Braking power: Rotor size, front vs. back

[Odinson]

Brakes.JPG

 

Pedal?!

 

It's a lever, boet!

[Odinson]

Let Sheldon Brown enlighten you: http://www.sheldonbrown.com/brakturn.html

[Eldron]

I have bikes with 203/180 180/160 and 160/160 and I have to say - I don't really feel a huge difference between the 3... I guess if you're doing massive stops from 60 down to 20 repeatedly it makes sense to take heat dissipation into account but for 95% of riders I'd say 180/160 is way more than you'll ever need.

 

As for when to use them. If the wheel is straight then both - as you start to turn the front wheel the bias should go quickly from front to rear - the more you turn the more the bias.

 

Don't drag your brakes for long periods - rather ride the stuff and brake when you need to.

 

 

Edit: For the original question.... As you brake your weight shifts forward - this reduces grip on the back and increases it in the front - hence the bigger rotor on the front. Even a 140mm rotor will lock the rear chippity chop under hard braking.

[rouxtjie]

203??

 

Jussie pal, eat less, poo more 

lol....come ride haka with us PAL.....wanna see skillz at work.

[Capricorn]

Quite simply, the back wheel requires less force to lock, as it's "dragging" through the dirt, and your weight moves from the middle of the bike (during riding) to the front under braking. The front wheel has all the weight pushing it into the dirt, and is where the majority of the braking traction resides. 

 

just for clarity, the reason the rear wheel locks up is that as Myles alluded to, braking of either front or rear wheel results in a rotation of the bike around the braking point. this lifts the rear wheel, while not entirely off the ground, it does reduce the normal force between the wheel and the ground. the normal force is a crucial aspect of effective braking.

Momentum also dictates that the rider mass is compelled forward, further unweighting the rear and sending it to the front wheel.

So the combined effect of rotation and mass movement conspire to reduce the normal force between the wheel and the ground, causing reduced friction between the rubber and road (which causes rolling), to the point that the braking force exceeds the rolling friction, causing the rear wheel to lock up.

 

The front wheel however, is a whole nother ballgame which Myles covered.

 

you can nicely demonstrate the rotation effect to yourself by hanging your bike  off the ground at the saddle (bike stand is ideal, but even works on a bike rack), spinning either wheel up, and then braking hard.

[dave303e]

Also please if you are right handed put your front brake on right hand lever,this whole bicycles doing it backwards is not ayoba.

Most people are right handed, most of your braking done with front brake so your best fine motor coordination should be on your front brake.

It's not about how hard you can pull the brake, its how hard you can pull it without locking the wheel or going over the bars...

[V12man]

203??

 

Jussie pal, eat less, poo more 

I also have 203/180 on one of my bikes.... 63kg.....

 

But slowcoaches like you don't need brakes....

[rouxtjie]

I also have 203/180 on one of my bikes.... 63kg.....

 

But slowcoaches like you don't need brakes....

boom....

[V12man]

Also please if you are right handed put your front brake on right hand lever,this whole bicycles doing it backwards is not ayoba.

Most people are right handed, most of your braking done with front brake so your best fine motor coordination should be on your front brake.

It's not about how hard you can pull the brake, its how hard you can pull it without locking the wheel or going over the bars...

We have had this debate before.... your front brake should be on the side you are less likely to panic snatch.... so for most people this is their non-dominant hand (left)

 

But there is no right or wrong way - put it whereever you feel most comfortable - just please don't tell me which side I should run it on - that's not your decision.

[dirtrider]

With Eldron on this argument . Also got a mixture of rotors and have tried different combinations . They all stop fast enough .

[Odinson]

With Eldron on this argument . Also got a mixture of rotors and have tried different combinations . They all stop fast enough .

 

It depends on your riding style.

 

If you're gravel grinding, even shoddy rim brakes will stop you, but when you're doing long, sustained gravity-assisted riding, larger rotors will remain cooler and provide consistent braking force.

[V12man]

With Eldron on this argument . Also got a mixture of rotors and have tried different combinations . They all stop fast enough .

This is true - they stop fast enough the first time - BUT - do they stop well enough after a 1500m of fast decent with lots of hard braking corners....  THAT is the question you need to ask yourself.

 

Bigger brakes work better for longer - but are unneccesary if you don't need the added capacity of the bigger brakes - so despite haveing monsters on one of my bikes, my XC bike runs 160/160 - and that has been enough for me on everything I am willing to take that bike on so far.

[Skott5]

I thought a rotor was a rotor was a rotor ..

 

Then I put XT rotors on one of my bikes on recommendation .. Gees what a difference.

Previously where I was running a 180 rotor, It was replaced with a 160 and was 10 times better !

 

It's not all about the size, but about the quality ...

 

Ja ja, and before I get chirped about the size thing ... 

I dont have the biggest boat in the harbour, but I row the fastest .. 

[RockCoach]

In hard braking scenarios: to counteract the body pivoting over the braking points on the wheels (thus going over the bars) move your body low and back as far as you can. Push your feet through the pedals (with your chest just about on the saddle). You are effectively pushing your body weight through the tyre contact patch with the ground with much improved braking stability and you'll soon realise why your front rotor is larger.

[Halfdoesyn]

To add: bigger rotors not only dissipate heat better, they have more leverage because the contact areas are further away from the rotational centre. Meaning you can stop with less force using a 180mm rotor than a 160mm rotor.

[rouxtjie]

It depends on your riding style.

 

If you're gravel grinding, even shoddy rim brakes will stop you, but when you're doing long, sustained gravity-assisted riding, larger rotors will remain cooler and provide consistent braking force.

totally agree....feel on 180 / 160 combo vs 203 / 180 on a long gravity descent is different.