Hydraulic Steering Assist

[RichardEntrekin]

Forgive me for bugging the conversion forum with this question, but the Newell crowd for the most part is not as mechanically adept as you guys.

Let me put the question in context. I am OCD about the way the coach handles going down the freeway. I am also a greasy knuckled retired and recovering engineer. I have been through the coach from one end to the other on the suspension in the effort to make it drive like a BMW (yes I know that’s not possible) I have replaced all the tie rod and drag link ends. I have rebuilt the steering gear box with ZERO lash. I have replaced the bushings in the tag and drive axle suspension, the panhard bar for the rear axle is new. I have torn apart the GM tilt steering column and replaced all the flimsy bicycle bearings with ABEC 3 sealed bearings. I have replaced the u joints in the steering linkage. All of these things have drastically improved the straight line stability. It has been aligned and verified by me. The tires are inflated to the correct pressure for the weight. The fronts are the massive 365/70 22.5 sized.

The newer coaches have a hydraulically powered cylinder that is either attached to the drag link, or to the right side steering knuckle. The cylinder is powered via the TRW steering gear box. Freightliner also uses this setup on some trucks. I am toying with the idea of adding one, but not to reduce steering effort, but to add some stability in the dead zero steering input.

My question is: in a zero input to the steering gear box, in other words no input to the steering wheel, will the hydraulic pressure on each side of the cylinder be 1. Free to move 2. Or static?

If the pressure is free to flow in and out of the steering gear box, then the cylinder would not resist and would not resist movement caused by the road or wind. If the pressure is locked, then the cylinder would act as a massive resistance to movement caused by the road irregularity or wind.

Sorry for the novel.

[buswarrior]

The wide steer tires are a devil.

Buses just don't track the way they used to.

Prevost and Van Hool double decker, are running these 365's and they just won't "sit" in the lane. The pavement irregularities drift them around.

I can't see adding more hardware helping with that?

Perhaps a trial? Put a pair of 12R on it and take it for a spin, and see if there's a change?
Perhaps a more aggressive alignment setting, at the expence of some tire wear? Call the tire manufacturer and find one of their experts, not a sales people, and pick that brain?

I too, like a bus that "stays the course" .
Well worth the effort!

Happy coaching!
Buswarrior

[chessie4905]

try adding a degree  or  1 1/2 degrees positive caster. Will lessen tendency to wander. Commonly done on GM's, at least. If solid axle, will need to tilt more

[RichardEntrekin]

I agree the wide steer tires are part of the issue. The coach originally had 11R tires which you had to run at 130 psi to handle the 17300 weight of the front end. The 365s give me a safety margin and I run them at 105 according to the Michelin table.

I have played a lot with the toe in. Any where from 1/4 all the way to 1/32 in 1/32 increments.

The best setup so far is 1/16 toe in, and 105 psi on the fronts.

Unfortunately, the castor is not adjustable. I wish it were, or I would have already done that. I do feel that lowering the air pressure on the fronts to the minimum mimicked the effect of increased castor . The lower pressure creates more drag on the tire which some people in the racing industry refer to as rolling trail. The front end is a DANA IFS 84.

Don’t get me wrong, it drives really good. The unrecovered engineer in me wants it to drive even better.

Besides, my wife says working on the coach is cheaper than a mistress.

Thank you for your replies so far.

[chessie4905]

until you get caster fixed, you won't get rid of the problem.

[RichardEntrekin]

I don’t disagree with you concerning caster. That is going to take some figgering, give the bolt up of the suspension carriage with the frame, and no caster adjustment from the factory.

But you have me taking a really hard look at that. Thanks.

[RJ]

I don’t disagree with you concerning caster. That is going to take some figgering, give the bolt up of the suspension carriage with the frame, and no caster adjustment from the factory.

Richard -

How many degrees of caster is spec'd for your DANA IFS 84? In doing some research, I found some HD IFS caster specs in the 4.5º +/- 1º range.

I suggest you look closely at how the lower control arms are attached to the chassis. You might be able to slip a shim or two in between the chassis and the front control arm to increase the caster. That's how they adjust the caster on a Halliwell IFS. Add a shim at the front, take away a shim from the rear adds caster, reversing that reduces caster.  You may be able to copy Halliwell's technique with your Dana IFS.

Obviously, don't get under your Newell without properly blocking the body first.

FWIW & HTH. . .

RJ

[chessie4905]

When you had the alignment done, did they give you what the current caster was?

[RichardEntrekin]

Yes, the data is from Josams in Orlando.

4.3 on left, 3.5 on right so some cross caster was built in.  The Dana Specs are 3.0 degrees.

Unfortunately the way the suspension is attached to the frame only allows for shims to adjust the camber. I would either have to take a die grinder and slot the mounting plate, or find
 an eccentric bolt of the correct dimensions.

And thanks for the heads up on cribbing the coach. I put 4 20 ton stands under it whenever I even think about going under there. One in each corner, placed under the recommended support points.

So back to the question about the steering gear box and the slave cylinder…………

I have an inquiry to TRW. If they respond, I will post the answer.

[fortyniner]

Most of the hydraulic power steering I have worked on has a free flowing circulation through the spool valve. The spool valve directs fluid to the chamber needing assist  by progressively closing the return port and opening the respective supply port.  When its not boosting the chambers are exposed to the same low pressure and seem to provide some damping. 

The funny thing about power steering is when engine is off and you turn wheel the assist is now a restriction making it even more difficult. I often wonder whether an electric boost pump might be a good addition.

Ill add based on the comments above the steering might not be deflected as much as
your are correcting for wander due to other factors. 

[RichardEntrekin]

I got an answer from a steering gear specialist at TRW.

When the steering wheel has zero input, the slave cylinder will have approximately 70 to 90 psi on each side of the piston. This essentially turns it into a pretty stiff steering stabilizer when the steering wheel is not turned. With 80 psi on a 2 inch ID cylinder, that would be about 1000 pounds force resisting movement.

He went further to illustrate how the pressure in the slave cylinder is tied into the main piston in the gear box by saying that if the pitman arm were disconnected from the drag link AND you physically moved the slave cylinder it would move the piston inside the steering gear.

Thanks for all your replies.

I am still noodling on the caster issue. On paper, mine looks like it should be OK. However, a comment on this forum about the new Prevosts and VanHools with the 365 tires not driving like the older skinnier steer tires combined with one of my observations makes me ponder if more trail wouldn’t help.

My two observations are that first lower tire pressure, increasing rolling trail by virtue of a longer contact patch, helps with straight line stability. The second is that a heavy road surface compared glass slick pavement contributes to straight line stability. The increased drag of the road surface again acts to increase the effective trail. Anyway that’s the fairy tale I am telling myself.

I am wondering out loud with the proliferation of 365 tires on Prevosts if their alignment centers are doing anything other than spec for caster.

[chessie4905]

Here is some more to chew on.

http://forum.prevostownersgroup.com/archive/index.php/t-5501.html

[RichardEntrekin]

Thanks, I read that thread some time ago.

There is a massively long one on the Wanderlodge forum about increasing the caster.