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Boulder Bicycle Lugged

Wednesday, September 11, 2013

Fender Safety Thoughts

Yep - the worlds worst fender line!  Here debris became imbedded in the rear tire, hit the fender strut, and rotated it upward.  The tire/strut junction area then was drawn in and locked against the tire, and the bike came to a skidding halt.  Had the lock-up been on a front wheel, it could have caused severe injury or worse...

So here are some thoughts on the topic...
We are continuing or efforts to gain a better understanding of the failure mode with fenders, and see if there are ways to reduce the risks associated with them.  Metal fenders unlike plastic fenders don't use break-away quick release mounts.  But as we've written elsewhere, we've learned of a number of failures where plastic fenders "quick released" away, but then folded up dramaticaly into the forkcrown and locked the tire or otherwise caused a crash.  One guess is maybe the fender disengages from the strut, but then bounces off the downtube perhaps and then gets sucked into the crown.  Another issue is that if the strut quick releases away, the strut might bounce into the wheel and suck the fender into a world where it shouldn't be.
One idea for metal fenders might be to change the mounting point of the strut.  As the fender strut's mounting point is placed higher relative to the axle, as the fender/strut rotates with the tire when locked by debris, the fender/strut is drawn away from the tire rather than into it.  There is some discussion and thought, however, that lock-ups with metal fenders may not always occur where the tire/debris/interface gets drawn into the tire, but rather that the fender may arch up and lock up at the crown (and then the fender/strut rotates back down a bit).  If that is the case, changing the strut mounting point might not be clearly preferable (we are skeptical though of this argument).  So perhaps it may be that a slight change in strut mouting location (by having a mounting point farther up the forkblade) may not make a significant difference, or might even make the risk greater.  We just aren't sure.  But our general thoughts which we will explore is that changing the strut mounting location might be a good idea.
So we are looking into this more - and hope that if folks have thoughts (or actual experience or good analytics on this) on this they will contribute to the discussion.  We know of many threads on this already on the web - what we'd love to get is more description of actual failure mode and exact details which will shed more light on this important safety issue.
So if it becomes clear that a different fender strut mounting point is better, we will make changes to our forks accordingly.  And we will hope that other builders will follow along. 

Another possible remedy might be to refine what "proper" front fender stay attachment entails.  One idea we're going to test out is to consider using Berthout plastic fender stay attachment blocks and "run them loose".  By loose we mean still held firmly in place, but not so firmly that a good tug wouldn't pull the stay out.  To do this, it is vital that an extra long bolt be used to attach the mounting block to the eyelet and that a nut be used to secure the "loose" block to the eyelet.  Also, the fender stay probably should be on the long side so that it won't jump free of the mounting block by accident.  Note, though, that having struts come loose via a quick release mechanism is not clearly better either - if the strut gets caught in the spokes then all sorts of bad things can happen too.  But it may be that the rigidity of the struts used on metal fenders enables them to effectively repel most debris challenges, and if the strut can free up only when really necessary, it may provide the greatest but not complete reduction of fender risk.

Interestingly, for the situation in the photo above, we think that a modified strut placement would not have helped, but if the stays were not so rigidly attached to the eyelet, then maybe the lock up would not have occured.
In the meantime, as we sort things out - ride carefully and be vigilant - do your very best to avoid debris if you make the choice to use fenders!