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Preventive Maintenance on Suspension Improves Tire Life



Would you like increase the life of your tires?

Since the expense of tire replacement costs are typically higher than all other replacement component costs put together, vehicle owners and maintenance managers are generally conscientious about well known tire PM procedures such as checking tire pressure and alignments.

A very important, but lesser known PM procedure is the replacement of shock absorbers on a preventative, rather than a corrective maintenance basis. Recent tests performed by the Society of Automotive Engineers (SAE) and the Michelin Tire company have shown that it's very important to replace worn shock absorbers before they allow excessive bouncing, which in turn causes excessive tire footprint distortion.

Suspensions Have Changed

In the last decade Shock absorbers have become one of the major causes of unnecessary tire wear, as well as the cause of jarring and vibration damage to many other components on class 8 trucks. The reason for this phenomenal increase in worn shock related damage is the advent of soft air suspensions on tractors and trailers, and taper leaf suspensions on steer axles. In the past, shocks were not used on the old multi-spring rear suspensions. They also were not a critical component on the multi-spring steer axles. This was because the stiffness and inherent dampening of these suspensions did not require the added movement dampening of shock absorbers.

The Old Days
Vibration Dampener
Travel limited to 2"-3"
    Today
Movement Dampener
Travel up to 13"
   

Now days, shocks have become a very critical component. Modern suspensions, with little or no inherent dampening, and up to thirteen inches of vertical movement, literally bounce and hop out of control down rough roads when their shocks become excessively worn.

According to SAE Study # 962152; "Worn shocks contribute to wheel hop and/or greater tire load variance of x 4 = (+/- 80%)". In other words, on bumpy roads worn shocks can allow the force, or weight on the tire to change from less than a thousand pounds at the top of the bounce, to several thousand pounds at the bottom of the bounce. This force variation will occur approximately 100 times per minute at highway speeds. The resultant large foot print changes of the tire caused by worn shocks then create excessive tire wear. Good shocks, on the other hand, are designed to keep this load variance to a small fraction of this amount.

It is generally easy to spot a tire that has been subjected to this excessive foot print distortion. That's because the rubber that can move the easiest (and therefore move and wear the most), is the rubber next to the grooves, and sometimes the outside portion of the tire. On steer and trailer tires this excessive wear next to the grooves is known as "river channel wear". On rear tractor tires, the wear pattern can often also be seen circling the shape of the irregular grooves, eventually causing these grooves to widen.

Early Examples Of River Channel Wear
Steer Tire     Drive Tire
   

The wear pattern that eventually results from worn shocks and rough roads is called "scalloped cupping". The wheel hop caused by this loss of shock dampening will often build up a resonance with the rhythm of a worn road. This in turn causes a "cupping" pattern that repeats it self evenly around the tire. This pattern takes longer to make it self visible. Once you can actually see, or feel this pattern, it's in an advanced stage and has already taken thousands of miles off of the life of the tire.

Prevention Works Best

It's extremely important not to let this irregular condition start to develop in the early stages of a tire's life. Once the tire develops this irregular wear, its out of round shape will continue to feed on itself (even if the worn shocks are replaced). Therefore, if this deterioration starts in the early stages of the tire's life, its life will be considerably shortened.

Advanced Wear Patterns Caused By Poor Suspension Maintenance
Steer Tire     Drive Tire
   

Recommendations

The ATA's Truck Maintenance Council's latest Recommended Practice on air suspension maintenance (RP 643), presented at the winter meeting in March, 2000) recommends that fleets "Establish a preventative maintenance procedure which changes shocks on a regular basis". But when should this PM be performed? The R.P. also states that; "Fleets have found it beneficial to install new shock absorbers when installing new tires to maximize tire life".

Truck tires and shocks have certain characteristics in common. They both wear out slowly, (i.e. a shock slowly losses it's damping throughout its life) and they both have approximately the same average useful life. When a new tire is installed on a truck, the shock absorber may look ok, but if it has been run over 150,000 miles, it will have stroked over 30 million times. Since the shock's hydraulic fluid does not have the lubrication quality of engine oil, it will have lost much of its dampening capabilities due to internal wear. When a shock loses over 50% of its dampening, it can allow the excessive wheel hop on rough roads that can cause irregular tire wear due to excessive foot print distortion. Since this cupping will continue to deteriorate throughout its life, (even if the shocks are replaced at a later date) the cost per mile of tire replacement will increase significantly.

More Miles

Fleets that have tested the concept of changing their shocks before they go bad have reported savings. When shocks were replaceed at the time of tire replacement the fleet reported savings of tire and component replacement costs equaling several times the added preventative maintenance costs. (These fleets also required 50,000 to 100,000 miles on the shocks to be eligible for change out.)

Bottom Line

"It just doesn't make sense to pair an expensive new tires with a relatively inexpensive worn shock". Besides increasing tire life, changing shock absorbers when changing tires increases:

  • Suspension component life
  • Chassis component life
  • Electric and electronic component life
  • Life of air conditioning connections

In addition, this practice decreases brake lock up (due to wheel hop), accidents, and driver fatigue. Like other good preventative maintenance practices, changing shocks when installing new tires does not cost money, it saves it.

This document was provided by Reagan Industries.

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