I recently built a new bike using a compact 2x10 drivetrain(48/34 front, 12-27 rear). I could have gotten away with a 1x10, but I occasionally need to use the 34t ring when carrying heavy loads. Given that, I spend a good deal of time in the big ring, and mostly ride using the 5 largest rear cogs.

Things are pretty much dialed in, but if I backpedal for more than a full revolution of the crank, the chain skips off the largest 2 cogs of the cassette and eventually jams up. When sighting down the chain, I can see that the chainline is way out in the big/big combo.

It appears to me that I can address this issue several different ways:

1) Use a shorter bottom bracket spindle. I can probably get away with moving the crank about 4mm inboard without any ill effects.

2) Go with a smaller big ring in the front. Putting a 46t ring on would probably allow me to spend more time in the middle of the cassette, and I'd be less likely to need a big-big combo.

3) Learn to live with it, spend less time cross-chaining.

Thoughts?

Don't backpedal...

Really this is more an issue with road bikes (short chainstays) and more rear cogs (narrower gaps between the chain and cogs) then other bikes. Changing chain line might gain you some latitude in one direction but make worse the other combos. Andy.

Not being able to backpedal on both of the large cogs may mean the chainline is a bit off. Unless you have already checked the actual chainline measurement that would be your first step. If the bike frame was new I would hope that the rear triangle alignment is OK. Moving the crank inward should be OK as it sounds like you are unlikely to use the small chainring with the smaller front cogs. But being able to backpedal more is not a great reason for improving/changing the chainline.

Back when I was a kid there were only two reasons to backpedal - braking or showing off that you had a fancy English racer (3 speed). Now there's really no function at all in doing so.

Yup.

Leave backpedaling to politicians and celebrities with bigger mouths than brains.

It's DEJA VU all over again (thanks Yogi).

We just got through discussing this very issue Save yourself some grief and us the time typing by reading the linked thread that explains your "problem".

I have never had a bike do that, even a folding bike with short stays for 20" wheel.
Clicks a little, not super smooth, but never jump, even fully crossed.

Check rear end alignment, chain line etc. If you have a square taper BB, you can go shorter, but not the hollow, external bearing stuff. You can remount the chainrings inboard with spacers. 46t ring would buy you about one cog shift in the rear. Make sure rear D has enough capacity, and front D can adjust inwards enough. Pop on a different wheel or cassette for test.

Assuming clean 10s chain, lubed, of good quality, normal spring tension, minimal wear on anything etc.

I think you've got it pretty much figured out already.

I use some combination of all three, myself. The crank gets about as short of a BB as I can get away with, since I'm a fan of using the big ring with the whole cassette. And I'll size the big ring so that I don't have any unnecessary gears on the high end that I won't use.

I don't have the issue of popping out of gear when backpedalling, though -- even in the big-big. If I did -- and needed the ability to do this -- I'd probably get in the habit of shifting to the small ring when I'm getting close to the bottom of the cassette, because that would put me nicely in the middle of the cassette again.

It's a minor issue; mostly when I'm locking up and need to spin the crank. I'll probably end up with a smaller chainring, as I'd really like to take advantage of the tighter spacing in the middle of the range.