Just how good should the brakes on my Ruby be?

[Ruairidh Dunford]

I don't think that is fully back Denis - I suspect the handbrake push is not free and stopping it from going all the way.

It should sit like this:

[Denis Sweeney]

A bit of a calculated guess here but the angle you have there in your photo looks about 30 degrees?
Not sure I have as much as that but I would guess it would be nearer 20 degrees, the photo I sent is deceptive.
However, I will check tomorrow, I am not sure where the handbrake push could be jamming, it does feel free.
The lever on the drivers side is very tight against the brake rod and not a lot of room to have a looksee.
Will let you know
Cheers

[Denis Sweeney]

Hi all, I realise I am opening myself up to ridicule and abuse here again but I had a bit of a Eureka moment this morning at the unearthly hour  of 4 o’clock regarding the set up of the brake lever arms attached to the Cross Tube on my Ruby, following previous comments
Despite edging nearer to my 3 score years and ten than I would like, I suddenly thought back to my applied mathematics and mechanics from my old school days.
As a result I am pretty sure that to apply the most force into the system the two outer levers on the cross tube must be as near to top dead centre of the Cross Tube as possible, in such a manner that the prior to application of the foot brake the leavers lean towards the rear and on full application of the foot brake they should be then leaning towards the front  i.e. the total arc length of Cross Tube rotation during application of the foot brake must be split equi-distant about top dead centre. I realise that some may say that the pull should stop at 0 deg but the further away from top dead centre the bigger the loss in pulling power due to an angular pull on the cable
Similar to what we all try to achieve with the brake cam levers on all four wheels only at the brake levers we want finish at 90 deg.
Similarly the lower central lever should be set using the same principles.
I attach a pdf of my scribbling which as I say are from a long time ago but I do not believe the maths or laws of physics have changed in that time, altho’ my maths may need checking!
Having stripped the system several times during the past weeks, the basic set up on Ruby has changed very little, and not surprisingly neither has the performance of the brakes, which may denote the onset of madness, keep doing the same thing expecting a different result.
So I have a couple of other thoughts, how do we know just how suitable the brake cables we buy from our normal suppliers are? This is no criticism of anyone, but how do we know if the steel used is of the correct grade, one assumes the steel would be such as to limit extension under load if it doesn’t an awful lot of force could be lost in cable extension.
Some may have seen a post on this thread by my brother who built an amazing Ruby based special in which he asked a similar question. He had no problems with his brake system until he replaced the cables and now the brakes are not as good as they were previously, he posed the question about cables relaxing under load.
Normally all steel has an elastic limit and only when this is exceeded does the steel become plastic and eventually fail.
The other question is the suitability of the brake lining materials how do we know they are correct.
All comments gratefully received.
Best regards to all
Denis S

[Mark Dymond]

Denis
While in no way suggesting you are incorrect in your conclusion about the best angle to set this lever to, for the sake of accuracy I think you are wrong suggesting force is a a maximum when the cable is 90 degrees to the leaver, travel is maximum here, force is minimum. Bear in mind one attribute of a "cable in tension" is all force is coupled along the cable. Its not the case where have a component of force along and one perpendicular, as if it where a rigid body. At least that's the way I remember it goes.
I'm just being pedantic - maximizing the travel is probably what you need here anyway. ;-)

[Tony Press]

Mark,

Not sure I understand what you are saying- when the brake cable is at right angles to the brake lever the force in the cable is at a maximum under the applied torque from the brake crossbar.

This also applies in reverse to the pull from the cable acting on the hub brake levers.

The 'effective' brake lever becomes shorter as the brake lever moves forward from the vertical (where the cable was at right angles to the lever) thus giving less force or pull in the cable.

[Denis Sweeney]

Mark, the upper brake levers on my Cross Tube are set at about 10 to 15 degrees towards the rear of the car and it had been suggested that the lever needed to be rotated further to the rear and that this would improve the set up. This I was finding impossible to achieve even by releasing everything on the system. Please refer to earlier photos. 
But in thinking about the system early in the morning my view was that the nearer to the vertical as shown in black on the earlier attachment this will give the greatest horizontal movement for any given rotation of the tube and I thought the greatest force. 
Being silly and taking it to the extreme, if the lever were set just off the horizontal, for the same given rotation there would be virtually zero pull on the cable and zero force and so my reasoning is that the nearer to top dead centre the better. As I say school was a long time ago.
Cheers
Denis S

[Mark Dymond]

Tony \ Denis


If you think of it like a classical lever, force is multiplied by making the distance from the fulcrum to the load small (at the expense of distance the load is moved). Halve the distance, double the force. In the case of constant torque on the cross bar you get maximum movement, minimum force at the magic 90 degrees. In fact in a frictionless incompressable system it approaches infinite just before it cams over (as the effective movement of the cable approaches 0). In the real world (on your Austin) the total mechanical advantage between the pedal and the brake shoe are balanced against enough travel to overcome the cable stretch, losses in joints and the need to move the shoes a certain distance.

The reality is you want to minimise the "force" on the cable component, to minimise the loses from the elastic properties of the cable. This is done by having the levers at 90 degrees to the cable (and why the leaver at the cross bar is longer than the lever at the hub)

Its a technicality really but it's worth thinking about.

Cheers, Mark

[Tony Press]

Not sure that this applies to the Austin Seven setup where the brake pedal pull on the cross shaft is most effective when the shaft lever is vertical - which causes the brake levers to be vertical when pulling on the brake cables.

This applies to the pull on the brake levers at the hub end which give the best pull when levers are vertical with a right angle cable pull and the cams have the least rotation before the brake linings meet the drum.

The original cables have most likely stretched as far as possible by now.

[Mark Dymond]

Tony

Yes, absolutely right. Theres a lot going on when to look at the system as a whole and I in no way want to suggest that the cable should not be 90 degrees to the the lever. I was just being a bit pedantic, correcting the statement the force on the cable was maximum at this point. I'ts a technicality really and I probably should have kept it to myself.

Cheers, Mark

[Bob Culver]

I am glad I stayed out of that round!
Assuming the old chestnuts of appropriate lining material and bedding have been addressed, the different leverages would seem to be source of reduced performance. It would be tedious to change the rear to short levers, esp as these require different cable ends. Assuming the front axle is positively located, probably best to ignore the usual advice to adjust with a lead.
The rear Giirling adjusters can be backed off a few clicks to provide a quick way of adjusting rear side to side balance and trail/lead cf front.