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  1. #1
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    Fat VS MTB Fork Geometry

    Im Building a Girvin style linkage suspension fork for my Fatbike. Generally speaking, should the geometry be the same as a MTB? For example, the distance between the centerline of the axle to the centerline of the head tube. Or the head tube angle achieved by the length of the fork legs.

    I have a Yeti 575 that I love. Ill copy its fork geometry if the fatbike will react roughly the same...

  2. #2
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    Interesting project.

    Why not make the offset adjustable? It would be easy to do on a linkage fork.

    That way you can fine tune it once it's built.
    As little bike as possible, as silent as possible.
    Latitude: 57º36' Highlands, Scotland

  3. #3
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    Due to the larger wheels, the trail will be different with the same head tube angle. This affects the handling. I have a theory that you should try to mimic the trail on your present bike to allow for better handling. 50mm rake is not unusual on fatbike forks.

  4. #4
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    Thanks for the replies. Ill start with what the bike is now, and make it adjustable.

  5. #5
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    While I'm waiting for the perfect rear hub to widen, I started building my suspension fork.

    I started with some .065 wall 1-1/2 inch tubing. I welded in some 1 x 3/4 steel spacers. Then inside those I welded some 3/4x1/2x1/2 steel spacers. I will wait till its almost done to do the dropouts, so that I can play with head angle and height.

    I realize its going to be heavy, but its my first attempt at a fork.

    Heres some pics:














    The connecting pics will be aluminum. These wont have the shock mounted to them, so I wasnt too worried about the 1/2" aluminum dowel being strong enough. I drilled and was taping the first link when I broke my tap. TIme to call it a night!





    And the teaser shot!



  6. #6
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    I got my link pins that go between the fork legs done today:



    Here is what they look like installed.



    The I made the forks links. They are made from 1 X 3/4 aluminum bar. I made them 5 inches long bolt center to bolt center. I know thats pretty long, but its a lot easier to make something shorter, than longer. I used nylon spacers that are a common size for bushings. They can be replaced with bearings later on if need be.



    I had an old junky double crown fork for that I stole the crowns from. I cut them down to size and will thread them for the proper bolts in the next few days.



    Here is where I am so far. I am really happy with how its turning out. I know its going to be heavy, but its also going to be SWEET! And it should easily clear a 5.0 tire.










  7. #7
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    I have a Fox Alps 4 that I will use as the shock absorber.

  8. #8
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    That's coming along well.

    Have you calculated the effect of the fork travel on the trail etc?

    And are you going to have the brake mount floating to get anti-dive?
    As little bike as possible, as silent as possible.
    Latitude: 57º36' Highlands, Scotland

  9. #9
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    I havent figured it out exactly, but I think fork travel is going to effect the trail way too much. I may end up having to shorten the lower links a bit to keep it under control.

    Floating brake mount? I had not actually though about that, but now that you mention it, its probably a good idea.

  10. #10
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    OK, so this is where I am at:



    I have 50mm of trail with the fork in the position above if the dropout is centered with the back of the fork leg. Depending on where I mount the shock I have I can get up to 5 1/2 inches of travel, but that seems like too much. Im thinking 4" would be ideal for it. Also, with the height I would need for 5 1/2 inches of travel, my head tube angle would be 63 degrees. This isnt a DH bike so thats too slack!

    Here are some other pics:





    Im going to use the Fox Alps in the pic below. Im sure it will be the best of these 4 shocks.


  11. #11
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    I like the one on the left. It's got a spring. Much better than any of those quality designed air chamber dampened shocks.

  12. #12
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    LOL, Ya thats true. Plus its probably a pound heavier. And we all know that the heavier they are the better!

  13. #13
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    WELL! My Fork is ALMOST done. I just need to make the dropouts and disc mount. Here is a video of the shock!



    Its real smooth!

    Its real heavy unfortunately, 9.4lbs. Most of the weight is in the legs. When I get some extra $$$ ill pick up a TIG and make them in Aluminum. I was shooting for 4" of travel and ended up with 4 and 5/16 so im really stoked!

    Here are some more pics:








    The business end:




  14. #14
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    Looks good. Are you using some plastic bushing/sleeves to center the shock? Still think you should use the coil-over, or better yet, a double coil-over. Can you imagine it, two shocks. AWESOME!!! 3 would be right out though.

    One of my biggest concerns is the bolts going into your old double crown fork. I assume they really weren't designed for that type of mechanical link. Under severe load, the aluminum might crack by the bolt holes. Hopefully fine for light trail use, but I have a feeling you won't limit yourself to gravel roads. If you hit something that requires all that travel, you could end up having a very bad day. When you re-do it in aluminum, might be worth a look at different crowns. Also, have you thought about boxing in the outer ends of the top link or welding a plate across the bottom link? It looks like the rods should be able to slide out easily, and there would be a substantial increase in rigidity.

  15. #15
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    Thanks for constructive comments!

    The pins cant come out. The sleeve that they go thru on the fork legs are 1"x3/4" and there is a 3/4 x 1/2 sleeve welded inside of that. The pins themselves are drilled and threaded on the ends and have double grade 8 washers that are a snug fit into the outer sleeve.

    There will be spacers to hold the shock in the center. I just haven't put them in because the fork will have to come apart when I make the dropouts.

    The bolts in the double crowns are my biggest concern. I am worried they will break too. The holes I drilled are less than half the total thickness, but if you have any ideas on how to reinforce them I would be happy to hear it! My biggest problem is that the crowns are aluminum and I can only weld steel with my welder.
    I was considering making a steel "cap" that slides over the end of the crown, or using some aluminum as a strap that goes between the crowns and would be attached with 2 bolts at each crown. The strap would distribute the load over both crowns.


    But you are right, I am going to ride this fork as hard as I can once I feel its safe.

  16. #16
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    You just need to make a crown. The one you're using there is no way safe. Think of it this way - would you ride the *original* fork the crown was on with 1/2" holes drilled through it in 4 places? And this is actually worse than that, since you're also loading the weakened area in particular with the bolt.

    Seriously, start over on the crown.

    -Walt
    Waltworks Custom Bicycles
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  17. #17
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    Walt,

    Thanks for the reply. How thick would you make the crown? 5/8 or 3/4 maybe? Would you suggest I do it in steel or Aluminum? Were talking upper crown I assume, since thats the crown that has the shock load. The lower sort of just guides the links.

    Right now the upper crown is 1/2 inch thick and the lower is 9/16. The holes are threaded 1/4-20

  18. #18
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    If you used good bolts, I'd be more afraid of the crown cracking. Hopefully, the bike should give you a litle warning though. Inspect it real close after your inaugural ride. In time, If you have the drill to make a 1 1/8" hole, I'd say it's worth it to make your own steel crowns. larger, heaveir, but worth it. You could also bolt a faceplate between them so they don't torque.

    I'd also drill/tap some small holes on the bottom of the lower swing arms and put an aluminum plate across it. Putting 4 small 4mm bolts along each arm wouldn't mess with their strength, and worth it from the rigidity standpoint. Not too much added weight, plus it will help keep kicked up mud/dirt off the shock. Could possibly do a smaller plate across the top arms with a cut-out for shock clearance.

  19. #19
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    Quote Originally Posted by blown240 View Post
    Thanks for constructive comments!
    I'm full of constructive comments.
    Don't crash!
    If one shock is good, 7 is better.
    Brakes, we don't need no stinking brakes.
    That bike needs a carbon fiber steer tube.

  20. #20
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    lol! Nice...

    I remeasured the crown thickness. The upper is 9/16 and the lower is 11/16. Ill get some 1/8"x2" Aluminum strap today and make an upper to lower brace on both sides. That will hopefully be enough until I can make some new crowns.

    I may do the link braces too....

  21. #21
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    prior example of similar project you might want to review;
    Custom Noleen Fatbike Girder Fork

  22. #22
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    Thanks! Ive seen that thread before. I think for the next set of crowns I will cross drill the bottom of the steer tube and use a piece of 3/4 DOM tubing with a solid steel rod thru it. Then have the lower links on that. For the upper crown Ill use a thicker plate. Either steel or aluminum.

  23. #23
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    I got a temporary fix for the crown issue. Last night I made strap that goes from the top to bottom crown and then wraps around the top of the crown. Im hoping that this design will at least warn me if the crown breaks before I smash my face into the ground.

    I will for sure make new crowns, but I want to test the geometry first to see if there are any other big changes that need to be made.








  24. #24
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    This is such an awesome project, I've always wanted to see someone attempt this. Don't get discouraged by the feedback, I agree with Walt about the crown. Keep at it, the end result will be one of a kind.

  25. #25
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    Thanks!

    Im hoping to finish it and test ride it today. We will see if I have time....

  26. #26
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    I got my fork mounted and I took it for a VERY short test ride. I ended up having to trim a little bit of the crown to crown brace for steering clearance. It actually rides pretty good. The geometry feel alot like my Intense M1.

    I still need to make the brake mount, cap the bottom of the fork legs, make a new upper crown, and decide how or if I'm going to paint it. I also may end up putting lighter oil in the shock to speed it up a bit.

    Here are some pics:






  27. #27
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    I love it!!!!!

  28. #28
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    How's the steering? at what rake, and HTA?
    If it is not satisfactory, and you are at 50mm rake, with a HTA slacker than the normal 71 degrees or so, I would dial in more trail.

    As for anti dive: if you use different length links to vary the angle of the legs through the travel, it could be built in. You must also consider the trail change as well as the angle change though. I think ideally, the rake would decrease as the HTA increases with compression to maintain a constant trail, and the angle of the legs would decrease with compression, so that brake torque would resist compression.

  29. #29
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    It steers pretty good. I havent had a chance to really take it out yet, but around the neighborhood it feels just like my DH bike. I would guess the HTA is about 65-66 degrees. Havent measured the rake. Brake dive is there, but isnt too bad.

    I got a chunk of aluminum that I am going to make a new upper crown out of. Its 1 inch thick, 2 inches deep, and will be 3 inches wide, and should be plenty strong!

  30. #30
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    I got the new upper crown made last night. Its beefy! 1 inch thick and the shoulder bolts sink in a full 1/2 inch plus the threads. This fork is ready for some abuse!




  31. #31
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    I like it
    As little bike as possible, as silent as possible.
    Latitude: 57º36' Highlands, Scotland

  32. #32
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    I am thinking of making version 2 of my fork. I am shooting for 7" travel. Since I posted this fork build, I have added rear suspension to this bike. It has 7" rear travel. The rear of this bike is SO plush, and the front is good, but it could be way better! At 7" front travel, both ends would be equal.

    I did some messing around last night with some scrap steel, and I can easily get the travel using a spare Fox Float R shock that I have and links that are 3 inches longer than the ones I have.

    The problem with the longer links is that it will lift the front of the bike 3 inches higher than it is. And its already TALL! I have the room to shorten the legs about 1 1/2 inches, to bring the front down a bit. That means I would have to make new legs too!

    SO Im thinking:
    1. If I need to make new links AND legs, why not just make a whole new fork?
    2. I can't weld aluminum yet, so would 1 inch .065 tubing be strong enough for the legs? It would be about 1.5 lbs lighter
    3. Would 3/8 aluminum pins between the legs be string enough? Right now they are 1/2 inch.
    4. Would 3/8 steel pins be strong enough for the shock mount pins.
    5. Where can I get a steerer tube?
    6. Is a linkage fork with 7" of travel going to be totally unrideable with the amount of rake its going to go thru during travel?
    7. Are there any examples of long travel linkage forks? Should I do a total design change?


    Sorry for the long post, but I need input.

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