Skidpad question

[DSM97GST]

hey quick question, what does the skidpad of .9 g mean? How is the skidpad performed on driving tests? Is it performed at a certain speed?

 

[jtmcinder]

You choose a diameter of circle and keep looping until you have found the minimum time required for a loop. The formula that converts the diameter and time to lateral g is quite simple and if you want I will find it and post it.

There are standard diameters that are used and this makes a difference. My 2G, for example, produced almost a full g for a largish circle and only .88 for a small one.

- Jtoby

 

[Defiant]

Standard for lateral G tests is usually a 200' diameter circle. .9 would mean that a 3000 pound car can exert a controllable, steady-state lateral force of 2700 pounds. Changing tires will change this value. Tire pressure will change this value. Road surface, tread depth, moisture, and probably which damned selection of noise coming out of the CD player will make it change. Further, steady-state cornering is only of any value on that specific pad at that specific time, and is only a very approximate number for use against other chassis' approximate values. You also won't get the same number in both directions, to the extreme of cars meant to run on oval or circle tracks.
The test is done by driving on the circle at increasingly higher speeds until the car is no longer controllable on that circle. Drifting doesn't count, because drifting is to lateral Gs as movies are to real life.
The slalom test, where transient grip is slightly better-demonstrated, can be of more value.

However, as you'll find with enough time in a given car and enough trips through the same bit of twisty, "cornering" doesn't often mean the same as "handling". And in the DSM, you still have the eternal situation of having all that weight so far forward.

 

[jtmcinder]

Taking Defiant's point a bit farther, the optimal set-up for skid-pad tests is often quite different from the optimal for, say, autocrossing. Skid-pad is all about sustained lateral acceleration at the limit. Autocrossing is much more about transient behavior. This is a very important difference to keep in mind as you read various texts on handling. Most of the "rules" for increasing handling were written from a road-racing perspective, which is much closer to skid-pad (sustained) than autocrossing (transient).

- Jtoby