hannah3dog wrote:Probably on here somewhere - I've read it too [You must be registered and logged in to see this link.] Thinking about energy store I think by maintaining the same pressure it prevents depressing the drawbar actuating tube too far which would initiate auto reverse? Kept me awake last night wondering what it did!
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Blow up the above illustration and it can be seen that while the coupling, the handbrake and the breakaway cable all act upon the the brakes, the breakaway cable does not act upon the coupling.
See: [You must be registered and logged in to see this link.] Remembering that these are ALKO brakes. I've not taken the Knott ones apart yet.
Also you can only initiate auto reverse when in reverse.
When reversing, the towing vehicle pushes in the draw
shaft of the overrun device. The brake shoes (1,2) are
pressed against the brake drum via brake linkage,
Bowden cable and expander clutch (3). The brake
drum turns backwards, taking the trailing shoe (1) with
it. The transmission lever (4) swings back and allows
the draw shaft to complete it’s full travel. The braking
effect is virtually cancelled out and the wheels can
revolve freely in reverse(Fig 3).
It needs It must be noted that when the handbrake is
applied, the vehicle may roll approximately
25 cm (10 inches) backwards before the
parking brake force is used to it’s fullest
N.B. Dandys (auto reverse ones) have ALKO couplings and Knott brakes. The above link is the first reference I've seen to the spring energy store as fitted to Dandys. Other than the clearance being given as a different dimension 1mm instead of 3mm it is as per the guide from Dandy. See link above Fig 11 for diagram.
For clearer diagram see. [You must be registered and logged in to see this link.] Fig 10
So as can now be more clearly revealed the coupling acts directly upon the brakes in a linear form. The handbrake and breakaway cable act upon the brakes through the energy store (spring). This must occur to provide a reserve force to prevent the trailer from rolling backwards.
(My Thoughts) The later spring and gas strut systems seem to have an unstable equilibrium or trigger form of activation. As stated in our set up with a breakaway cable we need the spring as an energy store. I'm tempted to think that there is a smoothing action from the cable acting through the spring. If the spring was unnecessary to prevent the trailer rolling backwards I believe a directly linear cable would be likely snap too early when the shock force is applied. I think that the spring acts as a shock absorber and this is also necessary.
I should try an email to ALKO to confirm my thoughts.
I've not been able to properly understand this for a good while. It's like a light coming on.
One disadvantage of this forum is that we tend to answer questions as asked. This thread has gone from a thought on a vintage Dandy to an in depth piece on brakes. This does make it a bugger for us to index things.
I have some more stuff on rod brakes for you and a solution stopping the need for your other half to be jumping in and out the car to activate and deactivate the brakes but I'm going to add it to a new or appropriate new thread.