Shimano Acera FC-M391 Crankset (2012)


It may come as a surprise, but the 2012 Acera components range looks better than ever. I say it’s quite surprising because it is Shimano’s entry level mountain bike range. And even more surprising is the new crankset which, at a first glance can easily be mistaken for a mid-level product. Does the good news stop here or are there even more chapters where Acera FC-M391 scores good points?

The new crankset has been designed to be used together with a 9 sprocket cassette, so progress is quite visible. Then, it comes also with an Octalink bottom bracket which is another progress as this system is, at least in theory, better than tapered square spindle, and should also have a longer service life. Basically the crank arm is mounted on an 8 tooth spindle and is locked with a larger diameter bolt.

The Acera logo has been revised, it now looks much better same as the crank finishes, as I was saying above, leaving competitors such as SR Suntour behind.

Chainrings are steel made, you can forget about aluminum or special reinforcing for this price category. But if you are in a shop and ask the shop assistant how much Acera FC-M391 costs, you’ll find out that you can buy it for only 30 euro, without Bottom Bracket, which is as decent as it could be. The steel chainrings are of course heavier, but also more durable, which is good news for such a bargain product. They can also be replaced individually when time comes, as all you need an Allen key to disassemble them.

The arms are not hollow on the inside, but at least their inner sides have been carefully sculpted in order to get rid of extra material and some weight. Even so, Acera is still penalized by a weight of 1.045 grams, which is a lot, as the bottom bracket weighs another 300 grams. Thus, the very heart of the transmission weighs no less than 1.345 grams.

The chain passes from one chainring to another relatively fast, but under no circumstances as fast as with a Deore, SLX or XT crank. It can be ordered with a tapered square or with an Octalink bottom bracket and the crankset is also available in two configurations, 44-32-22 and 48-36-26, for those who wish to be faster.

Shimano Acera FC-M391 crank is rigid enough not to stir any complains from its user. It’s well finished, looks durable and sells for a good price. Nevertheless it cannot compensate the heavy weight, which makes it lose important points. But it still is suited for mountain and Trekking bikes, and my final advice for those who made their minds to buy this crankset is to quickly get rid of the plastic protection.

Weight: 1.045 grams (without BB)


  1. good day. if i would upgrade my shimano altus, whats the difference between upgrading it to acera and alivio?

  2. Hello Dominik! Both, Acera and Alivio are a little bit lighter than the Altus. Of course, you have have also an option for buying a Octalink Bottom Bracket (Altus BB is squared).