Barber Pole Wear

August 16, 2007 at 7:53 am 2 comments

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.

Entry filed under: Balance & Alignment, Installing & Changing Tyres, Tyre Technology. Tags: , , , .

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2 Comments Add your own

  • 1. ludocb  |  October 11, 2007 at 11:07 am

    Hi,
    I read with interest your article on barber pole wear.
    I believe I have this problem on my 1991 Ford Telstar Ghia.
    In about 1995 is started to rumble very badly on one the drivers rear side, especially if I was cornering at low speed. I was advised to replace the bearing which I did. The old one looked alright to me and the rumbled continues.

    The tyres on the rear were both worn on one side on the rear ( I cannot quite recall which was worn on what side).

    I was advised to have a front & rear allignment done.
    Nothing significant was found.
    I once got rumbles in the rear wjhen the tyres wore down a bit.
    The next set of tyres went the same way.
    I had another front & rear alignment done and guess what, it has all repeated again.

    Typically, the tyre has done about 20,000 KM, one side of the tyre has worn perhaps 15%, whereas the worn side has perhaps 5% left.

    Questions that I have.

    a) Is the slow rumble on cornering at slow speed indicative of barber pole wear?

    b) Would it be normal for the rumble to come after
    the tyre has worn a bit, but not when it has plenty of tread?

    c) Should a rear wheal alligment not pick up if the wheels were out of square or is the accuracy of rear wheel allignment less then the accuracy that is required
    for wheel squareness?

    Regards,

    Lee de Vries
    Geelong
    Victoria

    Reply
  • 2. ludocb  |  October 11, 2007 at 11:08 am

    Barber pole wear occurs on a rear tyre, because it is trying to steer the whole car, because it is out of alignment.
    So the FRONT tyres develop compensatory shoulder wear, on the outside of one tyre, the inside of the other, trying to sterr it in the dirextion you really want to go. The rear tyre, meanwhile, from time to time “lets go”,(it has less load on it) hops across the road surface, still spinning, and develops a diagonal wear pattern right across the tyre. Very aggressive tread patterns develop noisy “heel and toe wear” around 10000 km., which is best evened out by a DIAGONAL rotation on your car.
    This wear pattern can be seen late in the afternoon if you park the car in line with the setting sun. The shoulder blocks will be seen to be bevelled fore and aft.
    But by the description, it sounds as though your Telsar has too much rear camber, probably because it is run without any load on the rear. All the rear tyres are doing is holding up the corner.

    David
    david@carbonblack.com.au

    Reply

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