Professor ZZZ

Now that it's warming up, I'm going to dump some money into new shifters, probably shimano 105 ST-5800. I currently have a 8-9 speed rear mech, and I'm wondering if I can use these new shifters (set for 10spd) together for any length of time (the set up inquestion: shimano STI 10spd + 8-9 spd rear mech + 10 spd cassette). My understanding of the STI system is that the crank drum inside the housing sets the distance of each shift, as it has the teeth machined out at a specific distance corresponding to the throw distance of the rear mech between gears. So if anyone with more advanced knowledge or experience of these components can give me a ruling, I'll have a better idea on whether I can pair these components for few weeks while I decide on a new rear mech.

rpenmanparker 

11-speed shifters like 5800 won't work on a 10-speed cassette. The 8 or 9 speed derailleur would if you had the right levers. (I hate calling it a "mech". Where on Earth did that come from?). Anyway the spacing is different from 10 to 11, and the shifters won't be moving the chain far enough on each shift for a 10-speed cassette

Carbonfiberboy 

The 10 speed brifters have a changed cable pull. The RD will move accordingly and properly engage your new cogs. Should work fine. 11 speed brifters won't work.

shelbyfv

5800 shifters will not work with your 10 speed anything. You can still buy new 5700 10 speed shifters that will work with the rest of your stuff. 8,9 and 10 speed road derailleurs are generally interchangeable as long as they are properly sized for the cogs or chainrings. The number of cogs on the cassette must match the shifter. Apparently with 11 speed, Shimano has decided we need ALL NEW STUFF, including wheels....

LesterOfPuppets

It came from Great Britain.

rpenmanparker 

That's what a friend from there just told me. But he could not explain why a derailleur is a mech, but brake calipers, crank, or levers aren't. Assuming mech is short for mechanism, I am a bit puzzled.

rms13

I only think I hate more than mech is gruppo. Nothing makes you sound like more of a pretentious db cyclist than referring to what gruppo you have on your bike

rpenmanparker 

Tell it like it is, bro.

mrvrsick

Buy the whole group, it's less than $350, and sell your old stuff.

I've also heard that 11 speed will work on a 10 speed wheel if you take off one cog.

rpenmanparker 

Yes, as long as the spacing of all the parts, the cassette, derailleur, and shifters are 11-speed. You can run the cassette with only 10 cogs to fit on a 10-speed freehub. Just set the limit screws to prevent shifting into the spokes or chainstay when looking for the 11th position.

Professor ZZZ

I think I'm going to go with the 5700 drive train. I'd spring for the full 'gruppo' but I also dislike that term, and I hate selling bike parts on the internet even more. I had intended on stringing the purchases of these components along for three months or so, but I'm just going to get it done. Oh and FYI: I guess I've never thought about it, but I assume I use the term 'mech' because I watch a ****-load of GCN.

Thanks for the quick and concise feedback!