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First ride: YT Jeffsy MKIII

Storage and size-specific geo update German brand's classic

Jeffsy was the first 29″ mountain bike from YT and, at the time, first real push out of pure gravity-driven riding and into the world of trail riding. At just shy of 150mm travel, it’s a long-legged trail bike, to be sure. But compared to the DH Tues and the bruiser Capra, it is the YT with the broadest appeal.

With the third generation, or Jeffsy MKIII, that all-around approach remains. YT isn’t transforming Jeffsy into a specialist or overhauling it into a little enduro bike. It is still a platform squarely focused on finding the most enjoyment on the widest variety of trails. It’s just a more refined version of the Jeffsy. And it has a downtube storage compartment.

Prior to its release, we had the chance to ride the new Jeffsy for a few days in properly damp British weather. Here’s what we learned about the new bike over a few days of splashing around  in the Forest of Dean and in Wales.

YT Jeffsy MKIII: What’s new

YT aims to keep the Jeffsy as an all-purpose trail bike. There are no crazy changes to make it lower and way slacker to turn it into a mini-enduro bike. YT already has a Capra for enduro and wants Jeffsy to keep its own personality and purpose. There’s also no mullet option, with all models coming with matching 29″ wheels. That leaves the Jeffsy to be a generalist. It’s good for everyday riding, whatever trails you want to ride. Jeffsy is squarely focused on fun, but with a supportive enough platform to tackle long pedals.

Jeffsy keeps the familiar YT silhouette, with simple lines brought together at the headtube by the German brand’s “head box” area, though that is smoothed out considerably compared to previous generations. Travel on the Jeffsy actually drops five millimeters to 145mm rear travel with a 150mm fork. But YT says that travel is more progressive at the end of its travel so you still get the “pop” out of corners without a harsh bottom out. In fact, the brand says it improved pedaling efficiency by upping anti-squad to 105 per cent (at 30 per cent sag), which drops off quickly to prevent pedal kick-back.

YT does make some small, but significant changes to the Jeffsy. There is a slightly longer reach to bring it up to date. Seat tube angles are size-specific, getting steeper as the sizes get larger with the goal of putting more riders in the best position while seated. YT aims for a 77-degree effective STA at each size’s average or typical seat height. Two chainstay sizes (one for S-L, one for XL/XXL) help keep the rider balanced in the center of the bike.

Frame details: storage and protection

Since in-frame storage compartments have become a must-have feature on trail bikes, both aluminum and carbon fiber Jeffsy frames come with a sizable downtube storage area. There’s also a mount under the top tube for attaching other accessories, so you really don’t have to carry a bag to have the essentials with you on every ride.

YT takes its own approach to storage, though, separating the compartment hatch from the water bottle mounts. This might seem more complex, but YT says it helps keep the compartment weather-sealed longer since the hatch lid isn’t supporting a 500-750mL water bottle. There are also two neoprene, zippered bags to keep tools and other bits stored safely, though the compartment is also closed off so you won’t lose any small parts into the frame cavity. YT also includes its Thirstmaster 6000 (620mL) bottle, though most standard water bottles should fit in the frame.

YT adds generous frame protection to the chain and seat stays to help quiet down any chainslap. More interesting is the bolt-on downtube protection, which squeezes a layer of foam between the carbon fiber frame and external hard rubber. This aborbs impacts but also helps keep Jeffsy quiet on the trails.

A slippery day in Forest of Dean

First impressions: YT Jeffsy Core 5 MKIII

Before Jeffsy MKIII launched, we had the opportunity to spend a few days on the bike in the U.K. The riding was split between some fast, flowing, low-angle trails in the Forest of Dean and some rougher, steeper trails in Cwmcrn, Wales. It was a great balance to test out YT’s claim the Jeffsy is an all-rounder. While the sun did occasionally show, the majority of riding was in proper British weather: wet roots, greasy dirt and generous puddles.

The sun occasionally shone, but the dirt was never dry

Sizing and spec

The test bike was YT’s top-end Core 5, with a Fox 36 Factory fork a Float X shock and SRAM’s XO T-Type, though YT opts for the newer rocker-style shifters instead of SRAM’s pods. Interestingly, YT also specs Crankbrothers Synthesis Enduro Alloy wheel. The Core 5 also uses a higher-grade Ultra Modulus carbon fibre construction, while the Core 4 and 3 use YT’s High Modulus material.

I sit right in the overlap between YT’s suggested sizing for XL and XXL, and ended up with the smaller frame for the test days. Sitting on the bike in the parking lot, the combo of steep STA and reach felt almost short. This changed when tires hit dirt, where Jeffsy quickly felt balanced and really easy to get comfortable on quickly. It is fun and zesty, instead of being overly long and cumbersome like some excessively long mini-enduro bikes can be.

Brits are used to wet weather so the trails were still running fast

Jeffsy and the Forst of Dean

I sit right in the overlap between YT’s suggested sizing for XL and XXL, and ended up with the smaller frame for the test days. Sitting on the bike in the parking lot, the combo of steep STA and reach felt almost short. This changed when tires hit dirt, where Jeffsy quickly felt balanced and really easy to get comfortable on quickly. It is fun and zesty, instead of being overly long and cumbersome like some excessively long mini-enduro bikes can be.

Pedalling is more efficient when seated. There’s some movement, as you’d expect from a bike in this travel range, and more while standing. The bike’s low overall weight – YT claims 14.8kg for a size small, and it does feel featherly on the trail – helps it climb quickly, though.

Not huge, but this compression definitely pushed the Jeffsy to full travel

YT’s decision to go for a more progressive suspension tune makes Jeffsy smooth through the initial travel. This makes it comfortable and added to traction in some seriously greasy conditions, while still having enough support to stay poppy and be exciting to ride. It does mean the Jeffsy can go through most of its travel fast, though it doesn’t get stuck deep in the travel and I didn’t feel any harsh bottom-outs. Trying to survive in proper damp conditions on unknown trails, I wasn’t exactly riding full-tilt, though. So it would be interesting to see how it would deal with more aggressive riding on more familiar trails.


Having ridden the last generation Jeffsy MKII and having quite enjoyed that bike, I had pretty high expectations for the new-look machine. YT keeps the all-rounder, fun-focused features that make the last generation so good while adding important fit features like size-specific seat tube angles and chainstays. They also add new must-have features like a storage compartment and, for those who want to run Transmission, UDH compatibility.

The result is a bike that keeps true to Jeffy’s focus on making any trail fun. On the mellower, Forest of Dean trails, it was happy to get airborne off any little trail feature. In Wales, it comfortably dealt with steeper sections and rougher trails without feeling anywhere near out of its depth. YT continues making an approachable bike that isn’t overly specialized for any one type of riding or rider. Jeffsy is a mountain bike, plain and simple.

Jeffsy Core 5 on location the UK

YT Jeffsy MKIII: Pricing and availability

YT is launching the Jeffsy in five different builds to start with. That includes three carbon fibre frames and two alloy models. You can expect additional limited Uncaged releases to follow eventually, as YT does with most bikes, but the five Core models cover quite a range of pricing. Each comes in five sizes from S through to XXL.

All models come with YT’s updated Postman dropper post and Maxxis DHF/DHR tire combo (new EXO+ casing and Maxxterra compound). The adjustable Postman V2 comes in different travel for each size: 125mm S, 150mm M, 170mm L, 200mm XL, 230mm XXL.


As mentioned above, the Core 5 comes with a mix of Fox Factory suspension and SRAM XO T-type shifting with Code RSC+ brakes. Crankbrothers Enduro 3 Alloy wheels and Renthal APEX / Fatbar round out the build.

Core 4 shifts to RockShox suspension with a Lyrik Ultimate fork and Super Deluxe Ultimate shock. SRAM still provides shifting and brakes with a GX AXS T-type / Code RSC combo. Renthal black bars and Crankbrothers Enduro 2 alloy wheels finish off the build.

Core 3 and the alloy frame Core 2 share a build kit. That includes Fox 36 Performance and Float X performance fork/shock and SRAM Code R brakes with a mechanical SRAM GX Eagle 12-speed drivetrain. DT Swiss M1900 alloy rims and Renthal bar/stem finish off the kit.

Finally, the alloy Core 1 relies on a mix of SRAM SX/NS Eagle drivetrain, RockShox Lyric Base fork and DLX Select+ shock to get you out the door and on the trails. SRAM DB8 brakes and SunRingle 329 rims with an e*thirteen base alloy bar complete the build.

Weight and pricing

Claimed weight is 14.8kg (32.56lbs) for the Jeffsy Core 5 (size small, no pedals), with the Core 1 still a respectable 16.0kg (35.2lbs) (again, size small). Pricing on the Jeffsy MKIII line:

  • Jeffsy Core 1 $3,900 (USD 2,800)
  • Jeffsy Core 2 $4,600 (USD 3,300)
  • Jeffsy Core 3 $6,000 (USD 4,300)
  • Jeffsy Core 4 $7,700 (USD 5,300)
  • Jeffsy Core 5 $9,100 (USD 6,500)

Jeffsy MKIII is available now directly through YT’s website.