braking

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.

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