air over hyd. trailer brakes ??

[SuperJoe]

In all towing Ive done the tow vehicle has had an electric adjustable trailer brake module. This is a very simple design, and easy to use and set up based on load. My deuce has the air brake ports on the back of it for trailers. I did not understand how air brakes worked so i searched out this very well put together video.

http://www.youtube.com/watch?v=42u4v-AbWA4

After i watched it i have full understanding of how the air system works. My question is in a deuce with air over hyd brakes what controls the sensor pressure so the trailer brakes are matched to the trucks?

 

[rosco]

Interesting question! I have thought around the edges of it, but really never got down to the explanation of it. Since you mention the Deuce, lets keep it simple (at least to start with), and use the M105, as the trailer, notibly the early 105, that doesn't have the "break-away" feature.

I think the simple answer is, that the braking capacity of the trailer, is engineered/designed to match the Deuces brakes. I have pulled the 105 many thousands of miles, with both the M211 & M35, re-built the air chamber/master cylinder, numerous times, and can attest that the brakes on the 105, do, in fact work. BUT I have never seen the tires skid, on the 105, under any conditions. Loaded or empty.

 

[swbradley1]

Excellent video except it still does not tell me why you have to have pressure to get an air brake equipped truck to move. The video in the left corner of the screen after the military video explained it perfectly though. Service and spring.

Next time we get a truck to smoking I'll know why.

 

[sandcobra164]

I'll go with rosco's explanation and I'll explain why. The M105's brakes are matched to the truck's braking system. I recently pulled an MK14 trailer home and of course, had the air lines hooked up. I barely had to use any brake pedal at all compared to normal unhooked driving. The trailer brakes on that combo were applied much more strongly than what my own truck was calling for, therefore the trailer was stopping the truck. I did not try a panic stop but I'm confident that I could have locked the trailer brakes long before I locked my trucks tires up. The MK14 was never designed to be stopped behind a Deuce but rather a straight air brake truck.

 

[SuperJoe]

So is there a way to make adjustable if the braking system should change? As in what if a deuce is bobbed, or converted ti disc brakes, or both?

 

[rmgill]

Joe that's where the tricky area of engineering and implied legal liability come into play.

 

[SuperJoe]

rmgill thanks for bringing that to attention. I would assume pulling around a loaded trailer without a matched set up would be far more dangerous than an adjusted trailer. Do you have any thoughts on how to adjust one?

 

[rmgill]

In my own case I built a system to run an electric brake trailer off my truck with a dash mounted brake controller allowing me to adjust the braking force as necessary. For something small like an M105 I"d just go with the air over hydraulic. With the Electric system towing a 5 ton laden trailer it demanded the ability to test the brakes and adjust for effect.

Take it out and test it. The only place where it failed me was when on a down grade on a dirt road which had a patch of ice and where the trailer was unloaded entirely. Brakes on the deuce worked, sort of brakes on the trailer didn't and the trailer nudged the truck a bit. Let go of the brakes and it was all fine.