Suspension Lift Kit Knowhow – Part 4

Suspension Lift Kit Knowhow – Part 4

by Jim Allen


Pinion, U-Joint, & Driveshaft Angles

All suspension lift kits alter the driveshaft angle and can result in vibration, a short u-joint life, or even sudden and total failure if you don’t correct it at the time a lift is installed. Basically, every degree of driveshaft angle above 10 degrees shortens the life of the universal joint and increases vibration. Consider a continuous operating angle of about 15 degrees the practical maximum at normal ride height. When the suspension flexes, about 30 degrees give or take depending on u-joint size, it's put in the area where it could bind and break. The torque capacity of a universal joint also decreases with angularity. The distance between the transfer case output flange/yoke and the pinion flange/yoke increases with lift and you may run out of slip-yoke travel as the suspension travels up or down. One cure is a long travel slip yoke, but often a longer driveshaft that puts the slip yoke in a position where it stays engaged is enough.

With many lifts, a new or modified driveshaft is needed. You can have that done locally at a driveshaft shop or via mail order. Ready-ma,de shafts are sometimes sold as an upgrade for specific suspension lift kits. A new driveshaft gives you the opportunity to upgrade to a larger, stronger u-joint and a longer-travel slip-yoke or CV (constant velocity) driveshaft.

A CV driveshaft has what’s called a double-cardan joint at the transfer case end. Its main job is to reduce vibration caused by excessive angularity. Imagine two standard u-joints in a special housing that splits the angularity. If you have a problematic 15 degrees of continuous angularity, the CV splits it into two 7.5 degree angles for each joint and that reduces vibration, increasing life and torque capacity.

Details, Details!

A connected sway bar is a vital part of the safe street environment, but it can restrict suspension travel on the trail. You can have the best of both worlds with sway bar disconnects, or tunable sway bars. Some tall coil spring rigs need sway bars even on the trail and that’s where those tunable systems can come in quite handy.

Suspension lift kits and big tires and wheels require corresponding improvements to the steering system. Those necessary for correction, such as dropped pitman arms, are most often included with the kit. Depending on tire size and the factory toughness of the stock system, you may also need to beef up the tie rod and drag link by using stronger and bigger parts.

Sticky tires, especially on hard surfaces, will put a heavier load on the power steering system. The first thing that will happen is an increase in power steering oil temps, the cure for which is a power steering cooler. The second part is that your power steering may not have enough power to steer the vehicle in all trail situations and maybe even the parking lot! The cure for that is an enhanced steering box and higher powered pump, maybe with a hydraulic assist.

Even if the power steering system has enough power, big tires and hard wheeling put a lot of stress on the steering box mounting areas. Some rigs are particularly weak – many Jeeps and older GM trucks to name two. You can prevent breaking the box of the chassis with a steering box brace and/or mounting reinforcement. In some cases, the reinforcement plate can act as a repair gusset as well if the chassis is already damaged. If you have increased tire size by more than about 15 percent and plan to do much wheeling, steering upgrades should be on your A-list.