Ferguron

I am replacing the chainrings on a 10 speed bike. The old ones are steel, the new ones are aluminum. They are 110BCD and connect to the cranks via a five bolt pattern.

Sounds easy enough but...when I finished and tested it I found that when on the small ring and the fastest 2 cogs out back the chain is catching the pins and trying to shift onto the big ring.

Some might call that cross-chaining and that I shouldn't be using those gears but with the old steel rings I had no problems. I was not really trying to turn this old bike into an automatic.

I noticed the tabs on the old ring are bent in a little to push the ring out a little. The new rings have no such bend.

Has anyone seen this sort of issue? Did I buy the wrong type of chain ring?...I guess I’ll try putting a washer or two under the big ring.

cxwrench

You shouldn't cross chain small/small for just this reason. Easy fix: Don't do it. Those gear ratios make no sense anyway.

JohnDThompson 

Rather than putting washers between the ring and the crank, which would increase the risk of the chain jamming between the rings, you could put a spacer on the bottom bracket to shift the whole crank outward.

Andrew R Stewart 

Most likely the new rings have lift pins that stick out a tad more than the cheap steel ring had.

Rings can have a dish to them so when mounted the right way the two rings end up having the correct center to center of teeth spacing. Because steel is both stronger and stiffer than Al is the steel tings are most often thinner, thus a dishing, or offset, is needed to maintain the C-C dimension (that was developed for Al rings first).

First thing I would look at is how the front shifting/chain rub (ft der adjustment) is. If this is good than I would not add any spacers between the rings. That would only make the shifting worse and increase the chain rub on cage potential. Next i would look at the chain line of the rings and how it compares to the cassette's chain line. I would only change that (by spacing the BB, as John said) if the chain line was not pretty close to the same ft and rr.

The no cost solution (but one that is far harder to do) is to not use those combos. Andy

Chain catching the insides of the larger rings (made worse by those lift pins) is a result of how much tooth count difference there is between the rings, how well the rings chain line matches the cassette's and how short the chainstays are.

79pmooney

A trick you can do while riding (if your shifter allows it) is to move the derailleur cage in a touch while in those crossed positions. Now the chain is held away from the pins. You hear the chain rub but everything works fine. (You will have BFers, both alive and passed on to the other world, rolling over in their respective beds or graves for intentionally riding with chain rub and dooming another precious derailleur to an early death. I both do this and set my limit screws to rub in the extreme gears to minimize chain drops. I used to kill Cyclones regularly in 5-10,000 miles. I make no apologies. For me, it's about the ride, not the bike.

Edit: To cxwrench's comment re: crossover - I do it on purpose on long climbs with short breaks of flat or downhill so I don't have to double shift twice. Again, for me, the bike is there for the ride and for me, not the other way around.

Ferguron

I took another look...the problem I'm having has nothing to do with the DR...there's no rubbing at all. The problem is that the rings are just too close together.

The large ring has zero dish. Just not using the smaller cogs seems potentially dangerous...if I accidentally find myself on those small cogs, the chain tries to climb onto the bigger ring but it's not what one would call smooth transition.

I think maybe I need to go back to the heavy steel rings...all worked well with them...but they are really heavy.

Andrew R Stewart 

I didn't say that there's a current ft der problem, just that with changing the rings C-C or chain line there could be. That there is no ft der or rub after shift suggests that the rings have the correct C-C now.

There's nothing dangerous about avoiding some cog/ring combos. In fact some here (and in real life have found out) that using the combos with issues can be dangerous. Andy