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Tires, Wheels, Brakes, Steering and Suspensions Discussion of preventative/corrective maintenance and other technical issues regarding your coach's Tires, Wheels, Brakes, Steering and Suspensions and related components.

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  #1  
Old 05-22-2009
Rich Johnson's Avatar
Rich Johnson Rich Johnson is offline
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Default Rear Air Pressure

Hello: My 1981 FC-33 manual, and coach placards say to inflate rear drive tires to 75 psi. However, my tire jobber says that is way too low. He says not to go less than 90 psi, and recommends keeping them at 100 psi in the dual rear driver tires. Any ideas or suggestions? Any reason not to inflate them at 100?
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  #2  
Old 05-22-2009
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Rich I also have a placard but ignore it. I go to the tire manufacturer site and look up the recommended tire pressure for my particular axel weights. I have added 10lbs to the manufacturer pressure recommendations because Michelin reps told me this was OK. They said the ride might be a little harder but the sidewall flexing would be reduced.
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  #3  
Old 05-22-2009
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Vehicle manufacturers, including BB, tend to set tire pressures at the lowest possible value for the tire size and vehicle weight. This is done because the ride will be softer. Sometimes, softer ride is achieved at the expense of poorer handling. Most vehicles (cars, pickups, SUVs, and BBs) should actually have 10 to 20% more pressure in their tires, based on actual weight carried and tire loading.

It is best to get actual weights for each tire, then review the weight capacity/pressure charts issued by the tire's manufacturer -- and then select a pressure suitable for your coach. Note that weights should be taken with the coach loaded as it would be for a trip.

Since side to side loading is rarely precisely balanced, it's best to get wheel by wheel weights. The only problem with this (on vehicles with air bag suspension like most BBs) is that there can be false reading caused by pressure imbalances that occur after normal maneuvers approaching the scales. So side to side variations should be considered, but taken with a grain of salt.

On my coach, the placard with suggested pressures is, as I recall, (F,D,T) 100, 110, 90 psi. After weighing my coach, I determined that 115, 120, 120 are conservatively safe pressures to ensure that the weight carrying capacity of the tires exceeds the weights normally applied.

The 120 psi for the drive and tag axles is based on a (possibly exaggerated) 700 lb. side to side weight variation. To ensure that the "heavy" side psi was sufficient, it required 120 psi. Since it is advised that tires on both sides of an axle carry the same pressure, that suggested that psi be used on both sides. This weight variation could not be 'fixed' by moving anything carried in the coach, as there was nothing on board that could have affected the side-to-side weight to that degree. Indeed, with the floor plan used, the heaviest items (refrigerator, washer) are all on the centerline. Items in the basement were reasonably arranged -- and, if anything, should have been heavier on the opposite side as measured. The only thing that might contribute to the imbalance was the water and holding tanks, but they are apparently shaped to cross the width of the coach.

Now, it is entirely possible that the 700 lb side-to-side variation is considerably less. To reach the point where the weights were taken, the coach made a 90 degree left turn (from the street to the shop) and the air pressures in the bags are affected by that maneuver. The weights were taken within a brief time after the turn. (The coach was turned from the street, pulled about 60' across the parking area, into the service bay, and was weighed axle by axle on portable scales as the coach was brought forward into the bay.) So, while I take the side to side variation with a grain of salt, I also felt the need to ensure safe tire loading, just the same. I haven't had to opportunity to re-weigh the coach since those weights were taken. Clearly, it would be interesting to do so, but the only scales I've been near, (since those weights were taken) could only weigh axle by axle, rather than tire by tire (and that wouldn't tell me much, so I haven't bothered).

I note, too, that I carry about 5 to 8 psi more pressure in my Toyota's tires than the placard recommendation. If that was overinflated, I should get tire wear reflecting that problem -- but I never have (indeed, when I most recently replaced two tires on my car, there was an indicaation of low pressure wear on one tire -- one that had suffered a "slow leak" for some time before I was able to have it patched).
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  #4  
Old 05-22-2009
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Quote:
Originally Posted by Rich Johnson View Post
Hello: My 1981 FC-33 manual, and coach placards say to inflate rear drive tires to 75 psi. However, my tire jobber says that is way too low. He says not to go less than 90 psi, and recommends keeping them at 100 psi in the dual rear driver tires. Any ideas or suggestions? Any reason not to inflate them at 100?

Rich,

On an FC of your vintage you will see an alarming difference in side to side rear axle weights because of the generator located just in front of the right rear duals, on my 1982 FC35RB 'Bird I believe that it is 6,000# heavier on the right rear, so I have the right side at a higher pressure than the left. Be sure to read this post of mine from another thread and weigh your 'Bird before deciding what tire pressures to run.

Quote:
Originally Posted by iamflagman View Post
Rob,

I'm not sure where the factory recommended tire pressures are posted somewhere on your 'Bird, but you should read;

Wanderlodge Owners Group > Links, Files, Old Yahoo Forum Database and Downloads > Links > Tires and Wheels and Weighing Your 'Bird

I would suggest that you find a public weigh scale similar to what is described in HOW TO WEIGH YOUR BIRD.pdf and get the weights then look up the recommended tire pressures for your particular weights at each tire(s) position.
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1982 FC35RB
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  #5  
Old 08-12-2009
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Quote:
Originally Posted by iamflagman View Post
Rich,

On an FC of your vintage you will see an alarming difference in side to side rear axle weights because of the generator located just in front of the right rear duals, on my 1982 FC35RB 'Bird I believe that it is 6,000# heavier on the right rear, so I have the right side at a higher pressure than the left. Be sure to read this post of mine from another thread and weigh your 'Bird before deciding what tire pressures to run.
The statement above about the difference in side to side weights on my FC was ALL WRONG, those weights were taken from my results shortly after I purchased my 'Bird using a CAT Scale at a local Pilot Truck Stop and I arrived at the results using the same method that I describe below as well as I talk about in Wanderlodge Owners Group > Links, Files, Old Yahoo Forum Database and Downloads > Links > Tires and Wheels and Weighing Your 'Bird >
HOW TO WEIGH YOUR BIRD.pdf why they originally came out that way, I can't explain but they did at the time, maybe it was operator error on my part.

On my recent trip Bill Garamella and I got to talking about him weighing his 'Bird and he contacted the New Hampshire DOT asking them about weighing his 'Bird on the state scales on I-93 south of Manchester between exits#4 - #3, so on my way south I stopped in and used the scale as it was CLOSED for weighing trucks, but they told Bill that he could still use it to weigh the 'Bird as the scales themselves could still be used and the axle weight readouts were always displayed. As you can see in this aerial photo of the scale, there are trucks entering the scale but using the outer bypass lane to go around the actual scale platforms at the direction of the scale master.


http://maps.google.com/maps?f=q&sour...02497&t=h&z=19

The split platforms that make up the scale itself are located on the lane closest to the building, so after getting a normal full combination on the scale weight, I had to back up and pull the left side off of the scale so that the rights side was on the scale by itself at the same time making sure that I left enough room for trucks that were occasionally entering the scale to clear me safely, then to weigh the other side it involved turning the combination around and then pull onto the scale going in the wrong direction with the right side off of the scale to get the left side weights, this only required moving some cones placed on the outside edge of the scale and then when I was done replacing them back where they were originally, this is the method the the NHDOT Enforcement officer told Bill to use.


I used the method described above including turning the 'Bird and trailer around so that I could weigh the opposite sides and here are my final results;

Front Axle Right Tire = 6,000#
Front Axle Left Tire = 6,260#
Total Front Axle = 12, 780# (weighed 160# more total with both tires of front axle on the scale together)

Rear Axle Right Dual Tires = 9,620#
Rear Axle Left Dual tires = 10,120#
Total Rear Axle = 19,620# (weighed 120# more total with both sets of rear axle duals on the scale together)

Trailer Both Axles Right Side = 4,140#
Trailer Both Axles Left Side = 4,340#
Total trailer Axles Weight = 8,580# (weighed 100# less total with both axles on the scale at the same time, I did not split the rear axles front to rear.)

The difference in the total axle weights compared to individual tire(s) weight could be just from the positioning of the 'Bird and Trailer on the scale, so I use the individual position weights to determine my tire pressures, using the tire manufacturers tire inflation guide.

My 'Bird will probably have a lighter front axle and a heavier rear axle weights because I had the trailer in tow at the time I weighed, as the trailer hitch/tongue weight will cause this to happen.
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1982 FC35RB
I'm NO EXPERT, but I did stay in the FINN'S INN EXPRESS last night
HOPKINS, SOUTH CAROLINA
VISIT THE FINN'S INN EXPRESS REMODELING ADVENTURE AND TECH. TIPS
I'M SO SLOW ON THE HILLS,THAT I GET TO SMELL THE FLOWERS AS I GO BY.....AND WATCH THEM GROW TOO!! NOT SO MUCH ANYMORE
Visit the WILD HARE RACING website
REMEMBER 9/11
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  #6  
Old 08-12-2009
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I just read John's post about weighing on the NH DOT scales last week. I weighed ours on the same scale a couple day later.

Here we can compare John's FC 35 RB with Our FC 35 SB. The following are the BB specs and actual weights for our 83 FC 35 SB

GVWR BB SPec 34000 Actual 32660

GVWR Front BB Spec 13200 Actual 12900 Drive side 6440 Curb Side 6540

GVWR Rear BB Spec 23000 Actual 19740 Drive Side 9520 Curb Side 9980

Tankage

Fuel BB Spec 265 gal Actual Full

Fresh water 100 gal Actual 50

Gray 65 gal Actual 0

Black 40 gal Actual 0

Propane 45 gal Actual 25

Based on the above, it looks like I would go over the front axel limits fully loaded with grey, and black tanks full, full load of passengers and supplies. I have never run that way.
But it’s interesting to know this does appear to be pushing the limits on the front.

The back seems to be fine even after adding another 50 gallons to the fresh water tank , it would be balancing things out.

After I weighed our bird I brought the coach in for a leveling adjustment. The drivers side was about a inch higher than the curb and both front and rear were the same. Now it is adjusted to where it should be with the rear being one inch higher than the front. I bet this has some effect on the weight distribution and I will check it again soon.

NH Bill
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Last edited by NH Bill; 08-12-2009 at 09:08 PM.
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  #7  
Old 08-12-2009
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Unless the scale platform is very level, there will be some error. A little tilt one way or the other can alter the amount of weight on one side of an axle. The more weight higher up (eg roof storage) will accentuate this difference. Fluids will also tend to slosh to the low side, increase the weight on that side.
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  #8  
Old 08-12-2009
davidmbrady
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I might add that the tripod setup of height control valves, 2 on the drive and 1 on the steer, along with having the left and right air bags on the steer axle tied together, works. You'll notice that the percent weight difference on the steer axle is a mere 0.7 % while the percent difference on the drive axle is over 2%. What's going on is that the drive axle levels the coach left to right and the steer axle levels the coach fore and aft. Having the steer axle air bags tied together virtually eliminates transverse weight transfer at the steer axle and altogether eliminates torsional stresses on the frame. This works cause the center of gravity is just ahead of the drive axle. The steer axle is happy to follow the terrain similar to how the steer axle on a John Deere tractor pivots to follow the terrain. Just thought I'd throw this out there.

Last edited by davidmbrady; 08-13-2009 at 10:05 AM.
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  #9  
Old 08-12-2009
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NHBill.
a 1'' rake from front to back will not show a different blance between front and rear.
to get a major weight change you would have to have the rear 30% higher that the front.

When I was racing stock cars we had a set of 4 grain scales,we were very picky about how our cars weighed,not what they weighed.
well,ok,we were picky about what they weighed too!
On a 2500LB rule,my car would weigh 2507LBs after a race.
Not a lot of margain.
We spent many,many hours weighing race cars,seeing what would happen when you raised one side,the front,the back etc,etc.
A one inch height adjustment on a car with a 108'' wheelbase would net no change in weight on the front.

Now,a coach with a tag axle will see a big change on the front and drive axles when you adjust the tag axle air pressure.
Then you are "jacking" the rear of the bus up if you increase air pressure,and loading the front axle.
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  #10  
Old 08-13-2009
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David,

Thanks for that explanation. I did not know how this system was set up.

Randy, knowing that it would take a BIG rake to make a real difference, in the weight distribution, I won't bother rushing back to the scales. This, plus your earlier explanation of the variables, makes sense and now know I understand why YOU don't bother with it.

One thing I did not see mention of in this thread is tongue weight and how it plays into the equation.

Thanks all,

NH Bill
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