I'm looking to buy a pair of Mavic Cosmic Carbone SLE or Mavic Ksyrium SLR. The Cosmic's are 50mm height, Ksyrium 25mm.
I'll only be using the wheel for racing.
How do they perform in cross wind. I'm thinking about performance (watts used) and also handling, can I control the bike in a group.
My simple logic tells me that the smaller rim, would of course be best choice in side wind. But then again, I suspect that the aero wheels perhaps might not just have been made for still winds?
I'll start by saying that finding solid data on wheel aerodynamics is tricky at best. Some of the better sources are worth taking the time to read in full, and provide much more information than I can provide here. For example, see the article from Tour Magazine [1] that covered the Mavic Cosmic Carbone SLR (similar, but not the same as what you asked about). Also see the testing done by Roues Artisanales [2] where he reviews many wheels including some older Mavic Ksyrium models. Of course, tests such as these are expensive and time consuming, so we typically can't find wide ranging, scientific test data across all of the current model wheels.
Having said this, there is also a lot of rider data that can guide you as to the ride-ability of these wheels. For example, on the weight weenie forums [3] SpinnerTim said that the Mavic CCs "have strictly traditional deep-rim stability issues in cross winds. These ride the way deep rims like mid-decade 404s or Lightweight Standards rode when V-sections were the rule...". In short, the CC SLE will provide some aero benefits from the front, but isn't the best in cross winds.
The Ksyrium is kind of the opposite. It isn't a particularly aero wheel, but will also have pretty normal / average performance with cross winds because there isn't much there to catch the wind anyway.
For improved aerodynamics at various wind angles, I would encourage you to look at some of the more modern wheel shapes. Things like the Smart ENVE System or the Zipp Firecrest. Even the Mavic CC 40C is likely to provide better cross wind handling than the SLE [4,5].
[1] http://www.tour-magazin.de/services/qtr/epaper_4_2011/page67.html#/96
[2] http://www.rouesartisanales.com/article-15505311.html
[3] http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=109139
[4] http://cyclecraft.wordpress.com/2013/05/23/tested-mavic-cc40-carbon-clincher-wheelset/
[5] http://www.bikerumor.com/2013/05/06/first-impressions-early-miles-on-the-mavic-cosmic-carbone-40-c/
