Trail bike’s kinematics and geometry revised, while carbon models gain frame storage

By Robin Weaver

Published: Wednesday, 25 October 2023 at 14:00 PM


YT introduced the first Jeffsy back in 2016 as its true all-rounder trail machine, capable on both the ups and downs. It was, in the brand’s words, a bike “that has your back” and “the only bike you’ll need”.

The essence of that original bike carried over into the second iteration in 2019 and, four years later, this third-generation Jeffsy.

In its latest guise, YT says the Jeffsy is more capable and versatile than ever, able to tackle a big day out in the mountains or simply rip around in your local woods.

It’s a trail bike through and through, with an obvious nod to YT’s gravity-oriented preferences.

While it isn’t designed to specialise in only one type of mountain biking, YT seems confident that no matter what you throw at the new Jeffsy, it’ll handle it and leave you smiling.

Five things you need to know about the new Mk III YT Jeffsy

  • Head angle reduced to 65 degrees, while seat tube steepens across size range to ensure you can maintain a 77-degree angle with your saddle set to the correct height
  • Reach and chainstays grow across all sizes
  • Three carbon models, two alloy models, all with 29in wheels
  • New internal storage on carbon frames
  • Pricing starts at £2,499 / $2,799 for the Core 1 alloy Jeffsy Mk III

YT Jeffsy frame and suspension details

At just under £5,000, you get a great wireless group set and Ultimate level RockShox suspension.
At just under £5,000, you get a great wireless groupset and Ultimate-level RockShox suspension.
Dan Griffiths/YT

The latest Jeffsy comes with a carbon or alloy frame and uses 29in wheels only.

YT says it has spent a serious amount of time ensuring the Mk III frames work well in both materials and developed them in tandem.

In doing so, particular attention was paid to the tube shapes, surfaces and edges to ensure the design would perform on the trail. YT assures us the alloy bike wasn’t just an afterthought included to hit a price point.

While YT has given this third-generation Jeffsy the standard longer, lower and slacker treatment, as you’d expect, the German brand has slightly reduced the amount of rear-wheel travel.

Its predecessor (the carbon-framed 29er, that is) pumped out 150mm at the rear via the ‘V4L’ (Virtual Four Link) Horst-link suspension design. The new bike, using the same platform, delivers 145mm instead.

This is then paired with a 150mm-travel fork up front.

However, there’s a little more to it than that.

Less travel but delivered more effectively

RockShow Super Deluxe Ultimate rear shock on the YT Jeffsy Core 4 full suspension mountain bike
Rear-wheel travel has been reduced from 150mm to 145mm.
Dan Griffiths/YT

Just as was the case on the Mk II Jeffsy, the latest frame delivers its 145mm of rear-wheel travel using YT’s ‘V4L’ Horst-link suspension design.

YT says it upped anti-squat (a measure of how the suspension resists pedal-induced bob) figures at sag to 105 per cent. This then decreases as the bike sinks deeper into its travel in a bid to limit pedal kickback.

This was done with the intention of keeping the Jeffsy feeling sprightly and accelerating quickly, while having enough give at the rear to maintain grip when needed.

At the other end of the travel, the new bike gets a little more in terms of progression. This is designed to ensure when you do really hammer it on the descents or overshoot the jumps, it’ll handle those impacts with a little more composure compared to its predecessor, better resisting bottom-out.

What’s inside the box?

There's now some downtube storage within the downtube underneath this neat little hatch which is totally separate to the bottle cage.
There’s now some storage within the down tube, underneath this neat little hatch.
Dan Griffiths/YT

With the best trail bikes now more capable than ever, YT knew it needed to up the stakes when it came to frame details.

For starters, those who hate riding with a cycling backpack will appreciate the new down tube storage on the carbon frame (the alloy frame forgoes this).

Rather than creating an opening directly beneath the bottle cage, YT has positioned the Jeffsy’s hatch further up from the bottle cage bosses.

Why? Well, if you’ve ridden a bike where the water bottle sits on top of the storage hatch, you may have experienced it getting a little wobbly over time.

YT Jeffsy Core 4 full suspension mountain bike
The Core 4 comes with the wireless, electronic SRAM GX Transmission drivetrain.
Dan Griffiths/YT

Separating these makes sense, although it can cause issues with space in the front triangle. YT managed it, though.

YT claims the lid and sealing system is very close to being completely waterproof and uses an Igus bushing at the lever to keep it working smoothly.

Inside, a separating wall has been created to prevent your kit sliding down to the bottom bracket junction. The bags used are made from neoprene and zipped, though YT has turned this inside out because it helps to reduce rattling and vibrations when filled with kit.

Internal cable routing has been tweaked to accommodate the new opening, too, with cables now bonded to the sidewall of the down tube.

Rider-friendly frame details

YT Jeffsy Core 4 full suspension mountain bike
Pop the QR out to reveal a handy 6mm Allen key.
Dan Griffiths/YT

Aside from the internal storage, there are bosses below the top tube (on both the alloy and carbon frames) to bolt a gear mount in place so you can carry even more kit. There’s also space in the frame for a 620ml bottle using YT’s Thirstmaster 6000 side-loading cage.

The new frame protection lives underneath the down tube. This toughened plastic belly protector is bolted firmly to the frame, but to improve impact absorption and damp any whacks a little more effectively, there’s now a layer of foam between the protector and frame.

There’s also a new, improved chainstay protector to help limit chain slap and shield the frame.

YT says it has also worked hard on a renewed internal cable guide system and improved access to the linkage pivots on the driveside for easier maintenance. There’s even a 6mm Allen key housed inside the rear axle quick-release.

YT Jeffsy geometry details

SMLXLXXL
Low / HighLow / HighLow / HighLow / HighLow / High
Seat angle (degrees)76.9 / 77.577.5 / 78.177.9 / 78.578.3 / 78.978.6 / 79.2
Seat angle @ saddle height (degrees)77.2 / 77.877.2 / 77.877.2 / 77.877.2 / 77.877.2 / 77.8
Head angle (degrees)65 / 65.665 / 65.665 / 65.665 / 65.665 / 65.6
Chainstay (mm)437 / 435437 / 435437 / 435442 / 440442 / 440
Seat tube (mm)385 / 385410 / 410435 / 435460 / 460485 / 485
Top tube (mm)578 / 562593 / 585609 / 607627 / 630645 / 652
Head tube (mm)100 / 100108 / 108116 / 116124 / 124132 / 132
Bottom bracket drop (mm)33 / 2533 / 2533 / 2.533 / 2533 / 25
Bottom bracket height (mm)343 / 351343 / 351343 / 351343 / 351343 / 351
Wheelbase (mm)1,191 / 1,1891,214 / 1,2131,238 / 1,2361,266 / 1,2651,289 / 1,288
Standover (mm)730 / 738728 / 736732 / 740738 / 746742 / 750
Stack (mm)613 / 608620 / 615627 / 622634 / 629642 / 636
Reach (mm)435 / 442455 / 462475 / 481495 / 501515 / 521

As with the previous bike, the Jeffsy comes in five frame sizes, ranging from small through to double extra-large, all of which house 29in wheels.

A flip chip enables you to tweak geometry between the ‘low’ and ‘high’ settings, and slacken or steepen the head and seat tube angles (along with altering other measurements slightly). You can also raise or lower the bottom bracket height, depending on the setting being used.

Rotating the chip will alter the head and seat tube angle by 0.6 degrees and bottom bracket height by a significant 8mm.

A bit longer, slacker and lower

Headtube on the YT Jeffsy Core 4 full suspension mountain bike
YT has worked on revised internal cable routing, along with a host of other, rider-friendly features.
Dan Griffiths/YT

The Mk III Jeffsy has seen its reach figure grow 5mm across all five frame sizes. That means the size medium now sports a reach of 455mm, which is on par with a number of other mountain bike brands.

Just as they did on the Mk II bike, effective chainstay lengths grow across the frame size range, too. They’re longer overall, compared to the older frame design.

Now, sizes S-L get 437mm stays, while sizes XL and XXL sport 442mm stays (that’s an increase of 2mm over the older bike). It’s not massive, but designed to make the bike feel more balanced.

YT has raked out the head tube angle to a relatively slack 65 degrees (in the low setting).

Postman Dropper remote on the YT Jeffsy Core 4 full suspension mountain bike
The Postman dropper post has been reworked in time for the launch of the new Jeffsy.
Dan Griffiths/YT

The seat tube angle is worthy of further discussion. YT wanted to ensure all riders, no matter their height, had a similar seated position when climbing.

Traditionally, due to seat tube angles remaining the same across frame sizes, as the saddle is moved higher to accommodate taller riders relative to the bottom bracket, the effective seat angle slackens off.

The new Jeffsy’s seat tube angle increases with frame size. While the effective seat tube angle on the size small is 76.9 degrees, it’s 78.6 degrees on the XXL frame (in the ‘low’ setting).

But with the average saddle height ranging from 620 to 820mm across the five sizes, all riders should be able to achieve an effective seat tube angle of just over 77.2 degrees with saddle height set to suit.

Finally, in the lowest setting, the Jeffsy offers a decent 33m of bottom bracket drop.

YT Jeffsy range overview

Maxxis Minion tyre on the YT Jeffsy Core 4 full suspension mountain bike
YT includes the same Maxxis tyre combo across all bikes in the range.
Dan Griffiths/YT

There are five different Jeffsy builds available, three of which use a carbon frame.

All bikes use the same Maxxis Minion DHF/DHR II MaxxTerra EXO+ tyre combo, which is great to see, especially on the cheaper models.

YT’s new Postman V2 dropper post features on all bikes. This has a lower stack height, uses a new, more adjustable remote and offers riders the ability to alter the amount of travel, similar to dropper posts from PNW Components, Canyon and Giant Bicycles.

Both the Core 2 and Core 3 builds use an identical spec but different frame materials. That means you can save yourself a healthy chunk of cash if you like the build but prefer an alloy frame.

It would be great to see this as an alternative on the top-end bike, too, though.

Here’s how the Jeffsy range looks:

YT Jeffsy Core 1

The Core 1 features an alloy frame, a mix of SRAM's SX and NX Eagle gearing, decent RockShox suspension and SRAM brakes, plus some great rubber from Maxxis.
The Core 1 features an alloy frame, SRAM’s SX and NX Eagle gearing, and RockShox suspension.
Dan Griffiths/YT
  • Frame: Alloy, 145mm
  • Fork: RockShox Lyrik, 150mm
  • Shock: RockShox Deluxe Select+
  • Drivetrain: SRAM SX/NX Eagle
  • Brakes: SRAM DB8
  • Wheels: SunRingle SR329 Trail Comp
  • Price: £2,499 / €2,499 / $2,799

YT Jeffsy Core 2

YT Jeffsy Core 2 full suspension mountain bike
You’ll find the same Fox Performance suspension spec bolted to this alloy frame as you will on the pricier Core 3.
Dan Griffiths/YT
  • Frame: Alloy, 145mm travel
  • Fork: Fox 36 Performance, 150mm
  • Shock: Fox Float X Performance
  • Drivetrain: SRAM GX Eagle
  • Brakes: SRAM Code R
  • Wheels: DT Swiss M1900
  • Price: £2,999 / €2,999 / $3,299

YT Jeffsy Core 3

YT Jeffsy Core 3 full suspension mountain bike
While the Core 3 shares the same spec as the Core 2, it uses a pricier carbon frame.
Dan Griffiths/YT
  • Frame: Carbon, 145mm
  • Fork: Fox 36 Performance, 150mm
  • Shock: Fox Float X Performance
  • Drivetrain: SRAM GX Eagle
  • Brakes: SRAM Code R
  • Wheels: DT Swiss M1900
  • Price: £3,999 / €3,999 / $4,299

YT Jeffsy Core 4

YT Jeffsy Core 4 full suspension mountain bike
The Core 4 gets you Ultimate-level RockShox suspension, wireless gearing from SRAM and Crankbrothers alloy Synthesis wheels.
Dan Griffiths/YT
  • Frame: Carbon, 145mm
  • Fork: RockShox Lyrik Ultimate, 150mm
  • Shock: RockShow Super Deluxe Ultimate
  • Drivetrain: SRAM GX Eagle Transmission
  • Brakes: SRAM Code RSC
  • Wheels: Crankbrothers Synthesis Enduro Alloy
  • Price: £4,999 / €4,999 / $5,499

YT Jeffsy Core 5

YT Jeffsy Core 5 full suspension mountain bike
At the top of the chain is the Core 5, which comes complete with Fox Factory level suspension and SRAM’s X0 wireless Transmission gearing.
Dan Griffiths/YT
  • Frame: Carbon, 145mm
  • Fork: Fox 36 Factory, 150mm
  • Shock: Fox Float X Factory
  • Drivetrain: SRAM X0 Eagle Transmission
  • Brakes: SRAM Code RSC
  • Wheels: Crankbrothers Synthesis Enduro Alloy
  • Price: £5,999 / €5,999 / $6,499