What does the load in the back of your rig or truck do to the way that the front wheels sit on the ground?
That is the question that Curt asked; “It would be neat to know the difference between pickup front ends when pulling that 16,000 lbs. down the road.”
Wow does that open Pandora’s Box. There is no one answer as it effects the front wheels in different ways and not just depending on the style of front suspension, IFS or beam axle. One thing that effects it more is the type of load and where it is placed on the chassis.
The first thing that we need to know is that the alignment angle most effected by load or rear chassis height or deflection is caster. Caster is forward or backward tilt of an imaginary line drawn down through the center of the king pin or ball joints. The best indication of caster is the swivel wheels on a super market shopping cart. When you push the cart along the wheels trail behind the pivot between the wheels and the cart. This means that the center line down from the pivot point leads the wheel and it trails so it is positive caster.
When the DW says, “back up here driver” and you obey the wheels swing around the pivot point because they try to maintain a positive angle. The wheels do not like to lead the pivot. Positive caster provides better straight line stability and better recovery from a turn.
Well on a vehicle the axles or independent suspension is all hung from the chassis. When a alignment is done on the vehicle the technician puts the rig on a very true and level rack. Then hopefully he will check all of the parts for looseness as an accurate alignment cannot be performed if the pivot points and bushings are loose. The he should check the riding height front and back. The alignment is a measurement of the alignment angles with the vehicle at rest against published angles by the manufacturer. Alignment is a starting point and very seldom will the vehicle see those angle when riding down the road.
In the case of Curt’s question lets us see what happens. Suppose Curt had the alignment checked while the truck was empty, nothing in tow, and an almost empty fuel tank. Let us only look at the caster angle for this example. Just for the sake of argument let us suppose that the caster and the tilt back of the king pins was 5 degrees positive.
Now Curt goes home and loads up for a trip. Now if he has a travel trailer hooked to a conventional rear tow the tongue weight is all behind the rear axle. The rear wheels become the center of a seesaw and the weight added attempts to lift the front wheels off the ground. The chassis drops in the rear and instead of riding as designed with the nose down a bit it is now riding with the nose up. This tilts the king pin back more at the top and adds more positive caster. It also lifts some weight off the front wheels.
Now caster is also the angle that causes the front wheels to lay over or change camber, the inward or outward tilt of wheel. The design of this is to add bite to the tire when rounding corners. However adding too much caster causing the wheel to flop too far over on turns will chew the outer edges off the tires in short order.
The trick to keep things working right? The proper loading of the vehicle. Add springs or air bags as needed. Even incorrect tire pressure changing the diameter of the tire can effect this situation. The proper hitch and the proper location of your fifth wheel in the trucks bed. There is more but that will have to wait till another time.
Next time we’ll look at what a 30” extension between the wheels did for Denny’s Super C Chevy Kodiak.
Brad Sears