120mm fork on frame designed for longer travel

bluben79
bluben79 Posts: 37
in MTB buying advice
I've got some bits an pieces that I'd like to put on a new frame including a 120mm Yari fork. I've been looking at several steel hardtail frames but most firms recommend anything from 130mm to 160mm forks. What are the disadvantages of running a shorter fork? Thanks

Comments

  • steve_sordy
    steve_sordy Posts: 2,446
    edited January 2020
    What length fork was the frame designed for?
    The length of the fork is different to the amount of travel. But some forks can easily be adjusted to have more or less travel. In those circumstances the fork gets longer or shorter by the amount of change in the travel (less the %sag).

    The reason I'm asking is that the frame will have been designed for a certain purpose (type of riding) and with a max rider weight in mind. Exceeding either will stress the frame beyond its limits and will risk frame failure. It's not a black and white issue, as in 2 mm over the limit and it will fail, but the more you exceed the design limit the more likely you are to get a failure. So putting a fork with 180 mm travel on a lightweight XC bike designed for 100 mm travel and then taking your 120kg arse to the double black DH trails on it will probably not end well.

    If the bike was designed for a 130-160 mm fork travel, then using a 120 mm fork will not stress the bike, quite the reverse - it will be less stressed because of the reduced leverage on the head tube. However, assuming the rest of the fork lengths are the same, the front of the bike will be lower down. This will steepen the head angle, making the bike more twitchy and less stable when descending. The bottom bracket and hence your pedals will be lower increasing the chances of the pedals hitting the ground. But the bike will be easier to control when climbing and you will be faster when dodging through the woods.

    It is all a matter of degree, small changes are unlikely to be noticed or even cause a problem. But the further away you get from what the bike was designed to do.... well, it adds up!

    Forks with less travel are generally lighter and don't deal as well with rougher trails. This is why the downhill guys operate with 180-200 mm travel and the XC guys with 80-100 mm. It is not a direct relationship however. I went from 140 mm lighweight trail bike with 26" wheels to a 130 mm more robust aggressive trail bike that had 27.5" wheels. The 130mm travel bike felt more capable on rough trails than the 140mm. This was mainly down to the geometry differences and the tyre and wheel spec, but with a big contribution from the suspension spec and its tuning.

    This is what makes mtb so much fun!! :)
  • swod1
    swod1 Posts: 1,639
    As steve says i'd see if you could get the air spring swapped and run it longer 140 or whatever you feel like.

    a lot of forks 120mm+ the stanchions length are usually ok to run longer travel 10 or 20mm more but just check first.

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