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  1. #1
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    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!

    Like many others, I didn't like the ultra heavy stock suspension forks that come on most kids mountain bikes. The weight is one thing, but the lack of function really sealed it for me. There are many informative threads on this forum lamenting this.

    I found some threads showing how to shorten suspension forks where they removed the axle lugs, trimmed the stanchions and re-glued the lugs back on. I thought I could do the same with a rigid fork and found one on Craigslist locally that I thought would work.. I believe it is a X-lite or Exotic aluminum 29er fork at 986 grams unshortened (compare to the 20" stock fork at 1,920 grams). Similar carbon forks are lighter and would also work, but are more expensive. Some come with v-brakes mounts, but all of these forks have disc brake mounts. These forks are about $80 on Ebay, but as tapered steerers are the rage, straight steerer forks should be getting cheaper and cheaper and easier to find second hand.

    Here are the two forks side by side.
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-bothforks.jpg

    You can see the difference in weights.
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-newfork.jpg
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-stockgrams.jpg

    Here are some more images with the forks lined up at the v-brake mounting locations to show the differences in lengths. Ditching the v-brakes makes the shortening process as you only have to match the axle-to-crown distance. Keeping v-brakes means that you have to match the axle-to-brake distance as well as the axle-to-crown distance.
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-bothforks2.jpg
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-bothforks3.jpg
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-bothforks4.jpg

    I cut the ends off the forks before removing the lugs to avoid damaging the rest of the fork with heat, but I don't know if it matters too much with aluminum. I drilled holes in the stanchion stubs just to be able to stick a bolt thru them to use when twisting them them off with heat applied. I just tossed the stubs in my grill and cooked them for awhile, then twisted like hell. A torch may be easier.
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-lug0.jpgRigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-lug1.jpg

    Then I shortened the stanchions with a hacksaw and re-attached the lugs with loc-tite retaining epoxy. I bolted a hub to hold everything square as soon as I got the dropouts on with epoxy. Worked almost like a charm. If I was to do it again I would make sure my wheel was true and centered and then use the entire wheel instead of just a hub so I could get it better aligned.

    Here is the final result. You will notice that the axle-to-crown is not exactly the same. This is because the brake-to-crown distance was longer on the 29er fork and I didn't want to mess with it. If I ditched the v-brakes I would have them exactly the same length.
    Rigid Fork Option for 20" and 24" bikes - shorten a 29er fork!-bothforks5.jpg

    Hindsight is 20/20. I found it was difficult to get the stanchions exactly the same length without some machinist measuring tools. I just tape measured, hack-sawed, and went for it, which was ok, but I should have spent some time with a file to get the exact length on both sides. I was probably only a millimeter off on one stanchion, but that was noticeably too much with the wheel installed. I had to dremel the inside surface of one dropout where it rests on the axle to effectively shorten one side.

    IMO, the aluminum doesn't seem to transmit vibrations noticeably more than steel, but others disagree.

    At 900 grams the shortened fork weighs a kilogram less than the stock fork. It is shocking to pick them both up at the same time and think that the heavy one was for a child's bike.

    I think it would be easy to do this for a 24" bike as well.

  2. #2
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    Nice job on the shortening. After getting the dropouts cut-off, you could likely use a large carpenter square to verify that the leg lengths are equal (square to the headtube) before pressing the dropouts back on. If you have any concerns about the locktite holding the dropouts, it is easy to drill a 1/8" hole through the dropout and fork leg and use a pop-rivit to ensure that the DO will never come off the leg. As much as an improvement as this was over a 2000g "suspension" fork, I suspect that the resulting 900gr fork is still overbuilt for kid use,

    By comparison, an interesting catalog offering for a somewhat lightweight 20" fork is the kinesis BL05B, aluminum 695gr with bosses for 20" rim brakes;
    Kinesis industry CO.,.
    Only problem, I dont find anywhere you can retail order this fork, probably would need to place a mininum 1000 piece OEM order for a shipping container full of them but I suspect that the per-piece cost for these would be very affordable if they were actually available retail.

    Also, e-bay is currently showing a 380gr disk brake chinese carbon 20" fork available for $70, definitely worth considering if you are interested in changing to disks;
    Cycling Bike Bicycle Motocross BMX Full Carbon Fork Caliper Disc Brake 28 6mm | eBay

  3. #3
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    Quote Originally Posted by jblockers View Post

    At 900 grams the shortened fork weighs a kilogram less than the stock fork. It is shocking to pick them both up at the same time and think that the heavy one was for a child's bike.

    I think it would be easy to do this for a 24" bike as well.
    I just set up a spare set of wheels for road/fire trail use...
    Currently "trail to road" is swapping the wheels and the chainring. The "road wheels" have the original tires, tubes and a 12-32 cassette in place of the 13-40.

    We then switch over the chainring from a 32T oval to a 34T.
    (It was pain mainly because the old 30T has to small an ID to go over the cranks so it meant pulling the cranks each time....) Haven't tried the 32 Oval yet.

    Anyway the road wheels are ridiculously heavy at the moment... (front was almost a kg heavier) with tires, tubes and cheap discs (didn't weigh the rears yet) as the cassette only arrived this morning but I expect that to be over 1kg heavier as its got the SLX cassette as well.

    I do however have 2 spare hubs.(D711/712).. I've been wondering what to do with... that I only bought because they were reduced by 60%....

    Because of the crank inconvenience I also just got some used cranks off ebay I'm planning to shorten. (I may not have to if the 32 oval can go on and off without but.hey..)

    Now you have me wondering if I can make a set of road forks as well.... though 900g is lighter I'm not sure its worth the hassle of switching forks when we are doing tracks instead of red/black trails?? (As in would I actually bother) and since we have the F1rst Air its lighter than the 20 one and really works

    but I don't know if it matters too much with aluminum
    Yes very much so.... the properties of bike alloys are very much dependent upon the annealing process.

    How much heat though would depend on the type of blade and a good TCT blade would probably produce next to no heat.. I used a really worn one to cut a really thick seatpost and also S600 cranks and it was barely warm to touch.... (the worn one is on a smaller 210mm saw next to the bike stand and the big one with the sharp blade is in the shed and a real pain to get in and out as its 50kg of saw that needs to be squeezed through the door)

  4. #4
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    Wow, this and some different tires/tubes and I can shave 4 POUNDS from the hotrock? Then a stem/aluminum bars saves more.

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