Author Topic: Rim brake pulsing  (Read 1042 times)

Rim brake pulsing
« on: January 07, 2019, 08:37:55 pm »
Hi all.  Having some trouble with a rim brake that has recently started pulsing, once per revolution.  The obvious cause would be a worn and bulging rim, or grease on the rim.  However, have:
  • Cleaned the rim meticulously four times;
  • Removed tyre several times to check for cracks;
  • Measured the rim thickness accurately with proper rim-checking vernier calliper (1.3 - 1.5 mm at the thinnest);
  • Cleaned and checked the pads.

It is a Brompton, but that is probably not relevant.

Any ideas before I give in and buy a new wheel..?  Thanks!

Re: Rim brake pulsing
« Reply #1 on: January 07, 2019, 08:41:38 pm »
Have you checked to run-out on the wheel when you spin it?
We are making a New World (Paul Nash, 1918)

Re: Rim brake pulsing
« Reply #2 on: January 07, 2019, 08:42:30 pm »
Yes, that is perfect.  Sorry should have mentioned that.  Thanks for the suggestion.

Re: Rim brake pulsing
« Reply #3 on: January 07, 2019, 08:46:31 pm »
It's only just started?  Are the pads positioned correctly, i.e. not grabbing the tyre or diving under the rim slightly?
Never tell me the odds.

Re: Rim brake pulsing
« Reply #4 on: January 07, 2019, 08:50:19 pm »
No, all is good with the pads too.

The tyre had only been inflated to a reasonable 50 - 60 psi too.  Nothing crazy for a Brompton.

I could try the over-pressure test and go for something like 100 psi, deflate and see the effect but I'm not quite ready to go that far yet.

Kim

  • Timelord
Re: Rim brake pulsing
« Reply #5 on: January 07, 2019, 08:51:39 pm »
If a Brompton wheel can't cope with 100PSI I'd have second thoughts about riding on it...
To ride the Windcheetah, first, you must embrace the cantilever...

Re: Rim brake pulsing
« Reply #6 on: January 07, 2019, 09:04:48 pm »
Yeah - what tyre are you using? All of the common Brompton types are rated to 100+ psi.

(and aren't much fun at anything less)

Re: Rim brake pulsing
« Reply #7 on: January 07, 2019, 09:35:26 pm »
The other cause of pulsing brakes I've had is dinged flanges, from hitting a pothole too hard. It is possible to write off a rim on a pothole without getting a snakebite.
I doubt it would be grease etc. That would spread along the rim relatively quickly, reducing the pulsing.

I'd turn the bike over and put a pointer on the fork blade or chainstay, truing stand style.

Re: Rim brake pulsing
« Reply #8 on: January 07, 2019, 09:43:52 pm »
Sometimes minor irregularities can be made worse by slack/flexibility elsewhere.

First thought was head bearings. That seems unlikely, given the long distance (AIUI) between the top & bottom bearings on Brompton, but it's easy to check.

Second thought was brake bearings. I tend to assume that it's the front brake, since I consider back brakes to be secondary on a solo upright bike, but ICBW. I doubt that cable stiction would be a cause, but a failing brake cable wih few strands left might. The last should be easy to check, since cables rarely fail in the hidden parts.

Vernon

  • zzzZZZzzz
Re: Rim brake pulsing
« Reply #9 on: January 07, 2019, 09:53:11 pm »
We had pulsing rim on the tandem that turned out to be a split rim where the spoke eyelet had started pulling through. It was rather difficult to spot, had to look rather carefully even when we got the tyre and rim tape off.

Re: Rim brake pulsing
« Reply #10 on: January 07, 2019, 10:20:04 pm »
Thanks all.  The grease theory seems to be unlikely, especially after all the cleaning I've done.  The headset and brake calliper itself are also fine.  I think I'm going to have to do another examination of the rim, as Vernon says.  I have a truing stand, and it's been checked in there a few times already.   Running the rim through bare hands I get no feeling of any irregularity..  Next, I'll take off the rim take and peek inside the 'well'.  I expect corrosion in there at least.

Re: Rim brake pulsing
« Reply #11 on: January 08, 2019, 01:01:18 am »
if you have Vernier calipers measure the external width of the rim, both with the tyre flat and with it fully inflated.  It is normal for the rim to flare slightly under pressure  but it should do so uniformly;  in each case anything over ~0.1mm variation in width may cause a judder. Wheeling the bike along with the front brake dragging may reveal exactly where the bad spot in the rim really is.

Also check the rim joint; this often gives irregular braking; sometimes just turning the front wheel the other way round can alter it.

cheers

Re: Rim brake pulsing
« Reply #12 on: January 08, 2019, 12:16:48 pm »
What is "pulsing"? Is it the effect experienced when braking on a rim with an irregularity?

If so, can you duplicate it on a stand or with the bike upside down by tightening the brakes until they are almost on the rim, and turning the wheel? Or is it something you can only feel when riding?

If the latter, could you mark the braking surface with something that would show up the irregularity (I've never done this, but maybe tippex would work)? Then, again with the block as close to the rim as possible, have a low speed potter without using the offending brake, and see if the tippex is rubbed off any particular spot of rim.

L'enfer, c'est les autos.

Re: Rim brake pulsing
« Reply #13 on: January 08, 2019, 12:34:51 pm »
The one time I experienced the rim pulsing you describe, this followed very shortly thereafter
  ,

  ,

Re: Rim brake pulsing
« Reply #14 on: January 08, 2019, 10:25:40 pm »
As you suggested Brucey, I have measured the rim width at the middle of the braking surface with about 80 PSI in the tyre, not without tyre, as I did previously.  Taking measurements around the rim, there is a short sector that is 0.27 mm wider than the rest.  This seems to be the problem.

I have already seen that the Genuine Brompton rim is no less than 1.4 mm thick all round the wheel, at the braking surface, which is worn, but nothing outrageous, I'd say.  1 mm would be a rim for the bin, I would say.

So that's all.  Just to buy a new wheel now.  Many thanks.

Re: Rim brake pulsing
« Reply #15 on: January 09, 2019, 03:01:10 am »
glad you have found the trouble, but it is disappointing that the rim flares in such a non-uniform fashion. This isn't uncommon, sadly.

The idea of having machined braking surfaces is that you can have a perfectly uniform braking surface when the rim is new.  This is meant to turn a nearly perfect rim into a perfect rim, if you like. However in practice this approach seems to be used to turn rims that are nowhere near straight into ones of uniform width. You don't need to be a genius to see that this will inevitably result in rims that have much larger variation in wall thickness than you would get with an unmachined rim.

Consequently they quite often become unacceptably uneven before they are completely worn out, e.g. because they flex unevenly when the tyre is inflated.

Also modern rims with machined brake tracks usually have much thinner walls than unmachined rims.  Furthermore the surface of an extrusion often has a fine grained structure and is the most wear resistant part of an extrusion; that is of course the stuff that gets machined off.  The final insult is that if you look at a set of new brake blocks that have been used for a few days on a new (machined) rim, the brake block is almost invariably peppered with bits of swarf that were presumably mashed into the surface of the rim and got pulled out by the brake blocks.

OK, when rims were not routinely machined, one occasionally found one with a bad joint that would be troublesome, but overall, I suspect that we are worse off with machined braking surfaces than we were without.

cheers

Re: Rim brake pulsing
« Reply #16 on: January 09, 2019, 10:10:58 am »
The one time I experienced the rim pulsing you describe, this followed very shortly thereafter
<snip>
I've had that a couple of times, but I stopped and checked when I first noticed the pulsing, at which stage the split in the rim was only ~3 cm long.
I let half the air out (to ~50 psi, on 28 mm) and headed directly for home, avoiding the use of the relevant brake, The first time was in Llandrindod Wells, with the car in Chepstow to get back to (but I got a lift from Llanstephan).

If you don't stop and check the pulsing, it gets worse and fails pretty quickly, as you found.
The tyre pressure pushes the top half of the rim wall outwards, then it gets pushed back in each time it passes the brake blocks. The back and forth wiggle extends the split, and once it gets long enough that the inner tube starts to push through, the rim just tears along the weak line.

Re: Rim brake pulsing
« Reply #17 on: January 09, 2019, 12:21:15 pm »
The one time I experienced the rim pulsing you describe, this followed very shortly thereafter
<snip>
I've had that a couple of times, but I stopped and checked when I first noticed the pulsing, at which stage the split in the rim was only ~3 cm long.
I let half the air out (to ~50 psi, on 28 mm) and headed directly for home, avoiding the use of the relevant brake, The first time was in Llandrindod Wells, with the car in Chepstow to get back to (but I got a lift from Llanstephan).

If you don't stop and check the pulsing, it gets worse and fails pretty quickly, as you found.
The tyre pressure pushes the top half of the rim wall outwards, then it gets pushed back in each time it passes the brake blocks. The back and forth wiggle extends the split, and once it gets long enough that the inner tube starts to push through, the rim just tears along the weak line.

Mine went in the car park of Sainsburys while I was moving at maybe 3 MPH, the noise causing all of the starlings to take off simultaneously, and small children to start crying.

The second one went off in the bike rack at wrok, causing the transport manager, who happened to be walking past, to have a brown trouser moment.

Re: Rim brake pulsing
« Reply #18 on: January 11, 2019, 12:41:56 pm »
... machined braking surfaces ... in practice this approach seems to be used to turn rims that are nowhere near straight into ones of uniform width.
I'd never thought of it this way; clearly this would be a case of customers receiving an inferior product while the manufacturers reject fewer rims from production.