Tires are strange beasts and the physics behind them are not entirely understood. Sometimes they behave in counter-intuitive ways. For example who in his right mind would think that delivering power to the wheels in short violent bursts instead of continuously would increase tire grip?
Experts say that you get max acceleration when there’s a small amount of wheelspin (a number often quoted is 10% speed difference). Don’t know why this is, but generally tire behavior cannot be explained by the traditional model of friction alone because there are a lot of secondary effects that cannot be easily taken into account. Since we are talking about drag racing, one of these effects is sidewall flex. Look at the first pic here for an example: http://www.competitionplus.com/2004_11_18/mt.html. The tire tread bites hard on the tarmac while the engine power keeps turning the wheels. That results in the sidewall “crumpling”, storing energy. When the car starts moving forward, the tire will “uncrumple” releasing that energy much like a spring. If a lower profile tire was used, the sidewall wouldn’t flex and the wheels would spin a lot sooner. That also partially explains how these cars can sustain accelerations of 5G+ during the launch
This seems to suggest that a driver, before a race, could write down the time – to the thousandths of a second – and be accurate a statistically significant number of times. Or that I could hand a good driver a stopwatch, and tell him/her to stop at X thousands of a second with regularity. Pretty impressive, if I’m not reading you wrong.
Note that this is different than it is in track and field. There you most certainly can be called for a false start if you actually get off the blocks the an exact instant after the pistol is fired-the rules actually take into account the time delays inherent in the human nervous system. I’d guess if track and field had multi-stage lights that might be different.
W/regard to the lights (Christmas Tree), there are two small yellow lights on top of the tree. The top one is an alert that you are 12" from being staged (on the start line). The 2nd lights when the front tires are on the start line. Below these lights are 4 larger yellow lights, one green and one red. For the Professional classes the four large yellow ones light simultaneously and when they go off the green comes on, .5 seconds later. In the Sportsman classes the yellows light one at a time, .5 seconds apart, and then the green. The red indicates a driver has left early (disqualified).
The subject slippage is interesting. An engine must be well into its power range for it to what you want it to do (move the car quickly down the track). Say, for instance a driver wants to launch at 5,000 RPM. The car is sitting at the start line at that RPM. To keep the RPM up at launch, something has to slip. With a manual transmission, tire slippage (vs. clutch slippage) is preferable.
Most cars these days, however, have automatic transmissions. With automatics, slippage of the torque converter (vs. tires) is preferable. The torque converter is a fluid coupler between the engine and the transmission. Think of it as a clutch. All automatic-equipped cars have one. The difference is stall speed (built-in slippage). A torque converter in a street car has about 1800 RPM of stall. A race car’s converter may have 5000 RPM.
SanDiegoTim, My impression is that pro top fuelers, funny cars, and motorcycles still have manual transmissions. So those drivers are looking at timing the Christmas tree, reacting quickly, judging the tire spin, going straight (which isn’t as easy as it looks at that acceleration), and shifting. I got the impression that you were mostly talking about amateur racing. Am I wrong?
I’m sure you’re correct about motorcycles, but the Pro classes (Fuel and Alcohol Dragsters and Fuel and Alcohol Funny Cars) all use automatics. In addition to leading to more consistent elapsed times, they are stronger. When you see a Funny Car or Dragster smoking their tires it’s because the clutch (their autos have clutches) has been set too tight. In other words, not enough slip adjusted into it.
Another interesting note is how some cars with autos launch. In essence, the transmission is locked in 1st and reverse. Fighting each other, the car doesn’t move. To launch (release the reverse) the driver simply presses a button. It’s an electrical vs. mechanical function.
Thanks Telemark, that’s new tech in the last couple of years, I guess. I’m something of a gearhead, so I pay attention to NHRA, but don’t follow it in detail.
SanDiegoTim’s last comment is correct. It’s called a “TransBrake”
In an automatic transmission, everything is controlled with fluids, controlled through a valvebody. With the transbrake applied (controlled with a button by the driver), the valvebody is applying fluid in two opposite directions…pressing against itself…the car is literally in first gear AND reverse at the same time.
The driver is on the line, revved up, with the transbrake on, lots of power sitting there, but going nowhere…
At the green (or as others have said…milliseconds before it) the driver releases the transbrake button…, relieving the fluid from reverse and letting the valve body slam all the fluid through first gear, launching the car very VERY quickly, but just as importantly…more consistently than with a normal automatic transmission/torqueconverter setup or with a clutch.
I’m kind of surprised more people haven’t mentioned the steering part. Even in my very ordinary 6-cylinder Saturn Aura, if I floor it from a standing start, it’s hard to steer – the thing wants to slew all over the place. I have to wonder how it’s even possible to do any effective steering in one of those top fuelers.
The top fuel classes (dragster and funny car) don’t shift at all during a run, do they? Telemark’s quote would seem to support that - they just let the clutch slip and gradually engage in a pre-determined way while the throttle is wide open (You can see the throttle plates, and they appear to be completely open from the get-go.)
Having seen them in person, the engine note is pretty much constant (and cataclysmically loud!) all the way down the track. There is certainly no audible gear shifting, which would involve a sudden change in pitch.
The Pro Stock class cars most definitely shift several times during a run; it’s quite obvious from the engine note gradually rising and suddenly dropping with each shift, much like you’d expect from a road car as it rowed up through the gears.
I don’t think traditional gears could survive with that much power flowing through them. But I had thought the drivers still hit the paddles during the launch. Perhaps that has gone away with the new clutch system.
