Launch techniques

[C M Smith <cmsmith@domain.elided>]

Acceleration is limited by the grip of the tires. It is understood that 
pneumatic tires generate maximum grip when rotational speed is 10 to 15 % 
faster than freewheeling speed would be, i e the drive wheels need to slip 
by about 10 to 15%and the speedometer reading would show 10 to 15% faster 
than the actual speed of the car. I take this figure from the various 
articles about how ABS works. The precise overspeed of the drive wheels 
required is hard to describe in words but easy to feel in practice. You 
feel the tires "hook up" as maximum traction is reached.

Acceleration is also limited by the torque available to drive those wheels 
at that speed. Once you have generated the wheelspin I refer to, any more 
torque produced by the drivetrain is wasted in heating the tires to no 
effect, and indeed will result in less traction being generated for 
acceleration.  Generally, with an open differential and 2 wheel drive only 
one tire will spin and that will be the tire that limits acceleration. Not 
always though, I've noticed, as you can get both drive wheels to spin even 
with an open differential if traction is very good.

Every car/tire/engine tune/weight combination will require different launch 
techniques for maximum acceleration. Front wheel drive cars are a little 
trickier to deal with because the weight comes off the drive wheels more 
quickly as you accelerate more quickly so you must not continue to squeeze 
down on the gas as acceleration picks up. Paradoxically, with fwd you need 
to ease out of the gas as the tires hook up in order to maximize grip for 
acceleration. More torque from the engine rarely translates to quicker 
standing start acceleration times, but of course does improve roll on 
acceleration times significantly.

Rear wheel drive with an open differential is much easier to handle as you 
can really get into the throttle as soon as the tires begin to hook up, 
confident that rear weight transfer will give those drive wheels more grip 
as acceleration proceeds.

 With an lsd both fwd and rwd become much easier to accelerate optimally 
as one more variable is reduced.

All wheel drive vehicles require high levels of skill to accelerate due to 
the phenomenal grip of all four wheels and the odd effects of weight 
transfer. Essential to good acceleration for awd is a lsd center 
differential, and a rear lsd helps a great deal also. Otherwise, what you 
gain at the back is lost at the front.

 Generally, maximizing launch with fwd will not involve any clutch 
dropping as the key is to get those front tires hooked up and keep them 
that way by reducing torque momentarily then getting the hammer down as 
soon as traction stabilizes. This is not hard on the clutch. Dumping the 
clutch either produces prodigious wheelspin which gets you nowhere, or bogs 
the engine. Occasionally, on a one time basis only,  there will be  a loud 
bang from the differential followed by very poor acceleration! Also, with a 
turbo, shifting UP into a higher gear early is sometimes essential for 
maximum acceleration if traction is less than perfect. My SAAB Aero 
produces 252 lb ft of torque at 1,900 rpm and you simply cannot use that 
torque driving only the front wheels (or wheel as is usually the case!). 
Once rolling however, a SAAB Aero will run with some pretty expensive and 
powerful machinery, and keep running with it to well over 100 mph. My 164 
cannot overpower the front wheels on dry pavement so quick acceleration is 
child's play: rev to about 2,500 and slip the clutch in while squeezing 
more gas just as the tires begin to break loose, then drop the rest of the 
clutch and stand on the gas. Reasonably effortless 0-60 times in the 8 
second range are easily repeatable (slower at higher elevations of course).

Practice practice practice is the only way to find the best launch for your 
vehicle with you in it.
Michael Smith
White 1991 164L
Original owner
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