Wednesday, August 13, 2014

Swift Carbon, BMC SLR, Cervelo old and new: geometry comparison

Last time I noted that Adam Blythe when he switched from the BMC Pro Tour team to the NFTO continental team his stem went to something truly disturbing: a giant horizontal protrusion from the front of his otherwise very nice looking Swift Carbon frame. The obvious explanation is frame grometry: the BMC allowed a lower, longer position than the Swift. So I decided to check this.

Here's a plot of reach (x-axis) versus stack (y-axis) as reported by various frame brands (I need to be careful not to say "manufacturers"). I compare the BMC SL01 with the Swift Ultravox Ti, along with two favorite references, Cervelo old (2008) and new (2014).


Slight digression: Cervelo decided to take a fresh look at geometry back in those days by setting a seat tube angle of 73 degrees for all frames. They reasoned that the human body when it's shorter or taller is still optimized by the same seat tube angle: the desire for steeper seat tubes in smaller geometries is more driven by the desire to avoid foot-to-toe overlap. But skilled riders don't worry about foot-to-toe overlap, they decided: if there was contact, then it was at such slow speed that it wasn't a problem anyway (turning at such a radius at higher speed would result in a crash, overlap or no). Their point was it was reach which mattered, and while a steep seat tube would allow for a short top tube without overlap, the fit advantage of a shorter top tube was squandered when the seat needed to be jammed back on the rails. So they advocated for stack and reach being the two dimensions by which geometry should be primarily judged.

I like Cervelo: I don't always agree with them, but they are dogmatically engineering-driven: everything on their bike designs except for paint job is justified by sound functional reasoning. It's no coincidence it's on paint and graphics that they tend to go terribly, terribly wrong. But I digress.

Anyway, they complemented their R-series frame, which tended to follow a low trajectory in reach-stack, with an RS series, which allowed for a more "relaxed" position. But they subsequently determined that shorter riders benefitted most from the R-series geometry, while taller riders more from the RS-series, and so they redesigned their geometry to follow a straight line in reach-stack. At shorter reach, the stack was close to the original R-series. At longer reach, it became similar to the RS.

An additional big change they made was to go to longer rake forks and slacker head tubes on the smallest sizes. The combination of a slacker fork and more fork rake increases the front-center while allowing for the same trail. Thus the toe overlap problem is minimized while retaining responsive handling.

Another change: they increased the chainstay length from 399 mm to 405 mm, the minimum recommended by drivetrain manufacturers. With 10 or 11 speeds in the rear, the chain deflection angle becomes large the shorter the chainstay length. With 399 mm, there could potentially be chain rub against the big ring on the small-small cross-gear. The change was just 6 mm, but at some point the next mm makes a difference.

Anyway, digression over. The point of this post was Swift versus BMC.

Of the four bikes on the plot, the Cervelos offer the widest range of reach. It appears from the plot that the 2014 R5 has less than the 2008 geometry, but this is misleading: the angle between the coordinates of the top of the head tubes, which determines stack and reach, is 82.6 degrees between the two frames. That means if you added spacers on the old bike to bring the bars up to the same level on the new bike, with the same stem, the bar on the old bike would actually be further back.

For supporting long-and-low, however, both the BMC and the Swift are well ahead of the Cervelo 2014. Compared to each other, the Swift is lower at short reach, while the BMC is lower at around 40 cm reach, the longest the Swift offers. BMC's largest frame is a full cm longer than Swift's, but this isn't relevant to Adam, since he doesn't ride a large frame.

One thing is clear, however, which is that Swift Carbon followed Cervelo's "new" model of designing to stack and reach. However, they did so using a "lower and longer" trajectory. Those who find the Cervelos too upright might find a better fit with these bikes.

How to reconsile this with the photo of Adam's bike remains a mystery.

No comments: