Carbon Cranks - How much flex???

There was an interesting comparison (often referred to in these threads) in VeloNews last Fall, detailing various technical observations over 2003-2004's top crankset models. In particular, the piece focused on stiffness as a point of comparison; the cynics may argue (accurately enough) that crank stiffness is a moot point to all but the most powerful and competetive riders, but when you start picking apart the finer aspects of biomechanics and dollar value, the results are interesting, at least.

VeloNews' mechnical testing revealed surprising flexiness in a number of high-end carbon cranks, at least in comparison to the new DA10 crankset. The tested FSA model, if I remember correctly, was no exception.

Again, those ratings don't mean much to most of us (other than as a value indicator -- the DA10 crank is a lot cheaper than the flexier Colnago carbon crank, at nearly $900), but if you're getting particularly scientific about the positioning of your feet under the heaviest loads, you may as well dig up the article. Good luck!

Has anything else changed? eg crank length, type of pedals or their position, saddle height etc. I'd be very surprised if crank flex was the issue unless you are the incredible hulk.

Quoted post said:

Originally posted by patch70
Has anything else changed? eg crank length, type of pedals or their position, saddle height etc. I'd be very surprised if crank flex was the issue unless you are the incredible hulk.

No other changes: 172.5 crankarm, saddle in same position, pedals same, cleats unchanged.

Although I haven't found the Velo News article mentioned above, I emailed Lennard Zinn and FSA.

FSA suggests that my previous cranks deflected and now I am feeling a solid crankarm. I don't buy that, as I don't believe that Campy Record cranks are all that flexible (in spite of the ongoing Shimano debates). FSA also suggested that if the crankarms were mis-tapped, then the off-axis pedal spindle would be noticeable at all times not just under heavy effort. I agree.

Any other ideas are appreciated. Thanks.

Quoted post said:

Originally posted by timdodge


Any other ideas are appreciated. Thanks.

Mortals do not flex quality cranks. How about different Q factor?

Quoted post said:

Originally posted by boudreaux
Mortals do not flex quality cranks. How about different Q factor?

Aw c'mon you should know by now that many riders in these forums are faster than a speeding bullet and more powerful than a locomotive!!😄

Quoted post said:

Originally posted by boudreaux
Mortals do not flex quality cranks. How about different Q factor?

270lb Rugby prop mortals flex cranks, trust me on this.

I've got an FSA Carbon Pro Triple, and certainly don't feel any flex or deflection. Also, I don't believe FSA would sell arms with the tread inserts out of alignment. Seems any spindle alignment issue would be easy to observe while turning the pedals slowly.

Assume you bought the cranks new from a trusted retail source.

Perhaps the wider Q-factor could account for knee problems, as boudreaux suggested....I really don't believe it's the flexing of the crank arms.

Quoted post said:

Originally posted by boudreaux
Mortals do not flex quality cranks. How about different Q factor?

Q-factor is a real possibility. The FSA Isis BB is 108mm and the Campy is 102mm. I wasn't able to ride yesterday, so today I will see how the cleat realignment goes. Stay tuned...

Quoted post said:

Originally posted by timdodge
Q-factor is a real possibility. The FSA Isis BB is 108mm and the Campy is 102mm. I wasn't able to ride yesterday, so today I will see how the cleat realignment goes. Stay tuned...

Q factor actually has nothing to do with spindle length. It's a function of crank spider, and arm design. Cranks using an identical spindle length can have widely(no pun) different Q factors. If you do not have the numbrs for each crank,it can be measured when cranks are mounted, or just compare distance from crank arm at pedal to chainstay clearance.

Quoted post said:

Originally posted by boudreaux
Q factor actually has nothing to do with spindle length. It's a function of crank spider, and arm design. Cranks using an identical spindle length can have widely(no pun) different Q factors. If you do not have the numbrs for each crank,it can be measured when cranks are mounted, or just compare distance from crank arm at pedal to chainstay clearance.

Yeah, I know. Lennard Zinn just gave me an education on that. My bad. He also suggested toe-in on the cleats (the same thing I accomplished by kicking my heels out slightly). Time to ride.

Quoted post said:

Originally posted by armchair_spacem
270lb Rugby prop mortals flex cranks, trust me on this.

Bottom bracket movement or crank flex?

[quote="timdodge"]I put some FSA carbon cranks on my bike last week.... My theory was that the pedal threads were not perfectly aligned, causing my pedal spindle to be (very) slightly off-axis....My question is this: Has anyone else experienced this level of flex in their carbon cranks? It is very slight, but after 15+ years of riding my knees know a good rotation when they feel it.


....Hi. I know this thread is old. But in searching a solution to my crank/pedal problem, I found this thread. I have new FSA Superlight ISIS crankset on my carbon/titanium frame. From the first rotation, my left, non-drive side pedal seemed to very slightly move around under the ball of my foot. Also a slight outward cant of the pedal platform. Brand new Campy Records pedals. I have been chasing this for a month now. My left knee is getting a weird twinge, but I can still ride ok for now.... but I need to fix the problem. Also, the crank/BB flexes a bit too much. I am ready to upgrade to external bearings and new cranks. Before I do, does anyone have any ideas on this?

I do have some power at 6-5 and 210lbs. But not enough to do this...

I also found the following thread.

http://www.roadbikereview.com/cat/drivetrain/cranksets/FSA/PRD_320500_2491crx.aspx

Did you ever solve this problem?

timdodge said:

I put some FSA carbon cranks on my bike last week, and got my first real miles over the weekend - about 150 total, similar to what I have been riding. I have never had any knee problems, but by Sunday afternoon I felt like an old man.

I talked about this with my LBS. My theory was that the pedal threads were not perfectly aligned, causing my pedal spindle to be (very) slightly off-axis. Their theory is that the cranks were deflecting under load, causing the same problem: pedal axis to be off horizontal when using strong force. Same symptoms, different cause. I agreed with the LBS that the pain was sharper under heavy load, especially climbing.

My question is this: Has anyone else experienced this level of flex in their carbon cranks? It is very slight, but after 15+ years of riding my knees know a good rotation when they feel it.

For what it's worth, the pain was somewhat alleviated by kicking my heels out slightly. I am going to try realigning my cleats by a degree or so and see if this helps. Footbeds might be another option, but I would probably just go back to my old (Record) cranks.

Thanks for your comments.

Since pedals have a lot of float and most ankles can pivot a bit, without any problem, hard to see how crank flex or pedal alignment could cause your problem. Just look at how the distance between a rider's knees and top tube varies when he is riding and this causes his ankle to pivot, unless his foot is doing it inside his shoe. I went from a Record carbon 39-53, to their carbon 34-50 and thought I felt different after climbing, but that doesn't happen anymore and I speculate it was because I could spin faster and I wasn't use to this when out of the saddle. I guess you could record exact measurements, including pedal and crank arm deflection under load. And then put your Record crankset back in and make a comparison. It would take a little ingenuity, but your problem sounds like it would be worth doing anything to solve it.

Assuming that each crank arm flexed 5 mm under load (I think this is a gross over-estimation) and the crank arm length is 172.5 mm, the deflection angle is ~1.5 degrees. A more realistic value for the amount of flex is about half, or 0.8 degrees. And then you have to subtract the flex of your old cranks to find out the difference in angle. I'm not sure that supinating at the ankle by such a small amount would cause knee pain. But it might?

Did the crank length change also? That might be a likely cause. Other position changes?

John Swanson
www.bikephysics.com

timdodge said:

I put some FSA carbon cranks on my bike last week, and got my first real miles over the weekend - about 150 total, similar to what I have been riding. I have never had any knee problems, but by Sunday afternoon I felt like an old man.

I talked about this with my LBS. My theory was that the pedal threads were not perfectly aligned, causing my pedal spindle to be (very) slightly off-axis. Their theory is that the cranks were deflecting under load, causing the same problem: pedal axis to be off horizontal when using strong force. Same symptoms, different cause. I agreed with the LBS that the pain was sharper under heavy load, especially climbing.

My question is this: Has anyone else experienced this level of flex in their carbon cranks? It is very slight, but after 15+ years of riding my knees know a good rotation when they feel it.

For what it's worth, the pain was somewhat alleviated by kicking my heels out slightly. I am going to try realigning my cleats by a degree or so and see if this helps. Footbeds might be another option, but I would probably just go back to my old (Record) cranks.

Thanks for your comments.

[quote="Big Tim"]

timdodge said:

I put some FSA carbon cranks on my bike last week.... My theory was that the pedal threads were not perfectly aligned, causing my pedal spindle to be (very) slightly off-axis....My question is this: Has anyone else experienced this level of flex in their carbon cranks? It is very slight, but after 15+ years of riding my knees know a good rotation when they feel it.

....Hi. I know this thread is old. But in searching a solution to my crank/pedal problem, I found this thread. I have new FSA Superlight ISIS crankset on my carbon/titanium frame. From the first rotation, my left, non-drive side pedal seemed to very slightly move around under the ball of my foot. Also a slight outward cant of the pedal platform. Brand new Campy Records pedals. I have been chasing this for a month now. My left knee is getting a weird twinge, but I can still ride ok for now.... but I need to fix the problem. Also, the crank/BB flexes a bit too much. I am ready to upgrade to external bearings and new cranks. Before I do, does anyone have any ideas on this?

I do have some power at 6-5 and 210lbs. But not enough to do this...

I also found the following thread.

http://www.roadbikereview.com/cat/drivetrain/cranksets/FSA/PRD_320500_2491crx.aspx

Did you ever solve this problem?

lks said:

Since pedals have a lot of float and most ankles can pivot a bit, without any problem, hard to see how crank flex or pedal alignment could cause your problem. Just look at how the distance between a rider's knees and top tube varies when he is riding and this causes his ankle to pivot, unless his foot is doing it inside his shoe. I went from a Record carbon 39-53, to their carbon 34-50 and thought I felt different after climbing, but that doesn't happen anymore and I speculate it was because I could spin faster and I wasn't use to this when out of the saddle. I guess you could record exact measurements, including pedal and crank arm deflection under load. And then put your Record crankset back in and make a comparison. It would take a little ingenuity, but your problem sounds like it would be worth doing anything to solve it.

Thanks for your comments.

Actually, just to be more clear, I am only worried about a twinge in my knee. That is a concern, but my real problem is either crank flex and/or pedal spindle alignment. When I apply good pressure, sitting or standing it is more noticable. 'it' is mostly an outward cant to the pedal. It is subtle, but it feels really weird.

I switched from old Campy Chorus pedal (from my older bike with Campy Record alloy cranks 175mm) to my new bike (with FSA Superlight, ISIS BB, and normally brand new Campy Record pedals, also 175mm, same Q factor) just to make sure the new pedal spindle was not the problem (the only difference beteen the chorus and the record is that the spindle is Ti on the Record as far as I know anyway). ... But with the Chorus pedals on the new bike/drivetrain, there was no difference. Same weird feeling.

Measuring the deflection of the cranks under load seems to me to be a tricky operation to get right. I can see some of it, but how to measure it?

Thanks again for you comments and experiences.

dhk2 said:

Not sure if you're feeling torsional flex in the crankarm, or lateral flex in the BB.

Suggest you put the bike against a wall and stand on the left pedal to see how much torsional flex you detect in the crank arm vs the lateral movement of the BB. I've got the FSA Team Carbon Pro cranks, and can detect some torsional flex in them with Speedplay pedals.

At your size, "superlight" anything isn't a great idea (IMO). Don't know about your FSA cranks, but the Team Carbons I have aren't really CF; they have an aluminum core with just a wrap of CF. Not the stiffest cranks on the market...again, my opinion.

Same applies to the frame of course. A superlight Ti/CF rear frame will likely have a good amount of BB flex. Not sure how much it really matters in terms of performance, but it could be enough for you to detect that also when stepping on the gas.

Thanks for your reply.

Here is the history. I have ridden a GIOS Torino Pro for 22years. 2 re-paints, new Campy Record Group in 2004. I love the bike and still have it and ride it. My new ride is a Colnago CT2 with FSA cranks and ISIS bottom bracket. This is Titanium main traingle, and carbon fork and stays. The new bike has some side flex when I put a side force on it, but not as much as my old bike. Plus the ISIS bottom bracket should be stiffer than my old Record BB. I never felt anywhere near the flex in the non-drive side crank/bearing of my old bike. It is subtle, but definitely there. Only on one that one side.

I agree with the concern about anything labeled superlight. My LBS says it should be ok. In any case, as I understand it, most all carbon cranks have an aluminum core with a carbon wrap. The only exception that I am aware of is the Zipp (300?) cranks. In any case they retail for a grand, and that is just too much for any crankset IMHO.

Thanks again for your suggestions.