From: truesdel@ics.uci.edu (R. Scott Truesdell)
Subject: Re: Adjusting cantilever brakes
Newsgroups: rec.bicycles.tech
Organization: University of California, Irvine
Date: 20 Nov 92 18:41:54 GMT
Followup-To: rec.bicycles.tech

In article <20242@acorn.co.uk>, steve@acorn.co.uk (Steve "daffy" Hunt)
wrote:

> What is the secret of adjusting cantilever brakes for maximum braking
> performance?  I have played with my Shimano brakes for some time, and
> haven't had very satisfactory results.

I am assuming you have the 'low profile' Shimano cantilevers where the main
brake cable goes all the way to one of the cantilevers and a short
non-adjustable wire goes from the other cantilever to a clamp on the main
cable. When properly set up, these are the most powerful production brakes
on the market. They are a compromise between power, ease of
installation/assembly, and univerality.

Fact: as braking performance increases, adjustment becomes more critical.
Mechanical advantage can be increased by lengthening the cantilevers at the
expense of losing some travel. In practical terms, the clearance between
your brake pads and your wheel rim will decrease. This means you will have
to keep your wheels very true.

Fact: the brake levers that you pull on have a fixed maximum amount of
travel and all the work at the business end of the system must be
accomplished with that fixed amount of cable travel. This means that the
more powerful your brakes are, the less travel and clearance they will
have.  ServoWave brake levers are a way of taking up a lot of cable
initially, then going to a more powerful pull near the end of their travel.

So what can you do?

First, the power of the Shimano 'low profiles' can be maximized by sliding
the brake pad all the way down towards the boss. This increases the length
of the primary side of the lever and decreases the length of the secondary
side of the lever. Unfortunately you don't have complete freedom with this.
If all brake bosses were attached to the forks in the exact same place and
everyone used the exact same rims, Shimano could make this adjustment range
a lot smaller and thus ensure more uniform braking response. The up-n-down
adjustment of the pads will be largely determined by the location of the
brake bosses and the rim you use.

Second, the stradle cables should come off the cantilevers at nearly 90
degrees. Because of the 'low profile', this means that the stradle cable
junction lies quite near to the tire. You must make sure you have the
correct length short fixed cable for your application. Refer to the brake
installation and spec sheet.


  cable angle   /
  too high --> /            \__
              /                \__ <-- correct cable angle.
             /                    \__
        __  /                        \__  __
       /  \/                            \/  \
       |   \      brake pad high        /   |
       |    \     for lots of travel   /    |
       |     \    and ease of adjust- /     |
       |      \   ment.              /      |
       |       \     v              /       |
       |        \    v             /        |
       |  ______ |   __           | ______  |
       | |  ||  ||--|  |          ||  ||  | |
       | |  ||  ||--|__|     __   ||  ||  | |
       | |  ||  ||          |  |--||  ||<----- slide brake pads as low
       | |______||          |__|--||______| |  as possible for maximum
       |         |                |         |  power.
       \        /                  \        /
        \______/                    \______/




> How important is it to have the special alignment jig mentioned in the
> Shimano instruction leaflet?

Not very important... just easier. Near the end of the sheet there is a
dimension for maximum width of the brake assembly. If you get this
dimension (it's not super critical, plus or minus a couple of mm should be
fine -- it's a guideline more than a rule) and the two legs of the cable to
the cantilevers the same length (assuming you've got the correct
fixed-length cable) then you will be fine. You really have to juggle to get
everything set up just right -- everything keeps moving as you try to
measure and clamp everything down. The jig makes the process a LOT easier.
(Just like you, I've never seen one of these jigs in real life.)


> How critical is it to get the right toe-in on the blocks?

Not very critical. New Shimano brake pads have little nubs on the rear part
of the pad that look to be about 1mm. The thickness of a plastic credit
card should be about right. More toe-in means less likelyhood of squeeling,
less toe-in means more immediate power/less mushiness.

 - - -

I know of several people who have scrapped the fixed-length straddle leg
system used on the newer Shimano brakes and reverted to the more
conventional adjustable straddle cable. It doesn't cost much. Here's the
deal: You need to set the straddle cables up for the same angles as the
fixed cable setup otherwise you will give up some power. See diagram.

The new fixed-cable system produces very accurate results very easily and
quickly IF you have the jig. IF YOU HAVE THE JIG, the brakes can be
installed and adjusted perfectly in one step. Subsequent adjusting is
rarely needed. If you don't have the jig, the new system is more difficult
to install and adjust than the old system.

  --scott
Newsgroups: rec.bicycles.tech