Crank arm angle away from bottom bracket (Q factor?)

[rearwheel]

Hi, I'm currently on the hunt for a left crank arm (https://www.retrobike.co.uk/threads/left-crank-arm-replacement-175mm-diamond-fitting.432396/).

I'm confused by the angle of the crank arms though; is it simply the case that MTB crank arms are angled away slightly from the BB whilst Road bike crank arms are straighter? I'm looking for a straight one.

 

[FluffyChicken]

Angled, away...

It's the different in the profile, from normal, low and super low etc.

Ring stay in the same place and pedals stay in the same place, unless they are tweaking them to be further out or in etc.

The profile dictates the bottom bracket length.

So when superlow profiles came out in the early 90s for MTB, you gained heal clearance, often needed more off road when you feet get bashed around.
You get less bottom bracket metal, you may or may not get a lighter/stiffer crank arm.

 

[FluffyChicken]

What make model is you other arm?

 

[rearwheel]

It's Dura Ace I believe.

 

[FluffyChicken]

There will be a model number stamped on the back of the arm.
Then search on that number.