YT INDUSTRIES JEFFSY CORE 4 REVIEW

Words by Calum Ross  |  Photos by Robert Johnston

We were excited to welcome the new YT Jeffsy into our testing fleet following an enjoyable first couple of days riding it for its First Ride Review at the end of last year. For the long-term review, we decided to give it to a rider who’d be able to log some impressive mileage on it and give a proper insight to its durability over the long haul, as well as some insight from a consumer perspective. While it’s not the enduro bike that Calum perhaps needed, it’s safe to say that testing the YT Jeffsy Core 4 was an absolute blast.

ABOUT THE YT JEFFSY CORE 4

FRAME AND FEATURES | The YT Jeffsy MK3 was updated at the end of 2023, with 145mm of rear wheel travel and a 150mm fork to suspend its pair of 29” wheels. YT offers the new Jeffsy in a choice of three different frame materials, depending on the spec level selected. Core 1 and Core 2 builds feature an aluminum alloy frame; the Core 3 and Core 4 feature their High Modulus carbon fiber frame, and the top-spec Core 5 receives the lighter weight Ultra Modulus carbon fiber.

Regardless of the frame material, the same high quality treatment is applied throughout. There’s custom rubber protection for the downtube and chainstays; internal cable routing, and room inside the front triangle for a water bottle on all sizes. Rivets on the underside of the Top Tube allow for a tool or spare tube to be mounted securely. The rear end uses a SRAM UDH for T-Type compatibility or easy derailleur hanger replacement; and there’s a 6mm hex key integrated in the rear axle quick release lever.

A Flip Chip is added to the lower shock mount, offering 8mm of bottom bracket height and 0.6 degrees of head tube and seat tube angle adjustment. Rounding out the universal features are extra seals added to the frame pivot points, helping to resist the ingress of mud and water into the bearings and keep them running smoother for longer.

The carbon fiber frames are now equipped with YT’s “Stash” internal frame storage, with a custom-made hatch and weather resistant neoprene bags inside to keep the valuable snacks and tools safe.

SUSPENSION | YT Industries continues to use their V4L Horst Link suspension design for the Jeffsy, now providing 145mm of travel to the rear end. Anti Squat sits at just over 100% at sag, and there’s a generous 30% leverage ratio progression to keep you off of the bottom out bumper.

GEOMETRY | YT avoided going too crazy with the geometry on the new Jeffsy, to keep the well-rounded, “do everything” nature intact. The result is a geometry table without any real stand out figures. The chainstay length grows from 437mm on sizes Small – Large, to 442mm on XL and XXL to maintain the balance on larger sizes. Effective seat tube angles get slightly steeper throughout the size range to keep the rider better centered, with the Large coming in at a fairly steep 77.9 degrees.

BUILD SPECS | The new Jeffsy is offered in five standard builds, from the entry-level alloy Core 1 at $2,799 / £2,499 / €2,499 the top-spec Core 5 at $6,499 / £5,999 / €5,999. YT also has their limited-edition “Uncaged” series builds, with a special mixed-wheel alloy Jeffsy Uncaged 13 available at the time of publishing with a unique look and paint job.

The Core 4 we tested comes in at $5,499 / £4,999 / €4,999 typically, though is available with roughly 20% off at the time of writing. It’s a SRAM-heavy spec that leaves little to be desired. Rock Shox provides an Ultimate-level Lyrik and Super Deluxe suspension package, offering high levels of tuning potential. There’s a SRAM GX Eagle T-Type drivetrain; and pair of CODE RSC brakes.

An alloy Renthal cockpit controls the front end. YT spec’s their Postman V2 dropper (based on the SDG Tellis) in long travel lengths throughout the size range, with the large coming with a 200mm unit. This is topped by a custom YT x SDG saddle.

Rounding out the specs is a Crankbrothers Synthesis Alloy enduro wheelset, which is wrapped in a Maxxis Minion DHF / DHR2 combo in EXO+ casings. The build is claimed to weigh in at 33.3lbs (15.1kg) for the size small.

THE DIRT

Calum tested the Jeffsy Core 4 over six months in the North East of Scotland, from local trail centre blasting, to big mountain missions in the Cairngorm mountain range. The North East is notoriously tough on bikes, thanks to its thick mud and harsh granite rock, and the Jeffsy has come out of the other side smiling and ready for more abuse. Here’s what he had to say in regards to its performance.

SETUP | At 178cm (5’10”), the size Large YT Jeffsy tested was a good fit. That said, I certainly didn’t feel as if the bike needed to be any longer. To get a comfy starting point on the suspension was a simple process, largely thanks to the RockSox suspension on the Core 4 spec. Setting the rear to around 30% and the front 25% sag with one token in the fork, nothing felt too quirky about the bike and it was easy to start getting into the nitty gritty of the bike setup. I left the flip chip in the low setting and never felt the need to change it.

CLIMBING | Climbing the YT Jeffsy was an enjoyable experience overall. The suspension platform performed well in both open and closed suspension modes, but sometimes lacked the all-out traction required to clean the loosest of technical climbs. A bigger travel bike can provide this traction, albeit at the expense of pep and efficiency. Overall, the Jeffsy was a nimble and dynamic climber.

I found the climbing position to be comfortable during everything from hour and a half long sprint rides, to multiple big days in a row with up to 5 hours of saddle time. The 200mm travel YT Postman dropper meant there was no need to compromise on my climbing seat height, with ample travel to drop the seat well out of the way for the descents.

DESCENDING | Ripping fast and snappy trail center style tracks was where the Jeffsy started to thrive, rewarding you when pushing the pace of the bike. It provided a very satisfying feeling, as the bike generated speed very well when forced into big berms and compressions, delivering easy speed and flow. The bike felt very maneuverable when tipping into the berms typically found at Scottish trail centers. It handled big jumps with ease, feeling balanced and confidence inspiring in the air. The suspension platform had a healthy level of progression, which reacted well to the cornering compressions and the occasional over-jumps that are part and parcel when riding these tracks. I would not describe the suspension as bottomless, but it provided a great balance of suppleness and support required for this type of riding. The Jeffsy was reactive, and I loved pushing it on this style of terrain, always opting for an extra lap or two!

However, there is a price to be paid for this lively style of descending. On natural, rougher enduro tracks, I often felt underbiked on the Jeffsy. It suffered from noticeable fore-aft pitching when tackling the chunkiest of terrain with axle high steps and rocks. I feel that over time a very dialed suspension set up could help improve the bike in this area. By no means did this prevent the bike from being capable of tackling this style of track or put me off riding them, but it was noticeable that the on-trail antics were becoming rowdy at lower speeds than on my normal enduro style bike. Often, I found myself not being able to brake as late or sharply into turns as usual, and it forced a lot more mistakes in this area of my riding. Be it slightly out-braking myself into turns, or not being able to commit to the corner entry at higher speeds or while trail braking. Often this led to me feeling more fatigue when trying to equal the speed of friends on long legged enduro bikes.

Of course, the Jeffsy is not an enduro bike, and so this may not come as a surprise. However, I felt it important to stress this, given that I know many riders would be tempted to switch out their enduro bike for one. You absolutely can reap the rewards in the correct terrain but know that it will come with some compromises when the terrain hits the gnarliest of heights.

My main gripe with the Jeffsy’s descending characteristics was some vague steering feel, with the front end having a tendency to deflect whilst turning on undulating terrain or over small rocks and roots. In my opinion this is largely caused by the selection of a 50mm stem. Swapping to a 42mm stem helped to improve the weaker aspects of the Jeffsy’s downhill characteristics, with no real compromise to uphill performance. I swapped to a 42mm stem for a lot of the test, and it greatly improved the turn in and mid-turn characteristics of the bike, by removing the wandering and feedback feeling I was experiencing through the bars with the 50mm stem. Additionally, a double down tire casing instead of an EXO+ may have given a more stable front end feel. Of course, the weight penalty that would result could detract from the overall objectives of this bike. After all, it’s marketed by YT as a 50/50 split between climbs and descents.

FINISH AND VALUE | YT’s finishing touches to the frame were second to none in terms of function and practicality. Handy on-bike storage with the Crankbrothers Tool stash paired with internal downtube storage meant all tool and snack carrying needs were taken care of, and with room to spare. The frame silencing was excellent throughout the test, with the large protector fending off the assault of small stones on the downtube, and only a small bit of mastic tape was needed to prevent some heel rub on the chainstays. There was no cable rattle to speak of on this bike, and the frame seals seemed to do a good job of preventing rattles and keeping out water. My main annoyance with the frame was the bottle holder placement leaves you stuck with a relatively small bottle, hindering you on the biggest days out. It’s a shame, as this is the type of riding that most suits the Jeffsy, so be prepared to add some form of pack to help support your adventures.

COMPONENT REPORT | YT describes the Jeffsy Core 4 spec with the following: “This CORE 4 spec goes big, harnessing bleeding-edge tech from RockShox and SRAM to bring control and elevate your trail-slaying game in the process.”

This might read as a classic mountain bike wince-inducing marketing line, however for the most part YT have spec’d this all-rounder pretty comprehensively, leaving little room for more than a few personal choice upgrades for most riders.

Overall YT provided a great value spec for 5k, with no standout component weaknesses. The SRAM AXS gears will appeal to a lot of tech fans, but can be a hindrance in some situations, such as river crossings. If wilderness rides are on the agenda, a mechanical groupset may offer better peace of mind.  It must be noted that the first derailleur on the bike suffered a failure of the battery terminal pins, and left the derailleur needing replaced after only 60 miles of trail riding! Additionally, it can be frustrating if you are left without any gears because the battery has gone flat. I found that the derailleur and battery would “wake up” if movement is detected, which resulted in a flat battery when left in for multiple drives.

The RockShox Lyrik and Super Deluxe Ultimate combination is an excellent starting point on any high-end spec list, and performed well in the test. With even more time, I think most riders would be able to fine tune a setup they would be very happy with.

The remainder of the finishing kit is well spec’d. I was initially sceptical about the SRAM CODE brakes, as I typically prefer a Shimano setup. After the duration of the test though, I couldn’t find much fault with the CODEs performance or reliability. I do find the bite point at the lever is more vague feeling compared to the sharper bite point of a Shimano, but that’s a personal preference.

The casing selection of EXO+ tyres might be an opinion divider, but I found them to be a suitable choice for the all rounder nature of the bike. Riders looking to push the capabilities of the Jeffsy in the roughest terrain will be better served by a Double Down casing. The tread combinations of Minion DHF and DHR are unlikely to upset anyone, and rode predictably in a mix of dry and muddy conditions as expected.

The Crankbrothers Synthesis wheels were completely new to me, so I went in with no preconceived ideas about how long they would last. They have survived the test without even needing to tighten a spoke or bend out any dings, even after several rock clattering moments which could have left me walking back to the car park with a wheel in one hand and bike in the other! There is a bit of a rumble from the front bearings now, but nothing a clean and re-grease would not fix.

Finally, it was nice to see a bike with a branded bar and stem combo – especially Renthal – which helped me feel comfortable on the bike from the first couple of runs! It was a shame about the 50mm stem, but that’s a small flaw in overall a very strong spec list.

For all out descending speed I would be looking towards the Capra. Similar spec with increased travel and the addition of the Zeb and Double down tires would certainly chop a few seconds off the time on rougher tracks, but this comes with the obvious compromise of being a more relaxed climber and less lively on the trail center and mellower bike park trails. A balance between the two could be to equip the Jeffsy with thicker casing tires and shorten that stem length to move the bias more towards the downhills.

For those looking for an all day, do-it-all ripper, I think the Jeffsy’s finishing touches, spec and price push it towards the top end of the trail bike category, with most other brands struggling to equal such desirable features at this price point.

The Wolf’s Last Word

I would be doing the Jeffsy an injustice without mentioning how good it performs as a normal mountain bike. Sounds stupid right? While testing this bike, I found it excelled at being an all-round “grab and go” bike, regardless of the day ahead. While it might not be as fast as an all-out enduro bike on the most technical descents, I could chase down my friends on most chunky tracks I steered the Jeffsy toward.

It breezed up long climbs, leaving you with plenty of energy for the descents, and I often found myself smirking while accelerating down tracks I had previously found mundane or a dizziness-inducing effort on an enduro rig. Most of all, I loved chasing challenging big mountain days, where all-out speed is not the objective, but enjoying the challenging climbs and lengthy mountain descents are the most rewarding part of the ride. 

Price: $5,499 / £4,999 / €4,999
Website: YT-industries.com