geometry calculations for difazio front suspensio 1

[torocycles]

we are in the process of building a motorcyle with a harley engine.We want to use the difazio front swingarm configuration, but we aren't quite sure of the geometries we must use in order to avoid bumpsteer. Any help or tips will be appreciated.

 

[zedrider]

To quote Tony Foale;
"The layout of the suspension links defines the locus of the end of the steering arm on the upright, to avoid bump-steer the steering drag link must be of an appropriate length and be located such that it describes an arc coincident with the end of the steering arm."

In other words, if you have upper and lower swingarms of identical length and they are parallel, the steering drag-link must also be parallel and the same length.
In practice - for on-road bikes - if you can almost eliminate bump-steer for steering angles less than 5 degrees of lock it will not intrude into the riding experience. Larger steering angles should only be required at walking pace or less.

I suggest you have alook at Tony Foale's book "Motorcycle Handling and Chassis Design" ISBN 84-933286-1-8 before you settle finally on a Difazio FFE.
Have a look at his website at

http://tonyfoale.com/

ZR