02/12: New Tire Discussion on Woodalls: LT versus ST and more.

[ROBERTSUNRUS]

Originally Posted by jdalrymple

Not so fast.

What is the:

Weight Capacity?

Speed Rating?

Acceptable Side Slip Angle?


Too many unanswered questions here.





Just making a point. I'll quit.

Hi, at least the tire pressure problem/debate would be over. And you can save by not having to buy TP monitors too. [not that I would ever buy one]

[slowmover]

Originally Posted by CrawfordGene

Rednax,

The question that never has been answered and I ask it every couple of months on one tire thread or another: why do auto and truck manufacturers always recommend less pressure in the OEM tires than is stated as maximum on the sidewall? Or, another way, why wouldn't they recommend maximum sidewall pressure if it is safer?

I am assuming (because I didn't read the linked thread) that recommending maximum psi stiffens the sidewalls and that is why we should run a maximum. Is that correct?


Gene

My understanding about tires has always been the load/pressure relationship. For a tire to have the longest life, greatest reliability and best performance in braking & handling, then a tire properly rated for the GVWR (and a bit more; why axle WR combined is higher than GVWR) must have an adequate reserve capacity, first. Second is that to find the best pressure was to weigh the vehicle and use axle averages to set pressure against the manufacturer Load & Pressure Table (not ever going over or under the Vehicle manufacturer recommendations) and adjusting for changes in climate & terrain as necessary; hot weather to cold weather.

The vehicle manufacturer doesn't want you to have an unnecessarily hard ride, nor to transmit extra shock to the vehicle. Shock absorbers have their [important] role, and chassis bushings theirs. It is all supposed to work together. We change the pressure mainly when we change the load, and that's the only "good" reason (is a way of seeing it).

Trailers, mainly at low speeds, experience sidewall deflection differently than cars or trucks. They can wind up with one or more tires really pushed hard into a position. Certain kinds of turns, and a heckuva lot of parking. Sometimes it is not obvious. On tandem and tridem axle trailers this is more of a concern.

Going down the road the TT is never master of its' fate, so to speak, as is a car or truck. It is always being pulled (it could be pushed, also) and the forces that allow a car, say, to "relax" don't work out for something at the end of a 20' pole pushing or pulling it (as it is almost not ever perfectly aligned with the force pushing on it). And then it sits having been levered into that position. Days, weeks, months.

And the transient thing I tried to do above. Sideslip angle is when we get into the forces acting against the trailer to lose it's grip on the road. Markdoane made reference to one of Bundorf's SAE papers [1965] in a post/thread a few years back (that is good reading). Stiff sidewalls make for longer life and predictable road manners is, again, my understanding (even if it is only information posing as knowledge).

Philosophically, I'm interested in the worst-case scenario: tired, ill, injured driver on dark night, in rain, inadvertantly at too high a speed. Broken or potholed pavement coming up (unawares) with a-hole drivers around the rig and the need for hard maneuvering coming up. Braking. Downshifting. All the dice rolled. So, hitch, tires, vehicles are not what I want to worry about having made compromises upon, and am now reliant on driver skill to get me out of it. Confidence cannot be overstated, and it is my experience (personally and professionally) that this confidence in the rig itself that gives the margins one needs (for lack of a better term) when the moment is upon us.

I may say that I will not ever drive under any one of those circumstances, but it is my preference that rig and driver perform admirably if called upon to do so.

So if the tire engineers are adamant, I'm fine with it. Tire quality via brand is more my interest now (as a method of determing size and load has better tools available). Henceforth I want to ask my questions from established baselines, hitch rigging or tires. Numbers. No different than in engine performance parameters.

That's the best of my understanding. Jammer would do a better job.

I don't know if I ever looked at your older Silverstreak so I clicked on the link. It was beautiful! It must have been very hard to part with it. Did you get all the work done around the house when you removed the SS temptation?


I got as much work done around that house as budget allowed. And a bit more. Things changed quite a bit for us over just a few years. I did repaint the exterior in a job I was quite proud of (revealed details formerly hidden). Letting that trailer go hurt to some extent, but a rear bedroom model was greatly desired, so a combination of circumstance, opportunity and different responsibilities made selling it a fairly easy decision.

Glad you liked it. A nice one for condition when we bought it, and the renovation showed it off nicely. Thanks. They're an excellent choice when an otherwise original one can be found.

.

[eleighj]

I e-mailed Barry (tiretech) regarding ST Tires and his chart and provided him with the following parameters:

"After reading your comments on the RV Forum and studying your spreadsheet for ST tires, I wanted to say thanks.

We have a 2008 19’ Airstream single axle with the following dimensions:

GAWR – 4300#
GVWR – 4500#

The trailer came with Goodyear Marathons ST225/75R15 LRD (2540#)."

Here is his response; straight forward and concise (good engineer):

"If you followed along at the beginning of my web page, I said you needed to weigh your trailer - and that you should pick the worst tire as the one you need to determine the tire load needed.

Your email below reports the tire loads as axle loads - and in that case, I said to use 110% of that value. That means the tire load your are trying to match is 110% of the GAWR = 4730# or 2365# per tire.

The maximum load carrying capacity of an ST225/75R16 LR D is 2540 and I'm recommending using no more than 85% of that = 2159#. That's smaller than 2365# so you need to move up to a Load Range E and use 80 psi, if the rim will handle it. That will give you about a 20% reserve capacity."

Serious "margins" (reserve) built into his logic and calculations.


Ed

[slowmover]

And the point should not be missed -- as in several posts back near the beginning -- that individual tires may be off from off others by as much 30%. So one needs to find that out. Wheel-by-wheel.

[Gene]

Rednax,

First of all, the SS you had was a classic trailer that made me wish I had been looking for it when it was for sale and it was in my driveway right now. But I already had a trailer. The interior is warm and homey and I like that feel. Of course, as soon as I got it, I would be remodeling. The big difference in feel is that all the cabinets and partitions in ours are a light colored Formica that looks like fake wood—there's no warmth in that. But we keep working on it.

But I digress, as usual…

Your answer to my question, as I dissect it, seems to boil down to this: "Trailers, mainly at low speeds, experience sidewall deflection differently than cars or trucks." Thus, higher pressure is safer. I'm unsure it would be any different at high speeds, but maybe more noticeable at low speeds and especially when backing. And, basically, tire engineers say that and they should know. I am something of a skeptic and believe experts aren't always right, maybe less wrong than the rest of us. I can see the point for higher pressure, but am not sure it matters not I am sure it is correct. It is best to rest this question for a while.

I like the worst case scenario except that oil on black ice on a blind curve would be even worse. I think someone had a thread last year where that's what happened and they crashed. But in that situation no tire could help. Of course, your point is that you want a big margin of safety and I concur. Everyone has moments of non-attention, tiredness, and we may come to a bunch of vehicles going slowly or not at all on a blind curve, perhaps in bad weather, and we have to use all of that margin. So overdoing it is a good thing because the moment may come when we need everything we can get.

Thanks for your explanation. Because you look at numerous factors when you analyze a problem, your posts are always complex and require some thought. I like the intelligence that goes into them. I think I would feel very safe as a passenger in any vehicle you were driving and I can't say that about a lot of people.

Gene

[DKB_SATX]

Re: tire deflection vs. speed... I think the assumption is that at higher speeds one is generally going in a straight line or only making gradual turns for lane changes and following curves.

[slowmover]

Thanks, Gene. I might wish for a less tendentious manner, less what others consider pedantic, but early training in abstractions tends to linger. (Writing advertising copy, per your recent link, is no easy thing, n'est-ce pas?)

The "thing" about numbers is this: when I run into a problem I want to be able to fall back on a baseline. A zero re-set. I'd like for guys on AIR to look at the scale tickets and reported numbers from my logbook to help me in diagnosis. And, on other forums, with other savvy contributors to also help troubleshoot in the same way. In the overlap of RV's, commercial trucking, racing, and vehicle-specific experts, there is impressive overlap of how to dissect technical problems. The Internet makes new things possible.

Longest life at lowest cost with highest reliability is the backbone of safe operation, and not just my idea of how best to plan in allocating money.

As interesting as trailers are -- and as central as tires are to the Number One function of a trailer, mobility -- questions about operational use should be to the point. No deviations from agreed upon practice. In that I make it easy for my interlocutors thus may I generate more and/or better responses.

The idiosyncrasies of any rig are otherwise enough to occupy our attention (thus my earlier comment about "mind").

Following that, modelling the trailer to our taste and use is far more interesting at the end of the day than the smell of burning rubber. Symphony conductors live to a highly ripe age. Worn down mechanics, not so much. "Mind" is much at stake, then, in re-creating ourselves.

Finally, the arguments (the search) for what distinguishes good from better from best is little more than assembling the clock pieces that they (and the driver) wind down slowly together. Used to one another, as it were.

The search, then, is to make the future easier. Predictable on these counts of things mechanical.

.

[Cracker]

Originally Posted by Cracker

I asked this question a long time ago (---around 2006) - but never got an answer:

Correct Pressure??
This discussion finally came around to my question! During my factory vist, Airstream replaced my original Goodyear Marathon tires, load range "C," with new Goodyear Marathon, load range "D" tires. As Porky pointed out, the capacity of the "D" tires is 2,540 lbs at 65 psi, as opposed to 2,150 lbs at 50 psi for load range "C" or "D." Airstream originally recommended that the tires be inflated to 50 psi for my 2000 30' Excella. Accordingly, in order to achieve the softest ride, I would assume that the lower 50 psi pressure is still correct? I don't believe that Airstream is willing to arbitrarily increase my CCC by 1,560 lbs!
__________________
Cracker

None of the current discussions have provided a definitive answer either - however, I continued to run the tires at 50 psi for the next 6 years (---as per my previous comments herein) without incident. So, was my assumption correct???

I decided to compromise with myself somewhat and run the new Marathons (Load Range "D") at 55 psi. Not very technical - but with over 10 years personal experience running Marathons at 50 psi - including a Florida trip - I believe this is a reasonable change. Besides, 55 psi is the same pressure I run in my duallys, when loaded for a trip, so I don't even have to think too much! I run the front tires on the dually at 65 psi.

[slowmover]

The sidewall maximum air pressure for TT tires is easy to live with when we observe certain conditions. And it needn't be remembered at all as it is front of us at every air pressure check.

I was moving my trailer a few days ago and the stresses in sawing back-and-forth were not pretty. Hard on the sidewalls . . wants to tear the tire from the rim. Conditions not amenable to less than top numbers.



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