Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 11-06-08, 02:28 PM   #1
dsmyers
Senior Member
Thread Starter
 
Join Date: Jun 2006
Bikes:
Posts: 97
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Sandblasted canti posts

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
dsmyers is offline   Reply With Quote
Old 11-06-08, 02:50 PM   #2
dsellinger
slowest!
 
dsellinger's Avatar
 
Join Date: Oct 2008
Location: NW WA
Bikes: Felt, Atala, Kona
Posts: 628
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
You could just get some new bosses they are pretty cheap.
http://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)
dsellinger is offline   Reply With Quote
Old 11-06-08, 03:01 PM   #3
dabac
Senior Member
 
Join Date: Mar 2008
Bikes:
Posts: 5,409
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 20 Post(s)
Quote:
Originally Posted by dsmyers View Post
So, I took my frame to be powdercoated, and the powdercoaters sandblasted the cantilever brake posts while stripping the frame. I'm wondering if I should do something to polish them back to a smoother finish,
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.
dabac is offline   Reply With Quote
Old 11-06-08, 06:36 PM   #4
mike_s
Senior Member
 
Join Date: Sep 2008
Bikes:
Posts: 201
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by dabac View Post
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.
It depends on the brakes. For example, Shimano BR-CT91s pivot on the bosses. BR-R550s have integral bushes.
mike_s is offline   Reply With Quote
Old 11-06-08, 07:39 PM   #5
cascade168
Klaatu barada nikto
 
cascade168's Avatar
 
Join Date: Sep 2004
Location: Southern NH
Bikes:
Posts: 1,453
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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 ;-)
cascade168 is offline   Reply With Quote
Old 11-07-08, 05:19 AM   #6
dabac
Senior Member
 
Join Date: Mar 2008
Bikes:
Posts: 5,409
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 20 Post(s)
Quote:
Originally Posted by mike_s View Post
It depends on the brakes. For example, Shimano BR-CT91s pivot on the bosses.
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.
dabac is offline   Reply With Quote
Old 11-07-08, 05:47 AM   #7
mike_s
Senior Member
 
Join Date: Sep 2008
Bikes:
Posts: 201
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by dabac View Post
But on this pic it sure looks like it's a bushing in there.
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.
Quote:
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.
Perhaps you should contact Shimano, and tell them how much more you know than they do.
mike_s is offline   Reply With Quote
Old 11-07-08, 06:01 AM   #8
dabac
Senior Member
 
Join Date: Mar 2008
Bikes:
Posts: 5,409
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 20 Post(s)
Quote:
Originally Posted by mike_s View Post
Perhaps you should contact Shimano, and tell them how much more you know than they do.
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...
dabac is offline   Reply With Quote
Old 11-07-08, 06:19 AM   #9
maddmaxx 
Small Member
 
maddmaxx's Avatar
 
Join Date: Nov 2003
Bikes: Leader home built hardtail, Diamondback Response
Posts: 7,136
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 148 Post(s)
http://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.
__________________
We are an empire. Use it wisely.
maddmaxx is offline   Reply With Quote
Old 11-07-08, 06:51 AM   #10
dabac
Senior Member
 
Join Date: Mar 2008
Bikes:
Posts: 5,409
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 20 Post(s)
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.
dabac is offline   Reply With Quote
Old 11-07-08, 07:25 AM   #11
maddmaxx 
Small Member
 
maddmaxx's Avatar
 
Join Date: Nov 2003
Bikes: Leader home built hardtail, Diamondback Response
Posts: 7,136
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 148 Post(s)
Quote:
Originally Posted by dabac View Post
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.
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..........
__________________
We are an empire. Use it wisely.
maddmaxx is offline   Reply With Quote
Old 11-07-08, 08:01 AM   #12
jgedwa
surly old man
 
jgedwa's Avatar
 
Join Date: Sep 2006
Location: Carlisle, PA
Bikes: IRO Mark V, Karate Monkey half fat, Trek 620 IGH, Cannondale 26/24 MTB, Amp Research B3, and more.
Posts: 3,345
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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
__________________
Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
--------------------------
SB forever
jgedwa is offline   Reply With Quote
Old 11-07-08, 11:06 AM   #13
Joshua A.C. New
Senior Member
 
Joshua A.C. New's Avatar
 
Join Date: Aug 2007
Location: Northampton, MA
Bikes: Iron Monkey: a junkyard steel 26" slick-tired city bike. Grey Fox: A Trek 7x00 frame, painted, with everything built, from spokes up. Jet Jaguar: A 92 Cannondale R900 frame, powder coated matte black with red and aluminum highlights.
Posts: 956
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
This sounds to me like a job for grease. Which it would be, even if the bosses weren't blasted.
Joshua A.C. New is offline   Reply With Quote
Old 11-07-08, 12:11 PM   #14
dsmyers
Senior Member
Thread Starter
 
Join Date: Jun 2006
Bikes:
Posts: 97
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
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

Last edited by dsmyers; 11-07-08 at 12:16 PM.
dsmyers is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 09:01 PM.