[Vwdiesel] Air Brakes

[Bryan Belman]

Thanks for a GREAT description of air brakes to hydraulic.  I had no idea the heat could build up that quickly in a school bus, but if it were to build up because of dragging brake drums, that would be the end of the brakes for sure, big safety issue.


 Bryan Belman, Pt. Pleasant, NJ
04 Jetta Wagon TDI PD, 100hp, 5sp -- running :)
92 Jetta 1.6 Eco-Turbo Diesel -- running :)
82 Diesel Westy 1.9NA -- running :}
70 Type 1 stock Beetle -- Not running :( 




________________________________
From: Kurt Nolte <syncronized_turbo at yahoo.co.uk>
To: vwdiesel at vwfans.com
Sent: Wed, December 2, 2009 12:05:06 PM
Subject: Re: [Vwdiesel] Air Brakes

Dave Cook wrote:
> I could never figure out why air brakes were better on the bigger busses than hydraulic.  Seemed like the mechanics were always needing to dink with something in they systems because the tank wouldn't work right, or there was a problem with one of the cylinders, or something.  Whereas on the regular busses, the only brake issues ever were typical replacing pads/rotors occasionally.  
>
> I can see on a semi tractor/trailer where air brakes would be beneficial, IE being able to hook up the trailer, and have brakes without the problem of bleeding hydraulic brakes or something like that.  
>
> But why on a larger bus would they be better?  Just seems like more possibility of things to go wrong.

One advantage is the inherent safety feature: something goes wrong, the 
brakes engage, end of story.

The other is the fact that the "working fluid" is constantly being 
cycled out of the system, and the temperature of the fluid makes no real 
difference. In a hydraulic system, you'll continually build up heat in 
the brake fluid as you engage the brakes repeatedly. Eventually you can 
build up enough heat to boil your fluid, then you're screwed. The 
heavier the vehicle, the faster you build up that critical volume of heat.

With an air brake vehicle, you're dumping that hot air out of the brake 
lines each time you depress the pedal, replacing it with cooler 
compressed air each time you release the pedal. You just don't ever 
build up the kinds of critical heat levels that you can in hydraulic 
systems, because you're cycling the working fluid out so frequently.

The bus company I work for has three hydraulic brake "school bus" 
chassis vehicles, and their brakes are /horrible/ once you get deep into 
a work shift. We use them for short trips and short shift replacements 
because of that fact. School buses aren't meant to run stop/go for 
8-9-10-15 hours per day like a heavy transit bus, so they can get away 
with having hydraulic brakes.

Most school buses, at least in the local districts, are air-brake 
vehicles now. South Carolina automatically tests you for air brakes when 
you take your CDL tests, unless you specifically request not to be. It's 
not an endorsement anymore, but you get a restriction if you don't pass 
that part of the test.

I'm not sure when you drove a school bus or how old the air tech on the 
ones that constantly had to be messed with was, but ours are pretty 
reliable. They do require more frequent servicing than a hydraulic 
system might, but that service is pretty limited to just draining 
accumulated water from the system and making sure the air dryer is 
refreshed on schedule.

-Kurt


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