Oh boy. I'm glad you made it out of there under your own power (and not on
the end of a hook, or in an ambulance).
Your experience illustrates why I was so skeptical of rear discs on a street
B when the idea was proposed recently. More rear braking is really not what
the doctor ordered*. If you could balance it with 12" slotted front rotors
and 4-piston calipers, you might have something (though on typically skinny
MGB street tires you'd mainly have 4-wheel lock-up). What you need is a
proportioning valve to dial back the rear braking... probably pretty much to
the point where it's exactly the same as the old drums.
My understanding was that a limited slip diff would promote understeer,
unless you had gobs of power. That seems to be what occurred when you
understeered straight off the track.
So now you seem to have a combination of power-on understeer and rear
lock-up under braking -- that doesn't sound like the optimum setup for
predictable handling (to say the least).
It looks like you are faced wth another round of upgrades: vastly improved
front braking, and greatly increased horsepower. Or, I suppose you could
just chuck it and go back to where you were. You might want to talk to
fellow racers with similar setups (if there are any) to see if they have any
suggestions.
*On my Barracuda list we had a similar discussion recently. Someone was
advocatiing upgrading the rear drums to larger diameter, wider units from a
full-size Chrysler. My response was this was unnecessary and foolish -- the
rears tend to lock up first, anyway, due to the front weight bias, and it
adds both rotating mass and unsprung weight. At least the rear discs are
presumably better on those two counts.
on 3/13/04 11:35 PM, Eric at eric@erickson.on.net wrote:
> OK, I was asking what I should expect from my new rear disk brakes and
> LSD - out on the racetrack.
>
> Well, I can tell you.
>
> After minimal practice laps (we only had four qualifying/practice laps
> in the morning due to delays in scrutineering) and I came off the track
> feeling very unsettled. It just didn't feel "right" and I really didn't
> push it too hard (brakes and/or cornering). It was my first time back
> on the track after a year off and I used it more as a recon. run -
> remembering the corners.
>
> I was expecting another round of practice laps - but they didn't come.
>
> So straight out into competition and I wonder how many of you have
> pulled on the handbrake at 100 miles an hour? Because that was what it
> felt like on our little back straight (ooops, too much rear break - too
> late to adjust bias then).
>
> That would have to have been the fastest spin I have ever had and it
> left me seeing WALL, TRACK AND CARS, WALL, TRACK AND CARS - one complete
> 360 and some say another - others just say a further 180 degree spin
> then a long way sideways until I grabbed it and got it back under
> control. This was off the track in the grass and dirt. I honestly
> can't tell you how many times I went round - but I travelled a long way.
> Instantaneous spin - I never had a chance to try to catch - it happened
> SO quickly.
>
> That wall was far too close for comfort.
>
> The LSD was another story. Later in the day I found myself braking too
> early into a corner (not used to the brakes and not remembering braking
> points) with someone closing rapidly behind me. I made some stupid
> spur-of-the-moent decision to just forge my own OFF-line through the
> corner and floored it... the limited slip diff did just that, ploughing
> straight through the corner on the line I took. Not turning, drifting
> or whatever, just straight through and out the other side - off the track.
>
> Man - I have some setting up to do and some getting used to this new stuff!
>
> Oh yeah, I was running on borrowed rear tyres that turned out to be
> garbage, they would "let go" at the weirdest places (after I had gotten
> through a fast corner). According to witnesses it wasn't diff lock...
> they just reached a point where they let off a puff of smoke and said
> they didn't want to play any more sending me tank slapping until I could
> grab it back again.
>
> Oh yeah - when I get the energy (calm down) I will find out what went
> "bang" in the front suspension on the final lap of the day - I finished
> the lap - but she is sitting a bit funny. I was too disappointed to
> look at it before heading home
>
> A hard day at the office - slow lap times - lots of grass clippings -
> lots of frustration. Why is it these things are never finished in time
> to test properly before throwing them into the action?
>
> I might go walk the dog!
>
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
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