Is it normal for the rear suspension to "crash" over every bump in a Class C?

[p38fln]

OP, Put more weight in the back. Unlike lesser vehicles the F250, E250, Transit 250 and heavier chassis are designed to run at maximum payload 100% of the time. This isn't like an F-150 where the max GVWR is going to make the headlights point at the sky.

Switching the rear suspension completely for an Air-Liner setup like a semi truck uses will tremendously help your ride quality when not operating at full load because the airbags soften up as load is removed, then stiffen up as load is added. This is of course the "not cheap" option. The cheap option is go down to Lowes and buy 7 or 8 sand bags and toss them in.

Please be careful messing with tire pressures, low tire pressure is either the #1 or #2 cause for tire blowouts. Remember to air the tires back up if you fix the problem by adding weight.

I think Michelin had a very good weight to PSI chart for truck size tires on their website, but I'm not able to look it up again right now.

 

[LMHS]

My husband installed Firestone Ryde-Rite air bags under our vintage 1977 Midas Class C. It was on a GM Vandura 450 chassis. We simply matched up the chassis (P30?) with the air bags.

The leaf springs were almost flat on the camper when we got it. I went from having my teeth jarred, along with everything in the Class C shaking, over every concrete seam and patched spot on the highway to a smooth ride.

It took three trips to get the air to the perfect amount (55lbs I think). I wrote the amount of air pressure on a sticker that got put inside a cabinet door. Once we got it settled, I didn't have to refill the air at all.

It also helped to level out the camper as well. Until we added the air bags, we didn't realize the Class C was sagging towards the rear.

We used the Class C (fulltime) for 4 years, then my daughter used it for 5 years (fulltime) before selling it to someone.

 

[Gary RV_Wizard]

I don't know where I would find an RV tire pressure chart. I tried getting a weight-to-psi chart from the tire manufacturer but they could not provide one. So I used one from some other popular RV tire and added some for safety.

You did the right thing - there is no such thing as an "RV tire chart", just a chart for each tire size & model. Since nearly all tire makers belong to and adhere to Tire & Rubber Association standards, the chart for a given size and style of tire will be essentially identical.

 

[Gary RV_Wizard]

OP, Put more weight in the back. Unlike lesser vehicles the F250, E250, Transit 250 and heavier chassis are designed to run at maximum payload 100% of the time. This isn't like an F-150 where the max GVWR is going to make the headlights point at the sky.

I doubt that is practical on a Class C - most van chassis are already heavily loaded once the RV body is put on. Weighing the axles and verifying the loaded weight is a wise move, but I suspect it will show the rear axle is already near max weight.

 

[stillRV]

.... but I suspect it will show the rear axle is already near max weight.

Here are the numbers. I'm around 1000-1400 lbs underweight in the back, depending on who I am taking with me and what we are bringing.



Here is a weigh in with myself and one passenger, coming back from a camping trip.


And here is another one with myself and 3 passengers, in the middle of a camping trip.

 

[p38fln]

You did the right thing - there is no such thing as an "RV tire chart", just a chart for each tire size & model. Since nearly all tire makers belong to and adhere to Tire & Rubber Association standards, the chart for a given size and style of tire will be essentially identical.

Here's the official RV tire inflation chart from Goodyear, I knew I had seen it somewhere - as you say it's not a generic chart but specific to each size and style of tire but you can probably guesstimate a similar tire from another manufacturer if you can't find their chart.

This chart shows the available load carrying capacity at various PSI

https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

Note the maximum speed - big tires aren't rated to roll very fast. The largest tires are only rated at 75 MPH.

 

[Ray-IN]

stillrv; run the pressures listed on the federal tire placard on the vehicle door jamb, not tire sidewall listed pressure. The placard air pressures are to support the vehicles GVWR.

The function of a shock is to keep the wheel in contact with the road surface. Incorrect shocks can either fail to perform (be too soft) the task or be so stiff they transmit the movement to the frame instead of absorb the movement.
Company engineers work hard to design a shock for best performance for a particular vehicle, that's what I buy.

 

[PerroSucio]

You need to install a camera on the belly pointed at the rear suspension and see what’s actually happening when crashing over those bumps…..

 

[John From Detroit]

If the think is bottoming out there are two possible causes

ONE: TOO MUCH STUFF.. you are overweight. Hit the scales

TWO: Busted/wore out springs.

Of course driving 90 over speed bumps will do it but I suspect you are NOT doing that.

 

[stillRV]

If the think is bottoming out there are two possible causes

I don't think it's bottoming out. The Suspension isn't moving very much. And I am underweight. It feels like the suspension is too stiff. Perhaps the softer part of the springs are worn out, and I am sitting on the stronger/more compressed part of the leaf springs. But the RV sits level so I don't know if that's it either.

 

[p38fln]

I did some serious digging and finally found the chart that lists your tire size. Remember the E-450 is more commonly used as a truck than an RV, which is why its tire size wasn't on the RV chart.

Here's the TLDR version:
When loaded to the weights in your 3 passengers middle of the camping trip scale ticket
Front: 50 PSI Load Range C or higher
Rear: 65 PSI Load Range D or higher

This is only for the weights you posted in your fully loaded CAT scale ticket. If you hook up a trailer or haul extra people or stuff the PSI will need adjusted accordingly.

Your RV, when fully loaded, is well under the GVWR for an E-450. You are absolutely not overloaded

Also, if you put your tires to the max pressure for the max load, you need to have Load Range D on the front (minimum) and Load Range E on the rear. Your maximum trailer weight is 7,920 pounds with load range D and E tires inflated to the tire pressures listed on the door placard.

Here's how I got the numbers:

https://www.goodyeartrucktires.com/pdf/resources/publications/2010_loadinflation.pdf

Here's how you use the chart. First take your fully loaded CAT scale weight of 3600 front and 8480 rear.
Divide the front by 2 to get the individual weight per tire
1800 pounds
Find 1800 pounds in your tire size on the chart I just posted - it's on page 44
The front tires are single (S) so use row 2
Scroll to the right until you see a weight at or above 1800 pounds
That's the 4th column over (C)
This means that your steer axle should be inflated to a minumum of 50 PSI and you need to be running Load Range C or higher tires on that axle.

For the rear axle you divid by FOUR - you have dual rear wheels since you have a 450, SRW isn't even an option at 450 and higher.
8480 / 4 = 2120 pounds

Using the same chart on page 44, but row ONE (D) instead of row two, find 2120 or higher. That's column 7 (D). This means the drive axle should be inflated to a minimum of 65 PSI and you need to be running load range D or higher on this axle.

If you want to prove I did the math right, you can find the maximum weight listed on the GVWR sticker on the chart as well, and you'll see Goodyears recommendation is the same as Ford's for the vehicle being fully loaded to max GVWR.


Maximum trailer weight is calculated on this vehicle rather than being a solid number, see page 21 of the 2006 Ford towing guide available here https://letstowthat.com/wp-content/uploads/2020/09/2006_ford_towing_guide.pdf

Weight is simply 20,000 pounds - actual GVW, in this case 12,080 pounds, which gives us 7,920 pounds of remaining GCWR.

 

[p38fln]

I don't think it's bottoming out. The Suspension isn't moving very much. And I am underweight. It feels like the suspension is too stiff. Perhaps the softer part of the springs are worn out, and I am sitting on the stronger/more compressed part of the leaf springs. But the RV sits level so I don't know if that's it either.

I don't think the E-450 has multi stage leaf springs. You can check yourself, if you don't see an oddball spring that clearly would engage before the rest it's just a single stage leaf spring design. Most heavy trucks just have 3 leaves on each side in the rear and no softer empty spring on top. I used to drive a semi with leaf springs on the trailer with that same setup...there's a reason we called running empty "bouncing". Old timers told me that the tractors used to be the same way and if you hit a bump with one running empty/light or bobtail you'd bang your head on the ceiling and pray it was still pointed forward when you landed. Thank God air ride tractors were extremely common before I started driving them.

 

[Gary RV_Wizard]

Here's the official RV tire inflation chart from Goodyear, I knew I had seen it somewhere - as you say it's not a generic chart but specific to each size and style of tire but you can probably guesstimate a similar tire from another manufacturer if you can't find their chart.

That's an "RV chart" in the sense that it has tables for the tire models that GY identifies with the RV-use application. The inflation values, though, are the same if you put those Endurance LTs on a utility trailer or car hauler. Or those G614's on a local delivery truck. Tire load capacity vs psi is the same whether the tire is on a truck or a motorhome.

 

[CharlesinGA]

Whether or not you think its not bottoming, you need to crawl under the RV and look at the bump stops, and at the axles where they contact. It will clearly show if it has been hitting, as the axle and the rubber stop will be clean. I'm betting it is hitting. I'm also betting your shocks are worn out. On a heavy vehicle like this, they don't last forever like a car. A set of new heavy duty shocks will stop bottoming out and improve the ride and side to side sway immensely.

Not sure if it was ever said what year this RV is or the mileage on it. The rear shocks on my 2007 Winnebago View on a 2006 Sprinter chassis were totally worn out at 20K miles, I mean totally. I replaced them with Koni adjustable shocks set to full stiff and also replaced the air bags on the Winnebago installed 3Tsrv add on air spring kit. This also improved the ride and sway issues immensely. The chassis already had a Roadmaster anti-sway bar installed on the rear.

Charles

 

[p38fln]

That's an "RV chart" in the sense that it has tables for the tire models that GY identifies with the RV-use application. The inflation values, though, are the same if you put those Endurance LTs on a utility trailer or car hauler. Or those G614's on a local delivery truck. Tire load capacity vs psi is the same whether the tire is on a truck or a motorhome.

Haha if you see the second post I made two posts above yours I actually found the tire chart with OPs actual tire size and calculated the correct pressure for the weight from the CAT scale

 

[Gary RV_Wizard]

Haha if you see the second post I made two posts above yours I actually found the tire chart with OPs actual tire size and calculated the correct pressure for the weight from the CAT scale

I saw that - well done. I was addressing the point about there being an "RV tire chart", which implies it is somehow different than some other chart.

 

[stripit]

Met a couple who were getting bounced around terribly with their Class C and said it gave them a headache driving. So we tried to figure out the casue, looked at everything. Their rig had factory intalled air shocks. In checking the pressure in those shocks, found them full of water, not air. Someone added air at some point and the compressor had a LOT of water that got transfered into the air lines of the air shocks. Cleaned them out and added air, made a huge difference and the rough ride was gone. I don't think you have air shocks, as you have said you replaced the shocks, but as others have stated, something isn't correct for such a harsh ride.

 

[stillRV]

Whether or not you think its not bottoming, you need to crawl under the RV and look at the bump stops, and at the axles where they contact. It will clearly show if it has been hitting, as the axle and the rubber stop will be clean. I'm betting it is hitting. I'm also betting your shocks are worn out. On a heavy vehicle like this, they don't last forever like a car. A set of new heavy duty shocks will stop bottoming out and improve the ride and side to side sway immensely.

Not sure if it was ever said what year this RV is or the mileage on it. The rear shocks on my 2007 Winnebago View on a 2006 Sprinter chassis were totally worn out at 20K miles, I mean totally. I replaced them with Koni adjustable shocks set to full stiff and also replaced the air bags on the Winnebago installed 3Tsrv add on air spring kit. This also improved the ride and sway issues immensely. The chassis already had a Roadmaster anti-sway bar installed on the rear.

Charles

I think this is the bump stop you are talking about? It doesn't look like it's been hitting. I can still see the lettering on it. I think the problem is the opposite of bottoming out - the suspension is just too stiff for the load. It's not "giving" much when rolling over joints in the concrete, and instead transferring all that energy to the interior. Basically the suspension is not doing it's job of absorbing changes in the road surface.

I have replaced the shocks. It definitely helped in the way you would expect new shocks to help, but it did not help with the rear crash over bumps. I've also adjusted the rear tire pressure down to 70psi.

I count 10 or 11 leaves in the leaf spring, could I have a leaf or two removed, would that help? I think that might affect the ride height? Maybe I should have the whole leaf spring replaced?

I've read something about taking the leaf springs apart and cleaning them and using plastic inserts, so the leaves can move, that is supposed to help them "work" better?

Thanks for your help.

 

[John From Detroit]

If the suspension is bottoming out on every bump then one of three conditions exists.
1: Serious overload.. (Too much "Stuff" in the RV.
2: Broken spring (Visual inspection can usually find those)
3: Wore out springs.
Oh there is one more possible answer but I assume you Not Driving like a Race Driver (Super fast).

 

[stillRV]

If the suspension is bottoming out on every bump then one of three conditions exists.
1: Serious overload.. (Too much "Stuff" in the RV.
2: Broken spring (Visual inspection can usually find those)
3: Wore out springs.
Oh there is one more possible answer but I assume you Not Driving like a Race Driver (Super fast).

If you look at my post #25, I am almost 1500lbs underweight in the back. I don't think it's bottoming out. The bump stops look clean and you can still read the part numbers on them.

I am wondering if these Sulastic Springs might help?

Sulastic Rubber Springs

Get a smoother ride with Sulastic Rubber Springs Shackles

www.sulastic.com