16" or 19.5" wheels and tires?

[GR8ADV]

There is much talk about tire capacity with many threads here. Most of the talk is at highway pressure. The thing I think about is that off road you are likely providing some of the greatest loading challenges to your wheel/tire combo. One thing that does not seem to get talked about is what is your capacity at the aired down pressure? You might want to check the tire capacity of your aired down tires. Just a thought. Maybe I am wrong and this does not matter?

 

[gait]

I have a big spreadsheet with max load at speed and pressure. No problem with my load, just get a bit hot if I drive too fast on low pressure.

 

[GR8ADV]

I have a big spreadsheet with max load at speed and pressure. No problem with my load, just get a bit hot if I drive too fast on low pressure.

So please excuse my ignorance. Is it only static load and speed? It would seem to me if load was an issue then impact factors would be a huge issue regardless of speed. I would expect the impact load factors to be in the order of a 150% ncrease in load if not more. Always open to learn something. Thanks for any insights.

 

[gait]

static and dynamic load, speed, pressure, temperature, traction, sidewall deformation, construction, road surface, gas, humidity, comfort, and no doubt a whole heap other variables. Load, speed and pressure are a useful, practical, pragmatic, approximation.

My materials background (I'm really a metallurgist with a lot of other experience) suggests that impact loading is more of an issue for brittle than ductile materials. Charpy or Izod test if interested.

Thinking about it my mental model of tyres suggests that I lower the pressure to gain traction on sand or mud surfaces. I lower it but not by as much for comfort on corrugations, and I raise it for prolonged high speed motorway driving for temperature and tyre life. Rock crawling I'm a bit mindful of the balance between traction, what may happen when the wheel drops off a rock, and uneven wheel loading, so tend to reduce pressure but not by as much as for sand..

But fundamentally, in my case my tyres (Toyos) are so overrated for my vehicle axle and wheel loading I have trouble keeping the pressures low enough to be comfortable. Restricted by poor (IMHO) regulation.

 

[LeishaShannon]

Gait, is your spreadsheet specific to your tyres or is the theory transferable? I have tyres with a combined maximum load of 9t, the truck weighs half that and I have no idea what pressure I should be using as the charts i've found online don't cover such a small load! I rang Kumho and the bloke said 100psi, before even asking what our weight was!
I have installed a TPMS this week so I can keep an eye on temperature/pressure, they're currently at 80psi and barely get above the temp of the road.

 

[whatcharterboat]

................But fundamentally, in my case my tyres (Toyos) are so overrated for my vehicle axle and wheel loading I have trouble keeping the pressures low enough to be comfortable. Restricted by poor (IMHO) regulation.

Hi Julian..........and your truck is only around 4ton.......is that right?

Hi Shannon, if Julian means the load/pressure/speed table that Toyo provide for the M608z tyres, I have a copy I can email. Actually I will try to post it up here if I can. There are so many calculations and factors on the side of it though. It is more than a simple table. I have the full version on my office computerator so will take a couple of days to get back there.

Regards John

 

[gait]

that's the table John, it took me a bit of time to rationalise what it meant. I'll let you distribute it (thanks). I was given it by ATW after asking too many questions. . My truck is around 4.5mt.

Its a long time since I looked, there are similar, though simpler, tables for quite a few other tyres/manufacturers available.

ETRTO is one of the relevant international organisations. Maybe worth a google.

Here's a generalised speed / load / pressure table derived from ETRTO on a Michelin (India of course ..... ) website.
http://www.michelintransport.in/Home/New-Retreaded-Tyres/Tyre-Basics/Speed-Load-br-Pressure

I interpret them as the max speed / load at a pressure. There's nothing to stop me going slower or lighter. Apart from comfort.

Its a source of amazement and amusement to me that the tyre suppliers don't refer to the tables when offering tyre pressure advice. In my experience the tendancy is to pump up to nameplate pressure at the same time as using the rattle gun on the wheel nuts.

Its also interesting looking at the tyre placard on my truck and cars in general. A single number for tyre pressure is a bit simplistic and IMHO a tad misleading. When it comes to this sort of discussion its probably more or less meaningless. First principles are usually better.

I was introduced to the subject by an OKA driver.

 

[whatcharterboat]

that's the table John, it took me a bit of time to rationalise what it meant.

All I can say if you took you a bit of time, it must have taken me 2 bits. It's why I have been a little reluctant to pass it on. You almost need a masters degree in engineering to interpret it correctly.

My truck is around 4.5mt..

Did I hear you say a long time ago it was 3.9t empty. So 4.5 is pretty much your travel weight?

 

[gait]

the 3.9mt would be before big wheels (added weight) and parabolics (reduced weight) but yes, I can keep it below 4.5mt when it matters. Leaving one rim at home (take a spare carcass plus one rim with tyre) is a good start.

The spreadsheet of M608Z data is a bit complicated ...... and if I recall correctly a few inconsistencies which added to the time.

Looks like I've mislaid my copy. A copy would be nice for the files! I'll be annoyed if I've really lost it as I drew a "simple" chart of speed vs pressure for my load. Had it on the dash for a while until suitably brainwashed.

Interesting that the only puncture I had didn't register a drop in pressure or rise in temperature. The only indication was a week of "tap - tap - tap ...." as the wheel rotated. Like most technology, its useful, but don't become reliant on the tyre pressure monitor - IMHO they are an addition not a substitute. Same category as gps "turn left at ....".

 

[westyss]

I used a different table and interpolated it down to my weight, seems like most tire manufacturers will only post information near the top end of the weight scale for that particular tire, we use interpolation of tables in aviation so cant see why I would not use for for tire inflation.

http://www.michelintruck.com/refere...ins-and-warranties/load-and-inflation-tables/

this one isnt the one I used but may still work out. On the rear tires I use around 60-65 PSI, the front tires I keep lower for comfort and expect the tires to wear a little faster on the outer edges, I guess just the cost of doing bussiness.

 

[gait]

Thanks Westsys. Michelin chairs ETRTO
http://www.etrto.org/page.asp?id=1755&langue=EN

Now I recall, ETRTO want money for their standards.

There's passenger, commercial, and other standards (like earthmoving etc).

My recollection is that speed / load / pressure isn't linear which made interpolation for my load interesting.

 

[ianc]

Just as FYI - I spoke to the Continental tyre helpline and they asked a bunch of questions and suggested that I run my (now replaced) 19.5 285/70 Conti HDRs at no more than 70psi at the rear (about 3.5mt on the axle) when on the road. Any higher and the thread will be distorted with the low load. The HDRs are rated at 2900kg per tyre at (I think) 105psi.

 

[westyss]

Just as FYI - I spoke to the Continental tyre helpline and they asked a bunch of questions and suggested that I run my (now replaced) 19.5 285/70 Conti HDRs at no more than 70psi at the rear (about 3.5mt on the axle) when on the road. Any higher and the thread will be distorted with the low load. The HDRs are rated at 2900kg per tyre at (I think) 105psi.

I dont know if I am taking a really simplistic angle on the whole tire thing, I thought this was a pretty standard way of finding your ideal tire pressure per axle, my tires came with wear indicators and monitoring those for inner verses outer tire wear came to a number that made even wear across the tire.
My front tires will wear faster on the outer edges with the lower pressure I use for comfort, a bit of a sacrifice I guess.

Is this not the way to do it?

When I am heavily loaded for a trip will run the rears at 68PSI.
When lighter 60-65PSI
Fronts get set at 48PSI

 

[yabanja]

Is this not the way to do it?

I think that is an excellent technique.

If setting up pressures on an initial build I would recommend getting an infrared thermometer and measuring temps across the tire tread from the inner to outer edge. You want the temps to all be as even as possible while not exceeding your target temperature. This is how we set up a race car for circuit racing. It will make the tires wear as evenly as possible.

 

[Flys Lo]

There is much talk about tire capacity with many threads here. Most of the talk is at highway pressure. The thing I think about is that off road you are likely providing some of the greatest loading challenges to your wheel/tire combo. One thing that does not seem to get talked about is what is your capacity at the aired down pressure? You might want to check the tire capacity of your aired down tires. Just a thought. Maybe I am wrong and this does not matter?

Low speed work is unlikely to induce a failure unless you strike an object that perforates the tyres carcass. The carcass is less likely to perforate when striking an object at lower pressures (to a point).

Main failure modes on tyres, outside of aforementioned punctures are
Delamination - caused by low pressure for a given weight at high speeds creating high carcass temps
Broken/Bent belts - caused by striking an object, typically at high speeds

The air pressure/weight charts are mostly created to prevent delamination at the tyres highest rated speed at high ambient temperatures.

I dont know if I am taking a really simplistic angle on the whole tire thing, I thought this was a pretty standard way of finding your ideal tire pressure per axle, my tires came with wear indicators and monitoring those for inner verses outer tire wear came to a number that made even wear across the tire.
My front tires will wear faster on the outer edges with the lower pressure I use for comfort, a bit of a sacrifice I guess.

Is this not the way to do it?

When I am heavily loaded for a trip will run the rears at 68PSI.
When lighter 60-65PSI
Fronts get set at 48PSI

That is a good way to do it, although it may take some time to notice you have under/over inflated your tyres if based on actual rubber wear.

I personally draw a thick chalk line across the face of tyre, and drive a couple of hundred metres (~1000ft) and look to see if the chalk has worn off evenly.