All mechanical components have a manufacturing tolerance, and the drum brakes we use have an out of round tolerance of 0 – 0.06 mm (0.002″). A lower grade of drum brake may have a tolerance of 0 – 0.12 mm (0.005″).
In my experience drums exceeding 0.06 mm out of round will give some degree of pulsing under braking, and those exceeding 0.12 mm can give quite bad pulsing. Some people claim that unequal spoke tension causes the drums to pull out of round, and that the drums can be pulled true by adjusting the spoke tension. I have found the truth of the drum changes very little during the spoking process, thus I believe most of the out of true cases are the result of drums which out of true to start with, and that attempting to true the drums by adjusting the spokes will in most cases be a waste of time, and at the best lead to only a marginal improvement in performance.
So while some people have claimed an improvement in pulsing by adjusting spoke tensions, as far as I can see, this would only rectify a case where there was a case of gross inequality in the spoke tensions, leading to distortion of the drum mouth. The other problem I see with this method of truing the drum, is that if the untruth is a result of manufacturing tolerances, then truing the wheel this way will lead to uneven spoke tensions and shorten the life of the wheel. Furthermore some years ago I had contact with an engineering firm who were developing a Quadicycle using the drum brakes, and they were unhappy with the untruth of the drums, which they traced back to the way in which they were manufactured.
Some time ago I modified some drum brake hubs for Brompton bikes by reducing the width to fit the narrow front forks. However the person to buy the last one I made, Mike Perrin, email@example.com had problems with it pulsing, and I was not prepared to make another one, due to the time it would cost me to modify another drum. Thus I suggested he attempt to grind the drum true in some way. He was ingenious enough to do this in a very simple way, which was successful, and which I would like to share with you.
All Mike did was to wrap a strip of abrasive paper around the brake shoes, and then turn the wheel a few times, slowly adjusting the brakes, until he felt he had got the high spots removed, and the drum true. Here in his own words is his procedure :-
“Since I last contacted you I have been thinking of possible ways of rectifying the vibration caused by the two high spots on the SA X-FD hub brake drum. I was reluctant to use a drum sander for fear of shaky hands making the matter worse, and even contemplated having the drum re-skimmed by a machine shop specialising in this procedure. Before doing this, however, I thought I would try a fairly simple procedure which even if it didn’t work was hardly likely to do any damage. Taking the brake assembly out of the drum, I cut a strip of 180 grade wet and dry abrasive paper 210mm long and 18mm wide, which was then wrapped around the brake shoes and held in place by cutting two centrally located tabs 6mm wide and about 7mm in length one at each end should be folded down 90 degrees.
This is long enough to encircle both brake shoes and with a width of around 18mm, will extend either side of the shoes by about 1mm. When the brake lever on the body is pulled toward you a gap opens between the end of one shoe and the operating cam into which one of the tabs can be located. With the strip pulled taut around the two brake shoes and the brake lever now pushed away from you, the other tab can be similarly inserted on the other side of the cam. See attached photo where the tabs are gripped either side of the cam.
Replacing the brake body in the drum and the wheel on the bike I lightly applied the brake by means of the cable length adjuster until I could feel (and hear) the abrasive strip rubbing on the two tight spots. After turning the wheel for a while in both directions it seemed to be turning freely, so I continued with the procedure shortening the cable two or three more times until I decided to remove the strip and test it on the road. The improvement was immediately noticeable, and I don’t think will require any repetition. If a little more needs to be taken off, however, I know what to do. For what it’s worth, the procedure took no more than 30 minutes and cost me 30 pence for the sheet of wet and dry! With the application of the brake during this procedure the cam is obviously being operated, but the tabs appear to remain held in place and I have not experienced any movement or rucking up of the strip.
To cure the problem so easily suggests that only a tiny amount of lining needed to be removed, but the two ‘high’ spots were enough to cause quite disconcerting vibration and I’m very relieved to have solved it so easily. I thought you would like to know of the satisfactory outcome, too. I am so grateful to you both for your concern, advice and time. I am just sorry that Brompton could not see the benefit of this braking system even if it meant a small weight increase. Perhaps if a few more owners toured with camping equipment in the Alps or Pyrenees, with inadequate stopping power and over-heating rims on long descents, they might be persuaded to think again.
I would stress, Ian, that this technique does seem to have solved my problem where irregularities in the drum lining must have been very small indeed. By all means try it out yourself or suggest it as a possible remedy to others, but I am not qualified to say that it will work well in every case. It almost seems too simple for that, and makes me wonder why on earth someone hasn’t come up with the idea before?
I passed on this information to a customer who was having pulsing problems with his trike, and this was his relpy :-
On 7 February 2013 02:26, William A. Sims
Although I didn’t make my normal 14-mile ride Monday, I DID get in 5-1/2 miles around the neighborhood this morning. The verdict? No brake pulsing! The sandpaper trick WORKS!
Thanks so much for remembering me and sending the info. The only problem now is that I may wear the brakes out because they’re such a pleasure to use!
So I think we can safely say, that this method is worth trying, and is probably superior to attempting to true the drums by adjusting the spoke tensions. Drums can be damaged in service by leaving the brakes locked on hard, after a fast downhill decent when the drums are very hot. I do not know if this method of truing would fix the problem in this case, but I think it would be worth trying.
While I was fixing the problem by replacing the drums, I made a device to test the drums by using an old kingpin, and a dial gauge, see photos:
As soon as I can find the time to repair some out of true hubs I will report on the difference in the actual numbers on drums repaired by Mike’s method. If other people would like to do some testing, I believe that Inspired Cycle Engineering are offering a dial gauge for sale for testing the truth of the drums, but it would seem to me with Mike’s method, it is not necessary to measure the truth
Many thanks to Mike, for sharing this simple fix to rare but an annoying problem!