YT Jeffsy Core 1 review

The YT Jeffsy Core 1 is the most affordable option in YT’s aggressive trail bike line-up. The fun-loving bike has been a top performer in each of its previous iterations, but does the third-generation Jeffsy live up to the hype?

Rolling on 29in wheels, the Jeffsy gets 145mm of rear-suspension travel – 5mm less than the previous version – and a 150mm fork.

YT has always been known for its value-for-money spec and the £2,499 Jeffsy Core 1 is no exception. It’s aimed at the budget-conscious trail rider looking for maximum bang for their buck.

Although it misses out on the flashy integrated storage of its carbon-framed siblings, the Jeffsy Core 1 gets the same geometry and suspension platform, as well as a solid spec list.

Out on the trail, the Jeffsy exceeded my expectations, taking demanding enduro trails in its stride.

The slack, balanced geometry puts the rider in a centred position between the wheels, boosting grip and inspiring confidence.

However, despite its capabilities, the Jeffsy retains its fun-loving spirit and rewards playful line choice.

YT Jeffsy Core 1 frame and suspension

The Jeffsy Core 1 gets a feature-packed aluminium frame. The internally routed cables have rubber ports at all entry and exit points and there’s substantial rubber protection on the chainstay and down tube.

Rivets under the top tube enable you to attach a tube, pump or other tools to the frame, and the Thirstmaster 6000 620ml bottle and side-loading cage are included.

Other features include a SRAM Universal Derailleur Hanger and a removable 6mm hex tool stashed in the rear axle.

The Jeffsy uses YT’s tried-and-tested Virtual Four Link (V4L) suspension platform to deliver 145mm of rear travel.

The frame’s anti-squat is now a claimed 105 per cent at sag, while the end stroke of that 145mm is more progressive than the previous Jeffsy.

Its pivots are sealed from the elements, protecting the bearings and thereby hopefully extending their service life.

YT Jeffsy Core 1 geometry

The Jeffsy’s geometry is unapologetically aggressive.

The 65-degree head angle (slack setting) on my size-large test bike is slack for a trail bike. It's paired with a 475mm reach and a steep 77.9-degree effective seat angle.

The seat tube measures a compact 435mm and the chainstays for sizes small through large are 437mm long. The wheelbase is 1,238mm and is matched with a 343mm bottom-bracket height.

A flip chip in the shock eyelet steepens the head angle by 0.6 degrees, also lengthening the reach and raising the bottom bracket when set to the high position.

YT Jeffsy Core 1 specifications

The YT Jeffsy Core 1’s spec is solid straight out of the box. Suspension is taken care of by RockShox, with the Deluxe Select shock and Lyrik Base fork.

Brakes and shifting are delivered by SRAM, in the shape of the entry-level NX Eagle drivetrain and powerful yet affordable DB8 brakes.

The cranks spin in a threaded bottom bracket, which is easy to replace.

The aluminium SunRingle wheels are wrapped in grippy Maxxis Minion DHF/DHR II (front/rear) tyres, albeit in the slightly harder 3C MaxxTerra compound.

The e*thirteen cockpit features a 50mm stem and 780mm bar, finished off with ODI Elite Motion lock-on grips.

YT’s own Postman V2 dropper gets 170mm of travel on the size-large frame and is fitted with an SDG Bel Air saddle.

All in, the large YT Jeffsy Core 1 weighs 15.86kg without pedals.

YT Jeffsy Core 1 ride impressions

Getting the Jeffsy ready to ride was a simple affair.

After setting up the suspension (74kg kitted-up weight, 82psi in the fork, 145psi in the shock) and configuring the cockpit to my liking, I didn’t have any further fettling to do and immediately felt comfortable onboard.

YT Jeffsy Core 1 climbing performance

The Jeffsy Core 1 is a comfortable climber, thanks largely to its steep 77.9-degree seat angle, which creates an upright seated position.

This puts you over the bottom bracket and allows for efficient power delivery while spinning up climbs.

On steep singletrack climbs or switchbacks, there’s enough weight positioned over its front-centre to prevent the front wheel from lifting or wandering around.

The MaxxTerra rubber compound of the Maxxis Minion tyres rolls well and the V4L suspension effectively manages pedal bob under power when sat down, so very little energy is wasted when winching up hills.

Although the RockShox Deluxe Select rear shock has a lockout lever, I never felt the urge to use it.

Left in the open mode, it irons out bumps on rougher singletrack climbs without upsetting the bike’s pedalling performance.

The shock also delivers good traction on steeper, technical climbs, while the roomy reach enables you to move your weight around if needed without ever feeling cramped behind the bar.

Shifting from the SRAM NX Eagle drivetrain proved to be crisp and reliable, with the 11-50t cassette offering plenty of usable range.

YT’s Postman V2 dropper is a smooth operator and the aluminium lever is compact, ergonomic and matches the rest of the bike’s clean, quality aesthetic.

While, at just under 16kg, it’s no rocket ship up the climbs, the Jeffsy Core 1 impressed me with its upright seated position and efficient pedalling platform.

YT Jeffsy Core 1 descending performance

Despite having 5mm less travel than its predecessor, the Jeffsy Core 1 punches well above its 145mm-travel figure on the descents.

The relatively slack 65-degree head angle keeps it stable at speed, while the 475mm reach gives plenty of room to move behind the bar and adds to the bike’s composed feel.

While that reach figure isn’t seen as radically long for a size-large trail bike anymore, I never found myself wishing for more. In combination with the 437mm chainstays, YT has created a superbly balanced-feeling ride.

No matter how rough the trail got, I always found myself positioned perfectly between the wheels, with my weight balanced.

When I got momentarily out of shape on the bike, I intuitively found myself returning to that sweet spot of central weight distribution, thereby benefiting from maximum traction when I needed it the most.

The Jeffsy’s balanced geometry is complemented by its impressive suspension performance.

The V4L suspension platform delivers responsive and supple sensitivity off the top, effectively smoothing out smaller bumps on the trail and resulting in a plush, well-damped ride feel.

When pushed deeper into its stroke, the suspension ramps up predictably, taking fast and heavy hits in its stride without bucking you off-line.

In this respect, it often felt as though I was riding a bike with more than 145mm of rear travel and I struggled to find features that could shake the Jeffsy’s composure.

On mellower trails, the Jeffsy is still fun to ride, despite its capable geometry. Here, the suspension provides ample support to boost jumps, play with the terrain and pump the trail to make quick direction changes.

The RockShox Lyrik Base fork does a good job of matching its rear suspension and feels smooth and composed throughout the test.

I also liked SRAM’s DB8 brakes, which supplied plenty of power and didn’t fade or squeal on longer descents.

How does the YT Jeffsy Core 1 compare to the Merida One-Forty 500?

The Merida One-Forty 500 is another trail bike that sits in the YT’s price range, costing £2,600. With 140mm of rear travel and a 150mm fork, the Merida rolls on 29in wheels, like the Jeffsy.

It also shares a similarly aggressive geometry, with a 65-degree head angle and a long 509mm reach in the equivalent size.

On the climbs, the One-Forty 500 shares a similar, upright pedalling position and an efficient suspension platform. However, its lower-profile Maxxis Dissector rear tyre rolls faster than the burlier Minion DHR II on the YT.

On the descents, the Merida feels playful, confident and agile. However, when things get rough, its suspension doesn’t offer quite as much composure as the YT.

The fast-rolling Dissector rear tyre, which wins its points on the climbs, can’t match the YT’s rubber when it comes to finding traction on loose, wet or steep trails.

I also found the Jeffsy’s Lyrik Base fork to offer a smoother ride than the Marzocchi Bomber Z2 on the One-Forty.

YT Jeffsy Core 1 bottom line

Overall, the YT Jeffsy Core 1 is a seriously capable trail bike.

While it isn’t the lightest, it impressed me with its efficient climbing performance, offering a neutral pedalling platform and plenty of grip on technical climbs.

On the descents, it shines with its supple yet supportive suspension that punches above its 145mm travel figure.

The balanced geometry inspires confidence and produces predictable grip and handling across a wide range of terrain and gradients.

The build is ready to ride straight out of the box and doesn’t hinder the impressive performance of the frame and suspension.

Riders looking for a capable trail bike on a budget, that also holds its own in enduro terrain, will find what they are looking for with the YT Jeffsy Core 1.