SantaCruz163

What is meant by 'trap speed'?

rskrause

There's a timing "trap" at the end of the strip. Two timers 60' apart. Computer measures the time to travel that last 60' and calculates the speed over that distance.

Rich Krause

simple

nother question regarding trap speed, why is it measured that way and not done with actual 1320' speed?

Anderson

Thats one thing I've always wondered. Never seen the advantage of using an estimated speed.

Stephen 87 IROC

You would need something like a radar gun in both lanes to measure the speed exactly when the cars crossed the finish line. There's no other way for a static device to measure the speed of a vehicle as it passes.

Since the trap speed is a calculation, it's not as accurate as having a Gtech in the car. The Gtech will display the true mph at the end of the 1/4 mile. The trap speed is a calculation however most cars are still accellerating between the 2 timing points. Since there no way to measure the accelleration it just calculates to time it takes between the beams. In theory you're actually going a little bit faster than what the time slip says.

Many many years ago the trap speed timers were also 60' after the finish line. An average was taken between the 2 60' timers to give a more accurate 1/4 mile mph however many tracks want you to shut down at the 1/4 mile mark not 60' past it so for safety reasons the 60' timers after the 1/4 mile were discontinued.

rskrause

Well, if you wanted your average speed over 1320' it would just be 900/ET.

15sec ET = 60mph
10sec ET = 90mph
5sec ET = 180mph

So, that data is already there. The main use of trap speed is as a tuining aid. It large reflects hp while the 60' time (first 60') is largely a reflection of traction. The ET reflects both traction and grip and it's what wins races (besides cutting a good light).

Rich Krause

rskrause

As long as it's always done the same way it serves the purpose (a tuning aid).

Rich

Capn Pete

Exactly. It's a constant that will never change, so you can't argue that someone else has an advantage over you. I think that the "trap speed" is only off by a few mph anyways. I think that people with G-techs who run mid 13's claim 108 - 110 mph ("actual speed"), whereas at the track may only see 103 - 105 mph.

sseeya

Actually the timing lights are 66' before the finish line. It used to be 66' before and 66' after the finish line. That's a total of 132' exactly 10% of 1320. I guess it made the caluclations easier. As the cars got faster, they needed more room to shutdown, so the NHRA cut the traps down to 66' so the cars could shutdown at the finish line instead of 66' after.

Stephen 87 IROC

I stand corrected

brain

Very interesting. I was aware that it was 66 feet, however the before and after I was not. Do you have or know of any direct comparisons of what trap speeds were before and after the change? It would be nice to know how to compare an old muscle car to a new one, especially if this is a variable. Remember the horsepower ratings of the 60s? Gross rating with no accessories. All those 450 HP 350s running 15s.

rskrause

Of course it's 66ft, DOH!. As I'm old enough to remember the 60's muscle cars, I can comment on the last post a little. Some of the cars of that era had under-rated hp, but many were overrated. In any case, ET's with street tires reflected crappy traction. The suspensions were also an issue as well as the tires. However, a few minor mods would really make a huge difference. Headers were also huge as most had decent dual exhaust systems but very restrictive cast manifolds. Tires and traction aids made a huge difference. Anyway, most of the factory cars would run 14's completely stock but you take a second or more off with headers, tires, and traction mods.

I'm just guessing, but it would surprise oe if the trap speeds were different by more than ~2mph for a typical street car. A lot of the 60's cars were geared very short nd were pretty much out of breath at then end of the 1/4m, unless they had breathing mods likes heads and cams.

Rich Krause

Injuneer

If you do the math, for a car running under 100mph, and pulling 0.18 G's at the end of the track, the difference between the "average over the last 60-ft" and the "actual speed at 1,320-ft" is less than 1 MPH. It doesn't change much for a 110mph car pulling 0.20 G's.

For a 300mph car, pulling 1.0G at the end of the track, the difference would only be 1.65 MPH.

The G-tech is not "off" by its usual 2-6 MPH because of this difference... it is off because of the iterative nature of the calculation of speed and distance from the only 2 parameters that it can actually measure... time and G's... and the limited accuracy of the G-meter, and/or the low sampling rate.