RCTIP Talk (1 Viewer)

lx200inAR

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I was doing a deep dive into the tire pressure tables because I got bored at work.

Anyone know how T&RA and ETRTO derive the data for the inflation/load tables? Is it real world testing at different speed/load and checking tire temperatures? It's also strange to me that these tables aren't tire model specific in any way. It's all calculated just from size and load ratings. And really the load ratings don't affect the tables, other than the max load. A load C and load E LT tire will have same RCTIP assuming the load is within range of both tires.

The guide that gets posted here regularly, doesn't really speak to where the data comes from. Also it says to calculate load between data points in the table you can do a linear approximation. But strangely, the data is not linear across all it's data points per tire size. I was going to make a quick cheat sheet to calculate RCTIP based on min and max load and min and max PSI, but I noticed pretty quickly the calculated data didn't fit the tables.

Either way, for everyone's RCTIP pleasures, I'm attaching the spreadsheet I made to calculate RCTIP. Looks like you can't add a .xlsx file to mud. Here's a link to it on Google Docs. I don't think Google Sheets will run it very well as I'm sure I'm using some Excel only features to make everything work.


It has a lot of tire data also that you can use to compare sidewall and sizes. There is no "real" tire data in here, only calculated from the tire size and matched to the inflation tables.

Based solely on the data I have gone through, I'd say RCTIP is probably just a good starting place, but certainly experimentation with different pressures around RCTIP are more than safe enough and in some cases may be more ideal than the standard RCTIP calculation. It seems the recommendations are far too broad (as far as the application) to be a concrete answer to safe/ideal inflation. It all seems very specific with the PSI to load equations and what not, but when 5 different manufacturers can make the same size tire, in AT/RT/MT configurations, and the same tables apply to all 15 of those tires, the specificity loses some of it's shine.

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gaijin

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Anyone know how T&RA and ETRTO derive the data for the inflation/load tables?

I think you are looking at the problem backwards - tires do not drive the TRA data, rather it is the TRA data that drive the specifications for the manufactured tire. Here is an excerpt from the very first page o the TRA Yearbook (available for purchase here: The Tire and Rim Association, Inc.):

The Tire and Rim Association, Inc. (TRA) is the standardizing body for the tire, rim, valve and allied parts industry for the United States. TRA was founded in 1903 and its primary purpose is to establish and promulgate interchangeability standards for tires, rims, valves and allied parts.

It all seems very specific with the PSI to load equations and what not, but when 5 different manufacturers can make the same size tire, in AT/RT/MT configurations, and the same tables apply to all 15 of those tires, the specificity loses some of it's shine.

It should be very specific - all tires of the same size, irrespective of brand, are built to the same specifications which are set by the TRA.

HTH
 
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lx200inAR

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I don't think your argument changes mine about specificity. Even if you flip it around, then the T&RA provides a spec the tire manufacturer must meet. Does that mean the manufacturers all "hit the mark" with each different tire they manufacture, or do they only have to make sure they meet a minimum requirement?
 

gaijin

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I don't think your argument changes mine about specificity. Even if you flip it around, then the T&RA provides a spec the tire manufacturer must meet. Does that mean the manufacturers all "hit the mark" with each different tire they manufacture, or do they only have to make sure they meet a minimum requirement?

I would refer you to FMVSS (Federal Motor Vehicle Safety Standards), the DOT (Department of Transportation), and the tire manufacturers themselves - all else is purely conjecture.

HTH
 

lx200inAR

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Well...conjecture is what the interwebs is for, right? I suspect you could argue the the manufacturer would shoot for the minimum requirement, because it would probably cost them less money in production to hit the specification than to overshoot. That's at least an argument in favor of the load tables being more specific.

Do you know of any documentation/articles, probably in the yearbook linked above, that sheds any light on how the tables were derived? And what the process is to verify manufacturers are meeting the standards?
 

Yossarian

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Funny, I did the same today. Downloaded the Guidelines for the Application of Load and Inflation Tables from the Toyo website. While I did not drill down into as much as you @lx200inAR , I've been running the numbers myself.

This is in anticipation of trying to go with a P-metric tire on the TRD Rock Warriors instead of the LT-E's. While I haven't convinced myself it's the way to go yet, the variables for any replacement are all over the place.

Analysis paralysis anyone?
 

lx200inAR

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Funny, I did the same today. Downloaded the Guidelines for the Application of Load and Inflation Tables from the Toyo website. While I did not drill down into as much as you @lx200inAR , I've been running the numbers myself.

This is in anticipation of trying to go with a P-metric tire on the TRD Rock Warriors instead of the LT-E's. While I haven't convinced myself it's the way to go yet, the variables for any replacement are all over the place.

Analysis paralysis anyone?
You can try out the spreadsheet if you like. Let’s you enter any TIP info and any non OEM tire and it will calculate the RCTIP. Not that it’s doing anything amazing.. the automation is the only real “fancy” thing about it.
 

lx200inAR

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The other interesting thing to me is that floatation tires are rated down to 25PSI, while still carrying similar max load ratings to LT tires..
 

Yossarian

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The other interesting thing to me is that floatation tires are rated down to 25PSI, while still carrying similar max load ratings to LT tires..
And apparently the Toyo Open Country ATIII in the P-Metric (not sure what size) can be run down to 27 psi while meeting the load capacity. Seems counterintuitive?

This is why I downloaded the charts and started running the numbers.
 

lx200inAR

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Yeah, it would be interesting to read about the different construction properties that allow these wide ranges.

A 33x11.50R18LT needs 46 PSI to match up with the P285/60R18 33PSI of the LC200. But is rated from 25-80PSI.

The quick notes seem to be (mass generalizations, I know, please don't flame me):
P Metric (NON XL) tires reach their rated load faster than any of the others, in AT sizes they seem to have about same load Max as a C-Rated LT, but at 35PSI, rather than 50.
LT has higher bandwidth than P Metric for different load ratings, but min RCTIP is 35PSI
Floatation seems to have the best of both worlds. Widest bandwidth of load ratings and can function from 25-80PSI. Does take relatively high pressures to get to load equivalence with LT and P-Metric.
 
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So with the talk of P-rated tires being able to meet their load capacity with lower PSIs (you'd think that LTs would do this instead at lower psi):

This is due to there is more heat generated internally when running LT tires. There is more rubber having internal friction, so in order to prevent that, a higher psi has to be run when under load. However with more rubber structure in an LT tire, there can be higher load rating but there needs to be higher psi.
 

lx200inAR

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I've read that's the reasoning for not going below 35PSI on LT. So what magic is going on in a floatation tire that it can have similar max loads to an LT, yet not need to maintain pressure as high as an LT? Do they have a more durable makeup, that allows them to contain less material but be just as strong? Or is it just a matter of their being more load ratings? I mean, a floatation tire generally has a higher RCTIP compared to LT for any given load.

This is from a post on Grassroots Motorsports. Seems to make some sense. Floatation is an in-between as far as stiffness. I guess the market for floatation tires is why they generally are produced with more aggressive tread patterns.

What you are calling "flotation" tires are more commonly called LT (Light Truck, aka 3/4, 1-ton, etc pickups, vans) tires and are specified by the american T&RA (Tire and Rim Association). These tires can have one of many multiple ply load ratings (C, D, E, etc.) in the same tire size. The standard also specifies the physical dimensions of the tire and the weight they are supposed to support. Higher load ratings are reached by adding structure to support more air pressure. This is typically done by putting in stronger body plies and/or an additional body ply.

The T&RA also has a separate set of tables of truck sized tires intended for SUVs and 1/2-ton trucks. The size matches, but isn't proceeded by "LT". The physical dimensions are very close, but the tires aren't required to support as much weight at equivalent pressures as LTs. They typically aren't rated to hold the 80-100 psi that LTs are either, also limiting the load they can carry. Thus they don't need the stronger/extra body plies and can sometimes have softer sidewalls.

There is a third standard that is common in the US, commonly known as P-metric, that is published by the ETRTO (European Tire and Rim Technical Organization). The sizes nearly match the non-LT T&RA sizes but are proceeded by a "P". Physical dimensions are very close. Load ratings vary by a few pounds here and there, mostly due to differences in the formulas to calculate them. But they are also intended for SUVs and 1/2-ton trucks.

There are other tire standard organizations out there from places like Japan and Brazil, but T&RA and ETRTO are the two we see in the US.

Between "normal" tires and LT tires there can be a few other differences.

  • LT tires can have a slightly different tread pattern and more tread depth.
  • The external profile can also differ slightly.
  • LT tires are typically stiffer and heavier.
  • LT tires can be had in larger and wider sizes than are tabulated in normal car sizes (38", 40", 48", etc) which leads to the 4x4 credibility factor.
  • Tread compounds can be different too.
 

TeCKis300

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Yeah, it would be interesting to read about the different construction properties that allow these wide ranges.

A 33x11.50R18LT needs 46 PSI to match up with the P285/60R18 33PSI of the LC200. But is rated from 25-80PSI.

The quick notes seem to be (mass generalizations, I know, please don't flame me):
P Metric (NON XL) tires reach their rated load faster than any of the others, in AT sizes they seem to have about same load Max as a C-Rated LT, but at 35PSI, rather than 50.
LT has higher bandwidth than P Metric for different load ratings, but min RCTIP is 35PSI
Floatation seems to have the best of both worlds. Widest bandwidth of load ratings and can function from 25-80PSI. Does take relatively high pressures to get to load equivalence with LT and P-Metric.

The min RCTIP of 35 PSI for LT tires is just a practical threshold for documenting tires nominally meant for load bearing applications which is a focus of the reference. It's also a limitation of the reference. It's not some figment that is somehow defining a critical floor to the operational envelop of LT tires. Practically speaking, of course the tires work below that, and many off-roaders do it all the time. People using them in lightweight trailers, lightweight passenger cars, or low speed off-road work regularly run them well below 35 PSI and into single digits. Just that we don't have handholding guidance that exists in a nice table. Could the data be created and document? Sure. Utility of it given all the different variables would be questionable, compared to Floatation tires that are more meant for that application.
 
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For me, i never run my LT tire’s pressure lower than 35 unless i am crawling and airing down…and at those low pressures, it is very slow speed all the way. I would never run low on highway. My (and my family’s) life is worth more than trying to play around with silly pressures at high speeds.
 

lx200inAR

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I’m less interested in anyone’s arguments about safety based on the load tables. I’m more interested in peeling away where the data comes from. That way we can all make a better decision on what is right for our own trucks. I’m not trying to start any arguments with this thread about the validity of the load tables (comments about specificity not withstanding).
Before joining mud, I’d never even heard of calculating RCTIP, so it’s a whole new world for me, although I can definitely see how this seems like beating a dead horse, considering how often it comes up around here.
 

TeCKis300

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I’m less interested in anyone’s arguments about safety based on the load tables. I’m more interested in peeling away where the data comes from. That way we can all make a better decision on what is right for our own trucks. I’m not trying to start any arguments with this thread about the validity of the load tables (comments about specificity not withstanding).
Before joining mud, I’d never even heard of calculating RCTIP, so it’s a whole new world for me, although I can definitely see how this seems like beating a dead horse, considering how often it comes up around here.

It may be useful to understand the above references - that they are references for specifications that establish technical standards for the industry. From which may reference more specific standards. It's simply a guide along with interchangeability/mappings between standards.

Nothing there is source data. There is no performance information to be had on any individual tires, what manufacturers internal safety margins are, how much they may exceed these specifications , etc. It's a reference and interchangeability guide for load standards with minimum inflation pressures. That's it.

It is not the holy grail you seek.
 

lx200inAR

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It may be useful to understand the above references - that they are references for specifications that establish technical standards for the industry. From which may reference more specific standards. It's simply a guide along with interchangeability/mappings between standards.

Nothing there is source data. There is no performance information to be had on any individual tires, what manufacturers internal safety margins are, how much they may exceed these specifications , etc. It's a reference and interchangeability guide for load standards with minimum inflation pressures. That's it.

It is not the holy grail you seek.
This is definitely how I've always taken it and after looking at it more closely my opinion on that hasn't really changed. I tried to reference that in my first post without starting a flame war. It would just be interesting if anyone could post some sort of article or study that shows where any of this came from.

Either way, I wasted a few hours making a fancy spreadsheet, mostly just to see if I could. And now all the data is in one easy to digest place, for me anyways.
 

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