Topic: Soap Box physics advice needed

[greenjeans]

Ok fellas,

I was just in a coffin race in my town over the weekend.  The only rules:  6" wheel max, steering and brakes.   That's it.   Some found 6" wheels that used tires to have a larger diameter.   Some used urethane scooter wheels (I did) and some used skake/longboard wheels.    There is a 10' push zone at the start and the slope is constant until the very end.   The coffins with scooter wheels seemed to do the best.   However, the initial push almost always determined the outcome.   

Aerodynamics are pretty much thrown out the door since this is more of an adult/fun/artistic kinda race

From my physics classes years and years ago, I know that a heavier car will keep it's momentum longer than a lighter car - assuming that there is no friction on the wheels.   That was the thing everyone seemed to forget.   The heavier cars seemed to go good right at the start, but to me, the weight was creating too much friction on the bearings thus slowing them down.  But - the lighter cars seemed to run out of gas too.

I used 4 wheels on the rear of my coffin and 2 on the front.    I was thinking for next year that I'd add even more wheels toward the rear - perhaps 4-5 per side.  Spaced kinda like roller blades/inline skates.

I know that ideally, the weight should be balanced on the wheels - at least I think it should be.

Do you guys think that adding 4-6 additional wheels to my coffin would help it carry more weight by spreading the load out ?   
Would I be adding friction by adding more wheels ?

Any ideas or suggestions here ?

Of course my coffin had a motorcycle related theme:

[333]

Yes, you add friction with more wheels.  But more important, is it a straight track?  Any curves will make multiple rear wheels bind.  If you use inflatable tires, over inflate them. A harder tire has less friction.

[heffay]

I think it's all about the bearings and how you prep them... back in my skateboarding days we'd use the usual stuff we had on hand.  Then I saw a buddy of mine just absolutely flying down a hill and I asked him how he was going so fast.  He had dry graphite powder that he used on his bearings.  I tried it and it made my board sickeningly fast and ridiculously hard to stop.  It is not a long term product though, but for one downhill, it was perfect!

Keep in mind, bearing tech has come a long way in *cough, cough* 25 years, but bearings are where it's at. 

[333]

You youngster, you!

But I had another thought.  Wheel alignment.  Make sure the rear wheels are a true 90 degrees from the center line, and the front ones exactly parallel to each other.

[754]

Machine a set of billet wheels with good bearings.. Use o-rings for tires..

Is that Knievels DIEcycle..?

[70CB750]

Machine a set of billet wheels with good bearings.. Use o-rings for tires..

Is that Knievels DIEcycle..?

Exactly!  If the track is straight, narrower wheels will go better.  Look up record making vehicles - like ultralights and such and copy what is possible.

[Duanob]

What an awesome helmet! 

[greenjeans]

Yes - the track is straight.  No turns.   The scooter wheels I used have an ABEC-5 rated bearing.   I can easily upgrade those to 7s or 9s but
the ones I have really seem to spin quite nicely.   They are sealed, and actually seemed to roll better the more we sent it down the hill.

I have it hanging in the shop and gave it a good cleaning before putting it up.   If you spin the wheels with the air nozzle from the compressor,
they will spin for a good 30-60 seconds or more.    The rear wheels are pretty darn square as are the fronts - that actually took me the most time.

I had thought about taking the seals of the bearings and blowing the grease out and going with graphite or just a very, very light oil.   But then,
I figured that I weighed 200lbs + the car weighs about 65-70lbs.   I just figured at some point the weight would get to the bearings.


So - more wheels = not good or ?

[heffay]

ABEC 5 should be just fine, and ABEC 9 won't make you go any faster... ABEC ratings are more about tolerances, load capabilities and speed capabilities (that should not be confused with "it will make me go faster").  Since you are only travelling under the force of gravity, ABEC isn't gonna help you. 

This has a lot of good info about ABEC and how it will, or won't, affect your performance.
http://www.lushlongboards.com/workshop/abec-ratings-explained-c-199_200.html

[greenjeans]

Interesting read on the bearings.  Thank god I didn't spring for different bearings which I almost did.

If you had 2 identical cars on an identical track would more wheels go faster than the car with less wheels.
Assume that the car with less wheels had added weight to offset the weight of the extra wheels and that both cars were balanced equally.

[heffay]

OH MAN... Now we're getting into airplane on a conveyor belt territory!   

[TwoTired]

If you had 2 identical cars on an identical track would more wheels go faster than the car with less wheels.

How much energy is consumed with tire deformation as it rolls?

More frictional contact leads to more total friction.  More wheels would only help if total deformation energy of all wheels is reduced.
This assumes the road surface isn't deforming under load as well.
Ideally you would have pizza cutter wheels.  But, the more the road surface gains traits of a pizza, the more resistance is incurred.

More wheels and tires fundamentally raise the total of friction elements.  Their function is generally to spread the weight over a larger area or bearing surface at point(s) of contact.

Soft balloon tires on mud/sand.  Hard tires on hard surfaces.

[Duke McDukiedook]

OH MAN... Now we're getting into airplane on a conveyor belt territory!   

And we all know the soap box plane won't fly!! 

[Don R]

The drag racers are spending crazy money on ceramic bearings. But,  we're a bit crazy. I agree on hard tires so they don't deform. Larger OD will bridge bumps and cracks better.