dsmyers

So, I took my frame to be powdercoated, and the powdercoaters sandblasted the cantilever brake posts while stripping the frame. The brakes seem to fit and pivot OK, but the posts are definitely "stickier" to the touch than they were before. I'm wondering if I should do something to polish them back to a smoother finish, and if so, what? Any suggestions?

Thanks,
Daniel

dsellinger

You could just get some new bosses they are pretty cheap.
https://aebike.com/page.cfm?PageID=30...and=448&type=T

Or, you could sand them down with fine sandpaper to polish them, but they are going to be smaller then and that might create some problems too. (but probably not)
I would start with maybe 180-220 and work down to 800/1000 (200,400,600,800 etc)

dabac

You don't need to do a thing, as the brakes don't pivot against the posts in the first place. There's a double bushing thingy press-fitted into the brake arm that gets locked in place around the post and does the actual pivoting.

On secon thought I'd recommend some grease for rustproofing, as disassembly might once prove awkward otherwise.

mike_s

It depends on the brakes. For example, Shimano BR-CT91s pivot on the bosses. BR-R550s have integral bushes.

cascade168

I have sanded, ScotchBrited, emeryclothed, and filed those posts to remove corrosion, nicks, dings, burrs, etc. As long as they are round and well greased they should work just fine. If, while manipulating the brake arm manually, you feel some resistance, then you need to deal with that. Then grease liberally and enjoy ;-)

dabac

I find that hard to believe, but w/o having one here it's hard to be definite. Unfortunately it's no longer available at Shimano tech docs.
But on this pic it sure looks like it's a bushing in there.

Having the brake arm pivot against the boss is just such an awkard engineering solution. If the arm is too wide, or the boss too short assembly will lock the brake arm in place. If it's the opposite you'll get play, and too much play usually results in brake chatter and squeal.

Apart from that it'd just be amazingly poor engineering practice to have a hugely important piece of machinery upon operation generating an unscrewing torque on a critical retaining bolt. The right side might get away with it, but on the left the only thing keeping the brake arm from unscrewing would be assembly torque.

mike_s

There is a bushing, I never said there wasn't. It's pressed in, and doesn't move. The other bearing surface is the fixed boss.Perhaps you should contact Shimano, and tell them how much more you know than they do.

dabac

Me, and before that, the authors and contributors of the half-dozen or so books on risk, performance and reliability analysis of mechanical systems that are some of the tools of my trade...

maddmaxx 

https://www.paul-lange.de/produkte/sh...91-97.pdf/view

Very often, the paul-lange site can be used to pull up documents on Shimano equipment that are no longer available on the main Shimano web site.

dabac

That site was a good tip!

Although it doesn't help in determining if the brake pivots against the post. Never seen one that does, so I'm still finding it improbable.

maddmaxx 

Yea, I know its still unclear but it is nice to sometimes pull up an old PDF. I would be even nicer if I spoke the language..........

jgedwa

Lots of crappy v-brakes through the years have had no bushings. The brake arm simply turned on the post.

But, the spirit of the original claim in this regard is sill correct. Used with an arm with its own bushing and the problem of the OP is greatly reduced so long as not too much material was removed by the blasting. And, I doubt it was.

Your powdercoater should be severely browbeaten, btw. I would ask for a partial refund.

jim

Joshua A.C. New

This sounds to me like a job for grease. Which it would be, even if the bosses weren't blasted.

dsmyers

Thanks for the comments, everyone. I greased up the posts and did the install, and things seem good. I have a set of (very bling) Velo Orange cantis which are somewhat old-school (wide profile for road levers, which is one of the reasons I like them) and do appear to pivot directly on the post.

I have the cable yoke slightly off-center on the straddle cable (it has set screws that let you do this, as the brakes themselves do not have a spring tension adjustment) to get the pads to clear the rim evenly, but with this done, the brakes pivot acceptably, so I guess things are OK. Per one of the above poster's comments re: unscrewing torque, I think I'll loc-tite the mounting bolts...

EDIT: actually, I'm not sure they do pivot directly. I wasn't able to rotate any of the hardware that fits over the boss holding the brake in my hand, but...
Daniel