ici: pour le M/C
http://forums.nasioc.com/forums/showthr ... ight=brake
++ info sur le travel:
Factors that contribute to lost brake pedal travel:
THIS IS JUST GENERIC INFO, NOT FOR ANY SPECIFIC VEHICLE.
Just some of my experience from being an automotive brake engineer in Michigan for awhile.
First some quick definitions: Brake Pedal ratio is the measurement of how much mechanical assistance you are getting from the pedal. Example; an ratio of 4.1 will give you 41 pounds at the booster input rod for a 10 pound load at the pedal itself.
Dead or lost travel is how much pedal stoke is required before you actually start stopping.
Dead Travel or Lost travel and overall poor pedal feel is made up of the following:
[(travel as measured at the brake service pedal assembly pin (where the booster connects)]
(imagine traveling from the pedal through the brake system to the rotor, all lost travel must be multiplied by whatever your brake pedal ratio is.)
1. Tolerance between brake pedal pin and booster input rod. This can be quite a bit for systems that use a pin mounted brake light switch. If you do have such a switch do not remove it or take up the slack as your brake lights will be on all the time. If you don't have a pin mounted switch just get a tighter bushing. If you do..you s**t out of luck.
* will be felt during first few mm of travel
2. Slack in the brake pedal assembly itself. To see how good or bad yours are, with the car off pump the brakes until hard (2-3 pumps) and then grap the pedal with your hands and see how much it moves around.
* will be felt through first few mm of travel (I hope)
3. Dash flex. This can range dramatically from vehicle model to vehicle model. Not much you can do about this.
*felt during medium and high decelerations stops on most cars, on Fords dashes flex with the breeze
4. Lost travel in booster. This is designed to be there to allow for booster expansion due to climate and use over time. Only adds half a mm (multilplied by the pedal ratio).
*felt only in the first few mm of travel
5. Flex of booster shell. Can be a real problem on some designs. All you can do is try and brace the booster or replace with a better product.
*felt on medium and high deceleration stops
6. Design tolerances in the Master Cylinder. Varies creately from one to another. Simply, if you want less lost travel in the TMC (tandem master cylinder), you have to pay for a more expensive one. A minimum lost travel of about 1-1.5mm is required for proper and safe operation. However, I've seen some with double that. (again multiplied by the pedal ratio)
*felt duiring first 10mm or so of pedal travel
7. The brake tubes and ABS unit. Maybe .0000001mm here. Don't worry about it.
8. Brake hoses. Get steel braided ones, there worth it. Rubber hoses flex quite a bit even under low pressure.
*felt almost all the time
9. Brake Caliper Piston Roll Back. This is usually the worst offender. Only way to get rid of these is get better calipers. Roll back is how far the piston moves away from the rotor when pressure is released. The bigger the gap, the more you have to push on the pedal to get contact.
10. Caliper Defelection. The caliper actually flexing under pressure, like 9 you can only improve this with better calipers.
*felt during medium and high deceleration stops
11. Brake pad backing plate. If this is flimsy it will flex and not allow a good contact between the rotor and pad forcing you to apply more pressure and therefore more pedal travel. Fixed by replacing pads with higher quality ones.
*felt most of the time
12. Brake pad material itself. If the material is low density it will compress like a sponge. And if it's a low mu (friction) pad it will require more pressure and therefore more pedal travel.
*felt all the time
13. Rare, but a problem on really cheap brake pads: The bonding process used to bond the brake pad material to the backing plate. A poor process will cause the material to "squirm" around.
Well those are the biggy, but not the biggest. The biggest is AIR in the system.. Before you do anything else do a really good and thorough brake bleed. And only use the fluid it says to use on the cap, DOT 3 or DOT 4 or whatever.
How I would go about improving a system:
1. Bleed system
2. High quality pads
3. Get steel brake hoses
4. New fixed calipers from a know manufacturer
5. Some bracing for the actuation unit (booster/ master cylinder/ pedal)
Also, changing the brake pedal assembly to one with a lower ratio. Remember all lost travel is multiplied by the pedal ratio, the lower the ratio the less dead travel at the pedal pad. This also firms up the pedal as your getting less mechanical assistance. Just be careful, because if you brake booster fails it will take more force on the pedal to come to a stop. Minimum pedal ratio is calculated using NHTSA FMVSS 135 which states the maximum force required to stop a vehicle in a certain distance when the booster fails.
Pedal feel can also be "tuned" with a different booster. I will not get technical, but basically the booster sets the Force-Force curve (Force into the booster vs. the force out). How this curve looks will dictate pedal feel (assuming all else equal). A F-F curve has certain points that are important, the 2-stage, cut-in, boost ratio and the run-out.
2-stage is what sets the intial point of boost (when it kicks in)
cut-in is what sets how much initial force
boost ratio is just as it sounds, it sets how much assit you get
run-out is the maximum assist
By lowering the 2-stage and increasing the cut-in you get a better intial bite sooner. Just have to be careful you don't go overboard and have the driver eat the steering wheel at every stop light (like an '80s Audi).
Difference between Feel and Performance
Just some more stuff:
I want to make it clear there is a difference between making a brake system feel better and actually perform better.
The stopping distance of a car is not necessarily directly related to the feel. A poor feeling brake system can have very good performance, i.e. Jaguar. While a great feeling system can have lousy performance, i.e. Ford Focus
The only way to decrease braking distances is to create more heat through friction and conduct that heat more efficiently.
Basically, you need a bigger contact area between the rotor and pad (bigger pads and rotors) with the best contact patch possible between the two. Also higher friction levels with higher mu brake pads.
And most importantly.........REALLY GOOD TIRES!
And you need a way to get rid of that heat at a faster rate. More rotor mass and/or better conduction (vented, etc...).
How a car stops is simple, it takes Kinetic Energy (energy created by motion) and transfers it to Heat (infrared) Energy. It does this by the mechanism of friction.
KE=1/2 the mass of the vehicle multplied by the square of it's velocity
Stopping distance is determined by the rate of KE to IE transfer, or also know as WORK.
Simply, to stop sooner you need to transfer Kinetic Energy into Heat Energy faster.
There are no other tricks...you need higher levels of friction and ways to dissipate the heat quicker (you can also get more massive rotors which are capable of absorbing more heat).
The big things to do to get shoter stopping distance:
1. Best Tires for the conditions (use common sense here, no Pilots in Feb in Alaska)
2. Higher Firiction Brake Pads
3. Bigger rotors with better heat conduction properties or better heat absorbtion properties
That's it!!!!!!!
The small things you can do that give you that extra advantage:
1. LOSE WEIGHT (the car I mean), less mass, less KE
2. Minimize rotational interia of wheels/tires. Get lighter wheels and tires (all else being equal). Note: the farther away from the center of rotation the mass is the great the interia, so a 17" wheel will have greater interia than a 16" all else being equal.
3. Try and balance out the braking, if the rears can do more work it spreads out the work. Less weight transfer to the front [edited 4/5] BY MOVING THE BATTERY TO THE TRUNK, stuff like that.
4. Calipers with more pistons and more piston area, this gets you that better contact between rotor and pad.
5. Pratice! Get to know how your system works so you can best utilize it.
6. Don't drive to fast, remember it the square of the velocity. The amount of KE increase from 40KPH to 80KPH is not 200%, but 400%.
