Rockshox RS-1 fork, converting 100mm to 120mm. Will it change bike geometry?
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  1. #1
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    Rockshox RS-1 fork, converting 100mm to 120mm. Will it change bike geometry?

    my mtb has the Rockshox RS-1 100mm, and I'm considering upgrading to the RS-1 RL 120mm. My understanding is that both forks use the exact same housing, the difference in travel is due to the inner air spring cartridge.

    My question is, is the 120mm fork will have a longer axle-to-crown height, and thus changing my bike geometry? My bike is currently a hardtail running the 100mm fork, and I'm hoping that the 120mm version of the fork will not alter my bike geometry much?

    thoughts?

  2. #2
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    You'll end up running a little more sag so you won't gain all of that 20mm in ride height, it won't change it too much, less than a degree.

    https://bikegeo.muha.cc/

  3. #3
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    Quote Originally Posted by harryman View Post
    You'll end up running a little more sag so you won't gain all of that 20mm in ride height, it won't change it too much, less than a degree.

    https://bikegeo.muha.cc/
    thanks for the link. Very interesting numbers. I'd need to think about this. The question for me now is, will this slacker geometry effect 15% grade climbing? And what about at 20%? as it is, I think I can live with 15%. But at 20%, this will be tricky as I can barely hold the front front down while putting down power, and a slacker geometry will means more work

  4. #4
    'brifter' is f'ing stupid
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    Who knows how you'll react to the change? You can't quantify how much worse it will be or not. How much time do you spend climbing silly steep hills? If it bothers you now it will bother you 'somewhat' more with a longer fork.

    It's not the slacker geometry that's going to cause you problems it's the weight distribution that's the issue. Your weight is going to move up and back.
    #promechaniclife

  5. #5
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    Quote Originally Posted by cxwrench View Post
    Who knows how you'll react to the change? You can't quantify how much worse it will be or not. How much time do you spend climbing silly steep hills? If it bothers you now it will bother you 'somewhat' more with a longer fork.

    It's not the slacker geometry that's going to cause you problems it's the weight distribution that's the issue. Your weight is going to move up and back.
    currently my lowest gear is 34x42, and with the current 100m fork, then at 20% gradient i'm pretty much at my climbing limit while my face is probably 6 inches from the bar. I don't climb this often but there are a few trails around here that have these 20% segments. For the 13%-15%, I can climb in a relatively relax position, meaning I don't have to hunch down so much and just let my body fall forward a bit and I'll be in a sweet climbing zone.

    Anyway, after talking with a couple XC guys, they both said no. Stay with the 100mm fork as this frame was originally optimized for such fork and usage. They say going to 120mm might not give enough benefits on the downhill and technicals stuff to offset what I might lose on the climb. But without actually trying the 120mm, I won't know the full pros and cons both forks to make a full comparison. However, I'm also not too keen on spending over $300 for a chance just to try 120mm. So, i'm staying with the 100mm one.

  6. #6
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    My MTB is a mid-2000s hardtail that came with a 100mm Manitou fork. A few years back it started leaking and I went searching for seals, but came across a 120mm Manitou Marvel on the cheap. I bought it and threw it on. It worked fine, nobody died. I didn't really notice the difference in riding position, but did notice it rode better. I'm not an accomplished MTB rider, so my opinions probably aren't worth much.

  7. #7
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    Quote Originally Posted by bilbo View Post
    My MTB is a mid-2000s hardtail that came with a 100mm Manitou fork. A few years back it started leaking and I went searching for seals, but came across a 120mm Manitou Marvel on the cheap. I bought it and threw it on. It worked fine, nobody died. I didn't really notice the difference in riding position, but did notice it rode better. I'm not an accomplished MTB rider, so my opinions probably aren't worth much.
    thanks for the input, but yeah i'm looking for more analysis than just "nobody died" at what i'm doing. Factors like

    - long and sustained steep to medium-steep climbing, and fireroad vs switchback climbing
    - braking, longer forks tend to feel noodly in straightline braking
    - handling, longer forks absorb bumps better but also less lateraly rigid

    every little change, I think I will feel it, the question to me is pros and cons and which side of the equation is more important to me.

    pretty much all the XC hardtails I see on the trails from guys, are equipped with 100mm forks. Seems like 100m is the sweetspot that major manufacturers go with, so I'm just gonna stick with it.

  8. #8
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    Sorry, I wish I I could offer more than that. I guess the change was pretty minute to me. I understand sticking with the known quantity of 100mm. I wouldn’t want to drop $300 on an experiment either. If I hadn’t found the fork I did on the cheap I would have done the same.

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