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At 04:35 PM 2/14/2000 -0700, James Nazarian Jr wrote:
>....
>.... I have been working on a formula to calculate tire size needed for a
given amount of power. What I am trying to do is figure out how much
rubber to I really need to have on the ground given a set of performance data.
Forget it. You have obviously spent too much time in school and not enough
time in the MG. (Sorry, no offence meant). Trust me, I AM an engineer,
and I thrash the crap out of my MG every chance I get. The tire size you
really want for the power of a V8 won't fit under the fenders, so you
settle for the largest size you are willing to cram in there. As a general
consensus, a 185 tire will usually fit all around without rubbing anything,
but will be very close to the inside of the rear wheel arch. For a 195
tire you probably have to roll in the lip inside of the rear wheel arch to
keep it from rubbing on the side of the tire. For any larger tire size you
have to dramatically flare the rear fenders. You might rather have a lower
profile tire on a larger wheel. A 195-60-15 is about the same diameter as
a 185-70-14 or a 165-80-14. The shorter sidewall gives dramatically better
steering response, and the larger width allows for lower tire pressure to
put down a larger footprint.
>...[snip all head scratcher formula]...
>
>.... Many would suggest just go to bigger tires, but since car is for
autocrossing low unsprung weight is important.
Stop worrying about unsprung weight. Most autocross is done on fairly
smooth pavement, so the tires don't bounce much anyway.
>Also big tires = big $$$.
Not necessarily. Big tires cost a little more than small tires. What's
really big bucks is real race tires, as opposed to street tires.
>.... I'll be glad to share formula with everyone, but I have to figure it
out first.
The formula is: Throw away the calculator and go get some competitive seat
time. Get a set of reasonably priced radial street tires (not real cheap,
but surely Traction Rating = "A" on the sidewal). Then go autocrossing
regularly for a year or two, at least a dozen race dates, not expecting to
win anything at first, just for practice. While you're doing that, keep
your eyes and ears open and pay attention to the fast guys, buy them dinner
occasionally and talk "autocross". Learn to take it easy to go fast, that
finnesse reigns supreme over horsepower, smooth is beautiful, and that the
racing classes are only about 1/2 second apart in a 60 second lap.
Sometime before your third race date you should go here:
<http://www.ntsource.com/~barneymg/mgtech/tires/tire1.htm>
and steep yourself in the practical aspects of oversteer, understeer,
traction, tire pressure, and the magic shoe polish. You won't run
consistently fast until you understand the theory and learn to apply it
(which isn't very difficult).
Then about the time you have completely worn out that reasonably priced set
of street tires you will be ready for a set of real racing tires. Take
this to heart: On a grip scale from 1 to 10, race tires are all 9 or 10,
and street tires are all 0 to 3, and there is precious little in between.
You will have to experience exactly one lap on race tires to understand
just how important that is, and I would be happy to lend you my car for the
trial. You will be absolutely flabbergasted at the grip of a race tire.
When the difference between 1st and 3rd place is often less than 1/10
second, the difference between race and street tires is more like 3
seconds, meaning always dead last for the street tire car.
Now the race tires will cost about $500 a set (discounted), and you
wouldn't want to drive them on the street on a daily basis, because they're
pretty much guaranteed to wear out in about 5000 to 8000 miles. The life
expectancy of race tires is more like 2 to 3 hours of actual track time,
but that may be at least 200 laps on the track. But then once you have a
taste of race tires you will understand how futile it is trying to find the
"best" street tire, and you may lose most of your enthusiasm for street
racing anyway, stick with cheap tires for the street and save you money for
race tires for the weekend competition events.
Also realize that low profile tires give a harsh ride, and the wheels
become suseptible to damage from pot holes and rail road tracks. For your
MGB on the street I would recommend 185-70-14, and for the track 195-60-15.
Of course you will then need that extra set of wheels as well, because you
will be swapping tires regularly on the weekends. If you are not prepared
to do this, don't get your hopes up about winning at autocross. Last
summer there was a virtual horde of MGB V8s gathered for the BV8 national
meet in conjunction with the British Car Festival in Champaign, Illinois.
My MGA 1500 running in Stock class on race tires put a whole field of V8
MGBs to shame because they all had the best street tires that money could
buy. Oh, and you will also want alloy wheels for the race tires, as the
grip through the turns is great enough to put stress cracks in steel wheels
in short order (BTDT).
Now you WILL want to read a rule book to decide what class you want to
compete in before you spend too much money on go-fast goodies. Road race
tires (treaded type, not slicks) and any front sway bar are allowed in
Stock class, but otherwise it doesn't take much to get kicked upstairs.
Tubular headers or wide wheels will put you into Street Prepared class,
where you have to run 3 seconds faster to be competitive (spend more $$$$
on more goodies). Any internal engine mod (like a hot cam) will put you
into Prepared class, which is another 2 seconds faster yet, but then you
get to run slick tires and strip the car inside and out to the bare sheet
metal (and get a trailer and tow vehicle because it ain't street legal
without headlights and windshield). If you do an out-of-make engine
transplant (ie Ford rather than Rover) you go into Modified class, where
the only parameters for classification are engine displacement and gross
vehicle weight, and you will cook your checkbook in a serious way.
For more flavor of the real world of autocross, go here:
<http://www.ntsource.com/~barneymg/mg01.htm>.
Start by poking the Tickle-Me-Elmo MG, and then continue to flip through
several pages of "normal" features of an autocross car. Then from the same
address above, a couple of links at the bottom of the page lead to some
interresting on-track photos. You will also find a link there to the SCCA
national web site, which not only provides lots of information about rules
and classes but also gets you in touch with the local clubs that run the
fun programs. And you will find that the SCCA SOLO-II rule book is only $10.
You might like to check out the Tire Rack web site at
<http://www.tirerack.com>. There you can browse for an hour or two
learning all sorts of neat stuff about competition tires, why they cost so
much, and why they grip so well, and why you get a really nice discount
from a distributor as opposed to a local dealer (and no sales tax if you're
out of state).
All that should keep you busy for a while. If you have any further
questions, just ask.
Regards,
Barney Gaylord
1958 MGA with an attitude
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