Subject: [Tigers] Stopping distance testing

From: Jeff Feit <[email-masked]>
Date: Thu, 20 Jun 2013 06:30:29 -0400

A few things about stopping distances, having been involved with this kind of
testing before.

Certainly the surface used and test conditions can vary a lot, which has a big
effect. The big magazines like to do most of their testing at the same
location when possible-- but they don't always. If they are testing a car at a
remote location, they make do with whatever they can find. My guess is that
Classic Motorsports doesn't have access to a good test surface.

More importantly, there are 2 factors that have a huge impact on the stopping
distance, that have nothing to do with the ultimate deceleration capability--
how the test is run, and how quickly the car can build up deceleration.

Think about this. The 60 mph starting speed is 88 feet/second. What do you
consider the start of the test? Do you use a pedal switch (this is what
automakers use, but magazines don't)? Do you signal the driver to start the
brake event (adding driver reaction time)? Trigger at .10g? .50g? The method
can easily account for 1/2 second difference, which is 44ft at 60mph.

Then there is the fact that a car can't instantly go from zero to maximum
decel, and the things that affect this often don't have anything to do with
maximum decel capability. Things like the suspension, shocks, stiffness of the
brake components (calipers, lines, pedal) will effect how fast the decel can
build up-- and at a starting speed of 88ft/s it makes a big difference.
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From: [email-masked]
Date: Thu, 20 Jun 2013 11:46:54 -0400 (EDT)

Those are really helpful points to consider, thanks.
 
The methodology aspect is really just about comparing apples to apples, but 
 the point about "stiffness of brake components" is key. It doesn't effect  
threshold limit, but compressing the time it takes the system to get to 
that  point is significant. Also, as I wrote earlier, it can pay dividends in 
more  accurate modulation of a non-ABS system/

 
In a message dated 6/20/2013 3:32:53 A.M. Pacific Daylight Time,  
[email-masked] writes:

A few  things about stopping distances, having been involved with this kind 
 of
testing before.

Certainly the surface used and test conditions  can vary a lot, which has a 
big
effect. The big magazines like to do most  of their testing at the same
location when possible-- but they don't  always. If they are testing a car 
at a
remote location, they make do with  whatever they can find. My guess is that
Classic Motorsports doesn't have  access to a good test surface.

More importantly, there are 2 factors  that have a huge impact on the 
stopping
distance, that have nothing to do  with the ultimate deceleration 
capability--
how the test is run, and how  quickly the car can build up deceleration.

Think about this. The 60 mph  starting speed is 88 feet/second. What do you
consider the start of the  test? Do you use a brake pedal switch (this is 
what
automakers use, but  magazines don't)? Brake pressure switch? What 
pressure? Do
you signal the  driver to start the brake event (adding driver reaction 
time)?
Trigger at  .10g? .50g? The method can easily account for 1/2 second
difference, which  is 44ft at 60mph.

Then there is the fact that a car can't instantly go  from zero to maximum
decel, and the things that affect this often don't  have anything to do with
maximum decel capability. Things like the  suspension, shocks, stiffness of 
the
brake components (calipers, lines,  pedal) will effect how fast the decel 
can
build up-- and at a starting  speed of 88ft/s it makes a big difference. The
effect would show more if  you trigger off a brake pedal switch than if you
trigger at  .50g.

So, while in theory a "better" brake system shouldn't improve  stopping
distances with the same tires, in reality it does. As Pirelli  likes to say,
"power is nothing without  control".
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