I read through the forum of stack/reach postings, and I read the primer, if I missed something along the way I apologize for this post. I have a few questions to better understand how to use stack and reach.
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How do you determine what a human’s stack and reach is based on their body measurements?
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How much play on avg is available when selecting a frame? I mean if somebody has a stack of X then they should be able to fit a bike with a stack of X +/- Y
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How do you determine the stack and reach of a bicycle based on tube length and angle measurements (e.g. from a company’s website)?
I plan on using stack/reach numbers for all my bikes: TT, track, road I’m curious to see how close I am currently, considering I only ride stock (but might be a custom candidate at 196cm height, 100cm inseam)
Thanks!
Dave
I read through the forum of stack/reach postings, and I read the primer, if I missed something along the way I apologize for this post. I have a few questions to better understand how to use stack and reach.
-
How do you determine what a human’s stack and reach is based on their body measurements?
-
How much play on avg is available when selecting a frame? I mean if somebody has a stack of X then they should be able to fit a bike with a stack of X +/- Y
-
How do you determine the stack and reach of a bicycle based on tube length and angle measurements (e.g. from a company’s website)?
I plan on using stack/reach numbers for all my bikes: TT, track, road I’m curious to see how close I am currently, considering I only ride stock (but might be a custom candidate at 196cm height, 100cm inseam)
Thanks!
Dave
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You don’t. Stack and reach are bike measurements. Specifically, they are the measurements of a given bike, and they are also the measurements of the bike that ideally fits you, based on being fit on a fitbike.
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Depends on who you ask. Remember, changes in stack will also change reach, so if you are already going to use, for example, an 8cm stem, then your fudge factor on stack is less than someone else. As a general rule of thumb, you can fudge about 1.5cm of stack and reach with spacers and stems.
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You can’t. Well, you can, if they give you enough information, but it’s never as accurate as getting the actual measurements from the CAD sketches.
Rappstar thanks for the reply…
I mis-asked my first question. I understand that stack/reach are bicycle measurements, my point was that each person would have a bike measurement that is their own. Just like a 61 or 63 cm seat tube is “my” measurement. Check out Tiemeyer’s fit calculator http://www.tiemeyercycles.com/fit.htm with a few body measurements he can determine (at least in a ball park sense) what the tube length and angles should be for your bike. I dont see why the same body measurements cant be used to figure out a stack and reach measurement. I’m missing something here. And if stack and reach can be figured out based on inseam, etc., then what is the formula for determining that?
Regarding ques 3, is it pretty easy to get CAD sketches? I’m guessing no. You’re making stack and reach to be much more complicated than I have it in my head. It seems to me that we are just talking high school geometry here, and I’m kinda assuming someone has already done the math. Again, I’m probably missing something here but I dont know what. If you have a 700c wheel as constant, and you know BB drop, front reach, HT length, and HT angle then it seems to me you can mathematically determine stack & reach. Fudging for various headsets (as previously written elsewhere) can be accounted for.
Thanks for the help…
Rappstar thanks for the reply…
I mis-asked my first question. I understand that stack/reach are bicycle measurements, my point was that each person would have a bike measurement that is their own. Just like a 61 or 63 cm seat tube is “my” measurement. Check out Tiemeyer’s fit calculator http://www.tiemeyercycles.com/fit.htm with a few body measurements he can determine (at least in a ball park sense) what the tube length and angles should be for your bike. I dont see why the same body measurements cant be used to figure out a stack and reach measurement. I’m missing something here. And if stack and reach can be figured out based on inseam, etc., then what is the formula for determining that?
Regarding ques 3, is it pretty easy to get CAD sketches? I’m guessing no. You’re making stack and reach to be much more complicated than I have it in my head. It seems to me that we are just talking high school geometry here, and I’m kinda assuming someone has already done the math. Again, I’m probably missing something here but I dont know what. If you have a 700c wheel as constant, and you know BB drop, front reach, HT length, and HT angle then it seems to me you can mathematically determine stack & reach. Fudging for various headsets (as previously written elsewhere) can be accounted for.
Thanks for the help…
The formulae the fit calculators use are so generic that trying to get stack and reach out of them would be pointless. You could develop a set of rules of thumb.
As Rappstar said, the quality of info from websites about geometry can be pretty poor. I use safe assumptions such as 368mm fork length, 68mm bb drop and accept that my figures may be a few mm out if the manufacturer has varied from the norm (usually the ones who vary have a reason and therefore they realise the importance of stating what they have done). It is just trig though, with a bit of effort you can create formulae that calculate stack and reach to within 1mm of cad drawing figures. Some manufacturers give great data and that makes my day when cleaning it up for adding to the tables.
You’ve seen the comparisons I do, they’re just based on high school trig.
Yes, there are formulas for, roughly, stack and reach. Well, rather, it is a formula for stem+top-tube length and a formula for arm-rest drop. They are both based on seat-height. if you are motivated, with some simple trig, you can figure out stack and reach. BUT, I certainly would not use these to figure out fit, since there is definitely some variance. But, if you want them:
(overall height - saddle height) x .61 = top tube + stem (top tube at approx 77deg)
C = .005D^2 -.2D - 1.5 +/- 1.5 where D is saddle-height, c is arm-rest drop. units are cm. .2D changes based on how much variance you have from 78deg of STA. But if you are close t0 78, you can use the formula as is.
It is hard to get CAD sketches, which is why do that work for you. However, not every manufacturer is willing to provide us with those details. Given that bike sizes vary only by about 1cm of stack and reach in many sizes, I am not comfortable making a lot of the inferences you need from provided geometry tables to get stack and reach. I.e., crown height for the fork is never given, but that can change by as much as 5mm. So yes, you can often ballpark it. But I’m not really comfortable with that. If you ballpark the stack and reach, and then you ballpark your stack and reach with the formula, you might end up fitting well. But probability goes down.