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

August 9, 2007 at 8:16 am 1 comment

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.

Entry filed under: Balance & Alignment, Choosing the Right Tyre, Installing & Changing Tyres, Tractors, Tyre safety & maintenance, Tyre Technology, When to Replace your Tyres.

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1 Comment Add your own

  • 1. Machining Center  |  October 5, 2009 at 8:56 pm

    You really are reminding me that I need to balance my cars tires right now.


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