Any Tire Experts In The House?

[Simusid]

We've had our new rig for a few months now. International 4300 chassis. It looks nice, it's big, giant box, chock fulla gear and probably would last 10 years if we don't drive it into a ditch first.

I just drove it tonight and the back end sways so much I yelled to the medic that I was going to sing sea chanties. For all intents it's all over the road between 40 and 55 mph or so.

The weight is something north of 9 tons. It's a single rear axle w/ 4 tires. The tires list "max tire pressure" of 120 PSI on the side wall and when it was delivered, thats what it was filled to. And was all over the road. Someone in our office called someone (no idea who) and they said that 120 would be for the max load of the chassis and we should be lower. Ok, I guess that makes sense, so down we go to 75 PSI..... still all over the road.

I'm wondering if 120 was really too high but now 75 may be too low. Anyone ever dealt with this?

 

[Cup of Joe]

In my SUV, I had always had tires leaking air issues, so I went to an "automated" pump and told it to fill each tire with 45psi (which was the max allowed by the tire). Machine filled it up to 90psi each. Drove it and the ride was all over the road. Skidding and drifting in turns, as well as making the ride much rougher. Brought it to the mechanic which is when I found out about the over pressure. They lowered it down to 35psi which was the recommended in that cars manual. I would say go as low as 65 and see how it does. Though if you had tire overpressure issues you would feel it in the ride and the entire vehicle would be behaving weird, not just the back.

Could also be the shocks. I'd really have to drive it to tell you what I think, but I'm thinking it could be the suspension.

 

[CollegeBoy]

I think the reason you would be feeling it in the back and not the front would be because of the large amount of weight on the backend in the form of the box. Admit it, most ambulances are a chasis modified to have a box on the back, they aren't neccesarily designed for carrying the box. Try lowering the rear tire pressures some. I would drop the pressure initially down to around 100-90, and test it, and then begin to lower it in increments until you feel you are at a pressure that is comfortable. I feel it is safe to assume it is a rear wheel drive?

Disclaimer: Sure I may be corn fed and born with grease on my hands, but theres only so much about vehicles even a country boy can tell you without actually physically looking at it.

 

[JJR512]

Admit it, most ambulances are a chasis modified to have a box on the back, they aren't neccesarily designed for carrying the box.

Most ambulances are built on chassis that were specifically intended to have a box put on the back. They aren't modified, they're built like that by the manufacturer (say, Ford). Whether the end vehicle is going to be an ambulance, dump truck, utility truck, moving van, or whatever, the frame rails are made on the production line with mounting points designed to carry an aftermarket something. In fact, at least with Ford, if you want the chassis to be built to hold nothing, to be just slick frame rails, you have to special-order it.

As for the original question...What the tire says is its max pressure is just that, the maximum pressure you can inflate the tire to. It doesn't mean you should inflate it to that pressure. The tire manufacturer has no idea what the end user is going to use that tire for.

Most vehicles (I don't know if there's a legal requirement for all vehicles to have this or not) have a sticker on either the driver-side door frame, or on the edge of the driver's door, that indicates the manufacturer of the vehicle, maximum weight ratings, and some other useful information...including the recommended tire pressures. For example, there's an ambulance sitting about eight feet away from me right now; it's a Horton ambulance on an International chassis. The sticker (on the driver's door) says the manufacturer is Horton, and indicates the recommended tire pressure is 80 psi when the tires are cold ("cold" not meaning chilly; rather, not having been driven and warmed up by road friction). So check for that sticker and see what it says.

If you inflate the tires to the ambulance manufacturer's recommended pressure and the ride is still overly bouncy, take it to your repair shop and have them check it out. You might have the wrong shocks or springs installed, you might have shocks or springs intended for another ambulance model that's heavier or lighter. If you can't find that sticker, contact the ambulance manufacturer or your repair shop and ask them.

 

[DesertMedic66]

Im not a car expert or anything near that but it might not be a tire problem. It could be a suspension problem or a sway bar problem. When our rigs start to go all over the road like that it seems that the sway bar is always bent or loose.

And with the stickers on the door for tire pressure, it is a good guide if it's still the original tires from the factory. Once you get a different set of tires on the rig then it's not always the best guide to use.

 

[Simusid]

Thank, I didn't know that there would be a sticker on the door with specs for our delivered unit. I checked it and lo and behold it says 95 PSI.

Now I'm trying to find out if the info I was given was true that originally it was 120 and then lowered to 75. I hope this fixes the problem!