Amount of rim flex under normal riding? How much space do rim-brake users leave?

I’ll try to start a proper thread for my current project soon (project), but right now I have one specific question that will help me move forward on it: How much does a well-built 36" wheel wobble within the frame under normal use? I’ve read people mentioning rubbing their rim on the brakes on hill climbs… how much of a gap is normally left for these brakes?

I need to mount two optical sensors as close to my rim as possible, but I’d like to minimize the risk of damaging them. My unicycle is broken at the moment so I can’t measure it myself, and I figured with all the brake users someone should be able to give me a good ballpark. How close can I go?

5mm minimun. 8mm would be better.

its not only the flexing, but a broken spoke!

short cranks flexes more than long, due the higher pressure.

much success!

I think the uphill brake rubbing comes from frame flex more than rim. Since you have a KH36 you should be fine to mount the sensors close to rim.

Ben, yeah I suppose that makes sense.

Jogi, yes! numbers! brilliant. Thanks!

I had brake rub with a Nimbus (pre)nightrider with airfoil rim on climbs and sharp turns with the brake pads about 4mm from the rim. I haven’t noticed any rub with the brakes set about the same with a KH frame and Coker rim.

And I agree that it is more frame flex than wheel flex

Interesting project.
I’m with the others on here, assuming your wheel is well built and tensioned, its likely frame flex that makes the pads rub at idle and on hard acceleration, but the KH frame probably flexes the least of all the 36er frames out there.
I run a hybrid HS33/HS22 “frankenbrake” on my single speed KH36 that lets me set the brake pads back further from the rim than the stock HS33 will allow. THis is more on this set-up: HS33 / HS22 Magura unicycle brakes

I just measured the clearances between both sides of my KH36er Airfoil rim and brake pads with a dial gauge: looks like about 5 mm of clearance per side. Set this far out, the pads don’t rub (I’m 200 lb road/distance rider with moderate skills and speed).

Hope that helps a bit.


Yes, it does help! Not being a brake user I really had no idea what would be in the neighbourhood, and mounting the sensor as close as possible has all kinds of advantages. Think I’ll shoot for 5mm to start.

I guess I set the title of the thread wrong. Haha, all I care about is the rim movement with respect to the frame at the point of the sensor mount (ie the brake mount). Whether that movement is as a result of the frame flexing or the rim, or the hub or the spokes, bearings, bolts…

Also keep in mind that the hub makes a difference. A standard hub with fresh bearing will allow for very little slop, whereas some Schlumpf Hubs have considerable slop that necessitates a greater rim to pad distance.