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PostPosted: Fri Mar 10, 2017 3:57 pm 
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Trying to work out how reach relates to top tube length (sorry if this is really obvious to some people) after having a recent bikefit and finding all my 52.5 cm top tube bikes need different length stems. Is reach top tube length - BB setback, i.e factor of seat tube angle? So a frame with a longer reach will require a shorter stem but a more setback seat post to get the same position?

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Posted: Fri Mar 10, 2017 3:57 pm 


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PostPosted: Fri Mar 10, 2017 4:10 pm 
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Simple 'reach' as defined by the frame itself doesn't take into account other factors that determine the stem length you'll need. Saddle position relative to the BB (which can be, but isn't necessarily, affected by seat tube angle), handlebar geometry, saddle to bar drop, shifter design, and shifter placement all affect how far you need to truly *reach* from the saddle to your preferred handlebar position.

If you are solely comparing frames, stack and reach allow you to quickly get a sense if you'll be able to fit on those frames. If you are comparing two bikes, you need to look at more than just stack and reach which is what you are finding.


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PostPosted: Fri Mar 10, 2017 4:17 pm 
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"Reach", as you see it in geometry charts, is simply the horizontal distance from a vertical line through the BB to the center of the uppermost plane of your head tube. It is only comparable between bikes if you measure it at exactly the same stack height. Since bikes have different headtube lengths and angles, if you want to compare, a better place to measure reach would be at the point along the headtube axis where you want the bottom of your stem to begin.
Assuming you're after the similar fits it's rather puzzling why you have fitters putting you on different length stems on bikes with the same top tube lengths unless the height at which the stems are clamped is quite different among the bikes. Or the reach of the bars could be quite different? Not enough info to really say.

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PostPosted: Fri Mar 10, 2017 4:30 pm 
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You pretty much have it. Reach is another name for what used to be called the 'effective top tube'. It is a number for how much of the top tube is forward of the bottom bracket center. It's easy to imagine the effect of STA on frame reach for a given top tube length. Imagine a STA of 45 degrees - a huge chunk of the top tube will necessarily be behind the BB center and the frame reach will be very low. OTOH, if you had an STA of 90 degrees, all of the top tube would be forward of the BB center and the top tube length and the frame reach would be the same.

If your 52.5cm top tubed frames need different length stems to get your bars in the same spot, they must have different STAs and, perhaps, different HTAs though the effect of that on frame reach is much less than is the effect of the STA.


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PostPosted: Fri Mar 10, 2017 4:49 pm 
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thanks for the reply guys.

My T1 had a measured STA of nearer 73 vs the manufacturers quoted 74, my Oltre is 74.5. T1's quoted reach is 37.7 vs the Oltre's of 38.3. Both have stem and bars at the same height (both Deda RHM bars and -8 Stem). Front of saddle now at 5.5 cm behind BB. The T1 after the fit had a distance from nose of saddle to bar centre of 51.7 cm vs the Oltre at 52.6 cm both with 10 cm stems. Saddle and saddle heights are the same.

On my Ghisallo which has a 52.5 TT and a STA of 74 I can only achieve a distance of 4.5 cm behind the BB for the saddle nose even with the saddle all the way back on a 25mm setback post and the reach feels short (51 cm from nose of saddle to bar centre) even with an 11 cm stem!! There are no official reach and stack figures for my 2006 Ghisallo to my knowledge.

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PostPosted: Fri Mar 10, 2017 11:15 pm 
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cmcdonnell wrote:
Trying to work out how reach relates to top tube length (sorry if this is really obvious to some people) after having a recent bikefit and finding all my 52.5 cm top tube bikes need different length stems. Is reach top tube length - BB setback, i.e factor of seat tube angle? So a frame with a longer reach will require a shorter stem but a more setback seat post to get the same position?


Reach does not directly relate to top tube length. Toptube length relates to seattube angle, Reach does not relate to seattube angle.

Reach is purely the measurement from the plane of the BB forward to the centre of the HT. This is why it's a more precise comparison between frames than toptube length. Assuming that you can achieve the same saddle setback between bikes, the precise seattube angle is irrelevant, therefore the length of toptube behind the plane of the BB is irrelevant, therefore the entire toptube length is irrelevant, and reach alone is what matters.


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PostPosted: Fri Mar 10, 2017 11:19 pm 
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BdaGhisallo wrote:
You pretty much have it. Reach is another name for what used to be called the 'effective top tube'. It is a number for how much of the top tube is forward of the bottom bracket center.

That is not what effective toptube is. Effective toptube is an imaginary horizontal toptube on a bike that actually has a sloping toptube. Effective toptube still relates to seattube angle.


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PostPosted: Sat Mar 11, 2017 12:46 am 
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cmcdonnell wrote:
thanks for the reply guys.

My T1 had a measured STA of nearer 73 vs the manufacturers quoted 74, my Oltre is 74.5. T1's quoted reach is 37.7 vs the Oltre's of 38.3. Both have stem and bars at the same height (both Deda RHM bars and -8 Stem). Front of saddle now at 5.5 cm behind BB. The T1 after the fit had a distance from nose of saddle to bar centre of 51.7 cm vs the Oltre at 52.6 cm both with 10 cm stems. Saddle and saddle heights are the same.


Stem/bars at the same height, but are the stack heights of the stem/bars the same?


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PostPosted: Sat Mar 11, 2017 12:24 pm 
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wingguy wrote:
BdaGhisallo wrote:
You pretty much have it. Reach is another name for what used to be called the 'effective top tube'. It is a number for how much of the top tube is forward of the bottom bracket center.

That is not what effective toptube is. Effective toptube is an imaginary horizontal toptube on a bike that actually has a sloping toptube. Effective toptube still relates to seattube angle.


That's what I recall it being used to refer to, dating back to well before Giant and their sloping top tubes appeared on the market back in 1998 or so. No doubt its usage has changed somewhat with the introduction of the term 'reach' into the cycling lexicon.


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PostPosted: Sat Mar 11, 2017 4:44 pm 
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Here's a great resource for you www.bikegeo.net


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Posted: Sat Mar 11, 2017 4:44 pm 


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