Friday, 13 January 2012

Grinding out Girder Details…….

Here is a discussion point for you. How girder forks affect the 550 rule.

For my non Australian followers we have design rule which bikes must comply with that states the forks may not exceed 550mm in a horizontal plane from centre of the head stem to the centre of the front axle.
If you have a bike that is exactly on the 550mm max point stock type trees and forks. and then switch to a girder you will be over by about 150mm due to the width of the offset to the axle to neck stem. 
I imagine the same to be true with a springer front end. Therefore in Aust if you want a max height and length but comply with the rules, these types of front ends are not your friend.

Again I am not commenting on handling or looks, etc. That is a whole other discussion but could be considered relevant due to trail reduction in longer front ends.

It just goes to show how a simple blanket rule really doesn't work for producing a correct handling bike.

The designs I just did for my frame started with 545mm, 40Deg rake, trail was about 5.6"

I swapped the drawings to a girder and ended up with 709mm, 40Deg rake, trail was about 4.5" on an XS650…

Basically I can't build this bike with a girder unless I keep stock neck height (0 up) which defeats the purpose of trying to make the bike a little bigger to suit my heigh
t.  It will also have very be right on the minimum trail.

Above are some examples

(Left) 40 Deg rake, 0 offset, Mid point neck 920mm vertical x545mm horizontal to axle, 225mm Trail.
 (Right) 437 Deg rake, 3 offset, Mid point neck 917mm vertical x552mm horizontal to axle, 140mm Trail.

Most of the change here is in the trail due to the 3 Deg offset trees.

(Centre) 40 Deg rake, 0Ddeg-140mm parallel offset, Mid point neck 700mm vertical x550mm horizontal to axle, 107mm Trail.

So due to the parallel offset you loose a lot of height keeping in the 550 rule.
Also if you wanted to have more height, so reduce the angle you will run into potential problems with short or negative trail.  As it is in the example above at just above 4" of trail you are looking at poor handling at higher speeds.

Basically you can't legally & safely run a girder in Aust!!!!

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