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Out of alignment axles and wheels can dramatically reduce tyre life, but there is a cure.
What do you think would happen if the axles on a tandem-axle trailer were set pointing to one side? Or worse, if the axles 'fought' each other with one heading off to the right and the other the left. What would this do to tyre life and fuel consumption?
Tests on a trailer with deliberately misaligned axles suggest that a 10 misalignment of one axle increases drag by 14(.7( and fuel consumption by 3%. Easily explained if you consider the consequences on a typical truck wheel with a circumference of about three metres. With 10 of misalignment the tyre will effectively be dragged sideways by more than 50inm on every revolution. If the axles are 'fighting' each other, fuel consumption is even worse.
And misaligned axles are all too common on trailers, tractive units or rigids.
The situation is more complicated on steered axles, with castor angles, king-pin inclination and toe in/out settings. Steered wheels are preset so that when a laden vehicle is moving without any input from the steering wheel the road wheels are upright and pointing straight ahead. But problems creep in during use. A brick hits the track rod and bends it slightly and the toe in/out is disturbed.
If the steering is toeing-out by 3mm on the road then every time the wheel travels ahead one revolution it effectively drags the tyre 3mm sideways. Not as drastic as on the trailer axle but on a journey from London to Edinburgh the tyres will have been dragged the equivalent of 600 metres sideways! Think what that does for tyre life.
While it is common sense that all the road wheels should be heading in the same direction, it is worth questioning if that is the case.
A perfect example arrived at Lasalign's Crick workshop recently: an ownerdriver's vehicle with a front offside tyre that looked distinctly odd. Its profile was not flat across the tread area as is normal on a truck tyre, it was convex as on a motorcycle. The owner was not a happy man. He had complained of excessive tyre wear to the main dealer who had fitted a new shock absorber and sent him to have the tyres turned on the rims and the offside wheel balanced. But the problem remained.
To start the diagnosis Lasalign jacks up the front axle, clamps lasers on the front wheels and target boards on the rears.
Then by spinning the front wheels the lasers are adjusted to make them parallel with the wheels and focus on the target boards. By equalising the readings on either side the steering wheels can be set in the straight ahead position.
The front wheels are then lowered on to turning plates and the toe-in/out mea surement checked. On this vehicle the front axle was toeing out by 3.5mm more than it should causing the inside of the tyres to wear rapidly. Having corrected that, the wheels were again set straight ahead and the power steering checked.
Power steering will always want to pull itself into its neutral position where the pump is not working and this should be with the wheels straight ahead. If it is not, the vehicle will always want to wander off to one side and the driver has to correct by putting some steering on. The result is that the inside of one front tyre and the outside of the other will wear rapidly, causing fuel consumption and pump wear to increase, There is always some indication to show when the box is in the neutral position. When the wheels were straight ahead on this vehicle the indicator showed the pump was working.
To correct this the drag link must be altered in length. On twin-steered vehicles the problem is compounded and even more detrimental to tyre life.
The problems on the vehicle, although they may have appeared small, were enough to ruin a tyre with many millimetres of tread in the centre by wearing the shoulders away.