In reply to a message from Wiggles sent Mon 28 Apr 2008:
Wig-man,
OK, here’s my take. I know you know that it makes a big
difference, since racing setup balances brakes very carefully to
get the best braking, and modern cars have dynamic balancing units
that rebalance the front to rear braking based on the weight
transfer – how much the rear suspension rises during braking.
The reason the rear brakes are smaller is that while E Type weight
is well balanced front-to-back, under braking the car’s weight
balance moves forward so quite a bit more of it is on the front
wheels. Interesting that the Porsche 911 has the same effect, but
its rear weight bias means it’s weight is more nearly evenly
distributed under heavy braking, one reason they brake so well.
(Also note center of gravity does not move, FWIW.)
The real world effect SHOULD be (see the mathematical analysis
building up? ; -) that at the maximum braking point, when the front
wheels are at the very point of locking up, without locking up. At
this point any more pedal pressure would lock the front wheels and
reduce overall braking force. A good driver NEVER locks the wheels.
With smaller than required rear brakes, the rear wheel braking
force then will be a bit less, and the rear wheels will not be near
locking up. This means that the braking force on the rear tire
treads to the road will be a bit less than it would be with optimum
brakes.
The braking force on the car is the sum of braking forces of all
four wheels, so overall braking at the limit will be less than
optimum, and so brakes won’t stop the car as quickly.
The optimum would be that all four wheels are just at the point of
locking up at the same moment. At that time the braking force is
the maximum that the given tires can apply – all four of them.
The other point sometimes discussed is what end of the car locks up
first. If the front locks first, the car slides straight ahead
without steering, but it stays stable front-first. If the rears
lock first, you maintain steering but anyone who has experienced a
snap-spin realizes that once the car gets a little out of straight
ahead with front grip and no rear grip, the rear end swings around
a lot faster than a human can control it, and remember we are
talking about a pretty extreme emergency situation so the driver is
not likely to be on top of things (otherwise the brakes would not
have locked up). As for me, having all wheels lock at the same
time is optimum, and you get maximum stopping power up until the
wheels lock to boot.
So it’s important on the road when a fool pulls out, as well as on
track. Cruising to the store or to Texas, we won’t notice any
difference at all. Braking less than max, no difference (?)
And you can do the swap without pulling the IRS.
Also, note how the braking effect is in the end determined by your
tires, once the car has done its best. Crummy tires will slide you
into the tractor, while good ones will grab better. Good
investment, expensive tires.
Jerry–
The original message included these comments:
I’ve yet to fully understand this ‘‘problem’’, in the real world (NOT
the mathematical one).
–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php