Determining distance travelled per rotation of the driveshaft

Atropia

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Looking for math help. I got this rally computer wired in around the drive shaft. Yes, other people (especially bikes) normally put them on the wheels, but it’s very protected here during off-roading and won’t get knocked off. It’s used to determine exact distance travelled by counting rotations of the tires (or in this case the number of rotations of the driveshaft). You calibrate it by entering into the computer the distance travelled (in mm) per one rotation of the magnet (on the metal finger sticking out) passing by the trigger which closes the circuit. When the magnet is mounted on the wheel, that is pretty easy.

However - this is the way I have it mounted. I can’t lay under the vehicle to count an exact revolution as the jeep drives forward…

The circumference of the circle that the magnet travels is 498mm (C=2PiR).

I have 4.88 gears, and 34.5” tires.

I entered 498 into the rally comp where the circumference of the tire should go, for lack of a better idea. I am off (using Jeep odo and cell gps app) by about 200-250 meters per mile.

how can I determine how far my Jeep travels during a single full rotation of the magnet around my driveshaft?

my best idea is with a plumb bob attached to my sliders, and mark on the ground when the computer/odometer ticks over .01 (indicating a magnet pass) and then again when it ticks over to .02 (indicating another magnet pass) and again a third, fourth, and maybe fifth time. Then measure between all the marks and find the average. Since I need mm, I think I should do this many times to get a good number.

but if anyone has other suggestions, I’d love to hear them.

thanks

 

smokeythecat

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But at what time did the train leave the station?

But seriously.... Working backwards, let's assume a real world radius to the ground on the tire to be 17" after compressing a bit. That means that one revolution of the tire would be 106.8". Divide by 4.88 for your gear ratio and that gives you 21.89" of travel per rotation of the drive shaft.

As always, I'm open to correction.
 

Renegade

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Can I ask why you would want to do that vs using the speedometer (recalibrated for tires/gears)? Also, the radius of the circle traveled by the magnet is immaterial. 1 rotation is 1 rotation.
 

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But at what time did the train leave the station?

But seriously.... Working backwards, let's assume a real world radius to the ground on the tire to be 17" after compressing a bit. That means that one revolution of the tire would be 106.8". Divide by 4.88 for your gear ratio and that gives you 21.89" of travel per rotation of the drive shaft.

As always, I'm open to correction.
Bet that's pretty accurate, 498mm is 19.6"

I'm guessing you're 200-250m short then, so you're at 1409m or 1359m instead of 1609m

Correction to current value based on observed distance:
498mm * 1609m/1409m = 568.69mm = 22.38"
498mm * 1609m/1359m = 589.61 mm = 23.21"

Edit:
Also, using BFG KO2 35s published 602 revs/mi
1609/602/4.88*1000 = 547.70 mm = 21.56"
 

BigGreen

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34.5*3.14=108.33/12=9.0275 feet per rotation
5280(mile in feet)/9.0275=584.88 rotations per mile

584.88 revolutions per mile/4.88 gears = ~120 spins of the drive shaft per mile?
 


Garry in AZ

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Yeah, I also get just a fraction over 22", using the diameter of the tire at the stated dimension.
There are several other issues you need to consider here... wheelspin or skidding for one. Any tire slippage in either direction will affect the accuracy of the calculation.
If you really need high accuracy odo readings, I'd use a GPS.
 
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Atropia

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Hey Big Green - I think you’re onto something - just not sure what…

why would I multiply the circumference of my tire by Pi?

im trying to figure out how far my Jeep travels each rotation of the driveshaft. In mm.
 
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Atropia

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But at what time did the train leave the station?

But seriously.... Working backwards, let's assume a real world radius to the ground on the tire to be 17" after compressing a bit. That means that one revolution of the tire would be 106.8". Divide by 4.88 for your gear ratio and that gives you 21.89" of travel per rotation of the drive shaft.

As always, I'm open to correction.
Cool. I’ll convert to mm and see what happens. Thanks!
 
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Atropia

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Bet that's pretty accurate, 498mm is 19.6"

I'm guessing you're 200-250m short then, so you're at 1409m or 1359m instead of 1609m

Correction to current value based on observed distance:
498mm * 1609m/1409m = 568.69mm = 22.38"
498mm * 1609m/1359m = 589.61 mm = 23.21"

Edit:
Also, using BFG KO2 35s published 602 revs/mi
1609/602/4.88*1000 = 547.70 mm = 21.56"
Thank you also! I will plug in these numbers and see if I get closer! Thanks!
 
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Atropia

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Can I ask why you would want to do that vs using the speedometer (recalibrated for tires/gears)? Also, the radius of the circle traveled by the magnet is immaterial. 1 rotation is 1 rotation.
It’s for a navigational rally. The rally is run in meters only and is very precise. There is a computer on the other end of those wires on the dash for my navigator to see and use.

I don’t know if the circumference of the magnet circle matters or not, but one rotation can’t be one rotation as far as the distance traveled goes. You will travel further on one rotation of 37s than you will on one rotation of 35s.
 


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Atropia

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Yeah, I also get just a fraction over 22", using the diameter of the tire at the stated dimension.
There are several other issues you need to consider here... wheelspin or skidding for one. Any tire slippage in either direction will affect the accuracy of the calculation.
If you really need high accuracy odo readings, I'd use a GPS.
Correct. I will have to drive carefully. Fortunately right now I am calibration on the streets, so slippage and spin aren’t factoring in at the moment. The rally I am competing in is NO GPS. Maps, rulers and compasses only.
 

GATORB8

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I don’t know if the circumference of the magnet circle matters or not, but one rotation can’t be one rotation as far as the distance traveled goes. You will travel further on one rotation of 37s than you will on one rotation of 35s.
It does not. The magnet is simply letting it count full rotations of the driveshaft.
 

GATORB8

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Thank you also! I will plug in these numbers and see if I get closer! Thanks!
If your intention is to calibrate to GPS, I wouldn't mess with trying to figure out the correct distance, just use the formula I mentioned before to get you direct to the GPS reading. You can repeat the process to get even more accurate, if necessary.

Corrected Value = (Current Value) * 1609.34 / (Current Meters Per Mile)
 

CaJLMetalHead

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another way could be to measure manually... on a flat surface mark the very bottom of the rear tire contacting the asphalt with chalk... then push the Jeep until you measure let's say exactly 20 revolutions of the shaft... stop exactly there... and then measure the distance from that current position of bottom on the tire to the original position.. repeat a couple of times and now you have real-world values of distance per rotation of the shaft.
 

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Calculate axle revs per driveshaft rev:
1 driveshaft rev / 4.88 gears = 0.2049 axle revs per driveshaft rev

Calculate distance traveled per axle rev:
34.5" tire diameter * 3.14 = 108.4" => convert to mm: 108.4" * 25.4 mm/in. = 2753 mm/axle rev

Calculate distance for 1 driveshaft rev:
( 0.2049 axle revs / driveshaft rev ) x ( 2753mm / axle rev ) = 564mm / driveshaft rev

 

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