Posted by Keith in Washington [24.41.55.59] on Friday, September 20, 2013 at 17:36:11 :
In Reply to: bulk of the work posted by clueless [201.202.29.210] on Friday, September 20, 2013 at 10:52:38 :
You need front brakes. They do most of the work. As you hit the brakes the truck shifts weight onto the front axle and the rear axle looses weight. Think about the nose dive you get when you hit the brakes hard. This will cause the rear brakes to lock up much easier than the front which has much more weight on it to start with due the engine and etc. the rear axle is generally supporting only rear part of the body wight unless it is loaded.
The proportioning valve helps the rear from locking up by reducing the pressure going to the rear brakes. If the front and rear brakes are apply equal braking power the rears will lock up and tend to come around on you. The object is to get maximum rear braking without locking them up. Some vehicles had a load sensor on the rear which detected spring compression ie load and adjusted the pressure going to the rear.
Single circuit systems were for drum brakes in general which were far less effective. Yes those systems were more prone to the rear locking up. That is in part why we have dual systems today.
I do not know about semi trucks. I cannot believe they do not have front brakes. They have much more weight on the rear axle when loaded so are less likely to break loose in the rear.