Some people have to use a 6 inch rotor in the back.

Well an lbs mechan. said taht the larger the rotors, the harder it is to brake, but it will have a smaller chance of boiling the liquid. He also went off a tangeant of how xtr deraillers are real smooooth.

Well that lbs mech must not be a downhiller, downhillers don't rant and rave about the smoothness of a deraillieur.

Hahaha...good point kona. My deraileur barely does its job ...and it is XT

All any braking system does is convert the mechanical energy of the bike's motion into heat. In a disc system, the more massive the rotors (e.g. larger diameter), the more surface area there is to allow the heat created by the friction of braking to dissipate into the air.

obviously fahrenheit right?, in celcius that would be as hot as a Car,

I have 8" f & r, and they get damn hot sometimes, just ask my fingers....

What makes you touch something that you know is hot?

Stupidity?

What makes little kids stick paper clips in power outlets? What makes them lick a 9-volt battery?

Yeah its farenheit. I forgot about our friends from diff countries. Sorry.

I use a 6" in the rear. And I DH. On Occasion. Yes an 8" would be ideal for DH. But it won't fit on my frame. And honestly (quoted from a WC DH racer)

"If you really want to win, you shouldn't even use your brakes."

So I ask why does the rotor size matter. My 6" rotor stops me perfectly fine. Plus most of the braking in DH should be front brake.

Imagine if the rotor was only 2 inches in diameter. It would get very hot then, for the reasons I described above. Your six inch rotor may work for you, and that's great, but heavier, faster riders with more accomodating frames will see better performance from a larger rotor.

Thats very true. But If someone downhills with a 2 inch rotor. I don't even want to look at them. 4" rotors are SMALL for XC. To see a 2" rotor, thats rediculous. (I know you said Imagine, but eh) Yes heavier, fastier riders prefer to use bigger rotors. But a 2" rotor should never be put to the extreme where it would be glowing.

F1 cars typically stop over a thousand pounds of metal from hundreds of mph. Remember ke=1/2mv^2? Mass doubles, ke doubles. Velocity doubles, ke goes up by 4. Their brakes heat up to over 1000F and are glowing. I'm not sure you can have a guy on a bike putting that much energy in their brake discs to make em glow. And if you poured water on em, I'd be surprised if the disc ever worked again.

8 inch rotors, in my opinion, have better modulation.
This is probably because the rotor moves faster when it is further from the centre of the wheel, therefore you can change the speed of this more gradually than when the grip spot is closer to the centre of the wheel.

Glowing Rotors? I wont believe that until i see it. So post a pic!

8" is better than 6" obviously because otherwise the pros would be using 6" rotors.

I am gonna go with that I wanna see it

Good question - I've also wondered about the disc temp during a descent. One way to check is with a remote thermometer that reads the infrared signature without physically contacting the rotor. Radio Shack sells one for $50, but it only reads up to 400F.

I think you can get temperature sensitive stickers for this task. Maybe a slice of one on the rotor edge would suffice? An infrared pyrometer is ideal though... Summit Racing sells a 500 F model for about $100 and a 700 F model for about $170.

30 years ago I used to be involved with Karts (Go-Karts to the unitiated) and was involved with the manufacture of Frames and Brake units for them. These are fast machines, and the 100cc class only has one brake on the rear axle. I did not believe how hot the disc's got until a meeting overran its time, and we raced just about on dusk. Depends on the material, but the stainless steel disks were not showing any glow, but those made out of Gauge Plate, definitely glowed Orange. Why Gauge plate by the way, which is a softer material? Because it worked better, even though it wore fairly quickly and got covered in rust between meetings. If any of you do think your Discs are Glowing, Do a fast hard descent in the Dark and then look at your discs.

Incidentally, My bike runs Hope Mono M4's on 200mm discs, is driven aggressively and I have never had the fluid boil, never had brake fade, never had any problems of any sort, other than at times they are a bit too effective. This is on a heavy weight machine, but for Lightweight XC machines, where weight is a priority, 6" up front and a smaller disc on the rear is quite sufficient, unless you are prone to finding the big long hills wherre speed and hence braking efficiency is required