M450 Steer Tire BlowOut

[davidbrady]

Thankfully Leroy Wilson and his wife are uninjured. They experienced a steer tire blowout in their M450 on the way to Quartzite. Leroy's an experienced driver; the bus veered into a muddy median strip and stayed upright. We wish them the best. Leroy's bus is one of the few and gorgeous M450 show coaches with matching trailer. He and his lovely wife took Suzana and I on a tour thru their bus at RITV 2007.

Details so far:

Cruise set at 65 mph, tires under 2 years old and under 7000 miles, pulling 3 axle enclosed trailer. Damage isn't severe: broken front cap, shock, and leveler. They should be able to drive it back to Alabama.

Good luck Leroy!

[mikebulriss]

It's always good to hear when someone gets through that experience and that they are safe. You would have to think that his experience and good reactions, especially coming off cruise, were probably very helpful in his recovering and getting it stopped safely. Where were they when this happened?

I do have some other questions if you talk with him. Can we assume there was no road debris that caused this? If it was road debris, forget the following questions.

1. What psi cold tire inflation does he run?
2. 315 or 365 tires?

Thanks,

[dentmac]

Hi,
FYI
Tire pressure was 130 PSI, so 315 tires. No road debris. 2 year old tires.

None of the 4 previous tire failures involved road debris either. (5 total )

[GregOConnor]

two objectives here ;how to prevent a tire failure and ; how to control one.

How to prevent a tire blowout can be helped with Ross Macs quest to learn what happened to cause the blow out. I add shocks and porpoise motion magnified by tongue weight and cadence. there must be great force on the two front tires during momentary weight changes going down a uneven road surface.

[patticake]

There are so many variables here that we(with older coaches) don't know about that we could fill a book with "if this happened.......", that I think Ross or someone with suspension knowledge of that type of suspension, and is able to look at that coach(and blown tire) to determine the cause, or we may never know.
Ross has said that there are others with this same failure, so we know something is going on, but what, we may never know, or he may not be allowed to say.
Moving on......

[dentmac]

[Thank you Steve.
There are many variables that play(ed) a role in losing control after a front tire failure.
Obviously trying to prevent a failure is paramount. The items here well known.


The cause of one or two crashes in a vehicle type can be just the luck of the draw. But 5 tire failures (with no contribution of road debris) ?

Control loss happens... but 100% would be worrisome.

Just some comments:

Michelins have no known issues.
65 MPH is reasonable
Tire pressure at 130 cold will go to 155 easily.

IFS has a Scrub Radius that changes with suspension compression. The concept is that while in a turn,the outside suspension compresses causing the wheel to want to center (by trying to turn out- understeer effect.) With a tire failure, the suspension on the one side collapses and tries to turn out...wrong way.
Perhaps a composite runflat (Bulldog) would help as the rolling radius would not decrease as much as the Tyrol runflat.
It is reasonable that an anti sway system would help

A steering system has to be designed for a given load, so one designed for 13000 GAW may have issues when pushed to accept 17000 +.

NHTSA may have a role here.

Ross

[davidbrady]

All I can do is point out some differences between my H3 and the M450. Given that Prevost has billions of road miles of experience this may not be for naught.

Here's a frontal view of my suspension.


You can see the Prevost kingpin that Ross talks about, circled in red. Having the kingpin inside the wheel gives the designers the ability to get kingpin inclination (KPI) and scrub radius as close to zero as possible - as opposed to SLA (short long arm) ball jointed suspension with uprights which necessarily have to fit outside the tire/wheel assembly. (Given the size of the hub, brake, wheel, and tire there are practical limitations). I don't know of any good result from either of these suspension characteristics with the exception that some KPI adds a degree of positive camber in a turn. Additional caster tends to add negative camber in a turn; with KPI the camber can be balanced out. KPI, caster, and scrub radius all lead to less steering feel, axle weight jacking in a turn, and greater loads on the steering system. The takeaway is that Prevost has more control over KPI, scrub radius, and caster, and Prevost takes steps to minimize stresses on steering components.

Another way they minimize stress is by adding a hydraulic cylinder to their power steering. Prevost uses a ZF power steering gearbox along with externally mounted hydraulic cylinder assist. This assist increases and balances the forces acting on the IFS steering system. Here's a view of the system:


The hydraulic steering box provides power to the bell crank; the power hydraulic cylinder provides power to the idle arm. The Prevost Docs go on to say:

"The hydraulic power cylinder provides an added source of assistance and being connected to the R.H. wheel, makes it such that the total steering forces are produced with minimal stress on mechanical linkages."

[dentmac]

For side by side comparison here is the Prevost and the Bluebird RIS 13/16 suspension

[GregOConnor]

I Wicked Scrub Radius http://en.wikipedia.org/wiki/Scrub_radius and have a bit more understanding. It seems that the discussion on all the king pin scrub radius, FIS has to do with control after a failure. I wonder what causes the failure. Does the design give more stress to one side tire in a turn then fail going down the road later?
My background is digging holes and I dont know much about the dynamics and geometry involved in this discussion but, has slow moving stresses been considered ? since the suspension is designed to add camber in turns and change scrub Radius for control what happens in a slow turn without the forces of inertia making these changes? could it be slow turns where the tire casings are being stressed?

[patticake]

Ross, on all these "accidents", was it on the same side, as Leroy's was on the left(from what I read in the first mention) or just a front tire?
Just a thought from a previous situation that I had with a car manufacturer some years ago, when I was a fleet manager. I won't go into detail at this point, as it may be a mood point. But it also involved front tire situations with blowouts.