How many times has this happened to you? You get a call from one of your best fleet customers, and the maintenance manager begins to yell at you for selling him some lousy tires. Apparently, on this one group of trucks he has, the wear on the tires is uglier than a warthog in tights, and the drivers are losing their fillings faster than you can say, ``Ahh.''
The last thing he says to you is: ``Get out here, replace these tires and fix the ride problems you caused!''
What do you do? Take off for the airport and that well-deserved vacation you were putting off? Throw several tires of a different brand in your truck and head on out to the customer's location? Or put on your Sherlock Holmes deerstalker hat and confront a new sleuthing challenge?
Well, I don't know about the deerstalker hat, but this is certainly an opportunity to test your detective skills. Of course, you have to have some before you arrive at the scene of the crime.
You first have to have a basic understanding of what causes ride disturbances. There are numerous sources of vibration. Some frequently encountered ones are:
Road surface roughness;
Driveline component balance or propshaft angularity;
Improper fifth-wheel position;
Since there isn't much you can do about road roughness, let's look at the rest of the causes.
Driveline imbalance is easy to determine. Any ride disturbance that can be eliminated by taking the truck out of gear at road speed is probably engine or driveline related.
The driveline must be balanced and may also cause a problem if the angle that the driveshaft forms between the back of the transmission and the front of the differential is too great. This condition usually is found on short-wheelbase tractors and trucks.
Fifth-wheel position and trailer influence also are easy to determine. Any ride disturbance that can be corrected by dropping the trailer or altering the fifth-wheel position probably is related to the trailer tires, wheels/rims or hubs.
Tire/wheel/hub non-uniformity is a little trickier to determine. Vibrations that can be felt at 50-60 mph but stop above and below these speeds usually are the result of tire/wheel/hub run-out.
Vibrations that get worse with increasing speed usually are related to mounting or balance problems. Brake drums, wheels, rims and tires can all cause vibrations that are felt in the cab.
All of these components must be balanced, be within run-out specifications and be properly mounted or installed (including torquing) to ensure the whole rotating assembly is concentric and in balance.
Now that you understand the ``M.O.'' of vehicle vibrations, begin looking for clues regarding the cause.
To diagnose a vibration complaint, find out as much as possible about the problem. Start with interviewing the driver and take a road test with him to understand the complaint. Then inspect the tires and wheels assemblies for:
Proper inflation pressure, cupping, irregular wear and concentric bead seating. Make sure they are matched by designs and diameters.
Proper wheel-nut torque. Loose wheels can cause excessive bolt-hole wear, wheel vibrations and metal fatigue.
Balance weights that are split between the inside and outside of the rim and are at the same position on each side.
Then inspect the rest of the vehicle. (If you don't feel qualified to do this, work with the fleet's mechanic).
Jack up the front end so the axle is unloaded and check for broken or worn parts, looseness, excessive suspension wear, steering system wear and concentric mounting. Examine ball joints, kingpins, shocks, wheel bearings, linkages and shims. Do the same on the rear axles and also check the springs, bushings and air suspension.
Lift the hood and check the radiator, motor and cab supports, and crossmembers and bolts.
By this time you should have been able to determine if the source of the vibration is driveline-, fifth-wheel- or trailer-related. If your information leads you to believe the vibration is caused by wheel-end non-uniformity, there's a lot more work needed to pin-point the exact cause. Don't change the tires out yet!
If, during your test drive with the driver, vibrations could be felt at 50-60 mph but stopped above and below these speeds or the vibrations got worse with increasing speed, have the driver disengage the clutch and let off on the throttle. Then try to identify the offending wheel position by pinpointing where the vibration is felt the most.
If it is in the steering wheel and/or the floor pan under the driver's feet, it is probably a front-tire problem. If the vibration is felt in the seat, the problem is usually a rear tire. On tandem-axle tractors, it is likely to be the rearmost axle.
If all the information you have gathered determines that the vibration likely is due to rotating wheel-end components, the probable culprit is either an out-of-balance condition or excessive run-out.
Balance is most critical in free-rolling wheel positions on steer and trailer axles. In general, checking the balance on drive tires is not needed. On-vehicle balancing with a spin balancer may aid in correcting the vibration problem by balancing the particular tire/wheel/hub assembly.
However, if the hub or brake drum is out of balance, when placing another balanced tire and wheel assembly on this wheel position, the entire assembly still will be out of balance. The solution is to balance these other components as well.
Excessive run-out of the assemblies on the rear axles is a common cause of vehicle vibration. Check the run-out of all the tires on the rear axles(s) with a run-out gauge. If the run-out on any tire/wheel assembly exceeds 0.125 inch, it will almost always cause a vibration.
However, run-out on a tire/wheel assembly of as little as 0.040 inch may cause a noticeable vibration in certain conditions on certain vehicles.
If the high points of tire/wheel assemblies in dual positions are matched, a ride complaint will result. If the high points are 180 degrees apart, they will cancel each other out. If they are 90 degrees apart, they may still create a problem.
If the high points of the assemblies are at the same place, remove the outside tire and rotate it so that the duals' high and low spots cancel each other out. This is called ``clocking'' the tires.
Try to pair tires within 0.25 inch of run-out of each other and ``clock'' them so that the high and low points are opposite. Identify the tires with the worst run-out and put them on the forward drive axle (being sure to ``clock'' them) since this axle is the least sensitive.
Re-gauge the tires and make sure they are ``clocked'' right. If the high and low spots have changed, the problem is not tire-related, but rather wheel-, stud- or wheel-installation-related. Make sure the wheels are tightened using a criss-cross pattern. If the wheel is installed off-center, a vibration problem will be caused.
Measure the run-out of the studs by marking the first stud and measuring with the gauge. Check all the studs. If run-out exceeds 0.015 inch the hub should be replaced.
If a stud is bent, replace it. If the hub is all right, mount the tire on another wheel and retest or take the run-out measurements of the wheel. If the wheel is a problem, replace it.
Always check the run-out again after making a change in the components. If, after checking the run-out on all the components on the rear axles, vibration still can be felt in the driver's seat, balance the drive tires.
investigating ride and vibration complaints takes a thorough detective who knows what clues to look for and is doggedly determined not to give up.
Correcting the problem can take a lot of work, but it's worth the effort rather than just changing the tires out. If that's all you do, you'll be getting that same call from the fleet manager two months later.
(If you need help diagnosing a fleet customer's ride complaint, contact your tire supplier for assistance from a field service engineer.)