Gravity Racer/Soapbox Charity race advice

[Banger Rally Man]

Been roped into building a soap box car for the local Hospice charity race in August.

Advice here in uk is small front wheel & axles such as disabled scooter (widened) with BMX thin rear wheel & tyres.

Any US Gravity racer advice on best wheel combination for speed & strength.

[R. Pare]

Have never even looked into building one, so I can't comment on some of the finest details that need to be sorted, but there are some basic things to be in mind of:

1- Aero. Since gravity is all you have for power, wasting a lot of it pushing air around is to be avoided. Airflow off the back of the car is just about as importent as airflow across the body. If the rules allow the wheels to be enclosed, do that as well - a large percentage of drag in any car comes from the wheels and tires. If you cannot enclose them, make the rims as smooth and close to flush with the tire sidewalls as possible. Tires probably also need to be solid, no tread other than maybe circumferential grooves, and skinny, both for rolling resistance reduction and aero drag.

2 - Rolling frictions. The type of wheel bearings uses, as well as the lubrication of them is BIG factor in reducing rolling resistance. i would expect that single-row deep groove bearings will be your best, with no rubbing seals, and as little grease (or oil) as possible. Karters may have more details there for you.


I would expect that there a lots of YouTube videos on designing and building these cars.

[Steve Demeter]

Many years ago I built two cars a a youngster.

No lube on the bearings.

Aero ids your friend.

Get the driver as far down in the car as you can. The head sticking up higher than necessary is bad aero

We all had the same wheels and would spend hours spinning them to get the bearing surfaces friendly with each other. the best set of wheels I had would turn for 15 minutes with one spin by hand.

The winners had each wheel carrying equal weight to the extent humanly possible.

[R. Pare]

Apparently the Boy Scout of America have some very nice, thin wheels that they mandate for their cars:

http://www.aasbd.net/cgi-bin/commerc...ion&key=P27-RW

No idea on how light they are, but lighter is always better, all else being equal.

[phantomjock]

Some interesting ideas regarding CG placement, aero, scaling the chassis...

enjoy:

https://www.youtube.com/results?sear...tegory=0&page=

Cheers - Jim

[lance3556]

http://www.soapboxracer.com/winners-circle/

This hobby is pretty widespread here in the Western US. A few teams due the tour thru California, Washington, Nevada, Oregon. This link was a few years ago the top 3 in Portland. Aero, Bearings, and a good launch system.