(sorry!) Another question about brakes....

[Darren C]

Hi

I'm biting the bullet and removing the servos from my car, but I'm having trouble deciding on master cylinder sizes (I've read loads of threads on here, but thats left me with more questions than answers, hence this post).

My setup is pretty common (i think), so would be great to hear from others who have already gone down this path!

My brakes are:
Front: Wilwood 4 pot (4 x 1.38" pistons)
Rear: Sierra Cosworth (1 x 42.8mm I think)

I'm going for a Wilwood bias pedal setup with 6.25:1 pedal ratio as it will give me great leverage in the absence of the servo, and puts the pedals at a really comfortable height.

Can anybody advise on what ratios would be best for the front and rear master cylinders?

Thanks in advance!


Darren

 

[Ian Anderson]

I ran 0.7 front and rear and adjusted the balance slightly on the balance bar.
But I had different calipers (AP4 pot fronts and Saab9000 turbo on the rear)

Ian

 

[Terry Oxandale]

I can only go by experience here, but without a servo in your case (assuming that is a booster), I feel you will have to use some inordinate amount of leg pressure for braking. For example, I do not utilize a booster on my car (similar to the '40 in configuration), so I use a 4-pot system with roughly 45mm pistons (2380 mm = total square mm for both staggered pistons), driven by a 19mm (283 square mm) master cylinder. That's roughly an 8.4:1 hydraulic ratio only, not including the pedal ratio, (which is kinda standard for the aftermarket pedal assemblies). I am using a Tilton balance bar pedal assembly by the way, which I believe is a 5.5:1 ratio. This total leverage I use feels natural to me in terms of foot pressure applied vs response by the brakes. For you to get the same ratio with your caliper sizes noted (1774 total square mm front), you'd need a master cylinder in the 10mm diameter (255 square mm) range (this takes into consideration your additional pedal leverage with the Wilwood assembly), which is pretty small (would required a huge stroke).

I would suggest the use of larger-piston calipers for what you're shooting for. It will be a balance of stroke vs leverage, and the larger the calipers, the better options and system you'll have. Your system's caliper area is about 62% of mine, but again, your pedal ratio is a little better than mine. Regardless, the .75" (and the .70" Ian noted) I think is the more commonly used sizes for non-assisted brakes, which will require larger caliper pistons in order to allow about a 1" stroke to provide both good moderation of the brakes, as well as leverage to lock them up effectively (my test for effective brakes). Last note, and this is without knowing the rotor diameters you are using, CG, weight distribution, tire sizes, etc, I was able to utilize the exact same caliper/rotor combination front and rear for my car, with very little bias adjustment off center...but again, there is a LOT of data not included in that tidbit of info.

Lastly, if this is starting to sound expensive, I would consider perhaps the use of the Wilwood 4-pots now for the rear, and shoot for larger front calipers, but then that may not be allowed depending on inspection or regulatory requirements where you live.

 

[Terry Oxandale]

Correction, your front caliper piston size is about 75% of mine.

 

[Chris Kouba]

There are online brake calculators which should be able to get you appropriately started:

brake calculator spreadsheet - Google Search

I've had friends who used them and had good results.

 

[Darren C]

I ran 0.7 front and rear and adjusted the balance slightly on the balance bar.
But I had different calipers (AP4 pot fronts and Saab9000 turbo on the rear)

Ian

Thanks Ian
That seems to be similar to what i've read elsewhere on the forum

 

[Darren C]

I can only go by experience here, but without a servo in your case (assuming that is a booster), I feel you will have to use some inordinate amount of leg pressure for braking. For example, I do not utilize a booster on my car (similar to the '40 in configuration), so I use a 4-pot system with roughly 45mm pistons (2380 mm = total square mm for both staggered pistons), driven by a 19mm (283 square mm) master cylinder. That's roughly an 8.4:1 hydraulic ratio only, not including the pedal ratio, (which is kinda standard for the aftermarket pedal assemblies). I am using a Tilton balance bar pedal assembly by the way, which I believe is a 5.5:1 ratio. This total leverage I use feels natural to me in terms of foot pressure applied vs response by the brakes. For you to get the same ratio with your caliper sizes noted (1774 total square mm front), you'd need a master cylinder in the 10mm diameter (255 square mm) range (this takes into consideration your additional pedal leverage with the Wilwood assembly), which is pretty small (would required a huge stroke).

I would suggest the use of larger-piston calipers for what you're shooting for. It will be a balance of stroke vs leverage, and the larger the calipers, the better options and system you'll have. Your system's caliper area is about 62% of mine, but again, your pedal ratio is a little better than mine. Regardless, the .75" (and the .70" Ian noted) I think is the more commonly used sizes for non-assisted brakes, which will require larger caliper pistons in order to allow about a 1" stroke to provide both good moderation of the brakes, as well as leverage to lock them up effectively (my test for effective brakes). Last note, and this is without knowing the rotor diameters you are using, CG, weight distribution, tire sizes, etc, I was able to utilize the exact same caliper/rotor combination front and rear for my car, with very little bias adjustment off center...but again, there is a LOT of data not included in that tidbit of info.

Lastly, if this is starting to sound expensive, I would consider perhaps the use of the Wilwood 4-pots now for the rear, and shoot for larger front calipers, but then that may not be allowed depending on inspection or regulatory requirements where you live.

Hi Terry
Thanks for providing so much info!!!
It turns out that Dick Townsend (who lives about 2 miles from me) is running an almost identical system in his car - i've driven his car and the brake weight is heavier than my daily driver, but very useable (and if anybody has seen Dick on track, they can vouch for the effectiveness of his brakes). We're going to check his master cylinder sizes, and start with a similar setup. If it doesn't live up to expectations, I like your idea of swapping the wilwoods to the rear and upping my front callipers.
Your help is much appreciated!
Thanks
Darren

 

[Trevor]

It sounds like you use the same front Wilwood calipers; 4pot Superlites but I have Wilwood Dynalites in the rear (also 4 x 1 3/8"). I use 0.625" Girling master cylinders front and rear with a pedal ratio of 4.5:1. The pedal is pretty heavy, but with your higher pedal ratio, you probably won't be too far off. You have a large difference in total piston area front to rear (about 2.7:1 if my arithmetic is correct). Ideally if it was a single circuit an even smaller bore master cylinder could be considered, but I have never found anything less than 0.625" . You could use twin 0.625 MCs but would probably have to play with the balance bar to get overall brake bias how you like it.

 

[Trevor]

oops just cheched my drawings; pedal ratio was 5.5:1

 

[Michel]

Hi
With those calipers you need to use a 5/8 front mastercylinder and a 3/4 rear if you really want to have a nice braking pedal which give you not only confidence when pusching to the pedal ( not too smooth !!) but also will give you much more range if using a brake balance stuff ( which is mandatory when going from servo to non servo )
Of course you will need to have a new pedal and a new front bracket to hold those 2 MC
"tandem" (say in line ) mastercylinder is not advisable at all ( beside being easy to fit !!) as you then will need to use and "hydraulic" brake balance which is not that easy and good to set correctly
Hope this helps

 

[Trevor]

Having reread Terry's post above, agree with his final conclusions, and would go for the larger piston bore option that Wilwood offered (hopefully still do) for the calipers. Your pedal ratio is not so different from mine (once I checked and corrected) and you will want the larger hydraulic ratio that the larger bore calipers can offer to avoid too heavy a pedal, especially as you can't go any further (smaller) on the MC side.

 

[Dr. Bob Woods]

Here is my analysis of the Tornado original brake system and the modifications that I had to do to it.

I have a relatively big cam and I don't generate enough vacuum for the servos to work. My maximum vacuum is 40 kPa. I have disconnected the vacuum with no change in braking performance. You could install a vacuum booster pump, but that would actually allow you to put way too much pressure to the original brake system if you pushed too hard.

I changed a master cylinder and rear brake calipers. That helped, but I had to put in a really good brake pads to be able to lock the wheels.

See report attached.

-Bob Woods
Tornado GT40 in Texas

 

[Terry Oxandale]

Thanks Bob. That pointed out some obvious stuff I hadn't thought of on the non-linear character of bias in deceleration.

 

[Doc Watson]

(sorry!) Another question about bra.... always gets me to click it...... (sorry!)