Brakes who needs ‘em?

Everybody has been working hard to make the bike go faster, is it just because I am a non biker that I seem to be the only person to worry about stopping?

Some say mechanical brakes are pointless at 400mph (and we should consider them more like a parking device) – but I am struggling with this concept. Sure weve got parachutes, one for high speed deployment, the other for low speed (circa 200mph), braking, but surely we should fit something half decent, if only to let us push this thing further during testing here in the UK on tarmac?

But before we all get excited about the high tech, shiny, multi pot, carbon, ventilated, race brakes, a few reminders: At the Bonneville salt flats we will have 5 miles to speed up, the measured mile, and then another 5 miles to stop. So acceleration and deceleration rates are perhaps not as important as one would expect. Testing the bike, on tarmac here in the UK … well how long is the longest runway we can use?… so stopping (without crashing into something), will be an issue.

At Bonneville

At Bonneville the tractive effort that can be applied through the brakes to stop the bike is limited by the coefficient of friction between the tyre and salt. In the same way as the acceleration is limited by wheel spin, so deceleration will be limited by the wheel locking up.

The shape of the bike and its suspension geometry will severely limit the amount of weight transfer from the rear wheel, and there wont be too much of that front wheel dive that you all worry about.

Consequently we calculate that maximum deceleration we can achieve by brakes alone will be of the order of 0.4g. The good news is that all the forces that weve been battling with to gain speed will now be acting in our favour.

Putting a few numbers down to demonstrate this…

If weight on back wheel = 600kg

Downward force tyre to surface = 600 x g = 5886 N

Coefficient of friction tyre to surface = 0.55

Maximum braking force possible = 0.55 x 600 x g = 3237 N

Maximum deceleration due to brakes = 3237 / 800 = 4 m/sec^2 ( about 0.4g) ( f=m.a)

So 3237N is the maximum braking force we can expect to achieve, and that figure will remain fairly constant at any speed. (This assumes back wheel braking only).

Members of the ABA can view a more detailed braking report brakes-2

Comparing this likely braking force with the other restraining forces placed on the bike, (mainly Aerodynamic and wheel rolling resistance forces) and ignoring the use of chutes for the time being. What does this say?

Well, the calculations show that without using any mechanical brakes, between 400mph and 300mph the bike decelerates quite quickly, but at about 250mph the rate of natural deceleration slows.

This tells us that without chutes, brakes become increasingly more significant below 250mph.

Testing in the UK

Testing in the UK, on tarmac where the coefficient of friction will be nearer 1.0, nearly twice as much braking force can be applied without locking up the bikes back wheel and consequently twice as much brake generated stopping power.

Stopping on tarmac will have similar aero help, but we will only have half the rolling resistance. So brakes become more of a necessity as a backup to the parachutes.

Conclusions

So, we have decided only to worry about using in anger mechanical brakes here in the UK. And that the length of track at Bonneville supports the view that brakes will be handy for parking.

Again there are further more detailed Brake Details for ABA members … click here to join……