I have a pretty new Shimano 105 triple crankset I'd like to put on my old Sakae 2500 frame. The Sakae is English threaded so the bb should fit fine.

Will the triple set in front cause any misalignment with the older 6-speed hub in back? Are there any other reasons (mechanically speaking) why this won't work?

My weak legs can't propel my bike and me up the hills, and I like riding in the hills. I need a granny gear.

Thanks for your insight and suggestions.

-Nik

Triples have been around since the 5spd era. Your triple is probably 8 or 9spd but should work with a 5-6spd chain. Your FD may or may not have the throw to cover a triple, some do, some don't. Down tube friction shifter should cover the range ok. Chain alignment is not likely to be a problem but you may not be able to cover the big-big or little little ends without some chain rub. Depends on where the BB puts the small ring, which should be within 3mm of the outside of the BB shell.
Steve

Do it.

You'll need a bottom bracket that matches your triple crankset. Chain length will probably be OK because the big/big combination probably won't change. Depending on what you have for a rear derailleur, you might not have enough slack take up capability for the little/little combination. In real life that's not usually a problem since you will probably only use the granny ring with the biggest couple of rear cogs anyway. Friction shifters will cure a lot of front shifting ills. A front derailleur with a deeper inner cage, that's designed for a triple will improve the granny to middle ring shift.

I'll echo Retro Grouch on this topic. I was able to convert my 12 to 18 by going with a slightly longer bottom bracket axle and new front derailer. The rear derailer was already a long cage model. Removing one link from the chain kept better tension on the granny combo and did not strain chain or derailer on the fast gears. I avoid the big-big and little-little combinations, but have no trouble using all six cogs with the middle ring.

Thanks everyone. The Bottom Bracket for the 105 cranks is 118x68, which is the size shimano recommends. I'll give it a shot as soon as I finish painting the frame.