Braking!:- Lets sort this out.

[Minlak]

Every thread that has the merest mention of braking turns into a shit fit of what is correct and what is thought to be correct. It is wet everywhere and a lot of people with time on their hands so lets finally sort this braking thing out.
*Co Efficient of friction:- the force required to slide one surface against another.
*Newtons 3rd Law:- Every action has an equal and opposite reaction.

Braking:- Simply the act of stopping or trying to stop your momentum.
Efficent Braking:- Having your momentum slowed effectively with out losing control
a) Skidding is losing control.
b) Having your wheel on the verge of locking up with out locking up is the most effective.
How:- The tyre against the ground ultimately decides when we skid and when we slow.
Actual Braking:- You pull a lever in that forces a piston to compress fluid inside the lines and callipers forcing another piston to apply pressure to a pad against a rotor.
Factors:-
Lever length:- A longer or shorter lever will change the force required from your hands it will not change the force required to lock the wheel up against the ground.
a) A longer lever will have a fulcrum effect on the piston in the reservoir / lever thus needing less hand pressure to apply the same force
Pads:- The surface area and construction of the pads will change how quickly you reach the lock up point.
a) Pad material will change the co efficient of friction
Rotors:- The surface area of the rotor will effect how quickly you reach the point of lockup.
1) Rotors that are solid have the greatest co efficient of friction and there for require less pressure from the pad and lever to halt momentum of the wheel.
a) The size of the rotor will have no bearing on the pressure needed as the pad contact is exactly the same.
b) The construction of the rotor has a massive bearing as it changes the co efficient of friction.
*Rotors have holes to clear the brake dust and to vent gas build up between pad and rotor
*Rotor construction and size will effect cooling which in turn effects braking efficiency due to changing the co efficient of friction by adding heat (and the properties of the braking fluid used)

The force required to make a wheel skid is exactly the same no matter what combination of pistons / callipers / fluid / rotors / pads you use.
What will change is the efficiency at which this force can be applied to the rotor for what period of time before the wheel either locks up or fails to lock up. This is what makes one brake set feel different to another brake set.

Modulation:- The "feeling" in the lever that allows you to apply the rest of the braking forces with out locking the wheel up.

*The rest I will modify as definitive answers come in and hopefully we can have an accurate answer to braking.

 

[marc.r]

i think you dealt well with the braking friction at the tyre/ground and the calliper. the only thing you didnt address is the effect of change in rotor size.

i did not write this but am quoting it from a thread on physicsforums.com where a user deals with the issue mathematically which i find most free from bias/personal anecdote/gut feeling.

The frictional force between the discs depends on the coefficient of friction( mu) and the Normal Force between the discs. Assume the disc has a radius of R
f= μN
Hence the frictional force per unit area between the discs= μN/(pi x R x R)
If we now consider a small ring on one of the discs of radius r and width dr then the
torque dT acting on this ring =( force per unit area of disc) x area of ring = μN/(pi x R x R) x 2 pi r dr
If you now integrate from 0 to R then the total torque acting on the disc = 2/3 x μN x R
Hence the total Torque is proportional to the radius of the discs.
If you double the radius you double the torque.

Read more: http://www.physicsforums.com

the summary is that braking force is proportional to the radius of the discs.

hence braking power can be practically improved by
1. increasing the co-effecient of friction
2. increasing the radius of the rotor
3. increasing the force applied

BUT only up to the limit of the friction between your tyre and the ground. after that you just skid.

The other issue as raised above is modulation. i.e. the ability to linearly increase power to the limit of your tyre ground friction limit without getting tired hands! this is the reason for more and more powerful brakes.

in my opinion. you want to exert as little force on your brake lever to create a maximum of braking power while still being able to maintain modulated control. its harder for us to control our hand muscles when we are using more strength, especially when we are tired so a more powerfull braking system helps!

 

[Minlak]

i think you dealt well with the braking friction at the tyre/ground and the calliper. the only thing you didnt address is the effect of change in rotor size.

!

Just so other people realise as well what I have done so far is not the final evolution I want input like Marc's to build this up to something we all agree on as fact ... Thanks Marc I will edit the main post a few times a day

 

[driftking]

I think we need some solid calculations of two riders one heavy and one light with the same running gear. Most of the debates on here seem to revolve around momentum and the effect of weight on braking distances. We need something clear cut.

Also is there not the possibility that in some cases the real factor is the brake rotor coefficient not the tire and the ground. While they have an obvious relation, if the pad and the disc lock up before the tire reaches its limit than hasn't that force become the limiting factor not tire and ground. So the brake we use becomes massively important to the whole debate. It seems like this opens up a tangled web. At best we can assume ideal situations. But than again dh is anything but controlled and ideal so I wonder if it will actually end the debates.

 

[moorey]

And what of this 'breaking' I keep hearing about? :noidea:

 

[The Duckmeister]

And what of this 'breaking' I keep hearing about? :noidea:

That tends to stop your progress in a more abrupt and long-term manner. It also tends to be a once-off occurrence, and often at considerable expense.

Modulation: This is affected by master cylinder (brake lever):slave cylinder (caliper) ratio. A significanly larger slave cylinder results in less piston movement for a given fluid displacement, resulting in finer control of the brake's power application.

 

[wesdadude]

And what of this 'breaking' I keep hearing about? :noidea:

It's what Avids keep doing :behindsofa:

Although aimed more at cars than bikes drivingfast.net provides some good information on braking technique and threshold braking.

I think the most important part of braking technique is where to brake. DON'T BRAKE IN THE CORNER! Glad to have got that one off my chest.

 

[SummitFever]

Rotors have holes in high performance applications to vent gas build up between pad and rotor. Bike rotors have holes primarily for weight savings.

 

[SummitFever]

...DON'T BRAKE IN THE CORNER! Glad to have got that one off my chest.

Totally wrong. Look up "force circle".

Effective braking is as much about how you start braking as how you stop braking. Your tyres offer grip. Your job is to divide that available grip between the various competing forces of braking, cornering and acceleration. The fastest way round the corner is for all of those forces at any instance to be just less than available grip. On corner entry braking is high but needs to smoothly drop off as your cornering force increases. At the apex there should be no braking force and all cornering force.

Don't bake in the corner only applies to the very moment you cross through the apex.

 

[kwikee]

Rotors have holes in high performance applications to vent gas build up between pad and rotor. Bike rotors have holes primarily for weight savings.

And clearing dust/debris from the pad/rotor surface

 

[slippy]

And to improve cooling.

There are just too many variables in this discussion. E.g. the effects of brake lever length dont take into account the lever internals such as servo wave or taper bore designs. Locking up is not only effected by braking force and tyre tread but also tyre pressure and what surface you're on. The effect of rotor size will also be effected by rotor design and temperature. Any exhaustive analysis of all these factors would fill a book, for one person's bike.

Can't we just say don't buy Avid?

 

[moorey]

Can't we just say don't buy Avid?

Stick a fork in us, we're done.

 

[marc.r]

I think we need some solid calculations of two riders one heavy and one light with the same running gear. Most of the debates on here seem to revolve around momentum and the effect of weight on braking distances. We need something clear cut.

Also is there not the possibility that in some cases the real factor is the brake rotor coefficient not the tire and the ground. While they have an obvious relation, if the pad and the disc lock up before the tire reaches its limit than hasn't that force become the limiting factor not tire and ground. So the brake we use becomes massively important to the whole debate. It seems like this opens up a tangled web. At best we can assume ideal situations. But than again dh is anything but controlled and ideal so I wonder if it will actually end the debates.

how does the disc and pad lock up if the tire is still rolling?

 

[steve24]

Has anyone seen any tests done on pad/ rotor performance?

I have always thought it would be interesting to see differences on braking dyno between Clarke, insert your favourite manufacturer, Goodridge, Swiss stop etc pads.

Also between rotors- Formula, icetec, Ashima....

Avid bashing is about as predictable as financialwar's next thread being utter drivel....
and will get similar number of pages of shit in response.

 

[slippy]

Avid bashing is about as predictable as financialwar's next thread being utter drivel.

And the reason for that is....

 

[pharmaboy]

Totally wrong. Look up "force circle".

Effective braking is as much about how you start braking as how you stop braking. Your tyres offer grip. Your job is to divide that available grip between the various competing forces of braking, cornering and acceleration. The fastest way round the corner is for all of those forces at any instance to be just less than available grip. On corner entry braking is high but needs to smoothly drop off as your cornering force increases. At the apex there should be no braking force and all cornering force.

Don't bake in the corner only applies to the very moment you cross through the apex.

Yes, so if you have 0.6g of grip you can use it to brake or to maximise corner grip or a combination of the 2 ( in cycling I'm omitting exit acceleration) - so max brake while approaching corner on straight line and slowly releasing brake as you approach apex where brake should be completely off.

For co-efficient of friction Facts in the English language

http://abyss.uoregon.edu/~js/glossary/friction.html

So very little to no relationship of surface area and friction is proportional to force , eg gravity times mass.

Surface area in brake pads has some relevance because surfaces are not perfect , but ultimately the force at the lever has to meet the kinetic energy of the object to slow it down, whether that energy is concentrated on a 1 cm ^2 pad or a 100cm^2 car pad is moot, it's the same energy required if the rotor and pad materials are the same , except in the situation where heat is reducing the friction of the 2 surfaces.

In the real world we use fat tyres because that gives us say 100cm^2 of tyre contact area, within that contact patch we might have 70cm^2 that is dust or pebbles over hardpack and the remaining area is solid hardpack and the grip comes from that section - that's why fatter tyres provide better grip on poor surfaces.

For the maths proof of much of this and different surface co-efficients

http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

In the real world for brakes, heat dissipation is one major problem of design with repeated and long braking intervals

 

[bowtajzane]

how does the disc and pad lock up if the tire is still rolling?

This is were the" break" bit comes into it...........
Don't brake through the corner, break before the corner. ...
Ceramic rotors should be the next thing companies start developing, especially for the larger
GENTLEMEN and the just plain crazy among us

 

[wesdadude]

DON'T BRAKE IN THE CORNER!

I say this because I'm sick of braking bumps in stupid places, especially 2/3rds around the corner where everyone grabs a bit because they're scared they're about to fall off the edge. I don't mind a bit of trail braking, and in some corners it is indeed faster. When I make this argument I'm assuming a corner with a constant radius.

Totally wrong.

While I don't fully agree with you, I don't want to make a statement like that. You understand the concepts but I think you may be incorrect in application.

Effective braking is as much about how you start braking as how you stop braking. Your tyres offer grip. Your job is to divide that available grip between the various competing forces of braking, cornering and acceleration.

Wholeheartedly agree, especially with the first sentence. But cornering is a complex balancing of forces and I think you've oversimplified. The effects of acceleration in a forwards/backwards direction (pedaling and braking) are more complex than just working against the centripetal force of cornering. They affect fore/aft weight balance which in turn affects traction. They also make the bike stand up and resist leaning over (and therefore resist cornering).

Tyres can only effectively apply traction in one direction. Any braking force moves this direction away from the centre of the corner which is where it needs to be. Braking forces rob cornering grip.

The fastest way round the corner is for all of those forces at any instance to be just less than available grip.

The fastest way would be for all available grip to be contributing to centripetal force. If you're not converting all possible grip to centripetal force to make the corner, you're not going fast enough.

 

[driftking]

how does the disc and pad lock up if the tire is still rolling?

The tire isnt rolling the pad and disc locks up which locks the wheel up.

What im saying is lets say the tire and ground has a breaking point/skid point of X and the brake has its own of Y (as there is a point where the pad will lock the disc) The brake might cause lock up of the tire before the point of friction is broken with the tire and the ground. There has been the point in other threads that more weight equates to a high coefficient at the tire but the brake dictates lock up as well so that needs to be taken into account, more weight to the rider might increase the tire and ground coefficient but it doesn't increase the brakes coefficient of the disc.

 

[pharmaboy]

When I make this argument I'm assuming a corner with a constant radius.


.

As always, it's the assumption that should be examined. When approaching a corner on a motorbike, the corner is taken as a part parabola with peak G force at the apex of the corner. This is because you can't immediately enter a maximum sideways grip turn because you have to lean the bike over and this takes time. This can be seen with lean angle which without brakes is a function of grip and cornering g force. Al world class riders turn in with increasing turn in and increasing cornering g force which is pretty much the point where the knee scrapes the ground . I am informed by keen fans and riders that the ratio of top riders that trail brake is 100 to zero.

It's this transitional time to apex that braking is used for, in the same way that power is applied and squeezed on during exit - ie you have passed maximum cornering, and there is now grip available to accelerate ( easiest to comprehend on a 4wd car)

So, yes it would apply if you drove round a corner like it was a semi circle, but you don't