Tire Pressures

Topic: Tire Pressures (Read 1170 times) previous topic - next topic - Topic derived from New Realm Review

Tire Pressures

September 29, 2015, 08:58:54 am

At Mothership yesterday for service and a new Realm was in stall by David's corner. They were doing something to the slide. Looks to be all Foretravel except for the chasis. Modern front look. Big coach. Where are the smaller coaches with smaller tires? Does not appear to be making any except by special order. Full line of IH45 out front. Lots of used coaches. Saw Elliot Bray as he was here for service also. Met with Wayne and had my front end cked. All OK but he insisted I had too much air so lowered to 85 steer. He says to ck hot. Going to try this as I have always had 100 lbs and when hot would go to 106 to 110. He wants it hot at 85. I know my front weight so in line with his chart. Leaving today to go to Roundtop, Texas for the big antique sale. Bigger than Canton 5 miles long.
Then on to San Antonio and back to Conroe for the 6 state FMCA Rally.

Re: Tire Pressures

Reply #1 – September 29, 2015, 09:46:18 am

Dan,

Sorry, but "checking tire pressure hot" is against every single tire manufacturer's recommendations.

And, for good reason: If underinflated, that generates more heat, which raises PSI (PV-nRT). So, the lower the pressure, the more the PSI gain from driving.

Actually, you should not follow our "feelings" or Wayne's. Go by what the engineers for your tire manufacturer recommend for your actual weight (that defines the minimum for that weight). Add 5 PSI as a safety factor based on heavier wheel position on each axle. If all you have is axle weights, add 10 PSI to account for safety factor and left/right imbalance (as long as it does not exceed the PSI molded into the sidewall.

Re: Tire Pressures

Reply #2 – September 29, 2015, 09:56:25 am

That advice from Wayne might be hazardous to your health. At 85 lb in the steer tires I would be 16 lbs under minimum psi for their load by the tire manufacturers load/inflation chart. My TPMS would be alarming at 85psi. I run 106 psi.

Re: Tire Pressures

Reply #3 – September 29, 2015, 10:01:32 am

I think Wayne is wrong here as well. I used to run 100 in my 270 and now I run 115 in my 42 ft which is 5 pounds over the amount by individual weighed positions fully loaded. If I take the handicapped scooters out! I can drop down five pounds. Don't bother to do that though as it drives just fine at 115. I am heavier then you are though. Also, all tires are checked cold too and not hot.

Re: Tire Pressures

Reply #4 – September 29, 2015, 10:05:31 am

Roger/Brett: Tks for the advice and that is also what I have always done and been told and read. Steer Tires this morning cold cked at 85 lbs. and 87 lbs. So will be at 95 once on the road. Will ck today and may stop and put in up to 95 cold. Tks DAN

Re: Tire Pressures

Reply #5 – September 29, 2015, 10:43:49 am

Another tire pressure discussion. I've been reading these for three years now. Here's my take FWIW...

When I bought my 40' GV320, 1 slide, tag, I read through the owners manual. Handwritten was a note about tire pressures. It said "per Foretravel 90 front, 80 drive , 60 tag". Since I had it serviced at MOT before I drove it home to California, I checked the receipt to see if tire pressures were noted. It said they set them to 85-75-75. I decided on 90-90-80.

When I bought new tires last year, I discussed tire pressure with the manager of the tire shop. He closely inspected my tires and said they were wearing perfectly, so don't change what you are doing.

I've checked the initial 90 lbs before leaving on a trip and then at different elevations and temperatures during a long trip. Living near the coast in California, traveling through the scorching deserts, through high elevations (all in the first few days of the trip) I have seen the tire pressure vary in a range of 35 lbs, from 80 to 115.

My points are...

1. There is no magic number that is the exact, and only safe pressure. Tires are made to operate in a wide range of pressures.

2. Manufacturers recommended pressures are some calculated result by someone who has probably never seen a tire that has been on a motorhome or truck.

3. I'll go by real world experience. The manager at the tire shop, the Foretravel and MOT people see the real world of tires every day.

Do whatever you want but its probably pretty hard to make a mistake unless you decide to be way out of that range.

Re: Tire Pressures

Reply #6 – September 29, 2015, 10:55:23 am

Quote from: Ed – September 29, 2015, 10:43:49 am

2. Manufacturers recommended pressures are some calculated result by someone who has probably never seen a tire that has been on a motorhome or truck.

Ed,

Sorry, have to disagree.

One of the guys on my FMCA Committee (I am Chairman of the FMCA Technical Advisory Committee) is a retired tire engineer.

Believe me (actually believe him) they DO know about trucks and RV's as well as their tires. Anyone who gives you advice on PSI without asking what the wheel positions weigh is NOT GIVING YOU GOOD ADVICE.

Re: Tire Pressures

Reply #7 – September 29, 2015, 11:09:04 am

Respectfully, Brett, I don't think what you are referring to is the real situation here.

Foretravel built the coach. Foretravel decided on what size tires to put on the coach. I have to believe that they had a pretty good idea what it weighs and what tire capacities are.

MOT services the coaches. Most people there are former Foretravel employees. I think they don't need to ask me what the coach weighs.

The tire shop manager sees the results of proper and improper inflations every day. I don't think he needed to ask what it weighs.

I don't think any of this was a "shot in the dark".

Re: Tire Pressures

Reply #8 – September 29, 2015, 11:32:12 am

Steering off topic here, sorry,

Ed what does your FT mfg plate say about pressures.

Mine says COLD steer @ 115 psi and COLD drive @ 94 psi. I am sure that is at GAWR with the tires it came with. i would have to drop 1800 lbs off the front axle to run at 85. Empty and dry wouldn't do that.

Critical safety issue here.

Re: Tire Pressures

Reply #9 – September 29, 2015, 12:20:32 pm

Quote from: Roger & Susan in Home2 – September 29, 2015, 11:32:12 am

Steering off topic here, sorry,

Ed what does your FT mfg plate say about pressures.

Mine says COLD steer @ 115 psi and COLD drive @ 94 psi. I am sure that is at GAWR with the tires it came with. i would have to drop 1800 lbs off the front axle to run at 85. Empty and dry wouldn't do that.

Critical safety issue here.

Roger, it says 115-94-70. But it also says "Tire PSI is at Maximum GAVR. See tire chart for less axle load".

The plates show tire pressures at Max GAVR. You have a 36' with no tag. I have a 40' with tag. Foretravel apparently installs the same drive axle on both coaches, thus both of our plates say 94 for drive PSI. But FT recommends 80 for my drive and MOT says 75. The plate doesn't differentiate between length of coach or tag vs no tag. So we are back to my point that real world is more important than specs, to me anyway.

My coach, through various owners, has been following real world guidelines. There have been no problems through all extreme driving conditions. Why would I now start inflating my fronts to 115?

My initial point is that tires are designed to operate at a wide range of pressures and temperatures. There is no IMHO an exact number that we should all be striving for. Don't be way low or way high and you will be OK. 5 lbs on either side of the middle of that range isn't a life and death decision.

But, as always, do what you are comfortable with. (By "you', I mean everyone)

Re: Tire Pressures

Reply #10 – September 29, 2015, 12:53:55 pm

Quote from: wolfe10 – September 29, 2015, 10:55:23 am

Believe me (actually believe him) they DO know about trucks and RV's as well as their tires. Anyone who gives you advice on PSI without asking what the wheel positions weigh is NOT GIVING YOU GOOD ADVICE.

Agree 100% with Brett.

And to add some to the discussion, our owner's manual, which FT created, has a separate section on tire pressure, including load and inflation tables specific to the tires that came on the coach, which was pretty much taken directly from Michelin's site, albeit for total axle weights rather than axle ends. And it includes this statement:

Quote

Coaches should be weighed with a full traveling load. Front and rear axles should be weighed individually and tires inflated cold per appropriate chart.

My takeaway from this is that at least at one time, someone at FT understood that one should use actual as-travel-loaded weight and the tire manufacturer's load and inflation tables for that model tire.

Recalling discussions of actual coach weights, the bulk of the real-world experience here on the forum has shown that typical front axle weights, particularly on 40 ft coaches with slides, have led to LRH tires and inflation pressures somewhere in the 100-110 psi range to have the appropriate safety margin.

ETA - oh, and probably would be a good idea to split the tire pressure posts out to their own, appropriately-titled, topic instead of having them buried/hijacking in the Realm discussion.

Re: Tire Pressures

Reply #11 – September 29, 2015, 01:14:40 pm

This topic started out as "New Realm Review" by Nitehawk. I did not see Tyre pressures mentioned in it? Maybe I am missing something??
JohnH

Re: Tire Pressures

Reply #12 – September 29, 2015, 02:13:52 pm

A thread goes off topic. Now that's never happened before!

Hello John. Hope all is well on your trip now. The upper Great Lakes region is my old stomping grounds. Beautiful.

See you in Quartzite again?

Re: Tire Pressures

Reply #13 – September 29, 2015, 02:57:57 pm

Ed, with respect for your opinions and experience ...

Front and drive axles have the same GAWR on your coach as mine. Your tag is rated at 9,000 lbs.

GVWR for mine is 34.880 lbs. GVWR for yours is 42,000 lbs. Tags were added to keep max axle wt at 20,000 lb.

I have had our coach weighed twice, fully loaded, full fuel, empty waste, two passengers. The PO had it weighed once.
I set my tire pressures based on max side to side weight on each axle + 5 psi as recommended by the tire manufacturer for that load. All three weighs were within 800 total pounds.

When we have left MN in the winter at -30° I have aired up to the psi I want. Two days later and almost 100° warmer I am letting air out every morning to get back to where I want.

So as many have suggested, weight each corner when you are appropriately loaded, set the tire pressure according to the tire manufacturers recommended pressure for the maximum side to side load on each axle, add 5 psi for load variations.

Most blowouts occur from sidewall failure, almost always caused by underinflation. Lots of damage, lots of risk to you and others.

You may chose to do differently. I hope it works out well for you.

Re: Tire Pressures

Reply #14 – September 29, 2015, 03:25:31 pm

Thanks, Roger. I'm sure we are both safety conscious guys.

Probably 90% of the times that I stop, whether for fuel or a break, the first thing I do is walk around the coach with an IR thermometer. I shoot the temps on all 8 tires, spindles, and calipers. I even do the same on the toad. Always looking for any irregularities.

My front tires (at 90 PSI) are running at about the same temps as the drive tires which are also at 90 PSI. Tags are about the same temps also.

That's one of the reasons I stay with 90 in front instead of the 115 the plate says. That, coupled with the tire manager, after using a straightedge and depth gauge on my old tires, saying don't change a thing.

Re: Tire Pressures

Reply #15 – October 05, 2015, 11:56:59 am

Now that we are on the road again, I have had opportunities to do some follow up tests.

Traveling up Interstate 5 through central California, we stopped in a rest area. I immediately checked tire temps. All were at 107 degrees plus/minus 1 degree. Air temp was 88 and asphalt temp was 111.

A few days later, I stopped at a scale after filling up with fuel. Front weight is 11,220. That's 5610 per side.

On Michelin's chart, that falls between 85 and 90 PSI.

This thread evolved because Dan mentioned that Wayne at Foretravel suggested running 85 PSI in the steers (the suggestion to check when hot is questionable). I had posted that I had taken FT's suggestion of 80 PSI and MOT's inflation of 85 PSI into consideration and settled on 90 as my steer inflation and some said that could be dangerously low. Seems like all is well. Running 105 to 115 in the steers, as several have recommended, would result in an unnecessarily harsher ride.

My experience and thoughts. YMMV

While there may be disagreement on the forum concerning technical issues, different opinions are thought provoking and that is always a good thing.

Re: Tire Pressures

Reply #16 – October 05, 2015, 01:32:40 pm

Based on what you acrually weighed the front axle and the loacd charts for LRH tires just under 90 looks correct. Assume you have a couple hundred lbs difference one side to the other and that would put you over 90. The difference between 90 and 95 psi is only 260 lbs. That is why folks suggest adding the 5 lbs. My actual weights of my LRG tires put me at 102 psi. I run at 106. If you get a chance up in Oregon at a weigh station drive through with both tires on the scale and another time with just the right on the scale. See how well balanced you are. The scales are on even if the station is closed. They read to the nearesr 50 lbs.

Thanks for the update. I am getting new LRH tires soon. I will adjust as needed.

Re: Tire Pressures

Reply #17 – October 05, 2015, 02:58:39 pm

Quote from: Ed – October 05, 2015, 11:56:59 am

A few days later, I stopped at a scale after filling up with fuel. Front weight is 11,220. That's 5610 per side.

On Michelin's chart, that falls between 85 and 90 PSI.

Running 105 to 115 in the steers, as several have recommended, would result in an unnecessarily harsher ride.

Thanks for coming back with your weights. This is an excellent example of why it's important to weigh your specific coach "as traveling" and why tire pressure recommendations need to take weight into consideration. Our 2003 40' coach front axle weight is 12,175, with the heavier end being 6200 lbs. That's nearly 1000 lbs heavier than yours (who knows the reason; could be the difference between a GV and a U). So we run 105 psi in our front tires (LRH XZA3's). On yours, that inflation pressure would be harsh, on ours, it's fine and it's needed for safety.

Re: Tire Pressures

Reply #18 – October 05, 2015, 06:23:44 pm

Great discussion
Real world.."your" corner by corner weights are a must. Location of the fuel tank front/rear, Tag/non-tag? 60-70% of that weight left the front tires with tank moved rearward.

David

Re: Tire Pressures

Reply #19 – October 05, 2015, 07:46:26 pm

Quote from: Michelle – October 05, 2015, 02:58:39 pm

Thanks for coming back with your weights. This is an excellent example of why it's important to weigh your specific coach "as traveling" and why tire pressure recommendations need to take weight into consideration. Our 2003 40' coach front axle weight is 12,175, with the heavier end being 6200 lbs. That's nearly 1000 lbs heavier than yours (who knows the reason; could be the difference between a GV and a U). So we run 105 psi in our front tires (LRH XZA3's). On yours, that inflation pressure would be harsh, on ours, it's fine and it's needed for safety.

Michelle, I think some of the weight difference is in the cockpit cabinetry (and the stuff in them) in a U320. I don't have those. Your front entry door and step mechanism probably adds weight up there also. That might explain some of the difference.

Re: Tire Pressures

Reply #20 – October 05, 2015, 09:15:48 pm

After looking at the weights of the coach axles I have settled on 95 for front steer and 90 each for the duals. Powers tire co said 90 on rears and 100 on front but 95 will work just fine according to the weight of my 295 on the front, about 11,250 #. I will set these when the tires are cold; therefore when running down the hwy they should increase about 8 degrees each. So the running psi for the front will be 103 and the rear 98. Dan

Re: Tire Pressures

Reply #21 – October 05, 2015, 09:54:57 pm

It is a good idea to weigh your coach, each corner plus tag sides, every couple years. Our coach weighs more now than when we bought it but subsequent weighs are close but somehow there is an upward creep. More stuff I guess.

Re: Tire Pressures

Reply #22 – October 06, 2015, 10:49:10 am

Ok so no u 280 tire pressures..

COLD TIRES
75° OUTSIDE
EXISTING.. full load ( too much)
75% water, full diesel, full propane, empty gret-black
Front 90
Rear 95

75% water, full diesel, full propane, empty gret-black
Weighed.. full load ( too much)
Front 95
Rear 95