Tire inflation pressure?

[Russ]

A comment on another thread made me think, "What is the proper trailer tire inflation pressure?"
And is it the same for dual axle trailers as single?

[Be Free]

The maximum cold tire pressure is printed on side of the tire. Don't exceed it and don't get much below it regardless of the number of axles.

An underinflated tire gets hot and a hot tire is likely to fail or have a tread separation (radial). Both of them will wear prematurely if they are consistently under or over inflated.

[kmclemore]

For maximum protection, yes, always keep it near the max inflation on the side of the tire. However, for maximum wear, a better and safer trailer 'ride' and for what might be considered 'proper' inflation for your trailer, what I do is to chalk the tires like I did when I was racing sports cars.

The procedure is... first, warm up the tires with about a 5-mile run. Then stop and use a large piece of kid's sidewalk chalk to make a clear, broad mark across each tire's tread. Now, take the trailer on a short normal road drive... about a mile or so.. making sure to do at least one left and one right turn... then check the chalk mark to see where it's worn off. If it's only worn off the outer edges of the tread, you need more pressure... if it's only worn off the center but there's plenty on the edges, you need less pressure... and if it's evenly worn off the entire face of the tread you've got the right pressure. Once you get the proper pressure set, give it one more chalk test to ensure you're spot-on.

This will give the trailer tires plenty of pressure to prevent tire failure while also not being so high that it doesn't 'give' over bumps and such, and it also won't overheat. If you go with the maximum pressure on the tire you'll never really go 'wrong' but your trailer will ride hard like it's on rocks, it will be subject to 'bouncing' and thereby less control, you'll wear out the center of the tire's tread prematurely, and you'll have less traction on the trailer's tires in turns, wet weather, braking or in an emergency situation.

The same procedure applies to cars when you're changing tires and/or wheels from the factory-supplied size.

[Tomfoolery]

Truck and Light Truck tires have inflation tables for each size, showing load capacity for different cold inflation pressures in 5 psi increments usually. The table for the LT215/85R16 tires on my van are on the last page of this document.

https://commercial.bridgestone.com/cont ... 8-2018.pdf

The van was riding like it was on wooden wagon wheels when inflated to the door post pressure of 61 psi (cold) in all tires, which prompted me to weigh it. I’m a ton light on the rear axle, and half a ton light on the front, which explained why. I dropped the pressures to 110% of that shown in the table for the measured weight and it’s a huge improvement. 10% over allows for side-to-side imbalance, since I couldn’t do a 4-corner weigh at the local CAT scale.



Small trailers are a little different, and since I don’t have a travel trailer, but my 2-axle boat trailer has under loaded tires, I’ve never done any serious research on the subject of inflation pressure. I run mine about 5 psi below sidewall pressure, but have never checked the contact patch.

But there’s a retired tire engineer who posts on the iRV.com forum named Roger Marble (user name tireman9) who also has a blog. It’s a goldmine of information, for anyone who wants to dig. https://www.rvtiresafety.net/

There’s a section on inflation pressure, another on trailer tires, and loads of others too. Anyone who wants to get educated should spend a day reading his stuff.

Just some random stuff, which hopefully spurs others on to do the research I haven’t done.

[Be Free]

Tom,

That looks like a great resource for tire info and safety. Thanks!

BIll

[Tomfoolery]

The first page I looked at more carefully than just a glance revealed the difference in load capacity and speed ratings between passenger car (P), light truck (LT), and special trailer (ST) tires, all of the same size. No wonder trailer tires, especially those carrying rated load (like travel trailers, or single-axle MacGregor trailers ) don’t hold up very well.


"Here are some facts from the Tire & Rim Association industry standards book

P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)

LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph

ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph"

NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.”

https://www.rvtiresafety.net/search/label/Trailers

And to kmclemore’s point, he also recommends inflating ST tires to sidewall pressure especially when there are two or three axles, due the large side loading when turning. That’s in there somewhere, too.

[Russ]

Wow! I never stop learning from the folks on this site. Roger Marbles' site has so much information. I could spend a day on that site.

I've been running my tires WAY lower than sidewall pressure and will correct that.

I know my tongue weight is too light (or the aft is too heavy). Could low pressure cause towing issues? The rig fishtails at high speeds unless I put a lot of weight in the back of my truck (sand bags).

[Tomfoolery]

I know my tongue weight is too light (or the aft is too heavy). Could low pressure cause towing issues? The rig fishtails at high speeds unless I put a lot of weight in the back of my truck (sand bags).

I'm not a tire engineer, but I do know the sidewalls stiffen as the pressure goes up. Stiffer sidewalls is one of a few things that reduce the tendency to fishtail. That's why two axles are better than one - more tires, with equally stiff sidewalls, assuming they're inflated at or close to sidewall pressure, even if it makes a harsher ride.*

I move everything to the front of the boat when I tow rather than add weight to the SUV. It's got enough to haul without bringing ballast along. But I also carry twelve gallons of ethanol free gas in the regular tank locker at the stern, so there's that. I don't want to have to gas up when I get where I'm going, plus I never know if I'll find ethanol free gas there, either.

*I have equalized leaf springs on mine, and the equalization makes for a much lower effective spring rate when hitting bumps. And since both axles are so lightly loaded, I reduced the number of leaves in the spring packs, and shortened them as required to divide the distance equally for the same bending stress at each step. I neither need nor want 7000 lb of spring capacity under this boat.

Yeah, I just can't get away from it sometimes.

[Be Free]

Wow! I never stop learning from the folks on this site. Roger Marbles' site has so much information. I could spend a day on that site.

I've been running my tires WAY lower than sidewall pressure and will correct that.

I know my tongue weight is too light (or the aft is too heavy). Could low pressure cause towing issues? The rig fishtails at high speeds unless I put a lot of weight in the back of my truck (sand bags).

I try to keep the boat loaded so that it sits pretty much "on her lines" so I don't move any weight around for towing. The biggest cause of fishtailing for me is not having the boat all the way forward on the trailer. Even an inch makes a significant difference in how it feels above 50mph.

[Russ]

For towing, I move ANYTHING that I can far forward into the V-Berth. I'll empty the fuel tanks if possible to reduce aft weight. I wonder if tilting the motor ALL the way up is adding more weight further back. It doesn't need to be tilted all the way up anyway.
Under 65mph it tows just fine. But over that it starts to get fishy. The problem is my son insists on towing it with his Duramax and he has a lead foot.

I really don't care if the ride is rough on the trailer so I'll push up the pressure in the tires and see how that works. Also, I'll make sure the boat is as far forward on the trailer.

Thanks for all the great input!

[Be Free]

When you tip the engine up you are moving more weight forward (the power head) than backward (the foot). Regardless, all of the weight involved remains behind the axle so it's not going to affect the tongue weight. You are adding a very slight amount of torque behind the transom but I can't imagine that it is significant given the 2+ tons of weight in the rest of the system.

[Tomfoolery]

Tilting it full up does move its center of mass forward. And it unloads the hydraulic cylinder both because of the improved geometry, and because the CG is very close to being above the hinge line, and that means the resulting moment on the transom mount is reduced from where it is with the engine down.

Not that it matters though, as the BF50 at full throttle and the boat at full speed is putting something approaching 1000 lb of horizontal force on the prop shaft to drive the boat, and that's a LOT more moment on the transom than trailering with it tilted full up.

[Be Free]

Since he was asking about the possibility that raising the engine could affect fishtailing I was actually thinking of the slight change in the torque to the trailer tongue due to the "arm" being extended slightly behind the transom instead of being parallel to it.

If you want to see some forces working on the transom drive across a rough railroad crossing with the engine down. I had a friend do that. The engine caught on the crossing and ripped the entire transom out of a 24' boat

[Tomfoolery]

Since he was asking about the possibility that raising the engine could affect fishtailing I was actually thinking of the slight change in the torque to the trailer tongue due to the "arm" being extended slightly behind the transom instead of being parallel to it.

Ah. Reading comprehension.

Easy enough to calculate - moving 150 lb 6 inches forward, if that, is not going to make much of a difference in the grand scheme of things. I get a tongue weight increase of about 4-1/2 lb, with the boat's CG moving forward about 1/4 inch. Not enough to worry about, IMO, but there are other reasons for tilting it up. Like not ripping it off the boat on a RR crossing.

[Russ]

The engine caught on the crossing and ripped the entire transom out of a 24' boat