Death Wobble
Q I recently put larger tires on my Jeep and not I've get serious death wobble on the street.
A When trying to diagnose death wobble, always start small and work your way up. Although the larger tires may seem like the cause of the problem, in reality they are often only making an already existing problem noticeable. Start by checking the front bearings. Simply jack the front tires up off the ground and, grabbing the top and bottom of the tire tread, try to wiggle the tire/rim back and forth. If any lateral play is noticed, check to see where it's coming from. The usual suspects are the hub bearing preload (or the unit bearings on newer Jeep vehicles) or the ball joints.
If the hub adjustment, bearings, and ball joints check out OK, move on to the tires, making sure they're properly balanced. You can try mounting the rear tires on the front and see if that helps. If not, take the tires back to the shop that did the balancing and have them make sure the balance is still true.
With the tires ruled out, move on to the steering linkage tie rods and/or rod ends. Check the tie rod ends, drag link ends, and the track bar bushings (if applicable) any one or combination of these can contribute to death wobble.
Finally, if your Jeep has been lifted, check the front axle caster. You generally want 4-6 degrees positive caster (bottom ball joint in front of top). Usually when a lift is installed the front axle may be rotated upwards to alleviate pinion angle and reduce driveshaft vibration. However, when the pinion is raised, caster is lowered. If your caster is too low, remove or replace the spring shims if your Jeep has leaf springs, or adjust the control arm length or (if applicable) the eccentric bolts to increase caster to the desired amount.
Odo Adj
Q I recently installed bigger tires/different gears and now my speedometer isn't accurate.
A It's simple math, but if you increase your tire size by 10 percent, your speedometer will read slower by 10 percent. If you increase your gear ratio by 10 percent, your speedometer will read 10 percent faster. Whichever modification you're working with, there are a few methods of correcting your speedometer no matter what year Jeep you have.
Older Jeeps with mechanical speedometers will need to run a Drive Joint with a specific ratio to either slow down or speed up the speedometer. The Drive Joint mounts in between the transfer case speedometer gear drive and the speedometer cable and is currently offered by Stewart Warner (stewartwarner.com).
Jeeps from the '70- to mid '90s that run fully mechanical or mechanically-driven electronic speedometers can have new speedometer drive gears installed into the transfer case tail housing. Replacement gears with 26-38 teeth are available to correct for several ratios and tire size combinations. The gears are available through numerous distributors including 4 Wheel Drive Hardware (4wd.com) and Quadratec (quadratec.com).
Finally, vehicles with fully electronic speedometers that use a Hall-effect pickup and even mechanically driven electronic speedometers can use an electronic conversion box, such as the Superlift True Speed or the Jet Performance Speedometer Calibrator. Or, there are a number of performance controllers and hand held electronic programmer boxes that can adjust the speedometer readings by accessing the factory computer software through the OBD II port.