strange problem with rim and brakes



G

Gary Smiley

Guest
I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and v-brakes
with pink Kool-Stop pads. When I apply the brakes, they grip harder every
time the pads go by the portion of the rim where the valve is. So when I
break, the bike goes thunk, thunk thunk each time the wheel goes around
until I come to a jarring stop. It's very annoying. So I figured the
obvious- that there must be a bulge in the rim where the valve is. I put it
on a trueing stand. I checked and double-checked. There is no bulge- the
wheel is true. It's as if the metal near the valve has a different
composition or something, so that the brakes grab it more in that spot- but
there is no bulge. Has anybody experienced anything similar?
- Gary
 
Gary Smiley wrote:
> I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and
> v-brakes with pink Kool-Stop pads. When I apply the brakes, they grip
> harder every time the pads go by the portion of the rim where the
> valve is. So when I break, the bike goes thunk, thunk thunk each time
> the wheel goes around until I come to a jarring stop. It's very
> annoying. So I figured the obvious- that there must be a bulge in the
> rim where the valve is. I put it on a trueing stand. I checked and
> double-checked. There is no bulge- the wheel is true. It's as if the
> metal near the valve has a different composition or something, so
> that the brakes grab it more in that spot- but there is no bulge. Has
> anybody experienced anything similar? - Gary


Yup. If it were welded, then machined, it wouldn't have this problem!!!

*duck and run*

--
Phil Lee, Squid
 
"Gary Smiley" <[email protected]> wrote in message
news:[email protected]...
>I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and v-brakes
>with pink Kool-Stop pads. When I apply the brakes, they grip harder every
>time the pads go by the portion of the rim where the valve is. So when I
>break, the bike goes thunk, thunk thunk each time the wheel goes around
>until I come to a jarring stop. It's very annoying. So I figured the
>obvious- that there must be a bulge in the rim where the valve is. I put it
>on a trueing stand. I checked and double-checked. There is no bulge- the
>wheel is true. It's as if the metal near the valve has a different
>composition or something, so that the brakes grab it more in that spot- but
>there is no bulge. Has anybody experienced anything similar?
> - Gary
>


Are you sure the brake pads are clearing the tire completely? Maybe the
wheel is out of round enough that pads hit the tire in just one spot. Or
the grabbing could be a bulging tire. Or pads that are just a bit too high
that are missing the rim and making contact with the tire. . . or pads that
are positioned too high and have now worn over the edge of the rim causing
them to rub on the tire. Maybe replacing the pads, checking that the wheel
is round, and/or repositioning the pads to insure they do not hit the tire
anywhere around the circumference of the wheel could help. . . .
 
Phil Lee, Squid wrote:
> Gary Smiley wrote:
>> I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and
>> v-brakes with pink Kool-Stop pads. When I apply the brakes, they grip
>> harder every time the pads go by the portion of the rim where the
>> valve is.


> Yup. If it were welded, then machined, it wouldn't have this problem!!!


He said the problem was by the valve. The rim seam is on the opposite
side of the wheel. I doubt that welding would have any effect on his
problem.

--
Dave
dvt at psu dot edu

Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)
 
I already checked for all those things. I readjusted the brakes. I checked
for bulges. The wheel is round and true. I just can't figure it out.
It's as if the rim has more "grip" near the valve.


"Wheels by BFWG" <[email protected]> wrote in message
news:[email protected]...
>
> Are you sure the brake pads are clearing the tire completely? Maybe the
> wheel is out of round enough that pads hit the tire in just one spot. Or
> the grabbing could be a bulging tire. Or pads that are just a bit too
> high that are missing the rim and making contact with the tire. . . or
> pads that are positioned too high and have now worn over the edge of the
> rim causing them to rub on the tire. Maybe replacing the pads, checking
> that the wheel is round, and/or repositioning the pads to insure they do
> not hit the tire anywhere around the circumference of the wheel could
> help. . . .
>
 
Gary Smiley wrote:
> I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and v-brakes
> with pink Kool-Stop pads. When I apply the brakes, they grip harder every
> time the pads go by the portion of the rim where the valve is. So when I
> break, the bike goes thunk, thunk thunk each time the wheel goes around
> until I come to a jarring stop. It's very annoying. So I figured the
> obvious- that there must be a bulge in the rim where the valve is. I put it
> on a trueing stand. I checked and double-checked. There is no bulge- the
> wheel is true. It's as if the metal near the valve has a different
> composition or something, so that the brakes grab it more in that spot- but
> there is no bulge. Has anybody experienced anything similar?
> - Gary


Dear Gary,

The grabby section near the valve may just be less worn than the rest
of the rim.

A badly worn rim can still look good on a truing stand.

But under braking, the worn and weakened sections of the rim bend
inward between the pads. If there's a rim section that isn't weakened,
it's stiffer and shows up as a "grabby" spot.

Cheers,

Carl Fogel
 
Well, the bike is about 3 years old, and has around 10K - 12K miles on it.
Maybe it's time for a new rim. How can I tell for sure that it's worn?

<[email protected]> wrote in message
news:[email protected]...
> Dear Gary,
>
> The grabby section near the valve may just be less worn than the rest
> of the rim.
>
> A badly worn rim can still look good on a truing stand.
>
> But under braking, the worn and weakened sections of the rim bend
> inward between the pads. If there's a rim section that isn't weakened,
> it's stiffer and shows up as a "grabby" spot.
>
> Cheers,
>
> Carl Fogel
>
 
On Thu, 27 Jul 2006 21:43:27 -0400, "Gary Smiley"
<[email protected]> wrote:

>Well, the bike is about 3 years old, and has around 10K - 12K miles on it.
>Maybe it's time for a new rim. How can I tell for sure that it's worn?
>
><[email protected]> wrote in message
>news:[email protected]...
>> Dear Gary,
>>
>> The grabby section near the valve may just be less worn than the rest
>> of the rim.
>>
>> A badly worn rim can still look good on a truing stand.
>>
>> But under braking, the worn and weakened sections of the rim bend
>> inward between the pads. If there's a rim section that isn't weakened,
>> it's stiffer and shows up as a "grabby" spot.
>>
>> Cheers,
>>
>> Carl Fogel


Dear Gary,

If I understand your posts, the wheel is true horizontally (no
noticeable side to side wobble) and also true vertically (no
noticeable hop), but it grabs noticeably every time the valve area
reaches the brake pads.

You might inflate the tire fully, put it in the truing stand, and try
measuring the rim width at a couple of points with dial calipers.

But this may not show what happens when the squeeze of the brake pads
fights the expanding force of the tire pressure.

Many rims include a wear indicator, but yours may not be placed where
the problem is.

The reason to worry about the rim wearing out is that it can fail
catastrophically, like David Van Tol's rim:

http://home.comcast.net/~carlfogel/download/rim.jpg

You wouldn't want to be riding when that happened.

Here's a picture of a smaller failure on the same rim that gives an
exaggerated illustration of how the brake might get grabby at one
section:

http://home.comcast.net/~carlfogel/download/small_crack.JPG

To reassure yourself, you might try borrowing a rear wheel that will
fit, leave the chain off, and see if the different wheel grabs the
same way when you push along the street in front of your house.

The different wheel isn't likely to grab, but if it does, then your
brakes have a very weird problem.

If the different wheel brakes without grabbiness, then you get to
decide whether the wheel is even worth worrying about. You've gotten a
few years out of it, but the rim may be worn or the wheel may have
some other strange problem. You could fight it, but a new wheel would
solve the problem and save you the trouble of posting pictures like
the ones above.

I wish that I could suggest a better test, but most of what I've seen
is just people saying that they can tell by eye and feel and so forth
that a rim is bad:

"I have eyes and hands that can see and feel a worn rim and am able to
gauge with good accuracy when a rim should be replaced."
--Jobst Brandt
http://groups.google.com/group/rec....?lnk=gst&q="worn+rim"&rnum=3#256409091ab97593
or http://tinyurl.com/zbahl

Cheers,

Carl Fogel
 
Gary Smiley wrote:
> Well, the bike is about 3 years old, and has around 10K - 12K miles on it.
> Maybe it's time for a new rim. How can I tell for sure that it's worn?
>


Usually by the depth of the concave wear region. A typical rim wall
thickness on the brake track is 1.5mm new. You want to replace them when
the thickness is down to 0.5mm or before. 1mm is not a lot, you can
eyeball this with a straightedge along the rim if you don't have a
caliper. You may also see the beginnings of circumferential cracks along
the brake track which may look like scratches at first glance.

10-12K miles would not be too little to have worn out a rim,
particularly if you ride in the rain -- it's the grit picked up by water
that wears a rim out.

Swapping a rim isn't particularly hard if you can find one with the same
"effective" diameter. Just tape the new to the old & transfer the spokes
one at a time.
 
You're probably right- I've gotten more miles from rims before, but I use
this one for city commuting, winter, summer, rain, snow, slush, sand, grit,
and I use the brakes a lot. Even if the rim isn't totally gone, nobody in
this thread has come up with a way to solve the original problem anyway. So
I guess it's time for a new rim. I'll try to get the same model, so I can
save the spokes.
BTW It was a great wheel- In the three years I've had it, it never needed
trueing, and it never broke a spoke, and I weigh over 200 pounds.
I'm still using it - I just don't like the way it grabs the brake pads at
the valve.

"Peter Cole" <[email protected]> wrote in message
news:[email protected]...
>>

> 10-12K miles would not be too little to have worn out a rim, particularly
> if you ride in the rain -- it's the grit picked up by water that wears a
> rim out.
 
Peter Cole <[email protected]> wrote in
news:[email protected]:

<SNIP>
>
> Swapping a rim isn't particularly hard if you can find one with the
> same "effective" diameter. Just tape the new to the old & transfer the
> spokes one at a time.


Same ERD is important, but also make sure it's the correct "handedness" -
that the spoke holes are offset on the same side of the rim as the old one,
or else you'll wind up with crossed spokes over the valve hole.

Going with the same model rim will sidestep this problem, unless the rim
manufacture is a real bunch of bastards.
 
Gary Smiley wrote:
> You're probably right- I've gotten more miles from rims before, but I use
> this one for city commuting, winter, summer, rain, snow, slush, sand, grit,
> and I use the brakes a lot. Even if the rim isn't totally gone, nobody in
> this thread has come up with a way to solve the original problem anyway. So
> I guess it's time for a new rim. I'll try to get the same model, so I can
> save the spokes.
> BTW It was a great wheel- In the three years I've had it, it never needed
> trueing, and it never broke a spoke, and I weigh over 200 pounds.
> I'm still using it - I just don't like the way it grabs the brake pads at
> the valve.


1. Has it always done that? Even when brand-new?

2. How often do you pump the tire?

Maybe the rim has a patina and you've buffed that spot with your hand
every time you put the pump on so it never developed the patina. Try
scrubbing the whole rim with a degreaser (take the rubber off first)
and see if it doesn't even-out the friction.

--Blair
 
Gary Smiley wrote:
> I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and v-brakes
> with pink Kool-Stop pads. When I apply the brakes, they grip harder every
> time the pads go by the portion of the rim where the valve is. So when I
> break, the bike goes thunk, thunk thunk each time the wheel goes around
> until I come to a jarring stop. It's very annoying. So I figured the
> obvious- that there must be a bulge in the rim where the valve is. I put it
> on a trueing stand. I checked and double-checked. There is no bulge- the
> wheel is true. It's as if the metal near the valve has a different
> composition or something, so that the brakes grab it more in that spot- but
> there is no bulge. Has anybody experienced anything similar?
> - Gary


It's still possible that there is a minor bulge at that spot. You may
not be able to judge with a trueing stand, especially if it is the kind
with a reference only on one side. Try some calipers. It doesn't take
much of a bulge to have a noticeable effect on braking.

Another strong possibility is that there is some surface contamination
and I would rule that out first. Try giving the entire braking surface
a light sanding. The stuff that is sold for use with PVC pipe is about
the right roughness. Just clean the whole braking surface to shiny
metal, then give it a good washing. As a side benefit, it tends to get
rid of any squealing you might have, at least for a little while.
While you're at it, sand the brake pads to fresh rubber, or take the
opportunity to replace them if they are old and/or hardened. This all
should take care of any contamination problem.
 
> Gary Smiley wrote:
>> I have a '04 Trek 520 with a Bontrager Fairlane A-Sym rear rim and v-brakes
>> with pink Kool-Stop pads. When I apply the brakes, they grip harder every
>> time the pads go by the portion of the rim where the valve is. So when I
>> break, the bike goes thunk, thunk thunk each time the wheel goes around
>> until I come to a jarring stop. It's very annoying. So I figured the
>> obvious- that there must be a bulge in the rim where the valve is. I put it
>> on a trueing stand. I checked and double-checked. There is no bulge- the
>> wheel is true. It's as if the metal near the valve has a different
>> composition or something, so that the brakes grab it more in that spot- but
>> there is no bulge. Has anybody experienced anything similar?


>

[email protected] wrote:
> It's still possible that there is a minor bulge at that spot. You may
> not be able to judge with a trueing stand, especially if it is the kind
> with a reference only on one side. Try some calipers. It doesn't take
> much of a bulge to have a noticeable effect on braking.
>
> Another strong possibility is that there is some surface contamination
> and I would rule that out first. Try giving the entire braking surface
> a light sanding. The stuff that is sold for use with PVC pipe is about
> the right roughness. Just clean the whole braking surface to shiny
> metal, then give it a good washing. As a side benefit, it tends to get
> rid of any squealing you might have, at least for a little while.
> While you're at it, sand the brake pads to fresh rubber, or take the
> opportunity to replace them if they are old and/or hardened. This all
> should take care of any contamination problem.


I'm sorry if this doesn't apply to you; my intent is not to offend.

Inexperienced riders sometimes yank on the valve during installation
leaving the tube pinched under the tire at the valve. That will cause a
bulging tire which can be cut in short order by a brake shoe.

Since you seem to have ruled out a rim problem, remount the tire and
ensure the raised line molded just above the braking surface is visible
evenly all around the rim.


--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971