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Re: Stiff Brakes

To: "mg's" <mgs@autox.team.net>
Subject: Re: Stiff Brakes
From: Max Heim <mvheim@studiolimage.com>
Date: Tue, 1 Aug 2000 10:41:04 -0700
Interesting. He may be overstating the case for rhetorical effect, but 
basically that is how I feel about brake pedals, too.

Look at it this way: there are two feedback mechanisms available for 
modulating braking effort. Both of them are feeding back through the sole 
of your foot and your leg joints, not the most precisely sensitive parts 
of the anatomy (few of us can type with our toes, for example). One 
mechanism is spatial positioning -- i.e. the distance the brake pedal is 
depressed. Those familiar with large American cars with power brakes are 
aware of the shortcomings of this approach -- it is very difficult to 
tell precisely how far one is moving the pedal with one's foot in the 
absence of resistance. One certainly can't be accurate within the 
quarter-inch or so that is required, unless one depresses the pedal very 
carefully. Of course, in an emergency, one tends to want to apply the 
brakes quickly, resulting in instant lock-up. Adding to this is the 
factor of one's weight transferring forward during heavy braking, 
sometimes leading to lock-up even when one had modulated the pedal 
properly to begin with.

The other mechanism is resistance. It is much easier to precisely 
modulate the amount of force one is applying with one's leg, compared to 
the amount of distance of foot travel. This is easily demonstrated by 
attempting to push some small object with your toe a precise distance 
across a smooth floor without looking -- it is difficult to get 
repeatable results. Then try placing one foot on the bathroom scale and 
applying a specific "weight" -- look away from the scale and try to match 
it without looking. Much easier, no? That's because you have tactile 
feedback. Then think about the emergency stop situation -- your initial 
brake application can be as quick as necessary, but the instantaneous 
feedback you get from the pedal resistance allows you to modulate it 
short of lock-up.

I think as a practical matter of brake pedal design that there will 
always be some pedal movement, but I feel it is very important that this 
be exactly proportional to pedal resistance, in order to provide useful 
and accurate feedback. I don't think the importance of tactile feedback 
can be overstated in this case, yet surprisingly many manufacturers 
persist in producing "dead", limp, soggy brakes, presumably as a result 
of customer preference. But then, human factors is still a neglected 
aspect of design, after all these years... I mean, look at Windows <g>.

Eric had this to say:

>Searching the 'Net to help out a friend of mine with BMW break problems
>(and who seems incapable of using a search engine - but that is another
>story) and I bump into the following statement.  
>
>"The one thing about brake pedals is that they can never really be too
>stiff. Anything that can be done to reduce brake pedal travel is going
>to improve sensitivity and confidence under heavy braking. Ideally
>pushing on the brake pedal should be like pushing your foot against a
>brick wall. The pedal would not move at all and you would modulate the
>brakes strictly by changing the force with which you pushed against the
>pedal. This is pretty much what a top level racecar brake pedal feels
>like (F1, CART ,WRC etc). Brake pedal travel and effort are adjusted by
>changing the amount of fluid moved by the brake pedal. This can be
>accomplished by changing the leverage ratio that the brake pedal has on
>the master cylinder or it can be achieved by changing the diameter of
>the brake master cylinder. The latter method is most common."
>
>Now personally I prefer a little bit of 'sponginess' at the top to give
>me more 'feel' of the brakes... but I do want to find (and know where)
>that rock hard, this-is-as-fast-as-you-can-stop, level is.  I would
>worry if I had no downward play in my brake pedal.
>
>Any comments?  Is this just personal preference on my part - or
>silliness.
>
>
>Eric
>'68MGB MkII
>
>PS For Sprinting, I have large MetalKing brake pads that certainly do
>stop the car :-)
>


--

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.


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