225/45/17 is 100% fine on a gc8 with the correct wheel offset, ride height and some mild guard rolling.

 

225/45/16 is 200% fine.

Edited November 8, 2024 by Loren