YT Jeffsy Core 4 review – MK3 version of the excellent all-mountain bike

YT launched the Jeffsy MK3 last October. As you'd expect, the new frame had evolved in a number of areas compared to the MK2 version. But while the prevailing trend in the MTB market is to make increasingly capable bikes, YT made the surprising move to reduce the rear suspension travel by 5mm to 145mm.

Having been very impressed by the all-round trail riding capabilities of the MK2 Jeffsy (one of our best trail mountain bikes overall), I was keen to try out the new version. YT obliged by loaning me the Jeffsy Core 4 model for a few months of testing.

Frame and geometry

While almost identical to the MK2 in its layout, the MK3 Jeffsy frame looks sleeker and more refined than its predecessor. The small brace between the top and seat tubes has gone and harder lines have been smoothed out all over the carbon frame. The new Jeffsy retains the two-position geometry flip-chip on the lower shock mount.

One of the more obvious changes is the addition of a storage hatch in the downtube. The hatch sits above the bottle cage and 620ml bottle – which replaces the previous magnetic Fidlock bottle mount. Inside are two zipped 'Stash' bags for keeping whatever you decide to store within relatively easy reach and help prevent annoying rattles as you ride. YT has also added a Crankbrothers SOS Tube Stash mount under the top tube. As well as a Velcro inner tube strap, there's also a tire lever with a tubeless plug kit beneath it. The bottom of the downtube and BB area gets plastic armor.

I usually ride medium-sized MTBs, but on the YT bikes (Izzo, Decoy, and Jeffsy) I've tested previously a large frame has been the best fit. While the latest Jeffsy has got longer, the large size remains the right size for me.

Geometry-wise, YT looks to have gone with... 'If it ain't broke, let's just tweak things a little bit'. On the large frame in the low suspension setting, reach has increased by 5mm to 175mm, and the 1,238mm wheelbase is 23mm longer than before.

Key angles have been made ever so slightly more radical too. The head tube is now a touch slacker at 65/65.6 degrees (low/high position), and the seat tube angle is now slightly steeper but varies across the frame size range. YT says this provides consistently effective angles of around 78 degrees. Chainstays have grown by 2mm, which means 435mm on sizes small to large, and 440mm on XL and XXL. BB height is 1mm lower than before at 343mm in the low setting.

While YT hasn't provided any frame weights, the brand has confirmed that the MK3 version is heavier than its predecessor due to the extra carbon required to reinforce the internal storage hole and the frame's increased dimensions.

Overall, my large framed Core 4 test sample weighed in at 15.6kg / 34.4lb including an AXS battery.

Components and build

Currently, there are five models of the Jeffsy, all sporting either Fox or Rockshox suspension. The Core 5 model comes with top-tier Fox Factory kit, while the Core 4 option on test here comes with an Ultimate level Rockshox pairing of a 150mm Lyrik and a 55mm Super Deluxe shock. Neither the fork nor the shock are the latest 2025 model year versions though, so the fork has the Charger 3 damper (rather than 3.1) and the shock doesn't get the new High-Flow oil system upgrades. YT has made the shock tune more progressive than standard, with more damping ramp-up towards the end of the stroke to help prevent harsh bottom-outs.

Drivetrain-wise, the Jeffsy gets SRAM's excellent, wireless GX Eagle Transmission, but instead of the usual Pod shifter, the gears are controlled by an AXS Rocker controller – which I prefer to the Pod. Braking is also taken care of by SRAM, with four-piston Code RSCs with 200mm rotors at both ends. RSC-level levers have brake pad contact point adjusters as well as tuneable lever positions

The 29-inch wheels come in the form of Crankbrothers Synthesis Enduro Alloys which cost around $600 / £600. They have slightly different internal widths (31.3mm front and 29.1mm rear) and spoke counts (28 front and 32 rear). Crankbrothers say a slightly wider front rim improves support for the wider tires typically run on the front wheel, while a lower spoke count gives a more compliant feel to help grip. This all makes sense, but I wasn't able to discern any differences from a regular wheelset. The 17-degree freehub pickup isn't the quickest, but you wouldn't expect instant engagement on more budget-orientated wheels such as these.

The front wheel gets a 2.5in Maxxis Minion DHF tire, while the rear gets a 2.4in DHR II. Both tires come in a MaxTerra triple rubber compound with EXO+ casings which are ideal for a bike of this kind.

YT has updated its Postman dropper and it now comes with shims that allow you to adjust the maximum saddle height by 5, 10 or 20mm. Full travel is 170mm and I had no need to change it. The seatpost is topped by an SDG Bel Air 3.0 saddle.

Cockpit-wise, the Core 4 Jeffsy gets a 780mm Renthal AL2014 aluminum bar paired with a 50mm Apex 35 stem. Grips are ODI's Elite Motion V2.1.

Ride and performance

My first ride on the Jeffsy Core 4 was in a very wet South Wales on the White's Level trail at Afan, which begins with a 6km climb with some fairly steep ramps. I hadn't weighed the bike beforehand but I remembered it was around 16kg / 35lb when reading YT's specs. I was surprised at how eagerly the Jeffsy climbed for a mid-travel, all-mountain bike that sits quite deep in its travel. My body position when seated felt spot on for powering up the climb without the need for much standing. There was a bit of front-end lift on the steepest sections, but it was easy to correct with more pressure on the bars. In fact, the climbing went so well that I thought the bike must be lighter than the weight I had in my head, but weighing the Jeffsy later, that wasn't the case.

Descending the wet rocky trails, the bike proved that it could handle itself too. The low center of gravity, well-balanced feel and plenty of traction gave immediate confidence and it seemed only natural to properly rail the Jeffsy through the turns. The handling felt responsive and agile too, while the suspension and tire grip helped me hold lines through the slippery rock sections. Hitting rock drops, there was no harsh bottoming out – even after darkness fell and I wasn't quite sure where the landings were until I slammed into them.

A great first ride, but examining the Jeffsy the next day, the matte black carbon finish had been badly scuffed up. A lot of Afan's trails are made out of rock, with everything from slabs to Welsh grit over the course of a ride. In the wet, you get sprayed with grit and there were scratches and scuffs from the scouring action of my knees and heels on the frame. To be fair, away from Afan's bike-eating dirt, the finish has proved less problematic on the woodside trails I usually ride in Britain's southwest, but I'd still expect a more resilient finish.

After almost four months of testing the Jeffsy, I'm genuinely gutted to be sending it to YT. It quickly became my go-to and I've hung onto the bike as long as possible. It's proved to be a capable performer on everything from singletrack blasts to enduro trail runs – and to be honest, I'd totally forgotten about the 5mm of travel that's been lopped off until it came to write this review.

YT has specced a very capable set of components. The Rockshox suspension does a top job when hammering through rough sections as well as soaking up hits. I've been riding bikes from GX Transmission since it was first released and it's also excellent, though it does shift across the cassette more slowly than standard AXS. The SRAM Code RSCs give plenty of reliable stopping power and a progressive feel too. The Crankbrothers wheels have proved to be decent performers with a comfortable bit of give in them.

Aside from the matte finish, my only other real criticism is pretty minor. While I definitely appreciated the in-frame storage, a fairly small opening makes things a bit fiddly when fishing the out Stash bags or stuffing them back in.

As for how the Jeffsy Core 4 stacks up against comparable bikes around its $5,499 / £4,999 / €4,999 price tag, only fellow direct-sales brand Canyon can match it on full-price spec with the Spectral CF 9 (which is also slightly cheaper and lighter too).

However, at the time of writing, YT has reduced the price of Core 4 model to $3,849 / £3,499 / €3,499 (there are big price drops on all the other Jeffsy models too), which is a very significant saving. At that price, the bike is absolutely outstanding value for a bang-up-to-date, super capable, sorted trail bike like this.

Verdict

The new Jeffsy is an excellent trail bike package that can handle tougher stuff too. Improved pedaling has made it a true all-mountain bike that I had a blast riding on a wide range of trails.

Component choices are excellent and, while the wheels maybe cheaper compared to the rest of the spec, they do a decent enough job. Canyon's latest Spectral is the only rival bike to offer similar bangs per buck at full price.

If buying the matte black version though, I'd factor in an extra $100 / £100 / €100 for the cost of a full InvisiFrame kit – or opt for the Jeffsy with the shiny red finish.

Test conditions

• Temperature: 50 to 95 degrees F / 10 to 35 degrees C
• Conditions: Everything from sodden to bone-dry trails
• Trails: Welsh trail center rocks and roots, local singletrack, steep natural enduro runs

Tech specs: YT Jeffsy MK3 Core 4

• Price: $5,499 / £4,999 / €4,999
• Discipline: Trail/all-Mountain
• Frame: Ultra Modulus Carbon, 145mm travel
• Head angle: 65/65.6 degrees (low/high)
• Effective seat tube angle: 77.9/78.1 degrees (low/high, size large)
• Reach: 475mm (size large)
• Chainstay length: 435mm (S to L), and 440mm (XL and XXL)
• Bottom bracket height: 343mm (low setting)
• Fork: RockShox Lyrik Ultimate, Charger 3 damper, 150mm travel
• Shock: RockShox Super Deluxe Ultimate, RCT2, 55mm stroke
• Wheels: Crankbrothers Synthesis Enduro Alloy, 29in, Boost width hubs
• Tires: Maxxis Minion DHF/DHR II 3C EXO+ MaxTerra, 29x2.5 front, 29x2.4in
• Drivetrain: SRAM GX Eagle Transmission 170mm crankset, cassette and derailleur, SRAM AXS Rocker shifter
• Brakes: SRAM Code RSC 4-piston brakes, SRAM HS2 200mm rotors
• Seat post: YT Postman V2 170mm with adjustable drop by 20/10/5mm
• Saddle: SDG Bel Air 3.0
• Bar and stem: Renthal AL2014, 780mm bar, Renthal Apex 35, 50mm stem
• Sizes: S, M, L, XL, XXL
• Colors: Matte black, gloss red
• Weight: 15.6kg / 34.4lb (large tested)
• Rival bikes: Merida One-Forty, Canyon Spectral, Specialized Stumpjumper, Santa Cruz Hightower