OK, Question for all you Tire experts out there

atlasb

Recently I replaced the tires on our TV and went with B F Goodrich (USA made branch of Michelin)  I see often in tire discussions that a common problem with tire failures is under inflated tires.  I contacted Michelin customer support with the question,  My tire side wall specs a max cold pressure and the truck specs a different pressure.                                                                                          

I asked at what pressure i should inflate to.  The pressure of the tire or the truck?  Also, at what pressure should you consider your tires to be under inflated?  Our tires are rated for max capacity at max inflation.  So, if you run your tires at a lower pressure as stated on the truck are you not also lowering your capacity for the truck?

Michelin responded to run the tires at the value called for by the vehicle manufacture.  How ever, they did not address the question of low pressure being under inflated or at what point they consider a tire to be under inflated.  They said to contact your tire installer for the answer to under inflation.  He is the guy whose job it is to sell a tire not design for test  them so how valid would his answer be?

So, now the question also comes, We have an early (July 17 mfg date) 2018 400 that I have replaced the axle with the 3900lb newer axle.  The tires on the trailer are rated for lower rating so if we also reduce the air pressure from the max spec on the tire are we also not lowering the weight capacity of the trailer that we can use? 

Horigan

The recommended pressures for a vehicle or trailer assume the maximum load for the vehicle/trailer.  Tires used on vehicles or trailers typically have load margin above the requirements of the vehicle/trailer and these are shown on the tire sidewall, along with the associate max pressure.  So the sidewall pressures will typically be above, or equal to, the recommended pressures for the vehicle/trailer.

Putting a higher rated axle on your trailer doesn't increase the load capacity of the trailer since other structural aspects also determine max load capability.  Just run your tires at the pressure recommended for the trailer and don't exceed the weight limit of the trailer.

You're under inflated if you run pressures below the vehicle/trailer recommendation when at or near full load capacity.

atlasb

Horigan,  Thanks for the reply!

Just a follow up,  The early 400's came with 3500lb axles and later they started using either a 3900lb or 4000lb unit both class as 4000lb by Dexter.  Usually the 4000lb is for the BD model with extra pitch.  I am not aware of frame change for added weight capacity until they moved the axle forward to gain better tongue weigh ratio.  My 3900lb unit sitting side by side with the original 3500lb unit appears to have additional pitch which I understand came later.   Still not the same pitch as the BD models.

The Tires that came are the Castle rock ST205-75 R15 and have a capacity of 1820 lbs each for a total of 3640lbs. they are also L rated for a speed of 75mph, and are 6ply. They call for 50psi and that is also on the placard on the cab of the T@B.

If I am following this correctly, you would have to gauge your weight capacity for the trailer at the lesser of the 3900lb axle or the 3640lbs of the tire rating.  Does that sound about right?

Before I changed the axle, I scaled the T@B ready for a trip at a weight of 3240lbs with the wheels and tongue on the same scale.  I had a tongue weight including 2 bikes mounted on the tongue, of 525 lbs.  Without the bikes the tongue comes in at 464 lbs.  this weight did not include any water as we travel dry of water in most cases. I normally run about 48lbs cold for our trips

My Tow vehicle has TPMs that I can follow on my inst cluster and I follow the settings on a variety  of items.  I do see an increase of tire pressure of around 5 lbs or more as they warm up.  From some information I found,  The tires are constructed to account for a cushion in pressure rise as the temp rises.

Thanks again for your information.  Interesting that a tire mfg would not give out the information.  Almost like trying to find the proper lug nut tightening specs. They all want to pass it on to the axle mfg, or wheel mfg.

N7SHG_Ham

Typically the vehicle placard is same as tire sidewall when new. If you change the tires to a different rating, then the vehicle placard obviously will be different.

I believe my pickup tires are rated max load at 80 psi, I typically run about 65. Those are LT tires.

Same definitely does NOT apply to ST rated trailer tires, those are to be run at max cold psi 💯 of the time, it is the different design than LT or passenger car tires.

ScottG

DISCLAIMER: Not a tire expert.  :-)

My experience has been different--the vehicle placard usually prescribes a pressure lower (typically quite a bit lower) than the max pressure indicated on the tire sidewall. I've heard differing opinions on what is "best" practice. Obviously this becomes more ambiguous when you replace the tires with something other than OEM.

For no particularly compelling reason I usually run my tires anywhere from a few pounds over the vehicle placard specs, up to splitting the difference between the placard and the sidewall max. I figure--perhaps incorrectly--that the placard specs are a compromise between ride comfort and performance, and my preference tends to tip a little toward performance. 

I guess the bottom line is do whatever floats your boat, so long as your tires are somewhere between the placard and sidewall specs, and you are not seeing wear patterns consistent with under or over inflation.

I do agree that ST (trailer) tires are a different beast. There does seem to be more consistent agreement that those should be run at the max psi listed on the sidewall.

Grumpy_G

Trailer tires are typically chosen to carry the gross vehicle weight with some buffer to spare. As such the label calls for the maximum pressure the tires are rated for. On other vehicles (cars, light trucks) the combined weight carrying capacity of the tires often far exceeds the gross vehicle weight and running at maximum pressure leads to uneven tire wear and a rough ride. Hence the manufacturer in cooperation with the tire manufacturer recommends a lower pressure. Tire manufacturers have tables showing what pressure should be used for how much load for a specific tire. 

Horigan

Agreed, though I recall my original 2014 T@b recommended tire pressure below the max rating of the tire, presumably to smooth the ride.  

@atlasb
Load your setup to the lower of the two weight limits.

Denny16

@atlasb said:

“Horigan,  Thanks for the reply!

Just a follow up,  The early 400's came with 3500lb axles and later they started using either a 3900lb or 4000lb unit both class as 4000lb by Dexter.  Usually the 4000lb is for the BD model with extra pitch.  I am not aware of frame change for added weight capacity until they moved the axle forward to gain better tongue weigh ratio.  My 3900lb unit sitting side by side with the original 3500lb unit appears to have additional pitch which I understand came later.   Still not the same pitch as the BD models.

“The Tires that came are the Castle rock ST205-75 R15 and have a capacity of 1820 lbs each for a total of 3640lbs. they are also L rated for a speed of 75mph, and are 6ply. They call for 50psi and that is also on the placard on the cab of the T@B.

“If I am following this correctly, you would have to gauge your weight capacity for the trailer at the lesser of the 3900lb axle or the 3640lbs of the tire rating.  Does that sound about right?”

Yes, your max load would be the tire rating of 3640 lbs, not the max axle rating.