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PostPosted: Mon May 07, 2012 6:35 pm 
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Joined: Wed Dec 07, 2011 1:26 am
Posts: 400
Location: California's country side
Looking into a power meter, if I replace my BB30 red crank with a SRM S975 am I downgrading the performance aspect? It looks similar and fitted with what looks like to be red chainrings.

I have good success with the red crank and heel clearance, and I am sensitive to Q factor.

Also I am not sure if I want to be tied into the BB30 powermeter crank because I may replace the frame sooner or later, and a powertap wheel I can move between my bikes when needed.

BTW, if I put praxis rings do I need to recalibrate anything.


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Posted: Mon May 07, 2012 6:35 pm 


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PostPosted: Tue May 08, 2012 6:48 am 
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Joined: Tue Nov 11, 2003 9:47 pm
Posts: 1637
Location: Santa Cruz, California, USA
I went from a Red crank (BSA not BB30) to S975 Quark. Performance wise, they're both cranks- push the pedals and the chain moves. I've ridden with flexy cranks and stiff cranks and I don't buy the idea that crank flex affects performance. But in any case these two cranks feel pretty similar.


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PostPosted: Tue May 08, 2012 8:32 am 
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Location: Denmark
According to SRM the SRAM spider is both compatible with GXP and BB30, so all you need to do if you change away from GXP is chaging the arms...

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PostPosted: Tue May 08, 2012 9:30 am 
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Location: Melbourne, Australia
Red cranks use just a wavy washer to preload the bearings. S975 cranks use a threaded collar which pushes the arms much wider at the bottom bracket. This significantly decreases heel clearance if that can be a problem for you. Q-factor (width at the pedals) is unchanged or very minimally increased.

The arms use a similar construction method except for the obvious fact that Red cranks have an integrated spider and the S975 is removeable.

Recalibration on the SRM shouldn't be a problem as long as you torque chainring bolts to the specified amount (10nm I believe). For reference sake the S975 Quarq does require recalibration which you can do with an iPhone and ANT+ dongle. The new 2012 Red Quarq (which uses the same arms as S975) does not need recalibrating when chainrings are changed due to the updated Quarq.

As for BB30 or GXP S975. Either fit on BB30 or 68mm threaded frames with the appropriate adapter or conversion bottom bracket. The only compatibility problem is that BB30 can not fit in BB86 (or other 24mm spindle based press fit standards).

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PostPosted: Tue May 08, 2012 12:05 pm 
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Believe me! 10 Nm is way to much for alloy chainring bolts! Even though SRM recommends this they don't even themself use this torque! 10 Nm will complete rip those bolts - talking of experience!

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PostPosted: Tue May 08, 2012 3:51 pm 
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Location: Melbourne, Australia
I've always torqued my aluminium chainring bolts to 10nm. A new chainring bolt will not fail at this torque.

SRM don't list it for the sake of saying a number. If they did, and everyone destroyed their chainring bolts as you say they would, forums like this would be full of people complaining about excessive torque recommended by SRM.

If an aluminium chainring bolt does fail before reaching this torque it is either
-user error
-the chainring bolt has been removed and installed too times and reached its fatigue life
-has a manufacturing defect

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PostPosted: Tue May 08, 2012 4:55 pm 
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Location: California's country side
My chain ring bolts is torqued to 7nm only don't seem to have a problem with too loose.

the aerozine alum bolt says 8nm on them.


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PostPosted: Tue May 08, 2012 5:00 pm 
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Location: Sunny So Cal
No problem tightening original Red chainring bolts to 10nm. Done that 2x.


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PostPosted: Tue May 08, 2012 7:55 pm 
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And if you really want you can just use one red crank arm for the other arm instead of the S975 :)

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PostPosted: Wed May 09, 2012 2:46 am 
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Isn't the original poster talking bout Quarq and not SRM?

Fwiw, Quarq for their SRAM 975 based powermeter 10Nm for steel chainring bolts and 8Nm for aluminum. Deviate from this and the calibration is way off. For some reason, Praxis chainrings have not been approved for use with the Quarq.


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Posted: Wed May 09, 2012 2:46 am 


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PostPosted: Wed May 09, 2012 1:52 pm 
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Joined: Mon Jul 26, 2004 1:06 pm
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Location: Melbourne, Australia
r_mutt wrote:
Isn't the original poster talking bout Quarq and not SRM?

PoorCyclist aka Original Poster wrote:
Looking into a power meter, if I replace my BB30 red crank with a SRM S975...

Definitely talking about SRM

itsacarr wrote:
And if you really want you can just use one red crank arm for the other arm instead of the S975 :)

This is not correct for the BB30 crankset. Red and S975 BB30 cranksets use different bearing preload systems.
It is correct for a GXP crankset though.

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