# What wheel setup on a small, solar powered car, would have the least rolling friction?

We are attempting to design a solar car. The solar panel 4 1/2" x 13".

I like to break these problems into their simplest form, in order to eliminate the least amount of possibilities. So, the goal of the competition, as I understand it, is to transport the (provided) solar panel, and the (provided) electric motor down a 40 foot long track (the car is guided with fishing line as a guide wire).

The texture of the track is similar to the surface of a skateboard, although slightly rougher than that.

Of course there will be many other considerations besides the physical design of the car (gear ratios, lenses and/or mirrors for the panel, guide wire receiver), but right now I am focused on the car itself, and more specifically, the wheel orientation. I am wondering: What is the optimal wheel setup for this car on this track? Some ideas I have found/had are:

• Less rotational inertia in wheels will mean more energy being invested into the car itself. Additionally, the car, overall, should remain light weight. So, less wheels, and less massive wheels will be better.
• This terrain is relatively rough. As such, wider and larger diameter wheels should be used (the idea that a LEGO car rolls down the driveway extremely bumpily, whereas a bicycle does not). So perhaps large (but light weight) barrel shaped wheels would be most effective.
• The height of the fishing line guide wire can be chosen by us. So assuming we have a low friction guide wire receiver (like a lubricated, plastic drinking straw), it seems like we should set the guide wire in such a way to support a significant amount of the weight of the car. (Due to the fact that the car is solar powered, it is extremely unlikely that it would "peel out".) Should this supported weight be changing throughout the course of the track? Perhaps as the car moves along the track, the wire should support more and more weight, as the car will have less torque at those higher speeds, although, on the other hand, the sloped up rope could slow the car down.

What would the most efficient wheel orientation/setup for such a car be?

• Bicycle wheels seem a popular chice - why do you think that is? Aug 16, 2019 at 4:22
• Sorry--I failed to clarify: Here is the general scale the cars tend to be at, as the track is only 18'' wide and larger just means more mass. However, I do think that's a good implied point that the thinner wheels cause less drag.
– user21489
Aug 17, 2019 at 3:42
• Have a look at the size of the tires and then think : w2.engr.uky.edu/solarcar Aug 17, 2019 at 5:44
• So what did you design? How did you get on? Given your comment about friction below you should have been doing well... At least accept an answer so this question is not continually "bumped"... Otherwise it takes up the space a useful question could be using... Sep 16, 2019 at 4:46

For a power-limited application, the largest diameter wheel with the smallest contact patch with the pavement results in the most efficiency, because it minimizes bearing and rolling friction.

Wheel inertia is only a factor at startup, when the wheel is accelerating up to speed. It does not affect the behavior of the car at constant cruise conditions.

As pointed out by Solar Mike, your best choice is to go with a racing bicycle wheel, as it is already optimized for best efficiency, and they are available right off-the-shelf at a bike store.

• The cars are generally not much larger than the solar panels they carry because the track is 18'' wide, and a larger car just makes it more massive (and thus take more energy to speed it up). Regarding your claims about friction, the "smallest contact patch results in least friction" is a claim I disagree with.
– user21489
Aug 17, 2019 at 3:37
• the frictional loss I am talking about here is internal friction caused by flexural hysteresis loss in the rubber, not scrubbing friction against the pavement. Flexural loss in the rubber is minimized by minimizing the amount of flex, which is done with high inflation pressure and a small contact patch. Aug 17, 2019 at 3:47
• Agree with @PureStress, high pressure yields low friction, that's correct and obvious. And it's true for a smooth surface. For a non-smooth surface, the bumping of that rigid tire would cost energy and it would not be so clear. Narrow tires do not necessarily have less friction, because at the same pressue, they will have a longer contact patch (same area for same pressure at same load). Anyway, a racing bicycle tire will count a really fat in your scale.
– Ingo
Jan 14, 2020 at 17:22