10speed brifter on an 8 speed cassette
 

Will it work?

This is related to a post in the road bike forum, but this is probably a more appropriate place to post it.

I am thinking of putting 10 speed Ultegra brifters on a road bike of mine that runs an 8 speed cassette and index shifters. Any reason not to do this?

If i shift 9 times, will the derailleur yank the chain off the gears, or will it just reach it's max cable tension and stop shifting once it reaches its limits?

You will have very poor indexing. If you set your limit screws correctly, then you will not shift off the cassette.
Best thing to do if you really want to use a 10 speed shifter is complete the job with a new cassette and chain as well.

You set limits with the derailleur so it won't over-shift; the real problem is that the indexing will not line up well at all.

A 10-speed cassette is about the same width as an 8- and 9-speed (actually 10-speed is slightly narrower); therefore, the space between cogs and the amount of cable that gets pulled for each gear decreases with the greater the number of speeds.

In general, the right shifter, cassette, and chain need to match in number of speeds. So replace the cassette and chain with the new shifters.

Ok, so scraping the idea of the brifters.

How about 9 speed bar-end shifters? Will those work on cassettes of lesser gears? I feel like they might since they can do the whole friction shift thing. But now time for a super stupid question. Can I use the friction shifting on barcons with index shifting derailleurs?

Yes, the friction mode on 9 speed (or for that matter 10 speed) will allow you to get the rear derailer in correct postions for your 8 speed cassette. You won't need to change anything on your rear derailer settings, just make sure the low stop is really in the right position.
I am a real fan of bar-end shifters because of the friction ability and total cost of ownership.

Yeah, I feel like they'll be great on this bike.
Thank you for the input.
And yes, I'll be careful about the low stop on the RD. I like my spokes right where they already are thankyouverymuch!!!

If your RD is Dura-Ace 7400, 7401, or 7402 (the old 6, 7, and 8sp), any Shimano 9-speed shifter will shift it at 8-speed spacing.

If your rear derailer is any other Shimano (including the new Dura-Ace 7700, 7800, and 7900) you can use this alternate cable attachment to shift 8-speed spacing with 9-speed shifters. http://www.sheldonbrown.com/dura-ace.html#indexing

BTW: If your derailer is Dura-Ace 740x, using 10-speed shifters *and* the alternate cable attachment will shift it at 8-speed spacing.

You can use a 9 sp brifter with an 8 sp cassette if you use the alternate shift cable attachment at the rear derailler described by Sheldon Brown.

My RD is actually DeoreLX. I know, not the usual for a road bike, but the LX's work fantastically with my gearing.

You can still buy 8 speed Ultegra bar end shifters new.