Throbbing front brake unsafe?

yes, very very unsafe. you need to fix it asap, and you also need to consider entirely replacing the
front brake caliper.

technology.open.ac.ukmem ccf2.htmlOpen ↗

this is exactly the scenario you are describing. if this has been going on for some time, you could
have initiated a crack in the mounting bolt that is just waiting for a dangerous place to fail and
severely injure you.

ensure you use a wheel whose rim has a machined braking surface to eliminate this problem. and when
you do, you'll still need to replace the caliper! fatigue cracks don't go away on their own.

jb

"jim beam" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

yes, very very unsafe. you need to fix it asap, and you also need to consider entirely replacing
the front brake caliper.

technology.open.ac.ukmem ccf2.htmlOpen ↗

this is exactly the scenario you are describing. if this has been going on for some time, you
could have initiated a crack in the mounting bolt that is just waiting for a dangerous place to
fail and severely injure you.

ensure you use a wheel whose rim has a machined braking surface to eliminate this problem. and
when you do, you'll still need to replace the caliper! fatigue cracks don't go away on their own.

If machining solves the problem of uneven rim surfaces, then it does so at the expense of creating a
non-uniform wall thickness. In this case, it's difficult to know when to retire a rim. A cracking
rim can have as bad consequences as a cracking brake/brake bolt.

Jeff Potter said:

For the past couple years my frontbrake on roadbike hasn't braked smoothly. It's kinda smooth but
has a throb to it. I'm guessing that there are bumps on my rim.

Using the brake pads as a guide, spin the wheel and see if the rim moves closer to the pad at some
point. The wheel may be out of true, or the rim may have bottomed out on a pothole and spread out at
the point of impact. Also, make sure the rim is free of gunk.

See:

sheldonbrown.comcreaks.htmlOpen ↗

Art Harris

Quoted message said:

non-uniform wall thickness

that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation of
"Y", machining off a fraction subtracts from the X and half the Y because you're machining just one
of the two faces, the inner and the outer side of the braking surface. so machining makes
dimensional accuracy better, not worse.

and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
slightly thicker in the places that are going to be machined.

jb

In article <[email hidden]>, Jeff Potter

Quoted message said:

For the past couple years my frontbrake on roadbike hasn't braked smoothly. It's kinda smooth but
has a throb to it. I'm guessing that there are bumps on my rim. It kind of chatters sometime but
not with much noise. If I look down I'll see the fork ends vibrating. When I went to the mountains
this summer I had to do lots of braking. When I ride around here in the wide open flats I don't
brake much. But this summer I did and it made me wonder if something bad was going to happen. I do
keep the brake bolt tight---if it gets loose this throb of course gets much worse.

Assuming the caliper pivot is properly adjusted, you should be able to locate the portions of the
rim's braking surface that are causing the brake pad to grab and sand them with super fine
sandpaper. You can also reduce the grabbing effect a little by using a harder brake pad, assuming
you are happy with the braking performance of a harder pad.

jim beam <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Quoted message said:

non-uniform wall thickness

that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
one of the two faces, the inner and the outer side of the braking surface. so machining makes
dimensional accuracy better, not worse.

and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
slightly thicker in the places that are going to be machined.

jb

However, illogical as it may be, the machined rims I've measured have greater variation in their
wall thickness than unmachined rims. My opinion is that the rims aren't always fixtured perfectly.

Thanks for the tips everyone. --The rim sanding, esp. I will try it and be judicious.

Should I be able to see hairline cracks? Do cracks have to be visible to be dangerous?

Oops, now I see you mean a crack in the bolt, not in the fork crown hole that the bolt goes
thru. Hmmm...

--

Jeff Potter
****
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Jeff Potter said:

Thanks for the tips everyone. --The rim sanding, esp. I will try it and be judicious.

Should I be able to see hairline cracks? Do cracks have to be visible to be dangerous?

Oops, now I see you mean a crack in the bolt, not in the fork crown hole that the bolt goes
thru. Hmmm...

Maybe just borrow another front wheel for a short ride and see if it is the rim or the brakes.

jim beam said:

Quoted message said:

non-uniform wall thickness

that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
one of the two faces, the inner and the outer side of the braking surface. so machining makes
dimensional accuracy better, not worse.

You, of course, are assuming that the rim is perfectly straight. If the outside and inside faces
both wobble in the same direction, then you remove material from one surface, then where that
surface stuck out, you now have a thinner wall.

Of course, if your rim does that, you've got major problems anyway...at least, if it does it a lot.

Quoted message said:

and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
slightly thicker in the places that are going to be machined.

I thought the suggestion was to machine an existing, non-machined rim?

Quoted message said:

jb

--
Rick Onanian

Quoted message said:

However, illogical as it may be, the machined rims I've measured have greater variation in their
wall thickness than unmachined rims. My opinion is that the rims aren't always fixtured perfectly.

yes, your milage my vary. all i can say is that measuring two of the unbuilt rims, cxp14
[unmachined] and ma3 [machined] i have in the bike closet right now, i get the following, in mm:

ma3 cxp14
21.10 19.88
22.10 19.92
23.10 19.90
24.10* 19.66*

* at the rim joint.

this is measuring at 90 degree intervals. this is /not/ a wall thickness measurement because i don't
have a suitably anvilled micrometer to measure under the hook right now, but i would expect wall
thickness to be more consistent on the ma3 based on the above.

"jim beam" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Quoted message said:

non-uniform wall thickness

that's not logical. if you have an unmachined rim with wall thickness "X" and thickness deviation
of "Y", machining off a fraction subtracts from the X and half the Y because you're machining just
one of the two faces, the inner and the outer side of the braking surface. so machining makes
dimensional accuracy better, not worse.

and you don't end up with a "thinner" rim because it's extruded with machining in mind and is made
slightly thicker in the places that are going to be machined.

Depends on how the rim is machined. There are obvious (and likely) ways it can be done to yield
non-uniform wall thickness (thin spots).

The rim I just retired, a Mavic Reflex "SUP", "UB-control", had about 1 mm of brake surface wear.
There were clear signs of cracking (circumferential) at the joint. This was the only place cracking
was evident. There are also clear signs of grinding on the inside surface and some degree of
misalignment of the joint. I don't think the cracking at that exact spot is a coincidence. Any
misalignment that's "squared up" with a post-weld machining is likely to leave a thin spot. After
all, it's a subtractive process, isn't it? If the joint were not welded and machined, any
misalignment would stand out, and the buyer could reject the rim. This process allows sloppy
fabrication with a cosmetic dressing up to hide it. What you see is not what you get. Despite my
displeasure with this, I had to fork out another $60 for a nearly identical new rim. Try to find any
not made this way any more.