mrt2you

i recently bought a 2014 specialized roubaix expert. the bike is sold with a FSA slk light carbon hollow crank when new.
i have bad knees and hips so i use 5/8" pedal extenders when riding. the extenders had eliminated my knee and hip pains when riding.
my question is should i buy a alloy crank, and sell the carbon one, to use the extenders or do you think the carbon crank can take the added stress of the extenders?
i am at best a recreational weekend rider, i ride 1500 miles a year. my big weekend rides are 50 -75 miles round trip. also a clyde and weigh 200+ lbs so to me the weight difference is irrelevant between the cranks
i know most here will say ride it as is until it breaks them replace. i really don't want to have a crank break on a ride.
i am thinking sell carbon, buy alloy and have cash left over.
your thoughts?

Andrew R Stewart 

If you will worry over this then replace the crankset. Andy

TimothyH

It might be worth calling or emailing FSA to find out the maximum weight spec for those cranks.


-Tim-

rosefarts

I bet you won't be able to sell it for nearly as much as you think. It doesn't answer the strength question but if you are expecting to come out ahead doing this, you'll need to reevaluate.

markjenn

I'm not a mechanical engineer, but I think cranks are designed to be very stiff for efficiency and transfer of power which gives them very generous margins with respect to outright failure. I'd be very surprised if the additional stress you're applying would have the crank anywhere near design failure limits. Maybe flexing a little more than optimal, but no where near breaking.

- Mark

rosefarts

I weigh 140 and am not a hard rider, and even I have managed to break on (kooka, mid 90s). Cranks are very breakable.

Road riding should be okay but those extensions will add extra leverage.

Spinay70

I would consult the manufacturer, or if you bought it locally, get their $.02 on the setup.

Whatever you decide, if you are going to wrench on carbon, get a decent torque wrench to avoid overtightening stuff.

Spinay70

Marcus_Ti

I wouldn't worry about the weight spec on the cranks....simple weight isn't the problem. What could be a problem is your pedal extenders, which cause more leverage on the crankarm inserts.

mrt2you

THANK YOU for the replies.
i have had several local riders i casually know tell me the extenders shouldn't be a problem on the carbon crank.
i just wanted a few other opinions on the subject.
in my mind i wanted to replace it just to be safe. most of my weekday rides are solo and if it breaks on one of them i will probably be stranded for a at least 2 hours. not getting stranded would be worth the time and $$ to me.

as far as selling i think i will end up alright. the bike originally had carbon handle bars and stem on it. i sold them and the buyer gave me a alloy FSA BB30 11 speed crank with worn out rings with the deal. i also have a new FSA alloy 10 speed compact crank i bought about 6 years ago laying around. it has the smaller bottom bracket bearing shaft, not the larger bb30. my plan is to take the rings off the carbon crank, they are marked 10/11 speed on the rings so i would assume 10 and 11 speed crank rings are the same, and bolt them on the alloy crank. they have the same bolt circle and the dimensions of both cranks are the same except for the crank arms. carbon 170, alloy 172.5. then install the new 10 speed compact rings on the carbon crank. i will check if they work on the bike before selling to be sure if 10 speed rings work on a 11 speed bike. on a casual comparison the rings look and measure with my HF digital calipers the same. used 10 and 11 speed carbon FSA SLK cranks seem to be selling for $75-100 on ebay right now and if i get near $100 for it i will be happy. with new rings it should do that. if it doesn't then i will put in storage for future use.