# 24" fork on Hotrock 20



## 05Willys (Jun 13, 2008)

Hi,
Have any of you put a 24" fork on a 20 kids bike. I was looking at the RST First 24" fork and figured my kids could use it on the 20 and 24 so it would be a fine expenditure. 1/2lb lighter than the Suntour Jr on the bike now and it actually works which would be a novelty. 

I was thinking I would swap in a 40mm 0 rise stem and a carbon flat bar to take up the additional stack height.

Does anyone know the difference in the axle to crown length of the 20" vs 24" forks?

Thanks


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## RMCDan (Feb 28, 2008)

The Hotrock Pro used a 24" Suntour XCR-AIR fork. The frame was identical to the other 20" Hotrocks aside from the color scheme so it seems like the RST should work. The geo charts show that it raked the HTA out to 65.4 but everything else is listed as identical. I don't see how it couldn't have raised the BB too and the photos make it seem like it does. They did swap in a short/low stem like you're thinking of.

If you open up these two links the HTA and BB differences are pretty obvious when you switch back and forth between the tabs
https://www.specialized.com/us/en/bikes/mountain/hotrock-20-pro/106528
https://www.specialized.com/us/en/kids/bikes/mountain/boys-6speed-hotrock-20/118247


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## backinmysaddle (Jul 27, 2011)

I have done a bunch of mod'ing to 20" and 24" bikes and generally this works fine. The one thing an engineer friend suggested to me though is that changing the HTA more than a trivial amount will result in stresses being put on different frame joints in ways not contemplated by the frame designer. For the most part, I think these frames are way over engineered for the weight of the kids riding them, so that shouldn't be a huge concern. This is especially true of the way that kids tend to ride, it's probably less the normal on the ground bump and grind and more big air landings that would really stress the head tube welds. But it's worth considering.

I found that the slackening that comes with either adding a suspension fork to replace a rigid fork or adding a bigger suspension fork to a bike that already had one translates into increased confidence on bumpy descents. No surprise there, that is what it does to us adults. In my opinion, most of the kids frames were getting geometry that was very XC-ish, if not sidewalk grade geometry. Every bike we have raised the front on has performed far better.


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## 05Willys (Jun 13, 2008)

Thanks. 
I am considering a rigid carbon frok from ebay. Disc mounts 380g 
24: rst first 1600ish g 
26" Sid with travel spacers to hold the fork down at 40mm travel 1300 g and I can use it again and again as he grows.

I need to hunt down the axle to crown lengths on each

On a related note, I was checking out some 20" rims but I understand there are 20" mountain rims and bmx rims that say 20" and are a different size. Is 406mm the mountain or bmx size?

406 20 039 039 32 Hole Folding Bike BMX Disc Brake Rim Wheel Wheelset Double Layer Rim | eBay


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## watts888 (Oct 2, 2012)

AC length difference going from 20" to 24" should be right about 50mm. If your kid is big and growing fast, I'd go all in and get a 26" pike and reduce travel to 40mm or maybe 60mm based on the bike geometry. Otherwise, I'd just go with a rigid fork and shove as big a tire as I could find in there.

For tire size reference. It sounds like you're possibly lacing up rims, so it doesn't really matter what size rim you get, as long as the tire matches. both 20" tires are considered "BMX" tires, but different rim diameter. Once again, I blame Specialized.
https://en.wikipedia.org/wiki/Bicycle_wheel#Mountain_bike_wheels


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## teamsloan (Jun 13, 2007)

I did a rough measurement of my sons 20" Hotrock fork (the cheapo RST) and came up with around 370mm. I'm trying to find a rigid disc fork (preferably carbon) that I could put on it without changing the HT angle too much. A little slacker would be okay, but definitely not steeper. It seems like I need to find something between one of those 406 Chinese folding bike forks on eBay and a cyclocross fork for 700c wheels. Has anyone found anything?


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## thesmokingman (Jan 17, 2009)

Why not mod the fork, its easy?

http://forums.mtbr.com/families-riding-kids/increasing-sr-suntour-40mm-travel-fork-60mm-729468.html


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## teamsloan (Jun 13, 2007)

My goal in swapping the fork is to reduce weight. Increasing the travel is at the bottom of my wishlist for this build.


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## jabrabu (Aug 2, 2010)

I was thinking about doing something similar on my son's Trek Superfly 20. I like that the bike is light, and it fits him well, and he doesn't really need front suspension at this point. However, I'd like to take him to a lift-assisted bike park, and they only allow bikes 24" or bigger since smaller wheels don't fit their lift racks. I was thinking that I could get a 24" suspension fork and front wheel and install it for bike park days. The bigger front tire, slacker geometry, and suspension should all help for downhill riding, and the extra weight wouldn't matter.


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## teamsloan (Jun 13, 2007)

Well, I found this:
CarbonCycles.CC :: Components :: eXotic Shop :: MTB Carbon 3K with 1.1/8 inch Steerer :: eXotic Carbon Rigid MTB Fork - Disc Only Post Mount :: CC-F0E390

It's a non-suspension corrected 26'er fork with disc tabs. Around 20mm taller though than the stock RST fork.

Also found this:
https://www.aliexpress.com/item/bou...dead-fly-road-fork-20er-fork/32789391976.html

It lists the length as 310mm to the bottom of the dropouts. So, 60mm shorter than the stock fork.


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## watts888 (Oct 2, 2012)

jabrabu said:


> I was thinking that I could get a 24" suspension fork and front wheel and install it for bike park days. The bigger front tire, slacker geometry, and suspension should all help for downhill riding, and the extra weight wouldn't matter.


That would really slack out the geometry. You're adding about 2" fork, plus 2" wheel. This would be a legitimate 4" of AC length added, and that's a whole lot on a 20" bike. I hate to say, I'd stick with the 20" for now, and ask the park about other options for getting a 20" bike up there. Maybe a 24" rental?


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## jabrabu (Aug 2, 2010)

watts888 said:


> That would really slack out the geometry. You're adding about 2" fork, plus 2" wheel. This would be a legitimate 4" of AC length added, and that's a whole lot on a 20" bike. I hate to say, I'd stick with the 20" for now, and ask the park about other options for getting a 20" bike up there. Maybe a 24" rental?


Yeah, I was also thinking of just getting a Specialized Riprock 24. It looks like a nice bike, but it's several pounds heavier than the Superfly. Also, my LBS has a deal where they will give me 40% of what I paid for the Superfly when I trade up. I was looking at the Trek Precaliber 24 they had, which was a decent bike, but it's pretty heavy.


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## POAH (Apr 29, 2009)

A2C of the boost fork on the riprock 20 is 400mm which is the same as the rst F1SRT air.


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## watts888 (Oct 2, 2012)

I like that riprock. The XCT fork is kinda a dog for that bike though. IMO, they should have gone with a rigid alloy with those 2.8" tires. It would probably be a case where a nice light 26" fork will clear 24"x2.8" tires (kinda like how people shoved 27.5+ tires into 29er forks). Reduce travel with some spacers, and you have a great little 24" bike.


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## slowride454 (Jan 11, 2014)

Did any of you guys do these fork swaps? I'm picking up a used 20" Precaliber tomorrow and then the craziness will begin. Somehow I managed to turn his $50 Hotrock 16 into a $500 money pit. I have a bunch of 10 speed stuff laying around so I'll build some wheels and swap drivetrains, but the fork is bothersome. I am trying to see if I can get a rigid Superfly fork, but a quality air suspension fork would be nice.


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