Brake shoe break-in

[FV80]

I don't think that will work Brian. The harder you push, the LESS you get out of it. If you want MAXIMUM breaking pressure when you need it, you have to set the pedal up so that it NEVER goes over center. Of course, if you have a LOT of pedal travel, you might have enough pressure to do the job anyway. I set mine up for the least travel I could manage - meaning it takes a LOT of pressure.

My pedal is at quite a high angle toward me at rest - I can just barely clear my tierod/pitman assembly (with my foot on the pedal) at rest. If it goes over center at all, I can no longer lock up the tires. I do have to adjust the brakes after every other session at least.

Steve

[Speedsport]

JS,

To answer your question about baking, when using the stock shoes I would place them in the toaster oven for 1 cycle. I think it was about 30 minutes at 400F. Additional cycles I would not expect to have any benefit. I would not, however, suggest you do this with Carbotech shoes. According to Carbotech, their shoes require decent pressure on the material to bed in. I think the stock shoes just needed to have some of the manufacturing oils baked out of them.

Michael

[hardingfv32-1]

No, it works great. Almost the feel of disc brakes. Imagine a pedal with a kink in it (shallow V) The fulcrum point where the master cylinder push rod attaches is about 20 deg beyond vertical (towards the master cylinder). When fully depressed the fulcrum point is about 40 deg beyond vertical. The effective distance of the lever arm is reduced, thus increasing the force being applied to the master cylinder for a given amount of foot pressure. Now it goes without saying that everything has to be engineered properly. You do not just readjust a normal pedal arrangement.

Low leverage/high travel for early shoe travel and high leverage/low travel for actual braking pressure. The braking system can be improved.

Need more clear explanation? I think I'm describing it correctly.


Brian

[jstoezel]

The tilton assemblies seem to have the pedals vertical when no pressure is applied

http://www.tiltonracing.com/pdfs/90.pdf

No, it works great. Almost the feel of disc brakes. Imagine a pedal with a kink in it (shallow V) The fulcrum point where the master cylinder push rod attaches is about 20 deg beyond vertical (towards the master cylinder). When fully depressed the fulcrum point is about 40 deg beyond vertical. The effective distance of the lever arm is reduced, thus increasing the force being applied to the master cylinder for a given amount of foot pressure. Now it goes without saying that everything has to be engineered properly. You do not just readjust a normal pedal arrangement.

Low leverage/high travel for early shoe travel and high leverage/low travel for actual braking pressure. The braking system can be improved.

Need more clear explanation? I think I'm describing it correctly.


Brian

[hardingfv32-1]

Are you implying that this means vertical is the correct way to go? Do you think the Tilton systems are designed with drum brakes in mind? Image how large a customer base that must be. No, you are going to have to think about this a little.

Break down what exactly is happening as you move the brake pedal. Is there a fair amount of movement when the peddle is first depressed? This is the shoes traveling out to the drum. Why not accomplish this with as little pedal movement as possible. The problem is that this motion ratio will not work when the brake pressure is actually applied after the shoes are in contact with the drum. The leverage has to increase to make it usable for the average driver. This is a form of bell crank system. You have to design in some substantial angles to make this work.

With my system, pedal force can actually decrease at the end of the pedal travel, near brake lockup. This is not the best feel for the driver, as it is opposite of what is normally expected.

The Strange Fast Fill master cylinder is the hydraulic way to get the same effect:
http://www.strangeengineering.net/catalog/index.html

Brian

[Matt King]

With my system, pedal force can actually decrease at the end of the pedal travel, near brake lockup. This is not the best feel for the driver, as it is opposite of what is normally expected.

Then why do it? If it doesn't improve the driver's performance, it's probably not the right thing to do.

On an unrelated note...At the motorsports show in Indy last week I saw some cool reverse action AP master cylinders with a built in bias adjuster. Rather than push the rod in against the piston, the pedal pulls the piston rod and the inlet/outlet fittings are reversed compared to traditional Girling style. These would be useful in a Vee if you wanted to move the pedals as far forward as possible and not screw around with mounting the masters ahead of the beam.

http://www.apracing.com/info/products.a ... _2383_2364

[hardingfv32-1]

The word I used was CAN, not DOES. The purpose is to have reasonable pedal travel with low pedal pressure at the maximum braking point. My driver can feel the brakes better with lower pedal pressures.

You might not have a requirement for this system at this point in your FV career.

Brian

[Matt King]

You said it was "not the best feel for the driver." That implies there is a better feeling available. Then you said there was better feel for your driver. So which is it? Regardless of anyone's driving career stage, does it result in faster or more consistent lap times? If not, then what's the point?

[jpetillo]

My pedal is at quite a high angle toward me at rest - I can just barely clear my tierod/pitman assembly (with my foot on the pedal) at rest. If it goes over center at all, I can no longer lock up the tires. I do have to adjust the brakes after every other session at least.
Steve

Steve,
The pedal pressure really shouldn't change that much a little past over center - the leverag loss is really not that much. Could it be that your leg reach may be the limiting factor? When you have to push even a little bit further you can lose a lot with the legs. For me, a little more reach really makes a big difference in how hard I can push the pedal. John

[FV80]

Well - I can't GUARANTEE anything, but my pedal only has to go a very small amount over center for me to loose significant braking power. It's not a matter of REACH - I have enough of that, but I haven't truly studied the mechanics or calculated mechanical advantage number for the various angles .. I just know that it makes a BIG difference in both of my cars. Of course the pedal setup in each is nearly identical.

I'll have to think about Brian's suggestion about a variable mech advantage setup - it certainly has merit if I can figure out how to do it .
Steve

[problemchild]

I will redo any pedal configuration that the brake pedal goes over-center ..... especially on a FV where brake shoe adjustment and wear often results in longer than ideal pedal travel. Any driver, new or experienced, young or old, needs to learn how to modulate a brake pedal. I want more braking force as I push the pedal harder.

[Hal]

Generally speaking I try to get the brake pedals on both my cars to acheive maximum force when they are pretty much straight up and down, effectively at a right angle to the master cylinder push rods. No mechanical engineer here but could certainly see how braking force would be lost if the brake pedal went over a certain point. I am average size and height so setting up the my brakes is not that hard to do but can see where shorter or taller people might struggle with this.


Hal
D-13 88
Citation 88

[CitationFV21]

Not that it relates directly, but if you are looking for a different take on this subject:

http://dviaviation.com/files/38801004.pdf

I came across it while looking for pictures of proper pedal angles.

In TUNE TO WIN, Carroll Smith warns about having the pedal go over center, but depending on where the pedal face is, it probably does not make much difference.....but.....you loose total travel and if your brakes wear or you boil the fluid you may have a problem. You want to have as much possible travel as you can get.

One thing I had to do to my brake pedal is make it higher as the origional pedal was putting too much pressure on the ball my foot. Pedals should come in sizes just like shoes....

ChrisZ

[Hal]

After I changed the brake mounting position on the Citation, moved it forward, I had to cut, bend back, and have welded the two steel flat pieces holding the pedal itself, without doing this the pedal was so far forward it would have been very dificult to race. Now the brake pedal is right where I want it in relation to the gas pedal and gives good leverage into the mastercylinders. Took a lot of experimenting and fitting but works very well for me. I don't have a lot of travel in either the brake of clutch pedal as it leaves the pedals to far back towards me and just uses up time and efficiency taking up the slack, and there is NO brake drag. I also have light springs on both pedals to hold the pedals forward a little bit to take out even a more slack. All my fancy footwork is very intuitive now as everything is just where I want and expected to be, give me a very good feel.


Hal
D-13 88
Citation 88

[jpetillo]

Chris,
Yes, I've seen that dvaviation report. It being an Air Force Lab project (Wright-Pat?), it was well done.

That result says that the pedal angle was best chosen to fit the angle of the foot for best force applied, and your foot had to be within +/1 10-15 degrees of it's natural position to give the best force - this is from memory. That was what I meant when I said a few days ago that you can run out of steam fast with a slightly further pedal travel. It has to do with ankle angle and that changes fast. It had little to do with the over-center angle. Carroll neglected to show/say why. I think his intuition is simply excellent, but his reasons are not always there, so I find that I need to dig in and figure out why when i read his stuff. He's not always right in his analysis, so be careful.

There is a lot of discussion on this overcenter thing - we need to be careful. Some of it is myth - some not. Any loss of leverage has nothing to do with the pedal being over center due to standard lever ratios, it has to do with the change in angles of the "rods" pushing against the brake pedal, and then the rod angles with what they are actuating. (For example, if the rod lengths are infinitely long, they will stay parallel to each other and there is no change in ratio vs. over/under center angle. Do the calculation and you'll see.) One of those rods is your leg and the other is the master cylinder push rod. Your hip-to-leg will not change angle much because it's so long, so no angle advantage/disadvantage there. If you have a long Citation/Speedsport type rod to the master, also there is not much angle change there, either. It stays pretty parallel with pedal travel. It that case there is little mechanical change in the ratio - simple lever theory - whether you're before or over center, no significant difference in ratio. However, with a short pushrod to the master (like a Caracal, etc.), the angle of the rod into the master will change fast (not be parallel) and degrade performance quickly as it's angle from parallel changes and it doesn't push straight into the piston. This is the angle you primarily need to worry about. Everything else is a much smaller effect. For example, even if it did happen to work where the leverage lost is from the loss of mechanical advantage as you go over center with your foot, considering that cos(30degrees)=0.8660, then the pedal over center by 30 degrees could only lose 13-14% of effort. We move our pedals much less than 30 degrees - probably more like 20, and 20 degrees gives 6% and 15 degrees gives a 3.5% loss of effort. I don't think that 3-6% loss would result in an inability to lock the tires since we lose at least that much ability to push hard from just stamina loss over the course of the race. But it doesn't work like that, anyway. I'm sorry that I said a lot but explained little - it would get too long and boring so I just stated the results. But I should be able to back up any of these claims to your satisfaction if you want to contact me offline - or just show me where I've strayed from reality.

So, concentrate on your ankle angle (best force will also give you best feel) and the angle of the short pushrod from misalignment with the brake piston and forget about over/undercenter. John

[SR Racing]

John is correct. Assuming something outside of some rediculous OVER or UNDER you would be hard pressed (no pun intended) to see any real difference going over or under. Do the math. The mechanical ratio is important but that ratio change is tiny until you get to extremes. As he points out, the extension of the leg and angle of the foot allows for the best pressure. As he also mentions, the geometry of the pushrod is important.

[CitationFV21]

... depending on where the pedal face is, it probably does not make much difference.....but.....you loose total travel and if your brakes wear or you boil the fluid you may have a problem. You want to have as much possible travel as you can get.

ChrisZ

Quoting myself - people don't worry about total travel - I have seen some brake pedals that are on full as they just hit the steering links - not good.

When you bleed your brakes - watch the travel - if you ever run into a situation where you have to pump the pedal, you better have enough travel.

I think my next car will have adjustable pedal faces - much easier to adjust.

ChrisZ