Wheels (Smaller is better)
 

I've been thinking on my electric car design again...

(And for those that do not worship the ground I walk on and thus have no idea what I'm talking about, (heathens), my electric car design is a ground-up build, with 4-wheel hub-motor drive, 4-wheel steering, 50KWh battery capacity, seating capacity of 6 AND a huge trunk, all wrapped in an aerodynamic shell with a planned curb weight of ~2,500 lbs. Yes, it is still a twinkle in my eye, bite me).

Anyway, I got to thinking about wheels. Looks will be a non-concern on this vehicle, as the wheels and tires will be hidden inside of the bodywork. So I got to thinking about using the smallest wheels possible, for weight reduction as well as altering the ratio on the direct-drive hub motors.

Now my design calls for high-performance, so occasionally I'll be launching off at 150KW, and reaching speeds well into the triple-digits.

How small can I go? I'm thinking there are still some viable tire options at 13", but I'm thinking of going even smaller. How about 10? 8? Is anything made other than low-speed trailer tires at these sizes?

On an electric, you probably want to select everything for efficiency, including tires.

No, smaller isn't better. 13 is the smallest practical size that you can get good passenger car tires for.

Smaller diameter tires will spin faster (more bearing friction) and handle bumps and potholes poorly.

I just got a set of 14 x 5.5 inch aluminum RX7 wheels for my Geo, running 175/65R14 tires.

I had assumed that as obvious. ;)

That's what I was afraid of.

I was under the impression that lighter wheels caused the suspension to handle bumps and potholes better... Does the physical size of the tire and it's attendant flexibility affect this as well?

The other consideration for size I have is reducing the size of the necessary hole in the underside of what would otherwise appear like a plane.

Lighter is better, but smaller diameter tires will sink deeper in potholes and move at steeper angles. Plus spinning faster.

Also, some of the bigger tires have less rolling resistance than smaller. Michelin and Nokian both told me 14 had lower RR than their 13 size in the same tire model.

Check out Loremo.
Intro*-*Loremo - low resistance mobile

These are my new wheels. Very light, and only $50 for all 4.

I am using Nokian H silica compound tires. Not only low RR, but also very light, 2 or 3 pounds less than average weight. Nokian H won first place in German tire test this year.
Nokian H wins the German ACE tyre test

All else being equal, rolling resistance is lower with a larger wheel diameter since the rubber isn't flexing as sharply where it contacts the ground. That's the reason you see large wheel diameters on racing bicycles dispite their greater aero drag.

What you want is the largest diameter tires that will fit in the body - and the smallest diameter rims (Yes, yes, but "wheels" is sometimes confusing and I want to be clear that I am talking about the metal part, not the rubber part!)

Then a check for load carrying capacity.

My guess is that you'll find out that this results in a 15" diameter rim, and a small 75 series tire - and this will probably also result in much more load carrying capacity than you need.

Just for reference, current practice seems to be to total up the maximum load each tire will be carrying (worst case), then add 15% - then that is the MINIMUM load carrying capacity needed for the tire at the pressure selected.

...look into some motorcyce wheels.

Those pieces of junk do not have the life-span I am looking for. And I should know, I'm on my 3rd rear with the Mule, and soon, my second front after only 24K miles. :p