Re: Cornering - road car v F1 car


"Dave Baker" <Null@xxxxxxxx> wrote in message
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"David W. James" <unend@xxxxxxxxxxxxxxx> wrote in message
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An interesting exercise, but I am not sure how useful it really is.

Skidpad g-numbers are good for comparing tires if you use the same
care, or cars if you use the same tires, or even the same car and tires
with different suspension settings. How usefully the numbers translate
to use on a track is another question.

Case in point: for a few years I autocrossed a BMW M-Coupe. At SCCA
and some other events I used various R-compound, DOT certified tires
(Yokohama A032rs, Kumho 700 and 710s and Hoosiers), while at BMW club
events I used the street tires (Sumitomo HZRs and some Michelins, if I
recall correctly.) I also ran a G-Cube, so I got readouts on my
cornering forces, both peak and sustained.

All four of the various R-compound tires produced sustained lateral G
forces of around 1.35G. The G-Cube software defines 'sustained' as over
.1 second in duration. Race cars, or even autocross cars, seldom do 100
foot circles, and even if you do a perfect circle isn't likely to be the
fastest way around it.

Whatever the fastest way is round a given corner, which may indeed not be
to maximise the bend radius in all cases, the analysis shows what the two
types of car will do in the radius actually chosen in that corner. The
reason why it might not be fastest to maximise the bend radius depends on
the acceleration capability of the vehicle. Very fast vehicles can gain
slightly from using a tighter corner radius so they straighten up quicker
and get back on the throttle. Slow vehicles just have to minimise the
speed reduction by maximising the corner radius because they can't build
that lost speed back again quickly. The late entry, tight turn and early
corner exit strategy was first exploited by bikes maybe 30 or 40 years ago
but of course cars, such as F1 cars, can generate similar acceleration
nowadays. Whatever, it really doesn't have much to do with my general
analysis to give people an idea of how the two types of car compare.


The street tires varied more, but the various Max Performance street
tires all produces about 1.15G sustained.

By comparison, I recall a friend's M-Coupe (set up for One Lap Of
America with a *much* better than stock suspension) "only" pulled about
1G on the skid pad at Tire Rack.

I think you've answered your own question. The G Cube isn't giving an
accurate representation of truly sustained, by which I mean a lot longer
than 0.1 seconds, cornering forces. Devices which rely on an inertia
mechanism to estimate cornering forces can be caught out by short duration
pulses and also by lean of the vehicle which they interpret as additional
G force.

At the end of the day if you have a vehicle pulling an accurately timed
speed round a corner of accurately measured radius the G force isn't in
dispute. It's a function of very simple maths. What an electronic gadget
says about that same measure is very open to doubt. If you think your car
could pull 1.15g on road tyres when every other measure says it can't you
have to look more carefully at your data.

PS - if a vehicle leans by 5 degrees in a corner, which is not a very high
angle of lean for a tin top, a g force meter will interpret that as nearly
9% additional cornering force. 10 degrees angle of lean would add 17% to the
measured cornering force. It really isn't that hard to get these devices to
show much higher readings than the car is actually pulling. Tip the box on
its side and it will think it's cornering at 1 g when it's actually
stationary.
--
Dave Baker
Puma Race Engines


.

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