What's wrong with my wheel? Thumping feel when braking (Possibly resolved)

[makoti]

I thought it was the rim, but it looks true and is smooth on the brake track. No obvious seam issue. Ok, thought maybe it was my brake pads so I swapped them over to another bike. It was unrideable on that bike. Thumped, shimmied, and grabbed with that brakeset. I don't feel any unusual play in the hub. Front wheel, if that makes any difference.
WI hubs (Campy)
Pacenti rims

[AngryScientist]

Originally Posted by makoti

but it looks true

Do you have access to a truing stand? I think the first step has to be actually checking true/round...

If you were local, I'd be happy to look at it for you.

Edit: A lot of Pacenti rims (though usually pre forza) had cracking issues. Take a close look at all your nipple holes, etc to see if you can spot anything...

[makoti]

Originally Posted by AngryScientist

Do you have access to a truing stand? I think the first step has to be actually checking true/round...

If you were local, I'd be happy to look at it for you.

Edit: A lot of Pacenti rims (though usually pre forza) had cracking issues. Take a close look at all your nipple holes, etc to see if you can spot anything...

Good call. These are Forza, bt I'll take a look at that. I'll toss it in my turning stand, but for this kind of action, it would almost have to be visibly out of true.

[thwart]

In the old days I would wonder if you had some Accelerade dried on the rim.

Do they still sell that stuff?

[raven]

Not 100% sure based on your description, but is there any chance your tire isn't seated perfectly on the rim or has a high spot? If so, it could be the tire hitting under the fork crown.

[chismog]

First thought would be broken or loose spoke, but you said the rim looks true.

Look/feel for a bent spot on the rim, maybe just one side? Sometimes you'll see extra brake pad residue at the problem point on the brake track as well.

[tv_vt]

Originally Posted by raven

Not 100% sure based on your description, but is there any chance your tire isn't seated perfectly on the rim or has a high spot? If so, it could be the tire hitting under the fork crown.

My thought, too.

[Tandem Rider]

A "flat spot" caused by hitting something hard and making a slight dent into the rim will cause the rim to actually be slightly wider at that spot than the rest of the rim. This will cause braking to be unnerving, especially if it's the front wheel. Can you try another front wheel on the bike to test? Often taking off the tire and looking closely at the rim in a truing stand will help you spot the issue.

[makoti]

Originally Posted by raven

Not 100% sure based on your description, but is there any chance your tire isn't seated perfectly on the rim or has a high spot? If so, it could be the tire hitting under the fork crown.

The thing is it rolls very smoothly if the brakes are not applied. I only feel this when I put the brakes on

[velobaba]

I had pretty much the same issue with a rear wheel when brakes were applied. In my case, it turned out to be a very small "dent" on the rim. The "dent" was quite minor and was not easy to see but I was able to feel it with my fingers.

Hope this helps.

Jalal

[osbk67]

Is the thump happening once per wheel rotation?

If so I’d ride it as slowly as I could to try to identify _where _ the rim is thumping.

Or walk push bike under front braking to do so.

Identifying the where should greatly assist the what.

I suspect it’s an imperfect rim joint.

[JMT3]

In days of old sometimes the area where the rim was pinned and or welded would be slightly wider, heat distortion, and create noise when braking. Usually cheaper rims.

[Peter P.]

Originally Posted by Tandem Rider

A "flat spot" caused by hitting something hard and making a slight dent into the rim will cause the rim to actually be slightly wider at that spot than the rest of the rim. This will cause braking to be unnerving, especially if it's the front wheel. Can you try another front wheel on the bike to test? Often taking off the tire and looking closely at the rim in a truing stand will help you spot the issue.

THIS.

Pretend your fingers are a brake caliper and run them around the rim. You might be able to feel the bulged section.

[mass_biker]

Had the same issue after changing a tube.
“Just riding along” and then “thunka, thunka, thunka”.
I pulled over to see if there was a rim wobble (nope) or anything amiss. So I rode somewhat gingerly, and when I got home, I took off the tire again, and sure enough, the rim strip had bunched up (over?) in one spot, so the tube itself wasn’t seated square in the channel.
So, I replaced the rim strip while I was at it, reinstalled tire + tube, aired it up, and no more thumping sound.
Good luck!
MB

[oldpotatoe]

Originally Posted by Tandem Rider

A "flat spot" caused by hitting something hard and making a slight dent into the rim will cause the rim to actually be slightly wider at that spot than the rest of the rim. This will cause braking to be unnerving, especially if it's the front wheel. Can you try another front wheel on the bike to test? Often taking off the tire and looking closely at the rim in a truing stand will help you spot the issue.

OR, if the rim is really old, a portion of the sidewall may be separating/bulging due to the tire pressure.