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This article was first published in 2008.
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There are two ways to use skidpan (or skidpad)
testing. Firstly, to assess and optimise the handling of an existing design of
vehicle. Secondly, to help in the development of completely new vehicle
concepts. Both are incredibly important uses – but first of all, what is a
skidpan?
Skid Pans
Throw out any ideas you might have of drift
merchants circulating a piece of bitumen in a lurid tail-out slide. Instead,
picture a skidpan as simply being a marked circle around which the vehicle is
quickly driven.
Rather than a place to do slides, think of it as
a corner that never ends.
The continuous corner idea immediately
highlights the importance of a skidpan. Instead of being able to use a
traditional cornering ‘slow-in, fast-out’ approach, you have to maintain the
cornering line. Rather than following a ‘racing line’, the line you follow is
prescribed. Rather than being able to get a higher cornering power for just an
instant, it must be held continuously. And to negotiate the never-ending corner,
the steering must be accurate and sensitive, the motive power must be able to
generate enough power to keep you going around as fast as possible, the tyres
must continue to develop adequate grip, and the oversteer/understeer balance
must be good enough to allow the driver to keep all the wheels pointing as much
as possible in the direction of the corner.
Because, if any of these things cannot be
achieved, you’ll be going slower!
And the aim of a skidpan is to go as fast as
possible around the marked circle of a known diameter.
Uses
If you know the diameter of the circle and how
long it takes the vehicle to get around it, you can then work out the vehicle’s
maximum lateral grip (“cornering g’s”). This number represents the fastest that
a vehicle can go around a continuous corner, or to put it another way, the
maximum sideways grip it can develop – usually, on a smooth, dry surface.
Being able to directly measure the maximum
sideways grip is a fantastically easy way of finding out something that is
otherwise very hard to accurately calculate.
Sure, if you know the centre of gravity position
(height and position both longways and sideways), wheelbase and track, you can
calculate the cornering force at which the vehicle would overturn.
Alternatively, you can tilt the whole vehicle so far the inner wheels lift, and
then calculate the equivalent force from the tilt angle. And in fact, some
people use these numbers as if they’re true “g” figures. But of course they
aren’t – they don’t take into account how much the tyres can actually grip, they
doesn’t take into account whether the steering is good enough to allow the
vehicle to actually be driven in that way – and so on.
When the discussion is of unusual vehicles (for
example, tilting three wheelers), skidpan testing represents an easy way of
sorting fact from fiction.
For conventional vehicle designs, or ones where
evaluating of handling rather than pure grip is required, the skidpan is again
extraordinarily useful.
On a skidpan the fundamental steady-state handling
traits can be explored – for example, the understeer/oversteer balance. In
addition, the response of the vehicle to power changes is very easily determined
- for example, whether the car throttle-lift oversteers.
It is much easier to feel these aspects when
continuously cornering than it is when only ducking in and out of corners.
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Downsides
Of
course, there are many aspects of grip and handling where a skidpan is useless.
Turn-in handling behaviour cannot be assessed, the poise of the vehicle under
hard braking is never judged, and it would be a fatal mistake to assume that a
vehicle with a higher lateral ‘g’ figure is always going to out-handle one with
a lower figure.
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Testing
I use skidpan testing a lot - and I do it in two
quite different ways.
The first could perhaps be called ‘informal’ –
whenever I have a new car to test, I make sure that at some point I drive it
very hard around a medium/large sized empty roundabout.
I enter the roundabout relatively slowly and then
accelerate to the point at which the car is sliding. In every current car the
front will lose grip first – it will understeer. I then lift the throttle fairly
quickly and see what happens when the weight transfers forward. In many
front-wheel drive cars, the front will tuck-in and in some, the rear will slide
into oversteer. In rear wheel drive cars, the application of more power will
cancel the front understeer and push the car into oversteer.
Clearly, sliding cars around public road
roundabouts is quite politically incorrect – and may in fact be illegal in some
jurisdictions. Any use by you of the technique is completely at your own risk –
and also note that it requires far more skill than is shown by many drivers.
However, as stated above, I am emphatically not talking about drift-style
slides; instead, an observer would probably not even realise the car was
sliding. If the steering wheel of the understeering car is turned so far that it
is ploughing, you have not been listening to the car. If the steering is turned
so far in opposite lock that it can even be noticed from the outside, you have
let things go too far.
The beauty of an increasing speed skidpan test is
that things happen relatively slowly and with plenty of warning; as a result,
the corrections of steering and throttle can be achieved with subtlety.
Exactly the same test procedure applies to cars
with stability control; in fact, this is a really good way of feeling its
characteristics. Some manufacturers allow quite a lot of driver control of a
sliding car, while others shut things down very quickly.
I also suggest that a roundabout skidpan test is
by far the safest way of road assessing a car that has had handling
modifications. Especially when making mods that result in lift-off oversteer
(for example, stiffer rear springs and/or rear anti-roll bar), it’s vital to feel
the car’s behaviour when the car is on the edge of sliding.
In these types of testing, the time taken to
negotiate the skidpan circle is of far less importance that using the test to
feel the car’s characteristics.
The other skidpan testing that I do is on the
Human Powered Vehicles (HPVs) that I have developed.
In the field of HPVs there are some very
innovative designs – three wheelers that have wheels that lean into the corner,
machines with ultra-low centres of gravity, and those that tilt the rider.
Assessing the cornering prowess of these machines solely from their
specifications is impossible, whereas a simple skidpan test will soon show the
maximum continuous cornering power. Finding out the on-road reality is
especially important when (apparently) some of these machines cannot be pedalled
while being cornered hard, and (apparently) others are difficult to balance when
being cornered at max potential.
It’s also illuminating to note that many of these
more exotic machines are said by their owner/developers to handle like “they’re
on rails”!
I use a 6 metre diameter circle marked with chalk
on the roadway outside my house. I am fortunate to live at the end of a quiet
cul de sac and while the neighbours look on with puzzled amusement at my
testing, no-one has ever complained. Timing is by an observer with a
stopwatch.
The equation to work out the maximum lateral
acceleration is:
39.48 x radius
----------------------
time squared
...where radius is in metres, time is in seconds
and the answer is in metres per second per second. Divide this by 9.81 to get
the results in g’s.
Skidpan testing has proved to be of great use to
me in making comparisons of variations in fundamental designs, tyre pressures
and ride heights. Especially on three-wheel machines that will tip when their
limit is exceeded, it is also a good test of steering sensitivity and precision
– good steering is needed to keep a trike balanced on the edge (or not balanced,
as the above pic shows!).
The following table shows some measured cornering
g’s obtained with recumbent pedal trikes on a smooth, dry, 6 metre diameter
bitumen skidpan.
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Machine |
Lateral Acceleration(g’s) |
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Greenspeed GT3 |
0.36 |
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Greenspeed GTR |
0.37 |
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Greenspeed GTC |
0.37 |
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Greenspeed X5 |
0.39 |
|
JET |
0.41 |
The following table shows the results achieved by
making changes to one machine:
|
Air 150, different configurations |
Lateral Acceleration(g’s) |
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30 psi tyre pressures, standard ride height |
0.36 |
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30 psi tyre pressures, lowered ride height |
0.39 |
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60 psi tyre pressures, standard ride height |
0.32 |
Conclusion
Whether you informally explore the concept when
negotiating a roundabout, or you’re testing a unique vehicle on a marked circle
under strictly controlled conditions, skidpans are enormously useful in
assessing vehicle dynamics – whatever the vehicle.
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