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Brakes.


davey1991
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This is mainly for cars but obviouslt the principle is roughly the same for everything.

I understand that larger discs have more leaverage and they cool quicker than smaller ones.

But surely if you can put enough power on the pedal to lock the wheels this doesn't matter?

I understand that softer compound tyres will lock later because there softer OFC.

But i still don't quite understand how a bigger rotor provides anymore stopping power if you can already lock your wheels reasonably easily?

Bring on the ridiculously simple replies guys, i await my being proven as an idiot.

Cheers.

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I think its all to do with modulation. Having bigger brakes means you can control the braking better, so you can apply the brakes much closer to the point of lock up, and keep it there easier.

As soon as the wheels lock up your not braking efficently, so its good to keep it right on the edge of lock up.

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laws of leverage, it means that to provide the same drag force as a smaller rotor will require less force (assuming same braking system, rotor design and pads). It's pure and simple physics. If you already have the power to lock the wheels easily a bigger rotor isn't necessary. Unless like punk said, you're in search of more control/modulation, which is easier to achieve with bigger rotors.

Also big discs cool faster and this reduces brake fade and other unwanted effects of heavy braking.

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laws of leverage, it means that to provide the same drag force as a smaller rotor will require less force (assuming same braking system, rotor design and pads)

also big discs have a larger moment of inertia which require a greater stopping force in itself ;)

In general bigger discs have better braking power as the increase in moment of inertia is much lower than that of the mechanical advantage gained. Smaller discs are usually fitted as standard to cars as they're cheaper, lighter and relatively simple

**Question answered with effort below **

The term to answer your question though is Braking Torque

torque-force.png

Braking Torque = Rotating Force x distance of pads away from centre

More technically:

The radius of the pads away from the centre is pretty self explanatory. You can have a massive disc brake, but if the pads are close to the centre, you dont gain anything (and waste alot of money) Obviously disc brake manufacturers dont do this, so dont worry

Force is slightly more complicated. As you can see from the diagram, the force acts perpendicular to the direction of rotation, but as you know the pistons act on the surface (or normal to the surface) with the equation

Normal Force = Applied Pressure x Area of pads

If you resolve forces on the disc, introducing the coefficient of friction μ, the frictional force (assuming perfectly normal to the surface, not toed in/out) is

Rotating/stopping Force = Normal force x μ

So the total torque that your brakes use IS:

Braking torque = Applied pressure x Area of pads x μ x distance of pads away from centre

So as you can see, just the pressure you apply on the brake pedal is not the only thing that changes the performance of the brakes. Pad material changes the coeffecient of friction μ, disc size as discussed alters the distance, and the pad size increases frictional area

Hope this helps :P

Edited by PaRtZ
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The problem here is that trials riders are quite blinkered into seeing brakes as just being things that lock wheels through their leverage and power.

However in the majority of vehicles the way a brake functions usefully is to convert kinetic energy into heat.

A larger disc has more capacity for this heat and a larger surface area over which to dissipate it so is therefore more efficient at converting a moving vehicles kinetic energy.

The benefits of larger discs begin to run out as you approach the limits of the tires on which they're acting.

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also big discs have a larger moment of inertia which require a greater stopping force in itself ;)

In general bigger discs have better braking power as the increase in moment of inertia is much lower than that of the mechanical advantage gained. Smaller discs are usually fitted as standard to cars as they're cheaper, lighter and relatively simple

**Question answered with effort below **

The term to answer your question though is Braking Torque

torque-force.png

Braking Torque = Rotating Force x distance of pads away from centre

More technically:

The radius of the pads away from the centre is pretty self explanatory. You can have a massive disc brake, but if the pads are close to the centre, you dont gain anything (and waste alot of money) Obviously disc brake manufacturers dont do this, so dont worry

Force is slightly more complicated. As you can see from the diagram, the force acts perpendicular to the direction of rotation, but as you know the pistons act on the surface (or normal to the surface) with the equation

Normal Force = Applied Pressure x Area of pads

If you resolve forces on the disc, introducing the coefficient of friction μ, the frictional force (assuming perfectly normal to the surface, not toed in/out) is

Rotating/stopping Force = Normal force x μ

So the total torque that your brakes use IS:

Braking torque = Applied pressure x Area of pads x μ x distance of pads away from centre

So as you can see, just the pressure you apply on the brake pedal is not the only thing that changes the performance of the brakes. Pad material changes the coeffecient of friction μ, disc size as discussed alters the distance, and the pad size increases frictional area

Hope this helps :P

Yes acctually it does, so the rotor having more momentom (SP) on the outside of them means that infact you can apply more braking and lock up later?

Intriguing, thanks guys.

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Yes acctually it does, so the rotor having more momentom (SP) on the outside of them means that infact you can apply more braking and lock up later?

Intriguing, thanks guys.

The limiting factor is the tyres. If you are already locking wheels easily with smaller rotors then bigger ones will not see much of an increase in performance. The additional inertia of larger discs won't make a noticeable difference to the point of locking up, plus you've still got to slow that extra inertia down too!

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You'll notice the difference with bigger brakes when you're coming up to a 30mph corner at 160+ mph. Normal brakes overheat and fade in seconds at those speeds.

Let me put it this way, if my Micra was doing 160 i would be f**ked anyway. :P

I'll worry about my suspension first and then my brakes then guys thanks.

It appears i lost the ability to spell and use grammar correctly in the previous post. :S

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to go back to your first post, you mention about locking the wheels up easily.....

at perhaps up to 40-50mph, I'd imagine any car would be able to lock up the front wheels if you REALLY wanted to.....

Big brakes only really come into their own when you're looking at repeated HARD stops over and over again, and form high speeds.

The standard brakes in my A3 will stop me very quickly, from anything up to about 70-80mph, ONCE, and ONCE only.

More than 1 hard stop from above 70 and they'll fade, and I'm left with no brakes.

Similarly, one hard stop from 130, and as you're comming down the speed, as you go below 40 you basically loose all brakes as they just overheat.

Also (and this isn't meant to be rude AT ALL) have you ever driven a REALLY fast car?

It's amazing how you can have brakes that feel incredible at 30-80mph, yet you apply them at 140 and feck all happens. I'd imagine your micra will be able to stop itself fairly well form any speed it can manange, although I bet it'd still fade after 2-3 hard stops from 80

My old A3 312mm brakes with upgraded discs and pads were like that, below 80 it'd stop VERY well, but above 100-120 no matter how hard you pressed the pedal it wouldn't kill speed all that fast. Better than most, but nowhere near quick enough.

As a result, out at the ring, I was finding myself braking from 130mph VERY early into the tighter corners, because I just didn't know if I would be able to stop in time. the whole lap was based around keeping the brakes alive for a full lap, so many people go out and cook their brakes in the first 5k, then have the cruise the last 15k brakeless!

Bigger brakes only REALLY come into play when you're looking at repeated hard stops, and serious speeds.

Should have some 330mm Porsche 911 C4 front brembo's on mine soon :)

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I'm pretty sure I saw a braking test from 100mph between some supercars and a Ford Focus - the difference was marginal with all cars stopping in just over 4 seconds.

You can search youtube for further info, also recall watching a vid where they took an Evo as an example and demonstrated how the stock brakes stacked up against uprated ones.

Edited by Greetings
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Now now boys i see some sneeky little bitching about the thought of a moddified micra.

I'm no chav with all the big wheels huge exhaust malarkey.

Nick, I haven't driven anything stupendously fast, but then again i have only been driving a year.

I've driven a (New.) Mini cooper on a track day and i noticed even on that, the brakes would loose it after the straight, i do understand brake fade and what not.

The plan was when i hit my 1 year NCB do a little to the micra to improve the handling without making it look like halfords fell on it.

I kinda like the idea of a sleeper but then again i look like a penis in it anyway so going past people with my hair on fire 8ft in the air in a bright red girls car wont help the image.

I may take this to the car thread shortly as i have some more questions about manifolds and intake stuff.

If i was to upgrade my front and possibly rears what options do i have avaliable to me?

This may sound like a really stupid question to those in the know, but i'm not in the know so give me some leeway, would i be able to use the old front callipers on the rear if i bought new front? Obviously i'd have to get disk hubs and what not.

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Better pads and disks would probably be a better mod over an increase in size. Ferodo DS2500 or DS3000 pads on a good quality clean disk will stop you in plenty of time from any speed on just about any size disk.

A friend of mine had a set of DS3000s on a fiesta on a 254 disk and it'd lock up from 120 with little effort. They rattled and squeeked like f**k and wore out in a month but you could abuse them all day long and they just kept working.

After inspecting today, it needs new disks, they're umm what you'd call thin, although quite a bit of pad left.

I'll look into those, i just want something that i know will work everytime rather than the worrying feeling that if i stamp on it its gonna keep going.

Without removing the servo is there anyway to make the brakes feel more firm, they have a fresh bleed which seems pretty good I just want to feel like im doing something not me pushing a button that makes something do something.

I know its a lot of effort for a little car but each to their own.

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  • 1 year later...

I have a silly yet interesting question regarding disc brakes, specifically rear disc brakes which are very rare on 26-inch trials bikes. I just stumbled upon

where a man named Toni Bou pulls some massive moves on his trials motorbike. I didn't think about it at first, but then I realised that the rear brake was a disc brake and the wheel was about 26 inches in diameter, and the rear brake on that motorbike seemed to lock up pretty solidly. So if this is possible on a motorbike, which is a lot heavier than a trials bicycle, why are rear disc set-ups so rare on 26" trials bicycles? Is this to do with the fact that the motorbike wheels are more rigid?
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I have a silly yet interesting question regarding disc brakes, specifically rear disc brakes which are very rare on 26-inch trials bikes. I just stumbled upon

where a man named Toni Bou pulls some massive moves on his trials motorbike. I didn't think about it at first, but then I realised that the rear brake was a disc brake and the wheel was about 26 inches in diameter, and the rear brake on that motorbike seemed to lock up pretty solidly. So if this is possible on a motorbike, which is a lot heavier than a trials bicycle, why are rear disc set-ups so rare on 26" trials bicycles? Is this to do with the fact that the motorbike wheels are more rigid?

I'll ignore the age-old thread and wrong location and answer anyway.

1. Moto trials brakes use a smaller diameter, thicker disk with a larger piston meaning braking power is stronger. The larger piston has to be closer to the rotor for it to work with the same amount of lever pull.

2. Spoke flex on 26" bikes is an issue, makes the back end feel less precise and more 'loose'

3. It puts a load of weight out the back of the bike which isn't ideal.

4. Larger wheel = more turning force and the less able the rotor is able to deal with it. so fine for 20", but the extra leverage of 26" wheels makes it a less effective brake

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I'll ignore the age-old thread and wrong location and answer anyway.

1. Moto trials brakes use a smaller diameter, thicker disk with a larger piston meaning braking power is stronger. The larger piston has to be closer to the rotor for it to work with the same amount of lever pull.

2. Spoke flex on 26" bikes is an issue, makes the back end feel less precise and more 'loose'

3. It puts a load of weight out the back of the bike which isn't ideal.

4. Larger wheel = more turning force and the less able the rotor is able to deal with it. so fine for 20", but the extra leverage of 26" wheels makes it a less effective brake

Thanks for replying.

I'll ignore the age-old thread and wrong location

First, sorry but how is this the wrong location? This thread is about brakes and so was my question.

Second, brake physics hasn't changed for the past... 40 years? So my post and the thread stay relevant. I assure you there isn't a 'thread resurrection' conspiracy. If you believe this thread should not be active and if it bothers you so much when people reply to old threads, how come you are still subscribed to it? Heck, why do you even bother replying?? Shouldn't you be contacting an admin to request this thread be deleted, as it's so old and therefore useless? Your reply actually contained useful information, and this information will still be valid for a long time, so other people might find it useful.

I guess it has become politically correct to criminalize people who reply to old threads.

Edited by Scoox
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Second, brake physics hasn't changed for the past... 40 years?

I think you mean, Physics has never changed. That's irrelevant though. Wrong place, old thread. No one would have minded you posting a new thread in trials chat, and I'm pretty sure with a slightly better search you wouldn't have even needed to post.

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i used to own a honda crx. which the front brakes were pretty much permanantly blue in colour and on track days they would smoke from the heat generated ( and when i say smoke like smoke pouring over the bonnet like a small burnout, basicly the brake pads were on fire)

i would not be fittng larger brakes unless this happens to your car on track days or you see blue ish colours on the originl rotors. other wise the pads wont reach there optimum temperature for good working operation.

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