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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!


  1. In mounting fenders, it's wise to only use button head cap screws on the fender-to-bridge or fender-to-fork crown or rack connections to avoid sharp edges that could possibly catch debris.

  2. In mounting fenders, it's wise to only use button head cap screws on the fender-to-bridge or fender-to-fork crown or rack connections to avoid sharp edges that could possibly catch debris.

  3. I'm wondering if there would be any advantage to a rigid but breakable fender stay? I'm thinking of an aluminum stay not unlike standard stays supplied with Honjo, VO fenders, etc. But they would have built in breakaway points every inch or two, in the form of concentric grooves. The stays would be rigid, but could snap easily at any of the concentric breakaway points.

    Manufacturing would be straightforward and the design would not require any other modifications to fender or mount designs.

  4. The German bike club ADFC had a working group back in the nineties in order to understand the scope of the issue and find possible solutions. They found that fender related accidents were not uncommon and their work led to the creation of the various breakaway fender stays, like the Secuclip by SKS. They also suggested that a higher placement of the fender stays might be a solution. A longer article about the issue by Dirk Zedler can be ordered from the publisher

    For metal fenders, I've seen this handmade custom solution:

    The thing is firmly attached to one side of the fender stay, but the other side of the stay has a little dent in it into which a ball is pressed with a spring. Apparently it took a lot of tries to get the spring tension just right, to be not too firm or light.

  5. Thank you for posting this topic, one I'm very interested in learning more about. I wish someone would perform some laboratory tests of various breakaway designs and different mounting options. Besides SKS, there are several other designs now. But testing costs money. One interesting test (on youtube) was by the British Crud mudguard company, where a rag, a stick, etc were thrown into the bike being pushed at low speed. Impressive results, but these fenders are small coverage, and only for very narrow tires. One idea is that it is safer to have the front fender clearance extremely tight (nearly touching tire) at the bottom, and larger gap up the fender. The hope is that few things will get into such a narrow gap, but if they do, there will be enough room to eject or pass by the remainder of the fender. Does that really work? I'm concerned because I use a traditional leather flap, very low on the front fender, and I fear that only increases the chance of scooping debris into my fender.

  6. My most recent fender break like this involved a stick I picked up in the woods. It got caught between the SKS fender and the tire. It rotated up, broke the snap-off stay end, bent and broke the fender right off, and then the stick itself lodged between the tire and the fork crown. I don't see how any simple change to the fender design would have changed that result. But I don't see it as a tremendous problem at all. You don't often pick something up that way and it seems to me you'd need a quite slippery surface to cause the front wheel to skid rather than simply slow down quickly. And I've had more problems with sticks in my spokes than things jamming in the fenders. I don't think this is as big a safety problem as it's cracked up to be.

  7. Thanks for taking the time to discuss this, I feel strongly about it and love learning more
    on this topic. If possible, as you gain expertise, would you mind updating your blog with more information? It is extremely helpful for me.

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  8. My friend, an experienced cyclist, had his front fender lock up yesterday when a twig lodged between his front tire and fender. He was thrown forward off his bicycle and suffered serious injuries. I have his damaged bike and am working on what happened and how we might prevent these incidents in the future.

  9. For what it's worth - as a boy I was whizzing happily down a long hill when my back wheel locked up. I didn't realise that's what had happened - as far as I remember, didn't even notice much braking effect, just that the bike started weaving a bit - so didn't stop as quickly as I might or should have, so probably skidded for 100 yards or more. Got off, propped bike against the hedge to see what was going on, and saw my sturdy steel back mudguard all bunched into a knot under the saddle. While I was staring at this, amazed, there was a loud bang as the tube burst - I'd skidded far enough to wear a patch of the back tyre all the way through. If there's a moral to my little anecdote it's what you probably know already - back wheel lock-ups, if you're travelling in a straight line, are usually harmless (to you if not the bike)

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