Improving Brake Feel

[Quasimondo]

I've flushed and replaced the brake fluid with DOT 3 synthetic, which improved the firmness in the brakes a little bit, but they're not where I want them to be as far as firmness goes. Is there anything that I can do to increase the firmness, or at least eliminate the sponginess that I feel within that first 1" of pedal travel?

 

[xveganxcowboyx]

Braided steel brake lines should help.

 

[Defiant]

Have your buddy push the brakes while you look at the firewall with the hood up. Flex.

 

[ahaidet]

How much wear does your pads and rotors have? Replacing these can help a lot. If they are significantly worn you are working at the end of calipers piston travel. The pedal travel increases when you do so.. that first 1" of sponginess my guess could be gotten rid of with new pads and rotors all the way around. I recently replaced all the rotors and pads on my car and the pedal feel was like a brand new car.. I need to flush the brake fluid out with fresh and replace the old rubber hoses with braided stainless. My brake feel would be unreal if I did that. Also if you can "pump" up the brakes a little bit then you may still have air in the system. Also try holding the brakes all the way down and see if the pedal continues to drop, if it does you have a leak in the system somewhere, which will leak air back into the system which causes sponginess. Hope this helps.

 

[Quasimondo]

Rotors were resurfaced last year, pads have about 50% material left on them. Pedal did sink to the floor before, but after rebleeding them, that problem went away. Pedal is firm, but only after it travels past that initial sponginess mark. I think the braided lines may be the way to go then.

 

[askpoc]

braided steel brake hoses will work wonders for you

 

[Defiant]

How much wear does your pads and rotors have? Replacing these can help a lot. If they are significantly worn you are working at the end of calipers piston travel. The pedal travel increases when you do so.. that first 1" of sponginess my guess could be gotten rid of with new pads and rotors all the way around. I recently replaced all the rotors and pads on my car and the pedal feel was like a brand new car.. I need to flush the brake fluid out with fresh and replace the old rubber hoses with braided stainless. My brake feel would be unreal if I did that. Also if you can "pump" up the brakes a little bit then you may still have air in the system. Also try holding the brakes all the way down and see if the pedal continues to drop, if it does you have a leak in the system somewhere, which will leak air back into the system which causes sponginess. Hope this helps.

Some of the most bizarre stories I've ever heard. Disc brake pads move about .010" regardless of how far down they're worn.
A pedal dropping when held down had better not be a leak in the system, or you're about four stops from piling into the next thing you try to not hit. No "leak" is going to allow air to suck back into the system, it's only going to expell fluid. There's not enough "suction" to pull in significant air. A dropping pedal is most usually a leaking master cylinder.

 

[ahaidet]

Some of the most bizarre stories I've ever heard. Disc brake pads move about .010" regardless of how far down they're worn.
A pedal dropping when held down had better not be a leak in the system, or you're about four stops from piling into the next thing you try to not hit. No "leak" is going to allow air to suck back into the system, it's only going to expell fluid. There's not enough "suction" to pull in significant air. A dropping pedal is most usually a leaking master cylinder.

0.010"?? Hmmm. Lets look at some calculations.

On a 1G allwheel drive according to the Factory Service manual there are the following specs.

Front:
Disc thickness: New 0.94" Lower usable limit 0.882"
Which allows for: 0.058" variation in the disc thickness (Assuming they aren't worn past this which certainly could be the case with a neglected brake system. )
Front Caliper Diameter: 2.375"
Front Caliper Area= Pi*Diameter= 3.14*2.375"=7.4575"
Front Caliper Volume= Thickness*Area=0.058"*7.4575"=0.4325"
Now that is for one Caliper so we multiply by two and get 0.86507 cubic inches

Rear:
Disc thickness: New 0.39" Lower usable limit 0.331"
Which allows for: 0.059" variation in the disc thickness (Same as above)
Rear caliper diameter: 1.375"
Rear Caliper Area= 3.14"*1.375"=4.3175"
Rear Caliper volume=0.059"*4.3175"=.0.2547"
2 rear calipers= 0.50946 cubic inches

Meaning a total volume that the master cylinder has to account for:

1.3745 cubic inches now to get the distance that the master cylinder will have moved down since new we divide by the area of the master cylinder

Now DSM's and all modern cars use a 2 part master cylinder which has two pistons inside so in case one fails you still have the other so we will multiply by 2

Master cylinder diameter=1"
Master cylinder area= 3.14*1=3.14"
multiply by two for 2 pistons= 6.28 square inches

Now dividing we get a difference from new to worn of: 0.21886" (nearly a 1/4" that the pedal has dropped with a system that is at the lower limit above could be higher if it is worn farther)

Our calipers are not spring retract and are of the sliding caliper design so the piston does not retract this full distance. However knowing that the system is a sealed system and that the master cylinder has a spring to return the piston in the master cylinder back to its un-applied position there is a vacuum applied to the fluid and it is pulled back into place somewhat assuming no leaks. This pulls the caliper piston back a bit... how much is unknown but the maximum it could pull it back is the number above or nearly a 1/4". Even if it only pulls it back your said 0.01" The pedal is sitting that much lower, a lower pedal can be perceived as spongy pedal.

Also on the note of the system pulling air back in.. when you let go of the brake there is a vacuum applied or brakes would stay permanently applied. This vacuum can draw air in. (Other wise there would be no reason to close the bleeder valve when bleeding brakes) On certain Wilwood Racing Calipers I have seen leaks that only seep tiny amounts of fluid not noticeable if the fluid is being burned off, when the brake is released it allowed tiny amounts of air to get into the system... Any amount of air will allow compression which causes a spongy pedal.

Not far fetched at all. I agree with you that there is flex in the firewall, the pedal, the pedal mounts, the calipers , and even the rotor.. All which contribute to a lack of firmness in even new brakes. But these are also possible contributors to a spongy or low pedal.

Sorry Defiant not trying to step on your toes..

 

[Todd TCE]

Better not be a math major....

That piston area is Pi R2^ or 4.4sq" for the front and 1.5sq" rear. Each.
Volume should be a bit harder.

Displacement is probably more on the order of .040" per caliper given they are a slider and not fixed. The displacement of .010 is pretty optimistic. Never the less.....I'll run it based on .040 (remember it has to both push AND pull so it's .020 per side)

Frt .177ci X 2 = .354
Rr .059 X 2 = .118

Total .472ci of fluid. Or about 12cc of fluid.

A 1.00 mc would need about .625" of stroke to perform this.

The MC has an area of .785sq". While there are two chambers they work in tandem, its one piston moving sealed volume of two areas. Each of the chambers is acting on only half the required displacement total. A staggered bore moves different volumes per end of the car and is used for pressure differential.


I have no idea what this has to do with wear limits of the pad or the disc. Your pedal and MC don't change either height or volume requirements with worn pads. The piston displaces the lost pad material and the reservoir lowers the same cc of fluid. The pedal still moves the same and feels the same if you run the pads to the backing plate.

 

[damax03]

I think this will be a perfect time to ask a question I have pondered for a long time. I am a master technician at a Mercedes Benz dealership, and understand how basic brakes work, but am uneducated in how the different designs of a braking system change how the car will act. I am alway frustrated with the amazing intial bite and feel that ALL of their cars have NEW or OLD when in good operating condition. I have braided lines, powerslot rotors, and metal master pads all around. If I push harder on the pedal I can stop very well, but I dont have the feel. Its like the mercedes gets all of its brake pressure already within the first bit of brake pedal travel and the rest is sort of gravy after that. If I am unclear I will try to explain more, but this is something thats has always bothered me and I must get to the answer. Even better maybe someone can tell me some information on what it would take to make a DSM act in that way. Thank you in advance for any info that I can gain from any one who knows what they are talking about.

 

[Defiant]

Sorry Defiant not trying to step on your toes..

You aren't. You seem to have no grasp on how disc brakes and their calipers work.

 

[Defiant]

I am alway frustrated with the amazing intial bite and feel that ALL of their cars have NEW or OLD when in good operating condition.

Friction material and caliper design, along with an engineering department which spends very much time (and gets paid quite handsomely for) on the feely-touchy end of automotive output.

 

[Todd TCE]

With the math behind us I'll try to toss out my .02 on the second question.

First off; FEEL is really no indication of brake performance. Feel is related to the hydraulics of the system. In the simplest form, a larger bore MC will provide a firmer pedal feel. A smaller caliper piston area will supply a firmer pedal feel. But...each will require more pressure to work properly. The pedal needs more leg, the caliper more input pressure.

Brake responsiveness also relates to feel. A fixed caliper will be more stable and require less fluid displacement as they are more efficient in design. The slider has a small amount of wasted energy as the caliper has to move on the pins/guides before it works. Loss of feel.

Caliper design will effect this as well. Recalling the light four pots of some of the early kits I offered the hard core users found their body flex not to their liking. While flex does not effect brake torque it can make for a softer pedal feel as you are moving more fluid than should be required. This is common on a lot of alum floating calipers as well. The heavier the caliper the less loss of response. One particular brand makes its calipers so heavy they can then talk about how stiff they are. OK, but not everyone wants or needs a 10lb caliper.

Pads provide the bite but it's not the firmness you feel it's the reaction time again. As you run track pads they often bite with modest force and then ramp up in bite as the temp goes up too. That means you tend to feel the car 'hunker down' with only modest changes in leg input. That's a nice pad. But it often eats up rotors if used on the street.

Hoses change the pedal feel buy both reaction time and size. A smaller hose has a firmer feel as there is less fluid in it to move. Most kits run -3 hose. There both -2 and -4 as well. Using -4 will make the pedal softer and a longer stroke where -2 makes real hard pedal and a lot more effort. The smaller hose is fine for a Kart or a Motorcycle but when tried on a race car it was often removed as the driver complained of 'lack of feel' or modulation. On the other hand -4 hose was used for year where big caliper piston areas required a lot of fluid to be moved to work properly and -3 just wouldn't get it done.

Most cars today run much smaller piston area than even ten years ago. Why? Because the leverage of the big rotor is more valuable for brake torque and pad technology has produced a much better pad today requiring less pressure to work.

As I call it: Larger rotor, less piston area, tune with pad. Pretty much what car companies have learned too. Of course larger wheels have helped their cause as well now.

Best I can offer, feel free to contribute.

 

[ahaidet]

At about 7:30 last night it hit me that I used circumference instead of area of a circle. (guess thats what happens when you work with tires all day) I wasn't near a computer to come back and fix it. And forgot that the system is constantly replacing the fluid. Oh well.

Ive been thinking about this far more than was healthy over the last few days.
On my car the pedal feel improved greatly after the new pads and rotors. The only explanation I could come up with was the one above. Finally I may have figured out what I was noticing. With the pedal level being located the same. (above is wrong) The pedal feel improved. By putting new pads and rotors on the car I replaced old fluid with incompressible metal. Brake fluid can typically be considered incompressible however the fluid in my car is older, the fluid is hygroscopic (meaning it absorbs water). Water boils when in a brake system because of the high temperatures and introduces compressible vapor. The reason my brake pedal feels firmer is because I have less slightly-compressible fluid in the system and more completely non-compressible metal in the system. If I bled my brakes and replaced the fluid completely I would notice even more improvement. (read Brake Fluid 1A below link)

Bottom line: Pedal feel is really unimportant to brake performance. Only to driver performance. If you really want to stop faster invest in a quality set of pads. Use stainless lines to help with pedal feel. And replace the fluid and bleed the system every few years.

For any other info on how brakes work, improving pedal feel, improving performance... I would highly recommend this website as it is full of information and can help you understand how brakes work.

http://www.stoptech.com/tech_info/tech_white_papers.shtml

The great race car engineer Carrol Smith has contributed a lot of useful papers to that collection. And he uses the correct formulas for area and circumference. (Feel like an idiot for that one, Sooo I am just going to go slink off into a corner.. and not use math for awhile)

 

[prostreetdsmx1]

hmmm, i don't know how this got to be so number crunched but generally we've seen this problem fixed with some ATE super blue and SS line kit. I think that will do wonders for you bud.

best of luck to you.

-marti