Thoughts on C2 rims?
11-02-10, 06:00 PM
#51
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^^^
I see a pretty picture. Means almost nothing. A program like that is pretty much for customer presentation only. Maybe first cut design.
I see a pretty picture. Means almost nothing. A program like that is pretty much for customer presentation only. Maybe first cut design.
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"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
11-02-10, 06:04 PM
#52
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Edit: Although if you can run on multiple processors the CFD stuff isn't as bad. We used COBALT, and it was pretty good/easy to run on several processors.
11-02-10, 06:04 PM
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I don't. Crr, CdA, traction, and puncture resistance all get thrown into the decision process for the tire; CdA and how it handles crosswinds for the wheel.
That chart is a funny animal BTW, at least relating to it from a TT perspective; a 50mm Bontrager tubular is a pretty odd choice for a baseline wheel. And combined (multiple yaw angles?) vs. averaged aero drag would seem to be way biased towards CdA and fuzzy math. At 30 MPH if my reading is correct it claims around 10 aero watts improvement for that tire compared to some of the others. That doesn't jive with anything I've ever seen, but the smallest thing I've run in the tunnel was 85mm.
11-02-10, 06:06 PM
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That's up to the rider and gear selection if it's got a test dummy, but pretty much static speed if it doesn't. In our case we shot for a wattage and cadence within a few percentage points to make it as much apples to apples and to replicate a TT effort.
11-02-10, 06:07 PM
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https://forum.slowtwitch.com/gforum.c...ring=;#3054556
...Basically the firecrest concept places the widest part of the rim in the half closest to the spoke bed, and in section view, the spoke bed has been optimized to act as the leading edge of the wing, people have likened it to a Kammtail type concept in the rear half of the wheel. In terms of how it compares to the toroidal shape, it IS a toroidal shape. The beauty of this project was that we learned from the CFD that the front half of our wheel was highly, highly optimized, but the rear half had lots of room for improvement. In optimizing the rear half, we took about 70-80% of the drag off of the rear half of the wheel, while maintaining or even slightly improving the front half aero, but more importantly we reduced the side force in the front half of the wheel. This allowed us to generate a center of pressure that is located essentially at the steering axis. So there is a massive improvement in handling, as you still have the side force, but instead of the sideforce acting in front of the steering axis (generally in front of the hub with most designs) the wind force has no leverage on the wheel to generate steering torque...and the steering torque is a much bigger factor in stability and handling (as well as rider fatigue) than side force is. Keep your eyes open for some data posting, but essentially, Firecrest allows for significant drag reductions and the near elimination of wind induced steering torque (we also optimized the shedding behavior of the airflow off of the spoke bed in the front half of the wheel to eliminate aerodynamically induced harmonics! ...
11-02-10, 06:23 PM
#56
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11-02-10, 06:24 PM
#58
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Hey, I mean, who am I to argue with someone trying to sell me something? I'm just responding to what you are showing me. I don't know who Mr. Poertner is or why I should trust his analysis. Being that he seems to be associated with Zipp, he's not going to get into the anything proprietary with me anyhow.
I'm just saying you aren't going to go about modeling this stuff with your PC in a couple of hours. It's several months of design/analysis time on a high end computer to come up with anything close to the resolution a windtunnel can get you. You can do a little something on the computer to get an initial impression, fab it up, make some assumptions/WAGs, but all you get is order-of-magnitude behavior. Oh, and pretty pictures you can show your customers when the wheel goes on the market. In the end, you make a stab, you try your best to analyze and optimize it in the windtunnel, and then you let the marketing guys make the best they can out of it to try to convince someone to buy it. In the end, not even the guys designing the wheel know precisely how the air flows around it (windtunnel just tells you drag coefficients) and if this or that clever shape is optimum/better/worse in all situations. And very few people have the resources to spend time in a wind tunnel, so we are all kind of in the dark about this stuff.
Kind of fun actually. Like when the Wright brothers were trying to figure out how to get a glider to fly.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
11-02-10, 06:26 PM
#59
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Oh, and is the trispoke tubular or clincher. If clincher, I could send you another 19C Aerowing TT tire to run on it. You might be somewhat surprised by that combo.
11-02-10, 06:31 PM
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Sheesh...I point you guys to a veritable "mother lode" of interesting info and you make me spoon feed it to you?
From another post later in that same thread:
"The side force isn't so much lower, but it does remain nearly constant after stall...so after stall it is slightly lower. but the big story is the elimination (or nearly) of steering torque. When you think about it, there are 3 causes of aero induced instability. Side force, steering torque (which is the side force acting on a center or pressure that is some distance from the steering axis) and shedding frequency (vortex shedding off of the trailing edges of the rim). With FC, we have addressed all 3 in that the side force is lower after stall...so small gain. The steering torque is nearly eliminated as we purposely designed the center of pressure to be farther back than anything previously done (up until now nobody has ever even considered center or pressure). And using the Strouhal number and some really intricate CFD, we were able to push the shedding frequency off of the trailing edge of the front half of the wheel much, much higher than previously possible. Some designs have shedding frequencies in the 2-4Hz range, with toroidal rims generally being better in the 6-8 range. FC demonstrates frequencies in the 20+ Hz range...so considering that speed wobble is a phenomenon with 2-3Hz natural frequency...we look pretty good. not to mention the amplitudes at our frequency are so reduced that we are considering them to be negligible to non-existant."
11-02-10, 06:34 PM
#61
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A lot of what Brian said above. I'm not saying that they didn't run the analysis and that the analysis wasn't useful, but it's an engineering tool, and like most engineering tools, you get out what you put in. If you have someone who's good and puts together a well designed experiment you can get some really good information out of CFD, but it's not a magic button that tells you the best design by any means, and it's not always cheaper than just mocking stuff up and experimenting, although it does look better in marketing. I can absolutely vouch that coming up with a pretty picture doesn't mean that you've gotten meaningful results at all, since I've gotten results that don't make any sense with regard to fluid dynamics but have a pretty picture before.
11-02-10, 06:34 PM
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Hey, I mean, who am I to argue with someone trying to sell me something? I'm just responding to what you are showing me. I don't know who Mr. Poertner is or why I should trust his analysis. Being that he seems to be associated with Zipp, he's not going to get into the anything proprietary with me anyhow.
https://www.zipp.com/support/askjosh/askjosh.php
AKA "Zipp's Head of Engineering"
11-02-10, 06:36 PM
#63
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Wasn't in that post. And until I see their experimental method, setup, etc. I'm not considering it a mother lode of information.
11-02-10, 06:38 PM
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A lot of what Brian said above. I'm not saying that they didn't run the analysis and that the analysis wasn't useful, but it's an engineering tool, and like most engineering tools, you get out what you put in. If you have someone who's good and puts together a well designed experiment you can get some really good information out of CFD, but it's not a magic button that tells you the best design by any means, and it's not always cheaper than just mocking stuff up and experimenting, although it does look better in marketing. I can absolutely vouch that coming up with a pretty picture doesn't mean that you've gotten meaningful results at all, since I've gotten results that don't make any sense with regard to fluid dynamics but have a pretty picture before.
I'm confident that Zipp has the resources and the "smarts" to do it effectively based on what I've been privy to...on the other hand, I think a lot of other wheel manufacturers are more "seat of the pants" oriented...
11-02-10, 06:46 PM
#65
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I understand that, I'm just skeptical of taking claims at face value, since it's pretty easy to prove whatever you want (within reason) if you set up a study a certain way. Since I don't know how the studies are set up, I'm reserving judgment until I either look up how the study was done or see independent 3rd party data (let's be honest, no one cares why it's faster, we just care that it is).
11-02-10, 06:53 PM
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I understand that, I'm just skeptical of taking claims at face value, since it's pretty easy to prove whatever you want (within reason) if you set up a study a certain way. Since I don't know how the studies are set up, I'm reserving judgment until I either look up how the study was done or see independent 3rd party data (let's be honest, no one cares why it's faster, we just care that it is).
11-02-10, 06:54 PM
#67
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I am not in the habit of considering an internet forum of any sort a "motherlode" of information. Being that he's Zipp's "head of engineering", and he's on a public internet forum, most of what he is saying is, well, probably not bullsh** and intentionally misleading, but is very likely rosy colored and glossy.
The Strouhal number is simply a dimensionless parameter describing the frequency of vortex shedding, in the same way the Reynolds number represents dimensionless velocity. They likely used a pulsed or sinusoidal function for the velocity function in a simplified CFD model to ensure that, on a first order basis, the frequency of spoke passage didn't land near the natural vortex shedding frequency of the rim profile. Spoke passage will enforce a vortex shedding frequency, and if this lined up with the natural frequency of the rim profile, then the effect will be amplified leading to increased drag and buffeting in a crosswind. The shedding frequencies they scanned over would be expressed in terms of Strouhal number so they could relate it back to correlations coming out of the wind tunnel data.
The Strouhal number is simply a dimensionless parameter describing the frequency of vortex shedding, in the same way the Reynolds number represents dimensionless velocity. They likely used a pulsed or sinusoidal function for the velocity function in a simplified CFD model to ensure that, on a first order basis, the frequency of spoke passage didn't land near the natural vortex shedding frequency of the rim profile. Spoke passage will enforce a vortex shedding frequency, and if this lined up with the natural frequency of the rim profile, then the effect will be amplified leading to increased drag and buffeting in a crosswind. The shedding frequencies they scanned over would be expressed in terms of Strouhal number so they could relate it back to correlations coming out of the wind tunnel data.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
11-02-10, 06:54 PM
#68
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That might work...I could send you my narrow Jet90 with the Bonty Aerowing TT tire on it to run head to head against the C2 Jet 9. That would be the most "apples to apples" test. What tire would you run on the C2 Jet 9? Oh, and is the trispoke tubular or clincher. If clincher, I could send you another 19C Aerowing TT tire to run on it. You might be somewhat surprised by that combo.
The VF Record threw me off. Forgot they do a clincher version as well. I'm cooking dinner so I'm distracted. Hence the combined thing.
That aero improvement "jives" with what I've seen both from independent sources AND my own field testing. The magnitude of improvement, even at just zero yaw, was ~3-5W over a 20C VF Record on both a borrowed 808 front wheel and also my own Jet 90. That testing isn't done at 30mph either, though
I'll let you know when I nail down the tunnel details.
Last edited by Racer Ex; 11-02-10 at 07:13 PM.
11-02-10, 06:59 PM
#69
Senior Member
Their wheels cost enough that there are very few 3rd party tests (I've only seen one, and that is years out of date by now). It's not like this is NACA, testing wing profiles for the government. They say in public what people want to hear. Case in point, I was looking at 'cross tires a few weeks ago. Except for the obviously file treaded tires, every tire "shed mud well" and performed well "in all conditions" and "rolled fast" on pavement.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --the tiniest sprinter
11-02-10, 07:11 PM
#70
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Even with the best intentions there are so many variables in wheel/tire testing that the output from the manufacturers is just a guideline. And the decisions within the test are subjective; most customers don't ride at 30 MPH on a jig.
Testing a wheel by itself would be fine if we were just racing wheels. Unfortunately they get stuck on a bicycle and that tends to change those numbers.
And some stuff just gets completely cherry picked, Speedplay's pedal data the best recent example.
11-02-10, 07:38 PM
#71
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Money?
Even with the best intentions there are so many variables in wheel/tire testing that the output from the manufacturers is just a guideline. And the decisions within the test are subjective; most customers don't ride at 30 MPH on a jig.
Testing a wheel by itself would be fine if we were just racing wheels. Unfortunately they get stuck on a bicycle and that tends to change those numbers.
And some stuff just gets completely cherry picked, Speedplay's pedal data the best recent example.
Even with the best intentions there are so many variables in wheel/tire testing that the output from the manufacturers is just a guideline. And the decisions within the test are subjective; most customers don't ride at 30 MPH on a jig.
Testing a wheel by itself would be fine if we were just racing wheels. Unfortunately they get stuck on a bicycle and that tends to change those numbers.
And some stuff just gets completely cherry picked, Speedplay's pedal data the best recent example.
I'm not going to get too far into the debate. But I will note the following:
- Hed Stinger 9 faired very well in our aero test even with a "too small" 21c tire (which were used on all six wheelsets). In fact it was still the best.
- Wider wheels were noticeably, significantly better in crosswinds. The FSA's (very narrow) were frightening. This has been backed up in my mind after riding a set of Firecrest 404 clinchers.
I'm completely sold on wide rims, particularly for clinchers.
And time, not money, is the prohibiting factor in doing such tests.
11-02-10, 07:41 PM
#72
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Money?
Even with the best intentions there are so many variables in wheel/tire testing that the output from the manufacturers is just a guideline. And the decisions within the test are subjective; most customers don't ride at 30 MPH on a jig.
Testing a wheel by itself would be fine if we were just racing wheels. Unfortunately they get stuck on a bicycle and that tends to change those numbers.
And some stuff just gets completely cherry picked, Speedplay's pedal data the best recent example.
Even with the best intentions there are so many variables in wheel/tire testing that the output from the manufacturers is just a guideline. And the decisions within the test are subjective; most customers don't ride at 30 MPH on a jig.
Testing a wheel by itself would be fine if we were just racing wheels. Unfortunately they get stuck on a bicycle and that tends to change those numbers.
And some stuff just gets completely cherry picked, Speedplay's pedal data the best recent example.
To the gild the lily comment, I'm willing to bet an awful lot of people are willing to just take their word for it, especially since zipps have that reputation now. I'm also guessing that the people riding "dentist bikes" aren't doing the research to find the best setup for their bike.
11-02-10, 08:21 PM
#73
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That's a fact jack. Really, the best way to test stuff for you is for you to test stuff. But there are some good starting points out there.
11-02-10, 09:57 PM
#74
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But which frame/rider you have on top of those wheels is such an important variable that there isn't really any better way to test them.
I'm not going to get too far into the debate. But I will note the following:
- Hed Stinger 9 faired very well in our aero test even with a "too small" 21c tire (which were used on all six wheelsets). In fact it was still the best.
I'm not going to get too far into the debate. But I will note the following:
- Hed Stinger 9 faired very well in our aero test even with a "too small" 21c tire (which were used on all six wheelsets). In fact it was still the best.
BTW, I have a narrow Jet 90 that I feel more comfortable using as a front wheel in crosswinds than a prototype 404 carbon clincher I've been allowed to use. Granted, the prototype isn't the final FC shape, but still...it points out to me that whether or not a wheel is a "handful" has little to nothing to do with the width of it's brake track.
Why? What "sold" you on it?
11-02-10, 09:59 PM
#75
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