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rims: box section vs. aero?(3 posts)

rims: box section vs. aero?stratslingr77
Mar 10, 2003 11:13 AM
Would somebody in the know please tell me the difference in ride/handling characteristics of a standard box-section rim and something a little more aero, specifically the Mavic CXP-33? I'm thinking about getting these for an extra set of wheels. I weigh 210 pounds and ride a steel frame. Will these wheels (with Chorus hubs) withstand my weight? I'm not a "weight weenie", by the way..like I said, I weigh 210 lbs....anyway, any thoughts or advice would be appreciated. Thanks!
re: rims: box section vs. aero?russw19
Mar 10, 2003 12:58 PM
Well, the easiest way to answer this is to stick within Mavic's own line and compare the rims they offer. Mavic offers 4 road rims in clincher designs and 1 tubular rim. I will eliminate one from my views right away and that's the CPX-22 because it's new and I have never seen one in person yet. However the Reflex (tubular rim) and the MA-3 (clincher) are both box section rims. Mavic calls these classic rims and they ride just like any other rims. The box section is simple and strong due to the double wall. It is also an eyeleted rim which adds strength to the spoke nipple holes. I would recommend everyone, no matter what weight, steer clear of non-eyeleted rims. The road vibration over time may cause the rim to crack. It was a common problem with Matrix ISO rims, which other than not having an eyelet, were very similar in design to the CPX-33's. Box section rims often use a bit thicker aluminum at the walls so they are stronger than aero rims. Usually this comes at a weight penalty, but in Mavic's line the CPX-33 and MA-3 are the same listed weights. The Open Pro is lighter than all the other clincher rims and is not quite a box rim, and not quite an aero rim, but in between. Aero or "A" shaped rims are stiffer rims. Think physics class and triangle shaped trusses. Box section rims are often more comfortable because they spread the rim load over a larger area. Some riders claim they can tell a difference in rims and how they ride, others can't.

But aerodynamics in rims is another story. I have read in the past on Zipp's website and from other wind tunnel testing that a rim has to have a wing profile to make it any more aerodynamic than a box rim. The number I seem to remember was that a rim has to be 3 times as high as it is wide to make any real aerodynamic impact. Otherwise, it's just an "A" shaped rim and is only stiffer, but not more aero. I don't own a wind tunnel, so I obviously can not verify that, but it's worth looking up for someone if they are curious. It does seem to make sense though, otherwise the tire itself is the leading edge and the turbulence off it will go where it goes unless the large aero rims clean up the profile of the airflow over the wheel.

I think if you are looking for a comfortable all day wheel, I would get a Mavic MA-3, but if you want a stiffer and more responsive wheel, I would build with the CPX-33. I have several Open Pro wheels, but they all have a "clicking" noise from the joints. It's common with that rim, and I think my size also has a lot to do with it. I am about the same size you are. As for Chorus hubs, they are fine and strong. I would recommend you use double butted spokes, but maybe not Revolutions. Just stick with 14/15 DB, and you may consider brass nipples instead of alloy. They will only cost you an extra 30 grams per wheel, although it's at the rim, but they are quite a bit stronger, and tend to hold true longer. If you want to offset some of that weight, do so with a slightly modified lacing pattern on your wheels. I would recommend you stay with 32 holes, but you may go 2 cross up front and 2 cross non-drive rear, 3 cross drive side. Or you could stay simple and stick with 3 cross the entire wheel. The 2 cross pattern means you can use a slightly shorter spoke, but it may only save you a few grams per wheel and may not be worth the time unless you are building your own wheels. It's a semi-tricky pattern, and if you are paying a shop to build them by the hour, stick 3 cross as it's a simpler pattern to build.

Hope that's what you were looking for...

Russ
General observationsKerry
Mar 10, 2003 5:19 PM
The higher profile rim will be heavier, typically stronger, and more sensitive to crosswinds. At your weight, you probably won't notice the latter issue much. A true aero rim (50-60mm depth) will be significantly more crosswind sensitive. Despite the claim in another post, you don't need eyelets to make a good rim - Velocity rims have a great reputation and don't have eyelets. Cheesy alloy rims should have eyelets, and there's nothing wrong with them on good rims too, but they are not required for quality or durability.