Another canti brake question, hopefully an easy one!

[Brucey]

If you are going to use the same brakes front and rear, I'd question whether it is worth changing the fork for one with wider bosses; IMHO the C510 brakes will set up (as a mid-profile canti) as well or better on narrow bosses.

If you use CX50 brakes front and rear they will set up (with different spacers) as well on narrow or wide bosses.

If you are not set on using the same brake front and rear (and you are sticking with the wide boss fork) then there is something to be said for using the R550 at the front and the C510 at the rear.

cheers

[531colin]

That'll stop you. The lower you set the straddle the higher will be the MA.

[531colin]

...........If you are not set on using the same brake front and rear (and you are sticking with the wide boss fork) then there is something to be said for using the R550 at the front and the C510 at the rear........cheers

I don't often disagree with Brucey, but.......
550 is always going to be low-profile, its always going to have a high MA initially which will drop like a stone as the straddle moves up (as the pads wear). I would leave that one in the spares box, fit 510 front (set as mid-profile), for the rear I'm quite happy to use CR720.....for 2 reasons, it doesn't squeal much, and for the rear, its powerful enough.....you can lock the rear wheel with it, and if you really need to stop you won't have any weight on the rear wheel anyway.

[Brucey]

FWIW if the OP's photo is anything to go by, I don't think the 510 will make a very good mid-profile brake when set onto wide bosses; in the pic there doesn't seem to be enough post length to get the arms out any further, and that is with brake blocks that are quite thick.

IIRC the 550 has slightly longer arms than the 510 and with careful setup (on wide bosses) can achieve a decent mid-profile geometry; it isn't far off the geometry of brakes that work pretty well, like CX70 or Avid Shorty, etc. I certainly wouldn't assume that it wouldn't work without trying it, anyway, if using the wide boss fork.

cheers

[gerrymcm]

this is the 510 on the original fork and I've now measured the studs 62mm centre to centre.
I've put on some worn pads, which I think will be a similar size to the clarks cp522 holders I've ordered.

"If" I've understood correctly then I need to get canti's arm as far out as poss as that creates the lowest straddle cable adn therefore the lowest straddle cable angle?

I've printed this pic off and measured it and then superimposed the info on the snap.

I wish i understood this stuff!!!!

thanks
Gerry

[gerrymcm]

Gaz sorry meant to say ta for those links, I think I have made a mistake on those clarks CP522, they may as you say be too fat for the space between the rim/fork.
cheers

[531colin]

arms roughly at 45 degrees (to horizontal) seems to give me the "best compromise"
You have all sorts of things to balance up.....
Brake power (mechanical advantage) ...maximal with low profile
Pad/rim clearance...maximal with wide profile, its the opposite of MA
....and "how easy the brake is to live with"....the major snag with low profile is the decline in MA as the straddle goes up, and any brake with high MA has limited pad clearance, this is worse on the back with longer cable, etc.
Other confounding factors are length of brake arms, as Brucey mentions, and cable pull of the lever....STIs will pull more cable (=less MA of the whole system) than my old-fashioned "stand alone" aero levers.
But you only have 2 things to play with on any one brake....angle of the arms and yoke height. (lower yoke = higher MA for mid/low profile)

[Brucey]

The 510 brake looks like you have scope to adjust the MA on the narrow forks within a useful range. As depicted the geometry may leave you with a powerful but slightly spongy brake; you'll have to try it and see; at least you have room for manoeuvre there, geometry-wise.

However those forks do look rather narrow and the rim is reasonably wide; it is going to be touch and go with the XTR/CP522 pads/holders. If it works OK you will have to deflate the tyre to get the wheel in and out; I can live with that but not everyone would. The pads/holders are 10mm thickness where they are likely to clash with the fork blades, and extend ~44mm back from the post centreline. Although I think they will clash in this case, they don't clash with every brake/boss because of where the eyebolt finishes up.

On one bike I own I can use the XTR/CP522 type holders (just) but I can't fit my favourite insert in them, because they are slightly thicker than some others!

cheers

[gerrymcm]

Brucey,
Thanks very much your knowledge is remarkable.

At least if I don't get on with these forks I can use my other set with wider studs I just need to get the bike built now and see if I like it.

I'm really suprised that the front forks are so narrow, I think where the tyre would sit it's approx 42mm wide internally which given it's a Mercian KOM seems very narrow as wouldn't most tourers be aimed at 32mm tyres?
I don't know if the forks are original though.

Cheers
Gerry

[Brucey]

for quite a long time a 'light tourer' would typically be designed to accept rims ~20-22mm wide [eg Super champions, or Mavic Module 3 etc] which would be fitted with 28-32mm tyres.

In that context the KOM (and others similar) makes a fair amount of sense, where by comparison with what many people think of as 'a touring bike' these days the clearances are small and the range of tyres that can be fitted isn't very large.

It is as well to remember that back then you might have struggled to find a really nice tyre in 35mm (or larger) sizes; most of what was on offer would have been heavy and slow in those kinds of sizes.

IIRC back then, I certainly wouldn't have thought to use anything that size unless I was planning to go offroad or something!

cheers

[scottg]

Shimano CX-70 install doc

There is chart giving rim width/stud spacing showing which spacers to use.
24mm rims and 60mm spacing aren't on the chart., 65mm & 22mm rims is the closest.

http://www.shimano.com/media/techdocs/c ... 749996.pdf

[Brucey]

The installation chart gives 'perfect' settings, that present the arms parallel to one another (i.e. with the brake blocks' swivel barely used) when the brake is on. It is usually OK to have the arms slightly more spread than this, (i.e. with a narrower boss spacing/wider rim) using the articulation in the brake blocks to bring them square to the rim. In fact this setup can give a more powerful brake.

IIRC the CX70 spacers are 18, 14, 10mm thickness respectively. I think you can use significantly narrower boss spacings than 65mm and wider rims than 22mm if you are prepared to

a) use an even thinner spacer/brake block combination (with a suitably shorter bolt) and
b) if necessary use a standard (shorter) straddle cable in place of the unit link

The thinnest spacer you could use would presumably be one of the curved washers that comes with current shimano road brake holders, which is around 2mm thickness. This would (in theory) allow up to a further 16mm reduction in boss spacing/increase in rim width over the 'narrowest setting' on the shimano chart.

cheers

[531colin]

Shimanos fixed length link straddle unit thingy is a failure.
The idea was to make cantilever brakes which anybody could set up and get it right first time......this is a good ambition, but life isn't always that simple.....rim width varies, cantilever stud separation varies. Sheldon has a picture of the old type in 5 different lengths http://www.sheldonbrown.com/canti-trad.html and I can't be the only one who remembers the previous debacle where the bike shop had a series of plastic widgets of different colours which "guaranteed" successful brake set-up....



Now, instead of having a range of straddle cables and plastic set-up widgets, we have a range of spacers and a single fixed length straddle! After you lose the instructions, you are stuffed!

Fixed length straddles take away the ability to adjust the mechanical advantage. We should be going the other way, we should be putting the barrel adjuster in the straddle so you can take up pad wear without tools and without compromising the mechanical advantage too much.

Just man up and find out how the damned thing works. Its a lever, for goodness sake, it isn't a moon rocket.
Set the arms at 45 degrees to the horizontal......thats your spacer selected, or how far in you push the pads on the studs.
Set the straddle yoke on the mudguard for maximum MA, set it up by 10 or 20 mm for more pad clearance.

[gerrymcm]

nailed it!
"Just man up and find out how the damned thing works."

The less polite abbreviation of it is MTFU!

I've decided that I'm going to swap the forks so cue plenty of questions re painting the forks, headsets, re-threading the forks etc etc.

Cheers all

[Brucey]

personally if it were my bike I'd try to stick with the original forks; they surely ought to best match the rest of the frame?

When they are set up right, cantis on narrower bosses are arguably better brakes, because the brake block maintains a more consistent contact with the rim as the brake blocks wear. In extremis, neglected cantis set on wide profile bosses can see the brake blocks 'diving under' the rim when they are worn.

cheers