P metric vs lt bf goodrich load c (1 Viewer)

[highfructose]

Which would be safer, running a 285 70 r17 bf goodrich trail terrain at 30 psi…or running a bf goodrich k02 load range c of the same size at 30 psi?

The former calls for like 26 psi and the later 40.

Is the p metric tire really more robust at 30 psi? Does anyone have experience running light truck tires around 30 psi?

 

[TeCKis300]

The reality is I don't believe you'll run either at 30PSI.

RCTIP is falsely assumed to address the whole picture...it only talks to load handling. It does not address tuning for ride, handling, and stability. For either tall profile 17" tire, you'll likely run 35PSI+ for sufficient sidewall stability. Otherwise the sidewall will roll-over too easily in corning (e.g. firestone).

Pressure doesn't suggest robustness. As was mentioned in the other thread, it's to manage heat of the tire carcass against load. LTs have more mass that can heat-up therefore need more air to minimize deflection and heat generation. Overall heat against the temp threshold is further dependent on variables of environmental, speed, and use. The LT tire surely will have more overall robustness at any pressure.

For the LC, there's no need to run 40 PSI for the LT tire. Use the LX interpretation for minimum pressure - 35PSI. The RCTIP methodology put forth has always been inherently wrong because the manufacturers original recommended inflation pressure never only assumed load handling. It's very possible to run lower than 35PSI still, but as said earlier for handling reasons, 35PSI may be a great starting point.

 

[highfructose]

The reality is I don't believe you'll run either at 30PSI.

RCTIP is falsely assumed to address the whole picture...it only talks to load handling. It does not address tuning for ride, handling, and stability. For either tall profile 17" tire, you'll likely run 35PSI+ for sufficient sidewall stability. Otherwise the sidewall will roll-over too easily in corning (e.g. firestone).

Pressure doesn't suggest robustness. As was mentioned in the other thread, it's to manage heat of the tire carcass against load. LTs have more mass that can heat-up therefore need more air to minimize deflection and heat generation. Overall heat against the temp threshold is further dependent on variables of environmental, speed, and use. The LT tire surely will have more overall robustness at any pressure.

For the LC, there's no need to run 40 PSI for the LT tire. Use the LX interpretation for minimum pressure - 35PSI. The RCTIP methodology put forth has always been inherently wrong because the manufacturers original recommended inflation pressure never only assumed load handling. It's very possible to run lower than 35PSI still, but as said earlier for handling reasons, 35PSI may be a great starting point.

Do you know what temperature the carcass might get to that is inappropriate?

Or can one assume temperature based on the rise in tire pressure? Like more than 5 psi rise on the highway is bad, less than 5 is good?

 

[TeCKis300]

Do you know what temperature the carcass might get to that is inappropriate?

Or can one assume temperature based on the rise in tire pressure? Like more than 5 psi rise on the highway is bad, less than 5 is good?

It likely varies. General suggested it's somewhere in upper 100 degree range. And low to mid 200s where structural damage begins. Working ranges are likely no where near those and probably something less than 150-175 degrees, with margin above.

Temp rise may be an okay cross check and what you suggested sounds reasonable. It's not a be all end all strategy either as it depends on environmental conditions which can't completely be controlled, like how cold the tires are at start, versus afternoon temps, and the type of use.

 

[highfructose]

It likely varies. General suggested it's somewhere in upper 100 degree range. And low to mid 200s where structural damage begins. Working ranges are likely no where near those and probably something less than 150-175 degrees, with margin above.

Temp rise may be an okay cross check and what you suggested sounds reasonable. It's not a be all end all strategy either as it depends on environmental conditions which can't completely be controlled, like how cold the tires are at start, versus afternoon temps, and the type of use.

Thank you for the range.

I’m going to use a laser temp gauge to check tire temps with various pressures.

Cold Psi 30 and cold psi 34 on the same drive at the same time of day with lt275 60 r20 both add about 5 psi during a 60 mile drive. I assume the same amount of heat is being generated. I’m going to measure the heat of the tire to see where I’m at. It’s very warm to the touch.

My goal is comfort and handling on road (and safety). 30 feels pretty good to me, though I don’t generally drive more than 60 and don’t have a lot of curves…feels pretty similar to what the vehicle felt like w oem tires/pressure.

Part of me wants to just get p metrics and run it at 27-29 as indicated, but there are so many reviews saying p metrics get damaged easily compared to lt. It’s hard to justify running at 40 psi when the equivalent p metric calls for 27. Just doesn’t seem right.

 

[TeCKis300]

Sorry if I'll be direct.

There's no need to monitor temp for reasonable pressures.

Anything 30 psi or less is not safe regardless of RCTIP.

If you value ride quality, KO2s even in P-metric wouldn't be a good choice.

 

[bloc]

Laser temp gun is absolutely NOT the way to go about this. You'd get surface temperature, not what is happening within the core of the tire tread, and that is what matters.

There are lots of strategies involving chalk on the tread, monitoring change in pressure as tire goes from cold to warm, just following mfg guidelines.. but you won't see monitoring tire temp as a good recommendation anywhere.

 

[highfructose]

Sorry if I'll be direct.

There's no need to monitor temp for reasonable pressures.

Anything 30 psi or less is not safe regardless of RCTIP.

If you value ride quality, KO2s even in P-metric wouldn't be a good choice.

P metric in 275 60 r20 calls for 27 psi. For 285 70 r17 it’s 26 psi I think.

Ride comfort seems to closely reflect tire pressure…the squishier the tire the squishier the ride.

Chalk test on lt275 60 r20 at 31 psi didn’t indicate it as under inflated. Considering a p metric of the same size calls for 27 psi this isn’t surprising.

It odd to me that the same size tire calls for 27 psi for p metric and 40 for lt. That’s a significant difference.

Is it excess tire contacting the pavement with an under inflated lt tire that causes excess unmeasurable heat inside the tire?

 

[TeCKis300]

With tires, it's a balance of concerns. Have to understand what is being balanced when modifying.

I'll caution again too low of pressures in a tall profile tire in road/freeway use. Firestone/Explorer is a good example. It likely met criteria including load handling and chalktest. But in an evasive moose test type maneuver, stability is the concern with possibly a rollover risk. Sure, you can do it based on your use and driving style, but just be aware.

On the other end, your intuition to question 40 psi for LTs is correct. RCTIP is bunk and it's a false requirement. Because the stock tire inflation pressure was never only about load handling. The factory pressure likely was tailored up for dynamic handling and stability to the earlier point.

The LX RCTIP interpretation is closer for full load handling, speed, and environmental temp. Yet even that has margin as most don't carry a full load, at 112 MPH, in extreme temperature days.

So 35PSI and you can tailor down to a degree if you're not using the full parameters for heat loads like 60 MPH.

Then tire models have their own sidewall stiffness that contributes to ride quality. Trying to compensate for a stiffer riding tire with lower inflation pressures is a compromise. As inflation effects further considerations like MPG.

 

[bloc]

Is it excess tire contacting the pavement with an under inflated lt tire that causes excess unmeasurable heat inside the tire?

It’s not directly the contact with the road, it’s what the tire has to do to itself to have more tire on the road. With a lower pressure the tire flexes more with every rotation and this means more heat within the structure of the tire.

It’s the same base reason a LT tire needs more pressure than a P for a given load rating.. even if the overall tire flexes/deforms the same amount per rotation, the more robust aka thicker construction of the LT tire means there is more friction within the tire/tread/sidewall/carcass itself than a P.. so you need more pressure to reduce sidewall/tread flex, to try and offset that thicker construction and keep temperatures in check.

Keep in mind those load tables aren’t about the tire physically failing with the vehicle sitting still as soon as you add 1# over the limit. They are about keeping the tire safe in light of many more factors, like Teckis said. Max rated speed, on a hot road with plenty of imperfections, hauling up to the max load the tire pressure is set for. And that’s simply to keep the tire itself from coming apart.. then you should consider handling dynamics like a marshmallow sidewall if you run those 285/70r17s at 26psi.

If you are going to run less than the mathematically derived RCTIP equivalent when you switch tire types, it would be wise to verify the pressure you intend to run keeps the tire load capacity above (even marginally) the weight you are likely to actually put on that tire/axle.

Also know that the energy that allows you to create friction within the tire that heats up a LT model is ultimately coming from fuel burned.. the more you lower pressure with a LT tire the worse your mileage will be. This is on top of LT tires lowering mileage significantly compared to P’s even when fully inflated to the rctip.

 

[Itsky]

I have 285/70/17 load c bfg ats and OME and the ride at 39-40 psi is fantastic. Have zero complaints. I personally prefer it at ~39 psi than 36.

Just a data point.

 

[highfructose]

I have 285/70/17 load c bfg ats and OME and the ride at 39-40 psi is fantastic. Have zero complaints. I personally prefer it at ~39 psi than 36.

Just a data point.

Do you think it’s because 40 psi in a 17” rim has loads of air in it vs 40 psi in a narrower thinner profile tire of the same height in a 20” means there’s more total squish occurring in the 17” rimmed setup? You might consider driving over some speed bumps in a large parking lot at 40 psi…then airing down to 30 to hit them again. 40 might not feel so good after that.

30 psi in my lt275/60r20 feels perfect when hitting bumps…40 was jarrin... 35 is borderline. The vehicle actually tracks better at 30 and the internal psi of the tires doesn’t increase as much while driving with the 30. I can only assume this is because more of the tire is touching the road at 30 thus dispersing heat…rather than riding on the center of the tire with a slightly overinflated tire at 40 psi…causing both squirreliness in the handling and too much weight being pressed in the middle of the tire.

My suspicion is that the 40 psi recommended is excessive because the engineers, for some goofy reason, require a higher load capacity given the exact same numbers when using the same tire on a truck vs passenger car…I don’t understand how they calculate things differently based on the vehicle rather than the actual load.

 

[highfructose]

With tires, it's a balance of concerns. Have to understand what is being balanced when modifying.

I'll caution again too low of pressures in a tall profile tire in road/freeway use. Firestone/Explorer is a good example. It likely met criteria including load handling and chalktest. But in an evasive moose test type maneuver, stability is the concern with possibly a rollover risk. Sure, you can do it based on your use and driving style, but just be aware.

On the other end, your intuition to question 40 psi for LTs is correct. RCTIP is bunk and it's a false requirement. Because the stock tire inflation pressure was never only about load handling. The factory pressure likely was tailored up for dynamic handling and stability to the earlier point.

The LX RCTIP interpretation is closer for full load handling, speed, and environmental temp. Yet even that has margin as most don't carry a full load, at 112 MPH, in extreme temperature days.

So 35PSI and you can tailor down to a degree if you're not using the full parameters for heat loads like 60 MPH.

Then tire models have their own sidewall stiffness that contributes to ride quality. Trying to compensate for a stiffer riding tire with lower inflation pressures is a compromise. As inflation effects further considerations like MPG.

The explorer thing is interesting. They bumped the recommended air pressure from 26 to 30 as a result of that fiasco…and then later lowered the explorer and, I think, widened its track…and then added rollover stability control…

It makes me want to put 0 offset wheels on my car to get maximum width…to offset the slight sensor lift I have as well as the slightly excess weight due to the bumper and tools in the back.

And I’m considering 295 70 r17 since the recommended cold pressure for those would be 35.

basically getting shortest tire that’s recommended to be inflated to 35 psi.

maybe more volume of air will mean more squish in the 295 70 r17’s at 35 psi vs 275 60 r20 at 40. Or even 30 for that matter.

 

[TeCKis300]

My suspicion is that the 40 psi recommended is excessive because the engineers, for some goofy reason, require a higher load capacity given the exact same numbers…I don’t understand how they calculate things differently based on the vehicle rather than the actual load.

Don't chalk this up to engineers. It was a first approximation then falsely made into gospel by certain individuals. That then is blindly followed because it got reiterated so many times.

Real engineers balance trades, make assumptions, test and iterate with feedback, to land on recommendations. Even then, final numbers are never gospel. And subject to tailoring for use case, priorities, or change of assumptions. There's never only one answer for complex systems.

Credit to you for applying some common sense and asking questions.

The explorer thing is interesting. They bumped the recommended air pressure from 26 to 30 as a result of that fiasco…and then later lowered the explorer and, I think, widened its track…and then added rollover stability control…

It makes me want to put 0 offset wheels on my car to get maximum width…to offset the slight sensor lift I have as well as the slightly excess weight due to the bumper and tools in the back.

And I’m considering 295 70 r17 since the recommended cold pressure for those would be 35.

basically getting shortest tire that’s recommended to be inflated to 35 psi.

maybe more air will mean more squish in the 295 70 r17’s at 35 psi.

A 295 may potentially ride better between air pressure considerations, and the more curved fitment profile of the sidewall on the same width wheel.

Regarding offset - suspension is another area with balances and compromises. 0 offset will do far more negative things to steering stability, handling, and durability to name a few. Can't just widen track the way you're suggesting as offset is integral to suspension geometry. Though a 33.3" tall tire that is a 295/70R17 would dictate an incrementally lower offset of about +45.

 

[highfructose]

Don't chalk this up to engineers. It was a first approximation then falsely made into gospel by certain individuals. That then is blindly followed because it got reiterated so many times.

Real engineers balance trades, make assumptions, test and iterate with feedback, to land on recommendations. Even then, final numbers are never gospel. And subject to tailoring for use case, priorities, or change of assumptions. There's never only one answer for complex systems.

Credit to you for applying some common sense and asking questions.



A 295 may potentially ride better between air pressure considerations, and the more curved fitment profile of the sidewall on the same width wheel.

Regarding offset - suspension is another area with balances and compromises. 0 offset will do far more negative things to steering stability, handling, and durability to name a few. Can't just widen track the way you're suggesting as offset is integral to suspension geometry. Though a 33.3" tall tire that is a 295/70R17 would dictate an incrementally lower offset of about +45.

When I researched the explorer thing and they widened the track to help with rollovers…and also lowered it…and I coupled rhat with some forum posts by Porsche drivers who said a wider track due to more offset would lead to better stability yet worse steering response, I surmised that if you make a vehicle higher (lift) or heavier (bumpers), you probably want a wider track.

What I might expect to lose in steering responsiveness, I gain in reduction of a tendency to rollover. After all, I don’t want a narrow track width and twitchy steering…if anything I want the widest track possible and the most sluggish steering…so, at least currently, a zero offset seems to be the widest it’ll get while still being able to stuff a 34 (34 being basically the biggest that’ll fit in the spare tire area).


If I want to counter the tippiness of a lifted heavier vehicle, wouldn’t I want the widest offset possible? By my math a Ford F-150 raptor with 35’s has a track width of 74 to our 64 for our 31.5’s…and with 0 offset wheels we can get to 69”. Back of a napkin math says that wouldn’t be bad for up to 34’s.

Wider offset sacrifices steering responsiveness but gains tracking and stability right?

 

[Itsky]

Do you think it’s because 40 psi in a 17” rim has loads of air in it vs 40 psi in a narrower thinner profile tire of the same height in a 20” means there’s more total squish occurring in the 17” rimmed setup? You might consider driving over some speed bumps in a large parking lot at 40 psi…then airing down to 30 to hit them again. 40 might not feel so good after that.

30 psi in my lt275/60r20 feels perfect when hitting bumps…40 was jarrin... 35 is borderline. The vehicle actually tracks better at 30 and the internal psi of the tires doesn’t increase as much while driving with the 30. I can only assume this is because more of the tire is touching the road at 30 thus dispersing heat…rather than riding on the center of the tire with a slightly overinflated tire at 40 psi…causing both squirreliness in the handling and too much weight being pressed in the middle of the tire.

My suspicion is that the 40 psi recommended is excessive because the engineers, for some goofy reason, require a higher load capacity given the exact same numbers when using the same tire on a truck vs passenger car…I don’t understand how they calculate things differently based on the vehicle rather than the actual load.

Idk. It works really well on pavement. No downsides. Good handling on dry, wet and snowy pavement. When I go on dirt I air down.

 

[TeCKis300]

When I researched the explorer thing and they widened the track to help with rollovers…and also lowered it…and I coupled rhat with some forum posts by Porsche drivers who said a wider track due to more offset would lead to better stability yet worse steering response, I surmised that if you make a vehicle higher (lift) or heavier (bumpers), you probably want a wider track.

What I might expect to lose in steering responsiveness, I gain in reduction of a tendency to rollover. After all, I don’t want a narrow track width and twitchy steering…if anything I want the widest track possible and the most sluggish steering…so, at least currently, a zero offset seems to be the widest it’ll get while still being able to stuff a 34 (34 being basically the biggest that’ll fit in the spare tire area).


If I want to counter the tippiness of a lifted heavier vehicle, wouldn’t I want the widest offset possible? By my math a Ford F-150 raptor with 35’s has a track width of 74 to our 64 for our 31.5’s…and with 0 offset wheels we can get to 69”

Wider offset sacrifices steering responsiveness but gains tracking and stability right?

While +45 isn't the single answer, you could go as far as probably +25 without too much impact based on others experiences. I can tell you for sure 0 offset will create more fitment challenges against the fender and body mount that you probably couldn't run a 34" tire easily.

When OEMs widen track, they do it comprehensively. The spindle geometry with the UCA and LCAs will be lengthened and modified. Offset may or may not even change. There's many considerations including roll center, scrub radius, steering axis, caster/camber/toe curves, and more. Even roll stiffness between front and rear axles needs to be tuned.

That's the challenge here. Can't isolate and change singular factors without consideration for other considerations in the system.

 

[highfructose]

While +45 isn't the single answer, you could go as far as probably +25 without too much impact based on others experiences. I can tell you for sure 0 offset will create more fitment challenges against the fender and body mount that you probably couldn't run a 34" tire easily.

When OEMs widen track, they do it comprehensively. The spindle geometry with the UCA and LCAs will be lengthened and modified. Offset may or may not even change. There's many considerations including roll center, scrub radius, steering axis, caster/camber/toe curves, and more. Even roll stiffness between front and rear axles needs to be tuned.

That's the challenge here. Can't isolate and change singular factors without consideration for other considerations in the system.

That makes sense.

If seen some of your other posts explaining how even if a zero offset might stuff, if you hit something with both wheels at the same time, due to the poke, those wheels are driving up into the fenders and causing damage. That makes sense.

I’m starting to think you’re right that 25 offset is the most aggressive you can go without the nevessity for bump stops.

Would anyone happen to be able to clearly explain, as if to a 5 year old, what the following stuff means as it relates to load capacities and light truck tires, p metric tires, etc?

“
OEM Size (285/50R20 SL) Load Capacity: 2291 lbs. @ 33 psi.
Passenger type tires are fitted with a 10% overage in load capability when used on SUV's, Light Trucks, and Vans because of the difference in load handling. This load overage can be removed when changing to an LT type tire.

That brings the effective load capacity to acheive down to 2083 lbs.”

How does one “fit” a 10% overage to a tire?

By load handling do they mean weight at higher center of gravity in the vehicle thus more tippy and needing a stiffer sidewall not to flex during maneuvers?

rather than tire separation issues with lt tires requiring higher psi’s, are we really dealing with the necessity of stiffer sidewalls to help with “handling the load” with vehicles that have a tendency to tip and rollover by stiffening sidewalls (and thus effective suspensions)?

Are they saying load capacity and really meaning something more like flexion reduction?

Tire for tire in 33” p metric and lt, the recommended pressure goes from 26-27 to 40. Is this mostly just a stiffening of a sidewall to handle a vehicle with a higher center of gravity (truck, suv, van) so that when it tips under load, the tire doesn’t flex and induce a continuation of the tip into a rollover?

Like are regulators doing something stupid like limiting the alcohol content of liquor in the hopes it’ll reduce people’s tendency to get too drunk…and then we are looking at their alcohol regulations and claiming that liquor over 80 proof is bad for you…when in reality it’s just nonsense government regulation trying to keep people from doing stupid things by creating arbitrary regulations?

And rather than pointing out that people are stupid with alcohol, we are instead saying there’s some intrinsic qualities to 90-100 proof liquor that is unhealthy…in the same way rather than saying an lt tire is actually fine at 30 psi, and likely stronger than a comparable p metric at 30, we are issuing opinions about an unquantifiable excess heat that magically develops in the lt tire at that temperature though not in the p metric.

Are lt tired at 35 psi minimum the equivalent of govt regulations that keep liquor generally at 80 proof?

Because if that’s the case, the people that run oversized lt tires based on chalk test and handling characteristics rather than the minimum 35 psi that lt tires are graded for may have it right…

 

[KZN185W]

....
If you are going to run less than the mathematically derived RCTIP equivalent when you switch tire types, it would be wise to verify the pressure you intend to run keeps the tire load capacity above (even marginally) the weight you are likely to actually put on that tire/axle.
....

May I know where can I get a detailed info on the mathematically-derived RCTIP equivalent? I'm interested in doing a derivation.
Thank you.

 

[highfructose]

May I know where can I get a detailed info on the mathematically-derived RCTIP equivalent? I'm interested in doing a derivation.
Thank you.

You can probably use the calculator here based on whatever your oem size and pressure is.

Tire Pressure Calculator

Easy to use tire pressure calculator. Use our tire pressure calculator to calculate the new tire pressure needed when changing tire sizes.

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