Why do rear mudguards always fail at brake bridge?

[Crofton]

My third set of aluminium mudguards in seven years has just failed in the exact same way as the previous two: cracked at the brake bridge mounting point. It’s driving me mad. What am I doing wrong?

I have read countless articles about fitting and done this with great care and attention to try and ensure no undue stresses etc. but I always end up with the same result after a couple of years. Vibrations eventually lead to the rear mudguard cracking and breaking into two pieces where it is secured at the brake bridge. I have tried a couple of pairs of Velo Orange guards and the latest were Honjos so it’s not an equipment quality issue.

[LittleWheelsandBig]

Al rear mudguards generally only have a single set of stays. Particularly with a small frame, there is a long unsupported length of mudguard between the seatstay bridge and the mudguard stays. A second set of stays midway along that length would prevent vertical movement sufficiently to virtually eliminate fatigue. Better-designed plastic mudguards (which are more flexible) have long been fitted with a second set of stays.

[rogerzilla]

Narrow ones (35mm) fail there.  Wider ones are inherently stiffer and rarely crack.

[Cudzoziemiec]

I've got SKS Edge aluminium mudguards on one bike, which look good (IMO) and do a good job of mudguarding, but the rear brake bridge fitting is flexible plastic, which clanks at every pothole, kerb etc, as it flaps around and the guard hits the bridge. I suppose eventually it will break and then I will simply drill a hole through the guard and bolt it directly to the underside of the bridge.

[CrazyEnglishTriathlete]

Because the cycling industry is happy to spend millions in microscopic improvements on gearing, chains, etc so they can charge punters more money, and diddly squat on mudguards that are useful for turning the bicycle into a practical everyday form of transport. No doubt you could provoke them into innovation by suggesting carbon fibre mudguards, which would probably fail at twice the current rate.

My rear mudguard failed in exactly this way on Tuesday, although it was provoked by a screw that got lodged in the tyre.  In failing the mudguard managed to progressively hammer the screw all the way into the tyre.  Ho hum.  Set Strava PBs like Johnny English setting off traffic cameras on the rest of the commute home, so I suppose it had some benefits 

[Kim]

It's aluminimum being shaken about.  It's going to fail somewhere eventually.

[vorsprung]

i would guess that an Aluminium mudguard that failed like this could be repaired (probably have to shrink a little) with a pop rivetter

[Ian H]

Stainless steel ones are pretty indestructible at 500gm approx.

[Danu]

You could have a boss fitted and bolt it to the stay assuming you have bridge between the stays

[Crofton]

Thanks for the comments. I am glad it’s not just me this happens to!

• I can see the benefit of chromoplastic mudguards with an extra set of stays but they are too ugly
• Wider mudguards also sound like a good idea (current pair are 41mm and the frame might just accommodate 45mm)
• I did wonder whether stainless guards might be less susceptible to this sort of fatigue cracking - will investigate further
• I fear any repair will rattle unacceptably!

[morbihan]

cork/rubber spacer to dampen the vibrations?

[aidan.f]

Buy a new mudguard identical to one wot broke.
Take the stays off the broken one
Fit another pair into place.
I presume your 'Chromoplastic is too ugly' refers to the material not an extra pair of stays.

[Crofton]

Buy a new mudguard identical to one wot broke.
Take the stays off the broken one
Fit another pair into place.

Had the same thought myself and a mechanic at the LBS also came up with the same suggestion so will try it.

[sparklyfish]

cork/rubber spacer to dampen the vibrations?

I had the same thought a couple of years ago, and have been using rubber washers as well as normal washers. Haven't had a mudguard fail since, but have had the rubber washer perish instead!

[Dickyelsdon]

Ive found GB stays to be very sturdy as they use two bolts rather than one single bolt to fasten the stay to the guard.

https://www.sjscycles.co.uk/mudguards/40-mm-gilles-berthoud-mudguard-stays-kit/

[fruitcake]

If you also run a rear rack, you may be able to screw the mudguard to that. That way the rack becomes an additional support stay. I've done this on a couple of bikes.

[Tod28]

It's aluminimum being shaken about.  It's going to fail somewhere eventually.

This - the fatigue life of aluminium is miserable. It will fail close to where it is securely fastened if the body of the material is relatively free to move about (albeit fractional movement). Stainless steel will have a significantly longer fatigue life in terms of number of cycles of flexure and the induced bending stress is a lower proportion of allowable bending stress also increasing the fatigue life in the same installation.

[LittleWheelsandBig]

cork/rubber spacer to dampen the vibrations?

I had the same thought a couple of years ago, and have been using rubber washers as well as normal washers. Haven't had a mudguard fail since, but have had the rubber washer perish instead!

The best mudguard washer material to damp vibration is supposed to be leather.

[Shuggie]

Oops, first time this has ever happened to me!




SKS Chromoplastic P-45 which had been on my Sabbath September AR-2 for about 1800 road miles and the rear mudguard broke on Sunday. I had added a B&M Secula battery operated rear light onto the back at about 800 miles which probably created much more stress. Several weeks ago I had to tighten the seatstay bridge screw as it had become really loose mid ride, maybe I did it too tight. I had thought of adding threadlock but this fracture beat me to it.

Have used SKS chromoplastics P-35 and P-45 with the same rear lights before on other bikes without a problem but the metal clips on the P-35 do wear the guards and have snapped in the past.

Will be replacing the guard but putting the rear light elsewhere .

Cheers, Hugh

[hubner]

I was wondering how these broken mudguards were mounted, direct mount with a bolt through a hole in the mudguard or with a crimp on metal/clip on plastic bracket?

Would it make a difference?

[Shuggie]

I was wondering how these broken mudguards were mounted, direct mount with a bolt through a hole in the mudguard or with a crimp on metal/clip on plastic bracket?

Would it make a difference?

This one was direct mount using the M5 bolt and washer provided with the frameset, I drilled the hole through the mudguard and was pretty pleased with its position.  Can’t see how easy it would be to use the metal clip bent to 90 degrees, or in this case the plastic one which does pivot, it would certainly and unacceptably reduce the tyre clearance IMHO.

Anyone else with better experience?

Cheers, Hugh

[hubner]

I'm just guessing; the hole, although small, weakens the mudguard at exactly where it is being stressed. Any vibration would get concentrated near the hole because that's where it's held rigid. Maybe a crimp on bracket allows some movement, so that vibrations get spread over a larger area.

Also, how thick is the mudguard? I would think anything less than about 1mm wouldn't be all that durable.

[Lightning Phil]

Never had mudguards break just the rear bridge fatigues sometimes.

[hubner]

It's better that the bracket breaks rather than the mudguard.

[Cudzoziemiec]

cork/rubber spacer to dampen the vibrations?

I had the same thought a couple of years ago, and have been using rubber washers as well as normal washers. Haven't had a mudguard fail since, but have had the rubber washer perish instead!

The best mudguard washer material to damp vibration is supposed to be leather.

I had some Velo Orange mudguards (can't remember now if they were aluminium or steel) which were supplied with leather washers. It didn't take long for the leather to become a slimy pulp. The guards themselves seemed strong enough, and they looked ace, but they didn't stay still due to poorly designed stays (at least for my bike). I replaced them with SKS Edge (aluminium but no leather washers supplied – in fact no washers at all) which clatter a bit but do their job.