Bike Flex at Higher Wattage

On the bright side, I’ve definitely been getting faster during quarantine, but I’ve started to notice a lot of flex in the frame when doing sprints at higher wattage. Many people around here are probably doing threshold or Vo2Max work at this level, so I’m curious if I’m imagining things, the bike is just going to do that, or if I’m doing something wrong.

Setup: Cannondale CAAD8 attached to Wahoo Kickr Core
Workout: Huxley - sprints at 318w, 384w

Is the frame flex I’m experiencing:

  1. Expected on a relatively cheap aluminum frame?
  2. Something wrong with the bike setup?
  3. Perhaps too much body English pulling through the handlebars?
  4. Something that’s always happening with your bike but you don’t notice outside?
  5. I’m in a deep dark pain cave and my perception of what the bike is doing is way off?
  6. Something else?

Weird - what are the sensations you are experience as flex?

Does it only happen on the trainer? Are you also noticing the “flexing sensation” while outside?

  1. Expected on a relatively cheap aluminum frame? Not generally - they should be handle sprinting watts
  2. Something wrong with the bike setup? Maybe? - is it connected correctly? Loose? not tightened down? is the cassette tight on the hub?
  3. Perhaps too much body English pulling through the handlebars? I don’t think so, but it is fixed to the Kickr so maybe more than if you were outdoors
  4. Something that’s always happening with your bike but you don’t notice outside? Bikes will flex, but a Caad 8 shouldn’t be flexing too much.
  5. I’m in a deep dark pain cave and my perception of what the bike is doing is way off? Maybe, aren’t we all - but again you are noticing something.
  6. Something else? I would take the bike off, put it back on, make sure all the connections are tight. Check the rear triangle and the frame for any cracks at any of the joints.

You might find that your trainer is rocking around slightly, or perhaps even flexing?

I assume you are standing if you are talking body English? That would put quite a lot of leverage/movement into the whole system.

Humblebrag. :stuck_out_tongue:

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Are you sure it’s frame flex? What does it feel like?

My assumption is that you are experiencing the normal rocking movement you get using an indoor trainer as noticeable frame deformation at 318w would probably indicate an issue with the frame.


The Kickr Core is a stiff little unit in my experience. On concrete you can feel the bike frame flexing under load.
I have various bikes that I’ve used on the trainer and my aluminium frame has the least amount of noticable flex.

Carbon bars flex and I’d consider that a good thing. Likewise, the carbon frames tend to flex, but mainly around the contact points.
The down tubes and bottom brackets are stiff as.

Buy yourself a foam yoga block and cut it up to make supports for the legs if you want the Core to move a little more.

I’m mostly seeing it at the front end of the bike, not really in the rear. I don’t notice it outside, but I’m also a lot more likely to be looking at where I’m going. I did another workout last night that was more threshold efforts, and I didn’t notice the flex much at all.

I’m thinking I might just be pulling too much on the handlebars during a sprint. I always stay in the saddle indoors, but I was definitely struggling at the end of that workout with finisher sprints.

:rofl: Not really a humblebrag to admit my 15 second sprints are Nate’s new FTP.

More just worried about either doing something wrong or an excuse that it’s time to upgrade to a carbon bike.

Bikes are build to be compliant, so a little flex is normal. Also, the power you put through pales in comparison of hitting a mild pothole during a descent. I have no information about your particular frame, but any well built frame should be able to handle the loads, specially if its an alloy one (so no unidirectional fibers you find in carbon bikes).

Edit: On the front? Is fork carbon? Maybe is your handlebars or wheel.

  1. Your bike doesn’t flex because of wattage (force (torque) x cadence), it flexes because of force. At lower torque, but higher cadence, your bike wont flex. It also won’t flex much if you’re not throwing your body around.

  2. On an AL bike, frame flex is a sign of quality. It’s easy and cheap to make a stiff, solid, aluminum brick.

  3. Can your front wheel move around? Letting it move around a bit will take a lot of stress off the frame. Put your front tire on a smooth bottom plastic plate.See if that helps. If you have both ends locked down it’ll wiggle in mysterious ways.

  4. 350w shouldn’t cause a lot of rocking unless you’re pedaling at 40 or 50rpm. You’re trainer is probably not on solid footing. You’d have to have a pretty big crack to have the bike wobble that much. Cracks in AL bikes go from unnoticeable to complete failure in only a few flexes, so your frame is not damaged.

If you’re worried about it, take the bike to the LBS. Try it on their demo trainer while someone watches.

Are you expecting your bike to be completely still when on the trainer? It will definitely move.