I have an AB oval chainring for my MTB - shimano 12sp chainring so can be mounted two ways according to which face shows out / in. However, because the ring is not flat (it sits in or out by a few mm depending on which face you choose) it fouls the chainstay on this bike when sitting inward. So it can actually only go on one way, sitting outward. But the ovality looks like it is in the wrong phase? What do you think?
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Yep, wrong way round. If it fouls the chain stay, it’s too big so won’t fit at all. Smaller ring required I’m afraid!
Lets not confuse two things here.
First - the phasing of ovality - its spot on in your pictures to me? Look at the top - crank arm is over the top dead spot, your leg is in its maximum power zone pushing down - and the effective diameter is biggest.
Bottom picture, your driveside leg is beginning to travel backwards, your opposing leg is going over the dead spot at the top - the effective diameter is smallest.
Now the 'back to front' bit. The chainrings have an offset (or certainly all the AB rings I have do). On the SRAM versions I have this is set so that the chainring outer is AWAY from the back of the crank arm, and TOWARD the frame (or OUT as you put it). Yours may differ - refer to the instructions.
Interstingly according to the AB website the Shimano rings appear to be flat, not offset like SRAM -
https://absoluteblack.cc/oval-direct-mount-xtr-m9100-chainring-for-shimano/
As long as you have the marking behind the crank arm, it can't be wrong either way round unless I'm missing something here?
They're not symmetrical about the crank arm so if you flip them, they turn into biopace.
Actually, having looked at it again, in conjunction with the photo on the website, I do still think its on inside out (and thus too big for your frame). The timing is suppsoed to be around 110deg after TDC - yours is showing as slighty AHEAD of 90deg after TDC, it should be slightly behind 90Deg post TDC.
Flipping the chainring round would achieve this.
What size is it?
EDIT - What @onzadog said, basically.
Thks - more of a sanity check as it looked wrong but I started to overthink things. It's a 34 so not silly big or anything, just there's not much clearance on the bike (a trance 29er). A spacer would sort things out but it doesn't look like the crankset has room for one.
Clearance on a lot of 29ers is tight for chainrings. With a lot of folks running 30t or even smaller, not many folks outside the XC race world run a 36t, which is effectively like a 34t oval.
I can't tell what the crank is, but check whether the splines are a regular pattern. If so, you can flip the ring to get clearance and then adjust the clocking. It will do weird things to your chain line though.