Markuson
SILVER Star
Tire Pressure recommendation (3 Viewers)
- Thread starter terdrocket
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It cracks me up that someone would come into this thread to question Gaijin’s numbers. I’m Asian and all the maths involved in doing what he does makes my head hurt. #everythingismaths
Thanks for continuing this discussion.
I went over the information in the link you provided.
These numbers posted seem to be for the correct tire pressure at that tire's maximum load limit. And there are also numbers posted for lower psi at loads under that tire's maximum limit.
Lets say my Cruiser plus my gear my wife and myself is 7,000 pounds total, that would mean each tire needs to be able to support a load limit of 1750. So, if 46psi supports 2294 pounds then it seems that 33psi supports 1645 pounds. I divided 2294 by 46 to come up with 49.87 pounds of load supported per each pound of pressure. So it seems that when I'm running my usual load, 35 psi would be optimal. And when empty 33 psi is still the recommended pressure.
.
I question everything. That's how I learn.
There's nothing sacred to me. It's all up for examination.
indycole
GOLD Star
I'm happy to be corrected if I'm wrong, but I believe the TRA-based process is based on the assumption that there relationship between stock GVWR, tire pressure, tire size, and implied load rating that must be kept in check. To keep matters simple, imagine the scenario where a bone-stock vehicle is carrying more weight than the GVM.
The 33 psi recommendation for 285/60r18 for GVRW of 7385 implies that there's a safety margin of ~36% compared to the maximum load limit (1 - (4 * 2512 / 7385)) = 0.36. Let's say you're 500 lbs over the GVRW (how dare you?!?!?!?!) — then, you'd need to find the pressure for that tire size associated with this load limit: (7385+500)*(1.36)/4 = 2681. This seems like it would be 35-36 psi using the Toyo tables. Going from P-Type to LT requires another step and changing tire size yet another.
I have no idea whether the above is absolutely correct, but the main point is about applying Toyota's load margin to the exercise.
I would imagine that inflating to barely meet the load limit would result in very high temperatures and dangerous situations, at least according to the TRA process. Most tire shops follow the same logic and will take you within 5 psi of the "by the book" pressure.
gaijin
GOLD Star
You are wrong. Look at the tire placard on your vehicle. The most your vehicle can safely weigh is 7,385 pounds. To safely support that weight, according to Toyota, each P285/60R18 tire - inflated to a Cold 33psi - must be able to support 2,512 pounds. This means the total Load Limit for all 4 tires is 10,048 pounds. That total Load Limit allows your vehicle to be safely operated at all weights from virtually unladen to fully loaded up to the GVWR of 7,385 pounds. This accounts for static loading as well as live loading which will place transient increases of load on the tires - think running over a pothole, hard cornering, braking, accelerating, etc. - all the forces that are more than just a static vehicle sitting on static tires. Since you are running a non-standard LT-Metric tire, we need to adjust these numbers by 10% so the Load Limit for all 4 tires must be >9,135 pounds or >2,284 pounds/tire. You will achieve the required Load Limit at 46psi.
Load is not a static number. It is not correct that you can weigh your vehicle and adjust the Load Limit of your tires to support just that weight.
If you run your tires at 35psi, even with only you in the vehicle with no cargo and an almost empty gas tank, at highway speeds your tires would overheat and self-destruct.
I can already sense you have stopped listening and have already solved the problems of this little bit of the universe by yourself.
I have outlined the science behind the numbers.
I have pointed you to an industry source that explains the science and the process of applying that science to your vehicle.
It is up to you to either ignore the science or learn from it - your choice.
HTH
I'm running 285/75R17 Ridge Grapplers (Load Range E). My 200 has Rear Bar with Swing gate, Drawers, a bunch of tools, water tank, roof tent, sliders, skids, Front Bumper, Winch etc etc etc. I saw earlier you reccomended 37 for this tire. Would you recommend I go with 37 with my added stuff? I think im around 42 cold front/rear right now. Thanks for the info buddy! Very interesting reading and info in your posts above.
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gaijin
GOLD Star
All I can recommend is that you run your vehicle at no more than the GVWR of 7,385 pounds. Up to that weight, 37psi F/R is the Recommended Cold Tire Inflation Pressure for your LT285/75R17 tires on your LC200.
Tires are just one component necessary to safely run our vehicles at the rated weight. Simply increasing tire pressures does not make up for all the possible weaker components when the GVW goes above the rated 7,385 pounds.
I know I have posted HYPOTHETICALLY in the past about what the tire pressure component MIGHT be when vehicle weights are increased, but that was a mistake.
Sorry.
Aw man. Now I gotta find one of your hypotheticals that fits my situation. Hahaha
gaijin
GOLD Star
My best advice is to go to a truck scale and weigh your truck - measure front axle and rear axle separately. If there is a CAT Scale near you (use the Cat Scale Locator: CAT Scale Locator | CAT Scale) you can download their App and never have to leave your truck to get a printout of the weights.
Once you know the actual weight, you will know for sure where your vehicle, as currently configured, stacks up against the GVWR and how that weight is distributed and compare that to the front and rear GAWR numbers. If all are below the max limits, then the tire pressure I gave you is fine. If you are over the max numbers ... well, that's for you to decide what to do. In any event, no sense getting all wrapped up about a problem you probably don't have. Weigh the truck, then let's talk.
HTH
Much appreciated tire ninja master. I’ll stick to 37 until I see any other indication from a scale.
I'm still listening. I like to know how things actually work.
No need to " sense" anything. I can tell you I am far from being finished with this topic. Otherwise, I'd be in my " little bit of the universe by myself" instead of engaged in a discussion on this forum.
When you state that by running my tires at 35 psi, at highway speeds, that my tires would overheat and self-destruct that's not been my experience in over 50k miles with these tires. And in all honesty and without the slightest hint of sarcasm, I wonder why not?
I have 180k miles on my LC - over 100k of those I put on in the past 2.5 years. The majority of that time I've been loaded up, and running at 33 psi. And in the past year, I've spent a lot of time in the Mojave and Death Valley areas, at speeds of up to 105mph on pavement and up to 60mph off road. At times it was over 115 degrees outside. I wonder why I didn't experience tire failure under these conditions. I would be less puzzled if I just cruised around town. It seems unlikely that I just got lucky for thousands of miles. That is why I'm skeptical of the 46 psi number.
I would like to know how these load limit numbers are calculated in the first place. Example: what calculations or experiments were conducted to conclude the load limits at a given psi?
I'd like to be able to check the work of the TRA. I see no reason not to question any claim made by any organization or individual, no matter how established or revered. Especially when I'm getting dramatically different results in over 50k miles of testing in the field- specifically:
1- tread wear pattern doesn't match that of an under-inflated tire
2-no excessive heat build up- psi doesn't increase by more than 4psi from cold starting pressure
3-no blowouts
My hunch is that IF you could take away the risk of lawsuits, the persons that actually came up with these industry standard numbers would tell us that within a certain range, it doesn't really matter what psi you run. I also wonder how low you'd have to go and for how long you'd have to drive before you did actually see excessive heat build up and eventually a blow out. I wonder if that number for my tires is more like 15psi rather than 33psi.
If I could find the science behind the load inflation tables perhaps I could work that out for myself. Or perhaps I'll just have to go run them at 15psi and at high speeds out in the desert until one of them blows. I'd prefer the former.
I believe the max load on the door placard is significantly higher than you quoted, but I'm going to wait and check tomorrow. The LC is parked about 1/4 mile away.
In the meantime, any ideas as to the science behind the load limit tables?
indycole
GOLD Star
TRA offers an engineering design book you can purchase if you're so inclined — The Tire and Rim Association, Inc.. Perhaps it contains answers to the questions you pose. It's hard to say whether TRA employs engineers, chemists, physicists, etc. to develop the tables based on their own calculations and models or if they defined standards based on cross-sectional industry data from each manufacturer's particular specifications and designs. I'd bet on the latter.
Either way, the inflation tables are standards and best practice is to apply those standards. I'd argue it's outside the scope of this forum to attempt to belabor the topic. Theoretically, if those tables did not exist, then the exercise to design around maximum load would require experts in multiple engineering disciplines (chemical eng, materials eng, mechanical eng at the least) with a healthy dose of pure science from physicists and chemists.
The kinetic conceptual model would look something like this (ignore the statics):
- The force on a tire is a function of rotational speed | more speed => more force
- The friction between the tire and the ground is a function of force and the contact area | more force and contact area => more friction
- The friction between the tire and the surface generates heat that is transferred to the air molecules within the tire | more friction => more heat
- Higher temperatures inside the tire mean that the air molecules within are moving faster
- Faster moving air molecules inside the tire mean that more force is exerted to the inside of the tire
- The more force exerted on the inside of the tire results in more pressure (this is the build-up) | more heat => more pressure
- At some point due to the physical heat of the tire and the pressure differential between the inside of the tire and atmosphere will result in mechanical failure and the tire will de-laminate
indycole
GOLD Star
(Gross Weight) * (1 + (Load Rating per stock sized tire at stock PSI) * 4 / GVWR) / (4 tires) / (1.1 as factor to go from P to LT) = target load rating per tire
7720*1.36/4/1.1 = 2386 per tire
From Toyo Table
At least that's how I use the tables. May or may not be right.
7720*1.36/4/1.1 = 2386 per tire
From Toyo Table
- 2315 @ 40 PSI
- 2510 @ 45 PSI
- (2510 - 2315) / (45 - 40) = 39 lbs per 1 PSI
- 40 PSI => 2315
- 41 PSI => 2354
- 42 PSI => 2393
At least that's how I use the tables. May or may not be right.
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We’re gonna have to rely on Cole for overweight trucks. I don’t think gaijin is gonna work in hypotheticals anymore. Hahaha.
gaijin
GOLD Star
OK, if we are talking hypothetically, let's all get on thge same page so we might actually produce something meaningful with all this.
First, @eizenjin91 you know you are over the allowable max weight for your vehicle - and that your weight is distributed too far forward. Let's assume we're not going to "fix" any of that. So ... yes, your tires probably need a little more pressure to support that weight.
@indycole has a pretty good idea, but I don't think it needs to be that complicated, and the numbers don't work for @eizenjin91 LT285/75R17 tires.
Hypothetically, since we are talking about an increase in weight less than 5% (7385 pounds to 7720 pounds), we may be able to safely assume that any "safety margin" built in to the stock numbers does not have to increase proportionately within this small percentage increase. This might be an easier way to go:
(Gross Weight - GVWR) / 4(tires) = Amount additional Load Limit required.
(7720 - 7385) / 4 = 84 pounds/tire
Load Limit at your Recommended Cold Tire Inflation Pressure of 37psi = 2301 pounds
New Load Limit for 7720 pound Gross Weight = 2301 pounds + 84 pounds = 2385 pounds/tire
From the Load/Inflation tables for the LT285/75R17 tires:
- 35psi => 2215 pounds
- 40psi => 2430 pounds
| (2430 - 2215) / (40 - 35) = 43 pounds per 1psi
- 35psi => 2215 pounds
- 36psi => 2258 pounds
- 37psi => 2301 pounds [Your Recommended Cold Tire Inflation Pressure for stock GVWR of 7385 pounds]
- 38psi => 2344 pounds
- 39psi => 2387 pounds [Your Hypothetical Cold Tire Inflation Pressure for a GVW of 7720 pounds]
Hypothetically, of course
HTH
indycole
GOLD Star
Ah, good catch. I pulled the numbers for 285/70r17 tires.
46psi for the LT285/60R18 tire on a 200 series Land Cruiser is, in my opinion, too high for most conditions
I make this opposing claim based on my experience in over 100,000 miles of driving in what were often much harsher conditions than those that our tires are subjected to in lab tests- and at much lower pressures than those recommended by the TRA. Typically i was running 33 psi. It is also based on my experience at 45psi, 40psi, 36psi, 34psi and 28psi.
The Tire and Rim Association is not an infallible organization. It is an organization run by several major tire and rim manufacturers.
The claims they make for load limits and tire pressure are in part, designed to protect the members from lawsuits. Now you might say, well that's great- if they think they won't get sued if they can convince us to run these tire pressures than that must be the best practice. Not necessarily.
Consider this, how much extra wear and tear on your body and on your vehicle's components are you being subjected to at 46psi? In my experience at 45psi, the Cruiser is a very harsh ride. Why deal with that if it's not necessary? You don't need to. I'm living proof of that.
If someone tells you there is only one correct tire pressure that you should be running, I think that's incorrect. Within a certain range, it seems there is plenty of room to experiment and decide for yourself. Closer to the truth would be to say that optimal tire pressure varies with the conditions.
One time when I still had my 60, I was running BFG AT 33x10.5, and after some light wheeling, I drove for about 45 miles at about 60mph and 20psi on the pavement through the desert at outside temps of around 90 degrees. I just didn't feel like using my slow under-powered on board air that day and so I went to a gas station instead. When i got there, my tire pressure had increased to 22psi. Hardly the excessive heat buildup and tire failure we read about on warnings in load limit tables. Would I have had the same success at 12psi? Perhaps. I've driven briefly at 8psi on pavement, but at that pressure I didn't like the sound of it or the handling so i stopped and spent a half an hour having fun with the nearly worthless VIAIR system and got it back up to 30psi, which was more than adequate for the ride home that day.
I spent several hours on tire research after exhausting the limited supply of knowledge here on the thread.
Here's a link explaining some of the tire testing procedures which tire manufacturers are required to perform: 49 CFR 571.109 - Standard No. 109; New pneumatic and certain specialty tires.
Here's a study that I found interesting: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/811617
An article on the physics of tires: http://www.motoiq.com/MagazineArticles/id/2026/the-physics-of-tires.aspx
@indycole 's description of how the tire would build up excessive heat and eventually fail was well written- with that in mind it seems that if your tire is in fact experiencing the excessive heat buildup that would eventually cause tire failure, this would be easy to monitor through your TPMS. If you see a rise in pressure of over 4psi above the cold starting pressure, it seems that would indicate that you are in the danger zone. I've not been able to create those conditions in the field. Yet.
Even though I disagree with @gaijin on what is the best tire pressure or that there is one " correct" pressure at all, I applaud his good intentions. And although I'm sure it was unintentional, I thank him for rekindling my interest in experimenting with tire pressure.
I'll post the results of my tire pressure experiments in a new thread so as not to continue to disrupt the quiet reverie of the status quo here on this thread.
I make this opposing claim based on my experience in over 100,000 miles of driving in what were often much harsher conditions than those that our tires are subjected to in lab tests- and at much lower pressures than those recommended by the TRA. Typically i was running 33 psi. It is also based on my experience at 45psi, 40psi, 36psi, 34psi and 28psi.
The Tire and Rim Association is not an infallible organization. It is an organization run by several major tire and rim manufacturers.
The claims they make for load limits and tire pressure are in part, designed to protect the members from lawsuits. Now you might say, well that's great- if they think they won't get sued if they can convince us to run these tire pressures than that must be the best practice. Not necessarily.
Consider this, how much extra wear and tear on your body and on your vehicle's components are you being subjected to at 46psi? In my experience at 45psi, the Cruiser is a very harsh ride. Why deal with that if it's not necessary? You don't need to. I'm living proof of that.
If someone tells you there is only one correct tire pressure that you should be running, I think that's incorrect. Within a certain range, it seems there is plenty of room to experiment and decide for yourself. Closer to the truth would be to say that optimal tire pressure varies with the conditions.
One time when I still had my 60, I was running BFG AT 33x10.5, and after some light wheeling, I drove for about 45 miles at about 60mph and 20psi on the pavement through the desert at outside temps of around 90 degrees. I just didn't feel like using my slow under-powered on board air that day and so I went to a gas station instead. When i got there, my tire pressure had increased to 22psi. Hardly the excessive heat buildup and tire failure we read about on warnings in load limit tables. Would I have had the same success at 12psi? Perhaps. I've driven briefly at 8psi on pavement, but at that pressure I didn't like the sound of it or the handling so i stopped and spent a half an hour having fun with the nearly worthless VIAIR system and got it back up to 30psi, which was more than adequate for the ride home that day.
I spent several hours on tire research after exhausting the limited supply of knowledge here on the thread.
Here's a link explaining some of the tire testing procedures which tire manufacturers are required to perform: 49 CFR 571.109 - Standard No. 109; New pneumatic and certain specialty tires.
Here's a study that I found interesting: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/811617
An article on the physics of tires: http://www.motoiq.com/MagazineArticles/id/2026/the-physics-of-tires.aspx
@indycole 's description of how the tire would build up excessive heat and eventually fail was well written- with that in mind it seems that if your tire is in fact experiencing the excessive heat buildup that would eventually cause tire failure, this would be easy to monitor through your TPMS. If you see a rise in pressure of over 4psi above the cold starting pressure, it seems that would indicate that you are in the danger zone. I've not been able to create those conditions in the field. Yet.
Even though I disagree with @gaijin on what is the best tire pressure or that there is one " correct" pressure at all, I applaud his good intentions. And although I'm sure it was unintentional, I thank him for rekindling my interest in experimenting with tire pressure.
I'll post the results of my tire pressure experiments in a new thread so as not to continue to disrupt the quiet reverie of the status quo here on this thread.
