Suspension Creak

[travelite]

If you are looking for a "softer" ride you are getting into an engineering issue. For example, the valving in our shocks and the diameter of the air bags control how harsh a bump feels. Lowering the bus ride height in essence reduces air bag pressure so the bumps will be less harsh, but in doing so the risk of the bus bottoming increases. Conversely, raising the ride height increases the air bag pressure making bumps feel more harsh and risking a rebound to the full extension of the suspension and as you surmise damaging the shock eyes or the attach bolts.

Jon,

I'm far from an expert but I think your quote above needs a bit of clarification. The air pressure in an air bag (air-spring) is essentially the load on the bag divided by the cross sectional area of the air-spring piston. This psi doesn't change as you add or remove air from the bag (assuming a cylindrical piston profile) because neither the load nor the cross section change. When you add or remove air from an air-spring you're simply raising or lowering the bus. The pressure stays the same, but there will be some ride stiffness change in exactly the opposite way as was quoted above. Long air-springs (lots of air above the piston) makes the spring rate softer; short air-springs (less air) makes the spring rate firmer. The ride stiffness (spring rate) is directly proportional to the cross sectional area of the air-spring piston and inversely proportional to the air-spring length (the column of air above the piston); i.e., increase the spring diameter and you've stiffened the ride, increase the spring length and you've softened the ride. Of course, a real air-spring is far more complex than my simple explanation, but the fundamental principles remain the same. Please correct me if I'm wrong.

[Jon Wehrenberg]

David,

It has been my observation that as my air pressure in the auxiliary system builds (and feeds the air bags) the bus begins to rise.

I can bring my air bags on my present coach up to ride height in the front with about 105 PSI. To increase the height to full extension I have to increase the pressure. Having said that you may be correct in that the internal pressure remains the same, but the volume of air increases until the bag reaches its limits in height and then the pressure internally increases.

I may have my foot firmly implanted in my mouth and the only way to verify this is to put a gauge on the air bags and see what happens to the pressure as the air bag rises or drops.

Has anybody done this?

[travelite]

John,

That's right. Say your coach is at ride height equilibrium with 100 psi in the air-springs. You crank open the auxiliary tanks and shove 125 psi into the air-springs. The air-spring then rises, increasing it's internal volume, until the pressure in the spring is restored to 100 psi. If you continue to shove 125 psi into the spring, it will continue to rise.

The thing is, when you increase the height of the spring, the spring rate has gone down and the ride has become softer. When you decrease the height of the spring, the ride becomes firmer. This is easier to grasp when you realize that regardless of the height of the vehicle, the air pressure in the air-spring is the same. It's at the 100 psi equilibrium required to carry the load. What's changed is the quantity of air in the bag. More air, softer ride; less air, firmer. A simplified spring rate formula looks something like this:

Spring Rate (K) = (internal Pressure) * (piston cross section area) / (height of air column above piston)

[BoaterAl]

Jon,Truk,jack,Rick.and travelite....Your comments are well taken and I appreciate everyones impute in this ride handling issue. Lots to be learned on getting the best ride as we already have the best built. Simply put minor adjustments make a big difference. I have an appointment at the shop tomorrow with a Prevost tech which will check the ride height. Bag height as per specs doing this on a level surface. The tech does the adj from a pit and rechecks at least 4 times deflate and recharge. I believe the front is low by about an inch and I've only said the coach does not sit level appearing low in the front. Normal inspection of front end parts, shocks,(new Koni's installed air) air bags and any signs front end damage will also be looked at. The way you have put "equal up and down travel and the steering geometry is optimized" well put and I would take that one step further as ideal ride ability.
Next stop the air pressure site.
Thanks
AL
2005 Marathon XL II

[travelite]

Al,

If you're really an inch low up front, which is a lot considering that compression travel is only around 3 to 4 inches, then you're in for a treat. Your ride will almost certainly be better, softer, more compliant. Please keep us informed. Good luck!

[BoaterAl]

This is the continuing adventure on ride height of an XLII......lets get it right. The air bag measured in at an height of 10 1/2 in the front and was adjusted to 11 3/4 by the tech. The rear was checked and that was OK at 11 3/4, both sides. Road test and recheck and all is OK, the coach sits level now. On the way back to storage there is a remarkable improve meant on the ride.
Did a very close inspection of the front end and found no problems, air pressure was checked and adjusted per the scale weighing last week. Also gave me a good tip on the unfortunate chance of a flat tire in the front on where "not" to put the jack... hollow suspension tubes. Bet the tech tell what the XL II weighed in his opinion on the front axle and was within 100 lbs. The bus is "more better".
AL
2005 Marathon XLII

[BoaterAl]

Is anyone riding on Road King shocks on front of an XLII ? I was reading the bru-ha-ha on there claims of bigger better best.
AL
2005 Marathon XLII ....with clear shield,... Koni shocks..., ride height adjusted,... air pressure adjusted to weight,

[BoaterAl]

Is anyone riding on Road King shocks ? I posted this message and then it disappeared. This is one thing I don't understand .....the midday wash of the messages.
AL
2005 Marathon XLII

[jack14r]

I weighed the coach today with 110 LBS in the tag and then re-weighed with 80 LBS. in the tag and as Jon said nothing would change,he is right.

[BoaterAl]

Good news on that item. Did the bus weigh in close to mine ? My front was at 17770.
AL
2005 Marathon XLII