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Old 09-27-2020, 07:06 AM   #1
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Wider Tires at lower pressure ?

Hello fellow RV'ers,

Yet another post on RV tires, looked up the threads but could not find a lead to my questions. So here it goes;

- My rig: 2012 HR Vacationneer 30ft; Ford F53 Chassis; 18,000 Lbs GVWR; 19.5" Steel rims
- Suspensions upgrades: Rear track Bar, Roadmaster Sway bar front and a secondary on rear axle, New Bilstein shocks, Sumo Spring front, Roadmaster Steering stabilizer
- Actual Tires (2 year old) are six Continental HSR 225/70R19.5, 90psi front and 100psi rear. Load on axles are 6300Lbs front and 12000 Lbs rear but would usually be a bit less as this was weighted with all tanks full (fresh, waste, fuel) and ready to go for a long outing.

With all the suspension upgrades, the RV goes straight and handles well. The last issue I want to resolve is the stiffness of the ride, without going to a DP and air ride. I know this frame F53 has it's limits and it will never handle like a car.

Questions:
- Would a wider tire at lower pressure provide a smoother ride. I'm thinking of going with 245/70R19.5 at 75-80Lbs Front and 90Lbs Rear ?
- Would Michelin"s XZE be smoother, any other brand/model ?
- Going larger would require new rims, does aluminum rims help that much ?

Thank's

Paul
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Old 09-27-2020, 07:15 AM   #2
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You can't (shouldn't) guess what the tire pressure should be. Check the load tables for the tires you have to make sure you're inflated properly for the weight. Then check the load tables for the tires you're thinking about buying to see what inflations is recommended for load. From there you can try to determine if you think this will improve the ride enough to justify the cost.
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Old 09-27-2020, 07:36 AM   #3
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At least in theory the tire with the lower psi ought to allow a bit more sidewall flex and maybe a slightly softer ride, at least under some conditions. In practice, I doubt if you could tell much difference in ride.


A certain amount of total pressure is needed to support the weight at each corner and that is calculated as the psi x the size of the contact patch (the flattened area were the tire actually touches the road surface). The product of those two numbers ha to equal or exceed the weight on that corner. The minimum total pressure is therefore the same for both tires, but the wider tire has a bit more in contact and thus can use slightly lower psi to achieve the same lift (weight support).

Looking at the Continental inflation tables, it appears that the 245/70 can run at about 15 psi less than a 225/70 for any given weight load. That might be enough to make a difference.


Remember that you can't just change tire sizes without some research. The rolling diameter will differ and that changes the revs/mile, affecting the speedometer and odometer. You also need to determine if the wider tire still fits in the front wheel wells in a sharp turn, and that the rear duals have enough spacing between them for the wider tire.
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Old 09-27-2020, 08:06 AM   #4
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Koni FSD shocks will improve the ride and complement your chassis modifications. Proper tire pressure to load weight will help.
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Old 09-27-2020, 08:16 AM   #5
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My 30ft Newmar has the 245 size tires with a G rating, Goodyear. Placard calls for 92/92. Goodyear chart for my weights are for 85 front and 80 rear. Ride is fairly good at those pressures.
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Old 09-27-2020, 08:20 AM   #6
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If your talking about rears, then you need to be really careful. Duals need space between the tires to prevent rubbing. Wider, low pressure could cause the tires to rub, thus creating heat, weat, and possible vlow outs.
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Old 09-27-2020, 10:41 AM   #7
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last issue I want to resolve is the stiffness of the ride

This issue in your case is controlled by the spring rate and/or shocks. An air ride suspension only provides enough pressure to support the weight at a certain height. Your suspension has steel springs that will support the axle capacity (lets say 10,000 lbs.), even though you may only carry 8,000 lbs. of weight on the axle. You're going to get the stiffness of the 10,000 lb. capacity spring. You could add 2,000 lbs. to the front end, or change to an 8,000 lb. spring, and/or a lower spring rate, so the load and spring are more matched, but both options are not real practical.

The other option is a more compliant shock that many here on the form have written about, search and you'll find lots of opinions to choose from.

You're asking about changing tires. On my air ride, with the air out, I can't get my fingers between the fender and the tire on the rear. Some coaches can't have front tires turned any when the air is let out without risking fender damage. You really need to closely check for adequate clearance, fender and suspension, with the suspension compressed before adding larger tires.

One thing I think most people don't know, or understand, is that a wider, lower profile tire will provide a harsher ride than a higher narrower, high profile tire does.

I consider being able to feel every little crack and irregularity in a level
pavement surface, coming up through the tires and into the chassis as "harshness of ride".

Taller tires with high sidewalls absorb a lot of this tread movement over the irregularity, and will reduce the amount of vibration transferred up into the chassis and coach.

The aspect ratio in both tires you are asking about is "70", so I doubt there will be any significant difference in ride harshness, even though a different pressure is needed to support the weight.

If you look closely at some really big coaches, not my Freightliner XC chassis type, but some more along the line of the Prevost chassis, you'll notice the tires on these coaches are very tall in the sidewall area providing a much more compliant ride.

In your case, I think a change in shocks would the best place to spend money, if you haven't already done this.
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Old 09-27-2020, 11:00 AM   #8
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"Pol6".....There is not much more you can do to soften the ride, other than to get the coach weight and adjust the tires.

My brother-in-law bought a 2017 Fleetwood Southwind on a Ford chassis after selling a DP he owned for 13 years. He immediately added ALL of the suspension components you added, including the double sway bars. His list of add ons matched yours to a "T".

He wanted the new coach to handle like his old DP and was looking for more stability. Well.....he got the stability, but with all those additional/upgraded components you stiffen the suspension to the point that it corners like a skateboard, but rides like a tank.

When people on here ask what they should add to their suspension on new or used coaches, I always tell them to add one thing at a time and drive the coach. This way, you find out if that one item resolved your issue before you add more and more suspension components.

I'm a nut about adding anything I think will help the ride and probably would have done the same thing to a gas coach as you did. Unfortunately, we all learn the hard way.

So...... I would leave the suspension alone, enjoy the new found stability, and adjust the tires to as low as you can safely go.
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Old 09-27-2020, 04:38 PM   #9
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Quote:
Originally Posted by Pol6 View Post
Hello fellow RV'ers,


Questions:
- Would a wider tire at lower pressure provide a smoother ride. I'm thinking of going with 245/70R19.5 at 75-80Lbs Front and 90Lbs Rear ?
- Would Michelin"s XZE be smoother, any other brand/model ?
- Going larger would require new rims, does aluminum rims help that much ?

Thank's

Paul
Yep that is what I run at, 78 psi front and 85 psi back on 245/70/19.5 tires.

However pressure is one thing and tire construction is another, I went from Hankook's AH11 14 ply to Firestone FS561 14 ply and the Firestones run softer, the Hankook's were 2 ply sidewall and the Firestone is 1 ply sidewall.
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Old 09-27-2020, 05:03 PM   #10
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We’re just finishing a 2000 miles trip in our 2017 F53 Adventurer Class A. It’s a 24,000 pound chassis and we travel at around 22,600 lbs.

I was thinking today as I was driving on some medium rough roads - what was making us feel all the bumps and bangs?

I considered the suspension and wondered what part is transferring all of this harshness. I have a steering stabilizer, Koni FSD shocks, beefy rear anti-roll bar and Sumo Springs all around. My tire pressures have been set by weight, though I did bump them up by 10psi for cold morning convenience.

Obviously it’s the solid axles and the leaf springs because that’s pretty much all there is between the tires and the chassis. But what would soften that connection? Thicker neoprene bushing at the shackles?

I know Kelderman tried air bags at the rear shackle of each leaf spring but has abandoned that approach. Surely someone has really done some work on this problem. It’s basic chassis stuff that isn’t all that complicated... is it?
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Old 09-27-2020, 06:09 PM   #11
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Since you are satisfied with the handling and now you want to soften the ride, I would suggest you look into the Kelderman 2-stage air ride system. I got to the same place you did with my 2008 Baystar. About a year ago I had the Kelderman system installed front and rear. With only about 400 miles on the system, I can tell you it made a world of difference in the ride. The unexpected benefit was how much quieter it made the rig. Expensive but worth it in my opinion.
As far as I can tell, nothing has changed RE handling. It stayed the same.
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Old 09-27-2020, 08:36 PM   #12
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Since you have 225 tires now, that means you have 6" wide wheels. 245's aren't recommended/approved (you'd have to google the exact restriction) to be mounted on 6" wheels, you need a 6.75" rim for them.
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Old 09-27-2020, 08:59 PM   #13
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Lower air pressure will induce more tire roll on corners, and perhaps more sway.
You are driving a truck chassis nothing will change that.
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Old 09-27-2020, 10:44 PM   #14
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Quote:
Originally Posted by tderonne View Post
Since you have 225 tires now, that means you have 6" wide wheels. 245's aren't recommended/approved (you'd have to google the exact restriction) to be mounted on 6" wheels, you need a 6.75" rim for them.
Yes you are correct Pol6 cannot go to 245's cause he will need all new rims at 6.75".
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