filing crankset spider tab to correct chainring wobble?

[SurlaCraque]

Hey guys, I've searched but didn't really find a good answer to my question. So here it is, I have a Rene Herse crankset where there's a little bit of lateral run out on the rings measured as they pass the front derailleur cage. Not more than one mm I'd say. So probably acceptable but I'm a perfectionist, also the truer the rings run the less FD trimming you have to do. I've examined the set up carefully and have ruled out the following possible causes:
- bent chainring
- bad BB bearings
- bent BB spindle
- not true crank arm mounting hole

Basically the big ring dips outward when one of the three chainring mounting spider arms spins past the FD cage. So it's not wobbling around a misalignment somewhere, it's more like one of the three spider tabs is not in the same plane as the other two. The tabs measure the same thickness using a micrometer, and it doesn't look like any of them are bent. I've fixed a similar issue before using very thin (0.2mm to 0.6mm) chainring bolt washers, but I'm planning on 11 speed chainrings with this crank which has ramps and pins and I'm hesitant to increase the spacing between the rings, for fear of degrading the shift quality or dropping the chain in between. Would 0.2mm be anything to worry about? I know it's not a lot, but that's more than half the difference in width between 10 and 11 speed chains - i.e. the width that the chainrings "see" on a shift between those two chains. I was also wondering about trying to file or Dremel grind a little bit off the outside of the spider tab that is a little too outboard and then using a thin washer on the inside to correct the issue. I would definitely be concerned about not getting the tab smoothly and evenly relieved using my home mechanic tools and skills... has anyone done such a thing successfully?

TIA

[Too Tall]

Before you do something you may regret. Are you positive you have torqued the crank bolt to spec.?

[Mark Kelly]

I wouldn't have at the crank spider until I'd checked it for trueness.

Here's one easy method: remove the chainrings and remount the crank spider to the BB. Take the appropriately sized Paragon Machine Works tube block and clamp it on the down tube so that it is square to the axis of the frame and the crank spider just overlaps. This will give you a datum from which to measure the distances to the faces of the crank spider with a simple set of calipers.

[ant_tip]

I have successfully straightened bent Campagnolo spider arms with adjustable spanner. Very easy, took about 5 minutes to get everything perfectly straight.

[j44ke]

I've used a plate of glass (well, a mirror actually but glass nonetheless) to check crank arm trueness. A bit awkward but helped to gauge the actual difference. You can use it to check from one arm to the next by using the surface of the glass as a horizontal "square" on the clamping surfaces of the tabs as well as lay the whole crank spider on the glass to measure distances (depending on design of the crank.) And you can also lay down the chainring that you've been mounting on the crank and check it for irregularities. The tabs on the crank and the tabs on the chainring could all be in alignment, but the outer edge of the chainring is in some way deformed. This is especially true perhaps with a Rene Herse crank where the mounting bolts are very close to the center of the crank. Also might be a good experiment to find some wafer-thin spacers/washers to insert between the chainring tab and the crank tab to see if aligning the other tabs to the one that appears out of alignment works.

If this is a newly purchased crank, I would be hesitant to modify it. If you figure out that it is truly irregular, send it back to the seller.



Don't think the tabs on this^ design are going to bend well or easily!