Archive for August, 2007

Barber Pole Wear

Never heard of it? It’s associated with a vehicle out of alignment, and this article has been prompted by the sight of a short wheelbase trailer’s mad gyrations behind an apparently blissfully unaware Winnebago driver out near Broken Hill recently.

The vehicle involved can be either a car, truck, or trailer. The misalignment is not associated with the “front end” alignment, but the rear.

Suppose a truck is built longer on one side than the other- don’t laugh, it has happened. In one case, the drive axles on one side were located in different holes drilled in the chassis rails on opposite sides of the truck. So two axles of drive wheels were not aligned to the front end of the vehicle – they wanted to drive the truck at a different angle to the direction it was intended to go. So the steering tyres, out-muscled by the 8 drive tyres, had to accommodate their thrust by steering at an angle, in order to get the truck to run straight ahead. The result was that both front tyres wore out, one on the inside, one on the outside rib, very quickly: less than 10000 kilometres, repeatedly. In another case, the jig on which the vehicles were built was out of square by 50 mm.

For a while there was a fashion that the high tensile front axles would be cold-bent to cope with what was thought to be camber wear, when the real problem was further back on the truck. Hopefully, this practice has been diminished by the introduction of laser alignment equipment, which can be used in broad daylight to align a truck accurately. Previously, such alignments had to be done inside in a semi-dark room.

Now align your thoughts to a front wheel drive, where the front axle does all the work of driving and steering, and the rear just trails along for the ride. Experience has shown that for the rear to track correctly, the build tolerances on the relative position of the wheels must be much tighter. If the vehicle “frame” is built out of square, then instead of being a rectangular plot of the wheel positions, you might end up with a parallelogram. Or one wheel position at the rear may have a damaged or misaligned suspension bracket, with the wheel no longer tracking straight ahead. This one wheel will then try to steer the whole car in the direction it wants to go, fighting the other three for control. Of course, it can’t win, but it can make the car steer to one side, which the driver then corrects for at the busy end, and so pursues a somewhat wandering course of constant steering corrections. In an extreme case, the rear wheel can build up so much steering force which the car cannot accommodate, it “lets go”, breaks adhesion, and skips across the road surface whilst still revolving, till it relieves the stresses in the tyre. Then it starts all over again.

This generates a wear pattern called “barber pole wear” after the striped red and white poles outside barbershops. An uneven scallopy wear pattern develops diagonally across the tyre tread, which once seen, is entirely indicative of the whole vehicle tracking out of line.

There are many reasons for this. Let’s go back to the Winnebago. The trailer had a motorbike, and at least four 20-litre fuel cans, all on one side of the trailer. This flattens the springs on that side, which lengthens them, altering the position of the axle on one side. It is no longer at right angles to the direction of travel. The trailer is captive at the towbar, wants to track sideways, so once again, the tyres have to “let go” to get the trailer back behind the campervan. Being relatively light, this was easy, but the gyrations were wonderful to behold.

Scale this up to a 62 tonne B-Double truck, where the axles at the far end of the second trailer are a long way from the steer tyres. If the trailer is loaded off centre, the load has a very high centre of gravity, or the road is highly cambered, then the suspension alignment can be altered in the same way as happened to the Winnebago trailer.
If the unit is known to be habitually operated under these conditions, then the trailer can be set up deliberately out of straight line alignment, so that the whole unit tracks straight.

Trucks that run out of Sydney across the Hay Plains and the Nullarbor to Perth, can attain very high mileages on their trailer tyres, and 200000 km is not unusual. Tyres wear out faster around corners, and there aren’t many of those on that route. Also note these are “trailer tyres”, where the load is a lot less on each tyre than for the steer tyres. The third point is the worn appearance of those high mileage tyres is always very smooth. Tyres that slip in service develop a crepe appearance on the tread face, best seen on a hard worked tractor tyre (which operates at around 15% slip). Close examination of this creped surface can yield a lot of information on whether the vehicle is tracking straight. Here’s what you do- make sure the tread surface is slightly dirty- rub some dust on it. Then stick about four inches (100 mm) of Sellotape strips across the tread, the 2 inch wide stuff is great, press down firmly, lift them off as one piece, and stick them to a clean sheet of paper. Mark the direction of rotation, and the wheel position. Do this for all wheels.
Close examination (a magnifying glass is handy) will show a “grain” present in the imprint of the tread pattern. The direction of the grain shows the direction in which the tyre has wanted to travel. If it’s not straight down the line of the tyre, then the wheel is not aligned.  Steer tyres may show mirror reversed grain divided down the centreline.

Tyres wear faster than they should when they are out of alignment, or the pressure on their tread surface is not evenly distributed across the face of the tyre. The latter can be due to camber problems, or tyre tread- design problems, where the load is not evenly distributed across the tread ribs. Keep in mind that even load distribution is not always possible, because of the shape of the tyre itself, and tyre designers can compensate for this by having the grooves of different depth. The aim is to have all the tyre tread face wear out at the same time.

My first example of “barber pole wear” was a Renault, which the owner had brought it three times to be aligned, getting increasingly upset each time. Spotting this wear pattern, I asked “which wheel had been in the smash?” “That one”, the owner indicated to the tyre with the wear pattern. “They did a real good job on the bodywork, didn’t they, you’d never know” he said, to which I replied “A pity that they didn’t straighten the bent suspension arm as well”.

To make the point, the only way that these problems can be rectified is by having all the wheel positions aligned. For cars, ask for a “four wheel alignment to the thrust line”. Don’t be surprised that it costs more.
For trucks, either buy the truck already laser aligned, or have the whole rig aligned in its working configuration.

Advertisements

August 16, 2007 at 7:53 am 2 comments

Chinese Tyre Recall in USA

There is a  Chinese tyre recall in the USA – only to be followed by toy company Mattel’s recall announced today from its Chinese made toys.

The Chinese tyres subject to a recall in the U.S.A. are listed below.

These tyres were also imported into Canada, which so far has not reached a decision as to whether they too will recall the products.

LT235/75R15 patterns CR861, CR857
LT225/75R16 pattern CR861
LT235/85R16 patterns CR860 CR861 CR 857
LT245/75R16 patterns CR860, CR861 CR857
LT265/75R16 patterns CR860 CR861 CR857
LT31X10.5-15 patterns CR857 CR861

The company involved is Hangzhoa Zhongce Rubber Company, the second largest tyre company in China. Up to 450,000 tyres are involved. Brand names of tyres include Westlake, Compass, Telluride, and YKS.

Our source of the information was www.npr.org, and is additional to an article July 2 on testing of Chinese sourced seafood for the American market, and details other recalls on petfood, toys, etc.

In statements made to the New York Times and Wall Street Journal, Hangzhou Zhongce has denied that the tyres are defective.

Please visit www.npr.org or http://www.tirereview.com/?type=wm&module=4&id=2&53L3c73d=149 (type in ‘recall’ in the search box)

Or watch the coverage on CNN http://www.cnn.com/video/?JSONLINK=video/business/2007/08/10/todd.china.imports.cnn&DPFPR=true&pks=ijsioc080607google

August 15, 2007 at 8:34 am Leave a comment

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.

August 9, 2007 at 8:16 am 1 comment

V8 Supercars stick with Dunlop until 2012

Dunlop has beaten rival manufacturers to win its third consecutive contract to supply the control tyre to teams participating in the Australian V8 Supercar Championship Series from 2008-2012.

Dunlop’s V8 Operations Manager Kevin Fitzsimons believes the quality of Dunlop’s tyre was the key factor for Dunlop winning the contract.

Dunlop’s control tyre, with just 4mm of tread and a contact patch of 80mm, is designed and tested to control a 1.5 tonne V8 Supercar capable of 620 horsepower at 7,500 rpm with a top speed of 295km per hour in temperature of up to 120°C.

The Dunlop V8 Supercar control tyre is manufactured at the company’s specialised racing tyre factory in Nagoya, Japan, where each tyre takes more than three hours to be hand-crafted.

But in a sport where technology and competition is everything – there has been no change to the tyres compound or structure since Dunlop took over the contract from Bridgestone in 2002.

The cars by regulation are equal which makes it hard to pass.  There have been numerous discussions with Dunlop and the technical board on the subject of how to improve close exciting racing and increase passing during the race.

Suggestion of two tyres, a soft and a hard compound have been dismissed as has the talk of changing to 18inch rims.  Kevin Fitzsimons claims there will be no change to the rim size staying at 17×11 inch until after 2012.

The tyre has a reputation of being reliable, safe and providing exceptional performance at every track and this is what the teams and drivers have raved about.

Kelvin O’Reilly, the general manager of TEGA, which represents the V8 Supercar teams, said: “The V8 Supercar tyre supplied by Dunlop is required to operate at maximum efficiency in the widest of racing environments. Whether it be hot or cold conditions, high speed or tight circuits, the Dunlop tyre has been excellent for our competition use.”

I guess this is a good argument for the if it ain’t broke don’t fix it mentality.

August 1, 2007 at 7:09 am 1 comment