Suntour XC self-energising / Pedersen-design cantilever brakes - how to disassemble ?

[hyweldavies]

Anyone know how to take these things to bits ? Inside the cantilever
castings is a spring and some alloy "plugs" for want of a better word.
The alloy "plug" is possibly a push-fit, and is threaded on the the
inside - perhaps to aid removal ?

Any ideas ? It's not the end of the world if I drill them out, but
don't want to damage the springs, nor the arms, and it would be
helpfull to not ruin the bits entirely so that I can reverse-engineer
the design.

By way of explanation, I've bought a couple of pairs of these off ebay.
St John Street (Thorn) cycles have obviously got far too many so are
knocking them out v. cheap if anyone's looking. These units have a
clever feature that they are self-servoing from the rotation of the
wheel. That is, as the wheel rotates, it pulls the unit against a
spiral cam, which tightens them onto the rim even more. It may be
debatable whether this is absolutely a good thing or not, and to be
honest, having fitted the back one, I'm not sure it's really any better
than the Avid Canti's I had before. However they are specifically made
for the back only: on the front they would be anit-self-servoing, so no
good at all! Apparently the equivalant on the front was too fierce and
potentially dangerous. So bearing that in mind, I had the idea of
making new parts to fit, but with the opposite "twist" so that they
could be fitted on the front. I have good metalwork facilities
(including lathe etc), but need to take the things to bits first.

I dare say none of the above explanation makes any sense to anyone who
doesn't have these - but then, unless you'd had a pair you wouldn't
know how to strip them down. And even then I doubt if there's any
reason to do so.

Thanks

Hywel

 

[Jim Price]

hyweldavies wrote:
> Anyone know how to take these things to bits ?

Sorry, no, I haven't taken mine apart. The SJSC web site does suggest
that it is possible to strip them down - try ringing them and asking?

> By way of explanation, I've bought a couple of pairs of these off ebay.
> St John Street (Thorn) cycles have obviously got far too many so are
> knocking them out v. cheap if anyone's looking.

That's where I got mine. I also considered what would be involved in
fitting them to the front. The conclusion I came to was the best way of
doing it would be to find a set of forks with the canti studs on the
rear. Such forks do exist, and others (like the Pace RC31) can easily be
reconfigured to achieve this. One problem would be making sure you have
enough clearance for the cable hanger. SJSC have their own solution to
this issue here:

http://www.sjscycles.com/thorn/26tandembrochure/tandembrochurewebp11.htm

Another problem, which may just be scaremongering but there is at least
a theoretical issue here, is that you are introducing a positive
feedback system to the front brake. The net effect of it taking over
would be the involutary dismount over the handlebars. This would lead me
to avoid mounting these brakes behind the fork, as there is still more
positive feedback invoked if the forks twist as a result of the torque
applied during breaking. Lots of toe in and front mounting would be the
safest way to do it if it is worth doing at all.

I think they are probably best suited to the rear, as the cable run is
longer, therefore you would expect a larger amount of cable stretch on
the rear. The self energising effect would tend to allow that longer
cable to provide the same braking effect at a lower cable tension, thus
making the rear brakes feel better at the lever than they would
otherwise - even more so on a tandem, which is where SJSC tend to
recommend them. If they don't feel better, its time to read up on
cantilever geometry, as they are very tweakable. My link to Keith
Bontragers article on canti adjustment has died, but thanks to the magic
of google's cache, the good bit is this:
<quote>
The set up basics for any cantilever brake system:

* Spread the arms at the top. Extend the pads out of the arms to do
this. It seems weird, but it's important. Don't get carried away and
extend them out so far that they are not held properly. Make sure they
are held very tight when you are done with toe in adjustments.
* Bring the straddle carrier down as low as you can. If you don't
know how close you can get to the tire, adjust the lowest point of the
straddle carrier even with the underside of the fork brace. This is a
clearance you use all the time and is a good landmark to shoot for.
* Adjust the pads first, then the carrier, them pull the straddle
cable across and make the final cable adjustments.

</quote>
Once all that has been done, you have cantilever brakes just as powerful
as V brakes.

I'm using them with Campag Ergo levers, and I get a reassuringly
powerful rear brake effect (I have mini V brakes with Kool-Stop pads on
the front). I'm not convinved the pads are the best available, but it
won't get a set of Kool-Stop pads until they have worn out, and that may
be some time as I agree with Sheldon Brown on use of the rear brake i.e.
don't.

So, my conclusion (which I now realise wasn't totally obvious from the
rant above) was not to implement self energising brakes on the front, as
I have got what I wanted out of the front brakes without the hassle of
implementing a potentially dangerous solution to a problem best used on
rear brakes.

JimP

 

[hyweldavies]

Jim - thanks for comments. I'm sure you're right that it's not really
worth the trouble, and perhaps not even wise. I guess I'm doing it as a
challenge more than anything. As a plan B I could convert them to
non-self-energising which might be easier, and also suitable for the
front.

Hywel

 

[hyweldavies]

Jim - thanks for comments. I'm sure you're right that it's not really
worth the trouble, and perhaps not even wise. I guess I'm doing it as a
challenge more than anything. As a plan B I could convert them to
non-self-energising which might be easier, and also suitable for the
front.

Hywel