daihard 

This is a thought process that I'd like to run with the BikeForum people. I hope I'm correct here...

Let's say you have a riser dropbar on your bike. It has a 15 mm rise.

Assumption: Your bike has a head tube angle of 71.5 degrees.

1. You replace the riser bar with a conventional dropbar with no rise. Let's assume everything else is the same (i.e. reach, drop, width, etc).
2. You lower your stem by 15 mm, either by flipping the stem or removing the spacers.

My observation is that those two scenarios yield different results in terms of the effect they have on the handlebar height and reach.

1. It lowers the handlebar height by 15 mm but has no effect on the reach.
2. It lowers the handlebar height by 14.2 mm (15 x sin 71.5 deg) and increases the reach by 4.7 mm (15 x cosine 71.5 deg)

It is because the change in bar height (i.e. riser vs conventional) is purely vertical whereas the change in stem height occurs parallel to the head tube.

Am I correct? Or am I missing something?

Eric F 

The rotation/angle of the handlebar would affect the result. Most often, drop bars are rotated up from horizontal (using the bottom of the drops as a reference) to some degree when installed, which would change the effective direction of the rise away from purely vertical.

In the real world, fractions of millimeters difference probably won't be noticed.

tFUnK

Depends on how you define "reach". The standard definition of "reach" in the context of frame geometry is defined as the horizontal distance from the BB to the top of the head tube, so it's not dependent on your bar height. If you define reach as purely the horizontal distance between some point of reference on the frame/seatpost to some point of reference on the top of the handlebar then you are correct. If you are defining reach as the actual distance between some reference on the frame/seat (eg, the front of the saddle) to the top of the shifter hoods, then this distance will also be changed when going from a 15mm riser bar to a 0mm riser bar, but not equal to reducing 15mm of steerer spacers while keeping the same handlebar, for the reason you described. You can calculate this if you know the saddle-bar drop distance.

Crankycrank

Use this stem comparison tool to give you height and reach numbers to get the measurements you want. Stem Comparison Tool | yojimg.net Great for showing how flipping the stem creates different reach numbers when trying to get the same height with different spacers.

abdon 

Kind of a moot point all around. I mean yes; it will change things somewhat, but we seem to be assuming that besides that 15mm difference every other geometry was 100.000000% correct and any changes would be noticeable. There is a good chance it won't be noticeable. There is an even better chance that the geometry was not indeed 100.000000% correct so it may end up better, or going in the opposite direction and still not be noticeable.

Iride01 

If you replace the bars, then what is the reach for the bars you will replace them with? Different drop bars have different reach. So if you go that route, then that'll make a difference. Otherwise you've got the gist of it. Although lowering your bars might have you wanting to do other things to tweak the position.

daihard 

Very insightful! Indeed I was referring to "raech" loosely as the horizontal distance between a certain point of the hoods and the front tip of the saddle, which is different than the reach used in frame geometry. And you're right, if we talk about the the distance between the front tip of the saddle and the TOP of the hoods, then yes, that distance will change because of the lower position of the hoods. Given the geometric reach of my bike, I figure the difference will be around 0.25 mm, which is insignificant.

daihard 

In this scenario, I assumed the other geometric values of the two bars, such as reach, height and width, were the same. As you mentioned, different reaches can make a huge difference.