Help with Stack / Reach Ratio - Mtb

[crossjunkee]

I have a bike fit question for the fit guy's. I'm a little confused on stack / reach ratio. The 29er on the left has lower stack, shorter reach, shorter headtube, and shorter top tube. But if I look at the ratio between the stack and reach, they're virtually identical. The almost identical ratio would tell me the bars would essentially be in the exact same place for both bikes. Am I reading that right?

The goal is to downsize from a large to medium frame for XC racing.

[unterhausen]

I don't understand how the ratio really matters at all. I have short legs relative to my torso and arms. So I find a bike that has stack appropriate for my legs and reach appropriate for my arms.

Have to admit I'm not a big fan of stack and reach anyway.

[crossjunkee]

I don't understand how the ratio really matters at all. I have short legs relative to my torso and arms. So I find a bike that has stack appropriate for my legs and reach appropriate for my arms.

Have to admit I'm not a big fan of stack and reach anyway.

I think the issue you could get into in your scenario would be standover clearance. If you size up because of wingspan. Not that it's a really big issue and really the only option. For XC racing standover is important.

[Mark McM]

I'm not sure where the idea of stack/reach ratio having any meaning when it comes to bike fit.

The location of the handlebars will also depend on the length & angle of the stem, and an spacers on the steerer. Although the frames you show might have the ratio of stack and reach, with the same stem and spacers, the smaller frame on the left will place the handlars lower (by 24mm) and futher back (by 6mm) than the frame on the left.

It is true that frames with different stack and reach can actually have similar effective size and fit. But it isn't the ratio of stack and reach that determines that. The actual comparison is more like this: The stack and reach are measured from the top of the head tube, so imagine a frame with an extendable head tube. As the head tube extends upward, because of the angle of the head tube, the top of the head tube also moves backward. So as the head tube extends, the stack increases and the reach decreases.

For common head tube angles, the reach will decrease by about 0.3 (30%) of the stack increase. So, for example, if you had frame with a 540mm stack and 380mm and you added a 10mm of spacers, it would be equavalent in size to a frame with 550mm of stack (540mm +10mm) and 377mm of reach (380mm - 3mm).

But as I understand it, you're not interested in figuring out these two frames have the same equivalent fit, you want to actually change your fit by lowering your handlebars. In that case, you can look directly at the stack and reach numbers. Because the smaller frame has a 24mm short stack, then with the same stem and spacers it will lower the bars by 24mm. Because the smaller frame has a shorter reach, it will move the handlebars back by 6mm (that's typically less than the increment in stem lengths), so it comes close to the goal of just lowering the handlebars. It appears that the frame designer has also managed to keep the same front center, but that was done by slackening the head angle a little bit, and also lower the BB a little bit, so these are also things to consider. (The slacker head angle will be slightly more stable at speed, but have slightly more steering flop at low speed; the lower BB will decrease ground clearance with the same length cranks, but maybe the designer was assuming a smaller bike will have shorter cranks.)

[crossjunkee]

I'm not sure where the idea of stack/reach ratio having any meaning when it comes to bike fit.

The location of the handlebars will also depend on the length & angle of the stem, and an spacers on the steerer. Although the frames you show might have the ratio of stack and reach, with the same stem and spacers, the smaller frame on the left will place the handlars lower (by 24mm) and futher back (by 6mm) than the frame on the left.

It is true that frames with different stack and reach can actually have similar effective size and fit. But it isn't the ratio of stack and reach that determines that. The actual comparison is more like this: The stack and reach are measured from the top of the head tube, so imagine a frame with an extendable head tube. As the head tube extends upward, because of the angle of the head tube, the top of the head tube also moves backward. So as the head tube extends, the stack increases and the reach decreases.

For common head tube angles, the reach will decrease by about 0.3 (30%) of the stack increase. So, for example, if you had frame with a 540mm stack and 380mm and you added a 10mm of spacers, it would be equavalent in size to a frame with 550mm of stack (540mm +10mm) and 377mm of reach (380mm - 3mm).

But as I understand it, you're not interested in figuring out these two frames have the same equivalent fit, you want to actually change your fit by lowering your handlebars. In that case, you can look directly at the stack and reach numbers. Because the smaller frame has a 24mm short stack, then with the same stem and spacers it will lower the bars by 24mm. Because the smaller frame has a shorter reach, it will move the handlebars back by 6mm (that's typically less than the increment in stem lengths), so it comes close to the goal of just lowering the handlebars. It appears that the frame designer has also managed to keep the same front center, but that was done by slackening the head angle a little bit, and also lower the BB a little bit, so these are also things to consider. (The slacker head angle will be slightly more stable at speed, but have slightly more steering flop at low speed; the lower BB will decrease ground clearance with the same length cranks, but maybe the designer was assuming a smaller bike will have shorter cranks.)

Thank you, Mark McM. Good info. The geometry spec's are very close IMO. I don't think that much of a drop will be that noticeable. I could be wrong and will find in a few days.