Braze-on Bosses For Centre-Pull Brakes

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jimlews
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Braze-on Bosses For Centre-Pull Brakes

Postby jimlews » 11 Jun 2016, 8:04am

For a traditional touring cycle (old Dawes Galexy et al), are these identical to cantilever/V-brake bosses and if not, are they still available?

gerrymcm
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Joined: 30 Oct 2012, 2:52pm

Re: Braze-on Bosses For Centre-Pull Brakes

Postby gerrymcm » 11 Jun 2016, 9:58am

I've just got myself a 1989 bike made for canti's and was suprised to see thay they had only one adjustment / spring catch hole in them. I don't know if this is typical of studs for cantis as I've never had a frame for cantis before.
I've had a few bikes with V brakes and they've always had 3 holes.

Ceeway sell this stuff
http://www.ceeway.com/Bosses.htm

Brucey
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby Brucey » 11 Jun 2016, 10:24am

mostly the bosses are the same (or close enough) but;

- recently some makers have been supplying brake bosses (and brakes that fit them) in a slightly different length

- the bosses come with a pedestal; the pedestals are made different lengths and angles so that you can achieve the desired boss spacing on variously different stay/blade spacings.

- depending on the brakes you wish to use, the boss length, the exact boss diameter, the spring holes etc may not be important; not every brake uses the spring holes at all, and many brakes have an integral pivot bushing too.

- a few (mostly much older) brakes used bosses that were really very different. For a while some Sun Tour brakes uses bosses with spring holes on the other side (very logical, but it didn't catch on) and other makers such as CLB stuck with their unique bosses well after everyone else had adopted a standard type.

The boss in common use today is essentially the MAFAC canti design, since this was for many years the most common type of cantilever brake. If anyone ever goes to use a genuine MAFAC boss, it is (IME) essential to weld or braze the boss into the pedestal properly. It is meant to be brazed in already but if often isn't done well enough and the boss can work loose in service if it takes the slightest knock. Not what you really want in a brake, that... :roll:

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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gaz
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby gaz » 11 Jun 2016, 11:22am

jimlews wrote:For a traditional touring cycle (old Dawes Galexy et al), are these identical to cantilever/V-brake bosses and if not, are they still available?

Just for clarity are you asking if vintage canti bosses (below the rim) are the same as modern canti/V-brake bosses or is this an enquiry as to whether canti/V-brake bosses are the same as centre-pull/U-brake/roller-cam bosses (above the rim).

If the former I agree with Brucey (I also have some '88 vintage Shimano bosses kicking about somewhere) although the spacing at which these are now set tends to be wider than in years gone by which can cause issues fitting modern designs of brake to classic frames/forks.

If the latter I don't know but IIRC there is some variety even between modern manufacturers.
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jimlews
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby jimlews » 11 Jun 2016, 11:47am

Brucey wrote:- depending on the brakes you wish to use,


I wish to use Weinmann Centre pull calipers but dispense with the pivot carrier-the yoke that bolts through the fork crown and replace this with brazed on bosses.

I am inspired by Tony Oliver's excellent 'Touring Bikes' book where he says 'it is not commonly known that braze-on pivots exist for these brakes'.He shows a picture of braze-on centre pull Mafac brakes and later of Dia Compe. The pivots of course, are right up at the fork crown.

My question is prompted by the onset of pain in my hands when applying the brakes (drop HBs) especially going down steep hills. Oliver says that the set- up is 'ideal for cyclists with weak hands'...'because they have a high mechanical advantage'. He cites as disadvantages the need for 'precise inter pivot spacing.... and the flats that align the tension springs must be positioned even more precisely' (presumably than with cantilever bosses). Also cited as a disadvantage is the need to run absolutely true rims due to having to position the brake blocks close to the rim to achieve best results.

The above suggests to me that special bosses are required but, a frame builder I spoke to said (in effect)that they are all the same for canti's, V brakes and centre pulls.

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gaz
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby gaz » 11 Jun 2016, 12:04pm

Hand wash only. Do not iron.

jimlews
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby jimlews » 11 Jun 2016, 12:18pm

Thank you Gaz.

That looks like what I am looking for.

And thanks also to you Brucey, you answered my question comprehensively based on the limited info I provided.

Brucey
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby Brucey » 11 Jun 2016, 12:20pm

some people (wrongly IMHO) refer to cantilever brakes as 'centre pulls' and since true centre pulls are so rarely brazed on, and you were talking about galaxies etc I thought you meant canti bosses.

MAFAC centre-pull brakes use a special boss. Weinmann brake arms won't necessarily fit the same bosses because the way the springs mount is different; there may also be other differences that I am not aware of.

Now TO was right in what he said, but

a) he said that many years ago now, and things have moved on somewhat in the meantime and
b) the brake MA is still limited by the design of the brake arms; all you can do is have it so that the brake block is in the top of the slot, and that defines the brake MA.

In the context of a touring bike this sometimes allows a higher MA (with any given brake arm) than you might get with the standard mountings but in the context of all the brakes there are out there, it isn't necessarily a very high MA.

High MA brakes always come at a cost, and that cost is that the running clearance to the rim is reduced. In addition such brakes may require very frequent readjustment.

There are plenty of other options for high MA brakes that fit to standard canti bosses, so if you have standard bosses I'd suggest that you try some of those before embarking on a brazing frenzy that mightn't do any good anyway.

Depending on the brake levers you want to use;

- full V's can have a system MA of about 8:1
- mini-Vs with (pre 2008) STIs can have a system MA of up to about 9:1
- mid-arm cantis (with a low straddle and suitable levers) can have a system MA of between 5:1 and ~9:1

I doubt that brazed-on CP arms will exceed these system MA values even with the correct levers.

Note that the boss height will cause variations in system MA with these brakes too. I have seen boss heights vary from setting the rim as low as 20mm above the bosses all the way to ~40mm above the bosses; this would result in a x2 change in brake MA with the same brakes fitted.

At any given system MA, a system with lower brake cable tension (eg full V brakes) will be more efficient and less prone to both friction losses in the cable and various flex losses. Older style brakes (cantis and CPs) work best with high MA levers which then generate high cable tensions. These brakes can work OK but they are more sensitive to the condition of the cables.

If you just want to experiment with system MA, you can fit a 'travel agent' to most brakes, to change the MA. However IME this also introduces a certain amount of extra flex in the system, so often, lever travel is used up that way instead of in a more useful fashion.

cheers
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jimlews
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby jimlews » 11 Jun 2016, 12:35pm

Thanks again Brucey!

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531colin
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby 531colin » 11 Jun 2016, 1:13pm

Jim, there is loads of information here...http://forum.cyclinguk.org/viewtopic.php?f=5&t=63112&start=45....from Brucey about the MA of various systems.
I'm not sure what you have currently got on a bike?
If you have a bike with canti. bosses, I think the best cable rim brakes for drop bars are full-size Vee brakes with the appropriate drop bar levers.
If you are using road STIs you can get almost as good a brake by fitting a Tektro RX6 mini-vee with a noodle which includes an adjuster, and i would only use one on the front, cantis are good enough on the rear.

LollyKat
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby LollyKat » 11 Jun 2016, 2:42pm

gerrymcm wrote:I've just got myself a 1989 bike made for canti's and was suprised to see thay they had only one adjustment / spring catch hole in them. I don't know if this is typical of studs for cantis as I've never had a frame for cantis before.

It may have been typical back in those days - the studs on my 1987 Mercian only have one spring hole.

gerrymcm
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby gerrymcm » 11 Jun 2016, 3:06pm

Lollykat thanks for confirming. I guess the 3 holes evolved from 1 hole? The frame I referenced from 89 is a Mercian, it's my first Mercian and I'm extremely chuffed with it.

Brucey
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Re: Braze-on Bosses For Centre-Pull Brakes

Postby Brucey » 11 Jun 2016, 3:23pm

for many years genuine MAFAC bosses had

- sheet metal pedestals
- markings of AV and AR (for avant and arriere, front and rear)
- one spring hole

like this

Image

however later versions, which were otherwise similar, had F/R markings and three holes in the boss. Possibly these were made by weinmann, who took over the MAFAC tooling and made a some brakes (eg cantis) to the MAFAC pattern but with weinmann branding for a while.

The markings are stamped deeply, so are often visible even with paint on them. Because the bosses are not handed per se, the markings are usually visible on the upper surface of the LH front and RH rear boss, and the lower surface of the other two

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~