Tire Pressure Question

[jon48]

Yet another tire pressure question...

I have 6 Michelin 275/80R22.5 on my rig (less than 6000 miles).  Based on the scale weight of my last trip Front Axle weighed 11k and my rear weighed 19k.  Now that I am home and the rig is unloaded I presume the weight on the axles is less.

I sat down and puzzled over the recommended tire pressures using the above scale weights and the chart in the Michelin book sez 95# is about right for my weight at that time.  So I dropped my pressures from tire max pressure of 120# to 95#.

The next day I noticed that the sidewalls of the tires directly above the contact patch are bulging out a bit on all tires.  In my mind this tells me underinflation.

So is it really underinflation or just a misunderstanding on my part?  My inclination is to increase the pressure to reduce that bulge.  When I was running at max pressure I did not notice (nor did I specifically look for it) that bulge in the sidewalls.

Just wondering what the consensus is regarding the 'bulge'.

 

[manholt]

Jon48.

How old are your tires (DOT #) and why are they at max inflation?  When your coach is packed for a trip, that's when it should be weight and tires filled with the PSI that is recommended on the chart.  Not empty!  No need to dump air when it goes into storage.

[rayin]

Radial tires all have a sidewall bulge, whether at proper pressure or under-inflated, unlike bias-ply tires. With your axle weights you should assume an axle weight imbalance of 60/40 side to side, and use the Michelin pressure chart for 60% of axle load in both tires. Now, Tireman9 on his website, who is the FMCA tire expert and a retired tire design engineer, recommends 10% above that Michelin chart load/inflation specification so the owner is not frequently chasing air pressure in tires on cold mornings.

Personally I keep it simple and go by the Federal Tire Placard in the MH, and am never concerned about low tire pressures. I have air springs and shock absorbers to handle road unevenness. When the road is to rough for comfort I slow down until it is comfortable riding in the MH..

[jon48]

The coach is one year old but I'll check the DOT date.  The tires were at max inflation when I purchased and I figured no harm in stating out that way.  The trip I took in the fall was a rough ride (I come from 5th wheel experience - not prepared for the different experience of traveling in the environment of a coach) and the Interstate certainly was not in the best condition but I figured part of the rough ride was tires may be overinflated for my load.  Now that I have some time to check some things I looked at my scale weight for last trip and compared it to the Michelin pressure/weight chart.  Even though the coach is now unloaded of stuff I figured no harm in changing the inflation to what the chart indicated for the last trip I took thinking it will be representative of future trips.  But then I noticed the sidewall bulge and decided to ask about it.  And next trip I will be better prepared.

Thanks for mentioning 60/40 axle imbalance.  Even though I've try to load even, putting heaviest items in the middle of storage compartments, I've wondered about side-to-side distribution and plan to do the 'one side' scale weight procedure to determine distribution.

[kaypsmith]

If you can get 4 corner weights is even better for getting the pressures correct. The 60/40 is recommended in the event that four corner weight can not be obtained. The tire bulge is going to be there with a radial tire, I wish that I could get my wife to understand that, but that is another story. Welcome to the world of motorhomes, it's a real experience after 5ers or other types of TT's. We love the travel convenience of our motorhome, DW usually cooks while traveling and that's hard to do in a trailer. Please note that I am not downgrading TT's, most of us have been there at some point, just pointing why we love the MH.

[wildebill308]

jon48, welcome to the forum. Yes it is better to get 4 corner weights, but just getting axle weights and using the 60/40 formula will help. I like to run a couple pounds (say 5lbs) over what the table says just for a small cushion. That automatically compensates for temperature change and elevation.

Keep in mind some tires/brands sidewalls flex more than others. This picture shows my gap and bulge with Continentals on my Coach.

Bill 

[manholt]

Bill, that's your old coach tires...you really need to mow better for your new to you coach.

[wildebill308]

I think we were on a trip someplace when I took that picture. But you are right, I have some plywood I park on when home. You know my coaches to well. I will have to take some pictures of the MA so we have a comparison.

Bill

[jon48]

1 hour ago, WILDEBILL308 said:

jon48, welcome to the forum. Yes it is better to get 4 corner weights, but just getting axle weights and using the 60/40 formula will help. I like to run a couple pounds (say 5lbs) over what the table says just for a small cushion. That automatically compensates for temperature change and elevation.

Keep in mind some tires/brands sidewalls flex more than others. This picture shows my gap and bulge with Continentals on my Coach.

Bill 

Thanks for posting this.  It is quite similar to what I see here.  I just boosted the pressure up to 105# and cannot observe a difference in the way things 'look'.

I will accept that the bulge is normal.  I just wanted to make sure.

[tireman9]

I suggest you use your measured loads to learn the minimum cold inflation as published in the tables.. This assumes you will be carrying similar amount of stuff.

Your morning "cold" inflation should be good at the "Minimum" + 10% just to avoid having to chase your inflation as Ambient temperature changes.