I have a 1969 Ford Camper Truck that no longer has the camper on it.

I use it for hauling bulky, heavy stuff over short distances, often on dirt roads. (It is always in VT, not anything I would bring into the city.)

I am one of two drivers who use it and I'm having a disagreement with the other driver about the tire pressure. Need advice.

The tires are embossed max. 85 psi.

They are currently at 40 psi.

I think they should be at 85 or, at least, a lot higher than 40 psi. It feels to me like the truck is fishtailing all over the place, especially when I've got something (anything, no matter what) in the back and am going arouind a curve on a dirt road. Lots of those curves are on hills at the same time.

BUT, the argument is that you only increase the tire pressure when the load is heavy.



I don't like this argument because (1) What do I have to do, weight each load I put in? (2) The truck itself is pretty heavy and I often get the fishtailing feeling when it is empty. (3) I've been in Hilda behind the truck when it is being driven and it looks like the tire walls are bulging out. This is baaaaad to me as it seems like this would increase the chance of a blow out before the tread wears. (4) It means I've always got to be changing the tire pressure and I rather just put it up high and drive it right.

Maybe I'm totally coming from left-field. Maybe the fishtailing is due to other things.

Opinions, please.

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Growth for the sake of growth is the ideology of the cancer cell. -- Edward Abbey

1984 300SD: Hilda the veggie vundercar

PS: I have always inflated tires to recommended maximums, but I'm getting a lot of people IRL telling me not to in this case. This is the first truck I've ever maintained so I've never thought about it before.

Haven't been able to locate a manual for this one, either, so don't know manufacturer's recommendations.

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Growth for the sake of growth is the ideology of the cancer cell. -- Edward Abbey

1984 300SD: Hilda the veggie vundercar

the only reason i can think to leave the pressure at 40psi would be that the low pressure is better for off-road use. it puts more rubber on the groung and gives better traction. more pressure would give you better milage and less wear esp. on pavement. i would just compromise at 60psi.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

The "Max." pressure stated on the sidewall of the tires is just that, the maximum safe inflation pressure for that tire. The 'normal' operating pressure is usually below that maximum. If these are truck tires (which I'm sure they are with an 85psi max.) then I would try running them at 60-65psi for regular use.

My W126 has tires with a max. pressure of 44psi but I run them at the manufacturers recommended 29 front 30 rear or a couple of psi above. Increasing the pressure will give a harsher ride, sharper turn-in (less flex in the tire) but worse grip as the contact patch reduces as the pressure goes up.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

I just read where someone stated this with the opposite conclusion. Their thinking was that the increased pressure (tire patch to road surface) would reduce slip. That caught my attention because I always thought the way you stated it. There would be diminishing returns in each direction I am sure, but where is the sweet spot?

There is no question as to whether or not reducing the pressure increases the grip - it does. Anyone who has ever driven offroad knows this and most of us decrease tire pressure for offroad use and then reinflate when finished (having an onboard compressor helps a lot). The very notion that higher pressure in the tire pushes it harder to the ground is absurd. Also, heavier loads usually dictate lower tire pressure, ie to keep from blowing a tire you would decrease the pressure a little. Dervman's advice is on the money. That said, the quality of your tires will have a lot to do with the side to side action. We run 10 ply sidewall tires on our F350 (single rears - not dually) and it holds great. What are you running on that truck? Tire quality matters!

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

You didn't follow my post. It is not about air pressure in the tire, it is about contact patch on the ground and loading. It is almost out of context in this thread and easily misunderstood.

That aside, on to another point.
This needs to be checked, the max loading is printed on the side of the tire and on every tire I have ever checked the max loading capacity is at the max tire pressure. Check this with your tire manufacturer, don't trust this to advice on a forum. A blowout with a max load can leave you in little pieces somewhere.

Also be aware that if you reduce the pressure on dual tires, they may touch and you are almost guaranteed a tire failure.

I don't get it. What does that mean "increased pressure (tire patch to road surface)..."? As the pressure in a tire increases, the tire patch, or footprint of the tire actually in contact with the ground (as I define it), decreases. So the person who wrote this must think as I stated in my last post, or else they think that less tire contact with the ground reduces slip. Either is absurd. What am I missing?

My understanding of the max load at a certain pressure is that it is the maximum load you can carry at that pressure. It does not mean that in order to carry that load you must be at that pressure (Is that what you think?). The other advantage of deflating for offroad is that you can slam your tires into things (like stumps and rocks) without blowing them out. The decrease in the internal tire pressure means you can apply more external pressure without exceeding what the tire can handle. Don't worry about me having a blowout - I would never run a tire anywhere near max pressure with a significant load. I always decrease the tire pressure (just like everyone I know). When I take my Jeep offroad, I run my tires at 25psi, on road at 42. If I were to take it off road at the rated max press (50 cold), I would not expect to make it back.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

To properly inflate your tires get the tire manufacturer's load/pressure chart. Every chart I have ever seen says more load = more pressure. The numbers on the side of the tire are for max load and the tire is rated to carry that load only at that max pressure. Always check pressures with cold tires.

Reducing pressure to carry more load is backward from everything I have ever seen. Once again, check the tire manufacturer's load pressure chart.

The max. load pressure on a tire is for heat concerns. Lower pressures create more flex, with a heavy load, and in turn more heat. You should bump up your pressures if hauling "heavy" loads. But only to "max" if you are hauling that much weight.

For every day driving, the rear pressures don't need to be much higher than the front (4-5 psi). In fact, too much pressure while unloaded can be detrimental also, making the tires more susceptible to impact damage.

(Lessons learned from towing a 5th wheel trailer.) If you would feel more comfortable speaking to someone face-to-face, talk to the service folks at an RV shop. They are used to towing close to max. ratings and may even have a published chart of recommended ratings.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Those are bias ply tires, most likely 6 or 8 ply, and should not bulge under load. 40 psi is fine for an empty truck, but you need more pressure with load, and that really means more pressure all the time since you're not gonna change the tire pressures every time you load or unload.

You need the pressure low enough to allow you to safely drive the truck empty and high enough that the tires don't bulge much when fully loaded -- else they will seriously overheat and blow out -- not fun.

Tires with low pressure give more "grip" offroad because they deflect more at the tread and more tread stays in contact when you run over rocks and such.. On the highway, you will get the opposite effect because you will get more sidewall flex and the tread will eventually lift in the center, reducing the contact patch. The soft sidewalls will flex too much, both rapidly overheating AND allow the tire to roll over badly, either dismounting it or lifting the tread. You get the best road traction at the correct pressure, wet, dry, ice, or snow.

Tires work by friction, and the amount of side force generated will be a function of friction coeffecient (determined by the rubber composition), tire patch size, and load -- the stickier the tire, the larger the tire patch, and the greater the weight, the more side force can be generated. However, some of these relationships aren't linear -- if the load and slip are great enough, the rubber melts, and melted rubber is a LOT slicker than unmelted rubber.

Complicating this is the fact that the contact patch, the part of the tire actually in contact with the pavement, changes shape and size with tire pressure and side load. At ideal pressure, the contact patch will extend from side to side and in an arc from edge to edge at the front a rear. Overinflation will lift the sides off the road, leaving a thinner and thinner strip down the center of the tire in contact as pressure goes up.

Underinflation will cause the fairly stiff tread to twist up in the center as the sidewalls bulge out and down, again reducing contact area. Underinflation will make the tire LOOK like more is in contact with the road, but the center is essentially out of contact once the sidewalls bulge much.

Side pressure will lift the unloaded side of the treat until the tire starts to slide (or pops off the rim).

Radial and bias ply belted tires have more stable tread then unbelted tires, and hence have larger contact patches. Radial tires have much more flexible sidewalls and always have belts, so keep more contact area.

Pretty complicated, I know --

My advice: Contact a good tire supplier who handles truck tires and ask (or an RV place), they will set you right.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

You are correct. I've done a lot of looking around about this and your statement is correct about pressure and load. I guess I'm going to end up learning from this thread. I was misunderstanding the increased flex, as SD Blue wrote, and interpreting my own offroad experiences and observations. However, it is still confusing that a high pressure shock, such as a pothole or rock/stump offroad will blow a tire at the higher end of the inflation range faster than one at the lower. Maybe psfred, or someone, can offer an explanation? Maybe there are several variables (heat, internal pressures, external pressures, etc.) Apparently, it's more complex than I thought. An interesting thing I discovered while researching is that tire manufacturers are hesitant to provide recommended pressures and always default to the vehicle manufacturer. It's possible that sohj's tire pressure is too low for just the weight of his truck then, right? Where does one find pressure/load charts? (BF Goodrich Mud Terrain T/A 31x10.5 specifically) Sorry for causing or at least contributing to the confusion.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

Sorry for steering you wrong (slightly) on truck tire pressures. The front tire pressures need to be HIGHER than the rear while unloaded.

Here is a good formula:

Axle Weight / # of tires on axle = Per Tire Weight

Per Tire Weight / Tire Weight Rating (sidewall) = Ratio

Ratio X Max. Press. Rating (sidewall) = recommended tire pressure

I know from this formula you have to weigh the axles of the truck. However, that is probably a one time thing for what you are doing. For most pickup loads you can usually estimate the weight close enough. Also, higher sustained speeds usually mean higher tire press. (note the higher press. for the Benz at higher speeds)

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Sam

84 300SD 350K+ miles ( Blue Belle )

Lower tire pressure will allow the tire to deform around a sharp object such as a rock rather than forcing the object through the tire, at the cost of broken cording in the sidewall or tread -- they really aren't that flexible. They will also compress more without exceeding the rupture strength of the body of the tire, at the expense of bent rims and cut sidewalls.

Tire won't fail right away, but will develop "lumps" from broken cording in the body or tread.

It's really best to use off-road tires for off-road use -- the big nobby ones with 6 or 8 ply bodies, they are designed for such abuse. They are not very good for road use, but I don't believe it is possible to optimize a single tire for such very different uses.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I am making the assumption that these are modern radial tires with a load range of "E" which would explain the 85psi max. They will look slightly "bulged" in normal use when inflated and loaded correctly. Most E rated tires will take a load of at least 3000 lbs each. My truck runs E rated tires and the manual calls for 55psi rear, 45 front when unloaded. 85psi rear, 45 front when fully loaded. Stock empty truck weight is 5500 lbs. With my custom bumpers, tool box, etc. it is 6100 lbs. The max GVW is 8600 lbs and I have had it loaded to that and past with the rears tires up to 85psi. With 12,000+ lbs of capacity in the tires there is quite a safety margin. If I am travelling any distance with a moderate load I will increase the rears to 65-70psi. Regarding running empty I have found that 50psi in all the tires works fine for general use. Cranking up the pressure to 85psi and running empty will result in a very unpleasant rough ride. You say you haul "bulky heavy stuff" What do you haul and how often? How heavy is it? To me heavy is 25 80lb bags of concrete or a load of firewood stacked to the sky. Then you need to think about 85psi in the rears. Anything else in between, 500-1000lbs 60psi or so should do you fine. 40psi is definitely too low and you will damage the tires. RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K