Caster Angle

[travelite]

Folks,

Over on the Wanderlodge Owners Group (WOG), some forum members have been experimenting with adding in some positive caster.

The idea is to provide an extra degree of self-centering for the steering so that directional stability is improved (less tendency to wander).

I thought I'd ask POG to see if you guys know what the typical caster setting is for the various prevosts. I'm interested in all models and all suspension configurations.

On my 2002 Wanderlodge LXi, the caster is set to 2.7 deg Left (street side) and 3.0 deg Right (curb side).

Thanks

[Jerry Winchester]

David,

I have to think that adding caster angle will make the steering somewhat heavier and harder handling. Now I know that if Jon could get enough caster angle in the old Liberty combined with his normal slow speed and the ruts you typically find when not driving in the hammer lane, it would almost be like auto pilot and he might give it a go.

I also think that the more aggressive the camber, the more "weight" you have to lift when you turn the wheel. But I took Vehicle Systems class in college about 30 years ago and I slept a lot back then.

[travelite]

Thanks Jerry,

This is really the fundamental difference between a Wanderlodge and a Prevost. In our Wanderlodges we're working on caster, in your Prevosts you're working on Crestrons! :)

I'd rather work on Crestrons!

[Jerry Winchester]

Well our company had a '77 Wonderlodge with something like an old Cat 3208 V8 (no turbo) that sat between the two front seats. I wish I had any happy memories of it. We took it on a fire in Mississippi and the customer liked it so well he bought it. Best I remembered it had an old Ross power steering unit on it that you basically used to herd it down the road. I'll bet yours is a quantum leap ahead of that one.

[travelite]

Oh gosh yes... I can actually take my hands off the wheel for 20 seconds or so, but I'd like better. It's amazing over on WOG that we have folks still buying and driving those 70's and early 80's forward controls. They just don't die, and they're cheap as nails until one loses an engine, then the bus is effectively totaled, unless you have a donor and the skills to put it back together, which many do. :)

[Orren Zook]

Increasing the amount of positive caster will increase steering effort and straight line tracking, as well as improve high speed stability and cornering effectiveness. Positive caster also increases tire lean when cornering (almost like having more negative camber) as the steering angle is increased. With power steering, you won't notice the increase in steering effort caused by positive caster as it increases in a turn. The effects of positive caster are pretty much "positive," especially increasing the lean of the tire when the vehicle is cornering while returning it to a more upright position when driving straight ahead.

On vehicles set up to drive on the "right" side of the road, the curb side is aligned with a little more negative camber (usually about 1/4-degree) and a little more positive caster (again, about 1/4-degree) to help the vehicle resist the influence of crowned roads that would cause it to drift to the right. Since most roads are crowned, cross-camber and cross-caster are helpful the majority of the time, however they will cause a vehicle to drift to the left on a perfectly flat road or a road that is crowned to the left.

[BoaterAl]

David, the first thing that comes to mind with that setting is allowance for the crown in the road. If my memory is correct that is not a hugh amount of caster to improve the overall handling. Doing the adjustments until I have the bus driving me at the end of the day.
Has there been any changes in the ride height ? I screwed around with the Country Coach Affinity for almost 2 years and then it hit me to drop the ride height. I have no idea where CC came up with there suggested ride height. We measured shock travel and even Koni was stumped on why it was at the high end.
When I had the current bus on the rack we left the factory adjustment in place except for a slight toe=in adjustment. But this was done after a correction on the ride height setting to factory spec. Overall I'm satisfied with the tracking and watching tire wear. IMHO the ride height must be checked first for correct setting. So reading in-between the lines of your post and my past trying different things, drop the ride height, maybe 1/2 inch all the way around.


AL

[travelite]

Oren, Thanks. That's great information.

Al, thanks for the input. The Ridewell suspension on my coach is probably the same as what was on you Country Coach. I believe CC was Ridewell's biggest customer; Ridewell sure does miss them! The spec for ride height on my coach is 8" +-3/8" measured at the air spring. I've been all over the coach and have done just about everything conceivable. I've owned it for 6 years and have 105K miles on it, and frankly, the ride and handling are quite good, I'm looking for that last 5%. I have the FSD Koni's, the ride height adjusted (although I could experiment with slightly different settings), tire pressures set, centramatics, TPMS, CrossFires, etc. I pretty much know this beast inside and out. Many former LXi owners complain of a rough ride, I even solved that problem. Turns out the steer axle sway bar was excessively stiff. I had a chance to climb around an H-Series Prevost to take sway bar measurements on a stick axle equipped bus and I found the LXi bar to be 3 times stiffer. With that remedied she's pretty well sorted. The icing on the cake would be a tad more directional stability, especially in heavy wind conditions. Thanks again for the input.

Folks,

Along with typical Prevost caster settings, toe-in settings would also be interesting to compare with Wanderlodge settings. I know I'm comparing apples to oranges, and I don't plan on replicating the settings on my Blue Bird, this is more for my enlightenment on what similar heavy vehicles use for specs.

Thanks

[BoaterAl]

That's interesting that you bring up the sway bar. I had 2 front end guys tell me that sway bars are usually spec out heavy duty and over stiff. This is done for the driver that trying to drive his coach like a race car on curves. I think this theory is probably has some merit as I read where plastic coach owners find broken sway bars ...then blame it on a defective weld. You know they only sway so much. That was the next improvement for my CC but never had a chance to try it as the RV-T... ad sold the CC.
Just for conversation the Koni's Reds were set on one ......full turn above soft.

[travelite]

Al,

I have the Koni reds on the steer axle and they're adjusted pretty close to what you describe.

What you said about your suspension guys thinking most bars are spec-ed too stiff is fascinating. When I say that my LXi sway bar is 3x stiffer than the H-series that I compared it too, I'm talking about wheel rates. The H-series came in at around 350lb/in at the wheel and my sway bar was 1150lb/in at the wheel. The bar that Blue Bird put on my coach was 2 1/8" thick and the stiffness increases by the fourth power of the diameter. Ouch. Now I know what most folks are thinking: sway bars don't effect ride quality, they only effect roll stiffness as the coach leans. Well, this is true when both front tires hit the same bump at the same time, as in going over a speed bump and hitting it square, but in reality this never occurs. Traveling down the highway has each wheel reacting independently to its road input. One wheel bump, as in hitting a pot hole with the right front tire, tweaks the sway bar and increases the spring rate at the wheel correspondingly. Sway bars do increase roll stiffness, but the tradeoff is you lose some of your independent wheel motion. After my mod, the ride and handling was transformed.

[travelite]

As another point of reference I found a paper that describes Class 8 Truck alignment specs. Caster is typically in the range of 5 to 5.5 deg positive. Anyone have specs for their Prevost? Time to get a Ridewell engineer on the phone.

Thanks,

[Jon Wehrenberg]

I am not coversant with the Prevost specs on alignments, or the underlying reasons for the specifics, but I do know that the Prevost is sensitive to having the correct caster, such that if it is off, the coach handles poorly.

I suspect the engineers develop the requirements based on many factors and that the handling quality of the coaches (both mine have been capable of hands off straight tracking) is a testament to the engineers' efforts.

[GDeen]

I am not coversant with the Prevost specs on alignments, or the underlying reasons for the specifics, but I do know that the Prevost is sensitive to having the correct caster, such that if it is off, the coach handles poorly.

I suspect the engineers develop the requirements based on many factors and that the handling quality of the coaches (both mine have been capable of hands off straight tracking) is a testament to the engineers' efforts.

To the point that you can walk back to the fridge for a diet coke while cruising down the interstate??

[travelite]

The only detriments to excessive caster angle that I've been able to find in the literature are:

1) heavier steering,
2) greater induced camber in tight turns which may lead to increased tire wear, and
3) possible steering wheel shimmy

Benefits are:

1) better directional control, and
2) steering wheel self centering

Michelin posts a preferred spec of 3.5 to 4.0 deg. I'm running 2.7 and 3.0 deg. It would be interesting to see what other buses run.

Does anyone have a contact within Prevost that I could call?

Thanks,

[Jon Wehrenberg]

To the point that you can walk back to the fridge for a diet coke while cruising down the interstate??

Not being one to work inefficiently, I usually get the coach aimed, set the cruise and then I go to the bathroom and pick up the Coke on the return.

It is actually not as unsafe as it sounds. Di's screaming usually covers up the noise of the guardrail grinding the stainless steel off my bay doors.

BTW, This hands free steering is only available on Liberty coaches so all of you that own SOBs (some other brands) are advised to not try this.

[GDeen]

Not being one to work inefficiently, I usually get the coach aimed, set the cruise and then I go to the bathroom and pick up the Coke on the return.

It is actually not as unsafe as it sounds. Di's screaming usually covers up the noise of the guardrail grinding the stainless steel off my bay doors.

BTW, This hands free steering is only available on Liberty coaches so all of you that own SOBs (some other brands) are advised to not try this.


Thats pretty good cept I know how Di keeps you on your leash.....fresh baked cookies.

[stevet903]

Alignment specs for XL and H3 series are here: http://prevostparts.volvo.com/technicalpublications/pdf/MI98-25.pdf

[travelite]

Alignment specs for XL and H3 series are here: http://prevostparts.volvo.com/technicalpublications/pdf/MI98-25.pdf

Steve,

Thanks, that's a great help! :)

Once again... The Power of POG!

[lonesome george]

David,
If you do experiment with the caster angle on your Blue Bird please post the result, this is the kind of stuff I live for.

[travelite]

David,
If you do experiment with the caster angle on your Blue Bird please post the result, this is the kind of stuff I live for.

You bet George. You and me both! :)

[stevet903]

If you ever wondered how they fix out of spec alignment angles on a "non-adjustable" beam axle, here is a quick read....

http://www.beeline-co.com/PDFs/Truck_Axle_Correction.pdf

[travelite]

If you ever wondered how they fix out of spec alignment angles on a "non-adjustable" beam axle, here is a quick read....

http://www.beeline-co.com/PDFs/Truck_Axle_Correction.pdf

Steve, that's amazing. I couldn't imagine trying to bend one of these axles. Let's hope they can do it with shims.

Speaking of which, other than Josams in Orlando Fl, anyone know of a shop that can handle a caster tweak? We'll be heading west on Highway 40. Someone in Tn would be great.

Thanks