18 replies to this topic

[hoffmanbrent]

Our track is 7ft shy of 1/8 because of garage size. We mostly do .7 sec index racing and the track record is .5524
What would these two ETs be on a 27.5ft track?

[Bill Seitz]

So, your track is 20.5'. Divide 27.5' by 20.5' and multiply that result by your ET's. That will give the ET at 27.5 feet. That's .939 for your index and .741 for your track record.

[hoffmanbrent]

That seems like a scale calculation, but I don't know, is why ask.
I estimate the car doing 30mph at the stripe, when running .55, which should be about .17 seconds to travel 7 feet
Does adding the .17 to the ET seem like fair estimate?

[Bags]

That seems like a scale calculation, but I don't know, is why ask.
I estimate the car doing 30mph at the stripe, when running .55, which should be about .17 seconds to travel 7 feet
Does adding the .17 to the ET seem like fair estimate?

 

Yes that’s about right.

[Dave Crevie]

The formula doesn't work out. It takes longer to cover the first 60 foot mark (2.5 feet) then the last 60 feet. (from the 25 foot mark to the finish at 27.5 feet) The speed and time does not average out over the full length of the strip. You can see that fact if your timing system has a 60 foot timed section like with TrackMate. The 60 foot time is going to be considerably longer than the the time it takes the car to go the last 7 feet. It would be real difficult to adjust the E.T. without knowing exactly what the trap speed is at the finish line. If you know for sure that the car is turning a full scale 55 mph at the finish, your correction should come out .07291,(or .073). 

[Phil Smith]

Dave, yes that's what I was thinking. I had no idea how to do the math on it.

[Bill Seitz]

I didn't consider the car accelerating. It's possible to determine the rate of acceleration, but that's an experiment or needs a lot of data that's likely not available, so there's no easy math way to confirm your conclusions. At best you have an estimate, because if the car is still accelerating at 20.5 feet, calculating the last 7 ft from the trap speed at 20.7 ft, will still not be accurate unless the rate of acceleration is known.

[Dave Crevie]

Generally speaking, the rate of acceleration in the traps for a slot car will be nominal. There are a plethora of other variables that also will affect the calculations. Every car will have different characteristics, so it really isn't possible to come up with a hard and fast rule. I was using the given data. I just wanted to give them a ballpark figure to use. Not prove or disprove the existence of supermassive black holes. (Which, by the way, I still contend is a little man in a darkened room looking for the circuit breaker with a flashlight) 

[hoffmanbrent]

The formula doesn't work out. It takes longer to cover the first 60 foot mark (2.5 feet) then the last 60 feet. (from the 25 foot mark to the finish at 27.5 feet) The speed and time does not average out over the full length of the strip. You can see that fact if your timing system has a 60 foot timed section like with TrackMate. The 60 foot time is going to be considerably longer than the the time it takes the car to go the last 7 feet. It would be real difficult to adjust the E.T. without knowing exactly what the trap speed is at the finish line. If you know for sure that the car is turning a full scale 55 mph at the finish, your correction should come out .07291,(or .073). 

[hoffmanbrent]

I'm estimating trap speed of 30 MPH at 20.5 FT
With the ET of .55XX
I designed and built the timing system, unfortunately I'm not able figure out how to get a MPH without having computer system

[hoffmanbrent]

Sorry for the double post

[Dave Crevie]

If you built the timing system, just add an additional timed section for the last 2.5 or 3 feet before the finish line. Assuming your system can do hundredths of a second, you can use that time to calculate the trap speed. ( at least to a reasonable level of accuracy)

 

Using the E.T. to calculate trap speed doesn't work because of differences in car design, gearing, motor, etc. Some cars build their speed early and are topped out well before the traps. Others are still accelerating while in the traps. Too many variables here.

[hoffmanbrent]

Yes, I have thought of to add another beam 33 inch before finish line.
But didn't like idea of manually calculating that time into mph for each run
Or maybe I'd print out some big conversion chart

[MSwiss]

You wouldn't even need to print out much of a chart.

Trackmate doesn't even bother to provide 100% exact mph times.

Trackmate provides what I affectionately call "gerbil with a stopwatch" times.

IOW, they like to round things up or down.

If you go say 60 mph, you'll get 5 or 6 different 60 mph readings.

IE-60.11, 60.34, 60.49, 60.72, & 60.96.

You never "go", say 60.25 or 60.63.

[hoffmanbrent]

Starting to get motivated to add another beam for MPH. Friend at work came up with a fixed value I can divide with time to get average mph for the last 33 inches of track.

1.8744 ÷ time = MPH

[hoffmanbrent]

Just ran across this.
Why is speed sensor at 30 in before stripe.
Shouldn't it be 33 in ?

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[MSwiss]

I think you can enter any #, but Trackmate recommends 30".

[Dave Crevie]

It doesn't really matter where the trap speed sensors are placed. You enter the distance into the computer, and it calculates the speed based on that data. Of course, the shorter the distance from the speed sensor to the finish sensor, the more accurate the trap speed will be. But there is a limit to how short a distance the computer can measure over due to the limitations of the computer's timing ability. Trackmate figured that out, and used 30 inches as a distance a typical computer can read.

[hoffmanbrent]

I was just thinking there would be a standard from track to track.