General Discussion Triathlon Talk » P2C BB and Powermeter compatibility Rss Feed  
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2015-08-19 8:45 PM


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Subject: P2C BB and Powermeter compatibility
So it's about time for me to invest in a power meter, and I've been researching the various types, brands, etc. Where I get confused is what PMs fit on what bikes. As far as cranksets go, it's based on what bottom bracket you have, right? On the same note, does anybody know what kind of BB a 2008 Cervelo P2C has? I'm getting mixed info from various people/sources, saying BB30, BBright, neither...

One more question, what does crank size refer to? I'm guessing it's the distance from the BB to the pedal, correct? If then, I currently use a 170mm crank and were to buy a power meter with a 175mm crank, would I need to get a new bike fit? How, then, do people switch to compact cranks and such for particular races without paying for a new fit every time?

Sorry for all the questions. I just can't seem to find the answers (or consistent answers, anyway) from Google & forum searches.


2015-08-20 5:38 AM
in reply to: jenniferpelota

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Elite
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Subject: RE: P2C BB and Powermeter compatibility

http://www.bikepedia.com/quickbike/BikeSpecs.aspx?year=2008&brand=Cervelo&model=P2C%2fUltegra

FSA Mega EXO BB according to that.  I've got a P3C from 2008 and that's what I've got so it makes sense.  You can get a Power2Max Classic for that for dirt cheap.  http://www.power2max.com/northamerica/product/powermeters/power2max-classic-fsa-gossamer-power-meter/

Switching from 170mm cranks to 175 (why?) means you need to lower your saddle by 5mm.  You mention compact cranks though, that's not the same as the crank length but rather the size of the chain rings.

2015-08-20 5:51 AM
in reply to: 0

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Subject: RE: P2C BB and Powermeter compatibility
Originally posted by jenniferpelota

So it's about time for me to invest in a power meter, and I've been researching the various types, brands, etc. Where I get confused is what PMs fit on what bikes. As far as cranksets go, it's based on what bottom bracket you have, right? On the same note, does anybody know what kind of BB a 2008 Cervelo P2C has? I'm getting mixed info from various people/sources, saying BB30, BBright, neither...

One more question, what does crank size refer to? I'm guessing it's the distance from the BB to the pedal, correct? If then, I currently use a 170mm crank and were to buy a power meter with a 175mm crank, would I need to get a new bike fit? How, then, do people switch to compact cranks and such for particular races without paying for a new fit every time?

Sorry for all the questions. I just can't seem to find the answers (or consistent answers, anyway) from Google & forum searches.


Changing crank length does affect fit.

a) If you decrease your crank length from say 170 to 165, your seat needs to go up 5mm
b) If you go from 170 to 175, your seat goes down 5mm.
c) THis is because at the bottom of the pedal stroke, your pedal is now 5mm further away from the seat, so you need to bring the seat down

But the interesting stuff is at the top of the pedal stroke. In scenario b), without moving your seat, your seat is now 5mm closer to the pedal at the top of the pedal stroke. If on top of that you drop it by 5mm because of c) it's like you decreased you seat by 1cm. Decreasing your seat height closes your hip angle and a closed hip angle = less power at one point.

Most people shorten cranks to get the seat up, flatten the back a little, open the hip angle for more power. Or they can possibly drop the front a little more (because they have opened their hip angle).




Edited by marcag 2015-08-20 5:56 AM
2015-08-20 10:09 AM
in reply to: jenniferpelota

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Subject: RE: P2C BB and Powermeter compatibility

Originally posted by jenniferpelota One more question, what does crank size refer to? I'm guessing it's the distance from the BB to the pedal, correct? Correct.

 

If then, I currently use a 170mm crank and were to buy a power meter with a 175mm crank, would I need to get a new bike fit?

Yes (need a new bike fit).  Personally, I wouldn't buy a crank-based power meter with a different crank length than what I was using unless my fitter and I thought the different length was a better option. ie. shorter cranks to open up the hips.

How, then, do people switch to compact cranks and such for particular races without paying for a new fit every time?  

As already mentioned, compact cranks refer to the big ring/little ring gearing and not the crank arm length.  Standard crank (53/39), Mid-compact (52/36), Compact (50/34) ... there are some variations on these but these are the most popular setups for the mentioned configurations.

 

2015-08-20 11:27 AM
in reply to: JoelO

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Subject: RE: P2C BB and Powermeter compatibility

Originally posted by JoelO

Originally posted by jenniferpelota One more question, what does crank size refer to? I'm guessing it's the distance from the BB to the pedal, correct? Correct.

 

If then, I currently use a 170mm crank and were to buy a power meter with a 175mm crank, would I need to get a new bike fit?

Yes (need a new bike fit).  Personally, I wouldn't buy a crank-based power meter with a different crank length than what I was using unless my fitter and I thought the different length was a better option. ie. shorter cranks to open up the hips.

How, then, do people switch to compact cranks and such for particular races without paying for a new fit every time?  

As already mentioned, compact cranks refer to the big ring/little ring gearing and not the crank arm length.  Standard crank (53/39), Mid-compact (52/36), Compact (50/34) ... there are some variations on these but these are the most popular setups for the mentioned configurations.

 

Standard and compact refer to the diameter of the bolt circle. The 5 circles arranged in a start pattern. These are 110 or 130 mm. A lot of people have butchered this concept and think it refers to the chainring size, hence the mid-compact which is a marketing creation. It's necessary to have a set of compact cranks to be able to handle the smaller inner ring sizes, like 34T and 36T, but the compact term previously wasn't assigned to a specific ring size. It's also not necessary to limit use to only those sizes. A 50/36 is used by numerous people as well. These are on the Compact crank set. A 52/36 is also on the compact crank. I use a 54/42, and this is on the Standard crank size. Same as the 53/39. Older compact designs (the 5 arm or "star") have had issues with the bigger rings, so tended to not go above 52. The newer Shimano ones (4 arm or "star") are able to go up bigger in chain ring sizes even though they are all compact (110). One of the reasons these things are a really nice design.

Back more to the OP's question, if the crank arm length (not the crank set size) is still the same then there is no difference in the fit. The front derailleur may need some adjusting, but not the fit for the rider. If people get a different crank arm size then the fit should be adjusted according to match up what they had before. Some may not have fit as well before and now fit better with everything else the same so don't touch anything. Some may just move the saddle some themselves as it's not necessarily a huge move to make. So there is some "it depends" in there.

2015-08-20 12:05 PM
in reply to: brigby1

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Subject: RE: P2C BB and Powermeter compatibility

Ah, good to know.about the 110 and 130 BCD...should have checked with Sheldon Brown before I posted.

 



2015-08-20 1:49 PM
in reply to: jenniferpelota

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Subject: RE: P2C BB and Powermeter compatibility

You can email them directly.  They are pretty quick to reply.  More than likely you have a threaded bottom bracket.  That is the BSA68 option.

 

2015-08-20 1:56 PM
in reply to: jenniferpelota


8

Subject: RE: P2C BB and Powermeter compatibility
Thanks, guys. I appreciate all the explanations about crank length, compacts, etc. Surprisingly they made a lot more sense than what the LBS was telling me. Sometimes it's hard for expects to talk at a noob's level
2015-08-21 12:28 AM
in reply to: jenniferpelota

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Subject: RE: P2C BB and Powermeter compatibility
Your BB is the BSA 68 English thread. You can use the FSA as mentioned above, Shimano HollowTech II, or SRAM GXP. They will all screw right in for you.

You can also get a BB30 adapter from ROTOR for your bike if you wish to run a BB30 crank.

Have fun with the PM. Best investment you can make for cycling IMO.
2015-08-21 12:32 PM
in reply to: scubabrad02


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Subject: RE: P2C BB and Powermeter compatibility
Hi all, slightly hijacking this thread, I have a 2009 P2C, it has a FSA SLK hollow carbon crank, it doesn't look like the spider is removable. So just to confirm in that case If I want to go with the power2max power meter I would need a whole new crankset (rings + crank arm) that fits BSA68.
2015-08-21 1:55 PM
in reply to: scubabrad02

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Subject: RE: P2C BB and Powermeter compatibility

Originally posted by scubabrad02 Your BB is the BSA 68 English thread. You can use the FSA as mentioned above, Shimano HollowTech II, or SRAM GXP. They will all screw right in for you. You can also get a BB30 adapter from ROTOR for your bike if you wish to run a BB30 crank. Have fun with the PM. Best investment you can make for cycling IMO.

A SRAM GXP will also need a new bottom bracket as GXP has a tapered spindle that is 24 mm wide on one end and 22 mm wide on the other.  The GXP BB will screw right into the threads of your current BB shell though without any issue.



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