wilwood master cylinders

[TurnpikeBoy]

It's come to that time where I need to rebuild or replace the masters; rebuilding is probably pointless, so replacement is in hand.

Anybody tried the 5/8" bore masters in replacement (instead of the 3/4")? Would help the amount of leg necessary to get the car stopped.

[pgermond]

Sorry, I don't have experience with the larger m/c. However, out of curiosity, how many miles do you have on the car?

[TurnpikeBoy]

Almost 11K miles. The brake pedals have begun slowly creeping toward the floor, altho the clutch master still seems sound.

BTW = my car was delivered with the 3/4" masters; do you have the smaller ones?

p.s. - I see you've overtaken Mr. Foushee as the #1 poster - Congrats!

[pgermond]

I believe they are the 3/4" (Sunday... a couple of beers... you know.... ). I'm stll waiting for my clutch slave to go. I have a new one sitting on the bench that I've been meaning to install all summer - it's either been too hot or, when it cools down, I'm driving.

Maybe next month Good luck.

[Jerry Cowing]

Turnpikeboy,

Call Wilwood before you blindly change anything on your brakes. You're gambling with safety when you mess with your brakes and not know what you're doing. I don't believe a 5/8 master will push enough fluid through the lines to fully engauge your brakes.

With a clutch you don't want more than 1/8 inch difference between the master and slave. I'm not sure what it is for the brakes -- call Wilwood.

[TurnpikeBoy]

That's a good idea, Jerry. No harm in asking the experts.

In truth, tho, while going to the 5/8" will require more pedal travel it will also impart more force at the caliper when the same pedal pressure is applied. I assure you I'm not going into this blind; hydraulic circuits are everyday fare to me, so I'm pretty confident I'm not gonna wind up in somebody's fence.

That said, there is no foul to getting a second opinion!

(btw - the 1:1 ratio you reference is simply to get enough movement at the clutch - be aware that most caliper pistons are 1-1/4" + in diameter. Disc brakes only require about .040 - .050" movement, unlike a pressure plate that needs about 1")

[rdorman]

Pretty big increase in travel! About 30% increase. Be sure the bias is set correctly and the bar is not binding (both a common issues) and be sure that the pedal is not overcentering (not less then 90 degrees measured from you hip to the pedal up to the pivot point) when fully depressed. You know how to do the calculations, but since I already did them... you probably have T compound pads in there. Going to a .7 master and a BP-10 compound will result in about the same reduction in force going to the 5/8 (31%). OK, so they will only reduce it about 25% but a nice job anyways. Going to the .7 or the BP-10 results in about the same reduction of pedal force (12 vs. 13% or so) with only about a 13% increase in travel.

[TurnpikeBoy]

Hhmm. Well, the drop to the 5/8" bore is only 16%; if the leverage ratio of the pedal is 2:1 then there would be a 30%+ increase in pedal travel. If it's more like 3: or 4:1, then a whole lot more movement can be expected.

Still.....if the actual pedal movement is 1/8" now, it becomes 1/4" at 2:1 and upwards of 1/2" at 4:1. And while I don't have love for that thought, the jump in system pressure - and, thereby, pad pressure - equates to nearly 20% - and that's huge, too.

Wilwood says either can be done - but should have plenty of brake with the 3/4". They suggest shortening up the master cyl rod to the rears and centering the balance bar for starters; the balance bar being for fine adjustment.

OK, I'll try that first before changing anything else.

[rdorman]

It is the area that dictates the increase in travel, not diameter. Increase in travel will be about 30%. Pedal ratio has no effect on the percentage of increase in travel when changing master cylinder bore.

[JerryBP]

You have to shove the pedal 30% farther with but 30% less Pressure.

[rdorman]

You have to shove the pedal 30% farther with but 30% less Pressure.

Bingo Jerry! How is the car coming? Going to have it ready for the picnic or fall cruise?

[TurnpikeBoy]

It is the area that dictates the increase in travel, not diameter. Increase in travel will be about 30%. Pedal ratio has no effect on the percentage of increase in travel when changing master cylinder bore.

Change in diameter IS change in area....Pi x r(2) = area. Decrease area and you must increase stroke to retain similar volumes. Pedal ratio directly affects the length of travel felt by your foot; any increase in master cylinder piston movement is multiplied at the foot by the distance between those two points.

The gain is applied effort. The same pedal pressure appplied to the smaller master cylinder results in higher unit pressure.

[rdorman]

Change in diameter IS change in area....Pi x r(2) = area. Decrease area and you must increase stroke to retain similar volumes. Pedal ratio directly affects the length of travel felt by your foot; any increase in master cylinder piston movement is multiplied at the foot by the distance between those two points.

The gain is applied effort. The same pedal pressure appplied to the smaller master cylinder results in higher unit pressure.

Really? You don't say? But it is NOT 16% change and pedal ratio does NOT effect the PERCENTAGE of change in travel by going with a smaller master cylinder.

Hhmm. Well, the drop to the 5/8" bore is only 16%; if the leverage ratio of the pedal is 2:1 then there would be a 30%+ increase in pedal travel. If it's more like 3: or 4:1, then a whole lot more movement can be expected.

Still.....if the actual pedal movement is 1/8" now, it becomes 1/4" at 2:1 and upwards of 1/2" at 4:1. And while I don't have love for that thought, the jump in system pressure - and, thereby, pad pressure - equates to nearly 20% - and that's huge, too..

3.14 x .3125 x .3125 = .306640625

3.14 x .375 x .375 = .4415625

.306640625/.4415625 = .694~ x 100 = 69.4%

1/6=.16~
1/7=.14285714

.14285714/.16666666=.85714287

lets use a sample travel of 2" for 6/1

2/.85714287 = 2.33~

2/.694=2.881844
2.33/3.357348

2.881844/3.357348=.858

Pretty big increase in travel! About 30% increase.


Originally Posted by rdorman
It is the area that dictates the increase in travel, not diameter. Increase in travel will be about 30%. Pedal ratio has no effect on the percentage of increase in travel when changing master cylinder bore...

Or perhaps you would like a copy of the software I wrote that helps design braking systems? It takes in account every variable from aerodynamic lift and downforce to tire/pavement coeficient of friction.

Never mind... I was just trying to be helpful

Good luck to you

[TurnpikeBoy]

Y'know, rdorman, you're right; never mind.

And, yes, since you're offering - I would love to have a copy of your software on brake system design. Not that I would have any clue what to do with it, but it really isn't that often that one can get expert advice.

[pgermond]

Geez, I was lost back on post #7..... what are you guys talking about? Brakes.... right?

[TurnpikeBoy]

Phil, I'm gonna follow your lead...and go have a beer or two.