Hydraulics and sums

[chris.richard]

I'm struggling with the concept of the equivalent set up with tandem and dual M/Cs :confused:
The 164 has a tandem mastercylinder of 23mm diameter.
If you change to dual (parallel) mastercylinders of 23mm diameter, and change nothing else, what changes?
Am I right that:-
For equivalent braking, the pedal travel will be the same, but the pedal effort will be double, as you're pushing twice the area?
Help! Car builder in distress! :(

[colin artus]

In the example you gave you are pushing the same piston area in both setups - there are still two pistons in the tandem cylinder. You are displacing the same amount of fluid at the same rate = same effort. The amount of fluid and the area are the same, as in effect, you have the same cylinders operating in line or side by side. The effort only increases when the amount of fluid moved is greater for a given pedal movement. There might be some minor increase in pedal effort for dual master cylinders due to mechanical inefficiency (pushrods operating at an angle) depending on setup.

[chris.richard]

In the example you gave you are pushing the same piston area in both setups - there are still two pistons in the tandem cylinder. You are displacing the same amount of fluid at the same rate = same effort. The amount of fluid and the area are the same, as in effect, you have the same cylinders operating in line or side by side. The effort only increases when the amount of fluid moved is greater for a given pedal movement. There might be some minor increase in pedal effort for dual master cylinders due to mechanical inefficiency (pushrods operating at an angle) depending on setup.

I'm not convinced that's right Colin.
In a tandem master cylinder there is a floating piston that separates the primary and secondary circuit, so they both see equal pressure. The piston just separates a single cylinder into two halves. Since the secondary (floating) piston occupies a constant volume, the total amount of fluid displaced from the m/c is purely the area of the primary piston times its travel. The total displacement is simply the cross section area times the primary piston travel.

If you replace this with two separate master cylinders mechanically connected in parallel the displacement is the combined cross section area times the travel, i.e. to displace the same volume of fluid requires half the travel. However, since the pedal force is now applied to twice the area, you have to press twice as hard on it to achieve the same psi in the hydraulic system.

So now I think the pedal travel will half, and the pedal pressure double, to achieve the same effect at the caliper.

[strat6v]

You educated people :rolleyes:

Chris, what you need to do is reverse engineer this situation a bit and look at the loads you are needing to move, not apply, then all will become clear.

Lets think of your master cylinder pistons being a strat :D ,If you parked yours at the side of stevie boys then turned around, got one shoulder against one car and the other shoulder against the other(balance bar stylee) and started pushing, your little legs would have to exert a certain force. ;) We'll call that force two 'oomphs' :eek:
Now if we line your car touching nose to tail against steve's and you started pushing at the back then you would need the same two 'oomphs' as you had before to move the same two cars the same distance. The weight of one car doesn't magically dissappear just cos you haven't got your shoulder against it ;)

Reckon i could be a physics teacher :p

Once you have tried that experiment maybe you could attempt this one. Grab yourself two buckets and place one in front of each foot. Orientate the buckets till the handles both face away from you ( all will become clear in a moment)Now firstly, put your right foot into the right hand bucket then your left foot into the left hand bucket. Grasp the two handles like you would normally when picking up a bucket of water and pull up on them. You will now magically find that you can 'pick yourself up' :)

Aren't i a clever chap ;)

I'll get me coat. :D

[chris.richard]

I got my feet in the wrong buckets, spun round and fell over. Does that mean that I need larger buckets? And will that mean I won't be able to lift myself so high off the ground? :confused: :)

[Arthur]

This sounds like fun.

Being a BP-trained mechanical engineer, I tried cheap plastic buckets, but pulled the handle off one, and jammed my foot in the other. Repeating with galvanised steel buckets, I ricked my back on the left-hand handle, then fell over. So no data of any value yet amassed. I believe I'll need carbon fibre buckets for the third trial, and possibly SG cast iron as a comparison - two ends of a graph, so to speak.

However, the master cylinders are a little easier.
Tandem cylinders - since the first cylinder fluid displacement is independent of the second cylinder (straight-through circuit) there's no change. Similarly, the tandem cylinder is independent of the first, so again, no change. However, you could conceiveably have different area cylinders, and could therefore generate a different pressure for the same pedal push (or lever pull) depending on the cylinder in use.

For cylinders in parallel, then think area. Double the area equals half the pressure for the same push in kilos. Twice the push in kilos would get you back to the same pressure, for twice the pedal movement (displacing more fluid). BUT - beware the pedal movement. With an F1 racing set-up, the pedal movement is essentially b****r-all, and double that is of course b****r-all.

But then cylinders in parallel allow balance bars - and the position of the pivot will allo variation of push applied to each master cylinder. Think O-level machanics "Levers". Pivot in centre of bar, equal load in kilos to each cylinder, each being half the total applied by your boot. Shift the pivot over one way, and the ratio of lengths from pivot to cylinder will give you the proportion of total push applied to each cylinder.

Arthur.

[Chris J]

Is Colin right?