Hi Paul. I have given this more thought. A free bowden cable does indeed have equal and opposite forces on its outer and inner, i.e. the inner tension equals the outer compression. "Bowdenex" cable was a refinement for high-force applications which used square section wire for the outer, to better resist the high compressive forces without deforming. Of course, in a conventional bowden arrangement the outer may not be completely free if it is for example clamped to the same chassis at both ends. In this case some of the compresssive force is relieved from the outer and borne by the chassis instead. In the extreme the outer would be clamped all the way along its length in the manner of a conduit, and 99% of the compressive force would be relieved from the outer and transferred to the chassis. Your setup relies on being a free bowden arrangement however.

With no friction, your setup has a tension of let's say 100 lb in both front inner cables at a particular pedal pressure, measured at the "sending" rear end of the cables. It has a compression of 100 lb in both front outer cables at the same point (from the brake pedal via levers). The tension in the common cable going back from the pulley centre to the rear will be 200 lb, and the tension in the rear cables would be 100 lb each were it not for the rear swingle arm. The rear swingle arm will reduce the rear tension, biasing the braking effort to the front.

If there is friction in the front bowden cables, the frictional force is subtracted from the tension at the "receiving" wheel end. To take an extreme case, the tension in the inner cable at the wheel end could be reduced from 100lb to 60 lb. The compression in the outer cable at the wheel end will also be reduced from 100lb to 60 lb. The rear brakes, being fed via friction free open cables, will still receive their full whack.

Its my experience that frictional losses in curved bowden cables build up disproportionally with the tension applied at the sending end. With a worn or poorly lubricated cable I wouldn't be suprised if one could even reach the point where there is a maximum tension at the receiving end which couldn't be exceeded no matter how much tension was applied to the sending end.

To digress slightly, one way to reduce the friction problem is to swap large cable tension with small cable movement for lower tension and larger movement. This is now done on bicycle brakes by the use of effectively longer levers at both ends of the cable.

Location: New Forest

After much thought and a fair bit of help from this forum, including Ian and Dave who contacted me by email, I've finally got the brakes working to an acceptable standard. I thought I should report back and detail what the problem was. In fact it wasn't one problem but a combination of things, which together caused an imbalance to the extent that the front brakes weren't coming on as all the effort was going to the back brakes. There was sufficient friction in the Bowden cables for the back brakes to "pull against them" without the front brake shoes reaching the drums.

Although when I disconnected the Bowden cables I could pull and push the inner cable fairly easily by hand the drag increased with the load which then caused the back brakes to come on. (A look at my rather rough schematic further up this thread might help in understanding this). The vehicle has been sitting in storage for some time so I suspected dry grease or dirt inside the cables so I removed the grease nipples from half way along the Bowden cables and using plenty of penetrating oil and an air line blew out all the old grease/oil or whatever was in there from the centre outwards. I did this many times and finally lubricated the cables with a thin oil. This made the inners easier to move than before - but the front brakes still didn't come on.

I couldn't see any straightforward way of making the fronts operate any more easily so decided that increasing the force needed to operate the rear brakes might correct the balance issue. I did this using two springs, one stronger one attached between the connecting bracket and the chassis which the front brakes pull against before the rear brakes move and a second weaker spring attached between the rear swingle tree and the chassis. The cable running to the front brakes pulls against these springs and the front brakes are now pulled on before the rears move. The two new springs can be adjusted for tension.

Whilst I still have some fine tuning to do all four brakes are now working, with the fronts coming on first. A huge improvement!

A big thanks to everyone who gave thought to this and offered helpful suggestions as to how it might be resolved.

Paul.

Location: Tauranga NZ

Hi Paul

I think you are trying to push it uphill. Some handbrakes do use the conduit in full compression but seldom applied at full force. In conventional applications friction is incidental to the pull and push. In the system you have the whole push is deliberately transferred from conduit to cable. One of the most experienced contributors regularly recommends abandonment of customary Bowdenex system on the basis that too much friction and no better than conventional. Yet this is just incidental friction from operating one wheel, not the transfer of half the total braking force from conduit to cable.

Whoever devised it deserves Brownie points for lateral thinking, if nothing else.

I would love to take it around the testing stations for a w.o.f! The regulatory authority proclaims that their ex bullock cart "machanics" are all expert.

Bob Culver

Location: Auckland, NZ

WoF time coming up shortly Bob. Could be a challenging time for all concerned!

Regards
Paul.

Location: Tauranga, New Zealand

Clearly its a cold winter in NZ with all these kiwis parked on the net..
In Invercargill we are awaiting news of the trip to the MOT with interest
cheers
Peter

Location: NZ

In NZ the Vintage Car Club (which is a national body covering all makes) issues, in accordance with the V.C.C Technical Code, what is known as a Vehicle Identity Card. Now of course this system is not perfect and can be abused by dishonest owners providing falsified information, however any application is vetted by the club register, and the car is inspected by Committee members of the local VCC Branch so holds some validity. In my experience if you present one of these Identity Cards to the Tester it greatly eases the WOF (MOT) inspection process, the tester is far less likely to apply modern car standards.

Some of us do still use our cars even during the darkest depths of winter, I must admit though, up here we barely have a what you can call a real winter, although I did get caught in a brief Hail storm last week!

Location: NZ

Thanks Ian that's good advice. I'll take my Vehicle Identity Card along with me in the hope of smoothing the passage through the Warrant of Fitness test. I'll report back in due course, but it will be after I've had a couple of weeks in warmer climes. I think "winter" is a relative term!

Regards
Paul.

Location: Tauranga NZ

On the NZ winter theme, in the 1960s I was transferred from windy Wellington to much colder Dunedin. I departed the ferry at Christchurch in my RP very early on a bleak winter morning and set off on the 240 mile jaunt. With the cold, and then no other traffic on a Sunday morning, on the trip across the plains I thought I was in Siberia. The car had no handbrake gaiter and other vents so despite wearing all the heavy clothes I owned it was bitterly cold. Near Dunedin saw snow close up for the first time. Curiously, despite living outside, and regular 25 deg F frosts, the car on 6v used to start OK every morning. Assisted only by an old coat over the bonnet. And no antifreeze!

(In howling cold southerlies for which Wellington is renowned, my car with SAE50 could not make top gear on the flat! But I was once pulled over for doing 60 mph the other way!)

Bob Culver

Location: Auckland, NZ

Paul,
Disregarding the foot pedal bits, if each brake lever required A given amount of movement to apply the brake, the way you have drawn the rear swingle tree lever you would have double that amount being applied to the rear brakes.
I realise your scale may be a little out but it would still multiply the distance moved as drawn.
If it was connected directly to where you have shown the handbrake lever (no swingle at the rear) then all brakes would receive the same travel.
Would this help to prevent the rear brake from having preference over the front ....
I am disregarding friction just thinking of levers and mechanical advantage, just looking at things from a different angle.

Roger

Location: Haverfordwest

Hi Roger,

I see what you're saying but looking at the swingle for the rear brakes, if the force pulling on the rear brakes moves over a greater distance than the distance moved by the cable coming from the front which is applying the force (which it is) then surely the force applied further out from the pivot is proportionately reduced? Put another way, imagine pulling on the swingle by hand adjacent to the pivot point. If the load is on the end of the swingle furthest away from the pivot (and your hand) the amount of force you're able to create at the far end of the swingle will reduce the longer the swingle is, even though it will travel over a greater distance?

Perhaps there's a mechanical engineer on here who can explain this?

Regards
Paul.

Location: Tauranga NZ

John Cornworth emphasised the point that once operated you can largely forget the movement and just consider the tensions, in which case the rears are less than the assumed frictionless fronts because of the lost leverage in the swinging link.

I guess most did levers at secondary school and most are familiar with torque wrenches. To operate grasped half way down shaft, have to pull twice as hard for the same "leverage".

Bob Culver

Location: Auckland, NZ