Ok, so my own coach does not have IFS and runs with a solid axle. But when my coach was built there were no chassis out there for a diesel pusher with independent front suspension. And it is true that when I bought this coach I down traded from a later model Southwind on a Chevrolet P-30 with IFS to an older Foretravel on a solid front axle Oshkosh chassis. But more on that in a minute.
In general the IFS suspension has so many advantages over the solid axle that the decision to go IFS should be a no brainer. Lets look at the differences. In a solid axle set up when one wheel goes over a bump and that wheel rises it lifts that end of the axle. If the other wheel remains on flat ground the tipping up of one end of the axle causes the wheel on the other end to tip out at the top. Now the center line through the wheel will no longer be parallel to the ground but will tip down and intersect with the ground somewhere out to the side. That wheel will not steer around that intersecting point and no longer roll straight ahead. This is called bump steer that the driver must correct for. Independent front suspensions are free of bump steer.
The engineer in designing the length of the suspension arms and placement of the pivot points can correct for a variety of handling problems. Roll center, or how the rig leans on turns, dive which is the nose down effect of hard braking are but two handing problems that can be controlled.
And finally alignment. While there are a few alignment angles that can be changed on the solid axle like camber, but the work involved in bending the heavy axle to make the adjustments is a bear. Caster is adjusted by adding tapered shims between the springs and the axle, but this effects both sides of the vehicle. Each wheel if the independent rigs can be adjusted six ways to Sunday for handling, ride and tire wear.
The down side of IFS is complexity. There are many more wear points on IFS than a solid front axle. The steering linkage is more complex and must have pivot points that match the geometry of the suspension. And as all of us who owned and drove the P-30 Chevy chassis rigs the original engineering can make or break the handling of the rig.
The old P-30’s had as a major fault a narrow front end. The front wheels were closer together than the rear wheels. The newer IFS rigs moved the front wheels out to the same tread width as the outside rear dual tires. This helped improve the ride and handling, especially in the front engine rigs.
There are two designs IFS on the market for the DP rigs right now. Dana builds a unit that is found under the Freightliner chassis that is equipped with ball joints, just like your car. Other makers use a king pin system that has more wear points that the ball joint system. Add to all this the decrease in turning radius that can easily be achieved with IFS, the system appears to have created a strong foothold in the industry.
Well now that we have beaten this dead horse, come on back next Monday to see what we talk about next. And don’t forget to let me know if you agree or disagree with my thoughts.
Brad Sears