The equation would be a geometry one. You could probably work all that out (Sin, Cos etc) if you sketched it out but there are online calulators to do it for you:

http://yojimg.net/bike/web_tools/stem.php

eta you probably meant the spacers part of it so maybe this:

https://www.slowtwitch.com/...ulator/stem_calc.php

10mm down on the steerer = ~3mm forward

Online calcs as linked above are good for comparing spacer height, but not if you change the stem angle as the commonly used formula has a fundamental error.

^^^This.

You are moving the stem along the angle of the headtube (around 73deg for a road bike).

Cos(73deg) = 0.292, so for every 10mm spacer you remove the stem will move ~3mm forward (it will also move down about 9.5mm, since Sin(73deg) = 0.956 ).

---

ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Won’t your site give me the info he needs???

https://www.velogicfit.com/frame-comparison

Yes, but I didn't want to go from saying that all the basic online calculators and formulae are wrong to then say that my site doesn't have that problem and is better than everyone else. Obviously I've tried hard to make it so, but I try to maintain a modicum of modesty ;-)

this might help: http://fft.tips/stem but if it doesn't at least we tried ;)

Sounds like you should really leave the bike fit to a professional.

Moving the bars up and down does slightly affect the for and aft of the bars, but if you want to change the reach then change the stem length. and leave the stack height adjustment to your stem spacers of the stem angle.

Most people can only make minimal if any adjustments to the stack height using spacers. Be very wary of not having enough steerer tube in your stem and stem clamp.

Thanks for the insight! I've been trying to get further back on my Pro Stealth saddle and with a 0SB post and the saddle slammed forward, I had to lower the seat and was able to find a pretty good position. Need more time on it but the saddle felt more comfortable and my hips and knees felt good. But my reach and bar height felt really thrown off.

I've actually got a lot of room when it comes to spacers. Right now on my CAAD10 I've got a 25mm dust cap and then I think 20mm of spacers under and 10mm over the stem.

Dude your app is awesome. I built one that's more fit focused, if anyone wants to try it out. It can draw you, on your bike and figure out your angles for you. It's bikefitr.net if anyone is interested.

---

See your bike fit. https://bikefitr.appspot.com/

Unless I’m reading this incorrectly - lowering your stem will move the handle bar away from you.
Do you have a flippable stem?

Yes, I’m stupid. I do have a flippable stem but I can’t stand the look of an upturned stem. I’m going to try a 2cm shorter stem with the same stack height and adjust from there.