Trail bike’s kinematics and geometry revised, while carbon models gain frame storage
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
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
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?
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.
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
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
| | S | M | L | XL | XXL |
|---|---|---|---|---|---|
| Low / High | Low / High | Low / High | Low / High | Low / High | |
| Seat angle (degrees) | 76.9 / 77.5 | 77.5 / 78.1 | 77.9 / 78.5 | 78.3 / 78.9 | 78.6 / 79.2 |
| Seat angle @ saddle height (degrees) | 77.2 / 77.8 | 77.2 / 77.8 | 77.2 / 77.8 | 77.2 / 77.8 | 77.2 / 77.8 |
| Head angle (degrees) | 65 / 65.6 | 65 / 65.6 | 65 / 65.6 | 65 / 65.6 | 65 / 65.6 |
| Chainstay (mm) | 437 / 435 | 437 / 435 | 437 / 435 | 442 / 440 | 442 / 440 |
| Seat tube (mm) | 385 / 385 | 410 / 410 | 435 / 435 | 460 / 460 | 485 / 485 |
| Top tube (mm) | 578 / 562 | 593 / 585 | 609 / 607 | 627 / 630 | 645 / 652 |
| Head tube (mm) | 100 / 100 | 108 / 108 | 116 / 116 | 124 / 124 | 132 / 132 |
| Bottom bracket drop (mm) | 33 / 25 | 33 / 25 | 33 / 2.5 | 33 / 25 | 33 / 25 |
| Bottom bracket height (mm) | 343 / 351 | 343 / 351 | 343 / 351 | 343 / 351 | 343 / 351 |
| Wheelbase (mm) | 1,191 / 1,189 | 1,214 / 1,213 | 1,238 / 1,236 | 1,266 / 1,265 | 1,289 / 1,288 |
| Standover (mm) | 730 / 738 | 728 / 736 | 732 / 740 | 738 / 746 | 742 / 750 |
| Stack (mm) | 613 / 608 | 620 / 615 | 627 / 622 | 634 / 629 | 642 / 636 |
| Reach (mm) | 435 / 442 | 455 / 462 | 475 / 481 | 495 / 501 | 515 / 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
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).
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
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
- 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
- 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
- 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
- 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
- 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