[Closed] What makes a rim braked wheel "pulse" when braking❓

Loose headset?

No, it's definitely the wheel, seems like somethings grabby in terms of the rim surface. No issues with the old wheel. I've cleaned the pads with alcohol. Even if I push the bike along when braking it grabs at a certain point of the wheels revolution, nowt obvious.

🤔

Rim joint, worn rim, rim surface

The rim joints smooth, it's not "knocking" more like the friction of braking varies at a certain point of the rim. Rims not had much wear. Rim surface seems ok. Newish pads worked fine on the old rim, I suppose I could chuck some new one in and see if anything changes [pops out to garage]...

Is the tyre on properly? I've seen brakes slightly off adjustment where the tyre rubs on the brake blocks.

Yea, tyres on properly.

New pads doesn't help, it's like the rims "gripping" at a certain section, more than the rest of it.

I've had the same issue. Rim true, rim clean, pads correctly aligned. Can only think there might be some residue absorbed into the braking surface although that seems unlikely.

Possibly tiny ding on rim ?

there might be some residue absorbed into the braking surface

That's what it feels like, but it can't be, can it....??? 🤔🤔🤔

Look at your brake track, is it concave or have circumferential cracks? Once a rim gets seriously worn it flexes under braking and can fail suddenly and catastrophically as the pressure of the tyre bead forces the rim apart.

No, not that, I've brought this wheel to replace one that's concave. This one's got plenty of life in it.

I think I might try mud on it.

Two things I have found:

One is oil or contamination on the rim.
Also i have had the rim walls bulging slightly on part of the wheel. This was probably due to overinflation of a fairly worn rim. It was almost imperceptible when feeling it with my fingers but it made a difference to the braking and slowly got worse until the rim cracked. A friend had the same which caused the wheel to lock up on skidded road surfaces.

I have seen a customers rim that varied in width, it pulsed under braking as the wide point went through the brake pads, there was a measurable difference using a pair of calipers.

Is the rim concentric on the hub?

I have seen a customers rim that varied in width, it pulsed under braking as the wide point went through the brake pads, there was a measurable difference using a pair of calipers.

Cheers, I'll check for this tonight.

Further to the point raised by Bigyan - is it a low spoke count wheel? There can be a difference in width between where the spokes are and are not which might influence this.

Halo Aerotrack 32hole radially laced.

The more I think about it, I think that bigyans comment sits best with how it feels. Could have been dropped by the courier and it's splayed the rim. I'll check it with a gauge tonight.

My money would still be on the join. If there's a bit with unworn anodising then it must be grabbing that bit less than the rest of the rim.

This time of year i find the tracks get gummed up with all sorts of crap. Every few weeks i take the wheels out and give them a thorough scrub with a bit of neat muckoff untill the cloth comes away clean then wipe with IPA/disk&clutch cleaner. Then sand the brake blocks down a fraction to bring up some clean rubber. Seems to bring the brakes back to life. Might be worth trying that if you think there's something contaminating the rim.

My money is also on the join. Can be hard to spot but ther will be a tiny imperfection In width.

I've had this or similar before. My rim had recessed wear track along it and the pads had worn so they had a line that ran in the track. This maybe made the pads drag when the wheel deflected or was not perfectly true. Fixed it by filing said track off of the brake blocks so they were flat.

Your bike doesn’t love you anymore. The only solution is a new bike (with disc brakes).

rim joint is ok but has a bit of unworn off anodising on it still

Is it pulsing around that area? Could be that the anodizing grabs more or less depending on conditions.

Would also check for dings/splaying of the rim.

some residue absorbed into the braking surface although that seems unlikely

Something like this happens on automotive brakes so it could be your problem.

It's not absorbed as such but you MAY have a small area of the wheel where you have a thin, invisible layer of brake pad material stuck to the rim. Heat and pressure cause the pad material to stick to the rim. Then when that area passes through the caliper you get an increase in friction.

I'd find where it is on your wheel and then gently rub it down a little with fine abrasive paper and some kind of cleaner - maybe automotive brake cleaner if you have some.

Check the tyre pressure and make sure it's below the manufacturers recommended max.

Just an idea but could you take tyre off and then have a vernier caliper set to just clear the rim at a 'good' point and then run it round until you can find where it's 'out'. If you've not got a caliper you could rig something up I should think.

It's a phenomenon I've noticed on pretty much every rim-brake-equipped bike I've owned. I guess it's down to minor inconsistencies in either the straightness, width or friction of the rim. Such inconsistencies perhaps then get amplified due to differential wear patterns. Sometimes you can feel the rim join through the brakes but that's a different phenomenon (a 'tick' rather than a pulse). I've never really though about it or felt the need to do anything about it, though!