30 replies to this topic

[mailman]

Hi,

I know that there is information around here somewhere but for the life of me I haven't been able to find it.

I'm very confused as I seem to have read in the Haynes manual that an increase in bore size increases pressure, and in an article on Minispares it suggests the opposite (which seems more likely to me).

Can anyone clarify for me the effect of changing wheel cylinder bore size?

And with that, to provide the best balance with my new 7.5" disc set up, should I replace my rear wheel cylinders with 9/16" (standard), 11/16" (as per the article on Minispares), or 3/4"?

[MRA]

Think of it like this.... 2 people each 100kg get in a lift of mass 500kg the total force is now 700kg now at the next stop 5more people each with a mass of 100kg get in the lift assuming no one leaves, you now have 1200kg total mass

at the next floor the lift suddenly double in area allowing another 7 people each with a mass of 100kg to enter the force is now 1900kg

does that help ?

[mailman]

I think it does. But in simple terms, are you saying that larger cylinder equals more braking pressure on shoe?

[midridge2]

I think it means its a magic lift. lol

[lrostoke]

I've no idea on the physics, but I switched from drums all round to 7.5 discs, I've got the 3/4" cylinders on the rear. Works a treat non servo.

Most of braking is done by the fronts anyway...

[bmcecosse]

Yes - larger bore = more force applied to the shoes. But problem with Mini is TOO MUCH braking force on the rear wheels.....causing nasty spins.... My advice - fit the smallest cylinders, throw away the horrible taper nose 'pressure regulator' and fit instead the original 'pressure limiter' in the line to the rear brakes (put it up on the bulkhead near / just below the master cylinder) - and you can then fine tune the max hydraulic pressure going to the rear cylinders by modifying the spring . It works - and yes - no servo - and no more spins !

[JustSteve]

larger cylinders (or calipers) means more force, BUT more travel on the pedal.

[Cooperman]

Yes - larger bore = more force applied to the shoes. But problem with Mini is TOO MUCH braking force on the rear wheels.....causing nasty spins.... My advice - fit the smallest cylinders, throw away the horrible taper nose 'pressure regulator' and fit instead the original 'pressure limiter' in the line to the rear brakes (put it up on the bulkhead near / just below the master cylinder) - and you can then fine tune the max hydraulic pressure going to the rear cylinders by modifying the spring . It works - and yes - no servo - and no more spins !

Very good advice, Roy.

[TopCatCustom]

I've been told (with my alloy minisport calipers) to fit tiny 1/2" rear cylinders. So I have a new pair ready to fit. I'm using a non servo yellow tag MC and a Wilwood brake bias valve (adjustable) on the rear line, mounted beside my seat.

[Dan]

Is this a road car? Or a car that has to see road use? Having the brake bias adjustable from the driver's seat is very much illegal.

[Gr4h4m]

With smaller RWC will give you a higher firmer pedal. This feel is often found to be more agreeable to the driver as the brakes feel right there.

I like the 1275 gt rwc.

[MRA]

I think it does. But in simple terms, are you saying that larger cylinder equals more braking pressure on shoe?

Yes the bigger the CSA (Cross Sectional Area) of the pistons CSA * Quantity the more force acting on the brake pad or shoe, however the larger the CSA the lower the displacement, hence why in certain circumstances it can be very dangerous and lead to double pumping of the pedal..

The opposite applies to the master cylinder.

[MRA]

I think it means its a magic lift. lol

You dear Sir are a p*** taker of the 1st degree

[mailman]

Some very, very useful information here. My plan is to re-route the brake lines when the engine is next out. Currently I have the standard (and original) diagonal split system.

If I can, can I ask for some really specific advice, I have the minispares 7.5" disc conversion with 4-Pot calipers. The original RWC's were/are 9/16", I was under the impression that I needed to change the size of the RWC to correct the pressure.

Is the thought behind this that I should increase the pressure at the rear to match the force at the front? Or, to decrease the pressure at the rear to counteract a greater pedal force applied? I seem to be wrapping my head around what the size changes do, now I'm not sure why and which way to go. The later ones were as big as 3/4" with discs etc. Why?

[mailman]

Foolishly forgot to include the specific information that I was after

If I can, can I ask for some really specific advice, I have the minispares 7.5" disc conversion with 4-Pot calipers. The original RWC's were/are 9/16", I was under the impression that I needed to change the size of the RWC to correct the pressure. Which RWC would be best, i.e. performance and safety.