"Reach" and "stack" are a relatively new measurement being employed to understand bike fit, so it's taking a while for the cycling community to absorb it into their collective wisdom. Using reach and stack for bike fitting starts with the idea that the distance from the bottom bracket (center of the cranks) to the top of the head tube (there the fork goes into the frame) is not adjustable on any contemporary bicycle. (Exemptions to this are exceedingly few.)
Since you can adjust all kinds of things about your handlebar and saddle position by means of seatpost height and offset and stem/ handlebar choice, the stack and reach of the frame are the best way to predict how a bike will fit when you set it up to optimize handling by means of cockpit components -- handlebar and stem choice.
If you start at the center of the bottom bracket and draw a line straight up to a line level with the center of the top tube, that vertical distance is the stack. The horizontal distance from that imaginary vertical line through the bottom bracket to the center of the head tube is the reach.
If you need help visualizing this (which is totally understandable), check out the way Transition Bikes illustrated it for the benefit of Vital MTB readers a few years ago .
The point of this is to remove the saddle position from the equation. Whereas effective top tube is partially determined by seat tube angle (a steeper angle would shorten the effective top tube, a shallower angle would lengthen it), reach and stack are about where your feet are relative to your feet. This is important because, when riding a bicycle at "fun" speeds, you have to stand up and get your butt off the saddle -- quite a bit! With the mainstream adoption of dropper seatposts, the saddle is often literally nowhere that affects how the bike handles at all.
A frame's reach and stack are static, but how that plays out are somewhat adjustable. You can raise, lower, shorten, or lengthen the resulting effective reach and stack (where your hands grab your grips on the handlebar). Effective stack can be adjusted by grip height (amount of spacers, stack height of stem, height of headset top dust cover, rise of stem, handlebar rise) and effective reach is the result of the length of the stem, handlebar setback (resulting horizontal offset based on length and backward angle of handlebar).
These two measurements have a proportional relationship to one another. Reach and stack form a right triangle, the hypotenuse of which can be easily predicted by Pythagorean theorem. Do you remember your first geometry lesson? If not, it's time to review.
It is my belief that you can use this effective downtube measurement to some effect to know how a bike will fit and handle based on previous experience with other bikes. If you don't have experience with other bikes, it's time to demo and borrow some bikes to find out what works for you.
If you want a sophisticated, detailed calculator to determine truly how a bike will fit you from your feet to your hands, check out the Lee Likes Bikes MTB school. I have nothing to gain from plugging his services, but Lee McCormack knows a thing or three about wrangling a bike. Chances are, his advice will put you on a bike that is "small" relative to the long-low-slack scheme of bike fitting, but it's worth exploring his methods to learn how effective reach and stack affect how a bike fits and handles.
I won't comment on the specifics of what kind of reach is ideal for any rider or terrain. For me, as a person who is about 174 cm tall (that's 5' 8-1/2"), I can find ways to fit on a bike with a ETT of 595 mm to 615 mm or a reach of 410 mm to 450 mm. that's a huge range 1-1/2 inches is a lot when it comes to optimizing a bike fit), but other factors come into play.
The point is that reach and stack are a good way to understand the way a bike will fit you, once you've ridden a bike and know what you like or don't like about the way a bike handles relative to the way your hands are positioned relative to your hands.
One final caveat, and it's a big one, is that reach and stack don't necessarily mean the same thing from one manufacturer to another. This is because the reach and stack (and other measurements, for that matter) will change dramatically depending on where the fork and shock are in the travel. Look on the manufacturer's geometry chart and find the point where (hopefully they specified this) it tells you if this is the "sagged" measurement or the "static" one.
Most manufacturers will list the reach and stack of a bike based on those angles at "sag," which is where the angles of the bike will settle when a rider is mounted on the bike. This is usually considered to be 20-30% of the suspension travel, assuming this is a bike with suspension in the front, rear, or both ends. Typically, the angles and lengths will change relatively little on a full-suspension bike, but change noticeably on a hardtail.
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