Stem Chart Tool

I've had a few chances recently where I've needed to figure out the right stem to buy for a bicycle.  In the past I've always drawn out the existing stem on graph paper and then marked where I'd like my handlebars to be and used that to figure out what new stem would work.  Here is an example:

In this case I was trying to figure out the right stem for my RB-T.  I had a 9cm stem on there with a 96 degree angle.  I wanted 2-3cm more rise and about 1cm more reach.  So I graphed out the existing stem, then drew a point 2.5cm higher and 1cm farther out and figured out what would work.  It turned out that an 11cm stem with a 17 degree angle worked well.  The line shows the top of the headset, in both cases I used 3cm of spacers.

This works great, but computers can make things easier.  I also wanted to figure out how to generate graphics on the fly from a webpage, so I made a new tool.  Here is the same thing done with that tool:

 

It uses the word "spacers" but you can think "exposed quill" if you are working with quill stems.  The default page shows how one might replace a Technomic style quill stem with one that has a little rise to have less exposed quill:

You can find the tool here:

http://alex.phred.org/stemchart/Default.aspx

It shows off my normal terrible design skills, but gets the job done.

Published 28 November 2007 02:07 AM by AlexWetmore
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Comments

# James Moore said on 28 November, 2007 10:44 AM

Hi Alex, thanks for making this tool. I'm sure it will come in handy. I've linked to it from my brand new (like less than a week old) free bike tools site. I also linked to your great series on rack building. Thanks for taking the time to put all of this online!

# Jimmy Livengood said on 28 November, 2007 12:17 PM

Nice! Is there one for head angle/fork geometry that exists? There are probably plenty of people like me who have an existing bike/frame and are considering a new fork. I have a couple of CAD type programs that I can draw it all in, but a little tool like your stem tool would be neat. And probably a lot of work, too.

# Rex Kerr said on 28 November, 2007 01:55 PM

I was recently in the market for a new stem to fit a bike for somebody else and created an Excel spreadsheet for the same thing.  I neglected to consider spacers since I usually end up working with pre-cut steerers and almost never want to go down, so mine just calculates the difference in height/reach when given head angle and stem length/angle.  I like the graphical approach though.

# AlexWetmore said on 28 November, 2007 02:02 PM

Jimmy -- www.kogswell.com/trail.php is what you want.

Rex -- Spacers can be important to consider if you are changing to a stem with more rise.  The first graph shown shows that my stem goes up by 26mm, but I only wanted about 20mm of rise.  So lowering the spacers by 6mm brings me right to the correct location.

# Jimmy said on 28 November, 2007 02:47 PM

Thanks! A quick google after I asked the question also turned up this:

www.kreuzotter.de/.../elenk.htm

Also, this one, which seems to be for motorcycles, but should work anyhows:

www.papkegroup.com/.../superbikesupensionkinematics1.html

This one is from some recumbent site, and includes provisions for approximate center of gravity, etc. Haven't played with it yet to see how it actually works out for upright bikes.

www.recumbents.com/.../trail.asp

# AlexWetmore said on 28 November, 2007 03:21 PM

BikeCAD (google it, I don't remember the website) is also great for messing around with this stuff.  It'll let you draw out an entire frame and can determine trail and other geometry for you.  You can also offset the handlebars from the bottom bracket and it will compute the right stem to use.  There is a free version that is good.

# Greg Weiner said on 08 February, 2008 05:14 PM
Awesome tool Alex! Using your tool I can see that a Nitto Periscopa will place the bars exactly where a Salsa P7 would. Since those aren't made any longer and go for big bucks on eBay I'm glad to see the inexpensive (yet high quality) Periscopa will fit the bill. Thanks for the time and effort it took to put it together.