Wednesday, November 28, 2012

De-squealing brakes

Its the season for howling brakes.  I thought I would share my understanding of the problem and see if anyone has thoughts to share.  A little squeal is okay, but if you see pedestrians cringing when you stop at an intersection or you find yourself avoiding the front brake to save your eardrums- time to solve the problem.  Although it can be nice to announce hard braking required when a car pulls in front of you...
A shudder or squeal occurs when the brake momentarily locks up and then releases the rim.  If this happens at a low frequency, shudder occurs.  At a higher frequency it is a squeal.  I think of it as being similar to a shimmy:  it is a resonant frequency that can be caused by many small things.  So there is not one universal fix. The main contributor is a bow string effect of the brake cable.  The longer the bowstring, the worse the shudder and squeal.  When the brake is applied, the fork deflects backwards.  This draws the cable tighter, like a bow string.  Which causes the brake to momentarily but suddenly lock the rim.  It does not happen as much on the rear brake because the frame doesn't move in response to braking as the fork does.  Also doesn't happen as much with sidepull  or V brakes because there is no bowstring- the cable attaches directly to the brake.
If you look closely at my commuter here (with the squeal issues) the cable stop is at the usual place on the headset.  That leaves about 12 inches of bowstring.  I solved the problem of brake shudder on my cyclocross bike with a fork crown mounted cable stop.  That leaves about 1.5 inches of bowstring, and totally eliminated brake shudder.  I've had the same results on other bikes.  The fork crown on this bike, which I had R&E cycles make after the demise of the stock fork, doesn't have a hole in the fork crown so its not an option.  I didn't think to specify that when I had the fork built because I assumed that any steel fork aside from maybe a track fork would have a hole.  I was wrong.  Its otherwise a great fork.

Frictional qualities between the rim and brake pad can also cause squeal and shudder.  A new rim can be quite grabby at first, until the surface gets worn in a little.  Tubular glue on the rim surface can cause squealing.  The brake pad material can make a big difference.  In my experience, salmon brake pads squeal less, brake much better in wet conditions, and don't wear the rims out quite as fast as black compounds.  Sometimes taking a brillo pad or sandpaper to the rim and brake pad can eliminate squealing.  In this case the brake pads were getting thin so I replaced them (Yokozuna salmon pads) and the problem is solved.

Finally, its possible that laxity where the brake pivots on the cantilever post or in the parallelogram linkage on V brakes may facilitate squealing.

The amount of fore/aft movement of the fork under braking influences shudder and squeal, as described above.  A properly designed fork can help eliminate the shudder.  Modern cyclocross carbon forks have gone to truly massive fork crowns and oversize lower headset bearings to minimize flex between the cantilever posts and cable stop.  It seems to me that a fork crown mounted cable stop is an easier, lighter, and more elegant solution.  For a steel fork- the fork blades should be oval in cross section at the crown and taper down below the cantilever posts to a nice small radius bend near the dropouts.  This makes it so that fore/aft flexion occurs mostly below the cantilever posts where it won't create the bow string effect.  This also makes it so that more up/down flexing occurs over bumpy terrain, improving ride quality.  The current trend of making straight-bladed steel forks bugs me!  Its done either to mimic carbon forks or save labor, but there is no functional reason for it that I am aware of.

A word on discs, because I know what you're all thinking...  I know, hydraulic discs are awesome.  I have used them.  They belong on suspension bikes because they have unsurpassed power and modulation.  I don't have my own yet but someday I will.  Hydraulic discs are overkill on a road racing bike or cyclocross racing bike.  But on a cargo, commuter, or mountain bike its probably worth the added weight and complexity for the truly awesome braking power.  Going down a steep hill in a rainstorm with a trailer?  I've done that many times, and you have to be really careful!

I'm not sold on cable actuated disc brakes. They have the same drawbacks as hydraulic discs in terms of weight and complexity but aren't as powerful because of cable flex. The pads have to sit really close to the rotor so they are more prone to brake rub if the rotors are warped. Cantilevers are maligned as much as they are because of tricky setup.  A properly set up cantilever is just as powerful, or nearly so, compared to a cable disc.  A poorly set up cantilever is admittedly awful.  No power, poor modulation, and noisy.  The most common pitfalls are a straddle cable that is too high, resulting in no power, or crappy pads.  I don't even bother with the stock black pads anymore.  They are immediately replaced with salmon pads.

So before you toss your cantilevers in the garbage, tinker with them a little.  Lower the straddle cable and change the pads.  You might be surprised.


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