Originally Posted by
hairnet
There was a whole discussion a while back about using cassette cogs for SS conversions. It basically concluded that it's a bad idea because those cogs are designed to make shifting easier, which means they sort of want to drop the chain and may not be totally safe. I would do it to see if I like the gear, but in the end I think I'd get a cog without all the shaped teeth and cut outs.
Maybe this isn't a problem on a frame with horizontal drop outs since you can keep the chain tight always. With vertical drop outs you'll need a tensioner, which means the chain won't be under constant tension and tight.
The only way the chain will derail is if a derailleur pushes it off the cog. There needs to be a side force from the derailleur jockey pulley to make this happen. If what you said were true, the chain would drop off the cog as soon as the chainline were not straight, which is most of the time on a multi-speed system. Also, the chain would be constantly falling off the chainrings, because they nowadays also have all kinds of shifting ramps, bevelled teeth and whatnot to facilitate shifting. Bottom line, cassette cogs work just fine for SS conversions.