Cycling UK Forum

Can anyone spell out for me just what measurement(s) to make in order to establish what size callipers would be required for any given combination of rim and frame, and particularly forks!

I've just replaced a rear 27x11/4 wheel with a 700c wheel. The existing centrepull brakes had enough visible vertical adjustment left to prevent me from worrying about it. I was somewhat surprised though, to discover that the shoes which had previously only just missed the tyre (narrow braking surface) now leave a mm or so of rim showing above the shoe when I pull the straddle cable to bring them into contact. I would have expected the opposite - the new wheel is a smaller diameter than the old isnt it? If anyone can straighten me out about this I'd be grateful! FWIW, the new rim braking surface is a couple of mm deeper than the old and the rim is a bit wider, and no, I havent gotten confused - both rims are clearly size stamped (I checked already!)

My problem is that I would like to replace the front wheel too, principally so that I can use modern puncture resistant tyres. The front brake has shorter arms than the rear and the shoes are already as low as they can go. Hence my opening question...

If your centrepulls are Weinmanns I think the number may tell you the drop. So the 750 is a 75mm drop 650 is a 65mm drop. Not sure but it looks that way. That is the maximum drop.
With my new 622 rims being much narrower than the old 27" ones the brake shoes have to come much closer together, this means the arms go past being straight and are pointing in towards the rim which makes the effective drop of the brakes shorter. So my centrepulls would only reach if I had brake blocks with lots of washers on the back to make them reach further in. The new dual pivots are designed to work with narrower rims and dont have this problem.
Despite that my Alhongas only just manage and I have to shave bits off the brake blocks to protect the tyres. The front is the opposite with the Alhongas being too long so I had to buy a pair of Tektros aswell.

Measure from the centre of the bolt hole to the centre of the brake track. As a guide the 700c rim diameter is 8mm less than the 27" so the brake reach needs to be 4mm longer.

PW wrote:As a guide the 700c rim diameter is 8mm less than the 27" so the brake reach needs to be 4mm longer.

Not on this occasion but thanks for telling me where to measure!

Meic: Looks like youre correct about the significance of the numbers on the Weinmanns - and with drops that big,precise measuring becomes a bit academic - there isnt much choice in alternative brakes.
The brakes dont seem too bad at present (I have fitted some modern blocks). If I splashed out on a set of Alhongas would I be likely to notice an improvement in power or control? This was supposed to be a bargain basement project but I do take braking seriously!

When I put the Alhongas on the difference was amazing! However they had new brake pads which were probably much better than the old type as they wore away in 2000 miles! Also it meant brand new cables as you need different lengths. So how much was due to the calipers? I dont really know.
I have put the old pads off the centre pulls on the Alhongas and they are noticably worse than the Alhonga ones. However I can not say if they are as bad as with the centrepull brakes because such feelings are very unreliable once you got used to a different standard altogether.

Dave, it sounds as though one of your rims was already 700c....

I promise you, clearly marked rims, installed tyres ditto, no mixup!
I think meic has probably suggested the cause, something to do with how far the brake arms have turned before the blocks touch. If they went a little past BDC with the old rim...
It looks as though I'm going to need to replace the brakes anyway - the existing front rim and brake, with new blocks installed, has no vertical adjustment left, so as the blocks wear and the arms swing further in I'm likely to end up braking on the tyre walls

DaveP wrote:If I splashed out on a set of Alhongas would I be likely to notice an improvement in power or control? This was supposed to be a bargain basement project but I do take braking seriously!

During my Super Galaxy upgrade from 27" to 700s I moved from the old Weinmann centrepulls to Alhongas. The improvement was striking. I was amazed that I had put up with such poor braking performance for so long.

. . . descending the Bealach na Ba was frightening enough with the Alhongas . . the Weinmanns would have been unthinkable.

I had the same problem when changing the wheels of my sons old bike. I hope this helps. Edinburgh bike Co Op sell Ka 49-57 dual pivot £19.95. These price are both for front and rear.

Thank you Graham, That's what I call an endorsement!

Thank you too Edwards, but I dont think they'll be big enough for this case!

SJS used to have some even longer drop Alhonga on their web site but did not show them last night, they were about £20 a pair. Might be worth phoning them as I have always found them help full when I have phoned in the past.

Spa Cycles have 'em - front and rear £45 posted

I did a web search to discover them. One of the things I turned up was indeed a page from SJS offering the Alhongas. I had already failed to find any reference by looking and searching the SJS site, so I came to the conclusion that this was probably historical. I wish I understood how this sort of thing can happen...

Bike shops that sell on line as well as over the counter often don't show all their stock lines on their web pages. They may have left the item out during more recent updates. Give them a call to find out.

If you need to cheat for a while until you can sort the upgrade, the slots on the Weinmanns will stand a slight tickle from a round file...

SJS sold extra deep Alhonga Dual Pived nut type brakes earlier this year. I bought a set in June I think, as I do not have the receipt the best I can do is say they measure 55 to 75 mm. It is certainly worth phoning them as they might be able to get the length you need in.
Try to explain to a computer is not the best, the people that work there I have found very helpful when I have phoned and on my 1 visit.