How to calculate tractor wheel slip
Slight wheel-slip is essential for correctly transferring the tractor’s tractive force while still preserving the pasture condition.
This article explains how to measure wheel-slip and provides some options to achieve the optimum tractive efficiency with various implements by effectively managing wheel-slip.
The degree of wheel-slip is equal to the ground’s resistance to the tyre’s lugs’ horizontal compression. This is measured as a percentage; the ideal slip ratio for 2-wheel-drive tractors is 10-15 per cent or 8 – 12 per cent for 4-wheel-drive tractors in cultivated soil. When the slip ratio is high, deep down soil damage occurs, and fuel usage will increase by up to 20 per cent.
Firstly, let’s consider a few points about traction and tractors. Traction is something that many tractor drivers often get either too much or not enough. And in both instances, the consequences cost money, money that you don’t need to spend.
To convert engine power into efficient drawbar pull the tractor requires the proper amount of traction. Without it, your money like your fuel goes up in smoke. Depending upon the application and circumstances, a tractor driver will require one of two things.
They could be interested in getting maximum traction which means having the highest possible drawbar pull. However, in today’s high-tech world of precision agribusiness, most strive for optimum efficiency to achieve the best performance per dollar of operating cost.
Therefore, getting down to the nitty-gritty means today’s tractor driver must think about tractive efficiency.
What is tractive efficiency?
Simply put, tractive efficiency is defined as drawbar power divided by engine power used. This is a neat way of showing how much of the engine power is being converted into drawbar power.
It is helpful to remember that drawbar power equals drawbar pull multiplied by ground speed.
To work out your tractive efficiency, make the following calculation:
Tractive efficiency (%)
0.0278 x Drawbar Pull (Newton) x Ground Speed (km/h)
Engine Power (kw)
In practice, tractor drivers never get as much power at the drawbar as the engine is developing. Why?
Simply because several other factors use up energy from the engine. Like the, alternator draw, air-conditioner, engine cooling fan, rolling resistance of the tyres and by the tractor’s transmission. However, the principal reduction in drawbar power is due to wheel-slip. And tractive efficiency depends upon the correct amount of wheel-slip.
So, What’s the difference between a high value of tractive efficiency and a low value of tractive efficiency?
The difference; wasted money.
With a high value of tractive efficiency, most of the engine’s power is being used to pull the implement. But with a low valve of tractive efficiency, much of the engine’s power is wasted, spinning the wheels without moving the tractor forward efficiently, which costs time, fuel and consequently, money.
Out on the paddock, it is possible to achieve a tractive efficiency as high as 85 per cent. It is also possible to accomplish a tractive efficiency of 50 per cent or even less. Since only the tractor operator can control tractive efficiency, it is up to them to determine how their tractor is set up in relation to the applicable soil conditions and implement being towed.
If we go back to the formula for tractive efficiency for a minute, you will see that you do not necessarily need maximum drawbar pull to get a high value of tractive efficiency.
In Table-1, you can also see by adjusting the implement width and operating speed you can achieve similar tractive efficiency. For instance, you can do just as well with moderate to high drawbar pull in conjunction with a higher working speed as in the bottom of the examples in Table-1. Therefore, depending on what type of tractor you are operating, you might choose a smaller implement at a higher working speed, instead of the broadest implement, your tractor will pull.
How to measure wheel-slip
How does a tractor driver know the tractive efficiency at which his equipment is working? The answer is to measure wheel-slip because tractive efficiency and wheel-slip are closely related.
Let’s look at the general shape of the curves in Diagram-1 for a moment. Bear in mind that these curves are illustrative. Nevertheless, they paint an accurate picture of how tractive efficiency varies depending on the soil conditions, and differing tyres and inflation pressures and of course tractor weights.
Consequently, it is probably surprising to note that the best efficiency is not obtained by having the lowest possible wheel-slip. The best efficiency is found over a relatively small range of slip.
Look at Table-2, and you will see the optimum range of wheel slip varies for 2WD and 4WD tractors.
As a rule, however, keep to the high end of the range when you are working with wet soils.
Whilst we are talking about wheel-slip levels, what is the difference between slip levels that are too high and those that are too low?
You guessed it: Wasted Money.
Slip levels that are too high cause power losses through reduced ground speed.
And as you would expect, this wears out tyres much faster. Alternatively, low slip levels indicate that the tractor is too heavy for the load being pulled. Subsequently, there is little point in wasting power by dragging an unneeded weight around the paddock.
Nevertheless, it would be great to measure wheel slip by merely watching the tractor in operation. Unfortunately, we can’t, so slip must be measured.
The following is an easy method suggested by many agricultural authorities. All you’ll need is a 30-meter measuring tape, some chalk or masking tape to mark the tyre, two stakes or pegs and a calculator.
Firstly, place the implement in the ground at the desired working depth. Then mark a reference point on the side of the rear tractor tyre with the chalk or masking tape. Walk alongside the front corner of the implement while the tractor is working, and when the tyre reference mark points down to the ground, place a stake in the ground just outside the front edge of the implement.
Continue to walk alongside the implement, count five revolutions of the tractor tyre and again place a stake beside the front edge of the implement when the reference mark on the tyre comes down on the ground. Measure this distance and call it ‘Id’ (for implement down)
Repeat the previous steps and with the implement out of the ground. The distance the tractor covers with the implement out of the earth will be known as ‘Iu’ (Implement up)
To calculate your wheel-slip in percentage terms follow this formula.
% Wheel-slip = (Id -Iu) x 100
Changing the amount of wheel-slip
If your wheel-slip measurements don’t correspond to those in Table-2, now is the time to take corrective action.
If your wheel-slip is too low, try reducing the weight of the tractor. Alternatively, you can increase the drawbar pull.
Depending upon the Implement you are using, you can either increase implement width or increase your working speed.
If your wheel-slip is too high, on the other hand, you could try increasing the tractor’s weight. Think of your tyres through. Be careful not to exceed their load limits, or the tractor’s maximum permitted operating weight.
You can also increase drawbar height. But take a special warning. Do not try to pull from a position higher than that provided by the tractor manufacturer.
The best way to lower wheel-slip is by reducing drawbar pull. To reduce drawbar, pull, decrease implement width by removing outer tines or if the job permits, work at a shallower depth.
The final result should fall within the range of percentages in Table-2. And if at first, you don’t succeed …
There it is then. You’ve started saving money. When you think of it in terms of time, fuel and tyres, it adds up to a lot.