Tiagra has only recently been upgraded to 10 speed and the new 4 arm spider, 4700 is what it's called? So makes sense you're only now seeing it. Having said that, 105 doesn't cost that much more and gives you that one extra gear and makes it a bit easier to upgrade to the other 11 speed groups (Ultegra, DI2, D-A)if you want to in the future. At maybe a $75 premium, I would definitely go with 105. I think in general TT bikes are not really "entry bikes" like road bikes are, they are for special uses, and so you will see even the entry bikes equiped with higher specs than you would on a road bike. I think even seeing 105 on TT bikes is a relatively recent occurrence.

3 reasons.


1. Can't use NEW TT shifters. Current Tiagra is 10 speed, and current tt shifters are 11 speed.

2. Can't use OLD TT shifters. Current Tiagra rear derailleur has been redesigned for a new pull-ratio. Old (indexed) 10 speed shifters would not pull the right amount of cable. This would lead to bad shifting.

3. The price of 105 rear derailleurs is really close to Tiagra.


....
Having said that, I've used Tiagra 4700 and think it's great on a road bike. Sure there's a weight penalty (The cassette alone is 100g heavier!), but it's great value / performance and much better than previous Tiagra versions.

I swear it's like they force you to upgrade everything. Entry level TT bikes and lower mid bikes are more and more becoming a thing of the past.

---

I still lapped everyone on the couch!

that sucks. I bought some Tiagra STI shifters for my road bike I'm upgrading to 10 speed with parts I took off my TT bike. I was going to re-use the Ultegra rear der. Hmmm... now I wonder if it will work. Guess I'll find out quick.

---

TrainingBible Coaching
http://www.trainingbible.com

Tiagra 4600 was also 10 speed, but with external shifter cables and 5 arm crank

You're right, I think I may have confused Tiagra with Sora here.

Unfortunately this won't work. The Big S made the new 4700 series use the same cable pull ratio as their 11 speed components. This means that teh 10sp 4700 shifters won't work with 10sp 5/6700 derailleurs. Its an interesting move, one I'm not sure about.

Interestingly though, I read that you could effectively use a 4700 RD as 11spd if you were to use the 11sp 5800 or 6800 shifters.

Becuas eof this you couldn't use older 10sp bar end shifters, although friction shifters would work.