Dual Master cylinders

[armandodiaz]

I'm building my own version of "Smart Racing's dual master cylinder kit" for my track car. I'm just about ready to modify my housing. One question I have is: Which MCs to purchase. I'm looking at the Tilton 75 series but not sure of the bore size?

My car is a 85 Carrera (3.4L) about 2200lbs wet, with driver.
It has 930 brakes all around. With Hawk Blues.

What else will I need to determine bore size?




thanks

[Bill Verburg]

Originally Posted by armandodiaz

I'm building my own version of "Smart Racing's dual master cylinder kit" for my track car. I'm just about ready to modify my housing. One question I have is: Which MCs to purchase. I'm looking at the Tilton 75 series but not sure of the bore size?

My car is a 85 Carrera (3.4L) about 2200lbs wet, with driver.
It has 930 brakes all around. With Hawk Blues.

What else will I need to determine bore size?




thanks

5/8 & 5/8 would be a good place to start

SC/RS used 7/10 & 5/8 w/ a robust balance bar

[Cory M]

The master cylinders themselves are pretty cheap and easy to change. I'd pick up two 5/8" and one 7/10" and experiment. I've got the Fabcar set-up with Tilton 74 series, I believe they are the same as the 75 series but a couple of inches longer and a few bucks cheaper. There are a few threads on Pelican with good info on this set-up and some homemade versions of it if you do a search.

[armandodiaz]

Excellent, thanks.

Does the bigger bore offer more stopping power? So the 7/10 would go in the front and 5/8 in the rear?

[wildcat077]

Armando,

Cory is right on for the master sizes at 7/10 and 5/8 ... you want to put the smaller master on the circuit where you want the most braking force .I have 996TT brakes on my car and it is heavily front biased which is why i'm doing the Fabcar install with a Tilton bias adjuster to help balance the braking forces.

I'm currently installing a brand new Fabcar dual master unit in my car and i'm just waiting on some parts from Patrick Motorsports to get started.
You will need some tees and fittings for the installation,i can take some pictures for you when i get my order from Patrick,it will save you a lot of guessing work !

Cheers !
Phil

[armandodiaz]

Thanks, appreciate the help.

After doing a search and watching this video:

Racing Brake System with Wilwood Part #1 - YouTube

I learned that the smaller bore actually creates more stopping power. So my question is, since our cars are heavy in the rear, do we want the smaller bore for the rear brakes?

Thanks,

[chris_seven]

There are just a few issues that you need to consider of you want the system to operate as a whole and do the job well.

As you have discovered the smaller the cylinder the lower the pedal force needed to generate a given clamping force at the caliper. (I am not sure it is entirely correct to describe this as stopping power).

The down side of this is that the smaller the cylinder size the greater the pedal travel so there is a compromise to made in this respect.

The next issue is where on the pedal the bias tube will fit as this will affect pedal ratio.

pedal ratio can influence cylinder choice and increased ratio reduces pedal force but increases travel so is analagous to smaller cylinders whilst reduced pedal ratio is analgous to larger cylinder size.

It is useful to have some idea of the pedal ratio compared to standard.

Just a safety note - If you look at pedal thickness on early cars without Servos they had 8mm thick pedals. I think this reduced to 5mm on Servo cars and if you increase pedal effort you may need to consider this point.

If you look at the brake balace of a standard car with a tandem master cylinder then it will have equal area master cylinders front and rear.

cars in this configuration have reasonable brake balance but I would agree that the ability to adjust is very useful.

If you put mismatched cylinders than your 'null' brake bias will move and you may restrict the useful adjustment range.

We have been making bias pedal boxes for years and we always use equal area cylinders so that when the adjuster is centred the brake balance is as standard and our customers who use these pedal boxes seem happy with this choice.

Good Luck

[armandodiaz]

I do believe equal sized MC on a street car would work great. Also on cars, race or street, that are equally balanced (ie; 944, Boxster, Cayman), equal sized MC would work. But on 911s, where they are so rear heavy, I got to believe that the MCs will have to be different.

[Quicksilver]

Originally Posted by armandodiaz

Thanks, appreciate the help.

After doing a search and watching this video:

Racing Brake System with Wilwood Part #1 - YouTube

I learned that the smaller bore actually creates more stopping power. So my question is, since our cars are heavy in the rear, do we want the smaller bore for the rear brakes?

Thanks,

The easy way to get it in your head about MC sizes relating to bias is to focus on the phrase, "Pounds per square inch". If I push on a piston with 100 lbs of force that has a bore area of 1 square inch then the system will generate 100 lbs per square inch. 100/1=100... Pretty basic.
So if you have the a piston with 1/2 the area you are still going by the standard of pounds per square inch but pushing half the area will double the pressure. Focusing the force multiplies the effect so you get 100/0.5=200.

But on to your bias question...
The 930 calipers are designed with the front/rear bias matching. The Carrera has the bias mismatched and used a proportioning valve for the rear. If we compare the front piston areas of the 930 caliper to the Carrera you will see that the 930 has 125% of the piston area of the Carrera. This would mean you would want a larger master cylinder to use with the 930 setup. You would also most likely be keeping the cylinders unbiased.
I'm using a .7" and 5/8" MC setup on the Smart racing setup with the stock Carrera calipers. You should expect to need larger so I'm pretty certain you will want a pair of the .7".
I would suspect the pedal will be a bit softer/lower-effort then mine but I think I like my pedal harder then most.

Oh! And make sure you remove the proportioning valve from the system.

[Bill Verburg]

Originally Posted by armandodiaz

Thanks, appreciate the help.

After doing a search and watching this video:

Racing Brake System with Wilwood Part #1 - YouTube

I learned that the smaller bore actually creates more stopping power. So my question is, since our cars are heavy in the rear, do we want the smaller bore for the rear brakes?

Thanks,

the small bore will provide a higher line pressure to that end of the car, this is turn increases brake torque at that end.

As I posted above SC/RS used 7/10" & 5/8", smaller one in front. But then the bias bar could be adjusted to give you more back.

the best way to adjust the balance bar is through a movable screw pivot which can be turned by a dash mounted knob which changes the effective length of the action arms

This is the base setup for 997GT3RSR, yours will be different due to the different m/c & caliper setup

[armandodiaz]

Thanks Bill I appreciate all the info.

[chris_seven]

The clamping force numbers are quite interesting if you do the calculations.

If you use a 0.625" front and 0.7" rear as Bill suggests was used on the SCRS you end up with the following:

Front Caliper Hydraulic Ratio 11.46:1 and Rear Caliper Hydraulic Ratio 5.7:1

This is a Brake Balance of 67/33 - very forward biased.

The pedal effort would also increase by around 35% from a system with equal sized cylinders.

If you switched them round and put a 0.7" on the front then:

Front Hydraulic Ratio 9.15:1 and Rear Hydraulic Ratio 7.14:1

This is a Brake Balance of 56/44 but again pedal effort will increase by about 35% from equal cylinders.

If you use 2 x 0.625" diameter you end up with a Balance of 62/38 and a lighter pedal.

We modelled all this with a spreadsheet about 4 years ago when we starting making pedal boxes.

We tried all combinations and the most popular has been equal cylinders of 0.625".

The most recent comment we received form one guy who used our system in this configuration on his race car was:

I actually just finished the pedal box install and test drove the car this past week. Everything is working flawlessly (including the bias adjuster) and the car actually stops better...less pedal effort stops the car much better

During development we tried a number of different cylinder combinations and if you use the 0.7" Front and 0.625" Rear you may find yourself easily locking the rear and having to move the bias a long way forward. This will reduce the overall adjustment that is available which with a short bias bar will be around 4 turns in each direction for the centre.

If you switch from a pair of 0.625" to a pair of 0.7" cylinders the pedal travel will reduce but you will increase pedal force by 25%.

The level of force needed assumes you don't change the pedal ratio.

[Bill Verburg]

Here's what I came up w/ I didn't include the effect of the balance bar here as I don't have the SC/RS specs for it


as you can see using the 7/10 & 5/8 significantly moves bias to the back

[chris_seven]

Originally Posted by Bill Verburg

As I posted above SC/RS used 7/10" & 5/8", smaller one in front. But then the bias bar could be adjusted to give you more back.

Bill, I am not sure how to interpret your spreadsheet.

If the small cylinder is in the front then surely brake bias will move forward.

If the small cylinder is on the rear then it moves backwards.

The pedal pressures must also change if you change even one of the two cylinders or have I made a fundamental error? as you show the same force for equal or unequal cylinders.

I calculated the pedal effort based on acheiving the same total clamping force by summing front and rear and then comparing the totals and then seeing what was needed to acehive the same levels.

I dodn't take pads and tyres into account and will have a small error based on rotor diameter which i also neglected as I was just trying to get into the right ballpark

[Bill Verburg]

Originally Posted by chris_seven

Bill, I am not sure how to interpret your spreadsheet.

If the small cylinder is in the front then surely brake bias will move forward.

If the small cylinder is on the rear then it moves backwards.

The pedal pressures must also change if you change even one of the two cylinders or have I made a fundamental error? as you show the same force for equal or unequal cylinders.

I calculated the pedal effort based on acheiving the same total clamping force by summing front and rear and then comparing the totals and then seeing what was needed to acehive the same levels.

I dodn't take pads and tyres into account and will have a small error based on rotor diameter which i also neglected as I was just trying to get into the right ballpark

Thanks I needed that!
There was a bunch of legacy code from the original street setups, I think I have it all corrected now

The only source of error I can think of in mine now is the pedal ratio, I used 5 untill I can get a better #, I also don't know the range of effects of the bias bar on the SC/RS set up, the data from the RSR is from the factory.


sorry for small size, it looks like maybe Wayne has further restricted the displayed image sizes

[chris_seven]

Bill,

I have measured a bunch of non-servo pedals and the projected lengths all give a pedal ratio of 5.6:1

I have never tried to measure a Servo pedal as they always seemed irrelevant to what we were doing.

With our Bias Bar box we have used a pedal ratio of about 6.2 to keep forces reasonable.

My Business partner used to be a Research officer at the Motor industry Research Agency (MIRA) in the Uk and was involved in Brake Testing for years.

We have tested our Bias Bar boxes in cars using an accelerometer and have tried to achieve a nominal decleration of 0.5g for 62kg (140lbs) of pedal force which is MIRA's nominal 'standard'.

With a standard floor mounted SCRS pedal box it is also difficult to fit a bias bar with a 66.5 centre and it helps to shorten the tube to allow 60mm centres.

It is also important if you mess with the brake pedal to consider the pedal board.

With non-servo systems it is important to refit a pedal board to have a throttle stop or the accelerator can overtravel and bend the revese link mounting. this can cause the throttle rod on an LHD car to jam on the tunnel side and hold the throttle open - normally at full throttle!

[Bill Verburg]

Originally Posted by chris_seven

Bill,

I have measured a bunch of non-servo pedals and the projected lengths all give a pedal ratio of 5.6:1

I have never tried to measure a Servo pedal as they always seemed irrelevant to what we were doing.

With our Bias Bar box we have used a pedal ratio of about 6.2 to keep forces reasonable.

My Business partner used to be a Research officer at the Motor industry Research Agency (MIRA) in the Uk and was involved in Brake Testing for years.

We have tested our Bias Bar boxes in cars using an accelerometer and have tried to achieve a nominal decleration of 0.5g for 62kg (140lbs) of pedal force which is MIRA's nominal 'standard'.

With a standard floor mounted SCRS pedal box it is also difficult to fit a bias bar with a 66.5 centre and it helps to shorten the tube to allow 60mm centres.

It is also important if you mess with the brake pedal to consider the pedal board.

With non-servo systems it is important to refit a pedal board to have a throttle stop or the accelerator can overtravel and bend the revese link mounting. this can cause the throttle rod on an LHD car to jam on the tunnel side and hold the throttle open - normally at full throttle!

Porsche published the pedal ratios for all of their street cars up through the last 911 in '89, after that not
is you 911 setup a copy of the SC/RS? if so then I can use 6.2.

also note that I standardized on 2 different scenarios
1) w/ 60# pedal effort I know that you can go higher but to keep things in line and check w/ factory data I like to keep line pressures ~50bar

2) at f/r line pressure of 50bar

w/ the street inline dual masters it's a whole bunch simpler

as I noted the bias bar I showed was just an example, it happens to be from the 997RSR

I also standardized on .5 μ for the pad friction coefficient, it can vary widely from that value

[chris_seven]

Bill, we didn't copy and SCRS pedal box as we wanted to keep a standard pedal board and the SCRS is different in this area.

We just used a similar concept and used a pedal that fits under a standard pedal board.

We use a small diameter'ball' on the bias rod so the tube diameter we weld to the pedal is quite small and allows the increase.

There is a guy in the Netherland that makes a fairly good copy and it may be worth asking him to share the pedal ratio information.

[armandodiaz]

How would 5/8 (0.625) and 3/4 (0.75) work?

[armandodiaz]

How would 5/8 (0.625) and 3/4 (0.75) work?

I'm looking at these and they don't offer the 7/10 (0.70):

Wilwood Compact Master Cylinder - Speedway Motors, America's Oldest Speed Shop