Ask the Mechanic: What makes some trucks seem misaligned?

Hello Paul, what is a centre bolt in relation to trucks and buses that somehow makes them appear misaligned on the chassis, posing a risk of brushing/swiping on-coming vehicles on a narrow two-way road?

Jack

Hello Jack, what you refer to as misaligned is commonly known as crabbing; when the back of a vehicle does not follow directly behind the front wheels but is slightly angled to one side or the other. That happens when the rear beam axle is no longer fixed at precisely 90 degrees to the main chassis (which is probably still straight) in exactly the same position on each side. And that happens when the linkages that attach the axle to the chassis are loose, bent, broken, or missing.

Clearly, a rear beam axle cannot be bolted or welded directly to the chassis or there would be no rear suspension. The air in the tyres would be the only cushion. That design is bouncy even on a donkey cart, and not good enough for a truck weighing several tonnes and carrying several times its own weight and at much higher speeds than a punda-plod.

So, the beam axle is attached to the leaf spring stacks only, using “U-bolts”, and it is the springs that are attached to the chassis, through a pivot eye at the front and a swing arm “shackle” at the back (to accommodate the fact that when a curved spring flattens under load, it gets longer).

Only the longest spring in the stack (the “main leaf”) is held by the pivot and shackle points welded to the chassis. The other progressively shorter leaves (including the final “helper” leaf at the bottom), are attached to the main leaf by the “centre bolt”, which passes through the middle of all the leaves. The whole stack is held by the U-bolts looped over the axle and fastened through a plate under the helper. That’s what holds the back axle precisely in place.

If the centre-bolt comes loose or shears and drops out, the spring stack is no longer a unified component and the individual leaves can shift position backwards, forwards and/or sideways, and are therefore no longer tightly held by the U-bolts, which, in turn, no longer tightly hold the axle which, in turn, will no longer have assured 90-degree symmetry on both sides of the chassis.

One back wheel will be slightly in front of the other relative to the truck’s direction of travel. This will have the effect of “steering” the back of the truck slightly sideways. The driver can correct for that by counter-steering slightly so the truck continues in the desired overall direction, but the front of the vehicle will be off-set from the back. It will “crab”. Even if the centre bolt is intact, loose or broken, U bolts will have the same effect.

Crabbing makes a truck’s bodywork unexpectedly wider for on-coming traffic, adds to drag on the engine and extra fuel consumption, places considerable additional forces on the (already suffering) linkages, compromises steering integrity and subjects the tyres to extra scrubbing. The truck needs to be stopped, unloaded, and have its entire rear axle and suspension realigned and properly rebolted. Pronto.

WHY DOES MY CAR CONSUME MORE FUEL?

Hello Paul, I drive a Mutisbushi Outlander 2008 but its fuel consumption is unbearable. What could be the problem?

Chris

Hello Chris, the price of fuel went up so we get less litres for the shillings we pay, yet we travel the same distance. That can create the impression that your car’s fuel consumption has gone up. Nevertheless it is possible for your car fuel economy to reduce if the engine is not well tuned. Air cleaner and spark plugs should be in tip top condition with a clean throttle unit to allow ample air intake for efficient burning.

The car tyres must have good treads as energy demand (fuel consumption) goes up as you accelerate to gain traction on worn out tyre treads. Your journey planning is crucial. If you drive in slow peak hour traffic, you will crawl in lower fuel thirsty gears. When you drive in non-traffic peak hours, you tend to drive in higher and more fuel economical gears. That saves fuel. The same thing happens when you are in a hurry and drive aggressively (as opposed to sensibly), higher engine revs call for lower gears which results in more energy demand. The reverse is also true.

Send sms: mycar (space) your comments and questions to 6933

Or email them to: [email protected]