I've just been on a track day at Silverstone. It was fun. The first few laps were OK then as the speed built up I noticed an odd problem. When braking hard from 120 MPH, the front right cycle wing went epileptic. Lifting off the brake stopped it. From then on it happened every time. In the pits, everything looked fine. Nothing was loose or broken. The wheel turned fine and neither disc or calliper were particularly hot. The car braked well. straight, with no binding or sticking.

I developed the habit of lifting off from 120 to 100, braking gently from 100 to 80 and braking hard from 80 to 50.

I tried a few emergency stops on an empty bit of service road and all seemed fine.

Later in the day, lifting off from 120 caused vibration in the steering wheel, a bit like driving over coarse tarmac. Reapplying the power stopped it. Again, I rechecked everything and it looked good.

At road speeds everything was just fine.

What can be the cause(s) of these symptoms?

it will be that bike engine, have you thought about replacing it with a car engine ?

better weight ditribution ?

mine do it around 100mph as well, its a bit like a basic speedo...again worse under fast breaking.

[Edited on 24/9/08 by graememk]

Possibly some sort of resonance effect...You colud have some form of small vibration in the brakes or wheel bearing that mathches the natural frquency of the cycle wing. This vibration may only occure at high speeds

Paul

PS - I think I have had too much to drink now so time for some sleep.

aaah i see you have the old problem of bike engine vibrational unbalance in the acoustical harmonics, this causes really bad vibrations in all parts of a kit built vehicle, and can cause serious health problems later in life, doctors wont treat on NHS as its all self inflicted, private hospitals charge a fortune, but if you ask the doctor soon he will suggest fitting a CAR engine

hmm, resonant frequencies of the cycle wings? makes the cycle wing vibrate in a particular way that makes it look eliptical perhaps. like twanging a stretched elastic band, but curved)

if you can fine a way to tune this vibration, try for 33hz interpolated with 66hz, and take a female passenger for a ride. she'll love it

So we're in the mood for helpful responses today then?

Seriously, it was wobbling by several centimetres at an alarming rate. It wasn't safe, but was it normal?

It is normal since your car, like every other object on earth, will behave according to the laws of physics.

If does sound like you have found the resonance frequency for the wing.

I see this on mine around 60 mph so I need to go slower or faster.

Try having your wheels balance, change tyre pressure, tyre type, adding/removing weight from the cycle wings or beefing up the stays. I cannot guarantee any of this will work since different road surface will impart vibrations of different frequencies.

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Originally posted by smart51
So we're in the mood for helpful responses today then?

Seriously, it was wobbling by several centimetres at an alarming rate. It wasn't safe, but was it normal?

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