Stack/Geometry Question

[JasonF]

My Vamoots has a stack of 557mm. This measurement, plus 2cm of spacers and a -6 degree stem is really comfortable for me. In fact, I'll probably run less spacers as the weather gets warmer and my saddle time increases.

I'm considering a frame that has a stack of 542mm. If using the same -6 degree stem, is it safe to assume that in order to get the bars to the same level as the Vamoots I'll need to use 3.5cm of spacers?

I know a newbie geo question, but just want to measure twice and pay once ;)

Thanks all.

[tuscanyswe]

That is the whole idea of stack n reach to easily be able to compare frames. But in my experience i wouldn't trust that measurement alone as it seem to be measured differently sometimes on some frames.

Even if measured exactly the same there are more variables, for instance i assume your vamoots has an external headset whilst the new frame may have an integrated one? Is that accounted for in your comparison? If not thats a good 3.2 cms extra gone missing.

Id never trust a single measurement alone on something as vital as size on a new frame.

[JasonF]

Thanks for the input.

New frame would be a King Cielo, so headset is external and you're right: a manufacturer's measurement could be off.

Effective top tube, standover, rake (trail) and angles seem to match up.

[tuscanyswe]

BB height on road frames and lengths of carbon road forks are fairly constant, they rarely differ a lot.

So to double check the stack:

The headtube length should be similar if the stack is similar providing the same headset solution is used on both frames. The small differences that may still be seen should be explained from small difference from the bottom bracket drop or small differences in lenth of the fork or fork angle.

Stack / reach is useful for knowing it will fit you but perhaps not so great for knowing exactly what to put on the frame in order for it to fit you well. Its simply doesn't tell enough info imo.

[SlackMan]

My Vamoots has a stack of 557mm. This measurement, plus 2cm of spacers and a -6 degree stem is really comfortable for me. In fact, I'll probably run less spacers as the weather gets warmer and my saddle time increases.

I'm considering a frame that has a stack of 542mm. If using the same -6 degree stem, is it safe to assume that in order to get the bars to the same level as the Vamoots I'll need to use 3.5cm of spacers?

I know a newbie geo question, but just want to measure twice and pay once ;)

Thanks all.

Unless I misunderstand your figures, I think you'll need more than 3.5cm of spacers. This is because angle of the headtube means that the additional 1.5cm of spacers will create less than 1.5cm in vertical height. I'm not sure I explained that well. Does that make sense to you? Try the website below:

http://www.bikegeo.net/

[Erik_A]

Or possibly just flip the stem.

[doomridesout]

Something to consider re: the Cielo. Is it one of the ones w/ a 44mm tapered headtube? If it's the same 1 1/8" external King HS on both, no worries, it's apples to apples. If the Cielo is the road racer, the top bearing is quite a bit shorter than an external 1 1/8" top. I don't have stack heights in front of me, but there's another difference.

[Mark McM]

Unless I misunderstand your figures, I think you'll need more than 3.5cm of spacers. This is because angle of the headtube means that the additional 1.5cm of spacers will create less than 1.5cm in vertical height.

While this is technically true, the actual deviation is fairly small. For a nominal 73 degree steerer angle, the height increase is 96% of the spacer thickness. So, an extra 1.5 cm of spacers will increase the stack by 1.43 cm - the difference is less than 0.1 cm.

[Mark McM]

I'm considering a frame that has a stack of 542mm. If using the same -6 degree stem, is it safe to assume that in order to get the bars to the same level as the Vamoots I'll need to use 3.5cm of spacers?

As noted in my previous post, yes, adding 1.5 cm of spacers will get you within 0.1 cm of the previous height.

However, you may also want to consider the effect of spacer height to the relative Reach dimension, as well. As the stem/handlebars is raised by spacers, the stem/handlebar is also moved rearward, due the angle of the steerer tube. For a nominal 73 degree steerer, the stem/handlebar is moved rearward by 0.3 cm for every 1 cm of spacer added. Adding 1.5 cm of spacers to the 542 cm Stack frame will increase the effective Stack by 1.43 cm, and also decrease the effective Reach by 0.45 cm. In addition to the spacers, you may need a slightly longer stem to create the exact same fit on the new frame.

[JasonF]

As noted in my previous post, yes, adding 1.5 cm of spacers will get you within 0.1 cm of the previous height.

However, you may also want to consider the effect of spacer height to the relative Reach dimension, as well. As the stem/handlebars is raised by spacers, the stem/handlebar is also moved rearward, due the angle of the steerer tube. For a nominal 73 degree steerer, the stem/handlebar is moved rearward by 0.3 cm for every 1 cm of spacer added. Adding 1.5 cm of spacers to the 542 cm Stack frame will increase the effective Stack by 1.43 cm, and also decrease the effective Reach by 0.45 cm. In addition to the spacers, you may need a slightly longer stem to create the exact same fit on the new frame.

Wow, excellent - thanks for all the replies!

Ironically, the reach on the two frames is identical (373mm) as is the head angle. Seat angle on the Vamoots is 74.25 vs 74 for the Cielo. Trail figures are the same using identical tires.

Only "x" factor is the axle-to-crown on the Cielo. My Moots has the Seven 5E standard reach fork, which has a generous axle-to-crown of 375mm. My guess is the Cielo isn't much longer but I'll have to check.

[drewellison]

What I recently realized in working with stack and reach, is that the same reach numbers with different stack numbers will give you a different handlebar position in relationship to the bb.

Assume you've got two frames and you want to set the rider position the same on the two. One from is 60 cm stack with a 40 cm reach, and the other is a 62 cm stack with the same 40 cm reach. You set your saddle positions the same height and setback from bb. From your butt's standpoint, there's no difference between the frames, other than the top tube is 2 cm closer to your torso on the 62 cm stack frame.

Now you set your handlebar drop the same on the two setups. Say, one handlebar is then raised 2 cm above the headset and the other is 4 cm above the headset (so the drop from saddle is the same). Since the headtube is angled, the 62 cm handlebar will be a longer reach from the saddle than the 60 cm stack bike. And the fits will not be the same.

To set up the same fit between the two frames, the taller stack frame will need a shorter reach.

[Mark McM]

What I recently realized in working with stack and reach, is that the same reach numbers with different stack numbers will give you a different handlebar position in relationship to the bb.

Assume you've got two frames and you want to set the rider position the same on the two. One from is 60 cm stack with a 40 cm reach, and the other is a 62 cm stack with the same 40 cm reach. You set your saddle positions the same height and setback from bb. From your butt's standpoint, there's no difference between the frames, other than the top tube is 2 cm closer to your torso on the 62 cm stack frame.

Now you set your handlebar drop the same on the two setups. Say, one handlebar is then raised 2 cm above the headset and the other is 4 cm above the headset (so the drop from saddle is the same). Since the headtube is angled, the 62 cm handlebar will be a longer reach from the saddle than the 60 cm stack bike. And the fits will not be the same.

To set up the same fit between the two frames, the taller stack frame will need a shorter reach.

Another curious thing about the Reach dimension is that for most size runs of a given model of frame, the Reach dimension changes only a little, even if the Top Tube length changes dramatically. Take for example a very common bike like the Trek Domane SLR 7 (http://www.trekbikes.com/us/en_US/bikes/road-bikes/performance-road/domane/domane-slr-7/p/1466000-2017/). Based on the geometry chart, the effective TT length varies from 51.9 cm on the smallest size (50 cm) to 59.3 cm on the largest size (62 cm), a difference of 8.2 cm. But the Reach dimensions for the smallest size is 36.8 cm, whereas on the largest size it is 38.6 cm - a difference of only 1.8 cm.

Most of the difference in the "cockpit" (saddle to handlebar) dimension is the difference in the setback, both because the larger frames have shallower seat tubes, and that even for the same seat tube angle, raising the saddle higher moves it rearward. But the seatback is by definition not included in the Reach dimension, so the Reach dimension only changes a small amount between frame sizes.