Tire Safety System

[Jon Wehrenberg]

Jim,

First a qualifier. I do not know if what I am describing below is standard on all shells, or if the converter has disabled the feature as I heard a certain large west coach converter did. Take the following with a grain of salt until you have verified it.

On 45 foot coaches that I have been beneath there is a device that is intended to prevent the rear of the coach from dropping when the tag air is dumped. Again, on the coaches I have seen from the underside, there is a switch that engages the tag axle air dump system when the steering wheel is turned to the extreme in either direction. When that occurs tag axle air is automatically dumped, and to prevent the rear from dropping air over hydraulic shock absorbers on the drive axle are brought into play, and they literally support the coach so it will not drop.

If you get under your coach, on the bulkhead forward of the drive axle will be a reservoir with a sight glass mounted in the middle. With the coach at the ride height, the reservoir should have the sight glass half full of Dextron III fluid. That is the hydraulic fluid which is forced into the hydraulic cylinders (which look like shock absorbers) that are mounted on the rear of the drive axle.

If you coach is dropping when the tag air is dumped, it could be because you do not have this system, the system is not functioning, or the system is low on hydraulic fluid.

Do not manually lift the tag as this may over-ride the operation of the system described above. There is insufficient information in my Prevost manuals to be able to say with certainty that what I am telling you is completely accurate.

[JIM KELLER]

Thanks Men !

[merle&louise]

How much air (psi) do you guys carry in your tag and how much in your drive axle tires?

I am told to put 110psi in the drive axle and 80psi in the tag. Any ideas why?
My rear tires are 11R80 24.5 Michelins and my front tires are 315/80 22.5 Michelins.

Why not have all three rear tires carry the same load?

[Jon Wehrenberg]

Bad advice was given to you Tuga.

The air pressure in the tires is related to how much weight they carry and the pressure that is required for that weight is listed on the Michelin web site under the specifications for your tires.

Because each of my axles carries a different amount of weight I have different tire pressures for the tires on each of my axles. If I weighed the coach from side to side I might even have more than three pressures.

Your first step is to have your coach weighed and once you know what each axle weight is you will find the correct tire pressures.

[merle&louise]

Jon,

How much air do you carry in the rear tires and what is the actual weight of the drive axle? How much air do you carry in the tag and what is the weight of the tag axle? How do both compare to the GVWR for each axle?

Do you think that I should weigh the driver's side rear of the coach and then weigh the passenger side rear? Would they carry different pressures if they differ in weight?

[Jon Wehrenberg]

Tuga,

Without going to the garage to verify I believe I am around 14,000 on the steer, 20,000 on the drive and 12,000 on the tag. When I looked up the pressures my steer needs to be 100 PSI, the drive 80 PSI and the tag 85 PSI.

I have not weighed the side to side on each axle because the scales I have seen don't have sufficient width alongside the scale to run one side of the coach on the scales at a time, so I over inflate my tires by 5 PSI. The actual pressures I run are 105, 85 and 90. Those were of a fully loaded coach with full fuel and water, all our stuff in the bays and closets full of clothes.

If you were able to weight the side to side differences you could be more precise on inflation pressures. I have to assume that is splitting hairs however because on our coach with a full fresh water tank the weight is biased towards the passenger side and as that tank is emptied and the holding tank is filled the weight shifts toward the driver's side.

As far as capacities we are approaching the limit of the steer axle, but we are below the other axle limits. I can't give you specifics without getting the weight ticket and the limits from the data plate. We do not exceed any Prevost limits.

[Jon Wehrenberg]

I did a little research in the Prevost technical diagrams and found the Tag axle unloading system is as I described. According to the wiring diagram the switch which activates it is sensing the position of the Pitman arm in the steering box. If the speed is below a certain value the air is dumped from the tag and hydraulic lock valve is energized. It is all automatic.

I have seen the switch actuator relating to the Pitman arm on my coach and the steering wheel must be almost at the extremes of its travel in order to actuate the system. I do not see any provision for supporting the rear of the coach if the tag axle is lifted manually via the switch behind the driver's seat.

[merle&louise]

Thanks Jon, I looked up the psi that Michelin suggests for my application and the front axle(315/80 R 22.5) is right on the money @ 130psi. It is rated at 17,640 and the actual weight shown by the scale was 16,920 (full fuel and water). Michelin shows with 130psi per tire the supported weight would be 9090# on each tire for a total of 18,180#.

I'm going to weigh the drive and tag separately as soon as I can get to it. I think that I will be o.k. because Michelin shows an 11R24.5 L supporting dual tires @ 12010# @ 105psi and a single tire supporting 5550# @ 80psi. All adding up to 35,120# and my actual weight is 33,180.

I am still going to weigh the drive and tag separately to see if the pressures need to be adjusted up or down.

Thanks for the help, I really appreciate it.

[Larry W]

The prevost manual that came with my 99 chasis states on the 45'xl the tag will not lift off the ground but will unload. 40' will lift off the ground. The manual also states to unload the tag when making sharp turns or for additional traction on slick surfaces. This worked for me in a rest area in Utah we slow me stopped with the drive wheels on ice.

Larry W
00 Royale Coach

[Orren Zook]

On the subject of wheel/tire balancing here's a link to a promising web page: www.dynabeads.com