YT Industries Jeffsy Primus: A Legit Trail Ripper for Rippers With Shorter Inseams

The component spec and geo on the YT Jeffsy Primus 26" kids bike solidly fits the bike's intentions, and the frame is built to blast.

If I didn’t have to learn how to ride singletrack on a crappy rigid Murray 26″ with road geometry I certainly wouldn’t have. Thankfully, my daughter’s generation has access to far better mountain bikes like this 26″ alloy Jeffsy Primus from YT Industries. The black frame is dressed in solid components, with a smart eye toward durability. Maybe the product managers at YT know how some kids will ride hard and leave their bikes to rot in the yard.

With 140mm of balanced travel, the one-size-fits-shorter-legs frame on the 24-inch-wheeled Primus model is said to fit people 135 and 150cm (53 – 59″) tall, and this 26″ version extends that range from 145 to 160cm (57 to 63″). My kid, who goes by Penelope, stands 155cm (5’1″) from the trail, and the larger-wheeled Jeffsy Primus fit her like a snug glove that needs replacing. The bike not only looks small for her body proportions but the fork steerer tube is cut too short to raise the bars. On multiple occasions, she mentioned that the front end feels too low, which is an issue we have run into with other YT builds. The bar height could be remedied with a higher riser bar, but it would be better if the company would leave the steerer tube longer, to begin with. Moving spacers under the stem is a free adjustment; handlebars aren’t. At $2,299, I would take a long think about fit before buying this bike for anyone who’s still growing.

Frame spec

YT Jeffsy Primus

All of the Jeffsy Primus cable and hose routing is internal, just like the brand’s larger bikes, and the rubber grommets kept it quiet while preventing scratches on the flat black finish. Welds all around this frame look just as solid as those on the size XL Jeffsy, and overall the bike seems poised to pass amongst siblings and eventually to the neighbor kids. In terms of construction and functionality, the Jeffsy Primus looks like a shrunk copy of the Base alloy bike we tested a while back, and that model proved a fantastic value for riders looking to save cash and ride fast.

YT Jeffsy Primus

There is a flip-chip on the Jeffsy Primus frame, and I set it in the low position and left it there just as I would with my own bikes. Penelope and I like to ride some steeper trails together, so there’s no need to fiddle with this chip. Given the low handlebar height, she needs every descending advantage possible to make the bike feel good with gravity. I would imagine most parents will set the chip to suit their local trails and leave it there, and maybe it’s helpful to have the option if you travel to ride with this bike’s owner.

26″ geometry

Angles across the Jeffsy Primus look good for a 140/140mm trail bike, closely matching the proportions of frames with larger wheels and longer tubes. The 65.5° head tube angle is a nice balance of nimble-enough handling for learning technical moves while being slack enough for any slope. With a 76.5° seat tube angle, riders are able to set the saddle fore/aft position where they need it to maintain traction while climbing and mitigate pain in their growing joints and muscles. That BB-drop number looks minuscule, but remember that the hubs are much lower to the ground inside those 26″ circles, keeping the distance from the ground reasonable for the bike’s 165mm cranks.

Component considerations

YT Jeffsy Primus

As mentioned above, this bike is built to outlast the growth cycle of its pilot. I wouldn’t mount SRAM SX level components on my personal bike, as I ride enough to be frustrated with the performance tradeoffs of that budget gruppo, but it’s an ideal starting place for my kiddo. The drivetrain is tough, and it functions well if properly adjusted. Sure, it’s not the crispest and most precise shifting, but it definitely gets the job done. Since many of the riders on this bike will be learning, crashing, and possibly breaking components, starting with this affordable SX Eagle build seems like the right call.

YT Jeffsy Primus

Depending on how hard you shred, the SRAM Guide R two-piston brakes may do the trick, or they might need a caliper swap. Penelope had no trouble slowing her mass with these brakes, and they should fit the bill until she’s comfortable riding a fair bit faster. For newer riders I like to set the rear brake lever up so that it bites a bit before the front. That way, if they suddenly grab both brakes the rear tire can do some work before the front grips and sends them in a flip.

The 760mm-wide handlebar is paired with a short 40mm stem that could likely be swapped out for something longer if you want to get a little more life out of the bike while growing. The 100mm of dropper travel could also be exchanged for something longer as riders grow taller, but it seemed a good fit for Penelope’s leg length.

YT Jeffsy Primus

Like the brakes, the dual Maxxis Minion DHR II tires may work out well or they may need to be upgraded with a heavier hitting casing depending on the pilot. If you ride hard on rocky trails it may be worth checking the availability of 26″ tires in the casing that works for your trails before purchasing a bike with this smaller diameter.

With a 140mm Manitou Machete Junit fork and RockShox Deluxe Select+ the Jeffsy Primus is easy to set up and get rolling. The fork offers externally adjustable rebound, compression, and air pressure, as does the custom-tuned shock. While these aren’t the highest level suspension components on the market, they fit well with the build and should provide the cushy ride a lot of newer riders are looking for.

Trail talk

YT Jeffsy Primus

On our first outing, Penelope said that the Jeffsy Primus “feels way lighter than the other bike.” That other bike was a borrowed Specialized Stumpjumper EVO, with a small carbon frame and 29″ wheels. At 32.3lb, the YT is definitely not lighter, but the fact that it feels lighter says a lot for the Jeffsy’s pedaling stance and rider positioning.

I didn’t ride this bike, but I followed my daughter nearly every time she did and I’m impressed by what she is able to do with it. Penelope has been able to clean rootier and messier climbs than ever before with this bike. Part of that progression comes with learning, and part of it is likely tied to comfort on the bike. She feels good aboard this steed, and the wide-ranging drivetrain allows her to power up and over some notably tricky rootballs. The more trail segments she can clean the more confidence she has to move on to the next challenge, and this bike has been a great tool through that learning process. The low handlebar likely helps while climbing, allowing her to keep weight over the front tire without thinking about it.

Despite the tight fit and low handlebar, Penelope enjoyed descending on the Jeffsy Primus. She didn’t slay any QOMs, but she feels relatively comfortable on the bike and enjoyed riding it down our local tracks. Like her parent, she’s a little more risk-averse than some of her friends, but she managed to launch some larger drops thanks to the travel and shreddy geometry this frame offers. She only somersaulted over the alloy handlebar a few times, which is a marked improvement. Riders who are a touch shorter than her will likely love learning and pushing their skills above these 26″ tires.

Pros and cons of the 26″ YT Jeffsy Primus


  • Nice durable build
  • Affordable
  • Appropriate geometry


  • Only available in one color
  • Size chart is a little off


Penelope is looking for a new mountain bike, and at the end of the test, she said she would go for a larger size of this same platform. The component spec and geo solidly fit the bike’s intentions, and the frame is built to blast. The next YT model that will fit her size and budget is the aluminum Jeffsy Core2 in a size small with 27.5″ wheels.