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Old 08-25-2020, 10:51 AM   #1
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Excessive tire pressure in my Class B tires

After getting my van weighed the other day with full fuel, water, and propane, and after reading hundreds of posts about tire pressures here, I decided to take a look at published pressures vs load for my Vanco LT215/85R16E tires. Much to my surprise, based on CAT scale axle loads, the 61 psi front and rear pressures from the door post are grossly high. Which I'm sure contributes to it riding like a truck (even though it is a truck).

So I plotted the table from Michelin, which is essentially the same as several others for the same tire size, and through regression, calculated corresponding pressures based on axle load inputs, just-because.

But a quick look at the scale axle loads shows what at first glance appears to be ridiculously low tire pressures from the tables, compared to what the door post sticker calls for. The van will scale higher with the two of us and our usual supplies, but still be a very long way off from the GAWR's of each axle since we travel light.

Maybe that's why the ride is so harsh, and why it wanders on flat highways without wind, requiring constant correction - like it can't figure out which tire to follow.

While the Sprinter 3500 has a high OCCC, the Ford Transit isn't far behind, based on its GVRW, and the fact that the front axle actually has an even higher GAWR. Most Sprinter tire pressure discussions I've read on this forum are in regards to small Class C's, with very little OCCC. But in the case of the vans like mine and the Fords (and maybe the ProMasters, though they have singles in the rear), it seems that door post tire pressures may be way too high for how they're outfitted and loaded.

I'll make some adjustments and give it a try this weekend to see how it rides, but I'll get it weighed fully loaded before our next trip this fall. I'm not expecting it to put on much weight.
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Old 08-25-2020, 11:18 AM   #2
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Just an FYI.....

NHTSA Recall ID Number : 20V479
Manufacturer : Forest River, Inc.
Subject : Awning May Deploy While Vehicle is Moving
Make Model Model Years
COACHMEN BEYOND 2020
COACHMEN CROSSFIT 2017-2019
COACHMEN GALLERIA 2016-2020
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Old 08-25-2020, 11:28 AM   #3
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Yep, 61 is definitely higher than necessary for the weight being carried and would account for the wandering and rough ride.
Personally I'd start at 50 front and 45 rear to see how that affects handling - which would still leave plenty of pressure for extra gear to be loaded.
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Old 08-25-2020, 11:50 AM   #4
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Quote:
Originally Posted by Winterbagoal View Post
Just an FYI.....

NHTSA Recall ID Number : 20V479
Manufacturer : Forest River, Inc.
Subject : Awning May Deploy While Vehicle is Moving
Make Model Model Years
COACHMEN BEYOND 2020
COACHMEN CROSSFIT 2017-2019
COACHMEN GALLERIA 2016-2020
Wow, I didn't know that. Now I have to find a dealer who will do whatever it is they need to do. It seems dealers who didn't sell it don't want to fix it.

Quote:
Originally Posted by Podivin View Post
Yep, 61 is definitely higher than necessary for the weight being carried and would account for the wandering and rough ride.
Personally I'd start at 50 front and 45 rear to see how that affects handling - which would still leave plenty of pressure for extra gear to be loaded.
Yeah, I'm going to dial them back and see how it rides and handles. But I will for sure take it back to the scale once loaded fully, with me and DW and the dog onboard. Probably on the way south in the autumn, since the scale is right next to the highway.
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Old 08-25-2020, 02:15 PM   #5
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With my made RV motorhome calcaculator, it comes to front 55psi and rear 48 psi
Then calculated with my own decided formula, that comes to higher pressures then the tiremakers use for their lists.
And I add first 10% to the determined axleloads, for things like unequal weight R/L, pressureloss in time, inacurate reading of pressure, etc.
Determining the weights in use, is a tricky thing in it all.
You did not have all the persons in it.
That Is why I also calculated for GAWR's I read from one of the pictures.

This gave F 4410lbs+5%/ 69 psi, R7720lbs+15%/ 72 psi. This is for if you did not weigh, and uses 15% rear to cover overloading, and only 5% front because never overloaded.

The 10% adding for given axel loads, gives if realy that weight, still no discomfort.
I concluded that to happen when real load on tire is lesser then 85% of the loadcapacity for the pressure.
Centrewear probably happens below 70%.

So in case 55/45 weightdivision RL on axle, on tire 100% used and other 81% so already a little discomfort, but the heavy side still no overheating expected at higher speed.
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Old 08-25-2020, 03:17 PM   #6
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Wow, I didn't know that. Now I have to find a dealer who will do whatever it is they need to do. It seems dealers who didn't sell it don't want to fix it.
I suspect your manufacturer will contact you with detailed instructions on how to get it resolved. I bought a travel trailer in Michigan in 2013, and not long after getting it home (Canada), it had a recall on the awning (motor). The awning company contacted me (or I called them?) and I had it done by a local repair guy, under warranty. No charge.
Agree about, no sell, no support. Even if you're paying fo the dervice, some balk at doing it. Maybe they sell junk, and can't afford to do other folks stuff, because their customers come first for repairs?
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Old 08-25-2020, 10:10 PM   #7
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Keep in mind, lower air pressure was the cause of the Ford Explorer roll-overs. Ford ignored the recommended air pressure from Firestone, and printed door stickers with lower air pressure to improve ride.
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Old 08-26-2020, 05:43 AM   #8
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Lower tire pressure, lower fuel economy. I run mine harder, and a few PSI up or down makes a difference.
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Old 08-26-2020, 07:28 AM   #9
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But 61 psi in the rears corresponds to a shade over 8000 lb rear axle load, with 5600 lb actual. If allowing +5% pressure, then 7630 lb rear axle load, which is close to the published 7720 GAWR.

Allowing +5% over actual corresponds to 40 psi, which is where I set it this morning. That's a long way from 61 psi. So is that front at 47 psi, which includes +5% and is also a long way from 61 psi.

Previous Sprinter vintages had 55 psi on the door post. I don't know why they changed it when the GVWR and GCWR hasn't changed, nor have the tires.

I'm not worried about fuel mileage, as this little diesel in a tall van sips fuel anyway. But handling is a concern, of course, and it handles poorly with those door post pressures. Harsh and twitchy and won't follow a straight line.

Next step after that is to get it aligned, loaded for travel with full tanks.
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Old 08-26-2020, 07:41 AM   #10
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I can't speak to the Sprinter but when we first started to run E range tires on our Jeeps we had the same ride and steering problems. Solution was to run a thick chalk like across the face of the tire and adjust the pressure until the wear was even after a short drive. Center wear, let air out: shoulder wear, put air in. I ended up with under 20 psi on one of my Jeeps and it worked great. I went even lower when off road, but I don't think that will apply to your Sprinter.
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Old 08-26-2020, 09:42 AM   #11
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I guess this is a highly subjective topic. My 10,500lb, tall, fuel sipping, loaded for travel, cab chassis handles fine, at all tire pressures, but I tend to keep them harder than door pillar numbers, as a preference for even more fuel sipping, with no noticeable change in ride or handling.
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Old 08-26-2020, 10:03 AM   #12
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Quote:
Originally Posted by Winterbagoal View Post
I guess this is a highly subjective topic. My 10,500lb, tall, fuel sipping, loaded for travel, cab chassis handles fine, at all tire pressures, but I tend to keep them harder than door pillar numbers, as a preference for even more fuel sipping, with no noticeable change in ride or handling.
It could also be that the alignment is off. I've heard (on the internet, of course) that these units are factory aligned as they're built, which means very light. Lots of discussion out there about realignment and replacement camber bolts, so that's next on my list of things to look into.

Gotta do something, though. That wander is annoying, and shouldn't happen on what's virtually a new vehicle.
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Old 08-26-2020, 02:05 PM   #13
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Quote:
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Keep in mind, lower air pressure was the cause of the Ford Explorer roll-overs. Ford ignored the recommended air pressure from Firestone, and printed door stickers with lower air pressure to improve ride.
To my conclusions, the 26 psi used for the Ford Explorer, would not have been to low, if they used a normal car tyre. The tires used where ofroad-looking tires with larger profile blocks, that covered a part of the sidewall, and so left lesser sidewall to flex.
Made a paintpicture to explain.only to show the idea, tires are of other brand.

Now this could also be the case here, that your tires have larger profile blocks, yudge that.
Then going from advice 55psi to 61 psi, was because they learned from the FF affaire, that these kind of tires should have lesser maxload, because of lesser deflection allowed, and so calculating pressure for the same GAWR, will give higher pressure.

Mind this is my conclusion, tiremakers will say I am wrong.
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Old 08-26-2020, 05:36 PM   #14
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I felt guilty enough that I haven’t had to add air to my tires in 3 years (except for the bad valve I had fixed), but now I have to worry because I follow the plate on the door. My 60# settings on my 3500 Sprinter drives great and delivers 18 mpg so I guess I’ll stick with it.
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