Front D rubs when accerelating

[Satyr]

My wife's bike is a secondhand mountain bike built by (for) Reebok, if that's relevant. There's a rubbing sound in the front Derailleur, but only when she starts off, or applies standing pedal pressure. Once she's at speed and pedaling casually, there's no rubbing.
Thoughts?

 

[Milkfloat]

Flexing frame springs to mind. You might be able to adjust the derailleur enough to stop this, but you could end up with it rubbing when not putting a lot of power down.

 

[twentysix by twentyfive]

With both chainrings?

 

[Satyr]

As far as I know, yes. Both chainrings. I've ridden it too, but the rubbing effect isnt as prevalent when ride it.

 

[rogerzilla]

The front derailleur on my old Thorn Nomad would reliably change from the big ring to the middle ring when sprinting!

 

[Pale Rider]

My Cannondale MTB used to do the same when I deployed full grunt.

It must have been frame flex.

The bike shop tried to adjust it out a couple of times, but they were never fully successful.

 

[Fab Foodie]

As long as cranks, chainrings and bottom bracket are tight, and the derailleur is properly aligned it sounds like frame flex as other have intimated.
However, the rubbing may be exacerbated of you are running 'cross-chain' gear selection such as big front cog and big rear cog or small-small at the same time because of the chain angle through the derailleur cage.

 

[Satyr]

Frame flex. You mean the frame is physically warped or twisted? Wow, that would be woefully inconvenient. :/

 

[raleighnut]

Frame flex. You mean the frame is physically warped or twisted? Wow, that would be woefully inconvenient. :/

No the frame bends to some degree when you apply a lot of force to the pedal, most bikes do it to some extent

 

[Vapin' Joe]

Unless Satyr's wife is a very strong rider frame flex will not be the issue.

 

[Drago]

Most likely scenario is, hopefully, the simplest - maladjustment.

 

[ColinJ]

My slightly flexy steel Basso road bike and hardtail mountain bike do it. My very stiff aluminium Cannondale and Specialized road bikes don't.

 

[Fab Foodie]

Unless Satyr's wife is a very strong rider frame flex will not be the issue.

Disagree, 2 of my 531 frames do exactly as described.

 

[PpPete]

I'm not particualrly strong, but out of the saddle I used to be able to generate FD rubbing on the Campag triple of my titanium bike - and that is considerably stiffer than either of my 531 frames. I could adjust to minimise it for most rear sprockets, but never eliminate it completely.
Since changed to a super compact double (out of vanity - not because of the rubbing ) and it's no longer an issue.

 

[Ajax Bay]

As long as cranks, chainrings and bottom bracket are tight, and the derailleur is properly aligned

Perfectly reasonable, but we don't know any of this.
Check that there's no lateral (ie as opposed to spinning) movement from the bottom bracket and that the right hand crank is secure. I'll be guessing it's square taper. But I'm on the same side of the roulette wheel as @Drago : FD needs tweaking.