So, the brakes in the challenge car suck right now, so I am trying to figure out how to fix the issue. (Engine swapped Spitfire for those that don't know me.)
To start, the brakes are upgraded all around.
Front: 4 piston Toyota calipers on other upgraded rotors. (Vs. Stock front ?disk? brakes)
Rear: FC RX7 Turbo ii disk brakes. (Vs. Stock Drums)
Master Cylinder: Stock Spitfire MC...
Right now the pedal takes a lot of travel till it really does anything. Given the upgrades above, it seems to me that the 4 piston front calipers would take more fluid than the stock calipers, and the rear disk brakes would also take more fluid than the stock drums. Soooo.... I would naturally think I need a bigger master cylinder and/or dual master cylinders for Front and rear.
Since this is a challenge car, and Wilwood pedal sets are not really challenge budget friendly, I am thinking about fabbing something up myself. So my initial questions:
-Do you all agree with my diagnosis that the master cylinder needs to be bigger or have a dual setup?
-Anyone know of a common, cheap master cylinder that is easily useable for custom stuffs? (Say, a 1995 Honda blah master cylinder only costs $15 new or something, and is easy to fit into a custom setup.)
-Any other tips from people who have made their own pedal setup?
I'd be looking to large american iron for cheap and big masters.
You can easily do the math, and you should. Figure out the volume of fluid that goes into your calipers (easier, measure current master pedal stroke in inches before some resistance and before you can't push any more, combine with diameter of current master cylinder). Then figure out your how far you want the new pedal to go (maybe 1/3 to 1/2 way to floor under as hard as you can push), and it's a simple ratio of cylinder diameter to pedal stroke.
That will get you in the right size range quickly.
Saron81
New Reader
3/15/18 11:12 a.m.
mlwebb
New Reader
3/15/18 12:12 p.m.
I put a new Tilton dual mc pedal set on my Datsun roadster, and would suggest the smallest MC's that will move enough fluid - the larger the MC the lower the line pressure. I put dual 5/8 mc's on my roadster (volvo/girling calipers on vented rotors).
Also if your fabbing pedals, pay attention to pedal ratio. Not sure about Spitfires, stock roadsters have a lousy ratio.
I have some aftermarket masters that may work for your application. I will look up the bore sizes when I get home. These are single circuit masters, but should mount up to the Spitfire location. I got them as a package deal for several used masters, so I can cut you a deal on one for your $15 request.
in general, if you are using a dual-circuit master cyl, match the MC bore and stroke with the front calipers.
if the balance is rear-biased (not likely), put an adjustable prop in the rear circuit.
if the balance is heavily front-biased (pretty likely given comparison of current components to stock components), you can look at smaller bore MC for the rear circuit. One off-the-shelf MC I have used is from the Porsche 944.
Before you/we make any decisions, we need to know bore sizes. What are stock Spitfire master, front caliper, and RW cylinder bores? Now what are the bore sizes and number of pistons in the new brakes? Is there going to be some kind of booster or just a dual circuit master? Finally, and this may be a tough one, what is the effective leverage on the pedal? Then we can do the maths. You want max line pressure of around 1300 to 1500 psi with 180 to 200 pounds of pedal pressure. Lower would be better. Then you need to decide what the balance should be. Aim for something like 63% front braking and 37% in the rear. Or, you can wing it and say brake pedal moves to far, need larger bore master. Brake pedal moves not much at all and pedal hard to push, need smaller master.
In reply to stafford1500 :
the wilwood is a complete body/reseroir with 7/8 bore. the tilton is a 1 bore and i have an extra resivoir for it. $15 for which ever one you want
In reply to stafford1500 :
Sent you a PM.
A quick search of the gargler shows:
84 Toyota pickup caliper. 4 piston, 34mm dia. (Stock Spit: 2 piston, 48mm dia). FC RX7 turbo has a floating caliper, 34.8mm dia.
Braking force is a ratio of the area, given 100psi line pressure, the Spit clamps 189lb when the Yoda clamps 267lbs. Neat, but what's the balance?
Stock Spit master is .812" dia (13/16). Assuming the pedal ratio is 6:1. You need 105bs (lots of assumptions here) force to stop the car. Smaller master would help reduce the force.
Maths. I believe I downloaded the spreadsheet from the locustusa.com forum. This shows that you might be a bit rear biased (hence the bigger piston recommended for the rear). Changing the front rotor size will only have slight impact on the brake balance - the equation is largely dominated by hydraulics.
FWIW, the Spitfire calipers and the Toyota calipers have almost the same piston area, so I don't think master cylinder sizing is your issue.
In reply to Brotus7 :
PM sent. Would like to get that spreadsheet and mess with it.
AWSX:
I have been away from email for most of 24hrs, but will check this evening and get back to you.
