YT Jeffsy Core 4 first ride review

The YT Jeffsy has long performed well in our Trail Bike of the Year tests, thanks to its versatility and fun, playful ride.

This third-generation Jeffsy is intended to build upon those do-it-all credentials but has been tweaked here and there in a bid to create a bike that’s more capable than ever.

YT’s love for gravity-focused riding still shines through in everything from the geometry to suspension tune, but if you’re keen on taking on a big day in the hills, the Jeffsy will handle it.

With tyres in the dirt, it’s obvious almost immediately that the Jeffsy’s lively ride feel hasn’t diminished. If you’re keen on skipping from line to line or making the most of every trail undulation in sight, this bike won't hold you back.

It is, however, a bike that now offers a touch more in terms of stability and composure when trails speed up. It certainly wasn’t afraid to tackle technical tracks at pace.

YT Jeffsy Core 4 frame and suspension details

Although the Jeffsy is available with an alloy frame, the Core 4 uses the latest Mk III carbon construction and is only available with 29in wheels.

That means it’s treated to internal down tube storage. Unlike many other bikes that offer a hatch into the down tube, enabling you to stash essential items (or just loads of snacks), YT has done things a little differently.

The Jeffsy’s hatch sits further up the down tube, separate from the bottle cage bosses to avoid any additional leverage or wear over time.

It’s more or less watertight, too, and features a wall at the bottom to prevent kit slipping down into the bottom bracket junction.

Zipped neoprene bags enable you to carry kit inside the frame without it rattling about.

YT has paid particular attention to the hatch lid, using an Igus bushing to ensure the lever can operate smoothly, and it should stay that way over time.

There’s also some external storage on the underside of the top tube, which is great for those reluctant to carry a cycling backpack.

That theme continues with the inclusion of the side-loading bottle cage (accessed from the non-driveside) and 620ml water bottle that come on every Jeffsy model.

A new down tube and chainstay protector should help to keep the frame fresh and prevent too much noise being produced when things get rough.

If you’re in need of a 6mm Allen key, never fear because the rear quick-release axle pops out to reveal one, which may come in handy.

In terms of suspension, while the ‘V4L’ four-bar linkage system remains, travel has been reduced to 145mm and altered slightly.

Anti-squat is now a claimed 105 per cent at sag, while the tail end of that 145mm is more progressive, with the aim of handling bigger hits with a little more control.

YT Jeffsy Core 4 geometry details

The Core 4, just like the other four builds, comes in five frame sizes (S-XXL).

All bikes get a flip chip at the rear shock mount, which enables you to tweak bottom bracket height by 8mm, while changing the head angle and seat tube angle by 0.6 degrees.

YT has lengthened the chainstay lengths, too. These grow as the frame size increases. While the S-L bikes get 437mm, the XL and XXL use 442mm stays (in the low setting).

Seat tube angles also differ across the size range (from 76.9 degrees on the small to 78.6 degrees on the XXL). This is to ensure a steep effective seat tube angle is maintained as saddle height increases (which slackens the effective seat tube angle).

The reach figures across all sizes have grown by 5mm compared to the previous Jeffsy. That leaves the medium frame with a reach of 455mm.

YT has slackened the head angle to 65 degrees, though, which should improve high-speed stability.

The bottom bracket sits at 33mm below the wheel axles.

YT Jeffsy Core 4 specifications

It’s great to see YT including the same Maxxis tyres across every build, including the cheapest Core 1.

The rubber combo sees a Minion DHF at the front in the lighter EXO casing, while the Minion DHR II at the rear gets the tougher EXO carcass.

These wrap Crankbrothers Synthesis Enduro Alloy wheels.

At £5,000, the Core 4 comes with some tasty bits of kit, including an Ultimate-level RockShox Lyrik fork with 150mm of travel, paired with a Super Deluxe Ultimate rear shock.

There’s wireless SRAM GX AXS Transmission gearing and top-tier Code RSC stoppers.

Interestingly, YT specs SRAM’s older-style AXS shifters rather than the latest Transmission pods.

The final component of note is the new YT Postman V2 dropper. This has a lower stack height and can now have its travel easily altered. It also uses a new remote.

How we tested the YT Jeffsy Core 4

I rode the new Mk III Jeffsy Core 4 trail bike during the YT press camp in the Forest of Dean, UK.

We spent a day riding on local, mostly natural trails, which were muddy, rooty and technical in places, but not particularly steep.

The following day, we ventured into Wales to ride sections of the Cwmcarn trail centre. This is man-made, fast and flowy, but quite rough. We also rode some steeper, more technical off-piste trails, which were loose, tight and more challenging.

This gave me a reasonable idea of how the new bike performs on the types of trails it’s aimed at.

We’ll follow up with a full review once the bike becomes available and we can spend more time on it.

YT Jeffsy Core 4 ride impressions

YT Jeffsy Core 4 climbing performance

Thanks to the MaxxTerra rubber compound on the tyres and the seated position created by the steep seat tube, reasonable 593mm effective top tube and 50mm stem, the Jeffsy is a comfortable place to be when it comes to winching up hills.

The suspension stays relatively neutral under power when sat down, so very little energy feels wasted when turning the pedals. At no point during my two days riding the new Jeffsy did I bother flicking the shock’s lever to firm things up, instead leaving the rear shock fully open for the duration of rides.

Thanks to the blocky tread pattern of the rear tyre, coupled with the supple suspension action, rear-wheel traction on shorter, steeper inclines is impressive, too.

YT Jeffsy Core 4 descending performance

While the Mk III Jeffsy might not be quite as compact or even as lively as its predecessor, I had no issues throwing it around whenever the opportunity arose.

On the natural, muddy trails of day one, I was impressed by the overall balance of the geometry and just how quick the suspension was to set up.

The front tyre lacks a little in the way of bite when it comes to gloopy mud, but it rolls well and provides predictable grip on loose gravel or hardpack surfaces.

On faster trails with hefty compressions, I was surprised just how hard I could push the Jeffsy.

Because we were riding on trails I knew, it was easier to gauge just how impressive this was because I tend to ride these trails on longer-travel enduro bikes.

Through one section, I managed to gap a small wooden bridge, landing into rocks. Here, I was impressed with how easily I kept the Jeffsy on its line and just how well it took this sort of rough treatment in its stride.

In some slower turns where I needed to get on the gas through the exit to maintain speed, I noticed the pickup from the freehub wasn’t quite as fast as I’d have preferred. This is a small niggle, though.

When tackling more technical trails, there’s certainly no lack of nimbleness or agility when it comes to carefully pinpointing and picking your lines.

It’s easy to loft the front end up and hop from obstacle to landing, despite how stable things feel.

As for the 145mm of rear-wheel travel, at times, it certainly feels as though there’s a little more on tap when you get really deep into the stroke.

However, it remains supportive through the mid-stroke to enable these quick changes of direction.

More time is needed on a wider variety of terrain to really get the measure of the Mk III Jeffsy, but so far, I’m suitably impressed.

YT Jeffsy Core 4 initial verdict

YT has done a great job of updating one of the most fun bikes in its line-up.

While it retains a lot of what made it a blast to ride previously, the changes help to make it more formidable when it comes to steeper, more technical trails.

The well-balanced geometry, suspension that's easy to dial in and have-a-crack-at-anything attitude make it a bike I’ll look forward to riding again. Soon, hopefully.