Jeff,
Congratulations on your interest in finding out what is hype and what is
real in the ongoing tire wars drama,Y2K version.
I'm here to make suggestions regarding a skidpad segment of your
evaluation.
First, 100 ft diameter is TOO SMALL! The steered wheel angle required
to follow that path will wear the tires pretty quickly. I know, many or most
autocrosses have turns with that type of size, but those are rarely taken
at a "steady state" cornering force. Instead, they are entered under braking
and power is usually applied from mid-corner onward. You can use such
a radius turn for a GEEZ! evaluation, but I would suggest a 180 instead of
a full circle. Then you can use it to judge maximum "Peak" lateral accel.,
as well as a braking/cornering/acceleration graph.
For a "Steady State" cornering evaluation, I would instead suggest 100 ft.
RADIUS. Get yourself a 100 ft. length of rope at any general store, get to
the flattest section of your lot, and have your assistant hold one end over
a center cone while you walk around and place cones on the perimeter
(which will form the inside of your testing lane). This will be closer to the
constant radius "sweeper" more commonly seen in Nationals type courses.
I expect that using R tires will get you up to between 45 and 50 mph. You
should be easily able to generate the data you want in 3 laps. Set yourself
up a sequence to follow with each tire set so that the data comes in a
defined order for each. I would suggest approaching the circle at a speed
which will not have you upsetting the car's balance at entry, say 25 mph.
>From there, gradually (but with deliberate additional throttle application)
add
speed until you get to the limit of adhesion, and a little beyond, if it
seems
warranted for the test. Do not use "hard" acceleration.
Note these important items:
1. Your steering angle will change as you increase speed. No surprise
here, additional slip angle is needed to generate more cornering force
as your speed goes up. KEEP TRACK OF IT! Do that however you
want; index mark the wheel, use a pointer attached to the dash,
whatever. At the very least you would want to record the steering angle
needed at the limit of adhesion, right before control goes away and the
tire can no longer respond to additional steering angle or speed. You
might want to record more points than that. My feeling about testing is
always to gather as much data as possible. This will give you valuable
additional ways to compare tire brands.
2. Force yourself to maintain an extremely stable steering wheel angle. If
you are truly interested in comparing tires, you must remove as many
dynamic changes as possible. Even a lot which looks flat will have
undulations which show up at 45 mph, and those may upset the car a
little. But if your compensations for surface irregularities are more
than
one or two degrees SWA, you will not be able to maintain a round circle.
Plus, your cornering force would then not be "steady state", but be
graphed as a weaving line, and harder to define as an upper limit.
If you wish, as the ending part of the test in each direction, see if you
can increase speed by using the sawing motion of the steering wheel
that would be taking the tire above and below its "critical slip angle".
The drivers who feel they can get just a little advantage by using the
wheel in that manner are legion. See if they are right or just fooling
themselves???? An electronic lap timer would be invaluable here......
3. Doing the test with several pressure levels, and even changing front/
rear pressure relationships will give you an excellent broad database.
You might want to make some subjective notes as you proceed, such
as the ubiquitous "feel" of the tire. Also note such items as "breakaway"
(hard/fast or soft/gradual) and recoverability. Don't depend on your
memory to handle that. At the end of the day, you will be really thankful
to have volumes of precise notes.
4. Definitely run each setup in both directions. If nothing else, it may
show
you something about your car. If there are notable differences in
directions,
see how they correspond to tire brands, and whether or not the averages
between the two directions are more indicative of total performance. If
you do get a big directional disparity, swap tires from side to side and
go again!
5. Expect to get better at the testing procedure as you go. It will not at
all invalidate your first part of the test, as long as you paid attention
to
being precise and steady with regard to your steering input. Rather, it
will just seem more natural to you.
6. If he/she has an Iron Gut, take along your helper. That person can keep
track of the information inside the car such as steering wheel angle and
speed. The driver's top priorities are keeping the circle round and the
the steering wheel steady. Harder to do if you are looking inside the car
at the wheel and speedo.
7. Gear up. Concerns about starving the engine for oil will be avoided if
you
just keep the rpm down. Why not use 3rd or (gasp) even 4th gear? As
long as you have the torque to gradually raise the speed to and through
the limit, why turn the session into an engine stress test?
8. If you should find a tire that sticks so well that you get wheelspin due
to
unloading the inside front before it washes out under cornering, we'll all
want to know about it! That brings me to......
9. SHARE your results with the rest of this list! (team.net be damned <G>)
Any questions?
Alan Sheidler
(been around and around and around more times than I care to guess)
Oh yea... I think the idea of maybe a 6 cone slalom for transient response
characteristics would be a very good idea too!
In a message dated 01/27/2000 1:01:39 PM Eastern Standard Time,
cashmo@spec-rx7.org writes:
> With all the technical knowledge found on the Geez! list, I figure it's
> the best place to ask these questions. Grassroots Motorsports keeps
> ignoring my requests (sigh) <g> to do a new DOT R series tire test so
> I'm looking for input on using Geez to help with my own testing since
> I've never done anything like this before.
>
> FWIW, I've heard all sorts of rumors regarding tires changing for 2000.
> That the Hoosier R3S03 compound is harder to last longer for endurance
> racing. That it hasn't changed but the A3S03 has. That the G-Force R1
> will have 4/32nd of tread this year so it lasts longer. That it will
> have less steel in the belt package and therefore be more compliant and
> less sudden in it's characteristics at the limit. That Kumho is coming
> out with a new tire called the X-something (X-ta?). So far it's a lot
> of talk, nothing confirmed. Even if they do tweak things, who knows if
> you'll get new tires or old stock when ordering.
>
> At my disposal I have an assistant, a FWD stock class car, a large
> asphalt lot (owner approves), approx 40 cones, stopwatch, air
> tank/pressure gauge, tire pyrometer, Geez!/laptop setup and multiple
> sets of rims for the different tire brands. My first idea was to set up
> a skid pad to try and measure max lateral grip. How large should the
> circle be? 100 ft diameter? Do I run the car up to and slightly beyond
> it's friction envelope then back off a bit, (3 laps max so the motor
> doesn't starve for oil?) stop measure tire temps and then do it again in
> the reverse direction? Then, measure tire pressures and try it +/- 5
> lbs? (I'm mostly concerned with how the front tires are doing) Then
> mount up the next set and repeat the procedure?
>
> Other than that, I'm looking for suggestions. I'd like to do a U-turn
> or slalom test but I'm afraid the stopwatch wouldn't be accurate
> enough. It would still be good for a subjective feel though. Perhaps I
> could setup a course similar to what GRM did for their plus-size tire
> testing in the Jan/Feb 99 issue.
>
> Are there any Geez! options that should be changed? Data based on
> absolute as opposed to sustained peaks? Raw unadjusted data?
>
> Any input would be appreciated.
>
> Thanks,
> Jeff
>
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