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  1. #1
    Liam123
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    Default how do you decide fork length? (harley 45)

    I haven't started an actual build thread because I still need some stuff. Just got my triple trees from Twinside cycles in the mail today and now I want some tubes (39mm). My question is how do you decide how long your tubes need to be? Using rake and wheel size? Also, not to get too sciency but how do I decide the fork tube stiffness and such? Surely I won't need anyhting too crazy for a souped-up flathead 45 with mayyyybbbbbbeee 40 hp. I've sort of just used Sportster parts on this bike because the size and weight seem similar. What are your thoughts?

  2. #2
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    Fork length as well as stiffness outside of stock parameters come down to aesthetics and comfort, so there’s really no universal do it this way answer. The original springer on a 45 compared to 39mm which are around 24.5” depending on the application are obviously different. What frame this fork setup is being mounted to as well as it’s rake wether stock or altered also plays a role. You will have to determine these things as well as the position you want the bike/engine to sit at. Front wheel size will also alter this dimension as well.

    Personally I’d figure out my current frame rake, determine a front wheel size, and then determine what kind of stance you are looking for and buy tubes accordingly.

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    If you hunt around on the web there are a few sites that help you calculate the length you need once you have the info that ECE was mentioning, mainly rake and wheel size. The calculation you need is based on getting the correct trail so the bike handles decently at speed and in turns.

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    IMO, that is a very good question and, since you're using a hydraulic front end, it is more complicated than one might think..

    Because I went the "sciency" route with mine and it didn't work out at all. What we want are aesthetics, right? And there's variables.

    Like the height and rating of the springs. The weight of your bike. (It's a 45, so you're in uncharted waters) The weight of you. Your rake. How much damper length you have to play with. The stance you want. Etc..

    If you're like me you can't wait to buy parts and see what it looks like, right?

    But my advice would be to go to swap meets and look on ebay till you find a set of used 39mm lowers and install them. Put the wheel on and the engine in and get it set up.. THEN look at it. Sit on it. Bounce on it. Put the seat, bars and the tank on it.

    And what you will probably find is that you want to tweak the rake a little. (You can get 3 degrees with a set of cups, and for cheap.) Then choose your tube length accordingly and you now have a reference point with the old tubes.

    The goal is to get it close enough you don't need to change anything, but.. If you want to, you can fine tune it with some PVC spacers and not have it thunking against the dampers all the time or being at a height you don't like. Because a poorly set up front end that is jacked around with spacers and doesn't work well is a PITA and you'll hate it.

    Then you can rebuild the old forks you bought, polish them up, whatever, and you have exactly what you wanted the first time and for a good price.

    Hope this helps.

    Attached Thumbnails Attached Thumbnails 20210311_110913_HDR.jpg  

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    PS: Any tube is stiff enough for a 45.

    Unless you're into cartoonish fork lengths? I wouldn't even worry about it.

  6. #6
    Liam123
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    Thanks for the advice! It's been hard to visualize without wheels so I might hold off, I'm not into redonkulously long front ends myself and I think I only have about 32-34 degrees of rake, just enough to look like a chopper but not enough to look stupid (I own a 10 over springer so I can say that). I've seen maybe 3 45's on the interwebs with hydraulic front ends so yea, it's not that common. I'm really chasing good performance to match the other nice stuff I got for this bike (4 speed trans, stainless bars, wr tanks, kr grind cams blah blah blah)

  7. #7
    Liam123
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    found this one, https://www.rbracing-rsr.com/advchoppercalc.html . I'll get an idea and post the results

  8. #8
    Liam123
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    Yea I was thinking a 19in wheel, I think the rake is 32 but im double checking now

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