Quote:
Originally Posted by Mr Gazza
I used to watch the forks bend backwards on my Ajay after converting to a hydraulic twin leader with Mini shoes instead of the 7/8" single leader. But I was still a single sided drum with the torque anchor on one fork only and there was no twist or pulling to any side. So the brake was essentially only anchored to one fork leg like a single disc.... I've never seen forks twist under braking.
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Was the Ajay still wearing a chunky mudguard?
70’s Yams and Hondas looked pretty with skinny Rickman style fibreglass front mudguards, but without a forkbrace, or the original mudguard bracket they’d twist when braking hard. A little wear in the bushes helped exaggerate this. Little trailies with high level mudguard also twisted all over the place.
Quote:
Originally Posted by Mr Gazza
If you're serious about this statement.. " Callipers in front or behind the fork leg change the polar moment of inertia about the steering axis, so affects the castor shimmy of the forks." I will happily listen to your explanation..  |
Going from memory:
The geometry for standard front forks relies on a castor effect to be stable when moving forwards. The contact patch of the tyre on the ground is behind the steering axis, so self centres when going forwards. Going backwards it is unstable and wants to be on full lock instead.
There are harmonic frequencies when castor effect wants to wobble. Generally around 35 mph and again at around 120 mph. The slower one used to be fun: no hands on a Honda G5 at 40 with no throttle would allow a full lock tank slapper to develop, but fade away as speed dropped to 30ish with no real worries. Never felt confident enough to play with a tank slapper at high speed though!
Handling improvements try to shift the wobbles above the top speed of the bike so it didn't happen, or occurs at lower speeds where it’s less dramatic.
To come back to calliper positions, the caster effect of the front end is affected by suspension geometry, but also by mass. Spinning mass which gives gyroscopic stability, and inertia about the steering axis. The inertia is the mass of the components multiplied by their radial distance from the steering axis. Larger inertias take more force to damp out the caster wobble, so if a wobble occurs it is more violent.
At this point help would be welcome from anyone who’s recently read up on chassis theory?
Large handlebar mounted fairings change the handling adversely, whereas bigger frame mounted fairings don’t have quite the same effect.