Hooray for this thread!
Have been discussing frame geometry recently in the light of my Custom build .... which went wrong ... BUT is being sorted.
Background:
Have always ridden road bikes, mostly decent 531 approx 22/23" frames or similar and with one exception always end up having the saddle as far back on the rails as it will go. The exception is a late 50s 23" Holdsworth with very slack seat tube indeed.
When I had a few quid I wanted a fast Audaxy style bike for long days in the saddle summer and winter. There were loads of good off the peg choices, but I still felt that I'd end up with the seat right back on the rails and me trying to sit on the rear rivets again, so I decided to go custom and after visiting 'Bespoke' chose a framebuilder. I was very clear in my explaination of how the bike should be with a slack seat-tube etc. At the measuring I actually took several bikes along to show that this was a consistent issue. We did the bike fit and after I still stated that for the finalised saddle position I wanted the seat-post laid back to give more rear-set saddle room. Somehow this major fact got lost in translation. When I saw the bike my first comment was 'That looks small' and when I saw the Brooks saddle as far back as it would go on a very laid back seatpost (ex-Giant donor bike) I knew they had it wrong. To be fair they let me take the bike and ride it for however long it took and see how it felt.
On the road after a few 30 to 40 milers, it felt too short. in the TT even with a longer stem than I had specified (the front end is another issue), but the major point was relative to the bottom bracket I could not get the seat far enough back to be comfortable. I wasn't balanced (Peter Whites Bike Fit article is brilliant here). My legs didn't ache particularly but they didn't work well either, it was hard work and not relaxed. My back was arched and energy wasted pushing myself back onto the rear of the saddle. My hands and neck hurt and because my elbows were straighter my head hurt after a while due to the jarring of the front end. It was a powerful position for a sprint but not good for a day in the saddle and not what I had specified.
http://www.peterwhitecycles.com/fitting.htmI got out the tape measure. Compared to my most comfortable bike '82 Holdsworth Elan, the seat to BB distance was slightly shorter. How to convey this after lots of measuring was difficult.
In the end when I returned to the framebuilder I took a standard layback seatpost on a wooden dowel (a virtual seat-tube), held the dowel end on the centre of the BB and the top with seat post attached at the point on the saddle rails where I needed it to be. We then measured the angle and the distance from the BB vertical to saddle-nose. The virtual seat-tube in the 'right' position for me comes out as 71.5 degrees and adds 2cm to the length of the TT behind the BB. The standard seat-tube angle is 73 degrees. It was an interesting process taling about position, and frame dimensions, putting the data into the CAD software it's a complex business and some of us humans are more non-standard than others and frame sizes are not equal.
Hummers situation seems the opposite to mine, but the issue of seat-tube angle i.s an interesting one in where it puts you relative to the BB. Interestingly in the early 80s there were a lot of tall (25") frames with short Top Tubes, Raleigh made a load, steep angles were also more in vogue in the later 70s too.
I think this is one of the most important dimensions of a frame to know. How much TT in front and how much TT behind the BB. I have a clubmate who like me needs extreme layback seatposts to make his bikes work for him (Velo orange do a 40mm post). Another clubmate upgraded from an older tope end Giant to the latest model and after very close scrutiny of the sizing and geometry data just could not make it work. So it's complex!
Am now waiting for my rebuild frame, I hope we got it right.
