[travelite]

With Koni's on my H3's steer axle the handling was on rails. I could place the coach on the white line and hold it there with my pinky on curves and bumps while passing semi's at any speed, but the ride was stiffer than I liked, so I went back to the original sachs shocks on the steer axle and I traded off some of the razor sharp steering for ride comfort. Seems there's always a trade off.

The IFS steer axle of the Prevost has an instantaneous roll center at road level; the roll center at the drive/tag axle is at the panhard rod. The roll axis inclines upward from steer to drive axle. This creates a roll/yaw couple at the front of the coach. As the coach rolls towards the outside of a turn it also yaws to the outside as if it's trying turn out of the turn. The wheels aren't turning away from the turn with roll (this would be bump steer), but it feels this way because the coach yaws outward. So what do we do, we add more steering. This is an okay thing to do in a turn but if the roll is caused by a wind blast and we're going straight we don't want to add in excess steering. The result is we tend to overcorrect.

My guess is the panel truck felt more precise because cargo carrying trucks have very, very high roll stiffness. Designers don't want trucks to lean or they'll tip over in turns. A truck and trailer suspension is set up to offer very high roll resistance; consequently, very little lean in a turn. No roll means no yaw. No yaw means no overcorrection. No roll also means no bump steer. It all depends on how well your bus handles roll, or more precisely, the effect of roll on steering. Also the wind blast hits the bus well ahead of it's center of gravity. The aerodynamic center of pressure acting well ahead of the center of gravity causes the bus to yaw. The panel truck also experiences this but I think the moment arm is quite a bit less because of the placement of the CG relative to the center of pressure. The CP may actually be behind the CG causing the panel truck to yaw into the wind.

The Prevost is a marvel compared to my 2002 Wanderlodge LXi and I mostly associate this to the IFS at the Prevost steer axle and the markedly higher torsional stiffness of the Prevost chassis. The Prevost seems to yaw into sidewinds. I remember literally holding the Wanderlodge steering wheel cranked 90 deg to counteract the sidewinds of Texas. I just returned from a 4500 mile trip thru Colorado via Texas and sidewinds barely affect the Prevost. I think the primary difference is the inclination of the roll axis on the Prevost as opposed to the Wanderodge's more horizontal axis also the placement of the CP relative to the CG. Remember, the LXi had a stick axle up front with the roll center at the panhard rod just like the drive axle. The LXi had roll induced bump steer and side-scrub of the steer tires as the roll forces kick the body sideways through the high roll center. This side stepping and tramp is a common malady of high roll center vehicles.

There's my wonkish explaination! LOL!

[encantotom]

David, glad to have you here!

thanks for the comments

tom

[Richard]

David,

Great to hear from you !!!!

Since the CFO is not signing the appropriation to satisfy my handling OCD with an H3-45, I'll just keep noodling on the Newell.

Translating your comments into my experience with the truck makes sense. The truck was sprung much much tighter in side to side motion even when overloaded. Couple that with shorter wheelbase and less surface area resulted in crisper straightline stability.

You talking about the sidewinds also reminded me of a not so subtle straight line behavior I have noticed. If I have a constant side wind (aka Texas) or a noticeable crown in the road, the coach requires less correction, than if the road is pancake flat or I have a head or tail wind. I always thought if I were working against a force that wanted to move the coach in ONE direction it was easier to hold it steady.

Again, don't get me wrong. My coach drives great. @77Newell went for a ride with me and commented on the lack of motion in my hands. I just want it to run on rails as you say, and it's a tease to see how much improvement I can realize.

I have the adjustable Koni's on the front. I may crank them up to see if I have the same effect that you noticed between the Koni's and the Sach's.

[HoosierDaddy]

That is very interesting, David ! Wonkish....You certainly fit in here!! Can you comment more on the torsional stiffness..The WL has a reputation of being "built like a tank". If I understand correctly the Prevost is considered to be monocoque construction. How does the WL compare?

[travelite]

Thanks Tom and Richard,

This past 10 months have been busy with our newborn in the house. It was a real treat getting the bus out for a 4 week jaunt this summer. We loaded all six of us into it and had a blast. The kids are 10 mos, 5, 7, and 10 years and they're already planning next years trip. Thanks for the re-welcome.

Richard, I know what you mean about the bus being more controllable when the suspension is slightly loaded by wind or road crown. I attribute this to taking some of the slack out of the roll stiffness as the bus takes a set to counteract the wind or road forces. It's like driving your dad's old caddy around the racetrack. On the straights is wallowy, but once it's tossed into a turn and the outside springs compress and take a set, the steering in the turn tightens up.

It's all a tradeoff and it sounds like Newell traded a smidge more in the direction of rider comfort and in the process took a smidge from steering preciseness. I think you can make it as precise as you wish it's just a matter of giving up something. If you add an infinitely stiff sway bar on the steer axle and eliminate all body roll you'll have the precision steering of a dump truck, semi, or panel truck.

Ever see an 18 year old drive a dump truck? They all think they're driving sports cars. Ever watch one in a turn? There's absolutely zero body roll. The things handle like go carts. When they hit bumps wheels on both sides of the axle come off the ground; one wheel bumps lift both wheels. They hop, skip, and chatter thru bumpy turns. If you suspend the body in the air and push up on one end of an axle the other end goes up and equal amount - infinite roll stiffness (or close to it). The designers don't care how they ride, they just don't want them to tip over in a turn. Put a bus suspension under a dump truck and give it to an 18 year old and he'll flip it in the first turn. Bus chassis are tuned for rider comfort at the expense of steering preciseness.

My H3 with Koni shocks did a great job of limiting body roll at initial turn in. This increased steering preciseness at the expense of rider comfort. With the Koni's it was like it was always in a slight crosswind or road crown as you described. But my tastes are more in the direction of rider comfort so off they went and on came the Sachs.

Another thing about the H is where the driver sits. I'm below living floor level so the effects of body roll and yaw are felt less by me than what passengers may feel. So I'm inclined to react less to them and to overdrive the bus to a lesser extent than if I sat at living floor level.

I think you can make your steering as good as you want; it's just a matter of what you're willing to give up. Tradeoffs.