Thorn Cycles Forum
Community => Thorn General => Topic started by: horizon on July 29, 2014, 07:05:57 PM
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Most manufacturers (e.g. Surly) now include the reach measurement as part of the frame geometry information. Does anyone know if Thorn do this anywhere or if it has been discussed before? Reach is the distance between a vertical line up from the BB and the head tube. It's hard to measure on a sloping top tube. I'm particularly interested in the Club Tour 580S. I'll email Thorn if necessary - they're usually amazingly helpful on things like this.
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Is it just me? I don't understand the utility of this measure, "reach", if it truly is the horizontal distance from the bottom bracket to the top of the head tube. What is it good for? Surely the cyclist's reach is much more influenced by the horizontal distance from the top of the seat tube to the top of the head tube, with adjustments from there to allow for the normal factors (seatpost length, layback, distance of handlebars above or below saddle, type handlebars, etc.) than by the distance, however measured, between bottom bracket and top of head tube.
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It took me a long time to work it out as well.
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I think the reach is:
VirtualTopTube - VirtualSeatTube * cos (SeatTubeAngle)
so for my Nomad, the 565L, that would be
605 - 565 * cos(73) = 439.8
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I think for the club tour 580s the reach would be
560 - 580 * cos(73) = 390.4
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That Club Tour reach looks almost identical to that of the corresponding Surly LHT.
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Is it just me? I don't understand the utility of this measure, "reach", if it truly is the horizontal distance from the bottom bracket to the top of the head tube. What is it good for? Surely the cyclist's reach is much more influenced by the horizontal distance from the top of the seat tube to the top of the head tube, with adjustments from there to allow for the normal factors (seatpost length, layback, distance of handlebars above or below saddle, type handlebars, etc.) than by the distance, however measured, between bottom bracket and top of head tube.
Andre, I think (though don't quote me) that sufficient reach is important when out of the saddle e.g. when cycling up hill, if there isn't enough reach the handle bars will interfer with the knees. I'm not sure that I'm correct but that is what I came up with when pondering the importance of the reach measurement in the past.
Reach is used in conjuction with stack when doing a bike fit, Stack being the vertical measurement between the BB and top middle of the head tube, giving you both X and Y co-ordinates of the top of the head tube in relation to the BB.
I think that they are handy additions to the other measurements when sizing a frame, I don't agree that these are the only measurements needed as some on the net seem to espouse.
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I think the reach is:
VirtualTopTube - VirtualSeatTube * cos (SeatTubeAngle)
so for my Nomad, the 565L, that would be
605 - 565 * cos(73) = 439.8
The stack measurement on your Nomad would be 565(sin 73°) = 540.3
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Andre, I think (though don't quote me) that sufficient reach is important when out of the saddle e.g. when cycling up hill, if there isn't enough reach the handle bars will interfer with the knees. I'm not sure that I'm correct but that is what I came up with when pondering the importance of the reach measurement in the past.
Reach is used in conjuction with stack when doing a bike fit, Stack being the vertical measurement between the BB and top middle of the head tube, giving you both X and Y co-ordinates of the top of the head tube in relation to the BB.
I think that they are handy additions to the other measurements when sizing a frame, I don't agree that these are the only measurements needed as some on the net seem to espouse.
No wonder I didn't get it! My everyday bike is deliberately designed to be used only while firmly seated, never, ever out of the saddle. And, not to put too fine a point on it, with the range of modern derailleurs and the Rohloff, I consider the bike mismatched to the rider if he ever has to stand or push, generally by an overly sporting choice of chainring and sprocket. (Different story for road bikes intended for actual racing. I'm talking about touring bikes and utility bikes.)
Thanks for filling me in, man.
The rest of this thread is interesting, but a little two-dimensional thinking persuades me that those two measurements, reach and stack, alone would not let you fit a road bike properly. Both of them presume in the first instance that the bottom bracket is properly placed in relation to the saddle and the rider's thigh length (this is the subtext of Jim's post a few above -- the old knee over the pedal rule of thumb), yet an optimum arrangement of reach and stack would tend to push the bottom bracket into entirely wrong contortions.
The sun shines and I'm off to ride, so I'll let Jim handle the mathematical proof!
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The three chapters on Stack & Reach here:
http://www.slowtwitch.com/Bike_Fit/Choosing_a_Tri_Bike_via_Stack_and_Reach/Stack_Reach_Primer_Chapter_One_95.html
and
http://www.slowtwitch.com/Bike_Fit/Choosing_a_Tri_Bike_via_Stack_and_Reach/Stack_Reach_Primer_Chapter_Two_96.html
and
http://www.slowtwitch.com/Bike_Fit/Choosing_a_Tri_Bike_via_Stack_and_Reach/Stack_Reach_Primer_Chapter_Three_97.html
they give the jist of the arguement for using Stack and Reach for bike sizing (I think that seat tube angle and head tube angle would need to be added for proper fitting).
Anyway I agree that one should not need to get out of the saddle, having said that, I find that on long hills I do like to now and then just for comforts sake. I live in the Scottish Highlands so plenty of hills to contend with. :)
Enjoy your ride, weathers a bit dreich here today, after days of glorious sunshine the plants were in need of a drink anyway.
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No wonder I didn't get it! My everyday bike is deliberately designed to be used only while firmly seated, never,
It's not to do with standing up in my case. Once you've got the relationship of the saddle to the BB sorted out, you can work on the bars. If you've got long legs and short arms (lots of women and some men), you might run out of backwards adjustment on the bars (i.e. even the shortest stem isn't short enough) and you will therefore be too stretched out for drops.
A smaller size bike won't work as it shortens vertically but not horizontally. Even though the top tube appears to get shorter, in fact the seat tube simply comes forward. On the smaller bike you then simply push the saddle back to keep the right relationship to the BB so you end up where you started.
The main irrelevance about the reach is that it's almost impossible to shorten it as you cramp the front wheel and cause toe overlap. So reach is almost always around 39 cm. But it can be more and it can be slightly less so it's worth looking at, maybe. My current bike seems to have a reach of 42 cm so there's a critical 3 cm in there that cannot be found anywhere else on the frame.
Because Thorn specify two frame lengths (S and L), it implies that the reach must be different. Simply lengthening the top tube by changing the seat tube angle achieves nothing: your legs still need to be at the right position for the BB so the saddle is moved back leaving your arms where they were in the first place. That doesn't mean that smaller bikes shouldn't have shorter top tubes, it simply means that you cannot solve this particular problem by buying a smaller bike in the range. So I'd like to know the difference in reach if there is one between the S and L.
The alternative is straight bars but you can still be stretched out on these. BTW most people never even know this problem exists as their body proportions fit the standard bike with drops. Other people usually go for straight bars, not getting on with drops due to the reach problem.
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The stack measurement on your Nomad would be 565(sin 73°) = 540.3
yeah, the height of the virtual seat tube is defined to be the same as the height of the real head tube. That looks right.
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If you've got long legs and short arms (lots of women and some men
So this is a measurement that is totally irrelevant to Italians. Anyone who has ever sat in the driver's seat of an Alfa Romeo, which seemed to be designed for dwarves with gorilla arms grafted on, will know what I mean.
The main irrelevance about the reach is that it's almost impossible to shorten it as you cramp the front wheel and cause toe overlap. So reach is almost always around 39 cm.
That might actually make it a good test for bikes that will fit the majority of people. But in that case it doesn't say anything about fitting a particular cyclist away from the bulge of the bell curve, because he must look for the exception to the rule, as you do. It still, therefore, seems to me pretty esoteric.
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It is a bit esoteric. Having said that, there are thousands of cyclists who don't take to drop bars and I wonder now how many simply find the stretch not right for their body proportions. 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. Even if you use the top tube as guide, you need to know the seat post angle as well.
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I'm particularly interested in the Club Tour 580S. I'll email Thorn if necessary - they're usually amazingly helpful on things like this.
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
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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.
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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.
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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
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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.
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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.