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What is 'reach'?
Last Post 12/11/2013 11:44 PM by 79 pmooney. 6 Replies.
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GJanney
Posts:76
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| 12/11/2013 06:05 PM |
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When looking at a bike manufacturer's site, what am I supposed to understand from the 'REACH' measurement?
I am trying to compare two bikes to determine which stem length to buy. I know I can compare the bike geometry (e.g. TT length) but am not sure what reach means or how I should factor that in to the equation. |
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Cosmic Kid
Posts:4209
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| 12/11/2013 06:33 PM |
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In short, reach is the distance from the center of the BB to a point at the front of the bike. That point can vary depending on what you want to measure. For a frame, it is to the centerline of the HT. You can also measure to your stem, or your brifter position. For a TT bike, you would measure to you pad position. See image below:
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| Just say "NO!" to WCP!!!!
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GJanney
Posts:76
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| 12/11/2013 07:50 PM |
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| CK - very helpful. Couple quick follow-on questions. The frames are different sizes (a friend's bikes) - a 60 cm and a 63 cm. That said, the TT lengths are 58.5 and 60 so he will need to compensate for those. I think from what you show above, I can take the reach from his original bike (63cm) and get a measurement which will help us determine the stem length he needs for the smaller frame. Odd question - what is a brifter? Brake/shifter (like Brangelina?)? |
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79pmooney
Posts:3180
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| 12/11/2013 08:22 PM |
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G, yes, good analogy! (Actually, I think of a brifter as someone you wouldn't want your daughter to date. I certainly don't allow them to hang out in my garage!)
Ben |
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79pmooney
Posts:3180
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| 12/11/2013 08:42 PM |
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There is one simplification this reach method assumes: that the shoes/cleats/pedals and hence ball of foot to pedal relationships are identical. This assumes that the circles spun by the feet around the BB axis are the same. Different crank lengths also change things, though many do not change their seat positions to accommodate different cranks.
Ben |
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Cosmic Kid
Posts:4209
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| 12/11/2013 10:51 PM |
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GJanney - yeah, a brifter is a STI / Ergopower brake lever / shifter.
And yes, you are correct - you can take the reach from one bike and use it to establish what stem length you will need for another, differently sized bike. The beauty of Stack & Reach is that it eliminates the variability of angles, etc. It is all based on horizontal distances from a fixed point - the BB.
IOW, two frames could have the same TT length, but different reaches due to differences in ST / HT angles. Using Reach is a much better measurement of frame size.
Ben, as I understand it, crank size does not affect Reach directly. It will impact saddle height, which will likely impact Stack, but not always Reach (at least not significantly). Yes, altering Stack will slightly impact Reach (thanks to the laws of geometry), but it is pretty insignificant. |
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| Just say "NO!" to WCP!!!!
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79pmooney
Posts:3180
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| 12/11/2013 11:44 PM |
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CK, headtube angles make basically VERY small differences unless the angles are very different and/or the distance from the toptube centerline to top of head tube is very large. (3+ inches from toptube CL to top of headtube and 5 degrees difference in rake only adds up to .7 cm of reach and that's a pretty extreme example.)
Seattube angles, however, make a real difference. (1 degree diff on a 57 cm seattube will move the top tub and reach almost a cm. Bikes that fit me have 74 seattube angles or steeper where 72.7 puts a normal post in the center of the rails. Hence a "normal" sounding toptube length puts my headset and stem well forward allowing the very long reaches I like. And, yes, my seatposts have a lot of setback!)
Ben |
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