First I need to say that I love my B17. That being said, I can't fit one on my Synapse because the rails are too short to get sufficient set-back. Does anyone know of another saddle similar to the B17 in terms of shape and width that might work out? I have heard a lot about the Terry touring model, but am not familiar with the saddle. Maybe Anatomica might be the answer?
Thanks

Look for a set back seat post, this is a post that has a longer head on it, so the connecting point is further back. Really though if you need the seat back further then the saddle will adjust, I wonder if the bicycle is actually the right size for you, it sounds a little on the small side.

Fizik Alianti.

There is a leather saddle, otherwise not too similar to a B-17, called the Selle Anatomica. It is highly adjustable and can be very comfortable. It has rails with a clamping area around 4 inches long, probably about twice as long as any Brooks. But if you slam back an S-A, I think you run the risk of bending the rails.

I have a Terry Liberator on one bike and a Brooks B17 on the other.
Both seats are comfortable.
But the rails on the Terry seat are shorter than the rails on the B17.

Give the Selle Anatomcia a close look,

The issue is unlikely to be bike size. It's just the way Brooks saddle rail are -- they don't allow the same setback as other saddles. Seatpost setback might a solution... but curdog needs to know what's on there already -- zero, 10mm, 20mm, 30,mm?

Velo Orange saddles are also made with longer rails for more set back, I've been happy with the ones I use. I've also had a B-17 and a B-67.

Marc

+1. The rails are much longer. If you go this route, follow the written installation directions carefully. My experience was that this is a touchy saddle to dial in.