Ride Height and Ride Quality and Ridewell

[davidbrady]

I find myself thinking about something Bennie Collier told me a long time ago. He said the LXi rides better when the steer axle ride height is set lower than the drive axle's.

When setting the axle ride height I've always blindly followed Ridewell's recommendation of 8" +-3/16" at both the steer and the drive axles. This seems reasonable because both axles use essentially the same air springs. The air spring part numbers are different but that's mostly because of differences in the air fitting location and mounting bolt holes. The bellows and the pistons are the same parts on both the steer and the drive axles, so I've always set my ride height to their recommendation, basically 8 inches.

We often talk about the Blue Bird Bounce or as some call it, Porpoising. We all know what it is because it's something all Ridewell suspension equipped Wanderlodges do.

The technical term for Porpoising is Pitch. Pitch is vertical displacement of the bus body on it's suspension but in opposite directions at each end. The front goes down while the rear goes up. At some point between the axles there's no up or down movement, that point is the pitch axis.

It turns out that pitch isn't dependent on the center of gravity location or the suspension geometry, it's dependent on the spring rates at the drive and steer axles.

Imagine an extraordinarily stiff drive axle suspension and a soft steer axle suspension. In pitch all of the motion will be at the steer axle and the pitch axis would be the drive axle. Alternately, we can imagine a stiff steer axle and a soft drive axle, now the pitch axis will be the steer axle and all the vertical motion would happen at the drive axle.

Without having "yet" done any experiments, I think our pitch axis is way back close to the drive axle. After all, the springs are stiff on the drive axle as compared to the steer axle. The effect of this is a lot of vertical motion at the steer axle as we all feel through our captains seats.

What does this have to do with Bennie's advice? Well lowering the steer axle ride height has the effect of stiffening the air spring. Now we've moved the pitch axis forward. In theory, we've reduced the vertical motion felt through the captain's seat.

Remember, this is all theory - I haven't yet experimented but I will! I'm going to play around with lowering the front in very small amounts followed by test drives. I seem to recall Bennie suggesting 3/4ths of an inch. I wouldn't go any more than that. In conjunction we could raise the rear slightly to soften it.