calculating ground speed

[Raspy]

tjc,

Eeek! You're right, pi X D equals circ. Wow, brain fade. :laughing:

Thanks.

 

[Baby Grand]

Remember, (Distance traveled= PI*D*#of revs) works great on hardsurfaces, but it does't give you good results in loose soil, where there can be significant tire slippage. For that measure you need to layout a known distance, mark one of the tires, and have a helper count revs by noting how many times the mark passes to get the slippage rate for that particular soil and tire combination.

 

[Redneck in training]

Remember, (Distance traveled= PI*D*#of revs) works great on hardsurfaces, but it does't give you good results in loose soil, where there can be significant tire slippage. For that measure you need to layout a known distance, mark one of the tires, and have a helper count revs by noting how many times the mark passes to get the slippage rate for that particular soil and tire combination.

That is where GPS excels. Most of them, even the cheap one, log the data such as time and date, location, heading, speed, vertical speed, elevation etc. every few seconds. You can download the data to a computer superimpose it on a map and see all the variables at any given time and location.

I used to know a guy who used to own a fertilizer application company. They had four-wheeler motorcycle with sample acquisition equipment on it. The technician would enter a field and take a sample at any point the computer with GPS told him. Then he would bring the magazine with the samples and flash card from the computer to the lab for analysis. The lab stored analysis data on the same card and gave it to the driver of the fertilizer application machine. He would enter the field at the same point as the four-wheeler and start the program. The computer with GPS would guide the machine over the whole field applying the fertilizer as need. No steering was necessary. I was impressed seeing it.

 

[PILOON]

I used GPS to clock my gear drive Mitsubishi and found that top speed was 7.5 MPH.
Slow boat to China, LOL!

 

[KennK]

I think you'll find that even the most basic GPS can provide a much more accurate speed than you'll get calculating based upon the wheel diameter ... though that is a great geometry lesson.

The GPS speed accuracy will tend to improve as the speed increases.

 

[JDGreenGrass]

I am no tech geek by any stretch but, I have heard that GPS is not a great measurement of speed due to dips, hills, mountains and such.??

In other words, when a car travels down a mountain and then up the next one it has travelled further than a straightline from mountain top to mountain top.

Have they improved.??

 

[pitt_md]

The ratio of the diameters is the same as the ratio of the circumferences. It's all a linear relationship and circuference is just twice diameter.

Not to be picky but circumference is Pi x D or 3.14 X Diameter

 

[no1greek]

my JD manual gives the ground speeds for all forward gears at 2400 rpm plus a conversion table for tire sizes other than 16.9 X 28. From that can't one assume that the ground speed at 1200rpm in the same gear will be exactly half?

 

[MHarryE]

Rolling circumference gets you close, but depending on soil and drawbar load slip can throw I speed off. GPS is better but is affected by terrain. Many large ag tractors are equipped with radar which is needed for optimum operation rates and is often integrated into the tractor systems for slip control.

 

[Redneck in training]

GPS works for most situation reasonaly well. I am just guessing there might be some more expensive units that will calculate correct distance and speed even going up or down hill. Friedn of mine has combine, large tractors etc all with GPS loggers.