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PostPosted: Mon Aug 22, 2011 10:26 pm 
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So I've decided to buy a crank based power meter. I've been using a compact on my road bike for the last year and love it. The problem is that I need a 55-42 setup for some of the TT's I do (Tour of the Gila). Does anyone make an aftermarket 55-42 setup for a 110 BCD? Yeah, Yeah, I understand it won't be as stiff as on a 130 BCD. Any help is much appreciated. Thanks!


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PostPosted: Mon Aug 22, 2011 10:42 pm 
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Location: it's raining, it must be uk
stronglight do them, for instance...

http://www.stronglight.com/stronglight/ ... ral_Double

also tt rings, should be stiffer, but max 54t...

http://www.stronglight.com/stronglight/ ... teauclm110


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Posted: Mon Aug 22, 2011 10:42 pm 


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PostPosted: Tue Aug 23, 2011 1:12 am 
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Stronglight is probably your best option with that second link above. 54t appears the limit.

One thing to note, switching rings on your crank based power meter is going to require re-calibration. The crank gets calibrated for a certain configuration. If you change spiders or chainrings, it throws that off. You might have already known that, but just in case you hadn't heard and imagined it being a 10mins operation the night before/after a TT stage.


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PostPosted: Tue Aug 23, 2011 4:29 am 
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nothingman is correct. If you go SRM you're screwed I think. If you go Quarq, you can self calibrate fairly easily. There's an iPhone app called Qalvin (free) that will talk to the Quarq and you can set the slope (if you know the right number for your rings) or you can recalibrate yourself (requires an accurate known weight to hang off the pedals). You have to buy an ANT+ receiver for your iPhone to do this though (about $75?). Details here:
http://www.quarq.com/qalvin

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PostPosted: Tue Aug 23, 2011 4:38 am 
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Thanks. I'll get the stronglight rings. I was on the Quarq website the other day and watched the video they have on how to change the slope using the Qualvin or Calvin(sp?). That was the deal breaker before, not being able to change the slope. Now though, its changeable!


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PostPosted: Tue Aug 23, 2011 11:30 am 
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lanierb wrote:
nothingman is correct. If you go SRM you're screwed I think. If you go Quarq, you can self calibrate fairly easily. There's an iPhone app called Qalvin (free) that will talk to the Quarq and you can set the slope (if you know the right number for your rings) or you can recalibrate yourself (requires an accurate known weight to hang off the pedals). You have to buy an ANT+ receiver for your iPhone to do this though (about $75?). Details here:
http://www.quarq.com/qalvin


Not true. I changed from 50/34 to 52/36 and different brands on my rings per a tech at srm advice and he said the difference in calibration would be 1 percent or less. I have the hollowgram compact srm by the way


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PostPosted: Tue Aug 23, 2011 1:08 pm 
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You won't need to recalibrate a Power2Max powermeter if you change the chainrings on it


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PostPosted: Tue Aug 23, 2011 1:14 pm 
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Ufgators I'd correct. The story that SRM needs to be sent back to SRM is a myth. To be fair, that myth has been exacerbated by SRM itself and comments expressly about the use of solid rings.

You can replace the rings on your SRM. As long as you torque the bolts to spec, you will be fine. Where you change the type of rings (eg., to a solid TT ring), the stiffness will be different. Any crank-based PowerMeter will need to be re-calibrated (whether or not they admit it). Fortunately, re-calibrating the SRM can be done simply by yourself. Thereafter, whenever you switch to those rings, you will have the slope for them. Torque them to spec and ride!


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PostPosted: Tue Aug 23, 2011 2:20 pm 
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Geoff wrote:
. Any crank-based PowerMeter will need to be re-calibrated (whether or not they admit it).

You'd better let Power2Max know that then, they don't even sell them with chainrings.


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PostPosted: Tue Aug 23, 2011 3:24 pm 
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lanierb wrote:
nothingman is correct. If you go SRM you're screwed I think. If you go Quarq, you can self calibrate fairly easily. There's an iPhone app called Qalvin (free) that will talk to the Quarq and you can set the slope (if you know the right number for your rings) or you can recalibrate yourself (requires an accurate known weight to hang off the pedals). You have to buy an ANT+ receiver for your iPhone to do this though (about $75?). Details here:
http://www.quarq.com/qalvin

You are not screwed, users can do this on an SRM and have been able to for the past decade without an Iphone.

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PostPosted: Tue Aug 23, 2011 4:10 pm 
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diarmuidc wrote:
You'd better let Power2Max know...

No need, they know.

Really, this is just simple physics and engineering. A crank-based PowerMeter has a number of potential error factors build-in to it. Among these are the chassis, the crankarms and the rings. If you change any of these factors, the slope can change. The issue then becomes one of materiality. The manufacturer will typically forward the PowerMeter with the chainrings. If not, they can substitute a common chainring (e.g., with a DA 7900 SRM, the stock Shimano rings) torque them to a known spec, calculate the slope and ship it with that information. If that is within the advertised error factor, then they are good-to-go.

If you change the rings, does it affect the slope? Of course it does. The question is: is that material? If you use rings of the same size from the same manufacturer, the variability is much smaller than if you change to rings of a different size from a different manufacturer and still greater if you change from a conventional ring to, say, a solid TT ring.

As long as you can re-calibrate the slope yourself, which you can do easily with an SRM, you will have no problems.

The other thing to consider is whether or not you need to even worry about it. The only reason that the accuracy of a PowerMeter matters is to ensure that the data generated is consistent. Aside from comparability year-over-year, it really doesn't matter whether it is measuring high or low. As long as you are testing to the same numbers and completing your workouts at prescribed wattage, that is what counts.


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PostPosted: Tue Aug 23, 2011 9:42 pm 
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Also, Miche makes TT rings in 110 BCD up to a 56. I know Cat 2's that use a 53 in that TT, so unless you're a severe wattage cottage I think you can find what you need.

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Posted: Tue Aug 23, 2011 9:42 pm 


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PostPosted: Wed Aug 24, 2011 12:41 am 
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We can make them to order at whatever number of teeth you need on a given pcd. There is a point where you start wondering why someone wants a 60+ tooth chainring on a 110bcd, which we have done, but sometimes it can be as simple as availability of a given crank length.

The stiffness issue is all relative as well. Sure, an identically designed chainring on a 110bcd will have potentially more flex than on a 130bcd for given number of teeth, but one manufacturers chainring at 110bcd might be stiffer than another manufacturers on a 130bcd for the same number of teeth.


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