Wednesday, April 8, 2009

Fender Mount Design Issues

In this post, where I began doing some stress testing on my Gran Tourer setup, I detailed some issues uncovered with the Velo Orange 45mm smooth fenders. To recap, my finding was that on bumpy surfaces at speeds I am likely to take them at, the fenders, in particular the front, would sway around enough to contact the wheel. I could regularly reproduce this, and it is not a desirable operating condition. In the worst case, the leading edge of the fender could catch on the tire, possibly causing a crash.

I found this a little suprising, the VO fenders being rather beefier than the Honjo's, where I've experienced no issues. Similarly, the Berthoud fenders have never displayed this characteristic.
So I became a little curious and began investigating.

First, I tested the lateral resistance of a set of Honjo's installed on my wife's Trek 720. This test with the Honjos, pushing the fender side to side at the mount, required markedly more effort than doing so with the Velo Orange ones for a similar amount of deflection. Again, this was very surprising to me, given that the Honjo fenders themselves are of lighter gauge alumininum and the stays are of virtually identical stiffness.

Hence, I started thinking about the mounting system. The Honjos use two fender to stay attachment points, as shown below.

The Velo Orange fenders use one attachment point.

When you push the Velo Orange fender from side to side, you can see the gap between the fender and stay where dual mounts would be shrinking and opening - the single mount forms a pivot point, whereas dual mounts would form an arm, requiring more of the fender and stay to distort when swaying side to side (and hence, providing more resistance to sway).

The effect of the differing attachment schemes is clearly visible when pushing the test fenders from side to side. The Honjo's stays visibly bow a good deal in their straight sections, whereas this effect is much more muted on the Velo Orange fenders, where the fender just sort of rocks back and forth, pivoting on the mount.

I then looked at lots of old pictures of French randonneurs with this sort of fender/stay mounting and all the ones I found had dual mounts. Given that a lot of those builders were pretty weight conscious and dual mounts seems to be the standard, or at least very common, there must have been some acquired tribal/experiential knowledge there that single mounts weren't adequate for their conditions, much of which was unpaved roads, cobblestone streets.

I've got a few extra of these eyelets around, so I'm going to remount the Velo Orange fenders with two attachments per stay. Before I do that, I'll repeat the test with no leather washers - though they look nice, they really are not effective and do reduce the rigidity of the assembly. I'll be reporting on the results of this. If it does stiffen them up, perhaps it will also quiet them down a bit; I found them to be somewhat noisier than other fenders.

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