In the coming months I will be doing more triathlon bike fitting and have been doing more studying on brand differences and sizing.
One item of interest to me has been the standardization of frame stack and reach to quantify frame size and future bike decisions.
I have used bar stack and reach to consistently set up my singlespeed mountain bikes for years and am going to start using frame stack and reach measures more often for my fit folks when it is time to ask the question, "Will my new bike fit?"
Firstly, what the hell is frame stack and reach?
The horizontal and vertical relationship between 2 points:
1. The center of the bottom bracket.
2. The center of the top of the head tube.
Take a look over this Velo News article for a primer - http://velonews.competitor.com/2012/05/bikes-and-tech/technical-faq/technical-faq-setting-your-bikes-up-identically_216035
X-Y (Stack & Reach) measurements of the center of the handlebar and the top of the head tube from the center of the bottom bracket. Mike Reisel illustration
The pic illustrates frame AND bar reach/stack but this post is concerned only with frame stack/reach.
The bigger question: How do I measure this at home?!
As you can see in the article...it is doable but takes some special equipment (I use a self leveling laser at the shop because it is so easy) or fiddling to keep it against a wall. The purpose of this post is to show how I do this at home with only a level and a tape measure. You will also need to use a triangle calculator app to obtain the reach measurement from the hypotenuse (diagonal BB to top of HT measurement). I like this site for calculations but any will work:
Time to get to work. Side note...this stuff is easier with a soft tape measure. It makes it easy to tell the zero mark is right in the center of your starting point. Just make sure to hold it taut when measuring so the flex of the tape doesn't throw you off.
1. Set your bike DEAD level with a 3 foot level between axle centers. Don't guess. A trainer is helpful here. By helpful I mean critical so don't lean it on a wall.
2. Measure the distance from center of BB to floor in mm. This is "d".
3. Measure the distance from the top center of the headtube to the floor. This is "e".
4. "e" - "d" = Frame stack. Easy. This will be "a"
5. Measure directly from center of bb to top center of headtube. This is "c". This is a diagonal measurement.
6. Plug "c" into the triangle calc app as the hypotenuse (diagonal) and "a" in as the frame stack or vertical dimension. Hit calculate and voila...there is "b" the frame reach.
*In the instance of a popular bike, a Cervelo P2 size 54, the stack "a" would be 512 mm and the diagonal "c" measurement BB to HT would be 661 mm. Calculated reach is 418mm.
This is pretty useful for any type of bike although be careful with your MTB. Rigid bikes are easy but front and rear suspension complicate matters once sag is thrown in. The ideal situation would be to have the rider mount the bike to sag it out then have a friend/understanding significant other get your measurements. Often a rigid and suspended bike will feel the same when both are set up right but have differing measurements.
Also be careful in comparing bikes with integrated vs. external headsets.
There is a good chance someone has already written this same thing but better. If this is the case... oh well. This is simply my experience and what I find to work well. It is simple, consistent, and requires minimal tools. Hope it helps.