Originally Posted by Dave Cutter

+1 You need [to practice on] the hills to develop strategy and technique.

Originally Posted by Carbonfiberboy

Because the speed on a climb is less, there's less inertia and that changes pedaling mechanics and thus muscle use. You're also in a slightly different position, though that's less important. Your cadence is also likely not to be freely chosen. Also because you're going more slowly, there's less cooling wind so it's harder to get rid of heat.

Originally Posted by gregf83

You're correct in that you don't need to train on hills to be a good climber. Hills, if available, are generally convenient places to be able to ride steadily without interruption but there a plenty of other ways to ride without interruption. If you can ride on the flats at 4W/kg you'll be able to ride hills at 4W/kg. Provided you have the right gearing the forces on the pedals are the same. The only difference is the intertial load at the cranks is lower while climbing or on a trainer.

Most people who ride a trainer consistently can get their power up to similar levels they generate outdoors provided they have sufficient cooling.

Most of my riding is on the flats during the year with only short (<5min) hills. A few time a year I'll do a hill climb and don't have any problem riding at threshold power. In fact, I generally use the power I generate on a 1 hr hill as my FTP. I rarely do a solid 1 hr ride at FTP on the flats.

So yes you are guaranteed to improve your climbing performance if you increase your threshold power/weight ratio on the flats. I find 6x5x1 intervals at 105-108% of FTP (6 - 5min intervals with 1 min rest between intervals) effective for raising FTP. No hills required

edit: I have no idea what strategy and technique are required on a hill. I just ride at a steady power and it seems to work out fine. Make sure you have low enough gearing to ride at 90-100 RPM even if your preferred cadence is lower. Grades are not constant so you need some flexibility in gearing.

last edit: Interesting case study on training for a hill climb: The Mt. Washington Hillclimb: A case study | ccbracing by Cameron Cogburn winner of Mt Washington hill climb.

Originally Posted by RChung

You're in SF and you're not near uninterrupted climbs?

Climbing is about watts/kg. You can increase your watts and decrease your kg even if you only ride a trainer in your basement.

Descending is different. It's hard to learn how to descend quickly and safely without actual descending, and it's hard to do actual descending without actual climbing.

[Edited to add:] A friend has a friend who was the captain of a container ship that traveled between China and the West Coast. He got a Computrainer and pedaled his way across the Pacific, many times. My friend says his friend was a killer on the climbs but couldn't descend for crap.