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Old 01-03-14, 06:59 AM   #1
MEversbergII
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Rear wheel flex

So I was swapping out my (rear) flat and redoing my brakes last night and I noticed that when I apply the brake, the wheel flexes a bit. I don't know if that's always been the case and I've just never noticed, or if it's a recent thing. Something to be wary of?

M.
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Old 01-03-14, 07:09 AM   #2
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Check the spokes with your hands to make sure they all are tight. You are so close to a really good shop with Pax Velo, I would just take the bike over there and ask them for a quick look. If the wind isn't blowing, you can even ride there
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Old 01-03-14, 07:49 AM   #3
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Check to be sure your wheel is completely seated in the dropouts.

My guess is it isn't and your brake is pulling the rim to one side. If that's not the case, your brake may need to be centered. I doubt it's a problem with the wheel itself.
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Old 01-03-14, 07:57 AM   #4
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NBD I bet. I assume you've ensured there's no play in the hub by wiggling the rim.

I'm tipping it's a road bike with a dual-pivot brake. The design of these means they can easily flex the wheel to one side if they're not centred properly.

Simply loosen the mounting bolt, apply the brake and tighten the bolt.

Or just leave it less than super tight, so you can centre it by hand; apply the brake and watch for one pad to hit first, adjust and repeat.

Or, you could use that dinky little 'centering screw' which IMO is completely superfluous on such an easily-centred brake design, unless you want to employ its true function which is to make the brake look all goofy when you wind it out.
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Old 01-03-14, 09:12 AM   #5
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Or, you could use that dinky little 'centering screw' which IMO is completely superfluous on such an easily-centred brake design, unless you want to employ its true function which is to make the brake look all goofy when you wind it out.
+1 I also never saw any need for those "centering screws" on any dual pivot design.
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Old 01-03-14, 09:30 AM   #6
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+1 I also never saw any need for those "centering screws" on any dual pivot design.
They're not necessary, but they do make it easier to fine tune the centering once it's mounted pretty close to perfect.
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Old 01-03-14, 11:33 AM   #7
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Check the spokes with your hands to make sure they all are tight. You are so close to a really good shop with Pax Velo, I would just take the bike over there and ask them for a quick look. If the wind isn't blowing, you can even ride there
Pax Velo has a shop? Or are you talking about Mike's?

Good to see another person from (or around) the area!

These are V-brakes. I swapped on a spare rear wheel (which has made me realize just how bad I let the bearings get on my other one...this is my workhorse bike...) and it also flexes. Thinking maybe I borked up something putting it in the dropouts. Rode in fine, though, so I'll check when I get home. While redoing the brakes I also discovered it's time to regrease the bearing nubs.

Busy bike weekend ahead...good.

M.

EDIT: While we're on the subject of centering screws, how do I 'reset' those v-brake centering screws that are on each pivot? They've managed to find themselves in interesting positions.
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Old 01-03-14, 12:11 PM   #8
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They're not necessary, but they do make it easier to fine tune the centering once it's mounted pretty close to perfect.
I think the point Kimmo and I are making is that they aren't needed even for that unless you insist on having the centering within 0.01 mm. Hand centering the calipers can be done more than accurately enough without them.
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Old 01-03-14, 12:24 PM   #9
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I think the point Kimmo and I are making is that they aren't needed even for that unless you insist on having the centering within 0.01 mm. Hand centering the calipers can be done more than accurately enough without them.
This is BF --- are you sure that centering to 0.01mm is good enough?

I don't think any of my bikes which have given me 10s of thousands of trouble free miles would pass muster with this crowd.
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Old 01-03-14, 01:00 PM   #10
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Originally Posted by MEversbergII View Post
Pax Velo has a shop? Or are you talking about Mike's?

Good to see another person from (or around) the area!

These are V-brakes. I swapped on a spare rear wheel (which has made me realize just how bad I let the bearings get on my other one...this is my workhorse bike...) and it also flexes. Thinking maybe I borked up something putting it in the dropouts. Rode in fine, though, so I'll check when I get home. While redoing the brakes I also discovered it's time to regrease the bearing nubs.

Busy bike weekend ahead...good.

M.

EDIT: While we're on the subject of centering screws, how do I 'reset' those v-brake centering screws that are on each pivot? They've managed to find themselves in interesting positions.
I'd try backing off the screw on the side with the widest gap between pad & rim.
EDIT: reverse the above to the narrowest gap.

A LITTLE tweek goes a long way.
When doing little adjustments like this (such as RDER's) I like to adjust a bit, ride the bike and "exercise" the "system" I'm tuning a few times and then recheck.
IOW, tweek a bit and then go and actually use the brakes a few times hard.

Last edited by Bill Kapaun; 01-03-14 at 01:26 PM.
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Old 01-03-14, 01:15 PM   #11
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Originally Posted by MEversbergII View Post
These are V-brakes..
V-brakes, ----This changes everything.

While V-brakes have spring adjustment screws to center when open, the forces when closed depend of simple geometry.

A quick physics lesson---
Each arm is a simple lever. Newton's third law dictates that the force applied at the end of the lever is equal. So the force at the brake shoe is determined by it's distance from the fulcrum (aka pivot bolt). If either shoe is lower than it's mate it will push slightly harder moving the rim over until the wheel's stiffness resists.

Minor flexing is normal and acceptable, because bicycles don't depend on being perfect to work. But if it bothers you get raise or lower one shoe to balance the braking force.
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Old 01-03-14, 05:11 PM   #12
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Originally Posted by FBinNY View Post
V-brakes, ----This changes everything.

While V-brakes have spring adjustment screws to center when open, the forces when closed depend of simple geometry.

A quick physics lesson---
Each arm is a simple lever. Newton's third law dictates that the force applied at the end of the lever is equal. So the force at the brake shoe is determined by it's distance from the fulcrum (aka pivot bolt). If either shoe is lower than it's mate it will push slightly harder moving the rim over until the wheel's stiffness resists.

Minor flexing is normal and acceptable, because bicycles don't depend on being perfect to work. But if it bothers you get raise or lower one shoe to balance the braking force.
Ah, it all makes sense now!

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Old 01-03-14, 07:53 PM   #13
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I'm actually a bit surprised that V-brakes are capable of pushing a rim across; this would be due entirely to the difference in return spring strength between the two arms if nothing else is wrong...

Sticky pivot maybe?
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Old 01-04-14, 10:50 AM   #14
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1st pad touches the rim , but only drags, until 2nd pad reaches the rim, and for brakeing,
you continue the pressure, to force the 2 together.
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Old 01-04-14, 10:54 AM   #15
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. . . the wheel flexes a bit. . .
How much?
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Old 01-04-14, 02:34 PM   #16
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I didn't read everyone's posts but this would be my approach:

- Check wheel is seated in dropouts.
- Install wheel backward to check for proper dish.
- If that checks out, center the V-brake using the set screws, checking for sticking pivots.

Quote:
Originally Posted by FBinNY View Post
They're not necessary, but they do make it easier to fine tune the centering once it's mounted pretty close to perfect.
Agreed. I like the centering screw.
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Old 01-05-14, 11:53 PM   #17
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1st pad touches the rim , but only drags, until 2nd pad reaches the rim, and for brakeing,
you continue the pressure, to force the 2 together.
Yeah, this is why I'm a bit surprised; seems a bit weird.
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Old 01-06-14, 12:17 AM   #18
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Pax Velo has a shop? Or are you talking about Mike's?
Sorry, I didn't mean Pax Velo. I meant Pax Adventure Center - Dave's.

http://www.paxadventure.com/
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Old 01-06-14, 02:05 PM   #19
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Oh, I can't cross the bridge cycling unfortunately. I do have Mike's on my side, though!

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