calculating ground speed

[defed]

my tractor lists the ground speeds in various gears and ranges using 11.2-24 r1 tires. i have 15-19.5 r4's. is the ground speed a simple ratio of the circumferences? the rolling circumferences are not that different (130 for r1, 117 for r4), but just curious how different the actual ground speed is.

thanks.

 

[Dmace]

If those circumferences are correct, then you should be able to multiply the given speed by 0.9 (90%) and that will give you your true speed with the smaller tires.

 

[Redneck in training]

If those circumferences are correct, then you should be able to multiply the given speed by 0.9 (90%) and that will give you your true speed with the smaller tires.

Then get GPS and measure it. Easy and fun project for HS kids.

 

[Tarheeler]

Then get GPS and measure it. Easy and fun project for HS kids.

Yep those GPS are neat

 

[AV8R62]

if you like this sort of thing , go buy a small digital bycycle seedometer they under $20
an in the set up instructions their is a formula for the size of tire .
Glue the magnet to the rim and mount the sensor where it will read the magnet when it passes and you are done. now you have more fun facts than you know what to do with like
total mowing time,average speed, instant speed and time of day

 

[Raspy]

defed,

Yes, it's a simple ratio of the circumferences. Or the diameters. Whichever you prefer. Measuring the diameters might be easier, or you might get them from the manufacturer's site.

 

[RoyJackson]

defed,

Yes, it's a simple ratio of the circumferences. Or the diameters. Whichever you prefer. Measuring the diameters might be easier, or you might get them from the manufacturer's site.

I'm curious how you figure the diameters come in to this (other then to calculate the circumference). Doesn't seem like just the diameters would help...

 

[MikeD74T]

I'm curious how you figure the diameters come in to this (other then to calculate the circumference). Doesn't seem like just the diameters would help...

He's just saying that comparing diameters is a linear relationship, double the diameter - double the circumference, etc. MikeD74T

 

[Raspy]

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.

Sometimes tire manufacturers give tire info in diameter and sometimes in revolutions per mile. (5280 X 12) divided by circumference in inches, is revolutions per mile. Or, for diameter to revolutions per mile, (5280 X 12) divided by (diameter in inches X 2)

Either way, once you decide on diameter or circumference, and get the numbers for both tires, you just divide the new into the old. This gives you a multiplier such as 1.08 for instance. Next, multiply your speed by that multiplier (ratio). If the new tires are bigger you'll go faster by the percentage difference. In this case the old speed X 1.08, just as an example.

If the new tires are larger, your ratio will be more than 1. Smaller will give you less than 1.

 

[tjc1989]

From my old HS days that I use often. C = pi D or circumference equals pi Diameter. If diameter is say 40 inches, multiply 40 x 3.1414 to get the circumference of 125.66 inches. I guess you could put a piece of tape on the tire, and time the amount of time the tire takes to make a complete revolution or several revolutions, then calculate according to how many feet you traveled in that time.

 

[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.