Tire pressure

Topic: Tire pressure (Read 1032 times) previous topic - next topic

Tire pressure

August 23, 2013, 10:20:48 am

I realize this is an emotive subject and all will recommend weighing coach fully loaded , but, as a starting point what are the recommendations for 2003 3620 no toad. Thanks in advance

Re: Tire pressure

Reply #1 – August 23, 2013, 10:35:54 am

Jimmy,

Toad will make no difference, since it only adds a few pounds to the coach weight (weight of hitch/tow bar).

Until you weigh it, go by the pressures on the GVWR sticker near the driver's area. The PSI on it assume each axle loaded to its GAWR.

Brett

Re: Tire pressure

Reply #2 – August 23, 2013, 10:43:28 am

Brett
Thanks for quick ,concise response

Re: Tire pressure

Reply #3 – August 23, 2013, 12:05:43 pm

I know others will disagree but I try go by what numerous truck tire professionals, including an engineer from Goodyear, have told me. By running the maximum pressure indicated by the tire manufacturer you will have the best chance of avoiding blowouts. Heat build up caused by friction during tire wall flexure is the most common cause interlaminary failure resulting in a catastrophic blowout.

Granted, the ride will be harsher by running this high pressure but for me its worth it.

Re: Tire pressure

Reply #4 – August 23, 2013, 12:34:12 pm

Quote from: Kent Speers – August 23, 2013, 12:05:43 pm

I know others will disagree but I try go by what numerous truck tire professionals, including an engineer from Goodyear, have told me. By running the maximum pressure indicated by the tire manufacturer you will have the best chance of avoiding blowouts. Heat build up caused by friction during tire wall flexure is the most common cause interlaminary failure resulting in a catastrophic blowout.

Granted, the ride will be harsher by running this high pressure but for me its worth it.

An awful lot of trucking, bus, fire departments, etc. agree with Kent. Here is an excellent quote that pretty much covers it:

"High performance and dynamic drivers often increase the tire pressure to near the maximum pressure as printed on the sidewall. This is done to sacrifice comfort for performance and safety. A tire at higher pressure is more inclined to keep its shape during any encounter, and will thus transmit the forces of the road to the suspension, rather than being damaged itself. This allows for an increased reaction speed, and "feel" the driver perceives of the road. Modern tire designs allow for minimal tire contact surface deformity during high pressures, and as a result the traditional wear on the center of the tire due to reasonably high pressures is only known to very old or poorly designed tires.

It may be, that very high tire pressures have only two downsides: The sacrifice in comfort; and the increased chance of obtaining a puncture when driving over sharp objects, such as on a newly scraped gravel road. Many individuals have maintained their tire pressures at the maximum side wall printed value (inflated when cold) for the entire lifetime of the tire, with perfect wear until the end. This may be of negative economic value to the rubber and tire companies, as high tire pressures decrease wear, and minimize side wall blow outs."

Pierce

Re: Tire pressure

Reply #5 – August 23, 2013, 01:17:50 pm

Was exactly my experience, three years running truck and off road tire shop, run max allowable pressure. On auto that might decrease the contact patch a small percentage, on our tires that reduction is negligible. Lower pressures are to improve ride.

Re: Tire pressure

Reply #6 – August 23, 2013, 02:01:44 pm

Hmmm...when we were at FOT in February I asked Wayne to recheck our alignment (he had just aligned it the previous visit) because I was chasing the steering to keep it straight. He did and then lowered my tires pressures and said that was all that was needed. He was apparently correct as it has been driving much better since.
I had increased the pressures.

Re: Tire pressure

Reply #7 – August 23, 2013, 02:49:10 pm

On my previous sob gasser motorhome, the mechanic liked to max out the tire pressures. I always discovered this on the way home. It was impossible to steer a straight line, darting all over the roadway. I suspect the front tires were having trouble gripping the road.

Re: Tire pressure

Reply #8 – August 23, 2013, 03:18:27 pm

In the alignment game, some general rule facts are
Dart & Dive is from frozen, tight steering components
Wonder & Weave is from sloppy worn parts
These are the normal game, there are always exceptions to everything.

Re: Tire pressure

Reply #9 – August 23, 2013, 05:14:00 pm

Hmmmmm, interesting......

Re: Tire pressure

Reply #10 – August 23, 2013, 10:23:38 pm

Quote from: Tom Lang – August 23, 2013, 02:49:10 pm

On my previous sob gasser motorhome, the mechanic liked to max out the tire pressures. I always discovered this on the way home. It was impossible to steer a straight line, darting all over the roadway. I suspect the front tires were having trouble gripping the road.

I would suspect front end alignment problems or the deficiencies Dave listed above. Sounds to me that the lower tire pressures masked the problem and when the mechanic raised the pressures the steering became more sensitive and along with it came a "wake up" call to take a look at the front end. With Dave's "dart and dive" the binding or frozen may be traced to the kingpins. Driving on wet roads along with lack of scheduled lubrication with cause these symptoms. Worn ball joints (again from lack of lubrication) will cause the wander and weave described. Poor front end alignment with toe out will require constant corrections with the steering wheel. A lot of these symptoms are magnified when passing large vehicles or cross winds.

Back in my stock car days, my car builder always made a point of making sure the wheel bearings were preloaded and the front end had toe in. With new tires (and high pressures) and a smooth track, it was not unusual to be able to drive with only a light touch on the wheel even at speeds over 150 mph. At those speeds, you can't afford to have the car wander around. Watching TV, you can see that even with a slow leak in one tire, a car's handling goes away in a hurry.

Maximizing pressures will insure the tires run the coolest, are the least susceptible to damage from chuck holes, last the longest, give the best fuel mileage and give the best emergency handling. The only exception is having fun on sand dunes, dry river beds or ATV desert/enduro riding.

Pierce

Re: Tire pressure

Reply #11 – August 24, 2013, 05:26:21 pm

I should add that sob motor home was a 20 year old basement model built on a John Deere chassis in 1987. In spite of being top heavy and riding on narrowly spaced leaf springs, lots of side lean, it drove pretty well when new. When a few years old, the shocks were upgraded and the sway bars beefed up. All was well until the steering gear box had to be replaced, after which it never drove well. Realignment, readjusting the steering box backlash, and putting more heavy items up front helped, but maxing out tire pressures made it very skittish. The final straw came when it needed new tires and the now obsolete skinny size was only available from a couple of manufacturers. The new tires were horrible from a darting standpoint. The good thing that came from this is what I replaced that coach with. I made it a point to move from one of the worst handling rigs in the road to the best.

Re: Tire pressure

Reply #12 – August 24, 2013, 07:55:08 pm

As usual, I'm trying to digest all of this great info and then react, so if I'm gettin' this correctly, maybe MUCH of our steering issues are related to tire pressures. I'm not surprised by this/also,am not as up on the details of alignment issues, as many of you are. However, If I'm interpreting the basic thread here correctly.... Determine the tire pressure YOU want to run in your tires and tell the Alignment team. yes/no?

IF we want to run pressures at the upper end, then alignments need to be set for "Those" pressures. Yes/no?

The 'alignment/tire pressure' discussion goes on and on for decades, but if we now have decent technology and understanding of issues, maybe we can get better handling in our coaches by just "Telling the alignment staff................"this is what I want and expect".

While at FOT, I was told my pressure choices of 100 front and 90 rear(cold) was too high and to lower them... I think I was correct and do not want to soften the tire, to mitigate steering or ride issues.
Note however...I truly love my coach and can deal with any of it's little glitches.. but I do think from what I'm reading, I or we... should start with a question from the alignment guy...., "What pressure to you want to use".

Re: Tire pressure

Reply #13 – August 24, 2013, 08:07:22 pm

Quote from: MemoryRoads – August 24, 2013, 07:55:08 pm

While at FOT, I was told my pressure choices of 100 front and 90 rear(cold) was too high and to lower them...

Whether too high or not would depend on the actual weights of the particular coach, at least at an axle level and preferably at each tire. It is extremely important not to run underinflated for the load at the wheel.

I believe the forum has generally found 100 psi or slightly greater is pretty common for the front on most of our coaches. Rear seems to typically range from 75 to 95 (or greater) depending on the coach (and whether it has a tag or not).

Our rvsafety.com weights showed we should be at 100 front, 75 drive, 75 tag* minimum and they recommended an additional 5-10 psi for loading variation. We run 105/85/75 cold inflation pressures.

* 75 is the lowest inflation listed for our XZA3+ LRH tires. We are well under the maximum weight for our tag at that pressure so we did not adjust upwards.

Re: Tire pressure

Reply #14 – August 24, 2013, 08:25:17 pm

Hi Michele, I do not have any data with me right now as I'm away from my coach, but my 2001 40' has the numbers stamped on it inside by the Driver's seat and the book for the coach also has numbers as does the tire manufacturer. I'll trust 'All of those sources' until i can get verification from weights individual wheels.

I guess my gut reaction to stay safe, is to keep pressures a tad higher rather than lower and am leary of an alignment gent- here or there- who would tell you to lower tire pressures to solve a problem. Certainly, adding some friction to where the 'rubber meets the road' can make life 'seem' wonderful, but.... I'm just thinkin' and askin'.

In the mean time, my tire pressures are up to where "I" want them to be and/but I still seem to need to tweak the wheel a tad more often than I thought I should. Just a tad...

yup, I'm not skilled in alignment, but my thought would be to add some toe-in, rather than deflate front tires.. "I'm still trying to learn!"

Re: Tire pressure

Reply #15 – August 24, 2013, 10:04:08 pm

1/16" to 1/8" toe in regardless of pressure is the usual big rig setting.

PIerce

Re: Tire pressure

Reply #16 – August 24, 2013, 10:12:41 pm

Eliminate any extra play in the front wheel bearings first. Then the drag link.

Hard to measure or quantify any play in the suspension bushings but I assume they do wear.

Also ride height valves that do not return to the exact position each time have the coach going one way versus straight.

If you inflate or deflate your coach and watch the steering wheel unwind you will see the issues. Far worse on Oshkosh because of the steering box mount.

Worn shocks allow the suspension to go through the travel too quickly and the same steering wherl unwind noticed in the ride height changes happens as the suspension moves up and down.

I used to dump and raise several times every coach I sold to verify how close the ride height returned to the same position every time.

Measure at a known point around the coach. Raise full height and check several times. Dump and check several times. Some stick from one direction not the other.

Tightening my Koni's helped the bump steer in dips a noticeable amount.

Biggest change was the wheel bearings. Took out the dead zone. All gone. No play.

Then we can talk about pressures and alignment issues after all the mechanical stuff is eliminated.

Wear in the tie rod ends is also an issue. Replaced my trucks tie rods brand new versus cup tires.....

Bob

Re: Tire pressure

Reply #17 – August 25, 2013, 07:45:39 am

Good Morning All;

Great info on tires, had been running 90 all around on our 98 U270 36'. Put them up to 100 yeaterday.

Now, question, have only been a FT owner since Jan this year. Where, other than FOT in Nac would one take the coach to for an alignment? I am in central Maine about 2000 miles from NAC.

Thanks

Re: Tire pressure

Reply #18 – August 25, 2013, 12:05:07 pm

I read this about tire inflation about a year ago and for me it has worked well. The MFG inflation guides are at the minimun pressure to be run for the weight that is on the tire. To be sure that you are running the correct air pressure , check tire pressure before starting out. Then after you have run at speed for 20-30 minutes stop and immediately check pressure. If it was at 90 PSI when you started and no more then 99 PSI now you are running enough air pressure. A 10% increase is appropriate. If the air pressure increase is more then 10% you need to add air. If the tire does not build up any air pressure after running it at speed you can safely lower the air pressure to improve ride and traction. It is always better to error on the high side. One more plus is that even if your gauge is not calibrated the 10 rule still works. Gam

Re: Tire pressure

Reply #19 – August 25, 2013, 01:50:53 pm

I like to lift the front end/tires off the ground when doing a lube job and then secure with big blocks or a proper stand. You can then grease the kingpins making sure you have grease coming out both top and bottom. I wipe off the extra. With someone in the driver's seat moving the steering wheel, you can check for play starting at the steering box out to all the suspension components. I have had kingpins so tight it took an air grease gun to force grease in. Good to make a map or photo where all the grease fittings are. Check brake pads (shoes) for even wear side to side while underneath. A partially sticking caliper (or slack adjuster out) can cause the rig to move around under braking.

Wandering or uneven handling can also come from the rear end not tracking straight. Good to go under and check for bent components (running over something or a previous accident). Not every truck alignment shop will check this so you have to first make sure they are up to speed and will actually check it rather than telling you they did.

Poor handling on older SOBs was frequently the result of leaf spring sag. The springs were of poor quality and as the years went by, they slowly lost their strength lowering the RV. With the engine up front, it usually dropped more. This changed the steering geometry, especially if one side was worse than the other.

Most will have the same tire on both sides in the front but different side to side tire brands on the front can also cause wander. A flexible side wall radial on one side with a stiffer on the other will see the most difference with tread pattern second. Steer tires vs all position tires at the front will also be a factor.

We just ran out our General Tire Altimax and replaced them with Cooper CS4 tires at 80K. The Altimax were outstanding but starting making noise at 30k and got louder as the miles increased until they howled. They had a very firm ride but stuck like glue in dry, wet or snow. The Coopers are very quiet, ride softly but not nearly as responsive. You can imagine mixing these tires front to back or worse, side to side in the front on a large vehicle.

Pierce

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