23 replies to this topic

[MarkWill]

Probably something easy to answer here. When a disk brake is applied, which part of the pad is doing the most work (leading edge, all of the pad, front surface)? I mean in reality, vs the ideal situation? This question comes around from a discussion I had with my colleagues re. bedding in the pads, when we started arguing about whether or not pads deform like sponges, or whether or not you grind them down against the disk. Also, why are some companies adding more pistons to the callipers, instead of using a bigger piston?

[alexbiker]

The number of pistons in the caliper is limited - have a look in the regs if you want. The size of the caliper relative to the disk is also limited, as are the cooling options.

All-round, it's limited!

Alex

[MarkWill]

To be precise, I dont just mean for F1 cars, I mean in general (eg Porsches have four- piston callipers, up from two before - why?). Also the regs dont answer the question about which part of the pads do all the work.

[mmmcurry]

From my limited understanding of mountain bike disc brakes:

The more pistons in the calliper means than the pads can be longer giving the braking force a bigger moment, which produces a larger braking force. Also on my Hope M4's the pistons size is different for the leading / trailing pots. I can't rememebr if this is to make sure the leading edge bites first, to put more pressure on the leading edge or something else.

Certainly my pads work much better when they have bedded in, also getting goodridge (F1 style) hoses increases performance over the softer ones that come with them. I think the pads grind down, or at least they do over winter in the Peaks.

Dunno how much help any of that is, its all from memory, not from knowledge.

Steve.

[MarkWill]

Hi Steve,

Thanks very much. Your answer is a bit along the lines of how the question came around - is the leading edge doing all of the work, or is the rest of the pad doing its bit as well (which makes it a bit useless otherwise).

IIRC, F1 teams have experimented with, and used, pads made from multiple sections (which suggests that the leading edge thing comes into play - more pads means more leading edges) but then this is in conflict with the bedding in of the brakes (unless its just to assure a uniform leading edge) because that would make it a useless exercise. Bedding in the brakes seems to be for two reasons - 1. to get the brakes to work optimally and 2. to increase the "feel" of the brakes. Which brings me back to the original question - which bit is doing all the work?

[xflow7]

If you visualize (or draw) a free body diagram of the pad you can see that the friction force on the pad produces a moment proportional to whatever the offset is between the pad supports and the rotor surface.

This moment has to be reacted by a moment generated by the normal force distributions of the pad/rotor interface and the pad/piston interface about the same support point.

So, for instance, if the piston/pad force distribution is such that it provides no moment about the support point, then the leading edge of the pad must have a higher normal force than the trailing edge, implying that it is providing relatively more braking effort.

Multiple piston calipers and the relative sizes of the pistons are, in part, aimed at manipulating the force distribution on the pad to achieve an optimal force distribution between the pad/rotor. I'll stop short of saying that the ideal is for the pad to be uniformly loaded on the rotor as I suspect that thermal considerations (and possibly brake "feel" considerations) may mean that some bias one way or the other is beneficial. I'm no brake expert, though.

The other reason for the multiple pistons is to allow larger pad area. Generally there is some constraint on the radial distance available due to wheel size restrictions, rotational inertia concerns, whatever. So, the only way to increase the area of pad/rotor interface is to expand circumferentially. Since the resulting pad is very elongated, it is not practical to attempt to load it with a single piston.

Dave

[MarkWill]

yes: Quote

"The other reason for the multiple pistons is to allow larger pad area. Generally there is some constraint on the radial distance available due to wheel size restrictions, rotational inertia concerns, whatever. So, the only way to increase the area of pad/rotor interface is to expand circumferentially. Since the resulting pad is very elongated, it is not practical to attempt to load it with a single piston."

Wouldn´t this imply that the rest of the pad is indeed doing its fair share of the work? This is getting to be a bit confusing, because I thought that discs were more efficient because of the fact that the leading edge length of the brake is longer than eg that used in drum brakes, but if the area of the pad is THE important factor, then a drum would be as good as a disc...no?

[xflow7]

Again, I'm not an expert, but anyway I fear I wasn't very clear. I think in a well designed system, the entire pad IS doing it's "fair-share" of the work. My comment showing the tendency for the leading edge to do more was intended to demonstrate the need for things like multiple pistons in order to get a "long" pad to work along its entire length to discourage excessive work by the leading edge.

On the drum vs. disc question, I believe the advantage of disc over drum is more to do with heat dissipation ability than length of the leading edge. Also, a disc can have significantly less rotational inertia.

[MikeTekRacing]

Originally posted by xflow7
On the drum vs. disc question, I believe the advantage of disc over drum is more to do with heat dissipation ability than length of the leading edge. Also, a disc can have significantly less rotational inertia. [/B]

true indeed

[AS110]

Mean effective radius is more important than just rotor area - so a large diameter rotor with say 3 small pistons is better than a fatter rotor with one large piston,the inner diameter is not doing as much work as the outer.Disc pads wear evenly over all their surface,unless there is something wrong in the caliper or mount,so I don't think they are being applied unevenly along their length.

[mmmcurry]

Originally posted by AS110
Mean effective radius is more important than just rotor area - so a large diameter rotor with say 3 small pistons is better than a fatter rotor with one large piston,the inner diameter is not doing as much work as the outer.Disc pads wear evenly over all their surface,unless there is something wrong in the caliper or mount,so I don't think they are being applied unevenly along their length.

Again, from looking at old pads from my bike the trailing edge appears to have more wear (down to the metal backing), although this is probably down to my bad maintainence.

Steve.

[AS110]

I'd say so - I own an automotive workshop and we can change several sets of brake pads a day...if we see uneven wear it's a sign of something wrong,not something right.Otherwise every set of pads we pull out would have uneven wear,and that's certainly NOT the case.

[mmmcurry]

Another question about disc brakes, how do you stop them from squealing? Mine are ok, but some people with Hope Mini's (2 opposing pistons) can't get them to stop squealing like a pig. The have had their mounts faced, cleaned the pads, rotors, put copper slip on the back of the pads, but still the noise persists. Any ideas?

Steve.

[red300zx99]

I work in a racing series that only allows a certain type of brake caliper, and it only has 1 piston. Brake pad wear is not even, the trailing edge is much more used then the leading edge. The use of multiple pistons in a caliper is to get even pressure from the brake pad onto the rotor. I am not going against anything that AS110 has said, uneven brake wear is bad, but improper caliper design or bad installations do happen. AS110, at the same time I question what would happen if one of those cars you work on w/ a new set of pads on goes racing round a circuit. The forces and heat would be much different then what people use in daily driving, id bet a nickle that such factors would show a different wear pattern then what youre used to seeing.

[AS110]

You're right,I'm just talking daily drivers,not racing,but then racing calipers are different from OE for obvious reasons,you are pushing OE calipers beyond what they were designed for.

Squealing is a big issue with brakes these days,from my end of the industry for sure....we get heaps of come backs for noisy brakes.Brakes were designed to use asbestos as a lining material,now we are not using asbestos,but still using the same design of brake,pad material is a major problem.Asbestos is soft and absorbs the vibrations that make noise...all we do is supress the noise.That black dust you see with modern pad materials is graphite,used to lubricate the pad and stop noise....this is a big problem - maybe someone here can give us some insight from the manufacturers end,not from me as an end user...

[Greg Locock]

So far as OEMs go there is no one size fits all solution. In no particular order:

add constrained layer damping to pad
change dimensions of pad
change stiffness of calliper
change geometry of pad to piston interface
change pad material
modify structure of rotor

CLD is a quick expensive fix but tends to degrade brake system stiffness which is a negative for brake feel, unless you make a cutout for the piston which reduces its effectiveness and increases its cost.

[Torquer]

I think it is time we evolve past horse and buggy friction braking technology.

[Christiaan]

Torquer, ever since I learnt of the braking forces caused by eddy currents when you spin an aluminium disk in a magnetic field, I thought hmmm! The heat dissipation may be way way better.

[Supercar]

I have designed a few brake upgrades for street and race cars, so I know a little bit about brakes. Yes, if the pressure on the pad was uniform, then the pad would have "dug in" and cocked and the leading edge would have been doing most of the work. But we want to have a uniform wear on the pad from the time when it is new until the time when it is almost fully worn, so for that reason the trailing pistons are always bigger than the leading ones. Larger trailing pistons make the trailing edge of the pad work just as hard as the leading edge.

The OD of the brake caliper is limited by the rim. The OD of the rotor (brake disk) is limited by the caliper strength. There should be enough pad area to concert al the kinetic car energy into heat. It is more efficient to apply the brake force as far to the outer edge of the rotor as possible, therefore the brake pads tend to be long (circumferentially) and not tall (radially). That sometimes requires 6 or even 8 pistons. But if the pad size is not a limiting factor (on most race cars that use race pads it is not) then 4 or even 2 pistons work just fine.

Drum brakes do offer more brake torque if the same braking effort is applied. But for most applications they have disadvantages that outweighed the advantages. They are difficult to modulate, to cool, to keep free from water, etc...

Philip

[nicholasc]

Originally posted by mmmcurry
Another question about disc brakes, how do you stop them from squealing? Mine are ok, but some people with Hope Mini's (2 opposing pistons) can't get them to stop squealing like a pig. The have had their mounts faced, cleaned the pads, rotors, put copper slip on the back of the pads, but still the noise persists. Any ideas?

Steve.

It's not the pads - it's the bike. That's why none of those 'fixes' work for any length of time. There is a harmonic vibration occurring in the frame or wheels. The only hope is to change something significant - add weight to the system (increases damping), tighten or loosen the spokes (changes the frequency of the vibration), change the pad material.
The lighter and stiffer bikes get, the worse the problem gets.
I've got a Cannondale F4000SL with Magura Martas. As the temperature drops before a storm they squeal a bit, but at no other time - so I'm pretty lucky for a bike that light. Thinner (worn pads) are also worse - I imagine due to the reduction in damping.
For me if the brakes squeal, they're off! If you can't cruise through the forrest in stealth mode, what's the point?

[Supercar]

Who cares about some brake squeal on F1 cars! Isn't this what we all drive here on this forum?

Brake squeal is caused by harmonic oscillations which result from the combination of brake pad material, pad stiffness, shape, size, caliper stiffness, rotor stiffness, car speed, temperature, air humidity and perhaps a few other things. Pads that are oversized for the application, race or "aggressive street" pads tend to squeal more if they are not used hard enough. The easiest way to stop it is to try brake pads from another manufacturer. It also often helps to chamfer the leading and the trailing edges of the pad, cut a slot in the pad or put some sticky goo on the back side of it.

The best internet resource for brake information comes from my favorite brake company, StopTech. I use their components to make many of my brake kits. It is worth reading their whole Technical and FAQ sections if you have questions. AP Racing also has some. Here are the links:

http://www.stoptech.com/technical

http://www.stoptech.com/faq

http://www.apracing.com/car/user.htm

To add to the leading/trailing edge discussion. StopTech now has a new small 2-piston caliper. It matches the street cars rear brakes best where brake rotors are usually already quite big (because they contain the parking brake inside). The pistons in those calipers are offset back, closer to the trailing pad edge of the pad in order to take the load off of the leading edge.

Philip

[MarkWill]

Thanks Phillip for these links. There really is a lot of information there (read it all with interest) and it really does answer a lot of questions. I was intrigued about the claim that a larger piston could actually lead to decreased braking - surely as an element in a braking system the shortcomings of longer brake travel and to much clamping force can be easily overcome? Also, I dont see how drilled rotors and slotted rotors are so different, although I`m prepared to believe them that slotted is better (nothing beats experience).

[Supercar]

Originally posted by MarkWill
was intrigued about the claim that a larger piston could actually lead to decreased braking - surely as an element in a braking system the shortcomings of longer brake travel and to much clamping force can be easily overcome?

One of my customers "upgraded" his front brakes with a kit from one big-name brake company. That was before I was officially in business. The pistons in that kit are BIG, one-size-fits-all. The only benefit that he got out of that kit was better hear dissipation. The drawbacks are premature front wheel lockup, longer stopping distances, longer pedal travel, not very good pedal modulation.

He asked me to make a rear kit for him and I am at a loss how to help him and keep the car balanced. The solutions are:

A) Also install BIG rear calipers, bigger master cylinder and a different proportioning valve. Or,

B) Replace his big front calipers with normal-sized front calipers and use a normal rear brake kit, or even keep the stock rear brakes.

Surely, one little problem could be "easily" overcome if the rest of the brake system is changed around it.

Also, I dont see how drilled rotors and slotted rotors are so different, although I`m prepared to believe them that slotted is better (nothing beats experience). [/B]

No modern race cars that run cast iron rotors run then drilled. Many run slotted to keep the pads from glazing over, but none are drilled. A through-hole is too much of a stress concentrator. Slots, on the other hand, are relatively shallow. In fact, they get almost worn out flat when the rotors are near the end of their life, so their effect is minimal, if any. The best rotors are plain smooth, if the pads that are used with them do not have a tendency to glaze over.

Philip

[coster]

Originally posted by MarkWill
[ Bedding in the brakes seems to be for two reasons - 1. to get the brakes to work optimally and 2. to increase the "feel" of the brakes. Which brings me back to the original question - which bit is doing all the work? [/B]

Bedding in the brakes is only done once. When the pad is new it has certain substances in the brake material which are required, or a by-product in the manufacturing process. Basically, you have to get rid of these impurities for the pads to work effectively. Gradual heating and cooling of the pad compound removes and burns these impurities out. If not bed in correctly and uniformly across the entire are of the pad. i.e. (too much heat or heavy braking and disc lock). These impurities become trapped inside the compound, rendering the pads next to useless.
And i just read this on the pad box!!
So for slow braking, without locking the wheels, the entire pad is basically equal in applied pressure and heat generation. Though this would logically change in the event of wheel lock and heavier brake applications.