YT Jeffsy 29 Core 4 Review


Words by Robert Johnston / Photos by Adam Lievesley

Young Talent or YT Industries surely needs no introduction by now, as a direct-sales powerhouse that has taken the mountain bike industry by storm. They’ve recently diversified their range to include ultra-fast xc/trail bikes through to enduro eMTB’s, but they’ve not left their long-standing models behind in doing so. For 2021 YT continues to offer their Jeffsy all mountain bike, with a frame that sees its third year of production, but with bang up to date spec and continued value. How does it stack up to some of the more recent additions to the market? The Eurowolf was excited to find out.


• Flip Chip Equipped
• HTA 66° / 66.5°
• STA 77° / 77.5°
• REACH 470mm (Large)

Price: £4,649 / €5,199 /$5,199

The Jeffsy is YT’s take on the bike for all mountains, designed to tackle steep ascents as well as the rough descents. Yt offers the Jeffsy in their Core range, as well as occasional special edition “Uncaged” models such as their Blaze spec and brand new Flight Attendant equipped Uncaged 6. The Core range currently features a Core 2, 3 and 4 (tested), with price tags from £2,649 /€2,999 /$2,999 through to £4,649 /€5,199 /$5,199 and two-color options per build. The Core 2 is built around an aluminum frame, whereas the Core 3 and 4 feature the lighter weight carbon fibre frame. All models are offered in sizes S-XXL to suit riders from 154-202cm (5’1”-6’8”) with 29” wheels front and rear. The signature YT V4L (Virtual Four Link) horst link suspension design delivers 150mm of rear wheel travel that is matched with the 150mm fork. For riders looking for a more playful ride, the entry level Core 2 model is also available with 27.5” wheels and a boost to 160mm travel.

YT Jeffsy 29 Core 4 Profile Shot

The Core 4 tested is YT’s highest spec Core offering, with an array of top-spec components that should satisfy all but the most bling-desiring riders. Fox takes care of controlling the 150mm of travel on both ends, with their Grip 2 damped 36 Float Factory up front paired to their DPX2 factory out back with Kashima coatings. Gearing and brakes are an all-Sram affair, with an X01 Eagle drivetrain driven by a Descendant Carbon DUB crankset, and G2 RSC brakes stopping with 200mm rotors. DT Swiss XM1700 wheels offer a strong package at a reasonable weight, with the 36SL ratchet, and are wrapped in a pair of 2.4” Maxxis DHR2 tires with an EXO casing Maxxgrip in the front and EXO+ MaxxTerra in the rear. Rounding out a capable package are the YT Postman dropper that uses proven SDG Tellis internals (125mm S/M, 150mm L, 170mm XL/XXL) topped by a SDG Bel Air 3.0 saddle, and a Renthal Cockpit comprising a 50mm Apex stem and 780mm Fatbar 35 with 30mm rise. The sum of this spec weighed in at 14.35kg for my XL test bike, without pedals.

This V4L suspension design is touted to offer a sensitive response off the top, with good mid-stroke support and plenty of end-stroke progression. This is accompanied by a “high amount of anti-squat” which should help to keep things firm under power and provide an efficient platform for pedaling. By my calculations on linkage analysis software, using an arbitrary center of gravity value for a typical height rider on the XL frame, anti-squat values sit at around 100% in all cogs at sag with a high level of progression in the leverage ratio from 3.5-2.3, and relatively low anti-rise figures at around 63%.

YT Jeffsy 29 Core 4 Rear Shock Linkage

Geometry numbers are conservative without being overly dated, with relatively steep climbing angles and healthy reach figures on offer across the size range. There’s a flip-chip on the lower shock mount offering half a degree of adjustment to the head tube and seat tube angles, with an accompanying 8mm change in BB height. Our XL had a 460mm seat tube and sported a 490mm reach paired to a 636mm stack. There are 20mm reach jumps between sizes with relatively short seat tubes across the board, which should allow for sizing up and down depending on rider preference. Common to all sizes are the slammed 32mm bb drop, 77-degree effective seat tube angle and 66-degree head angle in the low setting, which becomes 24mm bb drop, 77.5-degree seat angle and 66.5-degree head angle in the high setting. The chainstays are relatively compact for the small through large sizes at 435mm and see a slight bump up to 440mm for the XL and XXL to help to maintain the weight balance.

YT has got the frame details well covered by now, with a bolt-on downtube guard that wraps around the edges and well-covered chain and seat stays to keep things quiet and safe from damage, as well as key “chainsuck” areas protected to ensure a dropped chain doesn’t wreak havoc. Locking collets are used on the main suspension pivots to keep things firmly in place and backed with additional seals to ensure the bearings stay smooth through gritty rides and frequent washing. They opt for an integrated headset with molded carbon headset contact surfaces to optimize the weight and minimize the chance of creaking. Cable routing is almost entirely internal, with guided tubes to make things as easy and quiet as possible, and rubber plugs are used at the ports to keep things in place. A small bottle will squeeze in between the shock and BB area on the frame – YT offers a Fidlock-equipped solution as an optional extra called the Thirstmaster 4000 that’ll make best use of the space and give 600ml of hydration capabilities. The frame uses all of the typical standards, with a 148x12mm rear end spacing, 180mm post mount for the rear brake and 210x50mm metric shock. They opted to use a pressfit bb here, which will not please all but has been proved to work just fine.

YT Jeffsy 29 Core 4 Action

YT has begun to roll out some brick-and-mortar sites in key locations worldwide (currently Germany, UK and USA), which they call the YT Mills. These feature large showrooms that allow prospective customers to have a look at the YT range up close and personal and are manned by knowledgeable staff who are enthusiastic about their bikes and make killer coffee. I visited their UK mill in the Surrey Hills to collect my Jeffsy test bike, and after a comprehensive tour of their impressive facility I was given the treatment, a typical customer would receive when picking up a YT bike of their own in-store. Suspension pressures, saddle height and cockpit setup were all performed with the help of the kind employees, so I was ready to roll in no time. This is a valuable step in the right direction to removing some of that “alien” direct sales feeling, having a much more direct line of communication than the typical delayed email response from YT HQ you may have faced when purchasing one of their bikes previously. Another significant benefit of these YT Mills are the spare parts and warranty aspect, where YT are pushing hard to ensure that customers are taken care of much more effectively and efficiently than before.

Once this initial ballpark setup was out the way, it was time to hit the trails around their UK Mill – the famed Surrey Hills that can claim to have produced some of the world’s best mountain bikers including Brendog, Bernard Kerr and Olly Wilkins to name a few. This huge area of woodlands is littered with local-built trails and provides a great location to get familiar with the handling of a bike with a bit of everything from flat out jump tracks through to steep rutted turns. I hopped on the Jeffsy and instantly felt pretty comfortable trying to keep up with the YT employees, but there were a few things most definitely amiss with the setup that I managed to resolve over the course of the testing period. Most notable were the suspension setup and tire pressures, which were certainly on the “safe” side of things. This was the first bike I’d used my Motion Instruments data acquisition system on, and as such I was able to obtain a more detailed and optimized suspension setup than ever before. Further testing led to an increase in air pressure on both ends, a significant increase in rebound speeds and a healthy drop in tire pressures once the flimsy casings were addressed. The result was 28% sag out back with the stock volume spacers paired with 20% up front, providing a good compromise of dynamic ride height. Though there were big changes required to get the most speed and grip out of the bike for me, the initial setup had proved perfectly rideable and the kind of setup anyone could hop on and ride easily, which is absolutely the correct approach.

YT Jeffsy 29 Core 4 Action

The remainder of the test period saw the Jeffsy subjected to my typical mixed bag of riding, from downhill laps at Revolution Bike Park through to mellow mile crunching and everything in between, which the Jeffsy took in its stride. The climbing position is very comfortable, with what is now a relatively average seat angle sticking a good balance of front wheel weighting without feeling bolt-upright and too compact. Under power there’s a fine balance of support and compliance, with a subtle amount of bob that doesn’t reach the energy-sapping feeling, but enough movement allowed in the rear end to conform to the terrain below. I’d still tend to flick on that compression switch for the smoother climbs, but it wasn’t a necessity and made technical climbs a very pleasant experience. Overall bike weight is not super light but equally not too portly, falling into a happy middle ground where you’re not scared to climb or send. The “steep” head angle lets the Jeffsy maneuver well in the slower switchbacks, with the mid-length rear end maintaining a good halfway house of front wheel weighting without losing the all-important grip. Climbing the Jeffsy is damn good.

Much to my surprise, there’s a healthy amount of grip and sensitivity on offer that pushes the descending ride feel more towards the enduro side of the spectrum than I’d been expecting. This does lead to the Jeffsy feeling a touch more cumbersome when pumping through mellower terrain than other similar bikes as it is more eager to use that first half of its travel. However, it does lead to an impressively capable package when it comes to Enduro bike terrain, where both ends do a great job at suggesting there’s more than 150mm travel on offer and manage to avoid hang-up reasonably well over square edge impacts. This extends to the bigger hits, with high levels of progression requiring a serious compression before the bottom out bumper comes into play. It’s not to the extent that it’s the ultimate Enduro race machine, but it could likely handle all but the gnarliest races without too much of a complaint. This initial stroke sensitivity means it’s not the poppiest machine out there, but with a little more effort its relatively light weight will soar and let you make use of that progressive travel when it comes back down to ground.

YT Jeffsy 29 Core 4 Action

Geometry-wise, the Jeffsy feels quite poised on faster and flatter tracks, but that steep head angle can find its limits in the steeper switchbacks, with less steering confidence and an increased tendency to “tuck”. The flip side is the impressive ability to navigate the flat and tight turns, with less rider input required and the ability to change direction on a dime. I’d suggest the Jeffsy to be a great candidate for a bit of overforking to unlock a little extra confidence in the steeps – a 10mm longer fork combined with slightly more dynamic sag on the rear in the low geometry setting would lead to a slacker head angle without compromising the BB height and pedaling position too much, and I think it could push the Jeffsy towards an all-round “light enduro” destroyer if that’s what you’re looking for. This isn’t to say it’s incapable of riding steep terrain in its stock guise, only that slacker competitors tend to offer a touch more safety and confidence. I’d place the stiffness of the Jeffsy frame in the upper half of the scale for sure, but it’s not at the extent of deflecting everywhere or becoming hard to handle, only reassuringly sturdy.

What isn’t so sturdy is the tire spec. Although they could make sense for some customers, they’d be the first item I would swap out in order to add some extra capability to the bike at the expense of a little agility. Sram’s G2 brake surprised me with its performance, however, did begin to show its lack of ultimate stopping power for my 210lb kitted mass once the going got extra steep or the braking became sustained. Otherwise, there’s little left to be desired from the parts spec on the Core 4, with the remainder performing excellently. A particular relief in the parts spec is the 36t driver in the DT Swiss rear hub, which halves the slack from the DT hubs of the past and makes for a much more pleasant experience on technical climbs. Though the pricing is still reasonable compared to some brands, YT has begun to lose their insane spec offered at each price tag, however their frame quality has come on leaps and bounds at the same time so I’d suggest it’s not a case of greed.

YT Jeffsy 29 Core 4 Action

The Wolf’s Last Word

The YT Jeffsy Core 4 29er may not be the newest model on the scene, but still holds its own as a versatile all mountain bike that’ll happily crunch out mellower miles or tackle some tough enduro tracks. With a surprisingly sensitive and “bottomless” suspension feel, dialed parts spec (apart from flimsy tires) and good pedaling manners there’s a lot going for the Jeffsy, so long as you’re not looking for the ultimate steep terrain crusher.

Price: £4,649 /€5,199 /$5,199
Weight: 14.35kg/31.6lbs (XL, actual, no pedals)


Frame: Carbon | 150mm
Fork: Fox 36 Float | Factory | Grip 2 | Boost | 150mm
Shock: Fox Float DPX2 | Factory | 3-pos | 210×55

Brakes: SRAM G2 RSC | 200mm
Shifter: SRAM X01 Eagle | 12s
Stem: Renthal Apex | 35 x 50mm
Handlebar: Renthal Fatbar | 30R x 800W | 35mm
Headset: Acros | ZS44/ZS56
Saddle: SDG Bel Air 3.0
Seatpost: YT Postman | 125 mm (S-M) | 150 mm (L) | 170 mm (XL-XXL)

Wheelset: DT Swiss XM1700 Spline | 30mm | 110/148 | XD | 36SL
Front tire: Maxxis Minion DHR2 | 29“ x 2.4 | EXO Casing | Tubeless Ready | 3C MaxxGrip
Rear tire: Maxxis Minion DHR2 | 29“ x 2.4 | EXO+ Casing | Tubeless Ready | 3C MaxxTerra

Bottom Bracket: SRAM Dub pressfit
Cassette: SRAM XG 1295 | 12s | 10-52T
Cranks: SRAM Descendant Carbon | DUB | 32T | 175mm
Derailleur: SRAM X01 Eagle | 12s

YT Jeffsy 29 Core 4 Review

We Dig

Well-Rounded nature
Capable suspension package
Technical climbing prowess

We Don’t

Steeper head angle in steep terrain
Flimsy tires


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