Originally Posted by mtnbiker66
Ok.......So why do freeride and Downhill rigs come with 8" rotors front and rear? I've been running 8" rotors on the front with QR forks for a long time with no issues.
There is an engineering department, a manufacturing manager, a marketing department and a finance department. The engineer doesn't always get their way. Specing different size parts on the front and back is likely more of a hassle and increases the cost of the bike (I doubt that bike companies pay significantly more for larger rotors and spacers like we do). For an example of my point, look at inexpensive cars that typically come with disc brakes in the front and drum brakes in the rear. You really don't sacrifice anything since under heavy braking only the front wheels have enough traction to slow the car.
I have noticed recently that more and more bikes ship with assymetric rotor sizes. This decreases the overall weight of the bike and doesn't sacrifice any performance. On a 26er assymetric XC setups are 160/140 mm. On 29er XC bikes its a 185/160mm. Look for this to become the pattern as time goes bike.
For the person who suggested just using 203mm on a non-rated fork ... DON'T. You risk a catastrophic failure. And it will only fail at the WORST possible moment under emergency braking. If you're putting that much energy into the system, that fork probably would not survive anyway. At 280 pounds, I've bent two of them in crashes. OP, if this is your original KM, you may wish to consider going straight to a 20mm or 15mm maxel. I switched my KM to a suspension fork and ended up being stuck as I have three wheelsets, each of which is quick release. It would have been VERY costly to switch. At the very least consider a convertible 20mm front hub with an QR adapter.
Two parting thoughts:
1) Surly capabilities - the rear end of the Karate monkey has track ends with a rotating disc mount. You cannot get the wheel off using a round rotor. It is in your best interest to put a smaller rotor here. With a 160mm serrated rotor, you can get the wheel off without undoing the brake mount. I"ve heard limited success with larger rotor sizes.
2) I went riding this weekend in slightly muddy conditions. With a 185mm/160mm Avid SD7 setup for maximum leverage on the front, minimum leverage on the back ... I was STILL locking up my rear wheel too often for my comfort. The rear wheel IS just a helper under maximum braking conditions (downhill).
Last edited by BearSquirrel; 11-30-09 at 10:41 AM.