Motivation: I recently decided to change the cassette and rear derailleur of my Trek FX 7.5 bicycle to get a bigger granny gear. Installing and adjusting the derailleur is much easier using a bicycle work stand which suspends the bike allowing the pedals to be spun while shifting gears and watching the rear derailleur action.
Big Idea: I already have a workbench mounted vise, so all that was needed was a way to suspend the bicycle by attaching it to the vise.
Since my bicycle seat post diameter (and also many other bicycles) is about 27 mm AND the outside diameter of standard ¾ inch iron pipe is just barely under 27 mm, I just removed and replaced the bike seat post with a 5 “ long , ¾ pipe nipple.
A 90 degree elbow connects the nipple to a 18” long section of pipe creating a “L” shaped arm. The longer pipe is then clamped in the bench mounted vise - rigidly holding the bicycle about 6 inches off the ground (see photo).
Step 1: Check Bicycle Seat Post Diameter
For this idea to work, your bicycle seat post diameter should be about 27 mm so that a 3/4 inch pipe (nominally 26.67 mm outside diameter) will fit into the bicycle seat post mount.
To check your bicycle's seat post diameter, here's a URL (http://www.sheldonbrown.com/seatpost-sizes.html) to a bicycle seat post diameter database, or just measure your bicycle's seat post diameter, or just buy a 5 or 6 inch long 3/4" pipe nipple to test fit.
Step 2: Buy Hardware, Assemble, and Test
1) From a hardware store or big box store like Lowe's or Home Deport, buy one each 3/4" iron pipe:
90 degree elbow,
5" lg pipe nipple,
18" lg pipe threaded on at least one end
Total cost < $10 @ Lowe's (see receipt photo)
2) Screw the pipes and elbow together to create a giant "L" arm
3) Replace the bicycle's seat with the short end of the "L" arm, and tighten the seat mount screw. Avoid over tightening the screw as this might make it hard to reinstall to bicycle seat.
4) With the "L" arm now attached to the bicycle, clamp the long end of the "L" arm in the workbench vise. Position bicycle and vise so you can spin the pedals while working on the bicycle. Most vises have a pair a semi-circular jaws below the standard straight jaws for clamping round objects.
5) That's all there is to it !