What is the affect of a Disc Brake rotor size?

if you increase the diameter of the rotor, the surface area will also increase.....

you should ask the question a little better... maybe, "How does rotor size affect stopping power?"
Using a larger diameter rotor will increase your stopping power, but you should check with your forks manufacturer to see if the warranty will still be valid for using a larger rotor.

Not surface area. The greater stopping power from larger diametered discs is because the friction of the pads are applied farther away from the axle. Trying to stop a wheel near the hub. It takes much more force or friction then from farther out.. Rim brakes in theory are best (their braking power is applied as far from the axle as possible)yet are inferior because they work off of the pulling power of your finger w/o a mechanical advantage whereas disc brakes actuate from the power of a screw or hydraulic pressure that is multiplied, to some extent, by the power of your finger.

Did I make that confusing enough? hehe

we are talking lever arms and torques here. T= F * L or F= T/L.... as you can see from the second eq. as the Lever arm (rotor diameter) increases less Force is needed to "stop" the same amount of Torque produced at the wheel.

however it can get kind of complicated since there is another sort of artificial "lever arm" created from the head area on the bike to the tire contact patch on the ground. this is a totally dynamic lever arm obviously. as you ride it comes in contact with the ground to varying degrees to not at all.

so just think about the first example.

then of course you would have more surface area for heat dissipation. however, the torque scenario is the main reason

HOPE mono4 that made sense

i was only kidding about the surface area thing (even though it is true)...

The original poster worded his question poorly...

sorry to make you guys confuse. I was in a hurry when i wrote that. Anyway this means that the bigger the diameter the better?

Depends on what you use the bike for.
Riding down the side of a big mountain you will want the extra surface area and stopping power.
With an xc bike on flatter terrain the extra surface area is just added weight and the extra power wasted.

oh i see. Technically im a all type of rider. So which one you suggest?. Cause i dont really go for competition, i usually cycle around (terrain and non-terrain)

We use 8" rotors on the tandem. That's 203mm. Magura will warrant their brakes for tandem use with slightly smaller rotors. Moral of the story? Even those little 160mm rotors will stop a single with ease.

Get a set of Avid BB7 mechanical disc brakes with standard 160mm rotors. They are easy to use, easy to adjust, and have good stopping power. Unless you ride a 29er, then I would increase the front brake rotor to 185mm to keep the lever pressure about the same.

Well, the first time I ever used hydros was my friends Hayes Nines with 6 inch rotors. They had so little power I never wanted to use them again. A month ago he slapped some 8 inch rotors on his bike, and now they make me want to pay extra for them. I know Hayes Nines are not the most powerful brakes there is but just going from 6 inch to 8 inch really made them stick out.

actually, sometimes it is... especially in cars, when there is a greater surface area, it also allows more maximum cooling... thus increasing/decreasing brake power if your brakes are just right/too cold

I'm not sure what the terrain is like in Malaysia but if you ride huge hills, a 8" rotor is a really nice to have. I agree, as mentioned by other folks, that the advantages are more stopping power, less heat buildup and ultimatly longer brake pad life.

Below is a pic from the Jumping Pond Ridge/Cox Hill in the Kananaskis area of the Canadian Rockies. I can't image the desent with anything but 8" rotors. One of my riding buddies cooked his pads on his 6" Hayes mechanicals this day.

Cheers

T.J.

On the way up....


Shamless bike porn


The beginning of the descent



EDIT: If you are seriously considering 8" rotors, be sure to check with your fork manufacturer to be sure it can handle the added forces. Some forks are not rated for 8" rotors (I.e. All Fox <04, most X/C forks...)

If the Hayes Nines had no power, there was something wrong somewhere. You would have noticed if they were way out of adjustment, or had air in the lines. My guess is that the pads were contaminated, or not bedded properly.

Ok, I need to hit that trail bad.

Ns view, Malaysia track is something like that and sometime even worst but not very rocky. Its just that our track is very uneven and the gravel is unknown. And is there any other way to know wheter your fork support 8 inch? Cause i cant find any information on my fork

Boom, dead on. That looks so fun, slightly technical but you could pick up sooo much speed! Oh, and what's the fork on the Enduro posted here?

Usually the manufacturer will have this on thier site. What fork do you have?


This picture is from the top of Cox Hill facing north just a bit above the tree line. This trail quickly turns into a tight steep single track though the forest all the way to the botom that is full of loose baby heads, roots and little 1 – 2 foot drops. I didn’t take any pictures on the way down as I was having too much fun and forgot. I had a “close encounter” with a tree this day.

The fork on the Enduro is a 2004 Z1 FR SL. It is awesomely stiff and plush. It is much " burlier" than the orginal Fox Talas and the axel to crown height is slightly higher so it slackens the head angle just right for the type of riding I do. Also, the 04 Fox forks are not rated for a 8" Rotor. The 05 Fox's are....

T.J.

Here are a couple more shots from that trip if your interested.

On the way up Jumping Pond Ridge


Looking south on top of Cox Hill


Tequila Joe “Posing”

Nice pics. How much travel does the Z1 SL have? 130-150?

130mm

Gotta love the rockies. ...nice pics

How heavy are you? I am 240lbs and even down slight downhills the extra stopping power of an 8" rotor will come in very handy. On the big and steeps......8" or nothing! I am getting BB7s with 8" rotors on my soon to be new bike.

An 8" rotor with a 9mm QR is asking for trouble. Most manufacturers specify 20mm thru-axles if you want to run an 8" rotor.

However it is doable. My gf runs an 8in rotor on an old jrT with regular dropouts

I'm 6' & 168 in prime season and about 173 in the winter. At 240, I would recommend a 8" rotor.

T.J.