I recently built my first bike using a custom frame & sram components. After installing the bottom bracket and crank I thought there might be a little extra drag, but didn’t think it was too serious. After finishing the entire bike, I put on my powertap wheelset and got on my trainer. Immediately I noticed my power numbers were lower than I expected. Initially, I thought this was due to the layoff in training (3 weeks). After 3 weeks of workouts, my numbers are still 30 Watts below what I would expect. That is, my moderate exertion is 130W instead of 160W. My 3 min intervals are 200W instead of 240W.
Should I investigate the bottom bracket or is it more likely that my power has just fallen off after the holidays?
It is possible if the BB is screwed up enough it could be dragging that much. It would be obvious if you took off the chain and turned the cranks by hand.
Yes, the power numbers are from the same trainer. I did notice the drag before the chain was installed, but it didn’t seem like much, although spinning at 90+ rpm’s could make the friction a lot larger than when spun by hand.
It’s highly unlikely that a bottom bracket or crank install, even if installed wrong or too tight would suck up 30-40 watts. Like others have suggested, pull off the chain, spin the crankset, and see how many revolutions it spins. My guess is that your power is just down, and it’s difficult to produce power on the trainer. Or possibly your powermeter is reading low.
Not sure which bottom bracket, since it was an extra GXP bracket that my coach had, but the crank is SRAM force 22.
Masa757: My crank does not spin like that, so I’m thinking that’s the problem.
nickwhite: the rest of my set up is exactly the same: wheels, trainer, indoors. I last tested my FTP in December on my road bike and got 240W (same trainer & powertap) took off roughly 3 weeks, maybe four, but have been back on the bike since Jan 12th.
I will say my bb without chain spins almost 1 rev when spun I really do not like Wheels manufacturing BBs will have to see if there is any noticeable difference on trainer myself
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After looking at the video, I definitely agree that there is some excess friction there.
40W worth? I couldn’t possibly say - but having a mechanic look at it can only be a good thing.
Seems that it may be a bit over torqued. I have had this problem in the past with a similar set up using SRAM GXP and external GXP BB’s. My dilemma was, I torqued to MFC specification but there was excess play left over. Further torquing to the high-end of MFC specification to get rid of the play started to bind the bearings, adding too much resistance. The problem was worn seals that cover the actual bearings in the threaded cup. Try a new BB you should be fine. That’s my two sense
I will say my bb without chain spins almost 1 rev when spun I really do not like Wheels manufacturing BBs will have to see if there is any noticeable difference on trainer myself
I have Wheels Manufacturing BB’s on both my TT (PF30 outboard) and road (BSA threaded GXP w/ angular contact bearings) bikes, and they’re great.
Yeah, the bike is currently in my hotel room as I travel during the week. I don’t have tools to remove it and replace the BB so I’ll have to find an LBS. I usually leave the bike at the hotel when I travel home on the weekends since American charges $150 each way.
I’m thinking I may have lost 10-15 Watts but I’m pretty sure that BB/Crank install is costing my 20W or more.
Thanks for all the responses. Can anyone recommend an LBS in Nashville?
After seeing that, I wonder if I have mine setup right. I will have to pull the chain and give it a shot. Mine is just a quarq Riken on a SRAM GXP, so I can’t imagine there is anything else that is supposed to happen in the way of spacers.
Similar to what one noted above, if torqued to the point that all lateral play is removed (which is similar to the spec), then that is what I get. It does spin free-er if I leave it a little looser, but then I have some rocking there that can’t be good.
All that stated, no way I am giving up double digit watts to it, so OP must have some other issues.
https://www.youtube.com/watch?v=3gDrRAU1fPw
Here’s a video i took last night. Although the chain was still attached, I spun it backwards as hard as i could and only got 1 - 1.5 revolutions.
I’m going to head to a LBS and see what they can do.
Fyi, if you want to do a ‘spin test’ like in the other video in this thread, if you want to do even a very general comparison, you must do 2 things: remove the chain from the chainrings and also remove your pedals.