YT gives a leg up to the next generation of shredders with its impressive Jeffsy Primus.

Product Overview

YT Jeffsy Primus 24


  • Looks fantastic. Kid-optimised attention to detail. Suspension actually works. They’ll love shredding the downhills…


  • …but the climbs will be a drag on those fat Minions. Not cheap. Limited gearing range. Might not last a growing kid very long.


YT Jeffsy Primus 24 first ride review


Price as reviewed:


Those who think the youth of today have it too easy look away now, because this Jeffsy Primus is a kids’ bike at least as cool – if not cooler – than anything you could be buying yourself. And if your youngsters look like they could get serious about their riding this will give them an incredible leg-up, opening up burly terrain and big hits for even the youngest of shredder. Be careful what you unleash…this is one of the best kids mountain bikes out there.

Need to know

  • Available in 24-inch and 26-inch wheel versions, though not switchable from one to the other
  • Attention to detail extends to narrow grips, custom shock tunes, tubeless prep and more besides
  • Proper tyres open up serious terrain for proper little riders
  • Super active suspension that actually works, even for smaller kids
  • Virtual Four Link design carried straight over from adult sized Jeffsy range
  • No corners cut on the spec, top quality finishing kit from front to back

If you started out mountain biking on a rusty Falcon Sierra or wobbly old Raleigh Activator prepare for a shock at how far kids’ bikes have come. And how quickly a quality bike like this awesome looking YT Jeffsy Primo can unleash the proper ripper in your nipper. At a price.

YT Jeffsy Primus kids bike

Virtual Four-Link suspension is every bit as effective as the full-size version.

Like anything you get what you pay for, and at first glance you know there have been no corners cut shrinking down YT’s popular all-rounder for a younger generation of riders. And the deeper you dig into the spec the more impressive it becomes, the attention to detail underlining a zero-compromise approach that makes the price actually look relatively reasonable – particularly now it’s been reduced by over £400!

YT Jeffsy Primus kids bike

Flip chip helps tease a bit of extra top tube clearance for short legs. Down tube has been nibbled away to accomodate shock, but the Manitou McLeoed works well with lighter body weights.

It starts with the foundations of a sleek looking and well-finished hydroformed aluminium frame based around YT’s Virtual Four Link design, driving a Manitou McLeod shock with 130mm of rear-wheel travel and a four-stage compression switch to configure it to the terrain you’re riding. This is paired with a burly looking, 130mm travel Manitou Machete JUNIT front end whose Expert Air springing promises a kid-optimised response that’s often lacking in the basic forks found on most 24-inch bikes.

YT Jeffsy Primus kids bike

Up front there’s serious rubber and a very effective fork, even at low pressures.

Quality SunRinglé Düroc wheels are another welcome sight and save a chunk of valuable rotating and unsprung weight compared with the hefty hoops standard on the Marin 24in Rift Zone Jr we had on test at the same time. True, the Minion DHF/DHR tyre combo is probably overkill for all but bike park riding, but points to YT’s intentions, the fact they turned up already tubeless and with Peaty’s valves another satisfying detail.

YT Jeffsy Primus kids bike

One-by drivetrain could be wider range to help little legs on the way back up. Great to see short 155mm cranks fitted.

The relatively small 42t top cog on the SRAM 11-speed cassette is another indicator of the Jeffsy Primo’s gravity focused ambitions, even if the stumpy 155mm SRAM SX cranks will help little legs and are acut above the kind of no-name components often found on kids’ bikes.

YT Jeffsy Primus kids bike

Skinny SDG grips suit small hands.

Quality finishing kit from SDG completes the spec, the dinky saddle, 650mm bars and super skinny grips all optimised for the size of rider the bike is intended for. Which, disappointingly, is all too rare on many kids’ bikes. Even the reach-adjustable SRAM Guide brake levers feel appropriately small-hand friendly and, as always with YT, the thoughtful touches extend to properly sealed bearings, noise reducing chainstay protection and plastic frame guards to ward off rock strikes and other common damage. If anything the bike feels lighter than YT’s claimed 28lb (12.7kg) and, even with pedals, it’s going to come in well under 30lb. That’s a pound and a half lighter than the considerably more expensive Commencal Clash 24 that’s also likely to be high on your shortlist.

YT Jeffsy Primus kids bike

Rubber frame protection protects from inevitable scrapes.

Spec for the 26in version is pretty much the same, though the sizing and geometry is optimised for the bigger wheels and it gets a RockShox Deluxe Select+ rear shock with 140mm of travel instead. The price also increases by €100 in the process.

YT Jeffsy Primus kids bike

Florence was loving the Primus, whether on home trails, or shredding Les Gets and Morzine.

How it rides

The admirable, zero-compromise focus on creating the best 24in wheeled kids’ bike possible has clear advantages in unlocking the little shredder within, but it comes at a price.

On the plus side it was obvious seven-year-old test rider Florence gained huge confidence from the way the YT fitted her, the low-slung frame meaning plenty of standover. Any conclusions on our part about how the Jeffsy Primus actually rides obviously have to come from observation rather than first-hand experience, but it’s clear YT has absolutely nailed the sizing, the 24in version recommended for riders between 135cm and 150cm tall while the 26in accommodates those from 145cm to 160cm. A 65.5º head angle in the lower of the two geometry settings and long front centre gives a great weight balance, and the way Florence naturally positioned herself in the middle of the bike obviously helping her confidence through the corners.

YT Jeffsy Primus kids bike

Effective suspension, aggressive tyres and well considered details make the Jeffsy Primus a premium kid’s bike.

Also impressive was just how active the suspension was, even under Florence’s lowly 27kg. The Manitou shock and fork have both clearly been tuned for riders of her size, as evidenced by the obvious ‘flutter’ in the linkage over bumps big and small. Even a hunt online couldn’t dig up proper set-up instructions for the fork, but with a bit of trial and error we managed to balance the air chambers, the negative spring helping reduce the stiction light riders often struggle to overcome. Judging by the O-ring she was clearly getting all the travel, the suppleness of both fork and shock seeing her down top-to-bottom Alpine descents, and steeps in the local woods alike.

YT Jeffsy Primus kids bike

Flo quickly found her flow on the Jeffsy.

Judging by the combination of speed, airtime and the ease with which she was throwing the YT into the berms the Jeffsy was clearly working beautifully for her, the only mechanical issue we suffered coming when a spring in the rear mech came adrift and required an improvised trail-side strip-down to fix. This was a one-off on our initial shake-down ride and hasn’t recurred, so we’ll put it down to bad luck.

Review by Dan Trent and Florence Trent. 


And the limitations? They certainly don’t come from the bike itself, the spec or the way it rides. But by pinning the Jeffsy Primus so precisely to a narrow size-range of riders, there’s not a lot of growing room in it. It’s a blessing and a curse, given kids’ bikes that do permit upgrading from, say, 24in to 26in wheels look attractive for parents, but can mean compromises in terms of geometry and fit. No such issues for the Jeffsy Primus, but that could potentially mean a short lifespan – we’d be surprised if you got more than one season out of a rider of Florence’s age. Of course, looked after properly, kids’ bikes hold their value well and, assuming there’s no waiting sibling to take it on, there’s a thriving community of keen parents eager to buy second hand. That means equity to carry through into sizing up, but it’s another cost and faff to consider. The only other criticism you could level at the Primus is that its gravity focus could limit it to those with easy access to lift-assisted terrain at home or abroad, which is obviously a much more expensive way of mountain biking than just razzing around the local woods. Some lighter, faster-rolling tyres and a wide-range cassette would help broaden its horizons. But it’s clear what this bike is built to do.


Frame:Hydroformed aluminium Virtual Four Link, 130mm travel
Shock:Manitou McLeod (190x40mm stroke, air spring)
Fork:Manitou Machete Junit, 130mm travel (44mm offset)
Wheels:SunRinglé Düroc 110x15mm front/148x12mm rear, Maxxis Minion DHF 24x2.4in MaxTerra/Maxxis Minion DHR II 24x2.3in DUAL compound tyres
Drivetrain:SRAM SX X1 DUB crank 30t, 155mm, SRAM NX 11-speed shifter and r-mech, SRAM 11-42t cassette
Brakes:SRAM Guide R Custom two-piston, 160/160mm
Components:SDG Jr stem, 40mm, SDG Slater bar 650mm, SDG Jr seatpost 31.6mm, SDG Fly Jr saddle
Weight:12.7kg (28lb)
Rider height:127cm
Head angle:65.5º/66º
Effective seat angle:76.2º/77º
BB height:316mm/324mm
Front centre:650mm
Seat tube:315mm
Top tube:495mm