Monthly Archives: April 2007

minor note

I added the much touted soapbox racer construction manual to the Box widget on the sidebar.


After our experience last year I have been quite aware of the importance of good brakes. Those of you who were there know what I mean. You can use drag brakes (lever a pad down onto the road surface), bicycle rim brakes, or disc/drum brakes. The choice is open but the brakes will be tested on practice day and I have come up with some estimates to help determine if your brakes are good enough.

The race director has stated that there will be a time penalty imposed on cars that do not stop within some distance past the finish line. This distance has not been set, but 150 feet has been hinted at.

Update (April 25): The Race Director has just sent out mailing stating at least a 200 foot distance.

I’ve created a very simple spreadsheet calculating braking distance at various speeds and braking forces. WordPress doesn’t allow me to link the Excel file to the blog, but if you ask in the comments I can email it to you.

Update (April 25): WordPress now has new widget allowing more file formats to be shared, see the sidebar and you can download the spreadsheet.

The spreadsheet allows you to enter various car weights, velocities and braking forces. The car mass and velocities are straightforward enough, but how does one easily measure the retarding force generated by the brake system?

One way is to have the driver in the car, applying the brakes, while others try and push or pull the car. Measuring the force applied via a spring scale or the like will give you a pound or kg force number that you can enter into the spreadsheet.

Another way is to have the car on an incline, and if the brakes hold, then you have a fair idea if the brakes will stop the car within 150 feet.

Let’s assume the car can just hold itself on the 15 degree incline of the starting ramp. The “downramp” force component can be calculated by:

(note: max, car mass used, 375 lbs or 156 kg)

force = mass * sin(ramp angle)*(accel. gravity)

= (156 kg )*(0.2588) *( 9.8m/s^2)

= 395.7 Newtons or 40.35 kg-force.

Plug that into my spreadsheet, and at a velocity 55 kph the car would take a distance of 151 feet to stop.

I can imagine using some plywood to make a ramp at a steeper angle to determine the braking limit. It doesn’t matter if the tires skid or the brakes don’t hold, either one will indicate the limit. Measure the angle and then calculate the force. Its a handy estimate to see if you are at least in the ballpark.

I only hope I have got the angle/force formula correct, its been while since I have had to do trig. Oh and another thing, this estimate method does not, obviously, take into consideration any braking force generated by air or rolling resistance.

I wish…

I had these resources.


two approaches: cart-type steering where the entire front axle pivots to steer wheels (as in traditional soapbox racers), and kingpin based car type steering.

Cart type has the advantage of simplicity. The axle is controlled by cables attached outboard and either to a pulley and wheel control for driver or just 2 cables with handles (see soapbox derby construction manual). Two main disadvantages: the wheels will scrub more the tighter the corner turned. Maybe its not a big negative on the Canary Derby course, I think the tire scrub is really only a factor in turns less than 180′ radius (that radius comes straight out of my butt, can’t reference it). But the other negative is the effect that applying front brakes (if used) would have on steering. Most likely the brakes would not apply perfectly evenly to both wheels at the same time, resulting in a steering effect. This would be a problem if you had to brake suddenly. Similarly bumps on the road can have the same effect. A lesser disadvantage is that cart type steering does not provide any steering “feel” or self-centering.

Kingpin type steering has geometry that if set up correctly, provides steering “feel”, self centering, no bump or brake steer, and little if no tire scrub on corners. Its the type of steering used in cars. The kingpins are connected by a cunning arrangement of pivots and connecting rods, either to a steering box (rack and pinion often) or to steering “levers”. This type of steering is more complicated to construct and its debatable if it really has a big advantage over axle steering on the Canary course. Some cars last year did have it, looking totally cool and got a lot of compliments, two bonuses in my book. Have a look at the steering links on the side, and also Google for “Ackermann steering” for more info.

And a couple more points…

If you decide on the cart type steering, remember to install some sort of limit to the amount of travel the axle can make. Otherwise there is a possibility of major over steer (again, see soapbox derby construction manual).