Racecar Engineering - November 2005

V-Angles
“
THERE ARE FEW
PLACES YOU’LL
FIND RACING
ENGINEERS WHO
UNDERSTAND THIS
SORT OF TYRE
DATA
”
cornering force. At the pivot point of the boom was an
operator’s seat, engine controls, and an analogue strip
chart recorder. It was relatively crude and, I can
confirm, a nauseating job for the test operator.
Subsequently, Cornell Aero Labs (now called
Calspan) took its truck-mounted tyre measuring
experience into the lab, creating a high-speed surface
made up of a textured steel belt running on an airbearing
platen between two huge rollers. My exposure
to the Calspan tyre test data came again in the late
‘70s, while working on vehicle overturn simulations
for the US DoT, at a place called Systems Technology.
We sent dozens of tyres off to Calspan for the extreme
limit data we needed. After studying the results for a
few days, however, it didn’t seem to make sense.
Ultimately, I discovered that our procedure was too
abusive, and didn’t control for the abuse, and during a
single run the tyre would wear and overheat so badly,
as the slip angle and load was increased, that by the
end of the run it was essentially a different tyre. We
rapidly learned the importance of the A-B-A controlled
test, in which you frequently return to the baseline, to
see if it has shifted. This is still true in track testing –
and even more so, as the track is probably changing as
much as the tyre is.
You may wonder just how valid racing tyre data is,
when taken on a steel belt in a laboratory. But
consider how ‘noisy’ real track data is. It takes a lot of
signal filtering to eliminate all the track irregularities
from surface contamination and other surface
coefficient variations, while the high-speed belt is selfcleaning.
I have seen load cell hubs designed to isolate
the lateral force component on racecar suspensions.
But that still doesn’t allow you to accurately control
the camber or slip angle during a test.
And that brings us up to today, and why the topic
came up. Except for F1, Formula SAE and Formula
Student, there are few places you’ll find racing
engineers who understand this sort of tyre data. That’s
why Denny Trimble (University of Washington), Dr.
Bob Woods (University of Texas at Arlington), and
Edward Kasprzak (University of Buffalo) formed a
consortium of teams, and contacted Calspan about
running comparison tests on their tyres. Since the cost
is astronomical, Calspan agreed to a student discount.
Doug Milliken volunteered to handle the
money, he and Mike Stackpole volunteered
to analyse the data into Matlab and Pacejka
formats, and Goodyear and Hoosier
donated tyres. Ultimately, over 30
schools joined the consortium, at $500
each, to have access to all the data.
Most of the rest of the schools felt that their students
weren’t ready for that degree of sophistication –
although anyone can buy the data later.
Dr. Woods developed the test plan, with feedback
from Calspan’s test operator, Dave Gentz. Based on a
survey of member teams, they decided on seven tyres:
a comparison of two diameters (on 10 and 13in wheels)
of the same width, a comparison of two widths (6 and
7in) at the same diameter, all from both Goodyear and
Hoosier, plus one tyre from Avon. The standard test
procedure is to fix the pressure, load, camber angle
and speed, then during a run, sweep through
continuously varying slip angles, while recording six
components of force and moment, plus three infra-red
tyre temps, followed by a needle probe at the end. In
this case, the upper limits were 450lb load, four
degrees camber, and 15-degree slip angle, even though
the tyres seem to reach their peak at about six
degrees. A slip angle sweep starts slightly offset,
passes through zero to peak cornering force one
direction, passes through zero to a peak in the other
direction, than back past zero again. Five increments
of load and camber were taken to define a curve.
At press time, five of the tyres had been tested in
two days, and none of the raw data had been reduced.
Kasprzak was the attending test representative, and
some of his comments were ‘...they act like real race
tyres...very sticky...the test wasn’t too abusive...’ And
their budget affords one more day to test the other
two tyres, and to resolve any other questions in the
data. I asked him if there were any surprises in the
data that he could share, and he said he had been
more concerned with making sure the data was
complete and the runs were consistent. But he
admitted he was surprised that these tyres seemed
relatively insensitive to camber. That would be a
revelation, considering how much time engineers
spend using camber to balance a racecar.
This was a groundbreaking event for racecar
engineering students. The combined efforts to get this
data will make their modelling a lot more accurate.
And yet the data selected was primarily for design or
simulation engineers, and not much use for track or
development engineers, who more likely need to
know how tyre characteristics vary with temperature.
When I use a skidpad to study tyres, I record speed or
gs or Cf while watching infra-red temperatures (the
control variable), to resolve which tyres have the best
Cf at what temperatures. Then, you find the optimum
pressure and camber by running them in steps through
that temperature. This should be very easy to run at
Calspan also – just find the peak force slip angle, then
run there at a constant speed until the temperature
rises through the optimum. Maybe they’ll try that on
the remaining day.
As Kasprzak said, differences appeared small.
However, those miniscule differences are what wins
races in these days of otherwise nearly identical cars.
Next year we may see some of the teams running
different tyres depending on manoeuvre and ambient
temperature, or pre-heating tyres for short runs. RE
24 November 2005 Racecar Engineering
www.racecar-engineering.com