How do you launch a car?

[waters17]

I'm referring to a car with a manual transmission, and I'll use the e39 540i M62TU as an example, but the principal should be the same for any car, so reference whatever you like. I'm not really concerned with shift points after 1st, because they will vary with each car.

The specs give the following:
HP - 291@5,400 RPM
TQ - 340@3,600 RPM

What is the relationship between torque and horsepower?

Here are my thoughts.
Ideally you'd want to dump the clutch at 3,600 rpm, right? Wouldn't that cause wheel spin? If you dump the clutch before 3,600 rpm, you won't get optimal torque. However, If you go over 3,600 rpm, you lose torque, but gain horsepower.

Would you have to find a 'sweet spot' somewhere before or after 3,600 rpm?

[auaq]

Chuck (CSBM5) once explained how to launch a car and especially in the M5 over at M5board.com. I'll try to find the link and post it here.

[geargrinder]

Trebuchet.

[waters17]

Chuck (CSBM5) once explained how to launch a car and especially in the M5 over at M5board.com. I'll try to find the link and post it here.

Yay! Something to look forward to today!




Trebuchet.

I thought that method only works for a Peugeot.

[auaq]

Still searching for it but I can't seem to find that info. Pretty sure it was Chuck who explained it pretty well. Just PM'd him a while ago.

- - - Updated - - -

Found it thanks to him.


Your goal is to balance (in a very quick fashion) clutch engagement versus power supplied. That balance occurs during the initial engagement that creates the weight transfer to the rear. As the weight shifts to the rear you can quickly get the process done and be rocketing out of the hole with a hooked up clutch and hooked up tires. The devil is in the details of this of course. http://www.m5board.com/ubb/wink.gif

DSC has to be off of course or you can't even come close to a strong launch. Hence the reason you are only doing this on a closed course (autocross, dragstip) or your own property.

If you don't have previous experience with maximum launching of a RWD car, it would be ideal to practice such on a different car since the M5's clutch does not care much for a ham-fisted driver burning a few hundred HP through it when it isn't fully hooked up.

There are two big variables on traction during the launch. The first one is that initial weight transfer you create before you go for the whole launch. It's a very quick process, but it is critical to getting the most out of the rear tire's abilities. Mastering it just right will greatly increase "launchability". Take a look at the start here in this video of me driving at a BMW club autocross at the Performance Center (http://www.youtube.com/watch?v=zix2i_m9j9I). It's the only in-car video I have of a launch. Note the very small elapsed time between my initial engagement (the car lifting up), and then clutch hooked up and full power (almost) out of the hole with just a bit of wheelspin. This particular launch above is a perfect segue into the second and biggest variable: tire traction.

The above was done on R-comp tires in warm weather, so traction was superb. However, depending on the number of heat cycles your tires have done, the ambient temperature and the tire type, your launch parameters (starting rpm, rate of engagement and power application) will vary (perhaps dramatically for well heat cycled tires on cold pavement!). Not much more to be said here except brand new tires (after you have worn off the mold release compound) with few heat cycles will always have a ton more grip than well heat cycled tires. This last fact is what always makes me chuckle when I see a review of how great some new tire brand is compared to someone's old tires they just finished using up.

Some specifics to try... If the ambient temperature is reasonable and you have relatively fresh tires, shoot for a starting rpm between about 1800 to 2200 rpm. Use the throttle to balance the clutch engagement during that brief initial weight transfer period, and once you get weight starting to shift fully to the rear, use a LOT more clutch engagement coupled with more throttle. If you do this just right, you won't overdrive the rear tires but instead will be able to manage tire hook-up with the throttle while the clutch is fully out. On a hot track with R-comps, you can aggressively get into the throttle without too much wheelspin worry (as in video), but on street tires you have to be much more judicious with the throttle. As ambient temperatures fall below 60F, you have to be “exponentially” more careful, seriously.

The key to grip on launch is to NOT overdrive the rear tires. Once you loose that sweet spot of about 10-15% slip at the tire's contact patch, you'll have to really get out of the throttle to regain grip and the launch is blown. Practice feeling that weight transfer, developing the feel for how you can progressively (but very quickly) get the clutch engaged and the power down. A perfect launch will not be a wild, smoking mess (which the M5 is easily capable of doing), but will result in a brief small amount of wheel spin (maybe 1 to 2 car lengths on relatively fresh, high quality street tires) and a totally hooked feeling from behind the wheel.

Start small and work your way up. Work on clutch engagement. Get the feel for throttle response. Etc.. If you ever mess up and start putting too much power through the clutch before you have it hooked up, GET OUT OF THE THROTTLE IMMEDIATELY. Let the clutch cool and try again later. By the way, if you are trying multiple launches within a short period of time, you'll find that the clutch engagement capabilities will vary as its temperature rises, so give it some time to cool off.

That's all I can think of at the moment other than you're of course doing all of this at an autocross, the track or on your own property. At the strip, you'll have the additional variable of how the launch area is prepared which can make a HUGE difference in traction and how you manage this whole process.

[waters17]

Good stuff!

[dk500]

5k rpm and dump the cutch

[auaq]

5k rpm and dump the cutch

... and leave the DSC/ASC button on. Hehee

[waters17]

Too many times I've forgotten to turn off the dsc... Boy, was my face red.

[dk500]

Stallllll