Brake Pedal Pressure

[NF78]

Hi There,

I have searched for a answer but can't find any.

I have a question regarding floor mounted pedal boxes.

Going with that setup eliminates the brake booster,
does that mean that the brake pedal pressure is going to
be rock hard as if you pump the brakes while the car is not running?

If that is the case, how do you overcome it or is that one of the
things you have to live with going the floor mounted pedal route?

Thanks

Jean

[phoenix]

Originally Posted by NF78

Hi There,

I have searched for a answer but can't find any.

I have a question regarding floor mounted pedal boxes.

Going with that setup eliminates the brake booster,
does that mean that the brake pedal pressure is going to
be rock hard as if you pump the brakes while the car is not running?

If that is the case, how do you overcome it or is that one of the
things you have to live with going the floor mounted pedal route?

Thanks

Jean

Check the pedal ratio with the manufacturers and also the master cylinder size; The pedal ratio may be higher than the brake pedal you are planning to replace, and the master cylinder smaller in diameter, in which case there will be more mechanical leverage, so despite not having a servo, the brake pedal effort might work out to be much the same for the same 'line pressure' and therefore braking effect. However, if this is the case, the pedal will travel will also be greater for the same braking effect.

[NF78]

Thanks for the reply,

I will be running Tilton pedal set.
Any reccomendations on the ratio I should go for?

Will be running 4 Pot calipers up front and 2 Pots in the rear.
with regards to master cylinder, any thoughts on size.

Sorry for all the questions, just really lost on this subject.

[phoenix]

Originally Posted by NF78

Thanks for the reply,

I will be running Tilton pedal set.
Any reccomendations on the ratio I should go for?

Will be running 4 Pot calipers up front and 2 Pots in the rear.
with regards to master cylinder, any thoughts on size.

Sorry for all the questions, just really lost on this subject.

I would think in the region of 6:1 if you are losing a servo. I need to know the total piston area in your front and your rear calipers to advise on MC size with any accuracy - and how big your thigh muscles are!

If you know the caliper piston diameters, front and rear, I can work it out for you.

[NF78]

The 4 Pots have 1,25(31.75mm) & 1,12(28.45mm)
The 2 Pots have 1.38 (35.05mm).

Thighs should be ok, Hopefully.

[Tim Falce]

Originally Posted by NF78

The 4 Pots have 1,25(31.75mm) & 1,12(28.45mm)
The 2 Pots have 1.38 (35.05mm).

Thighs should be ok, Hopefully.

.70 front .75 rear and the pedal should be hard, you wouldn't want it feeling spongy would you.

[phoenix]

Originally Posted by Tim Falce

.70 front .75 rear and the pedal should be hard, you wouldn't want it feeling spongy would you.

This would give a brake balance of roughly 60% front/40% rear with the bias bar in the middle.

With a 6:1 pedal ratio and .254 mm pad/disc clearance the pedal would typically move about 28mm before the pads are on the discs. Braking effort at the pedal would be around 35lbs/15.5kg to give 34 bar line pressure. With a 1.9: 1 servo and a 4:1 pedal ratio you would require about 28lbs/12.5kgs of pedal effort, which means the non-servoed brakes would need 25% extra effort for the same stopping power.

However, if you don't like the idea of 28mm of pedal travel before the brakes come 'on', then choose either a slightly lower pedal ratio, or a go up a size on both front and rear MCs. Going to 0.75 & 0.825 would reduce the initial pedal movement to about 15mm, but would require about 44lbs/20kg of pedal effort - e.g. 30% more muscle, or 57% more than with the servo.

[Goran Malmberg]

I would suggest a total gear ratio of 250:1, which is, therefore, pedal and hydraulic "leverage" together. It provides a fairly easily trampled brake pedal. The biggest culprit, with regard to spongy pedal, is hydraulic system and hoses, and calipers on the flex. Is everything in order, so it will be very little flex. One example, a caliper piston area is 6000mm^2, the master cylinder area 300mm^2 and the pedal arm is 6:1. It is ((6000*2) / 300)*6 = ratio240: 1.
Goran

[phoenix]

Originally Posted by Goran Malmberg

I would suggest a total gear ratio of 250:1, which is, therefore, pedal and hydraulic "leverage" together. It provides a fairly easily trampled brake pedal. The biggest culprit, with regard to spongy pedal, is hydraulic system and hoses, and calipers on the flex. Is everything in order, so it will be very little flex. One example, a caliper piston area is 6000mm^2, the master cylinder area 300mm^2 and the pedal arm is 6:1. It is ((6000*2) / 300)*6 = ratio240: 1.
Goran

250:1 .... that is going in my little black book....

[NF78]

Wow

Thanks for the reply guys.
Quite a bit to take in and a bit of thought process to make the decision.

I really appreciate your assistance.

Thanks again

[Goran Malmberg]

I should add that I used a brake balance bar system as an example, which is the reason using the area of TWO calipers and one master cylinder in the equation.
Goran

[phoenix]

Originally Posted by Goran Malmberg

I would suggest a total gear ratio of 250:1, which is, therefore, pedal and hydraulic "leverage" together. It provides a fairly easily trampled brake pedal. The biggest culprit, with regard to spongy pedal, is hydraulic system and hoses, and calipers on the flex. Is everything in order, so it will be very little flex. One example, a caliper piston area is 6000mm^2, the master cylinder area 300mm^2 and the pedal arm is 6:1. It is ((6000*2) / 300)*6 = ratio240: 1.
Goran

Goran, I'd like your thoughts on what I have found:

I tried your 250:1 ratio with the (front) calipers specified in this thread, and even with the smallest master cylinder commonly available a pedal ratio of 8.5:1 would be required to get a ratio of 245:1

With a 0.7" cylinder (17.78mm) and a 6:1 pedal ratio, the overall ratio is 138:1 - far short of your target.

[Goran Malmberg]

Myself I have 4x1,3/4" calipers, 1 3/4" is 1551mm^2 x 8. This makes 12408mm^2.
A 3/4" master is 284 and a 5/8" is 198 mm^2.

With a 3/4" masrtere we got...
12408/284=43,4*6=262:1

With a 5/8 master...
12408/198*6=375;1.

Finally, with a 5/8" master and 4:1 pedal arm
12408/198* pedal arm 4=250:1.

Regards
Goran

[dtype38]

Goran, unless you have HUGE calipers all round then I'm thinking that your 8 x 1 3/4" caliper pistons are for the whole car. Your calculation therefore suggests you are using a single 3/4" master cylinder, not a balance bar. It would be difficult to get that ratio using a balance bar because the master cylinders would have to be half that area (not much more than 1/2" master cylinders).

[Goran Malmberg]

Originally Posted by dtype38

Goran, unless you have HUGE calipers all round then I'm thinking that your 8 x 1 3/4" caliper pistons are for the whole car. Your calculation therefore suggests you are using a single 3/4" master cylinder, not a balance bar. It would be difficult to get that ratio using a balance bar because the master cylinders would have to be half that area (not much more than 1/2" master cylinders).

You got 4 piston each caliper at 1551mm^2, in my case up front. That makes for a total piston area of 12408mm^2. The 3/4 master is 284mm^2. Where am I wrong? I am using AP calipers 4 x 1 3/4.

It is also so that we use another ratio for the rear since weight transfer occure to the front, needing more braking power. So another ratio for the rear is wanted. BUT, for the driver, a total that is about 250:1 makes for a fair pedal effort feel, if the car is also to be driven on the streets. In reality we could balans the 250:1 into the front and rear for the right proportions.

Now, a real racecar could use 100:1 ratio, but we will end up with 100kg pedal pressure to lock up the wheels. Just to know the size of the pedal pressure window.

No matter how we look at the equation 250:1 and caliper size issue, we need 250:1 in a manual system to have some manual braking comfort. We will still land at some 40-60kg for the leg to handle.

Goran

[dtype38]

Originally Posted by Goran Malmberg

You got 4 piston each caliper at 1551mm^2, in my case up front. That makes for a total piston area of 12408mm^2. The 3/4 master is 284mm^2. Where am I wrong? I am using AP calipers 4 x 1 3/4.

Sorry Goram, I was thinking you had 2 pot calipers all round and your calculation was for the whole car on a single master cylinder. But if you're considering the fronts only, I now don't understand your pedal ratio. If you have a balance bar then the force applied to the master cylinder is the pedal arm ratio x balance bar ratio. So if the balance bar is in the middle, then a 6:1 pedal arm gives a 3:1 ratio of leg force to each master cylinder. So I'm not clear on why you're using 6 as your pedal ration.. would that not only apply for a single master cylinder system?

[Tim Falce]

Originally Posted by dtype38

Sorry Gordon,

He should have gone to Specsavers

[GORDON STREETER]

Mmm

[Alex E]

Just contact Tilton technical in USA & they will send a form for you to fill in with all the info & they will tell you what cylinders to use. I have done this twice & both times only had to adjust the bias only approx 2 turns from centre to get right balance.
Alex

[Goran Malmberg]

Originally Posted by dtype38

So if the balance bar is in the middle, then a 6:1 pedal arm gives a 3:1 ratio of leg force to each master cylinder. So I'm not clear on why you're using 6 as your pedal ration.. would that not only apply for a single master cylinder system?

Let us say we have the front hydraulic system giving a 40:1 ratio, and the same goes for the rear. Then if you put the piston area of the two master cylinders together, the sum will be M*2. The sum of the calipers will also be C*2. C*2/M*2=40:1. So, the the front and rear together is still 40;1. The pedal ratio is also still 6:1. 6*40=240:1 right?

What is happen is that the ratio remains the same, but since your leg is working with TWO systems, the leg POWER NEEDED will be twice as much.

Does it make sence?
Goran

[dtype38]

Ah! Double the leg power. Now it makes sense.....

I best get off down to the gym then

More seriously though, achieving that ratio with commonly available master cylinders is only possible if you have quite large calipers (like yours). Coming back to the example given by NF78, he as a total front piston area of 5708mm^2 and rear of only 3860mm^2. to get the 250 ratio with a 6:1 pedal he'd need master cylinders of only 0.52" and 0.43". Does anyone make them that small?

[Goran Malmberg]

Originally Posted by dtype38

Ah! Double the leg power. Now it makes sense.....

I best get off down to the gym then

More seriously though, achieving that ratio with commonly available master cylinders is only possible if you have quite large calipers (like yours). Coming back to the example given by NF78, he as a total front piston area of 5708mm^2 and rear of only 3860mm^2. to get the 250 ratio with a 6:1 pedal he'd need master cylinders of only 0.52" and 0.43". Does anyone make them that small?

The "gym idea" is allways a good thing. The drawback of anything greater than
250:1 is that any flex within the system will create added pedal travel. The Gym + 150:1 is better. Or maybe the use of a very good designed power system.

The use of a pad with good coefficient of friction will also help as will larger diameter disc since it reduce the need for caliper clamping force.
Goran

[Goran Malmberg]

The weight of the car have a bearing on how much pedal power as needed, so if the car is easy, I would try with 5 / 8 "master cylinder. Then there was that about the brake balance.
Goran