08-17-2016, 06:53 PM
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!
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!
David Brady
'04 Prevost H3 Vantare
Asheville, NC