trek2

My Specialized Secteur has Avid BB5 disc brakes. Haven't had a flat yet ,but which way do I lay the wheel when fixing a rear flat? I'm used to laying it so the cog set is up, but with a disc on the other side, what do you folks do when changing a flat on the road? There's not alway a handy tree to prop things up.

1 Miyata Biker

You just need to turn the bike upside down on the seat/handlebars so the wheels are sticking straight up. Remove the chain and then use the bike as the "tool" to hold the wheel as you replace/repair the tube. That should keep everything vertical and off of the ground, out of harms way. You can usually find a soft grassy area to flip the bike over. Having a shade tree at that same location is a definite plus!

HillRider

Disc side up. You are less likely to damage the cogs than to bend the disc so protect it first.

Looigi

FWIW: Disc brakes can be pretty sensitive to contamination, a bit of oil or fingerprints can lead to squealing or chatter which can at times require some rather draconian measures to cure, so handle the wheel accordingly.

speedy25

I'll give hillrider the right answer award.

-SP

FBinNY 

Humans have two legs so they can hold things between them. (let's not go there)

Odds are that you can lay the wheel over on either side on a flat surface, and neither the disc or cassette will touch the ground. But dirt, grass or uneven surfaces can be complications, so the easy smart thing is to lean the wheel against something or hold it between your legs.

woodway

Flip the bike over onto the seat/handlebars. Pull the wheel. If you need to set the wheel down prop it up against the bike. Fix the tire. Drop it back onto the bike. Flip the bike over. Ride away.

davidad

Treat it like any other wheel repair. Be careful not to bend the disc. A friend trying to help one of our lady riders worked on the wheel, disc down and bent it.

asmac

I've changed many flats on disc brake wheels and never had a problem (or thought much about it). Just be reasonable and don't turn hydro systems upside down.

FBinNY 

Pardon the ignorance here, but aren't bicycle brake hydro systems sealed (not vented)? In which case turning the bike on it's side or upside down shouldn't make a difference.

I'm not challenging you here, just wondering if there is an issue people need to be aware of, especially if they lay bikes flat in their car trunks.

asmac

I stand to be corrected but understood that any air migrating to the caliper can lessen braking and turning it upside down lets any bubbles move in that direction. As you point out, it's not a question of leaking.

FBinNY 

That explains it. However air in the lines doesn't lessen braking since pressure is pressure, liquid or air. But an air bubble is compressible so it increases lever travel and gives a spongy feel to the action. A small bubble isn't a serious issue, but a larger one can use up too much travel and allow levers to bottom against the bar.

Where the bubble is doesn't matter, but you want to get all the air up to the bleed end so they can be bled out. In doing so you can use a trick that nurses use with IV drip lines. Tap the line, or flick it with a finger nail to jiggle it sharply. This speeds movement of bubbles upward against the viscosity or surface tension of the fluid. Transparent IV lines give the luxury of watching the bubbles move in response, so they know it's working. You don't have that luxury, so do this a number of times before bleeding to be sure all the air is at the bleed port.

asmac

Thanks, I defer to your much better understanding of this. I still hate turning my bike upside down; with any bike I have it seems to mush up the cables and shifters. Getting back to the original question I haven't found that rotors or cassettes are particularly delicate (assuming you don't hammer on them and why would you?) and have yet to damage anything while fixing a flat.

FBinNY 

I'm with you, and think people obsess over stuff that doesn't matter. I don't think that simply laying a wheel down on either side can do any harm, but I also can't see how someone can work on them that way. As for turning bikes over, this lost favor as drop bars with cables sticking out the tops of the levers became common. It was never an issue with upright bars, and nowadays with aero cable routing isn't for road bikes either.

I rarely turn bikes over, not because it bothers me, but because there are plenty of trees where I ride, so it's easy to slip the saddle over a branch for a "Mother Nature" workstand.

woodway

I flip my hydro brake equipped mountain bikes over on a regular basis in order to let the fluid in the front shock soak the top bushings to keep them from dying out (as recommended by the shock manufacturer). As FB notes, a bubble is a bubble no matter where it is in the system and turning the bike upside down won't really make a difference. A bleed kit will take care of the issue.

woodcraft

I've had to bleed the brake after standing the bike upside down.