Rims - Flo vs. Gigantex

[vmajor]

...and just to clarify we do also make a "U" shaped which indeed works well at low yaw angles with 23mm tires

Isn't that the most important criteria to consider when designing a rim?

Yes, but by low I mean really low, under 6 degrees for example. Also when you add a 25mm tire to it things get worse from an aerodynamic performance perspective.

The biggest problem with generic U profile rims (not talking about Flo, Zipp or us here) is that they are effectively flat so the stall is sudden and violent, greatly affecting handling. Thus it is not just about outright aerodynamics in the most common conditions, but stability in gusts or turns in cross winds.

...but, these are my opinions and I did ask for others' opinions and ideas, so thank you I will look into U shaped rims some more as one benefit of that shape is that is delivers very good radial strength so they can be made lighter than rims with more curved sides.

V.

[Hexsense]

Thank you for your insight.
I'm using a rim with 27.25mm width (25mm at brake track) so if the tire's actual width is around 25mm it should be fine, i guess. Which would be 23c-24c tires from most brand as 25c tire will turn into 27mm width, mounted.
But that is mainly when tire is the leading edge against the wind right? on rear wheel which draft behind the frame using 25c (27mm actual width) shouldn't be too bad?
picture: https://drive.google.com/open?id=0B2FoQ3WMz9LbNlB3U1Z5UGdOTXVOM1RCUnBkY1B3eHJuRnVR

PS. Do you think it look similar to Zipp's Firecrest (their older version, third one in the picture) ?.

[NovemberDave]

It's hard to tell from a picture, especially because the pictures posted on seller sites are inaccurate or at least not very detailed. Generally speaking, though, the rim you have is of a generally 404 Firecrest-ish shape. Very early development of max width (away from brake track), and fairly slab sided. Yours seems a little less blunt than a 404 FC but you're getting into shape details that pictures don't show all that well.

The testing we've done at A@ in NC mostly agrees with vmajor. We tested the FC404 and the ENVE 3.4 front, both of which are particularly slab sided, both of which had a fair amount of side force that was far forward of the steering axis. But both were faster as you moved to higher yaw, relative to rims they were tested against.

Trek's Speed Concept development white paper is very detailed about prevalence of various yaw angles. On the Ironman AZ course (low wind) you are constantly at very low (<5*) yaw. On the windy IM HI course, you spend the way majority of the time at low yaw (under about 7.5*) but spend some time at VERY high yaw in a few very windy sections so the mean angle is 10.6 from memory. On the IM WI course (medium wind) you spend most of your time right at around 6*.

[alastairb]

Sorry, could someone define stall (in lay terms) for me please?

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[NovemberDave]

Sorry, could someone define stall (in lay terms) for me please?

Fantastic question.

The condition in which wheels move through the air fastest and with least resistance is called laminar flow. The air flows evenly and regularly over the surface of the wheels. If you imagine riding through smoke, the smoke would part evenly around your wheels and flow over them, and the disturbance of the smoke left in your wake would be small. In stall, the smoke would actually separate well before your wheels got to it, and the smoke would kind of swirl around your wheels as they went through it, and the smoke would be all jumbled and disorganized in your wake.

In sailboat sails, you attach small ribbons (called tell tales, or ticklers if you are from eastern Canada - they've got all sorts of funny sayings there) to your sail at various places, and you more or less try to make them flow along the sail, which indicates that the air is moving along the sail in an orderly and organized fashion. If the telltale points up or down, you know the air flow is stalled and you know you need to make a change to your sail's shape or make a steering correction to correct it.

The reason why golf balls are dimpled is that if they were smooth, they would create a bubble of air around themselves that would make the air think the ball is way bigger than it is (thus have more air drag, and go slower, and thus not as far). The dimples create a small disturbance right at the surface that prevents this big bubble from forming. That's a simplified version of that, and the same principle applies to the dimples on Zipps.

Flow is fast, stall is slow.

[alastairb]

Sorry, could someone define stall (in lay terms) for me please?

Fantastic question.

The condition in which wheels move through the air fastest and with least resistance is called laminar flow. The air flows evenly and regularly over the surface of the wheels. If you imagine riding through smoke, the smoke would part evenly around your wheels and flow over them, and the disturbance of the smoke left in your wake would be small. In stall, the smoke would actually separate well before your wheels got to it, and the smoke would kind of swirl around your wheels as they went through it, and the smoke would be all jumbled and disorganized in your wake.

In sailboat sails, you attach small ribbons (called tell tales, or ticklers if you are from eastern Canada - they've got all sorts of funny sayings there) to your sail at various places, and you more or less try to make them flow along the sail, which indicates that the air is moving along the sail in an orderly and organized fashion. If the telltale points up or down, you know the air flow is stalled and you know you need to make a change to your sail's shape or make a steering correction to correct it.

The reason why golf balls are dimpled is that if they were smooth, they would create a bubble of air around themselves that would make the air think the ball is way bigger than it is (thus have more air drag, and go slower, and thus not as far). The dimples create a small disturbance right at the surface that prevents this big bubble from forming. That's a simplified version of that, and the same principle applies to the dimples on Zipps.

Flow is fast, stall is slow.

Thanks NovemberDave.

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[mpulsiv]

Specialized claims their CLX32 is just as efficient as their previous deeper section rims. It's like 28mm wide, designed for a 25mm actual tire. But I agree, we need 32mm wide rims to go with our 28mm tires. It will happen.

The Chinese U shape rim did pretty good in the Tour test, Issue 8.

Yes, Specialized made a splash by showing that 32mm wheel is faster than 40mm wheel. Depth is no longer the pivot point, it's all about the rim profile. I'm still waiting for other manufacturers to release a competitive wheelset next to Enve SES 4.5 at 31mm wide! Why so wide, some of you may ask. Well, 2017 is around the corner and tires still sit wider than rims. For maximum aero, it must be the other way around.
http://enve.com/products/ses-4-5-ar


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[alastairb]

Is Specialized trying to demonstrate that a super wide rim is fast, or are they meeting demand for a rim that transitions smoothly with a wider tyre? Presumably, if you're happy to run 23mm tyres then a ~17mm internal rim width is just as fast, right?

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[NovemberDave]

Presumably, if you're happy to run 23mm tyres then a ~17mm internal rim width is just as fast, right?

Yup. Plus you wouldn't want to use a 23mm tire on a rim even as wide as 23mm internal. Tire shape would get wrong, and there would be real concerns about the security of tire/rim interface.

[MattN]

Your wording regarding golf balls doesn't read right to me, perhaps it's a language thing.

The dimples on a golf ball create a turbulent layer of air at the balls surface which helps to keep the air flow attached to the ball for longer than it would a smooth one.

Through keeping the air flow attached to the ball for longer it reduces the wake behind the ball, the result of which is less drag.

[vmajor]

For those interested in going deeper and following the various articles on aerodynamics, the "turbulent layer of air" is called "turbulent boundary layer" cf. "laminar boundary layer".

Turbulent boundary layer benefits the aerodynamic performance of "bluff bodies", a golf ball being an example of such, while laminar boundary layer is the usual objective for streamlined bodies, commonly referred to as airfoils.

Thus a quick "tip" when reading about aerodynamic performance of various bike products, look for dimples, trip lines, texture or whatever other device that is used to create a turbulent boundary layer. If a body is streamlined, creating a turbulent boundary layer generally does not result in decreased aerodynamic drag.

[NovemberDave]

Your wording regarding golf balls doesn't read right to me, perhaps it's a language thing.

The dimples on a golf ball create a turbulent layer of air at the balls surface which helps to keep the air flow attached to the ball for longer than it would a smooth one.

Through keeping the air flow attached to the ball for longer it reduces the wake behind the ball, the result of which is less drag.

Just trying to help people understand and visualize the concept, not trying to convey what happens with great precision. I've spent a lot of my career as the interface between end users/lay people and engineers/technical product people, and have found that the typical lay person's "gloss over, eyes roll back in the head point" is pretty early. A rough concept broadly understood, beats an exact evaluation that few people get, in my world. I've been yelled at for expressing aero drag in watts rather than CdA, for example. Who understands CdA well enough to have any visualization of how it affects their on-bike speed? A few people on the forums. Who can understand the less precise/less technically correct value of things in watts and put that into an "on bike" context? Almost everyone.

[MattN]

I wasn't having a dig, just the wording of a plain ball creating a bubble etc. may have confused some.

Good wealth of knowledge here.

[NovemberDave]

I wasn't having a dig, just the wording of a plain ball creating a bubble etc. may have confused some.

Good wealth of knowledge here.

10-4, thanks