Bicycle Mechanics - Cage length advace determination??

Hi, is there a good way to tell in advance what length derailler cage is needed for a new drivetrain? Some formula somewhere? I know that I shouldn't cross the chain, but I often ride while drunk, so I don't want to trust myself.

I'm currently using:
52-40 in the front
11-32 in the rear (8-speed)
Suntour VGT rear der (med-long cage)

I'd like to move to:
53-40-28 front
11-20 rear (9 speed)

also, this question has been adressed in the forums before, but I'm not sure if it has been definatively answered: will old friction deraillers work with 9speed cassettes? I'm thinking of using an old campy super record, or a perhaps try to ebay a suntour superbe.

Hi, is there a good way to tell in advance what length derailler cage is needed for a new drivetrain? Some formula somewhere? I know that I shouldn't cross the chain, but I often ride while drunk, so I don't want to trust myself.

I'm currently using:
52-40 in the front
11-32 in the rear (8-speed)
Suntour VGT rear der (med-long cage)

I'd like to move to:
53-40-28 front
11-20 rear (9 speed)

also, this question has been adressed in the forums before, but I'm not sure if it has been definatively answered: will old friction deraillers work with 9speed cassettes? I'm thinking of using an old campy super record, or a perhaps try to ebay a suntour superbe.Well yeah Wrap requirement is front chainwheels tooth difference between large and small + rear cog large/small tooth difference. then you have to know the spec for your derailer. Your first example is 33 teeth,and the second is 34 teeth. If you can use the small/small in the first exampe with a bit of capacity left in the derailer,there should be no issue with the other. Easy enough to stay out of the small/small anyway.....Some old RD won't work with the wider 8/9/10 cassettes due to a lack of range between the limit scrws. Some will.

A derailleur with enough range to handle an 8-speed cogset should be able to handle a 9-speed, as well. I currently use a Campag. NR with a 7-speed freewheel, and it looks as though I have enough remaining adjustment range to handle another cog. The old SunTours should be up to task, as well, although not all shift levers will pull enough cable to reach all gears.

not all shift levers will pull enough cable to reach all gears.

What do you think about Suntour Bar end shifters?

What do you think about Suntour Bar end shifters?I have yet to see a shifter that did not pull enough,even small barrel campys.