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[ngrainqey]

hello,
I've bought some mahussive 9.5J AMG wheels... they're very yummy....

i'm trying to get wider tyres on the back than the front so turn in isnt like turning a barge by hand from the towpath lol

basically with tyre sizes and aspect ratios i can get it to within about 0.7% speed difference or 794.6revs per mile and 789.7 revs per mile

thanks in advance
Alex

[Kevin Wood]

thanks in advance

You're welcome. Now, what was the question?

Huge tyres aren't going to improve the handling, IMHO.

Was the revs per mile figure from the tyre manufacturer or an online calculator? Because most calculators are not accurate.

[ngrainqey]

LOL!.....calculator kevin,
your more than welcome to suggest a better way to calculate the two sizes needed for the front and back

i'm after narrower tyres for the front so its not daft and then wider for the rear seeing as its rwd and may aswell make the most of the 9.5J regarding rear tyre width lol

thanks
alex

[Agemo]

Just remember that if you have very wide tyres, there will be LESS traction,  i.e. your tyre contact patch will be wider with the same weight on it.
Hope that makes sense to you.

[Omegatoy]

dont make any difference to the abs if that what your asking?
on the mega meega, had everything from 10j 19inch wheels back and 8j fronts didnt like the way it drove
through 9.5j 18 inch wheels 8j front it drove ok
down to standard L.C. wheels and tyres, it was terrific on these
nothing fazed any of the systems

[Kevin Wood]

Ah, Ok. Just re-read the title. I would agree the wheels are close enough in size not to upset the ABS.

[2woody]

two issues.....

to answer the exam question, the ABS will certainly be getting upset at 10% difference.

Aren't AMG wheels going to be for a Mercedes - so they'll be 112mm pcd, which is not suitable for Omega B

the wheel-revs-per-mile figure is a measured figure, which must be obtained from the tyre manufacturer. A calculated figure will be anything up to 15% out.

Omega B was designed around 817 wrpm - I'd be confident going anything up to 820 and down to 815 or so.

What size tyres are you thinking of ....?

[ngrainqey]

245/40/18 front and 275 or 285/35/18 rear I've calculated the sidewall heights of all of them which iirc are 99 and 98.6ish meaning 49 and 49.6 or there abouts as the aspect is a ratio of the tyres width

[2woody]

245/40/18 front and 275 or 285/35/18 rear I've calculated the sidewall heights of all of them which iirc are 99 and 98.6ish meaning 49 and 49.6 or there abouts as the aspect is a ratio of the tyres width

'fraid that all means bugger all. it's all rather different once installed.

"your" 99mm when added to the 18" wheel gives 782 wheel revs per mile - which is 4% out from the measured figures. You've used the same maths as the calculators.

[ngrainqey]

hold on a minute, front wheels 18'' +99 rear wheels 18''+99.6 how is there a 4% difference there?

[2woody]

that's just the point - it's simply not 99mm.

with the tyre off the car, 40% of 245 is indeed 98mm, but with the wheel mounted and some weight on it it's a completely different story. With the car stationary, the figure is more like 76mm - you can see this in the sag of the bulge at the bottom of the tyre. With the car moving, it's different again - the tyre doesn't roll fully, it slips slightly, so you lose about 4 or 5 % of the motion you're expecting. This is less pronounced with lower-profile tyres.

245/40 x 18 is 811 wrpm, acceptably close to the "designed" 817

275/35 x 18 is 809 wrpm, which is probably too far away ( just over 1% )

285/35 x 18 is 802 wrpm, definitely too far away. ( 2% )