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Author Topic: Runaway bus?  (Read 1369 times)
windtrader
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« on: November 02, 2017, 10:08:07 AM »

An interesting incident yesterday got me wondering if anyone's heard of a bus losing traction while parked with brakes set?

The neighbor was getting a load of wood delivered. The 3 axle truck was backed up a fairly steep asphalted slope. After unloading about half the load, the truck decided to take off down the slope and into the property across the street. Fortunately, the truck stopped before ramming the house and just some shrubs got roughed up.

How this happened got me wondering if it can happen on a bus. The truck tag was down to help carry the load. As the weight came off, the tag stayed down while the drive axle started unweighting. That allowed the truck to lose traction and set it down the hill.

Is there anyway a 3 axle bus could somehow lose traction on the drives, is there any precaution one should take when parking a coach?
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Don F
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« Reply #1 on: November 02, 2017, 10:23:26 AM »

Were the truck's drive tires locked up and sliding or were the brakes out of adjustment so the spring brakes were not holding?

In reference to coaches:  The drive axle DD3's on older MCI's are notorious for not holding well when the coach is liable to roll backward (with park brake applied).  One should carry a wheel chock and use it if parking pointing up hill on grades.  Even if brakes are adjusted properly.  This was SOP when we operated MC8's and 9's.
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« Reply #2 on: November 02, 2017, 10:54:17 AM »

Anything can happen with a load on a slippery incline.  I've seen a single axle loaded farm truck slide down a driveway minutes after backing up that same driveway.  Once traction breaks the sliding resistance is less than the static resistance so the unit keeps sliding. In other words, the unit can sit there for a long time but once it starts sliding its not going to stop until it gets to the bottom of the incline or until something else intervenes to stop it.  In the case I'm referring to only one side broke traction so the wheels (duals) on that side actually rotated backwards as the truck slid down the incline.  Buses can't defy physics - they're big heavy pieces of machinery and gravity always wins eventually.
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R.J.(Bob) Evans
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« Reply #3 on: November 02, 2017, 10:59:40 AM »

If the coefficient of friction between the tire and the asphalt is low enough, the weight of the truck can easily overcome the force required to start moving(sliding).  Heat from the tires, oily residue on the asphalt, moisture on tires, angle of the slope, weight of the truck on each axle are all factors.  

I have "slid" off a 2 degree slope on a 4 wd  backhoe with all brakes locked!  The ground was damp clay.  I had to turn around and drop the hoe to stop.  Even putting the bucket down didn't stop me!  It was scary!

The precaution is to use "chocks", and when that fails ---run fast in the opposite direction  Grin

Steve
 
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Steve Toomey
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« Reply #4 on: November 02, 2017, 11:23:16 AM »

I worry about this also, as the Scenic has the ability to unload the tag axle which is where the spring brakes are located. So if the brake is set and the tags are unloaded on a slippery surface it could get interesting. I however always chalk and axle to be safe automatically while I park though.

Safe travels.

Jack
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eagle19952
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« Reply #5 on: November 02, 2017, 11:38:22 AM »

An interesting incident yesterday got me wondering if anyone's heard of a bus losing traction while parked with brakes set?



Is there anyway a 3 axle bus could somehow lose traction on the drives, is there any precaution one should take when parking a coach?

yes. adjust your own brakes. trust no one to do it for you.
the plaintiffs attorney will eat you alive.
blaming the mechanic doesn't work there.
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Donald PH
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« Reply #6 on: November 02, 2017, 12:45:15 PM »

When I was a young truck driver I had a Peterbuilt come back from the shop with new brake shoes and linings.  When coming down a long hill some white smoke came out of the brakes at the wheels as the brakes were applied. What happened is the new linings did not completely "arc" to the old brake drums and the linings were worn down to match the arc of the old drums. After I got to my first stop the brakes were set and the tractor started rolling; the brakes were completely out of adjustment.   The same will happen to an older MCI bus if the brakes are out of adjustment or if air leaks off the brake system. 

This will not happen to newer trucks or buses which have spring brakes or "maxi cans" because the strong springs holds the brake linings against the drums.  On the older buses, wheel chocks are definitely a good idea.
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brmax
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« Reply #7 on: November 02, 2017, 02:31:44 PM »

I dont really understand worn down to match the arc of the old drums.  Roll Eyes
But i prolly dont want to
I stress when adjusting brakes to have the wheel off the ground and able to turn.
Always an interesting subject for sure,

Good day
Floyd

ps: I think it’s a good thing to “see” and understand how your vehicles brakes work.
      Take the covers off any brake access points so to veiw a few brake applications.
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richard5933
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« Reply #8 on: November 02, 2017, 03:23:09 PM »

Speaking of chocking wheels... After doing lots of reading on the subject I found that chocks should be 1/4 the height of the tire to be most effective. That means 10" tall chocks for most of us. Not easy to find, but since my driveway has a decent grade I sprung for two. The 10" chock looks small in the photo, but next to what's usually used these chocks are huge.

Richard
1974 GMC P8M4108A-125 (Current bus)
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« Last Edit: November 02, 2017, 04:34:10 PM by richard5933 » Logged

Richard
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chessie4905
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« Reply #9 on: November 02, 2017, 04:27:34 PM »

Don't forget a leaking rear oil seal coating the lining.I still prefer synthetic grease and proper seals for the older coaches, at least, but that is another discussion.


Here's some, albeit lower, but they're cheap enough to get four or six? Also check your coach on a fairly steep grade,safely, to see if your parking g brakes are holding. Not a substitute for proper chocks, of course if desired. I have my coach chocked on level floor in shop while rebuilding dd-3's.
« Last Edit: November 02, 2017, 04:38:10 PM by chessie4905 » Logged

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« Reply #10 on: November 02, 2017, 04:30:36 PM »

"what usually gets used"

Is inadequate.

Be very careful following the herd...

What do the big boys with ample capital do?

happy coaching!
buswarrior
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Frozen North, Greater Toronto Area
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« Reply #11 on: November 02, 2017, 04:35:24 PM »

Weak parking brakes is a devil.

Both spring and DD3 chambers are capable of moving the actuating mechanisms, fooling all watching that something is happening, but they might not "squeeze" according to spec....

Brake dynometers, drive on rollers, are hard to come by in North America...

Test your parking brake, give it pull while rolling along on a quiet side street, see if you like the resulting stop.

Long ago, there was a pair of videos... I did a GM 5307 transit, another busnut did their coach, before and after "adjustments"...

Crumbs, who was it? And where did it get posted?

happy coaching!
buswarrior
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chessie4905
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« Reply #12 on: November 02, 2017, 04:50:35 PM »

If the big spring in a "maxi brake" breaks, or part of it, it obviously won't do it's job. Sometimes you can't tell spring is broken. Bus Warrior is right on. Check them at soonest chance, and if you are concerned of going through windshield, as some were concerned on a prior discussion, release them before you come to a complete stop, or right before nose contacts windshield.
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« Reply #13 on: November 02, 2017, 05:54:44 PM »

Just for fun...

The power of the archives of the Boards:

Google "buswarrior parking brake stop"

http://www.busnut.com/bbs/messages/233/18528.html?1187138090

Back when I typed in full and complete sentences....

happy coaching!
buswarrior
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buswarrior
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« Reply #14 on: November 02, 2017, 05:57:47 PM »

And from BCM:

http://www.busconversions.com/bbs/index.php?topic=1489.55;wap2

Where is Buffalo SpaceShip???

The site at the end of the link is long gone...

happy coaching!
buswarrior
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