Another Tire Pressure Question

[Jerry Emert]

For the last 2 years I have been ignoring the "great" tire pressure debate.  I haven't had an issue in my, mostly, local travels.  Ride is good, handling is really good.  Of course I've never driven another Class A so what do I know?   I've kept my tires at the plate recommended 120 psi. 
We are on our way West and some of these roads make it feel like I'm hitting 12 inch deep pot holes.  It's amazing that this coach is actually in one piece after this ride.  I have had a problem, that I figured out, with my leveling system, again!  It does not automatically go in to travel mode when I get ready to drive anymore.  I have to turn it off.  The ride before I figured that out was like a roller coaster.  Through all that though she went down the road straight and true, even when I felt like I was on the downward side of a 60 foot swell in the North Atlantic in the midst of running over 12 inch deep potholes.  So I am not inclined to think I have a shock problem, they are doing their job by keeping my tires on the road.  It is a ride problem at this point.
I can not find a tire pressure chart for my Hankook Ah-12, 275X70R 22.5 tires so I have maintained approx 120 psi.  I have never been able to get corner weights but I have been to a CAT Scale.  My weights are:
Front-12,140
Drive-24,120 with Tag up, I forgot to take it off Auto.
Gross weight 36,260
GVWR is 42,800
I have previously adjusted it to get the weight of the front axle down to where it is.  When I bought the coach it was overweight.
Finally, my question...is 120 psi too much for my weights or is 120 correct and... what is causing the coach to feel like it is smacking into 12 inch potholes when they are in reality only less than an inch or so high, normal crappy road patches?
Thanks as always.
Jerry

[Steve Huber Co-Admin]

Jerry,
This link supposedly contains the Hankook inflation chart. http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/24228710.cfm

Based on the data you supplied, 110-120 on the steer and 100-110 on the drive seems OK if the tag is down. However, you need to get the weight on all 4 corners to get the weights and resultant PSIs correct as neither the steer or drive axles will be evenly divided as to weight.  I suspect you'll be able to lower the drive PSIs somewhat but wouldn't do it untill I had the weights. Most scales have wide side areas so you can get all but one wheel off the scale to get the weight. Be sure to do it fully loaded and don't forget to get the tag weight. It takes some time but is worth the effort. I checked with a couple of truck stops to find out when they were normally not busy as you will tie up the scale for 15-20 minutes at least with the multiple weighs you need to get. You can adjust the tag pressure somewhat to vary the load on the front axle.
Steve

[Doug Allman]

Jerry, Ride problem could very well be the shocks. When we purchased our 2004 Marquis it had Bilstein shocks on it. Our 1991 Contessa prior to that had Koni's.
I even once tried out other shocks than Koni's on the Contessa and went right back to the Koni's (new set).

Would say we expierenced the same ride as you have and going up onto a bridge and then off the bridge we would porpoise at 32,000 miles on coach. Changed to Koni's and a totally different ride. All of the previous rough ride and handling just went away. At 82,000 on it now still that same Koni smooth ride.


If you have Koni's then you may just have some other ride associated problem which may be attributable to the air bag system.

IF you are thinking about changing your shocks to Koni I would suggest you take off old before ordering new. Our coach had 3/4" mounts on top and 1/2" on bottom mount. I have a son that has numerous metal machines so we just used a vise and a some very sharp drill bits to get hole on top mounts to correct size. We didn't spin the rubber in the mount. (We had called Koni and they indicated no problem with drilling out but next time check with them prior to order as they have the correct style but in a different part number that most no jobber will stock.)

[Gerald Farris]

Jerry,
Just like Steve said, no not lower the air pressure in your tires until you have accurate weights for each wheel location. A softer ride is not worth a blowout.

As for your ride quality, if you are in Louisiana, New York, or California, replacing your shocks in hopes a great ride is a waste of money. There is just so much that a shock absorber can do and road resurfacing is not one of them. If you are getting porpoising when entering or exiting a bridge (except in Louisiana were everything porpoises), you need shocks. But don't go and spend thousands of $$ trying to cure a stretch of bad road if your coach drives OK elsewhere.   

Gerald

[Jerry Emert]

Jerry,
Just like Steve said, no not lower the air pressure in your tires until you have accurate weights for each wheel location. A softer ride is not worth a blowout.

As for your ride quality, if you are in Louisiana, New York, or California, replacing your shocks in hopes a great ride is a waste of money. There is just so much that a shock absorber can do and road resurfacing is not one of them. If you are getting porpoising when entering or exiting a bridge (except in Louisiana were everything porpoises), you need shocks. But don't go and spend thousands of $$ trying to cure a stretch of bad road if your coach drives OK elsewhere.   

Gerald

Thanks Gerald, as I understand porpoising, my coach is not doing that.  Going on or off a bridge, the nose will dip, sometimes really abruptly, but just once, it does not do it 3 or 4 times.  If once counts then yes it porpoises all the time.  When it does it still handles well with no wandering.  I almost feel like I could let go of the wheel and hold on to something and it would go straight.  Don't worry, I won't.  The main issue is things like the actual divider lines on the bridges that feel like they are 12 inches high and the patches on the road that feel the same.  The roads that have the divider lines every 30 or 40 feet are a killer as I'm sure is probably universal for MHs.  If this sounds like shocks I'm all in and where can I get some between Arkansas and the Grand Canyon without waiting for a week?  From what I've read Koni shocks seem to be preferred or is that a ford chevy argument?  Thanks again as always.
Jerry

[Gerald Farris]

Jerry,
If your coach dips only once when you hit a bridge connector, it is not porpoising.

Yes, Koni shocks are normally considered the best for a class A diesel coach, but they are firmer riding shocks than the Bilstein units that your coach came with, therefore some owners go back with Bilsteins if they have a shock failure to maintain the softer ride. However, I am by no means convinced that the installation of a complete set of Koni shocks on your coach will do no more than lighten your wallet a couple of thousand $. Like I said earlier, no shock can resurface a road, not even a Koni.     

Gerald

[Jerry Emert]

Gerad, thanks again, I guess I'll just try to hold what I have until I can get 4 corner weights.
Thanks

[Bruce Sieloff]

Jerry,
At the risk of being woefully uninformed, I also have considered going to Konis, but for the front mounts only, as this is where the most noticeable dipping occurs when going off an abrupt surface. In the last year that we have been on the road I have altered tire pressures until I am now running at the max 125 psi for the Bridgestone R250s on the coach. At each increase in pressure the coach handled better until it now feels almost lithe on the road, at a cost of being increasingly sensitive to directional changes. I have to pay closer attention to where I'm going, not 50 yards ahead, but more like a 1/2 mile and then she centers out nicely. I think that for a 45 foot coach the initial impacts being lessened by a stiffer shock up front will mitigate the "upset" to the rest of the coach as the other 45 feet follow. Won't know until I try it, but it beats the cost of a full replacement. A steering stablilzer is also in my future. I will be in Bend next week and intend to ask their recommendations for such a modification. The coach does ride a bit harsher at higher psi, but the surefootedness is worth it.

[Jerry Emert]

Jerry,
At the risk of being woefully uninformed, I also have considered going to Konis, but for the front mounts only, as this is where the most noticeable dipping occurs when going off an abrupt surface. In the last year that we have been on the road I have altered tire pressures until I am now running at the max 125 psi for the Bridgestone R250s on the coach. At each increase in pressure the coach handled better until it now feels almost lithe on the road, at a cost of being increasingly sensitive to directional changes. I have to pay closer attention to where I'm going, not 50 yards ahead, but more like a 1/2 mile and then she centers out nicely. I think that for a 45 foot coach the initial impacts being lessened by a stiffer shock up front will mitigate the "upset" to the rest of the coach as the other 45 feet follow. Won't know until I try it, but it beats the cost of a full replacement. A steering stablilzer is also in my future. I will be in Bend next week and intend to ask their recommendations for such a modification. The coach does ride a bit harsher at higher psi, but the surefootedness is worth it.

Bruce, I had not really considered shocks until lately because I don't get that repeated rocking or porpoising when I hit a dip and the coach handles really well.  Its the harsh bumps that have me searching for a solution.  It does ride nice on good roads so it may just be my inexperience with these big machines.  I see the OTR trucks going by me at 70 + over the same roads without an apparent issue.  I've never ridden one of those either so again, maybe just me.  I guess I'll just suck it up and enjoy the ride.  I think that if I do get shocks it will be just the 2 in front to see what happens.
Thanks
Jerry

[Keith Moffett]

Bruce
Are you sure you havent exceeded the max pressure for the Alcoa wheels?  Most I have seen say max pressure cold is 120.

[Jerry Emert]

Bruce
Are you sure you havent exceeded the max pressure for the Alcoa wheels?  Most I have seen say max pressure cold is 120.

Yes, I run 120.  I had a mechanic try to put 125 in them once and had to stop him.  Thanks!
Jerry

[Bruce Sieloff]

The wheels say 130 psi cold and I keep the tires to the max 125 psi cold, and no more. Admittedly that is running a little close to the bone, but there is some engineering conservatism involved and it hasn't been an issue. I had the tires checked at a Speedco in Texas once and they indicated 136 psi hot, having just come in off the road on a hot day. I am more worried about generating heat from underinflation than the heat from external (roadway) sources.

[Jerry Emert]

The wheels say 130 psi cold and I keep the tires to the max 125 psi cold, and no more. Admittedly that is running a little close to the bone, but there is some engineering conservatism involved and it hasn't been an issue. I had the tires checked at a Speedco in Texas once and they indicated 136 psi hot, having just come in off the road on a hot day. I am more worried about generating heat from underinflation than the heat from external (roadway) sources.

Mine say 120 max.

[Gerald Farris]

Jerry,
Your coach has 4 front shocks, and 6 rear shocks for a total of 10 shocks, one for each air bag location. That is why you are talking about a couple of thousand $ in replacement cost.

Gerald

[William Jordan]

Must be different Wheels on different coaches , My 2003 Marquis are stamped 130. Psi cold