Reach measurements

[horizon]

Doing the trigonometry using the figures from the Thorn Club Tour Mk4 brochure.
580S has a reach ? 390.4mm 
580L has a reach ? 425.6mm

macspud: many thanks for that (and for the links in your previous post which were excellent). You rmaths is better than mine and the figures you produce seem to fit. What is interesting though is that Thorn make it absolutely clear that you need two different frame lengths for drops or straights. Who else does this? It seems that Surly for instance choose the short length as default, which is correct for drops but that doesn't stop people putting straight bars on a Surly. It also seams that the shorter length on a 580S isn't really shorter - it's the 580L that is simply made longer.

But this has made a direct comparison possible. The Sherpa frame though is still rated more highly than a Surly but that's another issue.

[Andre Jute]

But the push to use reach as a universal size indicator has some merit: take a look at a Thorn Sherpa 565L. You won't find anything that measures 565 on the bike - it's a virtual measurement.

Andy Blance has thought the fit through. The S or short top tube bikes are for drops, and the L or long top tube bikes are for straight bars, assuming sporting postures. But if the rider wants a more relaxed posture, the short frame with straight bars would be more suitable. So reach as a universal measure of fit would be confused by the posture and components desired, same as every other rule of thumb.

Most folk, I think, would be better off doing careful measurements, geting an intimate friend to check them, and letting Thorn worry about a precise fit to their desired posture and handlebar choice, which are clearly the controlling elements of fitting a Thorn bike.

[macspud]

The Sherpa frame though is still rated more highly than a Surly but that's another issue.

If you are interested in the Sherpa 565 comparisons then I calculate that:
Sherpa 565S has a reach ? 394.8mm
Sherpa 565L has a reach ? 435.1mm

[horizon]

That's a huge difference IMV in bike fit terms - just over 4 cm. That's the difference (for me at least) between comfortable reach and feeling too stretched out. You cannot gain this difference by moving the saddle forward (you lose the correct relationship with the BB) and a short stem still needs about 30 - 40 mm to accommodate itself. And once you have a short stem there's nowhere else for this adjustment to come from apart from reducing reach. That 39 mm is AIUI about the shortest you can go even on a 26" wheel frame for that size of bike. Surly seem to go down to about 37 mm on their smallest 26" LHT but I presume they are relying on the fact that the rider of that bike will have very short feet proportionate to their body size. This of course is all for drops.

[Relayer]

The idea is that with your hands on the hoods of drop handlebars your hands will be quite a bit forward of the front of the stem, with straight bars your hands will be level with the front of the stem or slightly behind.  i.e. S sizes for drops, and L sizes for straights.

This will eat up 4cm easily IMO.

If you're only interested in drops, an L frame might be OK for somebody with long arms and short legs.