I am not convinced that the geometry is of secondary importance. I have no argument with springs and shockers being of importance but! Below is the path that the front wheel spindle moves through as the suspension rises and falls.. The whole point of this is that when braking hard the front end does not rise up and prevent the suspension working.
These two curves were taken by putting a long rear wheel tommy bar axel into the front forks and then moving the forks up and down. In both case the spring boxes were empty, an empty modern AVO damper (no oil) was used in conjunction with the longer eyebolts, F57/2SS, at the base of the damper. These eyebolts were used on the series D machines and they were used with the Armstrong dampers, which are shorter than the original Vincent dampers. It is the combined length of the damper and eyebolts which controls the range over which the front forks can move.
The measurements are all in inches and it is the shape of the curves which is important. The exact value of the position along the horizontal axis is not guaranteed to be identical for both curves. Look first at the Standard curve and you will see that for the first 2.5inches/62mm of vertical movement the wheel has to move forwards by 0.5 inches/12mm. When one is braking the wheel is reluctant to do this and all experienced Vincent riders will have experienced the bike trying to ride up over the front wheel when braking. When braking hard the bike rises up over the front wheel, the wheel moves backwards to give increase trail which reduced the ability of the bike to turn into a corner and until the brake is released this situation cannot be recovered. This can cause a speed wobble wherein the front forks oscillate so violently from side to side that one cannot hold onto the handlebars and this often results in a crash. Note that with the standard front forks it is only after the first 62mm of vertical movement that the wheel can start to move backwards. It is interesting to look at photographs of the works racing machines in the early 1950s. They all have low front suspension which means that the first 62mm of movement has already been taken and the bike is operating in the top part of the standard curve 50 mm of vertical movement and about 8mm of backwards movement.
Compare the curve of the standard front suspension with that of the 'modified' one.
It would be good to see the same information for Vincent's design.
I did not consider it worth while designing the system around using eccentrics, That was a very ingenious design by the two Phils and served the needs of family men in the 1940s and 50s. If anyone wanted to fit a sidecar to a bike with a JE stem then I would recommend a short top link. The reduction in friction when one changes from Oilite bushes to ball or needle roller bearings is remarkable.
These two curves were taken by putting a long rear wheel tommy bar axel into the front forks and then moving the forks up and down. In both case the spring boxes were empty, an empty modern AVO damper (no oil) was used in conjunction with the longer eyebolts, F57/2SS, at the base of the damper. These eyebolts were used on the series D machines and they were used with the Armstrong dampers, which are shorter than the original Vincent dampers. It is the combined length of the damper and eyebolts which controls the range over which the front forks can move.
The measurements are all in inches and it is the shape of the curves which is important. The exact value of the position along the horizontal axis is not guaranteed to be identical for both curves. Look first at the Standard curve and you will see that for the first 2.5inches/62mm of vertical movement the wheel has to move forwards by 0.5 inches/12mm. When one is braking the wheel is reluctant to do this and all experienced Vincent riders will have experienced the bike trying to ride up over the front wheel when braking. When braking hard the bike rises up over the front wheel, the wheel moves backwards to give increase trail which reduced the ability of the bike to turn into a corner and until the brake is released this situation cannot be recovered. This can cause a speed wobble wherein the front forks oscillate so violently from side to side that one cannot hold onto the handlebars and this often results in a crash. Note that with the standard front forks it is only after the first 62mm of vertical movement that the wheel can start to move backwards. It is interesting to look at photographs of the works racing machines in the early 1950s. They all have low front suspension which means that the first 62mm of movement has already been taken and the bike is operating in the top part of the standard curve 50 mm of vertical movement and about 8mm of backwards movement.
Compare the curve of the standard front suspension with that of the 'modified' one.
It would be good to see the same information for Vincent's design.
I did not consider it worth while designing the system around using eccentrics, That was a very ingenious design by the two Phils and served the needs of family men in the 1940s and 50s. If anyone wanted to fit a sidecar to a bike with a JE stem then I would recommend a short top link. The reduction in friction when one changes from Oilite bushes to ball or needle roller bearings is remarkable.
