Wheel sizes -- pros and cons

[alga]

What are the pros and cons of different wheel sizes? I know for a fact that smaller wheels with higher tyre sidewalls are lighter (my daily Civic's winter stamped steel wheels with 175/70R13 tyres weigh 9 kg each, whereas my summer 195/50R15 alloys weigh about 16 kg each!) Is that why Caterham uses 13" wheels? What are the benefits of bigger rims with lower tyres (except for looks)? As for cons, obviously they make the ride harsher.

On a related note, what about tyre widths? What does extra width give? School physics suggests that the friction is proportional to the force, so when the contact patch gets bigger, the pressure on it decreases, and the friction stays the same. This simple model misses something, as wider tyres usually have more grip. Any ideas?

What do you think are ideal wheel size and tyre width for a sevenesque roadster?

Ideal wheel and tyre sizes are those that Caterham use but the Roadster can't wear 13" alloy rims due to the rear upright. Not sure if steel wheels will fit either.

[flyerncle]

195/55/15 seem popular as they fit under standard arches and are similar in size to sierra wheels for the speedo ratio.

[alga]

Speaking of Caterham, according to their sales materials they use:

• the standard Sierra tyre size 185/70R14 on the more pedestrian Classic and Roadsport models (100, 125, 150 bhp)
• 195/45R15 (smaller diameter!) on Superlight, R300 and R400 (175 and 210 bhp), and the factory built (EU-type-approved?) CSR (260bhp)
• 6Jx13 fronts, 8Jx13 backs (tyres 175/55R13 and 205/55R13 according to some forums) on the race-focused Academy (125 bhp) and Superlight R500 (263 bhp) models. Apparently these are used with slicks.

But why? What are the tradeoffs?

Alga,

I have put 15" wheels on my T25 camper(standard wheel size is 14"), I worked out the rolling radius based on tyre sizes etc as didn't want to alter the gearing or affect the speedo BUT for some reason even though there is only a few mm difference in the sizes my camper now sits 25mm lower.
I was then expecting the old fella to rev out after seeing how much it had been lowered as obviously it must have changed the gearing way too much......but no, if anything it's better now as pulls away easier and flies up to a staggering 60mph and settles down rather than sat screaming it's head off...........


So Mr Alga maybe it's just fashionable to have low profile fat tyres....I really don't know

[3GE Components]

The 13" rims on a Caterham is down to the choice of tyres available, in 13" you have a vast range of tyres, from high performance road through to slicks for motorsport use.

Kind regards

John

Best explanation of sillyness with tyres

http://www.motogp.com/en/videos/2011...itive+ice+race

[davidimurray]

Generally - you want to aim for a minnimum unsprung mass. The logic behind this is that when the wheel goes over the bump , everything on the wheelp/upright side of the shock/spring sees a force. Once you've gone over that bump you want the wheel to come back down and make contact with the road as soon as possible. However because you have an unsprung proportion of the mass, this has an inertia, which will keep the wheel moving upwards until the force is countered by the spring. All the time the wheel is in the air you are losing traction.

You can of course try and use stiffer springs, but then you get to the point where the wheel no longer moves up and down and you skip over the bumps, again losing contact with the road and hence traction.

What you need to find a happy medium between the softest spring settings and the forces produced by the unsprung mass. The more you can reduce unsprung mass the better. One of the easiest ways to do this is to reduce the wheel and tyre size as they are a massive component of the unsprung mass. You may have also seen some cars that have inboard brakes adjacent to the diff - this is the same logic but has the downside of introducing huge reverse cyclic loadings into your driveshafts.

[georgenewman1]

lighter smaller wheels = quicker acceleration bigger wheels= higher top end,
go for the smallest and lightest that will fit as the roadster seems to be fun in and out of the corners and not so much the straight line speed, thats how i see it anyway

[alga]

So the 14" steelies that fit might have an edge over some cheap, heavy 15" alloys?