# "V-ish" vs "U" shaped rims



## Lelandjt (Sep 11, 2008)

A couple years ago when aero rims went from the classic V to a curvier airfoil shape I got Farsports' 38mm version of this new shape and like them. Now I need a new wheelset and was just gonna get the same rim but I'm curious about their new U shaped offering. Do you think this will have less drag in no wind? Less drag at yaw angles or get less knocked around by crosswinds? They claim the same weight. Which would you get? I'm gonna do 38mm front, 50mm rear so here's cross sections:







vs








and







vs


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## MMsRepBike (Apr 1, 2014)

I'd go with the U shaped rims.

I don't know much of anything about the brand but as a general consensus in the industry the V shape is the old way of doing things. Once they got really serious in wind tunnel testing and such they found the U shape to be better. Now everyone's adopting the U shape more or less. Usually the U shape rims are being made wider too which is a benefit.

I think there's a lot more to it then V vs. U. Subtle design characteristics can make a big difference. But generally speaking I think the U is the winner.

I'm a lightweight rider and I can get thrown around pretty bad by the wind. The max I'd run up front is the 48mm one I have now, I would never go deeper. For the rear I don't think it really matters too much. I think a 38mm up front is a good choice.


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## coachboyd (Jan 13, 2008)

Actually, the full U shape is actually now the old style, and rim shapes are modifying a bit past that. A lot of the places that try to copy and paste rim shapes are now coming out with big bulbous U shaped rims. . .but new shapes are being developed that are shown to be faster than going that bulbous.

I know that rims from Enve, Bontrager, November, and ourself have rim shapes where the widest part of the rim is kept more towards the brake track instead of way down the rim. . .and it's proving to be faster than making a super U shaped rim.


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## looigi (Nov 24, 2010)

With a V shaped rim, the front of the wheel with the tire on it makes a nice streamline airfoil shape...but...the back half of the wheel has the sharp edge forward, so makes a backwards airfoil. The blunt inner edge of the U shape provides a better compromise drag-wise and also makes the front wheel less susceptible to being deflected by crosswinds.


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## woodys737 (Dec 31, 2005)

coachboyd said:


> Actually, the full U shape is actually now the old style, and rim shapes are modifying a bit past that. A lot of the places that try to copy and paste rim shapes are now coming out with big bulbous U shaped rims. . .but new shapes are being developed that are shown to be faster than going that bulbous.
> 
> I know that rims from Enve, Bontrager, November, and ourself have rim shapes where the widest part of the rim is kept more towards the brake track instead of way down the rim. . .and it's proving to be faster than making a super U shaped rim.


Is the wide part is where it is due to tire/rim aerodynamic relationship? If so care needs to be taken to select a tire that works well with the wheel. Crr v. aero starts to get confusing for those that care about such things. It's been fascinating to read what Tom A over on slowtwitch is doing in this regard...thoughts?


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## MMsRepBike (Apr 1, 2014)

woodys737 said:


> Is the wide part is where it is due to tire/rim aerodynamic relationship? If so care needs to be taken to select a tire that works well with the wheel. Crr v. aero starts to get confusing for those that care about such things. It's been fascinating to read what Tom A over on slowtwitch is doing in this regard...thoughts?


On this note, and especially with disc brake rims, we should be using those fairings that go between the tire and the rim interface to smooth things out. You know, the things that the UCI banned that Mavic and probably others were making. The UCI has no jurisdiction on my country roads so us mortals should have the banned stuff.


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## Bluechip (Feb 19, 2004)

My wife has an older set of 50mm Boyds (narrow) and I have a brand new (less than 200 miles) set of wheels I built with the Farsports 58x28mm U shaped rims. I never noticed any crosswind problems on the Boyds. Just a light tugging feel with strong gusts. The super wide Farsports seem to have a slightly stronger tug. I'm 200 lbs so it's not a problem. They are slightly deeper but not by much. I can't claim any aero differences. The Farsports are fit with 25mm GP4000sII's and are nearly flush with the rim. My wife's Boyds are also fit with the same tires but the tire is considerably wider than the rim.


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## coachboyd (Jan 13, 2008)

woodys737 said:


> Is the wide part is where it is due to tire/rim aerodynamic relationship? If so care needs to be taken to select a tire that works well with the wheel. Crr v. aero starts to get confusing for those that care about such things. It's been fascinating to read what Tom A over on slowtwitch is doing in this regard...thoughts?


Yeah, tire to rim interface is the biggest thing in terms of aerodynamics. A lot of people are running the Conti GP4000 in the wind tunnel because it shows good results. If you switched over and used a Vittoria the drag graphs would look completely different.

Brand new tires actually test way faster at higher yaw angles compared to a tire with just a little bit of mileage on it. From 0-10 degrees yaw, the graphs were identical, but then the slightly used tire got WAY slower.

I have not tested the Mavic system with the blades in place as I don't know anybody who has them to borrow. . .and I'm not that into buying a set of wheels to test once. I do know plotting in how the tire to rim interface affects air flow (especially in the gap between tire and rim) was one of the things we spent the most amount of time in in the CFD analysis part of the new rim design.


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