98TJ

I've read about the large chainring flexing on the standard cranksets but yesterday I got on the bike and put my foot on the pedal to get into riding position (was checking hood placement) and just happened to notice that there was a good amount of flex in the NDS crank arm.

I have 175mm cranks and never read about flex in the actual crank arms - only the large chainrings.

When I put weight on the arm (like I'm going to start moving forward from a stop) the outer portion of the arm will flex in toward the frame about 3mm.

I couldn't seen how much rotational flex the arm had, but this is disappointing.

I expected more stiffness from Red.

giantdefy

I have the compact that does not flex at all. Did u torque down the crank bolts to spec?

kleinboogie

It's probably your frame that's flexing, they all do. GL

topflightpro

You are more than likely flexing the frame, and not the crank arm.

98TJ

Yes, and had the LBS double-check the work.

It's not a matter of fastener torque, it's the arm itself deflecting in as load is placed on it.

Try this with yours, NDS crank arm at about 10 o'clock, hold the brake and apply a little pressure like you were going to set off from a stop. The crank arm will deflect in.

LowCel 

I remember reading an article a few years ago comparing flex in crank arms. The most flexy compared to the stiffest was in the fractions of a mm. Not nearly enough for most mortals to ever notice.

98TJ

Interesting. Frame is a 2010 Tarmac Elite.

I'm OCD like that. Once I saw it flex I was fixated on it and started inspecting the arm for cracks.

LowCel 

All frames flex in the bb area. The odds of your crank flexing a noticable amount is slim to none. If it is flexing that much then definitely do not ride it.

LowCel 

If you ever want to get an idea of how much your frame actually flexes around the bb area put your bike on a trainer, set up a camera and video tape it while standing and sprinting. You will be amazed.

98TJ

I might have to try that.

Jeepnut22 

How about a pic of the '98 TJ?

98TJ

It's in the workshop thread.

Fleabiscuit

You didn't mention it in your post but are you a fat slob? Unless you tip the scales at 500 lbs or so, I don't think you can flex these cranks. I'm no weight weenie and I can't flex the Red cranks as hard as I try. It's doubtful you are flexing them by just standing on them. I literally tried several times and was unsuccessful.

You may have a defective BB or maybe something isn't tightened down right. Try another bike with the same crank and see if you can flex it. If you do, congrats to you because you are a better man than I.

Cheers~

98TJ

I'm 6'1", 185lbs and something is flexing. After a few of the replies here I'm more inclined to think it might be a torsional twist of the frame in the BB area.

It's weird, though. I can't see a deflection in the drive side, but with the NDS I can see the flex.

merlin55

because the chainring hides the frame B/B shell.....

topflightpro

You are not a very large guy, so your size alone is not causing the crank arm to flex. I really think you are seeing the frame flexing.

It is often difficult to tell where on a bicycle a part is flexing. For example, during trainer sessions, it often appears to me that my seatpost is flexing, but I know there is no way I"m flexing a Thomson post. But the way everything is moving, frame, saddle and all, it gives the appearance that the post is flexing.