I have now built four different kids bike forks from 26" forks. First was an early manitou that I shortened for my daughters 20" bike, http://forums.mtbr.com/families-riding-kids/novara-pixie-20-project-853041.html
I then tried modifying a rockshox Mag21 into a 24" fork. I did eventually get the mag21 working but it was a fairly difficult conversion that I would not necessarily recommend repeating, details in the pixie thread above. Next up, I shortened a late 90's manitou spyder 26" fork into a 24" fork. Details of this one are at
http://forums.mtbr.com/families-riding-kids/trek-mt220-24-mod-build-894279.html and http://forums.mtbr.com/families-riding-kids/24-air-fork-884666.html#post10782625
The spyder came out as the best working fork I have made yet and it is really lightweight at below 1300gr compared to currently available 24" forks that start around 1600gr. THe spyder fork however was never sold in great numbers so finding one to do a conversion might be tough.
Another candidate conversion fork that was produced in large numbers starting back in 1997 and onward for several years was the Rockshox Indy series. I recently picked up an Indy S fork from local bike co-op for $8 to test out as a kid 24" conversion. The salient feature of the Indy forks that make them a good 24" candidate is that the dropouts are inline with the fork leg (not offset on tabs in front like on the Judy), and the lower portion of the slider casting is solid and has plenty of thickness to re-work into new shortened dropouts. The Indy forks came in various flavors from the lowly Indy S that used steel stanchions, 45mm travel and weight 1510gr, the intermediate Indy C, the Indy XC and SL that both utilized aluminum stanchions, more sophisticated spring/elastomer stack, 60mm travel, friction dampening and weighed as little as 1220gr (for the threadless steer tube SL). More info of the Indy forks can be found in the 1997 & 1998 RS product catalogs at Rock Shox Museum The slider casting on all the indy fork flavors is nearly identical, so even though I started with an Indy S, I may later try to upgrade it with lighter stanchions and innards from one of the better forks. Looking at the specifications for similar era RS forks, the quadra 21R fork looks very similar however it was reported that these are hollow (like the mag 21) and cannot be shortened.
The bike this fork is going onto belongs to a neighborhood friend of my daughters. He recently got a used Hotrock 24 that was about 8-10 years old.
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I was fairly impressed with the overall quality of this bike in comparison to some of the junky parts I have seen on other similar kid bikes. This era hotrock featured butted aluminum frame tubing, decent aluminum crankset with an aluminum outer ring, quick release aluminum hubs, a nice kids saddle, decent looking specialized tires, sun CR18 rims and aluminum seatpost, stem & handlebars. Original weight on the bike was just under 25 pounds, not too bad as a starting point for a modification project. Downside for this bike were an ugly kickstand plate welded onto the chainstay and it came with twist shifters that are hard for kids to operate. The single biggest downside to this bike however was the craptacular RST suspension fork;
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When I got the RST free of the bike, the bare fork weighed a full 1930gr on its own. It actually had an aluminum steer tube but RST must have packed the innards of the fork with lead or depleted uranium as weight ballast.
Here is the original rockshox Indy 26" fork before I got after it
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First step in the conversion is drilling a pilot hole at center of the new axle location, 25mm upward from the center of the original axle location.
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The outer side of the fork leg casting is scalloped inward and entire leg gets thicker as you move away from the fork tip, slightly complicates fabrication of a new forkend at correct distance from the brake post to suite 24" rim. If you lack access to a machine shop mill, easy solution is to use a 3/4" carpentry spade bit to cut in and level out the casting in order to form a new fork end. When you make the pilot hole, dont drill out to full size of the 9mm axle or the hole will be too big for the spade bit to stay centered. Use the spade bit to drill until you form a 3/4" circle into the tapered outer side of the fork that is parallel to the original dropout. There is a "tooth" on outside diameter of the spade bit that causes it to drill the outer ring a bit deeper than the main part of the bit.
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Once you have the new circle formed using the spade bit, you can hacksaw off the excess fork length. Drill the center axle hole out to 9mm and hacksaw slots to end of fork. Use sharp hand files to shape the dropout, get it all leveled out. End result should form a hood recessed into the casting that will provide enough clearance for the quick release when the axle is seated in the drop-out. I also used a dremmel tool cutter bit to help square off and clean-up the the hood area.
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The inside of the fork casting is tapered but not scalloped like the outside. It is easy enough to just use a large 1/2 round hand file to level out the casting above the new axle hole so that the hub axle nuts have a flat dropout surface to bite into. The new fork end needs to be parallel, you don't want the quick release to tighten down onto a tapered fork end or the wheel can easily fall out if the skewer loosens and slips. Also make sure to file the inside-to-inside dimension of the fork ends to exactly 100.0mm. If the inside dimension is different from the hub locklut-to-locknut dimension, it will cause binding of the forks movement.
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End result, a decent quality and lightweight 24" rim brake fork;
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The Indy forks are fairly easy to tune for stiffness with different springs and MCU bumpers. For kid use, just running a spring stack in one side is plenty. Final weigh on this fork was 1490gr, over a 400gr weight savings from the original RST fork at very minimal cost and it was a fun project. With no other modifications, this hotrock is now weighs 24.0 pounds. This was done just using a lower-end OEM grade Indy fork, but there were definitely nicer and lighter Indy forks available that could be converted in same manner. The conversion would be even easier if you had access to a drill press and mill but it can be done with just hand tools and some patience! End results are that it would be hard to tell that this fork was not originally factory made as a 24" without looking at it up-close.
I would encourage others to give a try at shortening 26" forks rather than spending $$ on a new 24" fork that is still significantly heavier than what you can easily fabricate yourself. Post your projects here, keep this thread going to share fork-mod ideas.
I then tried modifying a rockshox Mag21 into a 24" fork. I did eventually get the mag21 working but it was a fairly difficult conversion that I would not necessarily recommend repeating, details in the pixie thread above. Next up, I shortened a late 90's manitou spyder 26" fork into a 24" fork. Details of this one are at
http://forums.mtbr.com/families-riding-kids/trek-mt220-24-mod-build-894279.html and http://forums.mtbr.com/families-riding-kids/24-air-fork-884666.html#post10782625
The spyder came out as the best working fork I have made yet and it is really lightweight at below 1300gr compared to currently available 24" forks that start around 1600gr. THe spyder fork however was never sold in great numbers so finding one to do a conversion might be tough.
Another candidate conversion fork that was produced in large numbers starting back in 1997 and onward for several years was the Rockshox Indy series. I recently picked up an Indy S fork from local bike co-op for $8 to test out as a kid 24" conversion. The salient feature of the Indy forks that make them a good 24" candidate is that the dropouts are inline with the fork leg (not offset on tabs in front like on the Judy), and the lower portion of the slider casting is solid and has plenty of thickness to re-work into new shortened dropouts. The Indy forks came in various flavors from the lowly Indy S that used steel stanchions, 45mm travel and weight 1510gr, the intermediate Indy C, the Indy XC and SL that both utilized aluminum stanchions, more sophisticated spring/elastomer stack, 60mm travel, friction dampening and weighed as little as 1220gr (for the threadless steer tube SL). More info of the Indy forks can be found in the 1997 & 1998 RS product catalogs at Rock Shox Museum The slider casting on all the indy fork flavors is nearly identical, so even though I started with an Indy S, I may later try to upgrade it with lighter stanchions and innards from one of the better forks. Looking at the specifications for similar era RS forks, the quadra 21R fork looks very similar however it was reported that these are hollow (like the mag 21) and cannot be shortened.
The bike this fork is going onto belongs to a neighborhood friend of my daughters. He recently got a used Hotrock 24 that was about 8-10 years old.
I was fairly impressed with the overall quality of this bike in comparison to some of the junky parts I have seen on other similar kid bikes. This era hotrock featured butted aluminum frame tubing, decent aluminum crankset with an aluminum outer ring, quick release aluminum hubs, a nice kids saddle, decent looking specialized tires, sun CR18 rims and aluminum seatpost, stem & handlebars. Original weight on the bike was just under 25 pounds, not too bad as a starting point for a modification project. Downside for this bike were an ugly kickstand plate welded onto the chainstay and it came with twist shifters that are hard for kids to operate. The single biggest downside to this bike however was the craptacular RST suspension fork;
When I got the RST free of the bike, the bare fork weighed a full 1930gr on its own. It actually had an aluminum steer tube but RST must have packed the innards of the fork with lead or depleted uranium as weight ballast.
Here is the original rockshox Indy 26" fork before I got after it
First step in the conversion is drilling a pilot hole at center of the new axle location, 25mm upward from the center of the original axle location.
The outer side of the fork leg casting is scalloped inward and entire leg gets thicker as you move away from the fork tip, slightly complicates fabrication of a new forkend at correct distance from the brake post to suite 24" rim. If you lack access to a machine shop mill, easy solution is to use a 3/4" carpentry spade bit to cut in and level out the casting in order to form a new fork end. When you make the pilot hole, dont drill out to full size of the 9mm axle or the hole will be too big for the spade bit to stay centered. Use the spade bit to drill until you form a 3/4" circle into the tapered outer side of the fork that is parallel to the original dropout. There is a "tooth" on outside diameter of the spade bit that causes it to drill the outer ring a bit deeper than the main part of the bit.
Once you have the new circle formed using the spade bit, you can hacksaw off the excess fork length. Drill the center axle hole out to 9mm and hacksaw slots to end of fork. Use sharp hand files to shape the dropout, get it all leveled out. End result should form a hood recessed into the casting that will provide enough clearance for the quick release when the axle is seated in the drop-out. I also used a dremmel tool cutter bit to help square off and clean-up the the hood area.
The inside of the fork casting is tapered but not scalloped like the outside. It is easy enough to just use a large 1/2 round hand file to level out the casting above the new axle hole so that the hub axle nuts have a flat dropout surface to bite into. The new fork end needs to be parallel, you don't want the quick release to tighten down onto a tapered fork end or the wheel can easily fall out if the skewer loosens and slips. Also make sure to file the inside-to-inside dimension of the fork ends to exactly 100.0mm. If the inside dimension is different from the hub locklut-to-locknut dimension, it will cause binding of the forks movement.
End result, a decent quality and lightweight 24" rim brake fork;
The Indy forks are fairly easy to tune for stiffness with different springs and MCU bumpers. For kid use, just running a spring stack in one side is plenty. Final weigh on this fork was 1490gr, over a 400gr weight savings from the original RST fork at very minimal cost and it was a fun project. With no other modifications, this hotrock is now weighs 24.0 pounds. This was done just using a lower-end OEM grade Indy fork, but there were definitely nicer and lighter Indy forks available that could be converted in same manner. The conversion would be even easier if you had access to a drill press and mill but it can be done with just hand tools and some patience! End results are that it would be hard to tell that this fork was not originally factory made as a 24" without looking at it up-close.
I would encourage others to give a try at shortening 26" forks rather than spending $$ on a new 24" fork that is still significantly heavier than what you can easily fabricate yourself. Post your projects here, keep this thread going to share fork-mod ideas.
