2 Maintenance Questions

[zero]

JDE, I am just quoting what the owner passed on to me, you make a good point, after reading your post I agree that at 30 lb the tires should not be rubbing.

It could be that with the change in temps, 30 lb in summer could be lot lower in fall (1 lb/7 deg F). Hence my apprehension about keeping the pressure at factory spec.

Long story short, I am going to recheck my sidewall pressure rating and air them up to max:

it is going to take a while as I have a tiny 800 w generator and a 1 gallon pancake compressor.

I drive my diesel with 60 - 80, I am going to 80 - 80 this summer

You lost me a little. You're going to run 80 PSI in a 185R-14" tire or are you talking about something else?

The "diesel" part doesn't have any specific meaning to me. I run 24 PSI in one of my diesels and 75 PSI in another. One is a VW Jetta and the other is an 8800 lb. GVWR pickup truck.

30 PSI is all the rear tires need to ride as designed with a 5500 lbs. RV dually. Makes no matter if 6 ply rated tires or 8 ply rated tires. It just so happens that Toyota feels no need for 8 ply rated tires on a dual wheel setup. On a single wheel Toyota calls for an 8 ply rated tire. But with cab & chassis fleet trucks - maintenance is likely better than with motorhomes owned by non mechanically orientated people. And even those who ARE mechanics - like me - I don't check my tire pressures very often. I should, but it's kind of a pain in the a*s to check the duallies. I usually run my rears at 50 PSI which is over-inflated but gives some extra safety factor - like if one blows out while going down the road. I also have 8 ply rated tires, front and back.

I assume coach-builders that make motorhomes also give higher than needed PSI specs because they know maintenance might be lacking. Look at the mess Ford and Firestone got into when people were crashing SUVs with underinflated tires and excessive speeds. Ford got bad-mouthed for having correct PSI numbers on the door pillars because they did not have more "margin of error."

[shibs]

JDE,

sorry, my diesel = 08 chevy duramax CC long bed, door pillar says 60 front, 80 rear.

I am going to try 80 in the front also in this summer

As for toyo motorhome, I was going by the door jam when I went to "winterize" it few months back. I run 40 lb in my daily drivers for better mpg as I run mostly hwy, little bumpy but I pays at the gas pump, that works for me.

[Derek up North]

I'd expect you to see less improvement in MPG in the MH when running the tires at maximum pressures. The reduction in rolling resistance will be a much smaller %age when compared to the massive wind resistance of a MH.

I'd prefer to have the pressures (especially in front) set to give the best grip when braking. Generally, this will not be at maximum pressure. How many MPG do you want to be getting when you rearend someone?

[zero]

I read the results of a studies done by Michelin and Firestone (AKA Bridgestone) about over-inflation. Their tests show when tires are inflated under or over the "recommended inflation" by 20 PSI either way - there was a 2% change in fuel economy. Note that such "recommended inflation" is based solely on how much weight is on a tire versus internal pressure so it rides with the desired profile.

Take your Duramax Isuzu-powered truck. I assume it weighs around 7000 lbs. if 4WD and 6000 lbs. if 2WD.

If you have a 7000 lb. truck with a GVWR rating of 8600-8800 lbs. - GM is going to give tire pressure ratings that are good for that max weight when loaded. They do not trust you (or anyone else) to change pressures when needed according to load. So much of the time your tires are already WAY over-inflated and that makes them wear out earlier.

Let's say you've got 245/75-17" tires. They are designed to be run at 44 PSI with 2300 lbs. on them. How much weight do you suppose are on your rear tires when not loaded heavy? Probably no more then 1700 lbs. each. So at 44 PSI they are already quite over-inflated. At 80 PSI they are WAY over-inflated and some of the desired design characteristics of the tires are lost. Note that the tires at 80 PSI are designed to have 3200 lbs. of weight pushing against that 80 PSI.

Seems to me that most if not all newer trucks come with PSI ratings that go way over what it best for normal unloaded driving. But they do not want to get sued when someone puts two tons of weight in the back of their truck with tires at 44 PSI and then have a problem.

My 1994 Ford F250 extended cab, 4WD, diesel truck came new with 235/85-16" tires. Ford recommended the rear tires be at 44 PSI with a payload of 1300 lbs., 65 PSI with a payload of 2200 lbs. and 80 PSI with a payload of 3100 lbs.

Seems in 1994, auto makers trusted owners more when it came to tire inflation according to load. Ever since the Ford-Firestone tire debacle - I assume all auto makers are scared to trust anyone on the subject of tire inflation.

[shibs]

1. 2% increase in mpg with 20lb increase in tire pressure - conclusion reached by Ford engineers. Being an engineer myself I can found my gains to be higher than that.

For my driving style higher pressure yields me great mpg.

I do question the conclusion reached by the talented engineers at ford (I have quite a few friends at ford in Detroit) . It does not add up.

http://us.coopertire.com/Tire-Safety/Tire-Safety-Tips/Proper-Tire-Inflation.aspx

"Operating a vehicle with just one tire under inflated by 8 psi (56 kPa) can reduce the life of the tire by 9,000 miles and can increase the vehicle's fuel consumption by 4%."

2. I did not know that the tire pressure for toyo camper is behind the seat.

lower the tire pressure = more heat produced in tire due to increased friction.

3. Loss of Traction = Yes in peak winter with snow, higher pressure diminishes traction drastically. On my daily driver (Ridgeline Truck), I follow door jam if not lower if conditions are pooooopy. During summer, on paved surface, brakes work fine and I am yet to engage my abs.

4. Tire wear due to higher pressure, to date I have not had any vehicles have unusual tire rear. I have lost count how many sets I have gone through.

it all boils down to individual preference. most tires can take MORE pressure than the sidewall max rating.

what we have learned from the firestone/ford fiasco is that tire pressure LOWER than door pressure specs is dangerous.

if I have to err, I will err on the high side :-)

[Derek up North]

Hmmm. 1 set of experts says tires off by 20psi increases consumption be 2%, another says 1 tire off by 8psi increases by 4%. I can't imagine which is right.

[zero]

Hmmm. 1 set of experts says tires off by 20psi increases consumption be 2%, another says 1 tire off by 8psi increases by 4%. I can't imagine which is right.

The tests from Firestone (done with input from Ford) shows a 2% change in MPGs over a 40 PSI range, NOT a 20 PSI range. It was a 2% change with the tires either under-inflated by 20 PSI or over-inflated by 20 PSI. I.e - 40 PSI range all together. My father-in-law is a retired Ford engineer who worked on some of these tests. A 4% difference in actual fuel mileage with an 8 PSI change is ridiculous (in my opinion). That is unless one setting is zero (with a totally flat tire) and the other is partially inflated to 8 PSI.

Note that the Ford-Firestone tests were 20 PSI over or under the recommended correct tire pressure for the weight that was on the tires. No tests were done with tires being run 50% or more over. I don't doubt there is more gain there, but also a lot of loss. At those high pressures the tires wear unevenly and lose some of their handling and traction characteristics.

[Derek up North]

... tests were 20 PSI over or under the recommended correct tire pressure for the weight that was on the tires.

Could you clarify what you understand by "recommended correct tire pressure"? I assume these Ford/Firestone tests were done after the 'Ford Exploder' problems.

[zero]

Could you clarify what you understand by "recommended correct tire pressure"? I assume these Ford/Firestone tests were done after the 'Ford Exploder' problems.

Such tests were done before and after the silly Explorer thing. "Recommended" pressures come from the companies that make the tires. Those recommendations are based on the actual weight pushing against the tire, not the vehicle it is mounted on. The concept of the "recommended" pressure is to have enough air inside the tire counteracting the forces of gravity pushing against it. The goal is to have that tire in the range to get the best compromise of traction, sidewall flex, traction, and over-all tread life. I've read such specs from several companies and they vary by very little. Makes sense to me since air pressure and Newtonian gravity also vary very little here on planet Earth.

[Derek up North]

I'll only assume the following pressures are accurate:-

"Firestone emphasized the importance of maintaining proper inflation pressure. Firestone recommended a pressure of 30 pounds-per-square-inch (psi), whereas Ford recommended a range of 26 to 30 psi. Ford claimed its tests showed the tire performed well at 26 psi and that the lower pressure made for a smoother ride. However, Firestone claimed underinflation could put too much pressure on the tire, contributing to a higher temperature and causing the belts to separate."

http://wps.prenhall.com/bp_laudon_essmis_6/21/5556/1422345.cw/content/index.html

Testing the tires at 10psig (or 6psig) seems rather extreme. 50psig would be at the upper limit of an P235/75x15 (XL).

[zero]

I doubt Ford and Firestone always have the same agenda when making tire and/or inflation recommendations.

Regardless, it looks to me that car and tire makers as well as other information providers seem have been scared sh*tless over that Explorer thing. In my opinion neither Ford nor Firestone did anything wrong. But you get people who don't check their tires and then drive their SUVs like they are Aston Martins. And when they crash - they blame Ford and Firestone and/or anyone but themselves. I don't see it as much different then blaming gun makers when someone has a gun illegally and shoots someone illegally.

And since the Explorer/Firestone thing - it's gotten worse. I've heard numerous reports of busses and trucks crashing supposedly due to tire failures. Come on ! Once in awhile, OK. But most trucks and busses if driven at reasonable speeds in reasonable manners do not roll over and crash just because one tire blows out (front or back). Just as a 21 foot Toyota RV losing a rear axle also should not result in a roll over or sudden death (although losing an entire wheel and axle is certainly worse then just getting a flat).

This is a 14 year-old study but if anything, I bet things are even worse now - not better - when it comes to checking tires.

[Derek up North]

Just as a 21 foot Toyota RV losing a rear axle also should not result in a roll over or sudden death ...

I've never seen any proof of any fatalities. Certainly none mentioned in the Recall. Just injuries. Can't recall any reports of rollovers either. Any on the Forum and Groups who have snapped an axle have only suffered some bodywork, wiring and plumbing damage.

[zero]

I've never seen any proof of any fatalities. Certainly none mentioned in the Recall. Just injuries. Can't recall any reports of rollovers either. Any on the Forum and Groups who have snapped an axle have only suffered some bodywork, wiring and plumbing damage.

Looks like the Dolphin in this salvage auction suffered some sort of axle failure. Obviously did not roll over.