Spring Air Leak Shakedown.

[Kenneth Brewer]

If you can manage it, do not let that mechanic touch anything other than his locker and the car he drives home in. Prevost (or any other vehicle manufacturer) doesn't design for roads that might have a dip in them? What is a suspension is for?

The shock is moved at one end only in this case. The end that moves goes in and out with respect to the other end. That's it. The shock doesn't know whether the dip occurred in a corner or on the flat or if the vehicle was dropped the last few inches on a lift. Unless something else broke, like the wishbone, in which case the broken shock would have been the least of it. I have heard of cases where one or both ends were so tightly bolted in (misinstalled) that no rotational movement was available at the pivots and the shock was subsequently damaged (bent). But I seriously doubt (don't believe for a minute) that was the case here.

I'm sorry, but this fellow is.........misinformed. Please let me have his name so I may avoid him. Thank you in advance.

Thank you

[Jon Wehrenberg]

Dale, that is troubling. What you are suggesting is there is unlimited travel once the shock mount breaks and the air bag is ripped apart due to excessive front axle travel.

I will have to get under my bus and see what I can see, but I can't believe the times I have had my front axle hanging it was kept from dropping to the floor by either the air bag or the shock. I replaced air bags, shocks, and brake shoes on my 87 and in each case I believe I left the front axle hanging. I replaced air bags on my coach and helped Roger with his, and the front axle was hanging both times, but I honestly did not notice if the shocks were all that were holding it. I know it wasn't the air bags because they were off.

I still think the mechanic was blowing smoke and I still stick with my theory that the shocks that broke had insuffient travel.

FWIW I raise my coach front end to the fullest extent to clear the transition from the upslope to the garage floor. During that time the coach passes a driveway section that has an irregularity to channel water. The coach rocks sideways slightly and I have not broken a shock yet.

[Joe Cannarozzi]

Are some of the posts for the shock ends threaded all the way in. I am going to check the front on ours and see if that is the case it could be mine are too tight. They are double nutted on the bottom and I doubt it came like that. That could be my problem.

Usually the posts for the shocks have a collar where the thread ends and you bottom out a jam nut holding it on, right?

Dale the radius rods hold it back some, the 5th bar the most. It can only travel as long as the radius rods and as the side way "5th one" drops it will start to pull it one way the more the axle drops. Then there is only minimal "twist" in the other four, it will stop itself somewhere. I suppose the more the rod bushings are wore out the lower it will drop. If that happens before the shock fully extends is the question.

I have seen air ride trlrs with a strap around the axle housing limiting its travel. Possibly to keep from snapping shocks by overextending.

When I get the front apart on ours I will see if the axle will drop past the point where the shocks are extended out all the way. All that will be holding it will be the radius rods and a jack.

[Jon Wehrenberg]

I pulled my wheels off the front axle with the bus up on stands. I let the axle hang with the air bags and shocks still attached.

I measured the center to center dimension of the shock absorber mounting studs. It came out to 23 15/16"

I removed the bottom bolts on the air bags, removed the shock absorbers and let the axles drop if they could. They did drop. With the shocks removed the center to center dimension between the shock absorber mounting studs was 24 1/8", or about 3/16" greater. The shock absorbers do limit the suspension travel.

Despite being loose, the air bags did not lift off the bottom mounting plate so they do not limit suspension travel, nor are they so short that they stretch out when the axle distance down is at the limit of the shock absorber. With the shocks out I could bounce the suspension up and down a little so there is even more movement possible than when it just hangs straight down. Evenb with the extra drop from bouncing the air bags remained in contact with their base indicating they are sufficiently long in height.

So here is the moral of this story: If your suspension is fully extended, on either side or equally on both sides either because you have raised the bus or the terrain is so steeply sloped, you have the potential to break one of the rings from the end of the shock absorber. Dale has it exactly right. The mechanic that made the statement about how easy it was to break shocks has it correct. The dumb part is having a bus that has no mechanical limit to suspension travel.

If you do not want to snap the end rings off your shock abosrbers, do not transition any places where one side of the front suspension is forced to be fully extended such as having the rear of the coach level, while the front axle is "twisted" with one side up and the other down. Using the level low to level a coach on a sloped site may be OK, but do not drive with the coach in level low mode.

The reverse of this situation may also be true, but I doubt that. In the lowest position the air bags have internal "bumpers" to limit the down travel of the coach or the suspension. I don't believe the shocks are exposed to the weight of the bus in the minimum length position.

[Kenneth Brewer]

"The mechanic that made the statement about how easy it was to break shocks has it correct. The dumb part is having a bus that has no mechanical limit to suspension travel."

Well, it looks like I may be wrong, but though I hate to disagree, and would be reconciled (happily in this case) to be proven wrong on this one, as far as I am aware, all testing, and particularly for MIL spec tests (I will try to confirm this), has a section that tests full suspension extension. I am prepared to be very surprised that any vehicle relies on shock travel limit to retain/stop travel extension alone. Shocks I have removed on all vehicles I owned (except the Prevost at the moment), the unsprung weight was always mechanically hard linked by something other than shock absorbers or airbags. On the Bluebird, either end, the suspension travel is ultimately limited even if the shocks and airbags were missing.

A dip that would break a shock as described, at the least, would mean the front end was airborne long enough for the (airbag) spring forces to overcome the hydraulic damping of the shock, and with enough force to pop something in the shock/shaft (breaking). The rubber or elastomer cushioned shock ends should handle the impact of the sudden end of travel by design, unless improperly installed (the 3/16").

Aside from that, though, there is the mechanical restraint I spoke of with the upper wishbone and lower links. Vehicles I am familiar with may have the first (beginning) restraint/stop the rubber/elastomer bushing or eyelet insert, but the suspension was never held together by the shock only. If Prevost has designed their suspension to be limited by shock travel in such a manner, for such a large heavy vehicle, I will have once again been introduced, by force, to reality. I would rather think the shock travel was incorrect due to mistaken part number or production run change, and that broke the shock.

I would prefer to stick with Jon's earlier statement about insufficient travel. The signature Wrongagain is currently being used, and I hope I don't have to pay for the rights to use it.

[Joe Cannarozzi]

I am aware of more than one semi dump trailer manufacturer that uses a strap around the axle housing to keep it from extending past a given point.

[Orren Zook]

Joe & Jon are correct in their description of air suspension operation, I have customers who have modified their air suspension by welding a chain between the axle and a crossmember to limit downward travel to something less than shock length. I sell dozens of shocks every year to those who don't do this. Suspension manufacturers rely on the weight of the vehicle to keep the bags within their operational lengths when the tires are on the ground. I doubt we could see total bag travel unless trying to level the coach on very uneven terrain. Empty trailers as Joe described, especially those with spread axle suspensions will often lift an axle completely off the ground under certain conditions.