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Skidpan Testing

The never-ending corner

by Julian Edgar

Click on pics to view larger images


This article was first published in 2008.

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.

Click for larger image

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

  • Maximum Lateral Grip

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.

Click for larger image

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.

  • Handling

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.

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.

Testing

I use skidpan testing a lot - and I do it in two quite different ways.

  • Roundabouts

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.

Click for larger image

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.

  • Marked

Click for larger image

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.

Machine

Lateral Acceleration(g’s)

Greenspeed GT3

0.36

Greenspeed GTR

0.37

Greenspeed GTC

0.37

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)

30 psi tyre pressures, standard ride height

0.36

30 psi tyre pressures, lowered ride height

0.39

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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