Sunday, August 24, 2014

fitting a Cervelo demo R5?

On a recent ride, I found a shop local to the day job, Cupertino Cycles, had a Cervelo Demo R5. Cool! Since reading the RCa White Paper, I've always wanted to try one. Indeed, I've never ridden any of the Cervelo R-series.

Recall, perhaps, the Peloton magazine review of the RCa, the prototype which led to the present R5:

There is a fury to the way the bike reacts to power -- it leaps from under you, but the feeling continues beyond the initial acceleration. Each pedal stroke delivers a new surge forward.

Who wouldn't want to ride this?

Deal is the Cervelo geometry is, as I've described, a bit tall. So it isn't obvious what frame to try. But is low-stack over-rates?

So what I did is I started with the geometry for the Fuji SL/1, which is my race bike (since I've done little racing recently, the poor thing is getting neglected).

I started with the geometry of the size small, which I ride. I use a +6 degree 11 cm stem at present (I used to use a lower position but found that the lower bars didn't actually result in a flatter back, especially since I don't use shallow bars, but instead just caused my shoulders to roll forward, which causes back strain, so I flipped the stem). In conjunction with the 71 degree head tube, this results in an upward 25 degree slope for 11 cm on the stem, which results in the adjusted reach-stack coordinates shown in the plot below. I have 1.0 cm of spacers on top of a low-stack headset cap as well, not shown in the plot:


The original Fuji stack-reach is shown with the "x" on the open black circle toward the lower left, then the result of the stem is shown with a "*" toward the right.

Note this is not a relaxed position: I still have 9 cm of drop, which is plenty. Really the frame is by most standards "too small" for me. I was lured to the smaller frame by the lower mass and by the possibility for lower bar position. But I overestimated how much drop I really wanted. I'd have been better off with the next size frame as it would have bought me a steeper head tube, even if it had been slightly heavier.

From this point, I want to "undo" the effect of various stems to find the frame stack-reach coordinates which would give me the same position with various stem options. I picked stems from 9 cm to 13 cm long, with angles of either +6 deg, -6 deg, or -17 deg. Note the spacing of stem angles is 12 deg and 11 deg, so they are relatively equally spaced. The target for these stem angles were the Cervelo 48 cm, 51 cm, and 54 cm R5. They have head tube angles of 70.5 degrees (this is how they manage to get such short reach on the small frame -- super-slack head tube, then compensate trail with a long-rake fork), 72.2 degrees, and 73.1 degrees. In general, I consider a 73 degree head tube to be superior to a 71 degree head tube, since it results in less wheel flop: the wheel is less prone to flopping over when the bike is tilted, resulting in better cross-wind control. But I've never done controlled experiments to verify this.

Note if the trajectory from the Fuji-with-stem position passes over a frame size, spacers can be added to the frame to bring it up to that trajectory. Additionally if the trajectory falls under a frame size I could remove up to 1 cm of spacers to bring the fit down. There's blue dotted lines on the plot showing what points are accessible by adding spacers, with roughly orthogonal dotted blue lines marking 1 cm intervals. These lines are drawn for a 73 deg head tube, so would apply best to the 54 cm Cervelo, but would also be a good approximation for the head tubes of the other frames, which differ by at most 2.5 degrees.

Fit coordinates I don't consider to be an exact science, and all I need to do is get close. With this in mind, the 48 cm frame could fit with a 13 cm stem at +6 degrees. That's obviously excessive. The 51 cm stem would fit with an 11.5 cm stem at -6 degrees. That's acceptable, not too far from the Fuji except with the stem flipped. And the 54 cm Cervelo would fit with a -17 degree 10 cm stem. That might actually be the best option since it would give the steepest head tube angle and also a shorter stem which would stiffen the front end.

In any case, the shop doesn't have a 51 cm demo, so I'd need to pick the 48 or 54. Given the choices, a 54 would be the obvious pick. However, I doubt they'll have a -17 degree stem available for a test ride. I can check, however.

Also on the plot I show Cannondale and Trek Madone H1, although the stem adjustments aren't drawn for the head tube angles on these frames. I could fit each of these as well, with shorter stems.

Consider, for example, the Evo geometries:

The 52 cm size with a zero degree stem looks like it would work, or a -6 degree stem with some spacers. The 54 cm size could also work fine: I could remove 5 mm of my 10 mm spacer stack, perhaps removing all of them with a headset cap 5 mm taller, and get the same bar position, using a -6 deg stem. There's a small error in these projections due to the head tube angle difference.

This plot shows an alternate route to the same result. Here instead of starting with the target position, I start with each of the three Cervelos. I then extend the various stems away from these positions.


The 13 cm +6 deg stem from the 48 cm Cervelo scores a virtual direct hit on the position I presently have with the Fuji. The 11.5 cm -6 deg stem from the 51 and the 10 cm stem -17 deg stem from the 54 come in within 1 cm of that position, low. This is easily rectified with an extra 1 cm of spacer: no problem.

So again, the big stack Cervelos work. However if I could choose a Cervelo with the Trek H1 geometry I'd probably take that instead. It would shave several cm from these stem lengths. The 54 cm frame (3rd in the series) would provide plenty of options, using either a -6 deg stem with spacers, or a 0-deg stem with fewer spacers.

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