From: tan@lynx.msc.cornell.edu.UUCP (David Tan,Box 219 Baker Lab,54810,2720402)
Subject: Re: Force 40 Brakes [was Delta V question]
Originator: tan@msc2.msc.cornell.edu
Organization: Cornell-Materials-Science-Center
Date: Tue, 24 Nov 1992 02:52:34 GMT

From article <truesdel-231192140244@aux-1.ics.uci.edu>, by truesdel@ics.uci.edu (R. Scott Truesdell):
> In article <1992Nov23.211150.22849@src.dec.com>, jcollias@src.dec.com (Jim
> Collias) wrote:
> 
>> This is all simple physics.
> 
> No it's not. It's engineering and physiology (ergonomics?). And it's not

I missed the posts that started all of this, so I figured I'd just jump in
and start flailing...:)

In the sense that the amount of energy dissipated must be equal (except for
frictional losses in the cable and linkages) to the amount of energy expended
squeezing the brake lever, this really is just simple physics.  How tired
your hand gets, how much you have to stretch to reach the levers, etc., those
are a matter of engineering and physiology.

> that simple. For example, my girlfriend's hands are darn near half the size
> of mine. Even though she has a powerful grip, there's no way for her to
> pull as much lever travel as I can. Yet component manufacturers have to

[...]
 
>> The Force 40 system seems to stop better with 'less
>> effort,' but you're pulling more cable, so it all evens out.
> 
> No it doesn't. There is a complete dimensional order between distance and
> force.

What?  What does the second sentence mean?

The upshot of what Jim is saying is just this: energy = force x distance.
The amount of energy expended at the brake lever = energy applied at brake
pads (+ frictional losses, which I'll say are small in a decent setup.)
At the levers, longer cable pull allows lower force to be applied; at the
pads (situated very close to the rim in Force 40), short travel means 
considerable force is applied, for a given amount of energy.

> 
>> out, but it gives you great relief because your hands and your brain would PREFER
>> squeezing 10 pounds 100 times, instead of 50 pounds 20 times,
> 
> Where on earth did Jim come up with this ratio? I'm not sure, but I think
> the Force 40 is a 2:1 ratio over conventional setup. Max. Longer cantilever
> systems (e.g. Paul's) are more like 1.3:1 to 1.5:1. I AGREE that the more
> conservative increase in braking power (longer cantilevers) is preferable
> over the drastic 2:1 increase of Force 40. But then that's opinion, isn't
> it?
> 
> 
>> 	If it takes 'X' to stop your bike, then your muscles HAVE to put out 'X'.
> 
> No they don't! Mechanical advantage really does work. That's why we use it.
> That's why our bikes have gears and why 'spinning' outlasts 'hammering'.
 
Yes they do!  This seems to be precisely Jim's point above (10 x 100 is
preferable to 50 x 20.)  Hammering or spinning, your muscles put out the
same energy, for a given speed (and track conditions); it's just that your
muscles are better at sustaining low resistance/ high reps than vice-versa.
THAT's why we have gears

> One of the big advantages of increased leverage in a braking system is the
> relief of *sustained* effort. Brake pads are not brought to x force and
> then released. They are held at x force for varying lengths of time. After
> the movement has been supplied to apply x force to the pads, the amount of
> effort to maintain x force is reduced for an increased-leverage system. Any
> long fast steep decent will accentuate differences in braking efficiency
> that can be glossed over for short intermittant stops. It's after you've
> been on the brakes hard for 30 minutes that the difference really shows up.
 
> Jim keeps trying to make the point that the Force 40 system is somehow
> fundimentally different than longer cantilevers. I maintain the difference

My impression is that Jim is saying that Force 40's are fundamentally no
different from other cantilevers.  It's just taking the force/travel issue
to an extreme.

> is strictly a matter of degree (quantity). I agree that it seems like the
> Force 40 setup has gone too far in the direction of increased purchase, but
> I don't buy his explanation.
> 
>   --scott
 
 Dave