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Old 10-09-2012, 02:42 PM   #1
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Default Device for wheel speed checking?

In a nutshell: I want to be able to put new drivers in a slow class regardless of the equipment they have. The easiest way to do this is adjust the EPA of the throttle. But we hire the venue by the half hour and are really short on time at the beginning of the meets, so driving up and down the straight isn't the best option. I figure if I could just free-rev the cars and measure the wheel speed that would be pretty close to what I want. Does any one know of a cheap hand held device I could use to do this job?
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Old 10-09-2012, 06:09 PM   #2
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A timing light and a mark on the rim?
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Old 10-09-2012, 06:38 PM   #3
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In a nutshell: I want to be able to put new drivers in a slow class regardless of the equipment they have. The easiest way to do this is adjust the EPA of the throttle. But we hire the venue by the half hour and are really short on time at the beginning of the meets, so driving up and down the straight isn't the best option. I figure if I could just free-rev the cars and measure the wheel speed that would be pretty close to what I want. Does any one know of a cheap hand held device I could use to do this job?
Check under a load with a simple to build dyno.

http://www.rccartips.com/rc-car-dyno-cheap.htm

https://sites.google.com/site/simpledyno/the-dyno
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Old 10-09-2012, 06:40 PM   #4
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mark the rim and a strobe in a dark place?

Alternatively, do you have a radio with telemetry? I recall that the DX3S/SR3300T came with an RPM sensor, you just need a bunch of the stickers they use or similar...
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Old 10-09-2012, 06:53 PM   #5
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http://www.extech.com/instruments/pr...=21&prodid=252

If budget is a little higher, this would be good....but again, it doesn't test under load....
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Old 10-09-2012, 06:55 PM   #6
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How about a wheel to wheel drive, with a rear end off of a pan car, and a silvercan as a slave motor
Measure voltage from the slave motor=rpm
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Old 10-09-2012, 07:11 PM   #7
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How about a bicycle speedometer.
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Old 10-09-2012, 07:54 PM   #8
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Originally Posted by BlueGlowBoy View Post
http://www.extech.com/instruments/pr...=21&prodid=252

If budget is a little higher, this would be good....but again, it doesn't test under load....
Lots of good suggestions. I particularly like this one because it means I don't have to fiddle or build anything more complicated than a car stand. Also it's portable (we race at temporary venues).

It won't test under load, sure. But how much will that really matter in the long run do people think? I mean, after a short time it should be possible to get used to interpreting the reading, and the load isn't going to be too dissimilar between cars. I don't really need accuracy, just precision enough to know that the car will do about the right speed when it's put on track.
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Old 10-09-2012, 09:50 PM   #9
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Default digital tacho

my first thought was same as earlier posted. but try this a bit cheaper.

http://www.jaycar.com.au/productView.asp?ID=QM1448
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Old 10-09-2012, 10:46 PM   #10
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Originally Posted by RedBullFiXX View Post
How about a wheel to wheel drive, with a rear end off of a pan car, and a silvercan as a slave motor
Measure voltage from the slave motor=rpm
+1 for this idea, we use this on our bomber class.
invert the pan car tires on a wedge shaped board,hook up a multi meter to the silver can ,and read the voltage.works ok
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Old 10-09-2012, 10:53 PM   #11
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Quote:
Originally Posted by BlueGlowBoy View Post
http://www.extech.com/instruments/pr...=21&prodid=252

If budget is a little higher, this would be good....but again, it doesn't test under load....
That is exactly what I was going to suggest and I have actually bought the Jaycar version linked in another post above. Resolution is much more than you will ever need, and it's a piece of piss to measure each car.

But there is a problem. A good battery freshly charged coupled with a good speedo and a nice wiring job will give you a higher reading (no load revs will be higher) than an old tired battery etc. even if both cars have the same FDR.

I used the tacho to check there was no wear in my drivetrain that would cause the wheels to spin at different speeds (worn bearings, belts, etc).
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Old 10-09-2012, 11:30 PM   #12
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That is exactly what I was going to suggest and I have actually bought the Jaycar version linked in another post above. Resolution is much more than you will ever need, and it's a piece of piss to measure each car.

But there is a problem. A good battery freshly charged coupled with a good speedo and a nice wiring job will give you a higher reading (no load revs will be higher) than an old tired battery etc. even if both cars have the same FDR.

I used the tacho to check there was no wear in my drivetrain that would cause the wheels to spin at different speeds (worn bearings, belts, etc).
Thanks for the feedback. How much would you say is the difference between good gear, fresh charge over older gear?
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Old 10-10-2012, 03:10 AM   #13
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Thanks for the feedback. How much would you say is the difference between good gear, fresh charge over older gear?
I reckon it can be anything, but I think you need to think how much a tolerance you are ready to accept. 10% wouldn't surprise me and that is a lot.

That said, a good speedo is going to deliver and sustain much higher currents than your ordinary el cheapo, so not sure where that leaves you.
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Old 10-10-2012, 03:51 AM   #14
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I reckon it can be anything, but I think you need to think how much a tolerance you are ready to accept. 10% wouldn't surprise me and that is a lot.

That said, a good speedo is going to deliver and sustain much higher currents than your ordinary el cheapo, so not sure where that leaves you.
If what I measure correlates reasonably with car speed on track then that's ok. These will mostly be new drivers I'm slowing down, so I just need a ball park to make sure they aren't missiles. We can always make adjustments after the first heat, I just don't want to have to go to the trouble of straight line testing half a dozen cars if I can get close on a desk top.
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