Rims
 

So, getting ready to build up a new wheelset (it's a sickness, I know), and I was having some thoughts.

At what depth are there actual "aero" benefits?

The rims in contention are Velocity A23 with O/C rear, and Pacenti SL23. I know the finish on the Pacenti is nicer, but hear me out on the technical aspects.

Velocity A23: 23mm wide, 19mm deep. O/C rear (which I've built before with Aeroheads) makes for a very nicely tensioned rear wheel.

Pacenti SL23: 23.5mm wide, 27(?)mm deep. Depth will add some strength obviating a bit of the need to balance tension but it still won't be as balanced as the A23. Maybe some aero benefit as well.

The question; is there any "real" benefit at that depth? Width is comparable, so the tradeoff is the offset build vs 8mm of depth. If there's no actual benefit, my gut tells me that the offset build will probably be the better wheel.

What do You Think?

How fast will you ride? Average? Maximum?

I don't know. Very, very minor tradeoffs. Just trying to figure out, in the realm of measuring minutiae, which tradeoff will make the ever so slightly better wheel.

They'll both make really good wheels, of course.

My fast(ish) road rides with the club are around 20mph for 30 or so miles. Otherwise, depends on the day of the week.

That's the answer.

differing opinions on Minutiae can go on for many Pages Here ,, better any thing is just one ..

Sun CR 18 is a good value functional rim.

Yes, but isn't that why we have this forum?

I guess the bottom line is that if 27mm doesn't convey any more aero benefit than 19mm, then might as well run the offset rear.

Why do Hamsters have Exercise Wheels ?

You see aero benefits, but they depend on the width of the mounted tire, the amount of turbulence in the surrounding air, and most important the speed involved. IMO it's borderline meaningless to even think about aero at speeds below 20mph, and real benefits don't accrue until nearer 30mph in non-turbulant conditions.

So if you're a breakaway rider fighting not th get chased down, every bit helps. But if you're in the middle of the peloton, even at 30mph+, the aero differences won't be as meaningful.

Personally, I hate deep section rims because I ride in areas with gusty winds, and don't like the sail effect of deeper rims in crosswinds. So, if it were me (who rarely rides 30mph under power) I'd opt for the shallower profile, but this is your bike, so you'll have to toss your own quarter, and call it in the air.

I believe that for the speeds you ride and the shallowness of the 27mm "aero" rim you are considering, you will get no aero benefits at all. Therefore I would pick the offset one which you have found to have actual benefits.

+1. I'd go with the OC.

I don't know what that means.

Even 25mm can be significant. You just need to stay away from "bad" rims like the venerable Open Pro to have improved aerodynamics, especially at a 0 degree yaw angle. A23s are almost a box section rim.

From FLO's wind tunnel testing Note that 30, 60, 90mm deep rims with aerodynamic aspirations start out about the same pointing into the wind.
http://bikeforums.net/attachment.php...hmentid=450710

Yes. You'll get better aerodynamics and a rim that's harder to bend on potholes. Tension won't be as balanced, although you can usually go higher on deeper rims, and if you use thin enough spokes at least on the non-drive-side even that doesn't matter.

Those rims are close enough in shape that I would be shocked if you ever noticed any real difference in aero benefits so I would go with the nicely tensioned OC rear option instead.

Assuming equal quality I'd go for the lighter one.
DT Swiss has a newer OC rim. I have not tried to compare it to the other rims mentioned, but I'd guess it to be reasonably light. BTW, I've had excellent service from my old Open Pro rims

Thanks for all the input.

I'm going to go with both. I'll re-build a currently little-used wheel with A23, and also build up a Pacenti SL23. That should also match up the brake track widths close enough so that they'll be interchangeable as spares.

Aero is a far more important than weight on almost all terrain, and at almost all normal riding speeds - air resistance limits your speed far more than any other factors, even when riding relatively slowly. The most aero rim that I know of that is available as rim-only and has an aluminum brake track is the Flo 30: FLO Cycling - FLO 30 Rim It has actually been designed using CFD and wind-tunnel testing, most other deep alu' rims have just been given a triangular form that the designer hopes is more aero but has never actually been tested.

The two downsides of the Flo 30 is that it is quite heavy, 570 grams, but again the aero advantage more than offsets this, and it's not available in an off-center / asymmetric version, but the extra mass in the rim makes it quite easy to build with. The 30 mm depth will give no problems in gusty crosswinds, you have to go up to around 50mm before that starts to be a problem.

I've also built a couple of the new DT Swiss RR 440 asymmetric rims, which IMO are the best option for something lightweight and asymmetric, the tension on the NDS was up to 70% of that on the DS even when building with a 130mm road hub with 11-speed freehub body. However, the rim is much narrower and is less aero than the Flo.

Years ago I read something on Zipp's website stating that aero rims really don't matter much at depths below 38 mm (data from wind tunnel testing). I went with 28 spoke count hubs - already had 'em - Stan's Alpha 340 rims, and Sapim CX ray spokes. I used a Park tension meter, well worth it if you want to build your own wheels. I am very happy with the results, I truly can feel the difference accelerating and at speed. I weigh 160lbs. or so, I would go with an even lower spoke count if I was getting new hubs.

Quote Originally Posted by Banzai View Post
. . . depends . . .

for me the answer would not be depends but yes...... my question would be, how many people would notice it? :rolleyes:

I think that was before their Firecrest shape.

Yes, I posted before looking at the Flo wind tunnel graph. Pretty impressive.