originally i had thought that trap speed was just the exact mph you passed throught the finish line


then i heard that trap speed is calculated by how much you accelerate from the 1000ft mark to the finish.......and its not exactly the mph you were going.


someone explain it for me.

 

Its an average of the last 100ft whereas a GTECH gives you your instantaneous mph at the 1/4 mile mark.

 

Speed is measured in a 66-foot "speed trap" that ends at the finish line.

 

The last 66 feet is in the speed trap. Needless to say, you don't want to be in the middle of a shift on the long end

Doug D

 

ok, but what im wanting to know is


is the speed just an exact speed (like a single instant)


or is it an average of the speed you were accelerating throughout that whole 66 feet?

 

The trap averages your speeds as you break the first beam with the speed you are traveling as you break the second beam, 66 feet later. So it averages 2 speeds.

 

thanks, thats exactly what i wanted to know.

 

The speed trap starts 33ft before the finish line and ends 33 ft after the finish line.


edit; I did a search on Google and I'm wrong. Again I let logic cloud my thinking.

 

This is a direct quote from the NHRA website, in the section entitled "The basics of drag racing:"

Speed is measured in a 66-foot "speed trap" that ends at the finish line. Each lane is timed independently.


There is no way for it to know what speed you were traveling at the first light, because there is no other point of reference.

The speed trap needs two points of reference to calculate speed. It measures the amount of time it takes your car to travel from point A (the light at 66 ft.) to point B(the finish line) Then it calculates the speed needed to travel that distance (66ft.) in that amount of time.

 

Sorry Anaik, your edit posted while I was typing my reply.

 

I wonder if it's ever been the way I described, like back in the '60's? Back when I was a youngin.

 

Good memory.

I believe it was 66 ft before and after the finish line, not 33.
I suspect they stopped timing after the finish line because fast cars already have the laundry coming out as they cross the line.

Actually, there are two very powerful telescopes mounted in the stands; they read your speedo as you cross the line.

 

All these years I've been keeping my foot in it after the finish line so there wasn't a chance of me loosing a .01 or so. Oh well.

 

funny thing is, they chose 1/80 mi to make the math easy. As said above, it's the average velocity, i.e. (change in displacement)/(change in time). In this case, averavge velocity (mph) = 45/t where t is in seconds and measured from the time you trip the first light to the time when you trip the second one. sooo if it takes you .5 sec from the time you trip the first light 'till you trip the second one, your trap 'speed' is 90 mph.

 

And to add a little more info... for just about every class of vehicle, the difference between the "actual speed at the finsih line", and the NHRA "average over the last 66-feet" is not very large. For cars running 100mph, and pulling 0.20 G's (typical of many LT1's), the difference is only about 1 mph.

OldSStroker is right... it used to be measured before and after the actual finish line, and it was changed because the big dog top fuelers and funny cars were delaying braking and chutes to pick up that extra couple mph. As you probably noticed from the past weekend NHRA Pomona on TV, the top fuel dragsters are having difficulty stopping before the sand traps with loss of chute or brakes.......