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Fitting Thoughts – Serotta’s XY tool, Reach & Stack, and the A-D of Adjustment

Posted: February 18th, 2013 | Author: | Filed under: bicycles | No Comments »

I tend to be promiscuous with the bikes I ride and the equipment I use so accurately measuring my position and being able to reproduce position is important to me. Additionally, after a number of years away from regular riding I have once again ramped up the volume and in doing so have had to face the aches and pains and related position tweaks that come with riding a bike. As a consequence, I spend a lot of time fiddling with position and taking measurements. I want to share some of my observations.

1. Stack and Reach are great metrics. At a high level, it used to be relatively easy to get a bike your size because sizing methodology and geometry basics were very similar. That is not the case any more and as a consequence, the size of a bike, which may be 52, 54, etc. or S, M, etc., doesn’t tell you much. Stack and reach are measurements that tell you how tall the front of the frame sits and the reach measured from the centerline of the bb. A nice primer can be found on SlowTwitch. Looking at Stack and Reach numbers provides a very good and comparable metric for determining whether a given frame should fit. The metrics also show some funny frame design decisions. For example, the reach number for a Specialized SL3 Tarmac is the same when you move up from the 52cm frame to the 54cm frame. In other words, for a given seat height and setback from the bb, the reach will be same, the only difference will be the height of the bars.

2. The Serotta X-Y Tool is just ok. This tool is intended to be a relatively simple solution to reproducing bike fit numbers for ride to ride. Serotta decided to use the center of the saddle rails as the reference point for saddle height, which means that the numbers you get from the tool only apply to a given saddle, since the measurement from the center of the rails to the top of the saddle, you know where you butt connects, will vary from model to model and no doubt within a given model. The build quality of the tool is appalling, especially given the exorbitant price. The levels aren’t properly fixed in the tool and they regularly fall out during use. Really. Additionally, the tool uses two very thin plastic discs, presumably as washers to facilitate movement, but the construction isn’t documented, so who really know. Generally, It isn’t obvious where you actually the X/Y measurements, since the “>” markings on the tool clearly aren’t the correct point. The instructions aren’t helpful either. For example, for measuring saddle height, the instruction is to “[r]ead and record the saddle rail x/y number as marked on the x/y tool” but again, it isn’t clear where to read that measurement. I measured the distance using a tape measure, with the origin being the center of the x/y tool at the bb axle and … no number on the tool matched the observed number. The best number was off by 2-3 mm, which is a lot in a world where 2-3 mm is the typical amount of movement. Don’t try calling Serotta either. The number on the instructions isn’t correct and goes to some guy at another company. I called SICI, the Serotta group that works on these tools and left a few messages. No word back.

3. Use neutral settings as a baseline. As an starting point, hard to go wrong with Goldilocks settings – flat saddle, an inch or two of drop from the top of the saddle to the bars, two to three inches of saddle setback.  I don’t get why there are so many professionally fitted bikes with saddle tilt, whether up or down, suggests that the bike in fact does not fit. If it is angled more than two degrees either way, there is probably something else wrong with your fit.

4. Use the Cunningham Fit Like a Glove measurements. Charlie Cunningham uses a simple four point methodology based on your contact points with a bicycle. After spending a lot of time taking different measurements, I end up coming back to these. They are bar drop from saddle, saddle height, saddle set back and “reach” from saddle to bar. I add one more, which is saddle height measured through the center of the effective saddle rails, which is helpful for producing comparable saddle heights in a saddle and seat angle agnostic way. A pdf showing the measurements follows.

Cunningham Fits-Like-A-Glove

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