Wheel weight

[BluesFish]

I just discovered this forum today, 2 days before our PWD race. I've seen that the general opinion is that lighter is better but couln't find any discussion on weighted wheels. Could it be, that whells filled with, say, hot glue whould have a flywheel-like affect and help carry the car on the flat of the track?

Thought I'd ask the experts.
Thanks,

[Darin McGrew]

I just discovered this forum today, 2 days before our PWD race. I've seen that the general opinion is that lighter is better but couln't find any discussion on weighted wheels. Could it be, that whells filled with, say, hot glue whould have a flywheel-like affect and help carry the car on the flat of the track?

The heavier the wheels, the more energy it takes to spin them. That's how a flywheel works--it uses a heavy spinning wheel to store energy.

But in a derby, you don't want to store energy. You want to use it now to move the car across the finish line as soon as possible. So heavy wheels are a disadvantage.

[greg]

What about the diameter of the wheels? Are larger diameter wheels faster or slower than smaller diameter wheels?

[Stan Pope]

For equal wheel inertias, larger wheel diameters are better.

However, larger wheel diameters are ususally associated with higher wheel inertias.

Larger wheels reduce the energy loss when encountering bumps in the track. (By increasing the time overwhich the bump acts?)

[LD]

Does anyone have any ideas what would happen if wheighted wheels were incorperated in the "Grandpa's 'type 2'" (suspension) design?

My two sons "type 2" cars came in 1st and 2nd in pack today, with the third place car having weighted wheels.

I'm wondering if the combination would be even faster.

[ExtremePWD]

Does anyone have any ideas what would happen if wheighted wheels were incorperated in the "Grandpa's 'type 2'" (suspension) design?

My two sons "type 2" cars came in 1st and 2nd in pack today, with the third place car having weighted wheels.

I'm wondering if the combination would be even faster.

The concern being addressed by suspensions is that when a mass moves up and down it requires energy. You want all of your energy moving the car forward, not upward. The suspension attempts to isolate the upward wheel motion from the central car mass thus preventing using up energy to move the large mass. The small mass of the wheel uses much less energy. When you move the mass to the wheels you are now defeating the main mass isolation from the motion and forcing it to move with the wheels.

[LD]

Thanks for the explination, ExtremePWD. It makes a lot of sense.

Whould the "gyroscopic effect" associated with a heavier wheels ever be advantageous, particulalrly with a track with a long run-out?

[Stan Pope]

The "flywheel effect" becomes advantageous for tracks that are about 4' X 65' depending on several variables.

[LD]

Stan, can you elaborate? Thanks.

[ExtremePWD]

Stan might have some other insightful thoughts but I thought I would chime in while I was here:

The "flywheels" take longer to spin up because of the extra inertia which would make them a little slower at the start. They will tend to keep spinning longer due to the higher inertia so this helps on the flat. They will start to make up the difference from the slower start if the track is long enough. Typically, the "common" ~29-30 foot track does not have a flat long enough for the "flywheels" to make up the difference. The other bonus is that since more of the mass is now in the wheels instead of the body, the friction at the axles is reduced.

[Stan Pope]

Stan might have some other insightful thoughts but I thought I would chime in ...

You covered it very well... nailed both key points. Thank you.

[LD]

Thanks to everyone for your comments. I need to give the weighted wheel concept more thought.

Another question, off topic, but here goes: under what track type "conditions" would suspension type cars (ie Grandpa type 2) be advantageous?

Perhaps more directly, what is the down side to suspension?

Thanks again everyone.

[gpraceman]

Thanks to everyone for your comments. I need to give the weighted wheel concept more thought.

Another question, off topic, but here goes: under what track type "conditions" would suspension type cars (ie Grandpa type 2) be advantageous?

Perhaps more directly, what is the down side to suspension?

Thanks again everyone.

If you have a smooth track surface and well aligned track sections, the supension will do you little good. Where a suspension would be beneficial is on a track with a lot of surface irregularities and/or badly aligned sections, where the car might bump along.

[greg]

If you reduce the diameter of the wheel then you are also reducing the weight of the wheel, so wouldn't the smaller diameter wheel be faster than the larger diameter wheel?

[PWD]

I found the Grandpa web site last year. So we decided to build a suspension car for the fun of it with the barrel weight. The car was not that difficult to build but we had much difficultly getting the wheels to align. On our track the car was not any faster but rather much slower.

Our rules also prohibit such a car. In our rules there can not be any suspensions at all including built out of the car.

We still have the car and it is fun to talk about. It was a great learning experience for my boys. They can still explain the concept to others.

The grandpa site has a lot of interesting ideas. Including isolating the weight with foam. Another is the idea of having a majority of the weight focused on one point in the car.

There is so much learning opportunities for a boy with building PWD cars. It is a lot of fun. I love engineering and science and this allows my boys to become interested using a real life example.

It is too bad that most dads don't have an interest and miss out on a great opportunity.

-Stev-