Fish Tailing on Washboard Roads

[cgardnerma]

Hi Dan,

Pretty much any solid axle truck at stock air pressure will do this. Stiff sidewalls and higher pressures make it worse. Airing down to 20 PSI or less makes a HUGE difference. Independent rear suspensions, like the LR4 or even ay compact SUV do much better on washboard, but of course generally lack the heavy duty, towing capacity and articulation of a solid rear axle.

Chris

 

[Dan Higgins]

I finally received the "chart" back from Michelin/BFG. I have attached it below. As I look at it I am thinking that they told me 55 PSI because they were taking the curb weight of the LC and dividing it by two. At 55 PSI the max load on the chart is 2790. But this is for a single tire, right? 2790 x 4 > 11,000#. That is way beyond the GVWR of the LC. If I consider 2790 x 2 ~ 5,600 which is the curb weight of the LC (that was the weight I provided to them.)

So if all this is true, then the proper PSI for these tires when the LC is loaded at its GVWR is < 35 (2,035 max load).

All that tells me that I could drop the PSI to 35 and still be in good shape even with the LC at the GVWR limit.

Am I making sense of this?

 

[ikeb]

It doesn't add much to the overall thread... and, honestly, I trust @gaijin's math (well, not entirely true, I verified his arithmetic first)... but FWIW, when I fitted my BFG KO2's on my 17" RW wheels, I asked the 60+ year old man fitting my new shoes what pressure he was filling them to. His answer: "I filled them to 40 lbs". I thanked him for not blindly following the door placard and asked him why. He volunteered this: "...it's a big, heavy truck. 33 lbs is not right for this truck on these 'truck' tires. They need to be 40 lbs." I asked him how long he's been doing this (not how he knew this to be accurate) and he told me over 30 years. It matches the numbers that @gaijin has provided within 1 lb of pressure as well as my own research (I'm a math guy). I know my encounter with this gentleman is not scientific, but I do have a soft spot for folks that have been working within their craft for decades and take pride in their work and, err... it aligns with my calculations. Frankly, that's good enough for me.

 

[gaijin]

I finally received the "chart" back from Michelin/BFG. I have attached it below. As I look at it I am thinking that they told me 55 PSI because they were taking the curb weight of the LC and dividing it by two. At 55 PSI the max load on the chart is 2790. But this is for a single tire, right? 2790 x 4 > 11,000#. That is way beyond the GVWR of the LC. If I consider 2790 x 2 ~ 5,600 which is the curb weight of the LC (that was the weight I provided to them.)

So if all this is true, then the proper PSI for these tires when the LC is loaded at its GVWR is < 35 (2,035 max load).

All that tells me that I could drop the PSI to 35 and still be in good shape even with the LC at the GVWR limit.

Am I making sense of this?

View attachment 1342206

There's a lot to unpack here, so please bear with me.

Yes, you are STARTING to make sense of all this, but you are missing a few key points, and confusing some others.

First, the Load Limit/Pressure table that BFG sent you is correct. And, you are correct in noting that the Load Limits in that table are for a single tire.

Where you start to go off course a bit, is using GVWR as a basis for the calculations. The long story made short is Toyota recommends that the GAWR (Gross Axle Weigh Rating) should be no more than approx. 85% of the combined Load Limit for the two tires on that axle if they are P-Rated tires (OEM equipment). For the rear axle, which is the higher rated of the two axles, the calc looks like this: 4300 pounds [Rear GAWR] / (2 * 2,512 pounds) [2 * Load Limit for one P285/60-18 tire] = 0.85 (Approximately 85%). And, Toyota recommends the same tire pressure for the front axle, even though it only has a GAWR of 3,595 pounds - it is probable this is to enhance handling, not just for load carrying.

To translate this recommendation to LT-Rated tires, we must apply a correction factor of 10%. This is due to the difference in construction between P-Rated and LT-Rated tires which allows the LT-Rated tires to carry 10% more Load than an equivalent P-Rated tire. As a result, instead of a Load Limit of 2,512 pounds per tire for the P-Rated tires, we can safely substitute an LT-Rated tire that has a Load Limit of approximately 2,284 pounds (2,512/1.1).

This is where you can use the Load Limit chart that BFG sent you to determine what pressure you need for your tires.

Since your tires are LT-Rated, look in the Load Limit chart for your size for the pressure which yields a Load Limit of 2,284 pounds. It's more than 40psi, but less than 45psi, correct? At 40psi, the Load Limit is 2235 pounds and at 45psi, the Load Limit is 2425 pounds. The correct pressure to achieve our desired Load Limit of 2284 pounds is somewhere between those two. To get an idea of what those in-between Load Limits are, we can calculate the approximate Load Limits by subtracting the Load Limit at 40psi from the Load Limit at 45psi and dividing by 5 to determine the Load Limit increase per psi: (2425 - 2235)/5=38 pounds per psi. So, we can take the Load Limit at 40psi (2235 pounds) and add 38 pounds (2235 + 38) which will give us 2273 pounds at 41psi - close enough to our desired 2284 pound result.

Cliff Notes summary for the calcs so far:

Toyota recommends 2,512 pounds Load Limit for P-Rated OEM tires
That translates to 2,284 pounds Load Limit for LT-Rated tires
Load Limit table indicates approximately 41psi will yield the correct Load Limit

Important Note:

Your statement, "All that tells me that I could drop the PSI to 35 and still be in good shape even with the LC at the GVWR limit." is not correct. As we can see with the calcs, at 35psi the Load Limit for your tires would be only 2035 pounds - approximately 250 pounds/tire too little to meet the Load Limit requirements of the truck.

Also, there are no Load Limits shown below 35psi because the minimum recommended pressure for these tires is 35psi. Of course, this minimum does not apply to short term offroad use where airing down would result in lower pressures.

Hopefully this gives you the info you need to use the Load Limit tables on your own.

Key points for the 200 series Land Cruiser specifically:

P-Rated tires should be inflated to a Cold Tire Inflation Pressure that results in a Load Limit of 2,512 pounds.

LT-Rates tires should be inflated to a Cold Tire INflation Pressure that results in a Load Limit of 2,284 pounds.

Now I'm going to see if I can scare up some of Markuson's meds and get myself some sleep.

Of course, if you have any questions about the above, post them up and I'll do my best to answer them.

HTH

 

[Dan Higgins]

I am sure that no one else but me cares about all this stuff. But being a match/computer science guy (once upon a time - now I'm stuck in Marketing until I retire) I find the numbers helpful.

Thanks! So I'm back at 41. I could have simply trusted you. But I like to understand the why.

Then, again, maybe you're the 60 year old man that fitted ikeb's shoes.

 

[gaijin]

I am sure that no one else but me cares about all this stuff. But being a match/computer science guy (once upon a time - now I'm stuck in Marketing until I retire) I find the numbers helpful.

Thanks! So I'm back at 41. I could have simply trusted you. But I like to understand the why.

Then, again, maybe you're the 60 year old man that fitted ikeb's shoes.

Glad you found the info helpful

Like you, as a retired engineer I also like to understand the "why?" of these questions - especially when the answer can be found by the proper application of some of that science I learned so long ago.

For the record, I am not the 60 year old man who worked with ikeb on his tires - I flew past 60 a while ago ...