Lefthand crank arm for spinbike BH SB 1.8 i am trying to buy a new crank arm however there are 2 different type, a rhombic hole or square. After looking at the picture on Amazon, they look both the same. any help would be welcome.
Look at the square hole at the root end of the crank. You'll see that the square hole type has a square where two of the sides are kind of parallel with the crank.
Looking at the rhombic hole, the pictures will show that whilst the square hole is still square, it is turned 45 degrees and you could say it is more like a diamond, the edges of the hole are pivoted.
The holes will be the same, just turned by 45 degrees so it's important to match the existing good crank so they end up aligned. So if you are replacing just one crank, look at the square and see which way it is lying and match up accordingly.