Chainrings not in plane?

Which bike? 😕

xxamr_corpxx said:

Hi,

After inspecting my new bike in more detail than I did in the shop, it seems the chainrings aren't perfectly in plane i.e when I spin the cranks the chainrings seem to "wobble" side to side, they don't rotate perfectly in plane. The chainrings seem to be ok but I'm worried about the BB.

Is this "wobbling" due to the bottom bracket being incorrectly installed (in which case I'm not worried since I was going to replace it with a lighter one), or is it due to the frame's BB threads not being on properly?

Thanks

I'm guessing a bit, but I think it's more likely that the crank is not properly installed on the bottom bracket spindle. Most cranks these days have a small amount of interference fit at the bottom of travel, which could allow for minor deviations like you described. This shouldn't be too much of a functional problem, though you could try reinstalling the crank.

If it was a bent bottom bracket axle or non-parallel bottom bracket bearings, then there would be an incredible amount of pre-load and binding. I don't think it's the bottom bracket.

John Swanson
www.bikephysics.com

gclark8 said:

Which bike? 😕

I bought a $99NZD folder,single speed, steel frame.

xxamr_corpxx said:

I bought a $99NZD folder,single speed, steel frame.

You did not give a lot of specifics about your bike.

Let's assume a couple of things:

Your crankset is a triple.
Your cassette is a 7 or 8 speed.

Put the chain on the middle chainring and the middle of the rear cassette.

Now, get behind the bike and sight in the chainline. Yes, this is a somewhat crude judgement, but it will get you in the ballpark. Your chain should look (pretty much) in a straight line. You are now in the middle of your gear range and things should look like they are going in straight lines when you crank the pedals.

If your chainrings are not staying in plane then there is definitely a problem with the crankset installation, or the driveside crank itself. In this case, take the bike back to the shop and let them know your concern/s. The time to get problems fixed is when a bike is new. A shop is much more likely to fix your problem free of charge 30 days removed then they are after 6mos. or a year. Trust me on that.

Chainrings should stay "in-plane" when cranked. No if's, and's, or but's.

xxamr_corpxx said:

I bought a $99NZD folder,single speed, steel frame.

For a three piece crankset, BB shaft bent. 😉

For a one piece crankset, normal. :eek:

xxamr_corpxx said:

Hi,

After inspecting my new bike in more detail than I did in the shop, it seems the chainrings aren't perfectly in plane i.e when I spin the cranks the chainrings seem to "wobble" side to side, they don't rotate perfectly in plane. The chainrings seem to be ok but I'm worried about the BB.

Is this "wobbling" due to the bottom bracket being incorrectly installed (in which case I'm not worried since I was going to replace it with a lighter one), or is it due to the frame's BB threads not being on properly?

Thanks

So how wobbly is it?

If you're only talking about a couple of millimeters of run out, that's pretty common. If it's more than that, say 5mm or more, you probably have a bent crank arm. If that's the case you can try to bend it back but it's pretty hard to do.

You should know that the sculpting of the teeth to aid shifting makes the tips wobble in and out of the ring plane. My 105 ring teeth tips look like they're wobbling when I spin the cranks, but the ring itself is not.