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

For load capacity, we'll figure on just north of 2 tons gross. Perhaps as high as 2.5 tons.

As for fitting it into the body, the point of the downsizing exercise is to reduce the space in the body consumed by the tire, reduce the opening in the lower portion of the body, and eliminate any need for a wheel opening in the side of the body. So not really a matter of fitting the tire to the body, but actually of designing the body around a smaller diameter tire.

As for a 15" rim with a 75 series tire, I used to own a final-gen S-10, so I'm familiar with how BLOODY HUGE this combination is. Good god I don't even plan to have my hood that high off the ground! :D

I was afraid that would be the case. :(

Finding out how engineering is just a constant series of compromises?

You can buy some 9 lb aluminum 4x100 13" wheels from me :)

you can get 145/80R10 tires that the Honda N600's use, same tires the old Minis use, you can get 145/70R13 tires that NEV use, that is what I have on my electric car, they are a LRR (low rolling resistance) tire, you can also look on tirerack.com and see what is out there and the load rang of them.
if you go to small the you limit you brake rotor size, so I would stick with 13" or so.

At 100mph high speeds with 2500lbs you will need serious rubber just for safety, let alone required high performance. Minimum would be 185 13" 's I'd guess. How far will a 50kwh capacity battery travel with these power and weight demands?

I won't have brake rotors, so that's not an issue. I'll be downsizing my hub motors, but I don't believe that to be an issue either, as my estimated 65mph cruise will be 10KW. Acceleration can be dealt with through water cooling. :D

I estimate a 100% DOD range at 65mph of 250-300 miles, level ground. That of course is just an estimate, and my goal is a minimum of 150 real-world miles.

Of course, hypermiling techniques will be employed. :thumbup:

Compromise? That sir, is a vulgar profanity in my vocabulary. :p

Good luck with that!

What if your batteries are full and you *need to stop*?

No, you'll want the braking system to be able to generate 1000 HP or so, and you'll want it to be operable even if driveshafts break, electrics fail, and you lose power from the motor(s).

And wait a minute, you're going to water cool four hub motors? Two flexible coolant lines to each wheel, a water pump, an unaerodynamic radiator, umpteen gallons of coolant at 8lbs/gal...

There are very sound engineering reasons why most of the electrics on the road are tiny, low-performance machines. Start small.

The motors generate electricity, so even if the batteries fail, you still have electricity. I anticipate a failsafe which electrically jams all 4 motors. It wouldn't be a desirable stop, but it would stop pretty quick.

You can move just as much heat by moving half the coolant at twice the speed, and the radiator does not need to be either huge or constantly exposed.

Where are you getting your information?

I don't normally agree with frank, he has a good point, your facts seem to be based on alot of wishful thinking, for one coolant does not cool twice as fast if you move twice as much past it, this is why low speed pumps are popular for solar hot water systems, because the systems work best if you get as much heat out of the fluid as possible before putting it back to get heated, high flow pumps lead to over heating, also where are these motors that you are talking about using? or are you going to hand build them?

Hey! Waddya mean ya don't normally agree with me?!? :eek:

Don't you know I'm right 99% of the time?

One doesn't NEED to buy off the shelf components... It's just cheaper and easier.

Although it has been watered down in recent years, custom did mean at one time that something was made as a one-off for a single project. ;)

Skirt these in and he's golden!

http://i146.photobucket.com/albums/r...ixedthatvw.jpg

See, now that's what I'm talking about. Can I get a tire for a 4" wheel, that preferably will handle 200 mph? :D

Sound like you're after some sort of aircraft wheel!