Daniel J. Stern wrote:

> On Tue, 1 Feb 2005, Nate Nagel wrote:
>
>
>>>There are *many* ways of varying the mechanical advantage of the
>>>driver's foot over the disc caliper pistons.
>
>
>>True, but a lot of them involve tradeoffs, usually in the "stroke" of
>>the master cylinder.
>
>
> Yes...
>
>
>>A smaller bore in the MC will provide good pedal feel and acceptable
>>pedal effort, BUT is not often workable in modern braking systems as the
>>pedal will eventually "bottom out" on a full-effort, high-speed,
>>ABS-active stop - definitely something you don't want to happen.
>>Likewise, there may not be enough room to leave enough pedal left in a
>>one-circuit-failed condition.
>
>
> You've engineered more brake systems than I have, to be sure, but I'm not
> sure I believe that either of these must necessarily be the case. You
> might be able to convince me.

Actually, not really. I never engineered a brake system from scratch,
unless you count mixing amd matching parts on my own personal vehicles.
But I have seen what happens when you get something a little off, like
the MC bore size and your 70 MPH panic stop turns out to be a little
more interesting than you planned on when the ABS' hydraulic
accumulators suck all the fluid out of the MC. Whoopsie!

Keep in mind, however, that most of the stuff I worked on professionally
was light trucks - these issues may not actually surface for passenger
cars. The point that I was trying to make is, simply, that there really
are good engineering reasons to use a power booster, and that they
become more compelling when using disc brakes as opposed to drums.

>
>
>>Additionally, a litigation-sensitive corporation isn't going to release
>>a vehicle for production requiring *any* higher-than-normal pedal effort
>
>
> Sued for installing airbags
> Sued for NOT installing airbags
>
> =
>
> Sued 'cause the booster fails
> Sued 'cause the pedal effort was "too high"
>
> Remember, there are Federal standards for pedal effort.
>

Yeah... really, it boils down to how much room do you have between the
top of the pedal stroke and the floor of the car (don't want to have the
pedal too high, that would be bad ergonomically - you should be able to
get your foot on it without taking your heel off the floor) and can you
juggle your bore sizes and/or linkage to get a reasonable (or Federally
mandated) pedal effort within those parameters while not having the
pedal go to the floor either with one circuit failed or during a full
ABS stop from whatever your maximum design speed is. If you can't, then
you go to a power booster. I'm sure that someone could probably give a
rough, hand-waving estimate (like I have above, but a more informed one)
as to appx. what vehicle weight that starts to happen at... but the
point is, that weight would be significantly higher for a drum brake car
because a) they require less fluid movement (or should, if the shoes are
properly adjusted - since they have positive return springs you really
can't count on that though unless self adjusters are included) and b)
they require less line pressure.

nate

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