Rotor Sizing

[SteelyDan]

So I'm no engineer!

 

I have an FS with a 140mm rear rotor and a 160mm front rotor.

How come?

Is it about mechanical advantage?

 

[Capricorn]

 

 

yep. if you have a force (accelerative or retardent), and if you the increase the distance of point of application of that force from a reference point of rotation, the torque generated is increased.

 

Torque = force x radius.

More

whether you use the rear or the front brake, your weight distribution transitions to the front, thus reducing rear wheel traction which in turn reduces the effectiveness of the rear brake. Thus there is an opportunity for a weight reduction by making the rotor smaller.

When the rider's weight transitions forward, it provides the front with greater traction and thus becomes of the more effective means of braking.

Many beginner riders entrust almost all their braking to the rear wheel due to fear of being thrown over the handlebars. This might be true under certain situations, but it is completely avoidable, by shifting your body weight to the rear as required. ?Front wheel braking is just more effective. The physics dont lie?

Add therefore the bigger rotor.
Another benefit of a larger rotor is better heat dissapation under braking. This is fairly important especially for those who do a lot of steep, high speed riding such as Downhillers and Free riders. Heated brakes, depending on the design fo the brakes can result in what is refered to as 'brake fade', where braking becomes less effective.

Hope that helps

 

Capricorn2009-10-24 12:59:19

[SteelyDan]

Yup, that makes sense! Thanks!

 

[Johan Bornman]

yep. if.

Many beginner riders entrust almost all their braking to the rear wheel due to fear of being thrown over the handlebars. This might be true under certain situations' date=' but it is completely avoidable, by shifting your body weight to the rear as required.  Front wheel braking is just more effective. The physics dont lie 

[/quote']

 

Very nice answer. I'd like to add something to your observation about beginners.

 

Many beginners had bad experiences as children with front brakes. They applied the brake too forcefully and their overturning momentum  was less that the brake's decelleration force and they went over the handlebars. You can go over the handlebars in two "modes".

 

1) When you brake hard and you're arms are not braced. You slide off the saddle with your torso going forward. Your knees hit the handlebars and this causes the bike to overturn and results in a faceplant. The evidence is facial blood and two days later, two blue marks where your legs hit the handlebars just above the knee.

 

2) When you brake hard and you are braced. In this position a skilled rider can lift the rear wheel and balance on the front wheel, stoppie-style. The unskilled rider will hold onto the front brake and completely overturn even though he/she is braced. This results in a faceplant without the blue lines on the knees.

 

On good surface, you cannot skid a bicycle or motorbike's front wheel. The bike will overturn rather than skid. On a car with a low centre of gravity, the front wheels will skid.