Adjusting the Leveler Valves

[TomC]

If you don't have an exact measurement for ride height, you can do some measuring yourself when underneath-but as said before-make sure you have enough room underneath when the bags are completely deflated.  What you do is with full air tanks, disconnect the connecting rods on the axle that goes to the leveling valve.  First deflate it all the way down (you pull down on the rod to make the air escape the bags), measure the body to axle, then push up on the connecting rod and fill the bags up until they stop, and measure them again.  Set them at 5/8-3/4 the way up from the lowest point, and you should be there.  I run with my bags just one inch from the top-mainly to get the bus up as high as possible, and to have a better ride.  Unlike what most think, the higher the bags sit up off the axle with more air in them, the better the ride since the bags will have more air in them to cushion the ride.  Of course if you have an exact measurement from the factory, then use that. Good Luck, TomC

[DaveG]

I disagree with using the height control valves to raise the ride height to "where you want it, higher if you want"...I think that each manufacturer's suspension is designed for a specific ride height, and that is what it should be run at.

In addition, some consideration should be given to driveline (driveshaft/u-joints) angles...driveline angles seem to be more critical these days.

My suggestion would be to find the manufacturer's specs and set it accordingly...then if you have an issue, deal with the issue

I also believe that more air in the bag (to raise the bus) is more pressure in the bag and therefore a stiffer ride...but that is my opinion and like they say, "that's my story and I'm sticking to it."

I cannot argue those that believe that they are getting a better ride with the bags air up more, it just doesn't make any sense to me.

[TomC]

Dave-running more air in the air bags (higher ride height) makes the frequency of the up and down motions slower.  With less, or lower air bags, the piston of the air bag is closer to bottom so when you go over a bump, the pump up in pressure is more severe, hence it reacts faster making for a "rougher" ride.  I work for Freightliner as a new truck engineer and salesman-the air suspension components on Freightliner is made by Hendrickson.  I have had multiple conversations with their engineers confirming my statements about higher riding suspension getting a better ride.  If you have air suspension, you could experiment with the heights also.  An inch or two up or down isn't going to be that critical to drive line angle.  Good Luck, TomC

[DaveG]

No problem Tom, and I respect your input. I too am in HD trucking, and while Hendrickson or other suspension OEs may have a tolerance of the inch or two you speak of, I am pretty sure that Meritor or other driveline mfgs would differ...but isn't it always that way with those guys?!!

And I'm thinking back to the days when drivers (truck drivers) would adjust their ride height to suit them, without regard to the suspension or the driveline...I'm just thinking that the majority of end users here should be/would be better off setting their ride height at the bus mfgs suggestions/specs.

[gus]

Nick,

In case you think I'm really smart, I didn't know it either until two weeks ago when the guy who repaired my disconnected leveling valve link told me!!

I always thought the system attempted to bubble level!!

After he repaired it he told me to let it air up for awhile and see if it worked properly. I parked with one rear wheel in a small depression and expected it to level itself. That was when he explained it to me.

[RJ]

Nick, Chuck, Dave, Tom, et al -

I've posted this before (maybe it was on BNO), but when I was with our local transit agency, one of our drivers complained for months about a front end shimmy at 35 -40 mph on a New Look Flxible 40' transit bus he was normally assigned to drive on his route.

Operations thought he was nuts, but he'd wouldn't have the same complaint when assigned a different bus.  Other drivers also mentioned the shimmy.

Shop spent thousands of $$ and man hours changing brakes, drums, tires, dog bone bushings, steering prop shaft, etc., all to no avail.

Finally, one nite the tire company's tech guy was on the lot, and they ran this coach over the pit for his input.  (Agency leased the tires, very common.)

First thing he did was check the ride height on the front end - it was set 2" higher than specs!  Shop readjusted the front leveling valve to provide the correct clearance at rest between the bump stops and the axle (and checked the rears, too), then sent it out in the morning to see what would happen.

Bet you can guess the outcome. . .  (Three guesses, and the first two don't count!!)

Yup, no more complaining about the front end shimmy.

My preference?  Set it to OEM specs, with fresh bump stops if necessary.

FWIW & HTH. . .

[TomC]

On some older suspension designs, there were only one control arm making the axle rotate with the up and down movement-if the bus is set to high with this type of suspension, then I could see how it would shimmy since it would also effectively change the caster of the axle.  Most suspensions now (including my AMGeneral transit, GM's, etc) have full articulated control arms-meaning that the axle maintains the same relative position-especially the caster setting when it is going up and down.  I know on trucks on the rear suspension, you can actually see the axle housing rotating around the fulcrum of the suspension arms as they go up and down on a bump.  More sophisticated suspension systems-like the Kenworth 8 air bag, or Neway 4 bag have full articulation also keeping the axle in relative position in the up and down motion-keeping the drive line alignment in place-no matter whether the axle is in the full up or down position.  Depending on the length of your drive shaft-the shorter the shaft the more sensitive it will be to drive line angle.  Good Luck, TomC

[NJT 5573]

There is definitely a sweet spot for the drivelines to run. I guess if you don't get alot of vibration your OK.

You can probably find the sweet spot for the entire alignment and running gear package by using the same techniques and hunting around a little with each coach. Most of the manufacturers specs are pretty sloppy. I don't think there is anything wrong with an adjustment that makes things better.

I've done alot of fine tuning like that on my Model 20. If anyone wants to know how nice an Eagle can drive out they should take mine for a spin. It won't move around 2 inches in a 60 MPH cross wind at 75MPH. Chassis tuning is worth the effort.

My new dirt bike was real loose under power. It was within all the specs but it was a hand full to hang onto. Turned out the front suspension was too soft under power.

I think you have to drive each vehicle and give it what it needs. A mechanic will always give you the book + or - setting, but that is for the most part only a best guess that will get you out of the shop and down the road. The hard part is getting the experience to know what to fix and how to fix it. To do that, unless your Chad Knause you need to be both a shoe and a wrench.

[RJ]

Tom -

The '77 Flx New Look had a six air bag suspension - four on the rear axle, and two on the front.  Two leveling valves on the rear, one on the front.  Cheaper design than GM's New Looks, which were four & four, front/rear.  Three front dog bones, two lower, one upper.  Rear similar to GM, two lower, two upper.  Just FYI.

For me, the absolute sweetest riding air suspended coach I drove in revenue service were the 1978 GM P8M4905s in the charter company fleet.  Their 318" wheelbase, combined with Shepperd steering plus front & rear anti-roll bars, were heaven on the highway - even on LA's infamous freeway hops.  The MCIs felt "heavy handed," comparatively.  Interestingly, the Prevost X3-45 is the only 45-foot coach in today's market that has a wheelbase longer than the 4905. . . 

Interesting that you mention Neway suspension.  We had a bunch of Gillig Phantoms with the Neway trailing arm, two-air bellow suspension on the rear.  Those 24 buses caused more back injuries to the operators than any other coach in the fleet.  The rear axle acted like the fulcrum on an teeter-totter, and the drivers were on the lightweight end.  Eight hours a day of being flung up and down took it's toll.

Later model Gilligs came with four bellows on the rear, and the problem went away.

Eagles, with their Torsilastic suspension, are, of course, completely different than the air suspended coaches.  But, as any Eagle owner will tell you, without the suspension indexed correctly, and with poor shocks, the front end can buck you right out of the seat at speed.  Not so much on the 45-footers, tho.

FWIW & HTH. . .

[06 Bill]

I would like this pressure thing & ride height explained please. If it takes X psi to support X pounds @ a given height why does it take more psi to raise. The weight to
psi ratio is the same. However there needs to be an increase in volume. Adding air would raise the Bus and it seems the psi would level off to where it was.
06 Bill Pd 4106 2741.

[TomC]

RJ- I know your right about the Neway air suspension.  They designed it for maximum control on trucks, not comfort of passengers.  That suspension system is so stable, the military uses it on their tank transport trucks off road.  At Freightliner Custom Chassis Corp (FCCC), they were using the Neway suspension on their motorhome chassis.  It was so reactive, that with motorhomes, when hitting a rocking bump, it reacted so fast that dishes were being ejected out of cupboards and people being thrown around.  Neway redesigned the trailing arm.  The old arm is a one piece big J that is very stiff (great for stability-but not comfort).  The new arm is actually a 2 piece design looking like two separate arms side by side allowing some flex in the arm before being transmitted to the frame.  Much more comfortable of a ride.
On my bus, there is a bit of rocking side to side, but then when going into a curve, the big sway bars take hold and the bus doesn't lean at all.  Personally like a bit of a sloppy ride, makes for less jolting and being less tired at the end of a long day of driving.  Good Luck, TomC

[bevans6]

Here is my take on the PSI vs ride height thing.  It's just what I think, based on how I design race car suspensions.  Taking as an example a front axle with four bags.  It has total suspension movement of 6", and is set in the middle, 3" of droop and 3" of bump.  To make the math easy, it has a total load of 10,000 lbs, for 2,500 per bag.  The spring rate necessary to allow 3" of static bump with a 2,500 lb load is 2,500/3 or 833 lbs/inch.

If you want the suspension to set 1" higher, you have the same load - 2,500 lbs - and you need a stiffer spring so that it doesn't compress as much under that load.  the new spring rate is 2,500/2, for a rate of 1,250 lbs per inch.  The spring rate of the bag is (I think) directly proportional to the air pressure inside it, so to raise the height of the bus, you need to pump more air in.  If you had the suspension set in the middle with 30 lbs of air, you'd need to go up to 45 psi to raise the bus an inch.  If you added 2,000 lbs of passengers to the load, you'd need to add air similarly to increase the spring rate of the bag to get it to come back up to it's setting with the added load.  That's what the leveling valves are doing.  Another cool thing is that when the suspension is compressed as the bus goes over a bump, the air pressure goes up and the spring gets stiffer, a true variable rate spring.  

In my world, a stiffer spring equals harsher, quicker acting suspension and ride.  We even calculate suspension stiffness in cycles per second, or Hertz, as a way of relating a given stiffness to the load that it's carrying.  Intuitively I can't come to grips with stiffer suspension riding softer, but I haven't tried changing my bus either, so I'm willing to take it as a possible!  I do know that increasing the stiffness could easily result in a more controlled ride, which could be felt as a smoother, less aggressive, more comfortable ride.  I also think that if the suspension has too big a bag size relative to the load it's carrying, you'd have to run it at a relatively low air pressure, and the variable rate nature of the spring might result in too quick an increase, so the bus might feel harsh and over-reactive it you were running it too light at stock ride height.  This would be worse with double convolute bags that were plated and not using the air beams.

As always, having tried to figure it out on my own, I Googled.  This is an interesting article that adds to what I wrote above.  http://www.fsip.com/pdfs/newsrelease/MI-May-07-Firestone.pdf


Brian

[Homegrowndiesel]

Hey Nick
Glad to hear you got your bags in and are almost done. Be Safe.

Bill

[Nick Badame Refrig/ACC]

Hey Nick
Glad to hear you got your bags in and are almost done. Be Safe.

Bill

Thanks Bill,
I'm being held up by a nasty head cold and it wants Ransome!! I'll get back to leveling the bus later this week.

Nick-