M450 Steer Tire BlowOut

[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.