Questions/concerns about new fork on my trail bike

Forums Mountain Bike Forum Questions/concerns about new fork on my trail bike

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This topic contains 5 replies, has 6 voices, and was last updated by  Alvin Mullen 1 month ago.

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  • #290711

    Hey!  It’s my first time posting here after getting mixed responses from Reddit.

    Yesterday,I just picked up a barely used 2019 rockshox revelation along with a rockshox deluxe rct3 for a really good deal I couldn’t pass up on. The rear shock fits perfectly because it was the right sizing but the fork is 150mm travel. I ride a Marin rift zone 1 which is a 120mm travel trail bike. The original fork was a rockshox recon with 120mm. I got excited and threw it on my bike after I got it. I’m just mainly concerned about stress on the frame durability due to the larger increase in travel. I have the 29er version  which is a 120mm travel but the 27.5 wheel model comes with a 130mm travel fork (they share the same frame), so in my mind I thought it was ok because I’m only going up 20mm. Im running it super plush so that there’s less stress on the frame. test rode it yesterday on the street and it didnt feel weird to me. I know it changes the geometry drastically but Im willing to test it out first. My other option is getting a 140mm air shaft for $30 and swapping it instead of running it at 150mm. Haven’t gotten to test it on the trails yet because I need a new rotor adapter. Any suggestions/explanations?

    Should I keep it at 150mm travel while running it plush or just get a 130mm or 140mm airshaft and swap it? I could also flip it for and just get a different fork if I really wanted to.

    Should I be worried about frame damage?

    Sorry it might be a little confusing. Didn’t really know how to put it into words effectively.

  • #290757

    Ok lots to comment on there….

    If your bike is made for 120mm of travel, the most you would want to change that would be 20+/- either way. You’re right to be worried about stress to the frame. Most bike manufacturers will give you a range of fork travels that will work safely with their frame. If you go for more travel, you’re going mess the geometry up and affect the way it climbs, and you’ll be putting increased stress on the head tube. If your bike is under warranty you probably also risk voiding the warranty by going to a 150mm fork.

    If it were me, I wouldn’t go with anything bigger than 130mm in the front. 150mm is Enduro bike territory, and as you can guess, a proper euduro bike will make better use of all that travel than a 120mm trail bike could.

  • #290760

    For most bikes increasing or decreasing the stock fork travel by about 10mm won’t affect the geometry much or cause the frame to fail.  However when you jump up 30mm you will be changing the geometry quite a bit and you will be stressing the frame in ways it wasn’t designed for, which could lead to frame failure.

    In addition, using the 150mm fork will almost certainly void any warranties.  If your frame should fail and/or you got hurt, you would have no legal standing to collect damages for your bike or your own personal injuries.

    Well there’s the negative side!  In real life, most frames are overbuilt and unlikely to fail even with an extra 30mm of fork travel.  The geometry probably won’t be too bad and especially so if you set up the fork with a lot of sag, like say 30%.  You could certainly try it and see how it performs.  However, I would check the frame frequently for cracks.  Just remember, you are assuming all the risks.

  • #290780

    You should be able to buy a 130mm air spring for that fork, that’s the most travel I would run on that frame.

    make sure it’s a DebonAir air spring for your model year.

    It’s an easy swap that will give you a tad more travel and keep your bike in spec.

  • #290782

    As others have said, it may void your warranty – maybe get in touch with Marin directly or a Marin dealer to find out what they will allow there.

    Honestly I’d be more concerned about the geometry changes than any frame damage – frame failures at the head tube are VERY rare these days, even when over-forking bikes. Manufacturers don’t want to deal with lawsuits….

    In terms of geometry, it will make it longer and HTA slacker – good.
    It will also raise the BB a fair amount and slacken the STA – bad.

    As others suggested, you should be able to get an air shaft to reduce the travel to 130mm pretty easily and cheaply. If you’re a competent mechanic they’re easy to fit. That would probably be my preferred route.

  • #290813

    There is no need to worry about frame damage unless you are hucking off 6ft flat drops, in which case you could damage the frame with the stock fork. The actual increase in torsional stress to the head tube of the frame is minimal until the increase in length of the total fork is more than around 20%. Total length of a 120mm to a 150mm goes from around 510mm ti about 560mm, only 10% increase. and with the increase in sag it is actually less than that.

    Now that doesn’t mean the company won’t use it as an excuse to void the warranty, or that the change in geometry won’t cause it to handle strange.  But I wouldn’t be worried about any frame damage.

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