Determining the “lead” of your front tyres on a F.W.A. tractor.

Let’s deal with the first question- why do you need to know what the lead is anyway?
Before you start, read the article on ‘Tractor tyres for Front Wheel Assist Tractors‘. This should be ringing warning bells that a little serious study should be undertaken before you need to replace the tyres on your Front Wheel Assist tractor- most frequently the front tyres first.
When the time comes, you might find that these tyres are radials, made in Romania or somewhere (anywhere!), and that you can’t get them, except from the tractor dealer at an exorbitant price. Suddenly your tractor, now 3-5 years old, isn’t such a good acquisition after all.

So some homework is required.

If you fit tyres that don’t match the design engineers’ dimensions, then you run the risk of the tyres failing due to a torque buckle in the sidewall, the transfer case between front and rear drives failing (expensively), or in any case, the tractor chewing up fuel, and getting expensive to run.

There are two ways of checking the lead of the tractor with replacement tyres:

• The book method. If you have the rolling circumference of the front and rear tyres available from printed catalogue information, and are prepared to consult either the tractor handbook for that particular model, or even better, check the specification plate or get under the tractor to check the stamping of the drive ratio on the transfer case, you can do a “book calculation”. Aren’t pocket calculators marvellous! There have been many cases though where the manual says one thing, the transfer case another, so beware. Other forms of the same information are “rolling radius”, or “tyre diameter”, “static loaded radius”, or “revolutions per kilometre”. Note however, that neither “tyre diameter” or “static loaded radius” are fully satisfactory, because they do not allow for the flexing and slip of the tyre as it rolls. They are, as specified, static (stationary) measurements. However, the book method is a way of getting started on the choice of alternative tyres to suit your tractor, and will save you a lot of time in the field.

• The field test method. Our laboratory had a very expensive testing machine installed. On the front of the machine there was a very small plate. You had to lean well forward to read it. It stated “One test is worth a thousand opinions”. The advantage of a field test is that it tests the tractor in the configuration that it is actually going to be used in. The test is carried out on a hard surface. Disengage the front wheel assist and the differential lock to ensure that all wheels are independently free to roll. This may involve keeping it at idle, and check that F.W.A. is not automatically engaged if the engine is turned off. (check the handbook if unsure).

• This method requires a straight, fairly level hard surface up to 100 yards long. A tractor tyre has between 20 and 26 lugs on it, which extend down the sidewall of the tyre. This means that you can divide the circumference of the tyre into around 20-26 segments, so put a splodge of a bright colored paint on the buttress in contact with the ground at the time. (six o’clock position). Hammer a stake into the ground opposite the paint splodges, front and rear, or just lay it on the ground, if you’re sure you won’t trip over them.

• Then tow the tractor forward, with two people alongside, one counting the front revolutions, the other the rear, so you need three people. After ten revolutions of the rear tyres, stop, and lay a stake opposite the splodge of paint, which should be at 6 o’clock. The front tyre splodge won’t be at six o’clock, but probably somewhere up in the air. Count the number of lugs that it has completed in its last partial revolution, keeping in mind the direction of rotation.

Here you have two options. Either place a stake opposite the six o’clock position of the front tyre, and estimate the number of lugs traversed in the last partial revolution (for example 7 lugs out of 24), or better, roll the tractor forward till the splodge is at six o’clock, and lay the stake opposite.
The rolling circumference of the front tyres is given by the distance traversed by the number of revolutions (either partial or complete, the distances are different), and for the rear tyres, the distance traversed divided by ten, so you need at least a 10 metre tape and a couple of markers.

An example, using the whole number of revolutions for the front tyres:

Front tyre

Tyre Size: 13.6- 28
No of revs: 13
Distance b’twn stakes: 50.18 m
Rolling Circ: 3.86 m

Rear tyre

Tyre Size: 23.1-30µ
No of revs: 10
Distance b’twn stakes: 48.57 m
Rolling Circ: 4.857m

You’ve now arrived at the ratio between front and rear tyre rolling circumferences, in this case
4.857 divided by 3.86, equals 1.258
This ratio has to be lower than the transfer case ratio, which can be found on the tractor specification plate, stamped on the casing, or in the handbook. (The latter is the least reliable).

The objective is that the front tyres when driven, have to be rolling FASTER than they want to be when free rolling, and trying to lay down more track than the rear tyres. This generates a “lead”, which makes the tractor easier to drive and steer, it pulls better and is more efficient, particularly so in loose soil. On hard ground (roads), disconnect the front drive to avoid axle windup.

The front tyres are DRIVEN, on average 2% faster than the rears considering the amount of track they lay down. The extra 2% or so is taken up in soil slip, which is what makes it more efficient. In the example above, a transfer case ratio of 1.28 would give a lead of 1.75% (1.28 divided by 1.258, minus 1, multiplied by 100). By the way, the replacement tyres don’t HAVE to be radials, as long as they match the dimensions and the rim widths, they’ll be O.K. Be aware though, that bias ply tyres vary in diameter with pressure, while radials don’t. With bias ply, pressure adjustment can be used to give a final “fine tune” of lead, provided that the usual operating parameters are observed on minimum and maximum pressure, and that you can tolerate the harder ride that might result!

So vent out all that water ballast in your rear tyres. That’s “old technology”, and not applicable to F.W.A. tractors. Reap the benefits at the diesel bowser.

Tractor Tyres on Front Wheel Assist Tractors

The introduction of front wheel assist tractors probably caused more heartache to rural tyre services than any other recent innovation.

The traditional 2 wheel drive tractor had small front tyres, with just enough load on them to steer with, and around 75 to 80% of the weight over the rear axle. This was obtained by filling the tyres with water.  Weight over the drive axle translates directly into pull at the drawbar, which means that more work can be done by dragging more, or bigger implements. However, the extra weight chewed up fuel, which got more and more expensive.

Enter the front wheel assist tractor. These have all wheels driven, and the front tyres are around 2/3rds the size of the rear. The advantage is that all tyres pull. So water is not necessary for good traction, and fuel economy is improved.

A gearbox to transfer drive to the front axle as well as the rear, is incorporated. If the tyres are 2/3rds the size of the rear, then driving them 3/2 times faster means that they lay down the same track speed. But there’s a little more to it than that!

The front tyres are generally driven around 2% faster ( 2% lead) than the matching speed over the ground of the rears. This makes the tractor easier to steer, but more importantly, more efficient. The front wheels when working in tilled soil, operate more efficiently at higher slip than the rears, which are tracking in the soil consolidated by the front wheels.

Higher slip means higher wear, particularly on hard surfaces, so it is recommended that the front wheel drive is disconnected when on hard surfaces e.g. roads, where soil slip cannot take place. There is generally a switch on the dash to permit this easily. If the driven track speeds do not match, then the axles fight each other- the transmission “winds up”, and may break, or the tractor “bucks” in the field, and it uses more fuel because it’s not pulling as well as it should.

The introduction of these tractors has boosted the popularity of radial tractor tyres. The old design bias ply varied their rolling circumference with changes in pressure. Radials don’t- they roll around the circumference of the belts built into the casing, which stretch very little – as intended. This means the axle ratio chosen on the transfer case can be selected by the manufacturer to very tight tolerances, since the rolling circumferences of the front and rear tyres stay constant.

This does not mean that bias tyres cannot be used. Indeed, it may be that the imported radials are not available at all, so that you have to adopt another fitment. A matching tyre size can be chosen, and the inflation pressure varied within limits, both tyre casing limits, and ride comfort,  to achieve the desired “lead” of 2% or thereabouts (consult your manual) on the tractor. A later article tells you how to test your tractor on a farm road to determine the “lead”.

Be aware though, that because they are smaller, rotate more, and run at higher slip angles, that you will become aware of the need for replacements faster than you think. Be sure to consult your tyre expert. There have been some very expensive disasters.