At 04:23 PM 11/5/99 PST, Simon Matthews wrote:
>.... -- on my '57 MGA, it seems fairly easy to lock up the rear wheels --
is this normal? I have drums all round, the brakes were professionally
re-built about 2 years ago.
I don't think it's normal. I autocross a lot, usually on race tires nut
occasionally on street tires, and use MAXIMUM breaking regularly, which of
course transfers more weight to the front, which you would expect might
cause the rears to lock up, but they don't. I also run with nothing in the
boot, even with the spare tire removed, so it's not an issue of a heavy
tail either. Sometimes it locks up a front tire, usually in a turn, but I
don't recall ever locking up the rears alone (except in a handbrake test).
>I have always been a little concerned about overheating the brakes (being
drum brakes), but I now think I have my concerns mis-placed: I drove over
the Santa Cruz mountains during the tour before the Palo Alto show recently
with no problems,
One of my favorite roads is a trip through the mountains in Oregon from
Grants Pass to Gold Beach. Getting into the mountains is 15 miles up hill,
mostly in 2nd gear, followed by about 15 miles running the rollercoasters
along a the ridges (seldom over 45 mph in the fast spots). Getting down is
another 15 miles of very exciting twisties, and if one gets a little
overzealous an MGA with front drums can run into brake face in just a few
minutes.
OTOH, in a more recent experience I was hot lapping the two mile road race
course at Blackhawk Farms for 45 minutes with the race tires, which was
mostly flat out in 3rd gear with only a couple short periods in 4th. The
turns recuired a short but heavy stab on the brake pedal just AFTER the 100
foot marker to pull it down from 70+ to about 35 mph in a couple of
seconds, totaling about 10 seconds of braking for each 2 minute (plus a
bit) lap. I am happy to report absolutely no noticable brake fade through
the entire 45 minute run.
One problem I have had with the drum brakes is that after several years and
nearly 100,000 miles of my normal vigorous driving, including at least 100
autocross dates, the cast iron drums tend to get surface hardened from the
natural heat cycleing. This appearantly causes surface inclusions of very
small carbon nodules and some microscopic spalling and cracking. The end
result is that the front brakes were locking up at moderate to slow speeds
and only moderate pedal pressure when they really shouldn't. A thorough
cleaning and new brake shoes made no difference, and the drums were still
round within a few thousandths of an inch. The cure was to have the drums
turned to remove the hard surface. This required the removal of about
0.012+ inch from the radius (0.025 inch bigger diameter), as any lesser cut
would make the cutting tool chatter severely. After turning the drums
there is no more unexpected lockup.
A few years ago I converted an MGA from stock drum brakes to stock front
disk brakes (for a friend). I think this does reduce brake fade during
heavy braking under racing conditions, and may also help going madly (er,..
vigorously) downhill in the mountains. I also noticed the substantial
increase of unsprung weight from the cast iron rotors and cast iron
calipers, so for my own use I don't think the benefit justifies the
expense. ($0.02)
>but last weekend, when I drove my '97 Voyager to Muir Woods, we arrived at
the entrance (at the bottom of the hill) with burning smells coming from
the brakes and distinctly less braking. What good are ABS brakes, if there
is not enough cooling and the brakes have faded?
Uh, speaking as a sport car enthusiast, what you have there may be more a
matter of too much car rather than too little brakes. I don't know how big
the beasty brakes would have to be to haul a** down a mountain with a van
without fading or frying, especially if it's an automatic and/or you're not
downshifting enough. I would however be willing to give it a try just to
better understand what you're talking about. Betcha I can make 'em fade
quicker'n you can. <G>
(8^)-V----/ (stomp on it),
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
|