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07-08.-2007, 02:41 PM
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#1 |
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Registered User
Join Date: Feb 2007
Posts: 66
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Using PowerTap test to decide a new frame?
I'm wondering how one would design a meaningful + repeatable test for trying out several frames using my PT 2.4. Ultimately using this to help decide on a frame for my sprinter body type/style. (Pb 10s power on a flexy frame is just under 1300w @ 162lbs. Normal sprint is between 1100-1200.)
BTW: looking for a frame in the $3-4k range. Maybe some racing for fun but mostly serious club rides so 100+mi comfort is definitely a factor. Got some great suggestions in another thread promoting TIME, & BMC. Any others are welcome too. Looking forward to your ideas... Dave |
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08-08.-2007, 01:31 AM
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#2 |
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Registered User
Join Date: Jul 2003
Posts: 343
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Re: Using PowerTap test to decide a new frame?
I don't think you can. The range of variation in your sprint power is probably greater than the range of variation in frame stiffness. Add to that the fact that PT's (IIRC) aren't particularly precise at the shortest durations, and "meaningful & repeatable" moves out of reach.
If I were overly concerned with stiffness, I'd look at a) lab test numbers from Tour Magazin and b) how each frame felt when I sprinted (using same wheels, tire pressure, bar, stem, cranks.) Not sure you can find a shop that will let you do the complete version of b) though, so I'd settle for same wheels at same pressure. FWIW, Time frames are noodles in the lab. In your price range I'd look at Canyon, Storck, C'dale System 6, Cervelo R3, and custom builds from Crumpton or Ruegamer. |
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08-08.-2007, 04:04 AM
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#3 |
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Registered User
Join Date: Jan 2006
Posts: 262
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Re: Using PowerTap test to decide a new frame?
Yeah, but assuming you could find a route that would give you basically the same conditions, you could pedal at 250 watts for say 30 minutes and then look to see exactly how many feet you went. Do the test multiple times for each bike. If one bike consistently travels further, then you know it's transferring more of your energy into forward movement.
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08-08.-2007, 05:42 AM
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#4 | |
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Registered User
Join Date: Oct 2006
Posts: 1,505
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Re: Using PowerTap test to decide a new frame?
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08-08.-2007, 08:15 AM
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#5 | |
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Registered User
Join Date: Nov 2003
Posts: 577
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Re: Using PowerTap test to decide a new frame?
Quote:
How do you know your frame is flexy? |
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08-08.-2007, 09:44 AM
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#6 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
Quote:
Also, I notice on the trainer the front wheel rolls about an inch forward with a shift in weight. Also out-of-the-saddle in a sprint, I can feel the frame snap back after each pedal stroke. My arms have to do an exaggerated pre-torque of the bars to counteract the pedaling forces, and then act as shock absorbers mid-pedal stroke because everything is springing back. Dave Last edited by vladav : 08-08.-2007 at 09:53 AM. |
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08-08.-2007, 09:57 AM
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#7 | ||
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Registered User
Join Date: Nov 2003
Posts: 577
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Re: Using PowerTap test to decide a new frame?
Quote:
heavy and stiff is very easy to do. Light and stiff is not. Your frame is probably quite stiff. Quote:
For $1200, I doubt your bike has a carbon steer tube, but that is a source of flex that many can misinterpret as frame flex. "Feel" is not something that we can use to determine how flexy a frame is. |
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08-08.-2007, 09:59 AM
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#8 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
Quote:
I'm pretty sure a thrashing violent sprint would require a different aproach. And as Dave R W pointed out there's too many variables over that time frame. (And possibly same for what I want to do too...) Dave |
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08-08.-2007, 10:02 AM
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#9 |
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Registered User
Join Date: Nov 2003
Posts: 577
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Re: Using PowerTap test to decide a new frame?
Oh yea, and to actually answer your original question, power loss due to frame flex couldn't be more than a few watts (less than one I would expect). It's totally impractical to measure it at the rear wheel.
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08-08.-2007, 10:03 AM
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#10 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
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Agreed - which gets at the heart of this thread  |
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08-08.-2007, 10:07 AM
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#11 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
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08-08.-2007, 10:34 AM
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#12 | |
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Registered User
Join Date: Nov 2003
Posts: 577
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Re: Using PowerTap test to decide a new frame?
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It's an educated guess based on my experience and a degree in engineering. Remember that power is work over time. Work = force x distance. So, even though forces are high when sprinting, most of that force goes into turning the cranks so maybe your frame is flexing what, 5 to 10mm with every stroke absolute tops? So you've got power lost due to frame flex = Force(high)xDistance(very very small)xTime (slow, even at 120rpm) = a low number. Tire flex probably accounts for a lot more power loss. There's also speculation that at least some of the power lost due to frame flex would be returned when the frame flexes back. I believe if you search on biketechreview.com archives you'll find some discussions using actual numbers. edit: p.s. I haven't actually run the numbers so I will grant that I might be wrong and it could be as many as like 5w but I would still guess that it's closer to less than one. |
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08-08.-2007, 01:35 PM
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#13 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
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08-08.-2007, 09:15 PM
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#14 | |
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Registered User
Join Date: Nov 2003
Posts: 577
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Re: Using PowerTap test to decide a new frame?
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Before anyone yells at me too much, this should be: Force(high)xDistance(very very small)/Time (slow or relatively large, even at 120rpm) Sorry for the error. |
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10-08.-2007, 09:56 AM
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#15 | |
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Registered User
Join Date: Feb 2007
Posts: 66
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Re: Using PowerTap test to decide a new frame?
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Found www.tour-magazin.de site in German > translated on babelfish > still clueless. Of course googled too... |
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