M450 Steer Tire BlowOut

[dentmac]

Hi,
Failures have been 2 lefts and 3 rights.

(01-25-2015 21:42)patticake Wrote:  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.

[patticake]

That is weird, in other words, no rhyme or reason. Do you know if they were worn out on the edges(as in an alignment problem) or problems with the tires on the rim. Could the max air pressure(cold) have some bearing on the situation. Did the tires blow or did they come off the rim? Could the rims themselves, be the problem? Without examining them(in person or with pictures) we are all just guessing. Really need more information to make a rational thought on the situation.

[GregOConnor]

Usually tire failure is casing damage. A tread can blow off and still leave the casing drivable. The other situation of rapid air loss is when the bead moves off the rim. Are Bead-locks, Split rims, or tubes an option?

[cmillsap]

(01-25-2015 22:52)dentmac Wrote:  Hi,
Failures have been 2 lefts and 3 rights.

Ross,

As you know, this same Meritor IFS is also fitted to the M380. Although I have no data to confirm, I haven’t heard of this problem happening on the M380. I think that some parts of the Meritor IFS were beefed up on the units fitted to the M450. We also know that BB recalled the M450 and made modifications to lower the weight load on the IFS and front tires.

So the questions that should be asked are: Why is this happening on the M450 and not on the M380? How close are the steer axle/tire weight loads comparing the M450 with the M380? What (if any) are the differences in the Meritor IFS fitted to the M380 versus the M450? Are the alignment specifications the same? Is the wheel cut the same? Is the suspension the same? Is there some modification to the suspension on a M450 that allows too much weight transfer/load on a single steer tire around a curve?

If we can document the differences between the two applications, we may be able to better define the cause.

Chuck

[GregOConnor]

tag and drive tires force bus in straight line. when in a slow turn, require more effort and stress on front tire casings to make a turn??

[dentmac]

Hi,
It is exactly the same axle in both vehicles.
The recall was called as it was found the tie rods were manufactured with a lesser steel then the specs. This was learned when Bluebird tried to upgrade the axle to 18,000GAWR. The tierod had to be upgraded to pass the 17,000 rating. The tie rod is actually rated to 18,000. but the rest of the axle and components could not be up rated to more then 17,000 even with the reduced testing criteria used.
It was later realized that the tie rod was a safety issue to the 16,000 GAWR transit buses so a new recall for them was posted.
So the only official recall remedy was to replace tierods, change tires (same size and rating), alignment and add placards. Reducing weight had no methods listed but included using different weighing criteria and reducing tag load.


Ross,

As you know, this same Meritor IFS is also fitted to the M380. Although I have no data to confirm, I haven’t heard of this problem happening on the M380. I think that some parts of the Meritor IFS were beefed up on the units fitted to the M450. We also know that BB recalled the M450 and made modifications to lower the weight load on the IFS and front tires.

So the questions that should be asked are: Why is this happening on the M450 and not on the M380? How close are the steer axle/tire weight loads comparing the M450 with the M380? What (if any) are the differences in the Meritor IFS fitted to the M380 versus the M450? Are the alignment specifications the same? Is the wheel cut the same? Is the suspension the same? Is there some modification to the suspension on a M450 that allows too much weight transfer/load on a single steer tire around a curve?

If we can document the differences between the two applications, we may be able to better define the cause.

Chuck
[/quote]

[cmillsap]

(01-26-2015 18:10)GregOConnor Wrote:  tag and drive tires force bus in straight line. when in a slow turn, require more effort and stress on front tire casings to make a turn??

Interesting observation, Greg,
This is why I asked about the steer wheels cut. Remember that the M380 has no tag axle. I could turn my M380 on a dime. I don’t know what the degree of wheel cut is on this Meritor IFS but there has been a trend lately by motorcoach manufacturers to increase the wheel cut to attain a smaller turn radius. Some (I think) are up into the 60 to 70 degrees area. Extreme wheel cut on a tag fitted bus could put a lot of side wall pressure on the steer tires.
Chuck

[davidbrady]

Here's a first hand account of a blowout in an M380: "blew like a stick of dynamite I was fighting her to hold it on the road".

The differences are, of course, weight, wheelbase, tag axle, and power to weight.

I've never experienced a blowout in a bus so I can only imagine how violent the steering reaction must be.

Imagine rolling along in a totally relaxed state - everything seemingly working beautifully. Suddenly a loud gunshot as the right front tire blows. The steering wheel response goes numb as the tire loses 90% of it's air in less than a tenth of a second. There's nothing supporting that corner of the bus since the tire radius is a fraction of it's fully inflated size. The bus rolls to the side and heaves forward offloading the weight to the left steer tire and the right drive tires. Within a second the full load of the right front of the bus comes crashing down on the right steer wheel and what's remaining of the tire. The suspension fully compresses. Due to the combined result of the tire's air loss and the crushing weight of the falling bus, the tire's rolling resistance spikes producing a clockwise yaw on the bus. The yaw is exacerbated by any bump steer, roll steer, or scrub radius of the suspension. The driver has tenths of seconds to take notice, sit upright, get both hands on the wheel and prepare to countersteer the violent thumb-breaking pull the the right. He also has to get on the fuel to try to counter the yawing motion; that is, if he's not traveling so fast that there's little additional acceleration on tap. The driver also needs the physical strength to hold the wheel in a counter rotation for the 10 to 15 seconds required for the vehicle to stabilize, slow down, and stop. If he counters too hard he may over correct causing a loss of control due to oversteer. Certainly not a pleasant experience.

So, I can see how an M380 might be more adept at powering out and thereby countering yaw, and I can also see how the additional weight of the 450 falling down onto the deflated tire at 2G's or more of dynamic force, rebounding, and falling again can be more destabilizing than the M380. I can also see that a longer wheelbase of the M450 should result in a slower yaw rate, but when sitting at the front of the bus, the tangential velocity felt by the drive may be just as great (given the longer wheel base).

I'd really like to see some yaw stability control, but w/o a factory that's a pipe-dream. A Safe-t-Plus style steering stabilizer may not be out the question. (The power steering cylinder that Prevost has on the steering idle arm is effectively a stabilizer in that it absorbs road shock). I wonder too what impact 10.5" wheels with 365's may have - good or bad; they undoubtedly change the steering geometry.

[cmillsap]

David,

Do you know the degree of wheel cut on the Meritor IFS?
I wonder if the forward pressure exerted on the steer tire sidewalls when making sharp turns in a M450 would structurally damage them over time and cause one to eventually fail. Obviously, the more the degree of wheel cut, the more the odds of damaging the structure of the tire. Consider that all the blowouts on the M450s happened when they were traveling at +55 MPH at which time the damaged tire would be at its highest pressure, temperature and service load.

I never really paid any attention to the wheel cut on my M380 while I owned it but it sure does have an excellent turn radius given the shorter wheel base. I suspect it has a pretty wide degree of wheel cut also which may be just fine for a two axle bus but not good for a three axle bus.

Chuck

[davidbrady]

Hi Chuck,

The Document Library->Owners Manuals->2003 M380 Data Book.pdf has a max turn angle of 54 deg.

The Document Library->Brochures and Ads->450LXi_brochures_specs.pdf also has a max turn angle of 54 deg.

My 18000 lb steer axle on SpongeBob has a max turn angle of 56 deg.

Both the M450 and the M380 use the Ross TRW model THP-90 power steering gear box.