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Misc stuff

To: mgs@Autox.Team.Net
Subject: Misc stuff
From: Bill Eastman <>
Date: Thu, 05 Jun 1997 17:11:46 -0500
I've put another 200 miles on the A this week.  Lots of fun.  The kids are
dissapointed when I show up driving anything else to pick them up from
their various activities.

One thing that has surprised me is the relative rigidity of the chassis. 
The roads around here are pretty rough- potholes. frost heaves, etc. yet
the car exhibits very little cowl shake.  It is much better than the
Spit**re that I had and at least as good as the 84 Cor***te that replaced
it (and was replaced by the A).  I remember when the C4 chassis was
introduced it was considered to be much stiffer than version it replaced. 
Good old Syd and company did a very good job on the A chassis.  

When I replaced the emergency brake cable, I discovered a disturbing fact. 
The cable is tightly wound under a section of the frame. The rear brake
line also follows that section of the frame and, in my car, the two were
rubbing against each other!  The brake line was polished in that area but
didn't look like it had lost too much thickness.  I split a piece of 5/16
fuel hose and put it over the emergency brake cable to protect the brake
line.  I HIGHLY RECOMMEND that you other A drivers check this on your cars.
 It's scary.

I noticed a thread a while back on tire pressures.  In the early eighties,
Honda was developing their new Accord- what they called the NA (new
automobile) platform.  They listed the design goals of the team and one
goal was to have a lower hoodline than the Porsche 911.  Anyone who had
driven an 86-89 Accord or 88-91 Prelude knows that they have incredibly low
hoods.  The design to accomplish this goal was very advance but it did
result in some compromises.  These Honda's have very little front
suspension travel so they have very stiff tuning to avoid bottoming.  To
maintain an acceptable ride, they specified extremely low tire pressures
(26psi front, if I remember correctly).  This had some interesting side
effects.  The stiffness of the front suspension and the soft tires mean
that these cars understeer like crazy.  However, one can really tune the
car for a personal compromise of ride vs handling.  I ran mine at 36PSI in
front and turn in was pretty good.

What is my point?  In modern tires, tire pressure (within sane limits) is a
tool to tune ride vs handling and oversteer vs understeer.  It has very
little to do with tire wear any more.  My Sunbird, which was about the same
size as the Accord, specified 32psi in its tires and neither car wore their
tires funny. They just chose different solutions to the NVH compromise.

What are sane tire pressure limits?  Look at your tire.  It should list a
maximum load at a maximum pressure.  Divide the maximum load by the maximum
pressure.  Divide this number into the expected maximum load on your lbc to
get a minimum inflation level.  For instance, a tire rated at 1200 lbs at
30 psi  would carry 40 lbs per psi.  If you expect that you will never
exeed 1000 lbs per corner, then you could safely inflate to 25 psi as a
minimum (1000/40).  I expect that a similar calculation could be made to
estimate an upper limit for autocrossing (1200*30/1000=36psi).

Standard disclaimers apply.  I am assuming a linear reletionship when, for
all I know, pressures could by proportional to the inverse of the
hyperbolic sine or some other strange thing.  I run the A tires at 30 psi
because the chassis is stiff enough to handle the extra shock transmitted
compared to a lower setting.  That high of tire pressure on my Spit**re
would have caused the cowl to shake like a hula dancer.

Bill Eastman
61 MGA

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