Originally Posted by
invwnut
Isn't this backwards? Wouldn't a smaller seat tube angle make the effective top tube become longer? It angles towards the chain stays, thus lengthening the effective top tube? Please correct me if I'm not understanding this correctly
I know that the effective top tube length is very important. You can also change stem lengths to help. It is difficult to find smaller frames in stock. I'm 5'5" and ride a 49cm with an effective top tube length of 53.5cm.
The easy way to think about this is in extremes. Imagine a bike that has a seat tube angle of 90 degrees - i.e. straight up and down and claims at top tube of 53cm. A person would have to have a super super set back seat post to find the right position, hence, lengthening the reach to the handlebar.
On the other hand, imagine a bike claiming top tube of 53cm, but a seat tube angle of 45 degrees. In that case, you'd probably have to move the seat very far forward (somehow), essentially shortening your reach.
The position of your saddle to your crank should be unchanged regardless of seat tube angle. So on a steep seat tube angle bike like a Roubaix 49cm (sta of 75.5 degrees, tt of 518mm) - you may think it's small because of a short tt. However, you'll have to set back your seat to get right relative position to the crank. On the other hand, a Cervelo with 73 degrees sta, and tt of 532 on a 51cm RS, the reach is probably shorter then the Roubaix or pretty much the same.