A GTech is a small electronic accelerometer that mounts in your car (on the windshield like a radar detector) and measures acceleration and elapsed time to come up with a 1/4 time. They are only as accurate as the accelerometer used in their circuit. The math they use is very accurate so like I said they are only as accurate as their accelerometer and timer can be. As far as GTechs or track times being more accurate, Unit, your math makes sense, but every car is different, that is why people say their GTechs are consistantly off. Every engine has a different torque curve and therefore a different HP curve. So, when you say that a car in 3rd gear will accelerate at a lower rate than the car in 2nd gear, you're correct, assuming that the torque curve of the engine is flat (a constant) This is only true for electric motors, hardly ever for a gasoline internal combustion engine. So, the mph change in the last 60 feet would be different for a car crossing the traps in it's power band and a car not in it's powerband, for instance, with my gears, I shift into 4th about 300 feet in front of the 1/4 mile. So I am crossing the traps at about 4000 rpm, I would accelerate differently in the 60' in front of the line than a car crossing at 6000 rpm. Therefore the difference between average speed over the 60' and instantaneous speed at the end of the track will be different for every car. In other words, the track will be off by a different amount for every car. Now, I feel that I must make a statement about somthing that someone else said. Something about being able to get actual speed with a radar gun at the end of the track. Did you mean instantaneous speed? speed is defined as the (change in distance)/(change in time). Instantaneous speed is defined as the speed when the change in time = 0. Everyone knows you can't divide by 0 so it is impossible to measure instantaneous velocity, unless you use calculus which is what a GTech uses. When the GTech integrates to get velocity the margin of error becomes exponentially greater, so if the accelerometer is off by a small bit, the velocity will be off by a lot. Anyway, like I was saying, even with a radar gun, you are by no means measuring instantaneous velocity. The radar gun measures doppler shift. The change in frequency of a wave when it hits a moving object (listen to a car horn as the car is moving towards and then away from you, it's higher and it gets lower as it goes by right?) doppler shift. Anyway, the radar gun knows the wavelength of it's sent signal, it measures the wavelength of the received signal, and subtracts them. Which is still measuring a distance over time. So anyway, even though a radar gun is more accurate than a 60' average, it is still an average. My main point here is, nothing is 100% accurate, and all of this is for fun anyway guys, so GTech times, track E/Ts whatever, believe what you want about accuracy, it's all relative.
Oh yeah, Lizard King, what does the weight of your car have to do with a GTech measuring a 1/4 mile time?
[This message has been edited by QuantumMotorsports (edited 04-24-2001).]
|