Cervelo's "reach" concept

[LegendRider]

Cervelo advocates the concept of "reach" defined as the top tube length from a perpendicular line bisecting the bottom bracket to the head tube. Using this measurement you don't need to factor in the seat tube angle. For example, two bikes with 57cm top tubes will but different seat tube angles will fit differently. Cervelo's argument makes senses. Granted there are lots of other fit variables, but as for the top tube this concept of "reach" equalizes things. So, why isn't it the standard method of measuring top tubes? Will it ever become standard?

[Dave]

I asked Cervelo about their unusual geometry drawing and just got an answer today. Apparently Cervelo's use of forward-offset or curved seat tubes has resulted in a geometry chart that intends to show that all of their frames have approximately a 73 degree STA, by virtue of the location of the seat rail clamp, if a traditionally offset seatpost, like Campy or DuraAce is used.

http://www.cervelo.com/bikes/2005/R25-Team.html#Geometry

I initially saw no advantage to the idea of a measurement taken from the center of the BB to the center of the HT/TT intersection, until I took the time to draw an exaggerated graphical analysis using 85 and 65 degree STAs. What his revealed, is that for a given saddle position, relative to the bottom bracket, the Cervelo "reach" dimension tells you exactly the difference in horizontal reach between each frame size, or in other words how much difference in stem length is required to produce the same fit.

What is almost universally overlooked in figuring "reach" is the height of the handlebars, relative to the saddle. A rider who only has a 5cm height difference from the saddle to the handlebars might use a 120mm stem, while another rider with the identical frame and saddle height, but an 8cm drop to the bars might use a 110mm stem. Both bikes have essentially the same horizontal reach.

Another thing that is often ignored is the saddle setback relative to the BB. Rider of identical size and proportions on identical frames could choose different amount of saddle setback relative to the BB. Obviously, moving the saddle back will either stretch you out or require a shorter stem.

After looking at Cervelo's drawing again, I see that their "reach" dimension can be compared to any other brand of frame by taking the TT length of the other brand and subtracting the seat tube setback, if it's provided (like Colnago does) or the setback can be easily calculated. The seat tube setback is just the c-c frame size times the cosine of the STA.

[flydhest]

Dave,

It seems like Cervelo's thought might be as follows. Most people start off by getting the right placement of the saddle relative to the BB, that is butt-leg-foot-pedal connection right. If that distance is idiosyncratic to the rider, let that be the undefined part, but constant (presumably) for any given rider.

In the context of your comment, that it only matters if you also know about position relative to the BB, I think the opposite is their point. Set yourself up relative to the BB, and pick the bike that is the right length from the BB to the stem.

I do agree with you, though about the importance of the bars and such, including the drop and reach of the bars themselves.

Shortcuts are never as good as slogging all the way through all the dimensions to get the right fit.

[Dave]

I made a major revison to my original post regarding Cervelo's "reach" dimension. Drawing an accurate graphical analysis of frames with widely differing STAs showed that the reach dimension can be quite valuable. It's also pretty easy to use the concept on other frame brands, particularly those measured c-c.