Charles Wahl

I know this "belongs" in the Mechanics forum, but I don't often go there, so you all get a chance to solve it first:

I've just finished a thorough-going rehab on a Specialized Hard Rock (26" wheeled hardtail), keeping the original wheels, but installing a new set of Tektro "Oryx" cantilevers, including their pads. Everything else seems copasetic, but when braking (and only when braking) there's a "hitch" in the front wheel once per revolution (I'm fairly sure), more pronounced the faster I'm going and the harder brakes are applied. I felt each side's canti arm while braking (at my peril somewhat), and didn't really feel either of the arms moving, but it's strong enough to make the fork blades deflect visibly. I also felt the brake track all the way 'round both sides, and can't detect a lip at the butt join. When not braking, the wheel spins very smoothly, in a stand or on the road. The brakes don't squeal. Also, there's no rock stuck in my tire tread, because this bike has slicks on it (vintage "Avocet FasGrip City" 1.5 inch -- very smooth ride!). Toe-in angle is set by using a single credit card shim under the rear of the shoe when situating the pads.

Anyone have this experience on a cantilever-equipped build before?

Upon return home, I felt the canti arms, and while they're tightened really snug, one of them on the front wheel seems to have a bit more play fore/aft than the others, so I reefed down on that mounting bolt a bit more. Since I cleaned and waxed the top surfaces of the rims while refurbishing the wheels, I'm also going to try using some naphtha (would use isopropyl, but that's unobtanium right now) to re-clean the brake tracks, but the only other thing I can think of is that there's some bearing installation kerfuffle going on; but if that's the case, I don't see why it would happen, apparently, every revolution, and only while braking. I'll also re-tighten the brake pad bolts -- these pads are the Kool-Stop design with concave/convex washers to allow 3-D adjustment and toe-in. I suppose I could try flipping the wheel around too, since it's the front, though it's installed like it was originally oriented.

Maybe this is just something that will disappear as the pads seat, but this build is for my cousin's spouse, and I don't want to ship it off with a problem.

Thanks for any pointers,
--
Charles

Reynolds 

I had a similar problem once and it was due to some adhesive on the rim braking surface (new rim). Don't know how it got there (decal?) but it was pretty hard to remove. I had to sand it off.

Bianchigirll 

It could be, as stated some adhesive, or mor likely a small mismatch at the rim seem or a slight bulge in the rim walls. Did you reverse the direction of the wheel?

Charles Wahl

Not yet; it's installed as it was previously, I'm pretty sure (tire labels on drive side). Tomorrow I'll flip it. Thanks.

bwilli88 

I would take off the tire and measure the rim to see if it is bulged a one point. Take calipers and measure every 1 to 2 inches all the way around the rim

madpogue 

Check the seam of the rim.

oneclick 

Original wheels - check the wall thickness everywhere; if worn, it could be thin enough to deflect when the pad pushes.

If true replace the rims. It won't deflect many times before cracking.

flangehead

I'm having the same problem. I see black marks at the point of the "hitch" and I removed them with hand sanitizer and it got much better, but it is coming back.

Still troubleshooting.

L134 

Could the headset be loose?

mstateglfr 

A loose headset can create fork shudder when braking.
A warped rim can create similar fork shudder when braking.
A bend at the seam of a cheaper rim also can create the feeling of sticking/hitch when braking since the pad literally gets stuck on the bent seam.
Too much cable between the guide and the yoke of the brake is something I have read can cause shudder...basically the system isnt stiff enough. I use fork mounted cable stops for this very reason since my head tubes are quite long. Its a $12 component and I also like the cable bend a lot more than traditional headset spacer mounted cable stops.

Good luck.

alcjphil

Sounds like classic cantilever brake shudder. Installing a fork crown brake cable stop can greatly improve this:
​​​​​​https://www.velonews.com/gear/techni...fferent-bikes/

79pmooney

How old is that rim? If old, it may be giving you the warning that it is tired, You may be feeling the rim flex, like it's on its way to breaking. Put on d different wheel and see if the problem goes away. If it does, don't ride that first wheel until you have found the issue. (Sidewalls fail as an explosion, blowing a long jagged strip of aluminum off that can cut legs. scrape paint and deeply scratch carbon fiber. It will sound like a gunshot. Tire and tube will be destroyed. Usually happens while riding though I got lucky and had my one failure happen in the garage while I was inside. Scared the crap out of me.

The issue is probably something else though it sounds like a wheel issue. (Once per revolution.) But KNOW THAT, don't just assume it.

Ben

Charles Wahl

Rim is 20 yr. old, but I know its complete history, since it was in our family since new, only occasionally used, and never for anything technically demanding. It's a Weinmann 519: pretty heavy for a single-wall rim.


It has no perceptible offset at the seam on either side, and the brake tracks are not particularly worn.

May have a winner on the "loose headset" -- I did the "brake and rock" thing and there was clearly perceptible play there, so I readjusted that -- my bad and thanks for that mstateglfr . Will clean the brake tracks on the rims scrupulously, and the pads, and road test again this PM. I'll keep the fork crown cable stop thing in mind, but rider never complained about this in bike's previous life, and the brake function appears to be quite good -- first time I ever rode it.

79pmooney

My Mooney has at times had some canti shudder I looked into the fork crown mount hanger but only saw ones that required a shallow saddle wire. I like a long saddle so I'd have to make my own hanger. It's on my to-do list, but way down at the bottom. I might never get to it.

Ben

Charles Wahl

OK, here's what I did, that has not worked! Removed tire and tube, and found (with dial caliper) that the rim was about 23.65 mm plus minus 0.5 mm around most of the circumference, but 23.8 mm right next to the rim joint. Only 0.15 greater, but still.

Used a sanding stick with 180 grit sandpaper adhered to it, to level the area in the basic vicinity of the rim joint, both sides:


NDS (where I perceived the problem to be, but maybe not!) Drive side

Then I used the stick again, with 220 grit to enlarge and fair in the leveled area.

NDS left, DS right

Finally, I used 220 grit wet-or-dry and water to finish the entire braking surface both sides to match the "leveled" areas.

NDS left, DS right

And here's what the tube looked like roughly opposite (but not diametrically so) the valve:
o
Hard for me to comprehend what, exactly, was happening here -- tube twisted within the tire casing? But I noticed that the tube is 26 x 2, while the tire (which I was responsible for installing) is 26 x 1.5. Tube holds air fine when reinstalled, up to 85 psi.

In any event, after all that work, I took the bike out today for a test ride, and it's only a little better than it was. I can't countenance passing the bike on (as a present) this way, so I am seriously considering just replacing both wheels, and salvaging maybe the hubs, which aren't any great shakes, really.

zukahn1 

It's the wheel I have had the same trouble bucking hard on down with similar wheels But nicer a different set off wheel wheels all was fine. Check the rims. As for the above rims there fairly worn dinged with one bend I can see really the OP needs replace these wheels for safety .

dddd

I've had many vintage rims over the years which gave uneven braking force due to some subtle variation of thickness between the pads.

Sometimes this variation is due to the dowel or sleeve inserts at the rim joint (excess thickness/width there), or due to a welded rim joint having been overly ground "flat" after welding.
Sometimes it's just the anodizing being worn off unevenly, but most often in conjunction with one of the above two issues.

The fix I've used is to ride the bike getting some wet, gritty mud on the brake tracks, then dragging the front or rear brake to quickly machine the rim toward a much more even rim width, keeping the speed low.
A surprisingly small amount of wet mud can help achieve this, but it does dry off fast enough to require a few passes through the shallow mud puddles along the trail.

Here's one example after returning from some deliberate off-road rim machining. The improvement was huge and the bike cleaned up just fine.

Charles Wahl

Just to close this out: though the rims seem to me to be serviceable in all other respects, despite my efforts recounted above, turning the wheel around backwards, and replacement of the (new) Tektro pads with Kool-Stop salmons (with sanding to remove the new pad sheen in both instances), there was no way I could make much of a difference in the grab with each revolution. I happened on a "closeout" wheelset at Peter White's shop, brand new, being sold at a very good price, and bought them -- Deore M510 hubs with Velocity Aeroheat/Dyad rims and DB SS spokes -- so I bought that, and the problem has been fixed that way. Maybe it's just the power of suggestion, but my spouse, the former owner/rider of this bike, seems to remember that this was a problem before I built her a lightweight vintage bike to replace the Hard Rock. It's still a nice ride, though, and it's going to a good home.