After extensive research and personal experience in modifying a children’s bicycle, I wanted to share my insights on upgrading a Specialized Hotrock 20. My project began with a 2012 Boys Hotrock 20, a model very similar to the 2013/14 versions, with the primary visual difference being the addition of disc brake tabs in the later models. Opting for the 2012 green version, I initially overlooked the disc tab feature.
My initial aim was straightforward: to replace certain components of the stock drivetrain and braking system with higher-quality parts I already had. I also intended to improve the bike’s gearing to make it more suitable for varied terrains. I actively searched for deals online, focusing on closeout prices for 9-speed components and lower-tier high-quality bike parts such as X-7 and Deore.
Brake System Enhancement
Initially, I considered converting to disc brakes. I experimented with an A2Z disc tab adapter, but unfortunately, it was incompatible. As highlighted in a Pinkbike article earlier in the year which detailed a similar modification on a Scott bike, the round tubing on the rear dropout of the Hotrock prevented the A2Z adapter from fitting correctly. Since my son had also damaged his brake levers in several falls, I decided to upgrade the brakes to Avid SD7 levers and V-brakes. These levers are a great fit for smaller hands and offer excellent reach adjustability.
Drivetrain Modification for Performance
My objective for the drivetrain was to utilize a 9-speed Saint groupset I had available and implement an 11-34 cassette with a 32T chainring. This was a shift from the stock 14-28 cassette and 36T chainring, aimed at improving climbing ability and overall off-road performance. My son particularly liked the numerical gear indicator on the original setup, so maintaining an optical display was important. I found a Deore 9-speed shifter on sale and created a custom numbered label to replicate the original display. This setup required positioning the brake levers outboard of the shifters, which wasn’t ideal but necessary. Additionally, I replaced the stock cranks and bottom bracket with a Sinz 135mm crank and Shimano UN-52 bottom bracket. While perhaps not essential, it was part of the comprehensive upgrade. (Note: Specialized listed the original bottom bracket size as 68x128mm, which was incorrect; the stock size was actually 68x113mm.)
When planning to reuse a chainring, I overlooked the 5-bolt 110bcd pattern and ended up using a non-ramped 32T Sugino chainring. Options for wide/narrow 5-bolt rings are limited, especially if seeking smaller sizes. Raceface offers one starting at 38t for cyclocross, which was larger than I preferred.
Regarding the chainguide, the stock Specialized guide, while well-intended, had inconsistent performance and occasionally dropped chains. I opted to try a Bionicon guide and added a BBG bash guard—a cost-effective and well-made option.
Wheel and Tire Upgrade
Initially, I planned to keep the stock wheels and simply replace the cassette, assuming a standard bolt-on hub. However, upon removing the cassette, the hub unexpectedly disassembled, revealing loose ball bearings. It turned out Specialized used a hub where the cassette body doubled as the freehub, retaining the loose bearings. This design precluded a simple cassette swap and necessitated a new rear wheel. I’m still finding stray bearings in my garage!
Finding off-the-shelf 20″ wheels compatible with 9-speed (or higher) cassettes is challenging, prompting me to build a custom set. For the rear, I chose a SRAM X-7 disc hub and a Sun Envy 20″ 32-hole rim. I prefer cartridge bearing hubs for their durability, but 32-hole options under $50 were scarce. I also decided to replace the front wheel to move away from bolt-on axles, simplifying trailside flat repairs, as the stock setup required a 15mm wrench, not standard on most multi-tools. For the front, I used a Shimano Deore disc hub and a Sun Envy 20″ 36-hole rim (32-hole rims were unavailable at the time, otherwise, I would have used an X-7 hub). I chose disc hubs for potential future disc brake upgrades and their reusability in future projects.
The Sun Envy 20 rims were selected for their light weight (under 400 grams), 22mm internal width, and compatibility with V-brakes. Ensure the rear rim is used for both front and rear if utilizing V-brakes. The custom wheels turned out well, costing approximately $175 in parts, excluding labor (spokes were surprisingly expensive). I considered Sun CR-18 rims but opted for the wider Envy for a slight weight increase.
Tire-wise, I replaced the stock tires with Schwalbe Moe Joes, which are true 2.0 width and weigh under 400 grams each, a significant saving compared to the stock 700-gram tires. Lighter Schwalbe tubes were also installed.
Suspension Fork Upgrade
The original rigid fork was replaced with a Spinner Air fork. While it’s an improvement, further adjustments are still needed to optimize its performance.
Final Thoughts and Weight Reduction
Post-upgrade, the bike’s weight was reduced by a couple of pounds, now around 23lbs. This enhancement should significantly improve its performance on trails and handle singletrack more effectively.
Ultimately, while this Specialized Hotrock 20 upgrade project benefits my son, I must admit it was equally, if not more, for my enjoyment. It was a thoroughly fun and rewarding endeavor!
