Bent/oscillating chainring or not??
 

I recently built a bike up, and the crank is a TRUVATIV. Seems like a nice unit. When I pedal though, the chainring (at least the large one - have not checked the smaller one carefully) moves back and forth with respect to the cage. Years ago, when I used to mess with bikes more, this signalled a bent chainring, and you got out the wrench, or replaced the chainring.

It is new though, and I was told that some of the newer cranks have some sid to side movement, that somehow aids shifting. Is this possible, or do I have a defective crank/or chainring?

There does not seem to be any play in the bottom bracket, and it does this regardless of whether I am pedaling the bike, or simply turning the cranks by hand while in a repair stand.

Well? Any thoughts appreciated.

Or it could be some out of plane with the spider arm seatings. The only way to tell for sure would be to remove the chainring and use a dial guage on the spider arm mount points to check for runout and also to check the chainring for flatness using some good straight edges or a known accurately flat surface.

And just how much runout at the teeth are we talking about? A mm? 5mm's?

I have been told that all cranksets come with a little bit of wobble, and most bottom bracket spindles are not perfectly straight. If you're dealing with a square taper bb, try installing your cranks in different positions on the spindle and finding which position wobbles the least.

is it a tapered spindle on the bottom bracket? i remember reading somewhere (cant remember where) that sometimes when there is an unexplained "wobble" in the crank to take off the crank arm and rotate the spindle 90 degrees then reinstall cranks. sometimes there is a slight problem in the taper on the spindle. just a thought

wobble
 

great minds think alike, lol

ha!

Oscillation
 

Thanks for all the help. To answer your questions. Isis bottom bracket. Runout was over 1/8".

I ended up removing the chainrings. The spider arms (??) ... pretty good, except for one, which was out of plane with the others. I tweaked it some, it's pretty close now. I reinstalled chainrings, and the situation is much better. There is a slight wobble, but I think it's within the norm.

Thanks!

I have a big rubber mallet for situations like this one.