greg bender

http://www.thisoldtractor.com/gtbender

Steering dampers, aluminum spacer blocks, fork travel, and fender dents

I met Kim Schick at the 2006 MGNOC national rally in Elkader, Iowa. We got to talking about steering dampers and I learned that Kim had cast a number of the special aluminum blocks for mounting the steering damper to the underside of the triple tree. First class work and Kim hooked me up with one (Thanks!).

After examining numerous loops using the stock steering damper set up, Kim and I found that most (all?) had some sort of fender dent from the head of the bolt contacting the top of the fender on full fork spring compression. It seems that even the stock configuration did not fully account for the issue of full spring compression. After further investigation, Kim found that the pocket recess in the lower triple tree for the aluminum spacer block is different between different models. Specifically, he found the following differences:

After further contemplation, I decided to calculate the minimum space needed between the bottom of the lower triple tree and the top of the fender (thanks to Paul Linn for taking the time to measure the maximum fork travel on a set of uninstalled drum brake forks he had). Here is the math and logic I used...

  1. Total tube travel within each fork leg is 4.5" or 114.3 mm
  2. Part - but not all - of that total travel is taken up by the spring. The stock spring wire diameter is 5.76 mm and has 16 turns. When fully compressed, the spring would take up 92.16 mm of space (5.76 * 16 = 92.16)...or roughly 3.628". The Wirth progresively wound replacement spring has a wire diameter of 6 mm and approximately 14 turns, yielding 84 mm when fully compressed. Given that the spring can compress more than the total available travel, the spring will
    1. never completely compress or bottom out
    2. will never - in and of itself - prevent the fender from hitting the anything connected to the bottom of the lower triple tree
    I believe this to be a good thing as the spring should never completely compress, or bind (Thanks to Mike Tiberio for confirming my thoughts on avoiding complete spring compression).
  3. That leaves 22.14 mm (or a little less than 1") of available free space. Were a person to install a preload spacer on top of the springs, 22mm would be the absolute maximum you would want to go...and that thick of a spacer would be very close to completely compressing the spring upon maximum fork travel. I don't have a preload spacer installed on my Ambassador. But if I ever do install one, I think I would limit it to about 10 mm.

The key point from all this math and logic is that there needs to be a minimum of 4.5" between the bottom of the triple tree and the top of the corresponding point on the fender. If there isn't, then there is a very real risk of denting the front fender.

It is easy to check your machine. Jack up the front end so that the front wheel is off the ground. This extends the front fork completely. Then, just measure between the triple tree and the fender. I checked mine and my steering damper bolt was about 4" from the fender...hence my dent.

The bottom line is that there are steps that must be taken any time a steering damper is added in order to avoid dents in the top of your fender. Here are a few suggestions:

  1. THE WRONG WAY: Install a preload spacer on top of each spring that takes up more than the available 22 mm of free space. Doing so will limit total travel. The downside is that there is a very real chance of the spring completely collapsing and binding. THIS IS BAD. DON'T DO IT.
  2. Modify the steering damper mount/bolt so that you have a minimum of 4.5" of space between it and the top of the fender. Here are some ideas...