Author Topic: Reach measurements  (Read 5806 times)

horizon

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Reach measurements
« on: July 29, 2014, 07:05:57 PM »
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. 

Andre Jute

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Re: Reach measurements
« Reply #1 on: July 29, 2014, 07:53:36 PM »
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.

horizon

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Re: Reach measurements
« Reply #2 on: July 29, 2014, 10:22:43 PM »
It took me a long time to work it out as well.

« Last Edit: July 29, 2014, 11:56:41 PM by horizon »

JimK

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Re: Reach measurements
« Reply #3 on: July 30, 2014, 04:46:02 AM »
I think the reach is:

VirtualTopTube - VirtualSeatTube * cos (SeatTubeAngle)

so for my Nomad, the 565L, that would be

605 - 565 * cos(73) = 439.8



 

JimK

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Re: Reach measurements
« Reply #4 on: July 30, 2014, 04:49:37 AM »
I think for the club tour 580s the reach would be

560 - 580 * cos(73) = 390.4

JimK

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Re: Reach measurements
« Reply #5 on: July 30, 2014, 04:53:46 AM »
That Club Tour reach looks almost identical to that of the corresponding Surly LHT.

macspud

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Re: Reach measurements
« Reply #6 on: July 30, 2014, 07:43:09 AM »
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.
« Last Edit: July 30, 2014, 08:28:08 AM by macspud »

macspud

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Re: Reach measurements
« Reply #7 on: July 30, 2014, 09:06:31 AM »
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

Andre Jute

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Re: Reach measurements
« Reply #8 on: July 30, 2014, 09:37:13 AM »
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!

macspud

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Re: Reach measurements
« Reply #9 on: July 30, 2014, 10:09:20 AM »
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.

horizon

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Re: Reach measurements
« Reply #10 on: July 30, 2014, 10:14:17 AM »
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.

JimK

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Re: Reach measurements
« Reply #11 on: July 30, 2014, 01:34:23 PM »
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.

Andre Jute

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Re: Reach measurements
« Reply #12 on: July 30, 2014, 05:59:51 PM »
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.

horizon

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Re: Reach measurements
« Reply #13 on: July 30, 2014, 11:38:36 PM »
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.

macspud

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Re: Reach measurements
« Reply #14 on: July 31, 2014, 04:34:27 AM »
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