Nosnibor

Hi, I have inherited an old steel road bike with high end Campag groupset. I can't post pictures yet and will do when I can as I have no idea what it is, rides nice though!

It does have a bent arm on the campag GS drive side crank spider that causes the chainring bolt to rub the frame slightly. It is acceptable to simply bend them back using a bar with an ID that fits over the arm?

3alarmer 

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...personally, I would not bend straight the arms of an aluminum alloy crank, and then ride it. I'm sure that you will find at lest one or two people here who are fine with it.

AFAIK, Campy Gran Sport is not considered "high end". There's a real possibility that straightening that crank arm will lead to eventual failure in use. It's not as catastrophic as something like a front wheel collapse, or a fork failure. But if you happen to be standing on the pedals, or otherwise exerting significant force on the pedals when it happens, it can certainly throw you off the bike. If you do decide to just go ahead and do this, check that arm of the crank regularly for any telltale cracks that might be starting.

seagrade 

I have seen workers in the Vicenza factory micro-adjusting new RHS aluminium crankarms forgings for alignment. The cranks were mounted to an axle and the spider tab flats were checked against an indicator gauge and where necessary cold set using a custom tool not unlike an adjustable wrench in principle. It was a very routine-looking process, from memory for each and every forging, although I doubt anything was moved more than a few tenths of a millimetre. Chainrings were not fitted and the crankarm itself was left untouched. It took the worker only a few seconds to complete a very practiced process for each arm.
Memory fades, but my recollection is of Nuovo/Super Record arms, but as I didn't visit the factory until 1988, by which time I think they had ceased production, it's possible they were a newer model.
I'd be surprised if the base forging for GS cranks, once they went to five-arm, was any different to Nuovo/Super Record but I stand to be corrected in this respect.
I saw dozens of broken Nuovo/Super Record RHS crankarms in the 1980s and early 1990s and would now hesitate to ride fluted versions other than for theatrical purposes. The final version without crankarm fluting and with laser-etched rather than engraved shield logos I'd still use in relative action, were I to find any.
Reflecting on all that, I'd have no more concern about riding said crankarms after micro-adjustment to any of the three spider arms not adjacent to the crankarm itself than without. Almost all the breakages I saw were in the "V" between the crankarm and either spider arm adjacent to it, although there were occasional failures in conjunction with the engraved shield.
Hope that helps, I can't overthink it any more than that...

Nosnibor

Wow, a fountain of knowledge. Thanks!

​​​​​​The arm only mm out of true.

Getting scientific, I'm not sure of the grade campag used for forgings but bending slightly and blow torching to anneal it after I reckon is absolutely fine 🙂 just wondered if this was a common issue.

Miloburnz

Be careful with blow torching as the crank will have a protective coating on it so will not look so good if you burn it all off. If it's only a few millimetres out I'd just warm up the bent spider arm with a heat gun put it in a vice and push it straight or tap it straight with a soft hammer then dunk it in cold water.

T-Mar

Are you sure that it is bent spider? Have you removed the crankarm and examined it on a know flat surface, such as a surface plate? Chainring rub can be caused by other issues, such as bent chainrings, bent spindle, crankarm flex, spindle flex, frame flex, etc.

Schreck83 

Impressive first post!

repechage

By 1988, Corsa Record or one of the other
Valentino era groups.
in thinking about the order of operations, after machining would be deburring, polishing and anodizing. I guess aggressive polishing could move the metal.

as the bike will be new to you, take the cranks off as suggested, evaluate that and rebuild the bottom bracket.

Road Fan

Allow me to assist with the over thinking (I will overthink so you don’t have to!)! Microadjusting this way is cold-setting, which is the bending of an object past its elastic limits in order to change its shape in a post-stressed state. Aluminum degrades its structural integrity whenever it is cold-set. For it to have been done once or at least within the confines of the Campy factory makes it Campagnolo’s responsibility to prevent breakage while in the subsequent customer’s hands (that’s us!).

A manufacturing company can take this on because they can test the effect of aligning flawed parts and measuring any loss of strength. They can even find out how many times this can be done before the durability is compromised. We don’t have a method or tools to do that, or knowledge that would be needed.

3alarmer 

...I told you you would be able to find some people here who were OK with it. Most people who post stuff like this on BF are just looking for reinforcement, for something they've decided to do anyway.

If the rings are running true enough for you to be comfortable using them as is, have you considered a slightly less cumbersome repair ? If the minor interference is the only thing bothering you, it's not a big deal to fit and install a longer BB spindle, moving that drive side arm out a mm or two. Actually, you could probably accomplish this with a spacer on the drive side, but I'd need to see what you're working with. Regardless, the advice to remove the arm and evaluate on a flat plate of glass or milled steel is probably how I would start. YOu might very well save yourself some grief with a more thorough evaluation.

Nosnibor

​​​​​​
Very true. Thanks.

tiger1964 

An actual crank arm -- I'm behind that 100%; it takes 100% of the thrust load each revolution. Certainly a "there's a bad moon on the rise" situation.

A lateral tweak of one or more arms of a spider? Not sure that would bother me. Done it myself and, so far, no catastrophe; I presume each arm of the spider takes 20% of the load. And an out-of-true chainring causing chain drag on the FD is truly infuriating, to the point that I recently considered converting my entire fleet to 1x setups.

Nosnibor

As a follow up to this. I whipped the crank off, lay it flat, one of the spider arms was very slightly out by ~1mm.

Tweaked it true enough to stop the rub and re-fit to the bike.