Saturday, August 23, 2014

Cervelo geometry: R5Ca versus R5 and RCa

The old R5Ca, Cervelo's first version of a super-high-end stock bike ($10k for the frameset alone, and people, to my amazement, actually paid it...), used a relatively novel geometry whereby the downtube was offset forward of the bottom bracket. With the nominal 72 degree seat tube angle, this in effect gave a seat tube angle which was steeper for shorter bikes, more laid back for taller bikes. In this case by "shorter" and "taller" I mean bottom bracket to saddle.

With most manufacturers the same thing is accomplished by changing the actual seat tube angle from steeper in the smallest size (for example, 74.5 degrees) to more laid back (for example, 72.5 degrees) in the largest. But of course the effective STA doesn't then change with the amount of seat post showing.

Here's a plot I did:

image

But this was an experiment they didn't pursue. Maybe it caused too much confusion to dealers. I don't know...

In the present version of the R5, and the RCa prototype bike which preceded it, the seat tube appears to conventionally intersect the bottom bracket at its axis:

image

They've gone back to 73 degrees for all frame sizes.

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