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What's the reasoning behind smaller wheels? Unsprung weight?

By reducing the diameter of the tire you instantly lower the center of gravity and the roll center of the suspension with no other modifications - that's probably the major appeal for most.
A smaller-diameter tire also has to turn faster at a given road speed, resulting in it running hotter (less time for cooling before the same area contacts the pavement again). If your tread compound adheres better when warm, as most do up to some point, that'll improve your potential cornering speed in the earlier part of an event.
On smooth pavement, unsprung weight is not a huge factor, and there's typically not that great a change in weight anyway - 13" steel rims w/tires can weigh more than 15" alloys w/tires - but as a rule lighter is better, whether it be sprung or unsprung.
Smaller-diameter wheels/tires do reduce the gyroscopic effect slightly, since it's an exponential function of the distance the mass is from the center. Theoretically that should also make the car have more nimble steering response when making sudden direction changes at speed.

andy198712 wrote:What's the reasoning behind smaller wheels? Unsprung weight?

Hi Andy, my reasons are a lot less technical than Marc's explanation above.

I'm currently running 8x18" fronts & 9.5x18" rears. They looked great but I'm getting bored of the look of them now. German Look - been there, seen it, done it!

They're also heavy in comparison to the 4 lug brm's I used to run. My way of thinking about this is that wheels on cars are kinda like shoes on your feet.... You wouldn't choose to wear heavy steel toe capped boots to run in a sprint race!

I've been looking at a lot of mk1/2 Ford Escorts and their choice of rim seems to be 13". I think they look cool in a very retro way and you can't help but notice their fantastic handling.
I think this is attributed to the smaller wheels bringing the axles closer to the ground (making 18" rims seem like a set of stilts in comparison!)

I can run higher profile tyres (but not too high - planning on a 60 profile) to give a more comfortable ride and more predictable handling.
The rubber band tyres I run at the moment give great grip but don't give me much notice when they're running out of grip, so have a tendency to give me a surprise sideways moment.

I also think that there will be less rolling mass with the smaller wheels and tyres, with a little less rolling radius, which will give rise to better braking and acceleration, at the expense of a little bit of top speed. - But who needs a road car that can do 120mph when the law restricts you to 70mph? I'd rather have fun at the traffic light grand prix!
The car should still be capable of getting over 100mph if I need a bit more speed for track days.

Finally, I like to be different. I've not seen any other bugs in the UK rolling on 13's... I think the UK VW scene could do with a new "look". Perhaps it could be called a Retro Racer look!

A trend setter


Sounds good though!

The picture I have in my head of how my Bug will look is pretty cool. The wheels are just part of the changes I have planned.
Whether it looks as good in real life as it does in my head will remain to be seen! Lol!

Another effect of smaller-diameter tires is that the brakes' "leverage" is better, improving their efficacy. The contact patch of the tire will be shorter/smaller for a given width, though, which may limit the ultimate stopping power under some conditions, but on such a light car it's unlikely to be a problem.

Frankly I've never found the "pregnant rollerskate" look resulting from short tires that don't fill the wheelwells, but all things considered it does improve performance - brake cooling won't be as good, but if fade (or fluid boiling) is a problem one could always add some air ducts.

Marc, I was telling this to my friend the other day about the leverage on smaller wheels being better as the clamping force is closer to the outer rim.
He disagreed and said this was determined by the size of the brake disc...
I agreed to disagree, but will be sure to show him any difference I get once I've swapped my wheels.

http://www.racecartuner.com/03/307.html

When changing tire size, width and off-set don't forget to accommodate this.

Dan Dryden wrote:...He disagreed and said this was determined by the size of the brake disc...

Angular momentum of a rotating object is proportional to the product of the radius squared and the linear speed at the perimeter, which is why there's less gyroscopic effect with a small tire - even though it's rotating faster, the difference in radius is squared so it has a greater effect than the rotational speed.
For the purposes of comparing braking effort angular momentum can be ignored since both the tire and the rotor/drum are rotating at the same RPM. Imagine the case where the wheel is stopped, sitting on a big piece of cardboard...essentially we're interested in comparing how much braking effort must be applied to prevent someone pulling the cardboard out with a constant force when we change the diameter of the brake or the tire.

The force applied at the disc times the radius of the disc must equal the force exerted between the tire and pavement times the radius of the tire.
It's the relative diameter of the tire and the brake rotor/drum which determines the effort needed to achieve the same braking effect - changing the diameter of either will have an effect.

Let's say the rotor is 10" in diameter, and the tire is 24" in diameter. If you were to upsize the rotor to 12" with the same tire, the force required to be applied by the brakes to result in the same force where the rubber meets the road would be reduced by 20% because 5x12=60 and 6x12=72....72/60 = 1.2
So, your friend is correct, the size of the brake disc does affect the effectiveness of the brakes - no argument there.

But do the same math assuming the rotor stays at 10" and a 20" tire is replaced by a 24" tire. Here the required force is increased by 20% because 5x10=50 and 5x12=60....60/50 = 1.2

If given these examples he should acknowledge that a larger tire will require a proportionally larger brake rotor/drum for there to be identical braking forces; from that it follows that a smaller tire with the same rotor/drum will require less force at the brake for the same force at the pavement.

yeah, the tarmac has less leverage on the brake disk (easy way i looked at it?)

andy198712 wrote:yeah, the tarmac has less leverage on the brake disk (easy way i looked at it?)

That's the short version, yes - I was just giving you some ammo for the debate with your friend.

Dan, eyeballing my Tarox calipers id say 13"s would clear easy, they bolt right on but are a pricey caliper, like your '03, id need to space away from the lower ball joint on my drop spindles or go back to stock spindles.

Dan, eyeballing my Tarox calipers id say 13"s would clear easy, they bolt right on but are a pricey caliper, like your '03, id need to space away from the lower ball joint on my drop spindles or go back to stock spindles.
Marc, good real world experience and input, good to see how we did it before fashion and 'look' ever appeared, Dan, i have used stock calipers with 13"s, and relieved them to clear, the wheels concerned were revolutions, also back in production for a year or two now.

I've been to Hi Spec brakes today to see if they could successfully fit their Ultralite 4 pot caliper within the 13" Alleycat wheels.
The caliper will work with the standard discs but won't fit under this particular 13" wheel due to the taper of the inner dish reducing the clearance by too much.
Their calipers seem very nicely made, very light in comparison to the standard cast iron ones and give an increased pad contact area which won't be dissimilar to my rear discs, so I've ordered a set and hope to have them in the next 6 weeks.
Looks like I'll be selling these 13" wheels on and having a look around for some lightweight 15" wheels instead.

Why not try for some 14" rims instead? That way you're half way there?