E30 Zone

Anyone got any comments on how wheel & tyre size affects acceleration and top speed?

My old 318is with original 14" BBS's (195/60R14 tyres) seemed to accelerate better than my newer lower mileage one which has (as yet unidentified) 15" alloys with 205/50R15's. Both are unmodified cars except the newer one has a K&N induction kit - wouldn't expect this to make too much difference though.

I kind of expected the smaller wheels to give better acceleration as technically the tyre size alters the effective final drive ratio. I was a bit surprised I couldn't find any info about this though.

I also thought it odd so many owners seem to go for larger wheels. Admittedly bigger wheels with lower profile tyres often look better on e30's than the 14"s, but if it affects your ability to burn off GTi's, is it worth it?

There again maybe it's my imagination - the older car's engine's noisier so therefore feels faster!

hi m8 yer you figured right smaller wheel more off the spot acceleration
bigger radius wheel more top end speed something like that
so i think if you got a 15'' wheel you want 55 tyres and 17'' youd want 40s
to get the right radius

only in a 215/40 17

It's the overall diameter that matters. In your case, a 195/60/14 is actually very slightly (about 4mm!) bigger than a 205/50/15, so any difference in acceleration isn't down to that...

In theory, a narrower tyre's got less rolling resistance than a wider one, so should produce a higher top speed, but it'll hardly make much difference & will also have less grip so I wouldn't go out and buy some 105/80s!

HTH

The overall diameter does matter as it affects the gearing, but I was under the impression the diamater of the wheel (not tyre) also makes a difference.

For a larger wheel more weight is at a greater radius, so I assume this gives the wheel a greater moment of inertia i.e. it acts like a heavier flywheel, and takes more energy to accelerate it to a certain speed. And more energy going into turning the wheels means less is being put into the rest of the car i.e. slower acceleration.

Confused? I always think of the ice skaters spinning on the spot with arms held out, then spinning much faster as they brought their arms in.

This does not affect top speed, but means you will have to put more energy in the larger wheels to get there, and of course your brakes will have that bit more to do when slowing them down too.

I could be wrong of course, and I'd be surprised if it made much difference but that's my explanation!

N

I see what you're saying, but are you sure that a smaller wheel with a higher profile tyre doesn't weigh more than vice versa?

The smaller wheel has afterall got rubber in the same places as the larger wheel, just more of it further in too... & I'm pretty sure that just adding weight in the middle of the wheel ain't gonna make you go faster! ;)

In total it's probably not much heavier, if at all, but I suspect the weight is further from the centre on average.

The difference may be negligble, but I see no other reason for people to report slower acceleration with larger wheels even when overall diameter is kept identical.

Foir acceleration, you want the narrowest tyres possible that are capable of putting the power to the road.
My daily runaround 318i gets whatever wheels and tyres are available, and at various times has run on everything from 175s to 205s.
Makes a difference of several MPH to my terminal speed up a certain local hill. Even more so in the wet.

Perhaps the tendency to go for wider tyres when upping wheel size should also take some of the blame

TheDutch wrote:In total it's probably not much heavier, if at all, but I suspect the weight is further from the centre on average.

The difference may be negligble, but I see no other reason for people to report slower acceleration with larger wheels even when overall diameter is kept identical.

I do; it's a different car he's talking about!

His 15s may well be 6-tonne monkey metal jobbies though... Whichever way round the wheel weight is, it's really unlikely to be noticable on an arse-dyno, at least not in a straight line anyway...

was going to get 225s so wud i be better off with 215 on 17' alpina bk reps