Motorcycle front suspension design 1

[PTwizz]

I am in the early stages of designing a race motorcycle front suspension. Looking at those available, all appear to have a bottom yoke substantially thicker than the top yoke, with up to four pinch bolts on each side, clamping the fork legs. A couple of simple force diagrams show that a stiffer bottom yoke imposes much higher loads on the fork leg, ultimately to the point where the top yoke takes no load and the full bending moment applied to the leg is taken out over the thickness of the bottom yoke.
(Deep breath)
Does anyone know of a reason for the popularity of this layout?

Pete.

 

[TonyFoale]

Pete,

It's a pity that I didn't know that you were there. Apologise to Rob for my rather sudden departure, but I suddenly realized the time and had a plane to catch. In fact I just got there as it was boarding.

Tony Foale.

 

[Milanoguy]

Well, I don’t quite understand your objection to the common set up where the lower triple clamp carries all or most of the weight. But as to your question why this setup is so popular; I think the answer has to do with ease of adjustability/maintenance of the steering stem bearings.

On most motorcycles you can lift the front of the bike up on a stand so that the front fork swings freely. It is now easy to check for any play in the steering stem bearings. If any play is detected you remove the top triple clamp and tighten the adjusting nut. Since the wheel, forks rotors and so on are still attached you can get a good feeling if the bearing is too tight or to lose by seeing how freely the whole assembly will move.

On most sporting/racing motor cycles the clip on’s(handlebars) are below the top triple clamp. Thus when you remove the lightweight top triple clamp you haven’t disturbed any of the heavy components in the front suspension. So when you or the rider tests the bearing setting on the stand it is a very good prediction of how tight the steering will be on the track. This is also a fairly fast process which is good as it may take several laps for the bearing races to fully seat.

 

[PTwizz]

Milanoguy,

All that you say is true and indeed very practical. However, the loads that I refer to are bending loads (in the fork legs), rather than the loads created by supporting the machine mass.

Pete.