Brake master cylinder bore vs stroke

[my427cobra]

I've been advised by Tilton that my front/rear brake combination requires a 3/4" front master cylinder and 1" rear master cylinder.

What I'm wondering is what is the relationship of bore to stroke?

Some m/c's I'm looking at have strokes ranging from 1.0" to 1.1" to 1.25" to 1.4".

How will the stroke length affect the pedal pressure/firmness?

Confused.
Greg

[Ralphy]

A smaller bore will translate into higher brake pressure. A larger will increase volume with need to push harder/less pressure. I think 3/4" front and rear are fairly common. With a balance bar to move a greater foot pressure/force to the fronts. A smaller bore will translate to greater travel.

Ralphy

[DanEC]

Greg - I'm not too sure the stroke matters a whole lot as long as there is plenty of MC capacity in comparison to the brake caliper displacement - and that is going to vary from brake setup to brake setup. Unlike the clutch MC you shouldn't be fully stroking the brake cylinders or otherwise you could run short on brake pressue and also have no reserve capacity.

I don't think the stroke will have any effect on the pedal pressure as long as you don't run out of MC capacity - I believe the MC piston diameter and caliper piston diameter will most influence pedal pressure.

It's interesting that they recommend a larger MC for the rear brake circuit than for the front one. From what logic I understand on these things, your rear brake calipers must be of larger piston diameter and displacement than the front brakes. Probably has somehing to do the number of pistons in the calipers. More pistons are often smaller diameter.

[Rick Parker]

Your pedal pressure will be affected by the volume of fluid transfer from the master cyl to the caliper and the pedal ratio. Did the Tilton techs ask which pedal assy you were using? The smaller diameter master will have less pedal pressure but more line pressure with the same pedal effort IE. pedal ratio. It's all about the diameter of the master VS the diameter of the caliper pistons with the pedal ratio (foot travel) thrown in.

[my427cobra]

Hi all. A little background. I have 4 piston calipers on the front and single piston calipers (Ford Lincoln Versailles) in the rear.

My current m/c setup was advised by brake company X and I was told to use 3/4" m/c's front and rear. My pedal pressure has been low to moderate and brake performance acceptable but I would like a harder pedal.

Contacted Tilton with all my info (they have a comprehensive question sheet) and they came back with a ¾” front and 1” rear (due to the size of the single rear pistons).

So now to find a 1” compact m/c (due to my space limitations).

Most compact m/c’s have a short 1’’ or 1.1” stroke.

So I’m wondering if that will be enough.

I’ll call Tilton again tomorrow.

Cheers
Greg

[Ralphy]

A smaller MC diameter creates a higher pressure not larger diameter. Or are wanting to push harder with your foot/leg? If you want more braking and you have a pressure regulator on the rears? Toss it out and get a balance bar system. You can move a higher force lbs. to the front without limiting any pressure.






Ralphy

[my427cobra]

I want to push harder with my foot (have a firmer pedal).

[Ralphy]

Not sure of your whole setup. But then your right! Maybe you need to increase both front and rear equally so you don't throw the balance off.That's a first I've seen here, most posts want the reverse. Do you have any power assist? You should get less travel with a bigger MC, what is the travel length of your 3/4" MC?

Ralphy

[DanEC]

Ralphy has a good point in that a larger dia MC has more displacement for a given stroke as a smaller dia MC. So you can probably get by with a little less stroke on a 1 inch MC.

I think (a dangerous proposition for me) that for a muti-piston brake, you have to look at the total surface area of all of the pistons together to evaluate the resultant MC piston diameter that would be most appropriate and pedal effort (along with the pedal ratio as Rick Parker pointed out).

The 1 inch MC will increase pedal pressure over a 3/4 inch MC and that may require a little tuning of the front/rear balance. I don't know if it will be a great deal of difference since the front is remaining a 3/4 inch. I think you can roughly approximate it by looking at a ratio of the before and after MC piston area. Before - two 3/4 inch MC have .883 inches of combined piston area and 1-3/4 inch MC and 1-1 inch MC have 1.227 inches of combined piston area - a 39% increase which should relate to a similar pedal pressure increase given a constant pedal ratio. But the net result will depend on how your front to rear balance is set.

Something Bob at ERA pointed out the other day - with new brake pads, the pedal will seem a little soft until the pad surfaces get fully bedded in and conformed to the discs.

[my427cobra]

Hi Guys. Thanks for your input and guidance. What DanEC says is what I'm trying to achieve. I'll be offline for a few days but will let you know what I find out from Tilton when I'm back.

Cheers
Greg