7 sp standard double to compact double
I have an early 1990s steel road bike with a standard double crankset, 53/39. It's a 7 speed drivetrain with downtube shifters. The FD/shifter is friction. I would like to switch to a compact double 50/34. The only compact double cranksets that are available are for 9 or 10 speed. Will a 9 sp compact work with the 7 sp drivetrain?
The only other item you should have to change is the chain. The ramps on a 9/10 speed crank will interfere with your wider 7 speed chain. The more narrow 9/10 speed chain (your choice, the 9 speeds are much cheaper) will work just fine with your current rear derailler and may require a slightly longer throw on the front derailler to shift (we're talking tiny adjustments here). With downtube shifters, you can easily make this adjustment to your shifting technique.
Do note that I'm assuming in the above that you have a compatible bottom bracket for the new compact crank. Most of the compact cranks out right now use outboard bearings. Shimano makes you buy them seperately but FSA and others include them. The FSA Vero is a square taper compact crank but requires a 110mm spindle. If you already have that size spindle, it seems like your cheapest option. Even the Sora level compact will cost you more once you include the bearings.
Thanks for the info joejack. It has a square taper BB. Sugino and FSA make compact square taper cranksets that I think will work. I guess I need to measure the spindle first. I may end up going with an outboard bearing BB setup but I'd rather not replace the BB if the I can use the existing BB.