I have read almost every thread I could find on upgrading a hotrock 20 and though I would share a few lessons learned in the process.
I started with a stock 2012 Boys Hotrock 20. Only real difference from 2012 to 2013/14 is it looks like they added disc tabs in 2013. (I had the option to go 2012 or 2013 and liked the green color and missed that tabs at the time)
My original goal was to replace some of the stock drive train and brakes with better parts from my parts bin and improve his gearing ratio.
I looked online for deals as best as possible. Lots of close out on 9 speed parts and lower top tier bike parts (x-7, deore, ect....)
Brakes - My first thought was to go disc. I tried an A2Z disc tab adapter with no luck. (There was a pinkbike story early this year which went this route on a Scott). There is a bit of round tubing on the rear drop out that prevents the A2Z adapter from fitting the Hotrock. My son had bent his brake levers in a couple of crashes so I went ahead and replace the brakes with Avid SD7 levers and V-brakes. They fit his hands well and have a excellent reach adjustment.
Drivetrain - Goal was to use 9 speed saint I had sitting around and go 11-34 with 32 T ring vs the stock 14-28 and 36T ring to improve climbing and off road ability. He really likes the numbering on the stock gear indicator and it quickly became apparent that he wanted an optical display. I found a deore 9 speed shifter on sale and printed a label to give him the numbers. I did have to run the brake levers outboard of the shifters which was not my preference. I also swapped out the cranks and bottom bracket for a sinz 135mm crank and shimano Un-52 bottom bracket. This was probably not needed but I was on a roll. (Specialized listed the bb as a 68x128...this is wrong... stock was a 68x113)
I planned to use an existing chain ring but did not notice the 5 bolt 110bcd pattern. I ended up with a non ramped 32T suigino ring.
There are not many options for a wide / narrow 5 bolt ring if you are hoping to go that route. Raceface makes one but it starts in a 38t for cyclecross which was bigger than I wanted to go.
Chainguide - The specialized guide is a nice idea with marginal execution. It still dropped chains on occasion. I am trying a bionicon guide. Added a bbg bash guard...cheap and well made.
Cassette and Wheels - Since I was not going disc I had planned on using the stock wheels and swapping the cassette. It looked like a standard rear bolt on hub till I pulled the cassette and the hub started raining bearings. Specialized used a hub that the cassette doubled as the freehub body and held the loose ball bearings. This meant I could not simply swap cassettes and needed a new rear wheel. I am still finding bearings in my garage.
There are not many off the shelf options for a 20" wheel with a 9 speed (or 10, 11...) compatible hubs so I built my own. For the rear I went with an Sram X-7 disc hub and Sun envy 20" 32 hole rim. I like cartridge hub bearings but not many 32 hole options to be had for under $50. I decided to ditch the front wheel too while I was in a building mood and get away from the bolt on wheels. This should make changing flats trail side easier since the stock requires a 15mm wrench (not found on our average multi tool). I went with a Shimano deore disc front hub and Sun envy 20" 36 hole rim. (I could only find one Sun Envy 32 hole rim or I would have gone with an x-7 hub up front). I opted for disc hubs just in case the opportunity to go disc presents its self. I can always move them to the next build.
I picked the Sun Envy 20 rims based on weight (sub 400 grams), width 22mm internal, and machining for v-brakes. Make sure to get the rear rim for both front and rear if you plan on using V-brakes. All in all the wheels turned out pretty nice. I probably spent $175 on all the wheel parts not counting my labor. (its hard to find spokes cheap) I did look at the Sun Cr-18 but the Envy was wider with a modest weight gain.
I swapped out the stock tires for some Schwalbe Moe Joes...true 2.0 width and sub 400 gram weight. The stock tires were closer to 700 grams each. I also went with some lighter weight schwalbe tubes.
The stock fork was swapped for a Spinner Air. Its an improvement but I am still working on that one.
The bike dropped a couple pounds and is sitting around 23lbs. I think he will enjoy it more on the trail and it definitely will handle single track better.
Most importantly...I can be honest and say the project was 25% for him and 75% for me...I had fun!
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