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FOR LONG SERVICE TYRE WEAR
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Tyres with prematurely worn shoulders, or steering that pulls strongly are all-too common problems, and ones that fleet managers have grappled with for some time. But one man has decided to grasp the nettle and do something about it, regardless of the indifference of certain chassis manufacturers. Bryan Jarvis travels to Immingham to see him prove a point.
There can he nothing more frustrating than seeing tyres on new trucks or tractive units scrub away when you are sure that you've checked everything thoroughly. Operators have made huge efforts to counter front tyre shoulder wear and the 'pulling left' syndrome, which seem to be industry-wide problems.
But one man who believes he has some answers is Richard Fisher, manager of H&L Garages' extensive bodyshop facility at Immingharn.
He's firmly convinced that, while steering axles are all built within tolerance, what's really needed is a finer tuning of the steering alignment, especially castor angles.
He's so confident of his findings that earlier this year, he persuaded the garage, which supports a major Mercedes truck dealership, to add a highly specialised £40,000 Josam steering geometry gear to its chassis straightening pit.
It now has a dedicated lane with 20M straightening pit, weardetection plates and everything
needed for altering a truck's axle geometry.
The investment, especially in turntables for measuring smaller axles and independent suspensions, enables the garage to do full-chassis straightening and alignment checks on any truck or 13.6m semi-trailer, or handle two different wheel-alignment jobs at the same time.
Using a universal front axle support, the rig can accept any make of truck.
In his experience, Fisher never assumes that because it's a new truck, everything is in line, and some tyre wear problems may not just be down to poor alignment either.
For instance, tractive units with pusher or tag axles are not always positively located on a centre bolt, so on high-mileage machines they sometimes slacken and shift slightly.
Carry out a very close examination, he advises, before doing a toe-check; you might miss something very obvious otherwise.
Always start at the back axle and work your way forward.
Australian view
These days most drive axles have rear air suspension, and are usually easily adjusted. Fisher prefers the Australian view, which is that if you drive on the left with a right-hand drive truck, you shouldn't have a negative offset rear drive axle (ie the right rear wheel is slightly behind square).
Zero is ideal and 2.0mm/rn positive is also okay.
Severely crowned roads do affect a vehicle's steering but, even on flat surfaces. many trucks
still drift and the scrub persists. Reversing tyres on the rims will help to minimise the effect, "but it still doesn't resolve the over
0 riding problems", Fisher stresses. "What's generally needed is a simple change in the castor angles," he explains.
In the US and Australia, wheel alignment is a part of routine maintenance but is less common here or in Europe.
Operators try everything they can to extend tyre life, yet will baulk at paying for a thorough wheel and axle alignment check.
It's well accepted that misaligned truck or trailer axles create unwelcome forces that adversely affect steering, fuel economy and tyre shoulder wear.
In a large long-haul fleet, that could add up to a lot of money in fuel and tyres alone.
Alignment method
When H8r1 accepts a truck for a full alignment check, its tyres are checked for condition, suitability and properly balanced pressures. Ideally, they should be new or in a near-new state.
It's also given a short test drive to gauge the extent of any steering pull, and then a series of pre. liminary checks, which sometimes includes hydraulic system pressures.
H&L's technicians will then drive it onto the steering axle wear-detection pads at the rear of the straightening pit. This will highlight any unwanted play in the steering axle bushes or bear ings.
If it's a 63(2 unit with mid-lift axle, its alignment and U-bolt torque settings are also checked.
With these preliminaries out of the way, the mechanics next start measuring drive axle alignment.
Different manufacturers hold differing views; both Daf and Mercedes, for example, work on average settings of zero, with a tolerance of +/4.0mm/m of wheelbase; Renault's Premium is +/3.0mm/m, while Scania drive axles are set at a more moderate +/-t.omm/m.
But should a truck drive axle be a few millimetres out of line, the driver will be subconsciously countering forces that want to steer the vehicle from the back. Rolling a cone on a flat surface illustrates this reaction.
It also causes the opposing inner and outer tyre shoulders to scrub off too.
Fisher once encountered an Iveco attic that was giving tremendous shoulder wear on the fronts, and found a drive axle that was almost iomm/m out of line. Once it was realigned, the problem was largely resolved.
Toe-in adjustment
The next stage is to determine the camber and castor angle values, and then a check of the toe-in, To adjust the latter, simply slacken the contra-threaded track rod end clamps and rotate the bar in either direction.
Our sample Actros was found to be toeing in the required 0.2mm/m and needed no adjustment, unlike the two main alignment angles.
Camber was well within the MercedesBenz spec of -0.25° (+/-o.5°) nearside, and +0.25° (+/-o.5°) offside.
This is within tolerance, but knowing that camber will affect shoulder wear, Fisher prefers to see 0.5° on the nearside, and o° on the right.
Checking the castor angles on both locks, the unit was found to have 2.4° on the left side and 2.2° on the right, well within spec.
To fine-tune the angles, his technicians progressively apply hydraulic pressure on the axle beam to cold-twist it, a few degrees at a time, up to 12° (6° each way).
"I'm looking for an approximate 1.5° difference between the left and right castors, always more on the left than the right," adds Fisher.
A final re-check of the Josam gauges show that the desired castor angles are 3.5° on the nearside and 2.o° on the offside.
Satisfied with the final settings, the torsion bar is remounted and after inserting the appropriate castor wedge under the spring pack, the U-bolts are tightened and the axle re-measured.
Vehicles that have been realigned are taken on a short test drive around the South Killinghohne area, prior to being signed off.
The Mercedes Actros worked on here certainly followed the straight line set by the driver, without a trace of pulling towards the nearside, so there is some obvious success.
Of course, the vehicle will have to be monitored by the owner, but Fisher is certain that any steering and tyre wear problems will not materialise. "There's likely to be fuel improvements too," he adds confidently.