Anyone ever built a leaf sprung front end?

Discussion in 'Some Assembly Required' started by shinyribs, Jul 15, 2019.

  1. shinyribs

    shinyribs doesn't care

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    I've got a project bike that I'm collecting parts and information on. I really want to build a leafer front end for it. The project will be worthless without it. I have the tools and ability to fabricate this, so please don't try to talk me out of it. I just need to learn how to engineer the geometry so it'll function and handle properly. I have no clue where to start as far as geometry for things like the rocker and what rake and trail works well for these setups. I'm not even positive how things like rake and trail are exactly measured with these style forks.

    This is what I have in mind. Any info would be greatly appreciated!
    8372729497_3280b7480d_b.jpg
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  2. VX Rider

    VX Rider Long timer

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  3. VX Rider

    VX Rider Long timer

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  4. Z50R

    Z50R Not lost yet

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    I have done a decent amount of reading on the subject of motorcycle and bicycle dynamics. From what I can tell, there is still a lot of “we know it does this with this geometry” but there still is not a simple formula where you can plug in a desired performance output and the formula will yield the required geometry. I have not paid the big money for the good frame building books nor am I an industry insider so I could be wrong (and would love it if someone could post up formulas and field questions).

    My approach is to emulate the numbers on bikes I like riding. A steeper fork angle will make for a twitchier ride and a chopped angle makes for a more sluggish ride. Caster (how much a plumb line from the axle the axle is behind the axis of the steer tube when they touch the ground) is necessary to prevent the bike from throwing you off. The more caster the easier it is to ride with no hands and the more force needed to steer.
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  5. VX Rider

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  6. Z50R

    Z50R Not lost yet

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    Most modern forks are telescopic forks. These are used because they minimize the effects of braking forces on steering dynamics. The forks you have posted are trailing link forks. The axle pivots about a point in front of the axle. Since applying the brakes while the motorcycle is in motion creates a moment around the axle and the trailing link suspension is designed to react to such a moment, applying the brakes on flat ground causes the suspension to compress, more-so than you might be used to with a telescopic fork. This reduces the available suspension travel should you hit a bump and therefore makes the ride harsher under braking than it otherwise would be. It also has the effect of steepening the steering angle under braking making the bike a bit twitchier under braking. None of this matters much if you are working with small drum brakes like in the picture but could be a real, noticeable issue if you go with large supermoto disks.
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  7. VX Rider

    VX Rider Long timer

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    He said....."don't try to talk me out of it"
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  8. Z50R

    Z50R Not lost yet

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    Make no mistake, I am not trying to talk him out of it. He asked for information, I am trying to get him some. I want to see this thing made. If I can find sources to back my understanding of trailing link forks, it would hint that an intentionally slacked steering angle would be desirable - something you’d want to know about before cutting and welding.
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  9. shinyribs

    shinyribs doesn't care

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    Hahaha :lol3 I didn't think Z50R was trying to to do that, really. I intended that comment for the "if you build your own forks they will break and you will die" types of folks. Sound advice regarding the construction is exactly what I need!
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  10. shinyribs

    shinyribs doesn't care

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    I know you are right about all these things, but I'm not sure how much translates between telescoping forks and these girder setup with the rocker arms ( leading or trailing links) at the axle. The reason I say this is, if you look closely at the bike above, it appears the yokes on that from end have very little offset forward of the neck. In fact, it almost appears to be offset BEHIND the neck. So to figure trail on a trailing link like this, I wonder if you use the steering neck centerline as the basis for caster/trail...or the centerline of the center pivot of the rocking link. I really don't know, but my gut is telling me that plays in some how.
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  11. VX Rider

    VX Rider Long timer

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    They will be along shortly I'm Sure...

    And you're probably smart/careful enough that you'll make then 'as if' your life depended on them.
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  12. CopyCat

    CopyCat I am NOT like the others Supporter

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    Here's a board track racer @Rebuilder did for me. Cut a trailer spring in half
    olds.jpg
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  13. Z50R

    Z50R Not lost yet

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    Scroll down to “brake dive”

    https://en.m.wikipedia.org/wiki/Suspension_(motorcycle)#Brake_dive

    The reason leading link suspension exists is because the braking moment on the leading link combats the weight transfer that compresses the suspension. If you go with trailing link suspension, you amplify the brake dive effect rather than combat it. Again, not a “don’t do it comment” but rather a “be aware and design with it in mind comment”.
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  14. shinyribs

    shinyribs doesn't care

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    Good point on braking energy having an effect on the suspension. This is one of my main concerns. I imagine the pivot point ratios on the axle rocker can be manipulated to increase/decrease this effect. I'm also not dead set on a trailing link fork. A leading link would be fine as long as the aesthetics work, which it should. Leading link vs trailing link...all this is the stuff I am trying to learn.

    I really appreciate you guys posting up! VX Rider, I haven't gone through your links yet, but I will. Thanks!
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  15. shinyribs

    shinyribs doesn't care

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    That's a beauty! What exactly are we looking at. For a moment it look like a Honda XL engine, but the plug location is wrong and it's a single exhaust port.

    Curious what the design reasons are with this style front you have where the pushrods ( terminology?) that run from the axle rocker to the spring run criss cross past the girder vs straight up the front like the Indian above has. Again, this is the stuf I want to learn.
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  16. Z50R

    Z50R Not lost yet

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    Steering geometry does not care what kind of suspension you have. The geometry’s effect on handling is independent of the method or even if there is suspension (hence bicycle dynamics can be substituted freely). The fact that you have suspension means your geometry changes depending on how much your suspension has compressed. The type of suspension you choose changes how the geometry changes as suspension compresses and changes the effects of braking forces on suspension compression.
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  17. Z50R

    Z50R Not lost yet

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    The rods are where they are on that fork because it makes the look of the trailing link suspension like you posted while it is really a leading link fork.
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  18. CopyCat

    CopyCat I am NOT like the others Supporter

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    It is a 76 Xl350. For exact geometry discussions I would strongly suggest you PM @Rebuilder. He designed and built it. He's pretty darn good at what he does. Subject change:He's just finishing this one for me too
    speedway 5.png
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  19. shinyribs

    shinyribs doesn't care

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    The axle is still aft of the fork legs, so I believe it's still a trailing link. But the support rods running from the leaf to the axle rocker would be under compression during bumps vs under tension like the Indian above. Notice how the Indian leaf pack is stacked to bend downward (main leaf up top,additional leafs under) whereas CopyCat's leaf is stacked to bend upward(main leaf underneath, additional leaf above). Though the Indian does seem to have one additional leaf above the main leaf for "rebound".

    Man, I'm probably butchering all this terminology!
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  20. CopyCat

    CopyCat I am NOT like the others Supporter

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