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I have noticed that my rear wheel does not rotate freely at all. If I had to guess, I bet it probably is sucking .5 hp or maybe more from what is already not too crazy a motor to begin with, especially at 10-13,000 ft. altitude on extremely steep slopes. You probably already know this or have solved this yourself, but I finally got around to addressing it. When my KDX is on its work stand and I rotate the rear wheel to clean it or lubricate the chain, it would rotate very minimally, enough to indicate it's either the X-ring 520 chain, possibly wheel bearings, a bent brake disc, or the brake pads dragging. I'm comparing this to previous MX bikes which had non-O-ring chains, which were excellently maintained and lubed, and would spin substantially. I removed the X-ring chain, which is fairly new, meticulously clean and well-lubricated. The rear wheel rotated just a little bit more freely, but not much. I checked the lateral runout of the disc and it is within the factory spec and not bent. I then removed the caliper and the rear wheel free-wheeled on and on and on, like a bicycle wheel, confirming the wheel bearings were not a problem and the brake was. Per the factory service manual, I disassembled the caliper. The pins which hold the brake pads were pretty "corroded", enough to where it was no longer a smooth surface which the pads could easily glide back and forth on. The main caliper pins which connect the two parts of the caliper were in a similar condition, to where they could not glide in and out. They were also dry, with no silicone grease remaining on them. I sanded the pad pins and caliper pins to a very smooth finish. I also cleaned up the holes in the calipers where the caliper pins go, using some fine sandpaper wrapped around a pencil, rotating the caliper around it many times. Regarding the piston, I removed it and used a dishwashing Scrubbie to clean off some corrosion on it and get it mirror smooth. I then took CRC electrical parts cleaner, which is safe on rubber and other parts, and hosed all parts of the caliper out. I lubed the caliper pins with silicone grease and installed new EBC pads. I then went into the master cylinder, which I recently rebuilt, and following the factory service manual, ensured that the return hole was clear of any blockage. If it is not, the manual says it will cause the pads to drag. I reassembled the master cylinder and mounted the caliper on the swingarm, then filled and bled the system. The caliper and pads returned freely, so that the wheel rotates very well with almost no drag. When I installed the X-ring chain, the wheel rotated a lot less freely, but I think that is to be expected given that it is X-ring and lubricated properly. In any case, I've eliminated one source of substantial drag, and generated a little "free" hp. Hope this helps.

Nice write-up

thats the nature of sealed chains, even brand new they won't spin nearly as free as a standard chain like those used on motocrossers, and they're heavier. but as you might already know they've got a much higher tensile strength and last much longer, thats the tradeoff. also when you're spinning the wheel with the chain still on you're moving transmission parts inside the motor as well which also have some resistance. Good write up on rebuilding the caliper, I take the same procedures when rebuilding mine

Sealed chains are heavier, and due to the O-rings have higher rolling resistance as the side plates don't move as freely. They do not have a higher tensile strength and in fact non O-ring chains are just as strong as O-rings (they are made of the same materials). The primary difference is that the sealed chains require much less maintenance to continue working well, while non sealed require regular cleaning and lubrication. Properly maintained a non sealed chain will last just as long, problem is most people don't properly maintain their chains. Excellent write up on troubleshooting the dragging problem.

yeah you're right, the only non-sealed chains I was thinking of were the cheaper ones that stretch after a couple hard rides, my bad RK and pro taper make some really good standard drive chains

great write up! couple other things to consider: rear master cylinder adjustment. if it's too tight it will always apply a bit of pressure. also, there needs to be a return spring or the weight of the brake lever itself can slowly engage the rear brake, and/or it wont return properly. make sure the caliper pins are not bent as well, and that the rubber boots are in great shape as to not let water in the caliper pin hole.