Seeking suspension advice -- unique case

[gaijin]

@gaijin - How does the Toyo PDF work? I'm assuming since I'm rating for an axle with 2 wheels I use dual?

If I look up my size (LT285/17R17) it says single is 2215# @ 35 psi, dual is 2015#. The LC is rated at 7395# with 4300# for the rear axle IIRC. 40psi dual is 2210, so if I take the tire load difference divided by the psi difference I think I get approximately how many pounds of load per PSI

(2210 - 2015) / (40 - 35) = 39

Then if I split the rear axle weight between both tires (4300 / 2 = 2150) I'm guessing I should be running 38.5 PSI?

2015 + (39*3) = 2132
2015 + (39*4) = 2171

2131 < 2150 < 2171

(Note: I would've thought you'd use the "single" values but IIRC you said the correct PSI for my tires was 37 psi and single would imply I should run something like 33psi which seems very low)

There's a lot to unpack here, so let's take it step by step.

I assume your tire size is not LT285/17R17, but rather the same as in your sig, i.e. LT285/75R17, right? And that we are talking LC200 and not LX570, correct?

In that case, the RCTIP for the LT285/75R17 tires on your LC200 is indeed 37psi F/R. So, how did we arrive at that pressure?

You can't just jump into the tables and start picking out pressures willy nilly - and no, you should not use the "Dual" rows, those are for vehicles with dual wheels per side or 4 tires per axle. You should be using the "Single" rows which are for vehicles with a single tire per side or 2 tires per axle.

Now, forget everything you wrote about GVWR and GAWR numbers - they do not figure into the calculation. They certainly have a bearing on what your vehicle limits are, but nothing to do with calculating an LT-Metric tire pressure based on a vehicle that came stock with a P-Metric tire (please read page 11 P-Metrc to LT-Metric in the Toyo pdf).

First you need to determine the design Load Limit for the tires that came stock on your LC200. Look at the placard on the Driver side door jamb which lists tire size and cold tire pressure. You will find P285/60R18 tires @33psi. Go to the P-Metric tire Load Inflation tables and find P285/60R18. Then go across that row until you find 33psi - oh no, there is no 33psi column just 32psi and 35psi - no problem, take the 35psi Load Limit (2601) subtract the 32psi Load Limit (2469) to yield 132 pounds. That means there is a 132 pound Load Limit difference between 32psi and 35psi. So, divide 132 by 3 to find the Load Limit difference per psi and you get 44 pounds/psi between 32 and 35psi. Since we want the Load Limit @33psi, simply add 44 to 2469 and you get 2513 pounds as the Load Limit @33psi. This means that 2513 pounds is the design Load Limit for P-Metric tires on your LC200.

Now, referring back to page 11 of the Toyo pdf, we find that when installing LT-Metric tires on a vehicle which was initially spec'd with P-Metric tires, we must reduce the design Load Limit by a factor of 1.10. That means that the design Load Limit for P-Metric tires of 2513 pounds must be divided by 1.1 to determine the equivalent Load Limit for LT-Metric tires: 2513/1.1 = 2285. This means that 2285 pounds is the design Load Limit for LT-Metric tires on your LC200.

Since we want to find the RCTIP for LT285/75R17 tires on your LC200, simply go to the LT-Metric tire Load Inflation tables and find LT285/75R17. Follow the Single row over until you find the first value greater than 2285 - that would be 2430 pounds in the 40psi column. Since 2430 in more than we need, but the next lowest column, 35psi, is not enough at 2215 pounds, we must do the same simple calc we did above to find the Load Limit values between these two columns. So 2430 - 2215 = 215. Since 40psi - 35psi = 5psi, then 215/5 = 43 pounds/psi between 35psi and 40psi. To reach the nearest whole psi value greater than or equal to the desired 2285 pounds, we need to add 2psi or 86 pounds to 2215 to yield 2301 pounds @37psi. This means that 37psi is the RCTIP for LT285/75R17 tires on your LC200.

Clear as mud, right?

Give a man a fish... teach a man to fish... now go forth and catch all the fish you want

HTH

 

[TeCKis300]

Good stuff. You and I won't agree here and that's just fine. If there were others that may like catching different fish and wish not to put up with a unnecessarily harsh ride.

First you need to determine the design Load Limit for the tires that came stock on your LC200. Look at the placard on the Driver side door jamb which lists tire size and cold tire pressure. You will find P285/60R18 tires @33psi.

The assumption that the stock tire pressure of 33 psi for the LC was established solely on the basis of minimum load limit is simply not correct. The additional knowledge gleaned from stock pressures on the LX tells us otherwise.

LX derived pressures can always be safely used for the LC. It it wasn't safe, it would not be safe for the LX either.

 

[04UZJ100]

Jesus. We need a mod to automatically take the average of both your recommended inflation pressures and end it.

If tire pressures were so vitally important, there would be a boatload more blowouts on a regular basis from the general population who don’t know a thing about cars.

 

[MTKID]

Back to @BloggerL ’s original question, you may want to consider some of the aftermarket bumpstops available as an inexpensive stepping stone to smooth the ride on more harsh roads. Durobumps, Timbren and Wheeler’s Offroad are the three that come to mind. These may function at the expense of a little up travel.

Also you may find it helpful to learn accurate ways to lower your air pressure and safely raise it again when you get back to pavement. I find the faster you can do these things, the more likely you are to take advantage of the smoother ride of reduced air pressure when you’re offroad. The Apex Designs 3.0 Rapid Deflation Valve is one I just purchased to try and I have had both compressors and Co2 tanks and the Co2 tank is a little faster usually, but compressors don’t run out or need refilling.

Hope this helps.

 

[ramangain]

Good stuff. You and I won't agree here and that's just fine.

 

[NY2LA]

Back to @BloggerL ’s original question, you may want to consider some of the aftermarket bumpstops available as an inexpensive stepping stone to smooth the ride on more harsh roads. Durobumps, Timbren and Wheeler’s Offroad are the three that come to mind. These may function at the expense of a little up travel.

Also you may find it helpful to learn accurate ways to lower your air pressure and safely raise it again when you get back to pavement. I find the faster you can do these things, the more likely you are to take advantage of the smoother ride of reduced air pressure when you’re offroad. The Apex Designs 3.0 Rapid Deflation Valve is one I just purchased to try and I have had both compressors and Co2 tanks and the Co2 tank is a little faster usually, but compressors don’t run out or need refilling.

Hope this helps.

This is a new concept for me. Would these help smooth out bumps on paved roads? Do you keep them on when going off road?

 

[NY2LA]

LX derived pressures can always be safely used for the

Jesus. We need a mod to automatically take the average of both your recommended inflation pressures and end it.

If tire pressures were so vitally important, there would be a boatload more blowouts on a regular basis from the general population who don’t know a thing about cars.

What I don't understand about the tire pressure debate happening here is how airing down for offroad driving fits into the equation. Seems like folks air down to extremely low pressures without issue. So is the safety question related to higher speeds on the highway?

 

[NY2LA]

To get the damping you want and be good for wheeling or anything else, Kings or any other tunable set up would definitely be your best bet. Going to cost more than a gereric OME, ToughDog, Bilstein. But you get that custom valving specific for you.

I'm happy with my OME setup, it feels smoother on FS roads and quicker gravel pace compared to my stock suspension. But, I feel you would need better with what you're describing

I will look into these--thank you.

 

[gaijin]

What I don't understand about the tire pressure debate happening here is how airing down for offroad driving fits into the equation. Seems like folks air down to extremely low pressures without issue. So is the safety question related to higher speeds on the highway?

Yes.

 

[bloc]

So is the safety question related to higher speeds on the highway?

Correct. Highway speeds flex the tire carcass at such high frequency heat builds up and if not kept in check can cause the tire to disintegrate. More pressure generally limits this heat buildup.

I am one of the few that would suggest looking a different direction with your tires.

Your KO2s require 40+ psi to operate correctly, whereas the stock ones were 33. This alone can make a large difference in something called tire spring rate. Gaijin shows how to do the calculations above, but ultimately the reason is that P-metric tire is less substantial and generates less internal friction, so it doesn’t need as much pressure to get the same load capacity.

I was very unhappy with how my stock suspension handled LT285/65r18 KO2s, between the harshness from the increased pressure and the much higher unsprung weight than stock. I eventually switched to a 17” rim so I could run P285/70r17 p-metric toyo ATs, and was very very happy with that setup. This was actually slightly larger diameter than the LT285/65/18, around the same weight as the stock P285/60/18 road-oriented tire on stock wheels, and allowed less than 30psi rctip which does a great job of isolating harsh pavement hits. Yes a p-metric tire is less robust off-road, but I haven’t been easy on them and they’ve done great. No, the tread lugs aren’t as aggressive as the toyo AT2 in LT-metric flavor.. it looks less “cool”, but the ride quality and very noticeable mileage benefit make up for it. And I try to avoid deep mud anyway.

Which isn’t to say you should necessarily switch to 17s, but that there are other tire construction options that might do what you want well. Thing is not many AT tires are offered in p-metric flavor.. and your sizing will likely be limited.

Otherwise I’m another happy king 2.5 customer. The vastly improved damping ability should allow me to switch to a LT-metric tire and not suffer the ride quality penalty, and that opens up more tire size options like 285/75/17 which I think will be the biggest I’ll go.

For the record the shock shaft is not hard to protect, and having it pointing down allows for front remote reservoirs that have a few benefits, the 2 biggest of which are improved shock oil cooling and taking up less space within the main shock body, which allows for a larger piston and more shaft travel for a given shock length. Plus, while expensive, kings are fully serviceable and to some extent shock shaft damage isn’t even the end of the world. See filthy’s king rebuild video on YouTube. I plan to do my own rebuilds when the time comes.

Your question about supple damping for road and progressive for big hits immediately made me think of king’s internal bypass setup, but when I asked they don’t offer it on our front shocks.

 

[linuxgod]

I assume your tire size is not LT285/17R17, but rather the same as in your sig, i.e. LT285/75R17, right? And that we are talking LC200 and not LX570, correct?

In that case, the RCTIP for the LT285/75R17 tires on your LC200 is indeed 37psi F/R. So, how did we arrive at that pressure?

Ah yes, a typo sorry, you are correct. And this makes sense and helps a lot, thank you.

 

[linuxgod]

Good stuff. You and I won't agree here and that's just fine. If there were others that may like catching different fish and wish not to put up with a unnecessarily harsh ride.



The assumption that the stock tire pressure of 33 psi for the LC was established solely on the basis of minimum load limit is simply not correct. The additional knowledge gleaned from stock pressures on the LX tells us otherwise.

LX derived pressures can always be safely used for the LC. It it wasn't safe, it would not be safe for the LX either.

I'm not a mechanical engineer but I suspect the actual pressure is OK to run but the difference in recommended pressure is not due to load carrying ability but to handling. Similar to why Toyota says 33psi in the front and rear tires even though the rear axle is designed to carry more weight than the front... presumably the higher front tire pressure helps compensate for to much understeer

 

[linuxgod]

If tire pressures were so vitally important, there would be a boatload more blowouts on a regular basis from the general population who don’t know a thing about cars.

Firestone and Ford tire controversy - Wikipedia

en.wikipedia.org

 

[TeCKis300]

I'm not a mechanical engineer but I suspect the actual pressure is OK to run but the difference in recommended pressure is not due to load carrying ability but to handling. Similar to why Toyota says 33psi in the front and rear tires even though the rear axle is designed to carry more weight than the front... presumably the higher front tire pressure helps compensate for to much understeer

You got it.

Load limit establishes a safe minimum. To your point, OEMs also have to consider things like handing, understeer, tire sidewall rollover, etc. Therefore the LC OEM tire pressure spec may be elevated from minimum load limits to establish recommended pressures for those other considerations. We can safely assume that because LX utilizes lower load limit pressures - therefore load limit wasn't solely the reason for the LC stock pressures.

What I don't understand about the tire pressure debate happening here is how airing down for offroad driving fits into the equation. Seems like folks air down to extremely low pressures without issue. So is the safety question related to higher speeds on the highway?

Most of this got answered by others. I'll add that tire limits are largely established by a max temperature threshold due to heat created between two primary factors - speed and load. Increased tire pressures (up to the tire max pressure as dictated by tire construction) controls the heat generated.

RCTIP and stock pressures recommended here are good to the full GVWR of the 200-series AND to the max speed of the chassis or tire rating.

From that, some key points
- Reducing max speed reduces required pressure - this is why we can run far lower pressures when off-road
- Reducing load reduces required pressure
- Increasing load above GVWR requires higher RCTIP pressures

That last point, for those that have built heavier than GVWR, should really run higher recommended pressures. Saving grace is most of those builds aren't running around at max speed. And of course, all these recommendations have internal margin. When I tow, I do increase my tire pressures 5-8 PSI higher than just RCTIP or derived pressures. It has the added benefit of more sidewall stability. Tailoring pressure upwards is always a reasonable thing to do.

 

[linuxgod]

That last point, for those that have built heavier than GVWR, should really run higher recommended pressures. Saving grace is most of those builds aren't running around at max speed. And of course, all these recommendations have internal margin. When I tow, I do increase my tire pressures 5-8 PSI higher than just RCTIP or derived pressures. It has the added benefit of more sidewall stability. Tailoring pressure upwards is always a reasonable thing to do.

For the masses reading this, technically speed and load ratings are separate. If your C load tires are rated at 2500# at 50psi and 100mph max you shouldn't run 3000# on them but drive only 75 to compensate. Overloading (or underinflating) can cause a failure regardless of speed. But I get your point that both weight and speed affect the amount of heat generated and so there's likely more tolerance to (slightly?) exceeding the weight if you're not also generating the maximum amount of heat from tire speed too.

I do (or did) the same PSI increase, from my normal 38psi I run up to 44psi when towing. After once weighing when towing in with the rear axle at almost 5000# I've since gone further and this summer I ran my E-load tires at 48psi when towing. Why 48 psi? Pseudo-scientifically because 4960# is 2480# per tire if it's perfectly evenly balanced left-to-right; however much like how you down-rate a dual axle trailer by 10% I did the same because there's no guarantee my left-to-right weight is perfect. As such 2480 * 1.1 = 2728# and if I use that Toyo table I'd need ~47.5 psi. Also Icon re-rated my wheels to 2750# load capacity which by the table would be 48psi, so that feels like my absolute max. I'd gladly take input here though.

Background from a weigh-in I did in 2018. Fully loaded cruiser (2 adults (300#), 3 kids (240#), 1 big dog (75#), full cooler )75#?), 2 kayaks on the roof (70#) and mostly loaded trailer (normal gear, propane, battery, 2 bikes in the front of the trailer and 3 in the rear). I've got sliders, front bumper, and roof rack plus a heavy tool bag (50-60#) in the trunk

Front Axle: 3500
Rear Axle: 4960
Truck weight: 8,460 (with WD hitch)
Trailer Axles: 5140
Total weight: 13,600

 

[NY2LA]

Correct. Highway speeds flex the tire carcass at such high frequency heat builds up and if not kept in check can cause the tire to disintegrate. More pressure generally limits this heat buildup.

I am one of the few that would suggest looking a different direction with your tires.

Your KO2s require 40+ psi to operate correctly, whereas the stock ones were 33. This alone can make a large difference in something called tire spring rate. Gaijin shows how to do the calculations above, but ultimately the reason is that P-metric tire is less substantial and generates less internal friction, so it doesn’t need as much pressure to get the same load capacity.

I was very unhappy with how my stock suspension handled LT285/65r18 KO2s, between the harshness from the increased pressure and the much higher unsprung weight than stock. I eventually switched to a 17” rim so I could run P285/70r17 p-metric toyo ATs, and was very very happy with that setup. This was actually slightly larger diameter than the LT285/65/18, around the same weight as the stock P285/60/18 road-oriented tire on stock wheels, and allowed less than 30psi rctip which does a great job of isolating harsh pavement hits. Yes a p-metric tire is less robust off-road, but I haven’t been easy on them and they’ve done great. No, the tread lugs aren’t as aggressive as the toyo AT2 in LT-metric flavor.. it looks less “cool”, but the ride quality and very noticeable mileage benefit make up for it. And I try to avoid deep mud anyway.

Which isn’t to say you should necessarily switch to 17s, but that there are other tire construction options that might do what you want well. Thing is not many AT tires are offered in p-metric flavor.. and your sizing will likely be limited.

Otherwise I’m another happy king 2.5 customer. The vastly improved damping ability should allow me to switch to a LT-metric tire and not suffer the ride quality penalty, and that opens up more tire size options like 285/75/17 which I think will be the biggest I’ll go.

For the record the shock shaft is not hard to protect, and having it pointing down allows for front remote reservoirs that have a few benefits, the 2 biggest of which are improved shock oil cooling and taking up less space within the main shock body, which allows for a larger piston and more shaft travel for a given shock length. Plus, while expensive, kings are fully serviceable and to some extent shock shaft damage isn’t even the end of the world. See filthy’s king rebuild video on YouTube. I plan to do my own rebuilds when the time comes.

Your question about supple damping for road and progressive for big hits immediately made me think of king’s internal bypass setup, but when I asked they don’t offer it on our front shocks.

Great food for thought. Thank you.

 

[MTKID]

This is a new concept for me. Would these help smooth out bumps on paved roads? Do you keep them on when going off road?

These replace your small factory bumpstops with one that is thicker and a softer compound material. The factory bumpstops will engage only at the very end of the shock stroke/uptravel in order to prevent any harsh impact of suspension components and to prevent damage to the actual shock from compressing too far and too abruptly. The thicker aftermarket ones will typically engage sooner in the travel but because they are softer, they typically help soften the impact of larger bumps. There is a little difficulty in determining how much less up travel is experienced by using these because you have two types of impacts, both fast and slow. If designed well, a fast impact would likely compress the bumpstop to the normal OEM position, but during a slow crawl, you probably don't have enough energy to compress it completely, therefore you lose a little up travel.

And yes, they remain on your vehicle all the time.

The front of our 200's use two bumpstops that are the same part in two different locations. They protect the front suspension progressively, albeit harshly, with the front most impacting first, then the rear. Sorry the part is kind of in the shadows in the middle of this photo. When you look at the last photo you should be able to find the part in this first photo...

These ones are the Durobump, however they are intended to be installed in the rear of the two locations (on the front suspension) instead of the front, with less impact on up travel. He has warned me that this location could damage their bumpstop, however I am actually trying to reduce up travel a little to run a larger tire. I also have my "normal" height AHC of the 570 adjusted up a little from the common sensor adjustment. These were immediately noticeable to me, however if you are at stock height, I would recommend installing them in the rear mounting position as he intended. With them in the correct rear position, I think you may only notice them on larger dips and bumps rather than small, but I can't be certain.

They are inexpensive enough and easy enough to install that I think they are a great stepping stone to try. Ultimately, an adjustable coilover will be vastly superior when tuned correctly.

 

[bloc]

The way Ben at Filthy described it to me, if the shock valving is appropriate for how you are using it you shouldn't be hitting the bump stops in the first place.

Also, depending on the shock setup you use those might not work in the front position. As it is I had to grind off some of the metal outer ring of the stock bump and even some of the bracket that supports it, to clear the spring on my King 2.5s. I'll try to get a picture of how close everything is once you put a large spring and shock in there, but for now I only have a picture of the metal removal area. Before alignment even this had ~1/16" of room to the coil at full droop. After alignment that went up over 1/4".

 

[MTKID]

If the budget allows for it, King’s sound like the best solution and I’d agree with Ben’s assessment as long as you aren’t catching air in the truck on a regular basis like in the Middle East (which I am very impressed with).

Also, it seems with the number of guys running Kings, you could get the spring rate correct the first time and have a lot of assistance with tuning

 

[NY2LA]

So for the folks who recommend Kings, would you also suggest a tire change? I do love the tractions of the KO2s. For me, they've been great in the rain on paved roads, and great in my 3 limited offroad tests so far (loose dirt, light mud, wet forest trails). Or would I, even with the Kings, still be experiencing a rude ride over small bumps on paved roads?