Review: 2024 YT Jeffsy Core 4 - A Good Friend

With five years to cook, it makes sense that the third version of the Jeffsy came out with a pretty polished package, now sporting a whole new feature set, along with updated geometry and kinematics. After a few months and a wide variety of terrain, I've come away with some thoughts about Jeff - read on to find out just what, exactly.

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Frame Details

Jeffsy the third comes with either an alloy or carbon fiber frame, with a slightly lighter "Ultra-Modulus" carbon available with the most expensive build kit. The alloy and carbon frames share a 2:3 split between the 5 available builds. Those build kits are given the designations Core 1 through 5, with cost and parts spec climbing with the numbers. Each level gets a smartly-specced kit of parts, with a couple key differences between the carbon and alloy frames that really set them apart.

The carbon Jeffsy now features a well-designed hatch above the bottle cage that allows for in-frame storage, with additional accessory mounts underneath the top tube. Alloy options only have the latter feature, eschewing the frame storage. There's obviously a weight penalty to the alloy frame as well, to the tune of about 600 grams across identical builds (Core 2 vs. Core 3).

Aside from the frame storage, the rest of the Jeffsy frame is satisfyingly normal, without any weird proprietary bits or compromised areas to accommodate the changes they made. There is a flip chip in the lower shock mount that can be used to alter the geometry slightly from the stock Low position. In that alternate High mode, the bottom bracket is raised by 8mm, and the headtube and seattube angles steepen by 0.6°. This is a fairly subtle shift, but enough to make a difference on trail.

I almost forget to mention the addition of a universal derailleur hanger to the frame, as it's becoming so ubiquitous as to start to seem like a given. Popularity aside, this is great to see, both for access to common spares and for the potential to run Transmission drivetrains.

Geometry & Sizing
Coming to a decision on the "right" geometry for an all-mountain bike feels like a fraught problem to solve, as there are so many approaches to achieve the same goal. As opposed to pushing things to the extreme and compensating elsewhere, YT instead gave the Jeffsy pretty typical trail bike geo and kept things reasonable within the broader spectrum of options.

With five sizes available, reaches ranging from 435 to 515 millimeters, there's a wide berth of fit options in the Jeffsy lineup. That fit scaling isn't just constrained to the front end either, as YT implemented a semi-size-specific rear center to keep the bikes feeling balanced across the size range. Similar to the Smuggler I recently reviewed, the Jeffsy stretches two chainstay lengths across the range, with 437mm on S, M, and L - and 442mm on XL and XXL.

Another number that changes with each size is the seat tube angle, climbing ever steeper as you move up the size range. Where the Small sports 76.9° effective angle, the XXL's is nearly 2 degrees steeper at 78.6°. This may sound like an extreme swing, but it achieves an average 77° seat tube angle at each size's average saddle height.

The head tube angle is squarely set at 65° (perfect for a 50-50 up-down bike, IMO), the bottom bracket drop is 33mm, and the stack heights are in what I'd call a normal range, growing about 7mm per size. There really aren't any geometry figures that are out of the ordinary, everything just seems well considered and suited to the nature of a do-it-all mountain bike. I might have made the chainstays a touch longer, but that would detract a little from the easy pop and slow-speed handling that make the bike so easy to ride.

Suspension Design

The new Jeffsy continues YT's use of their 4VL (aka Virtual Four Link) suspension system, which is a fairly typical Horst link layout that they implement in some way or another on all of their full suspension bikes. This iteration of the Jeffsy uses a 210x55mm shock to deliver 145mm of rear-wheel travel.


They've increased the frame's progression over the prior model in hopes of keeping riders from using up all the travel too often, as this was a complaint some had about the V2 bike. Anti-squat is around 105% at sag, and sag is meant to be set around the 30% mark.

There's not too much else to report on here, as the Jeffsy has received updates and tweaks over time that simply improve on the initial concept, not re-inventing the wheel with each pass.

Climbing

Over the past few months, I've managed to put some heavy mileage on the Jeffsy, which around here means gobs of climbing. Typically steep, sometimes technical, and often quick as daylight becomes scarce. The seated position on YT's all-mountain bike feels perfectly suited towards just that - riding all over the mountain. While I get along well with seat angles on the steeper side of things, the relatively neutral 77° on the Jeffsy puts you in a great position for steep climbs and long traversing sections alike. I've spent many long days on board, and the long-term comfort is certainly there.

Pedaling characteristics are definitely on the softer side of things, particularly when the terrain isn't perfectly smooth. I liked that attribute for scraping up technical climbs, and when the trail is steep enough to warrant an out-of-the-saddle effort, but on some dirt road climbs it can feel less efficient. I used the climb switch a handful of times, on long consistent road climbs servicing some of my favorite descents. That lock position is very firm, which I tend to prefer to more vague lockouts; I want to be certain that it's on.

While not specifically a climbing characteristic, the storage options in and on the new Jeffsy frame really make the bike a great all-day partner. The in-frame storage is very well sealed, with an opening large enough to get a small jacket and some tools in, the top tube mount comes with a handy little tool wrap (on the Core 4 & 5 only), and you can just squeeze a large water bottle in the cage. I didn't love YT's included tool bags, as they lack any sort of organization, but I simply used a Trek tool wrap and called it a day.

Descending

I received the Jeffsy at the beginning of what I can only describe as the Week of Perfect Dirt, where every trail is in peak form and you can do no wrong. These conditions are dreamy, but can bias you a little as everything just feels so damn good.

To the detriment of trail conditions, but the benefit of the full-spectrum review, the rains came shortly thereafter, with some regularly programmed slop sticking around for the weeks to come. This was where I spent a lot of time getting to know the Jeffsy, and where it really seemed to shine. The grip of the suspension and neutral ride height under braking meant the bike felt comfortable and composed in steeps and slippery chutes, without sacrificing too much travel. Thanks to a well-sorted build kit, I didn't feel the need to swap out a single component for the duration of the test, even as things got quite grim.

In supportive, faster-moving terrain, I found the suspension active but supportive enough, though you definitely notice your feet moving under you as you pump the bike. Though this sensation bothers some, I don't mind the feeling, as the benefit is typically better bump absorption when that smooth terrain comes to an end. Mid-size and chattery hits are handled very well by the bike's kinematic, keeping things composed, comfortable, and quiet. My biggest gripe with the suspension performance of the Jeffsy was in particularly rough and fast terrain, where you're really pushing the bike to its limits. Obviously this is typically more the purview of bigger, more downhill-focused bikes, but having found myself in that terrain on more than a few occasions it felt worth commenting on.

The sensation is mostly just an unsettled feeling underfoot as you run out of travel, which should be a familiar feeling to most people who push trail bikes into more serious terrain. I had a few hard bottom-outs on bigger single hits, but adding two volume spacers (up to the 4 token max) helped alleviate that. I think this bike would also benefit from hydraulic bottom-out, as that end stroke ramp and return is part of what makes the chassis feel unsettled as you push deep into the travel on consecutive hard compressions.


As alluded to above, the Jeffsy really was an exceptionally quiet bike. In addition to the fully-guided internal routing, there are cable clamps where cables pass freely over the frame, as well as rubber plugs that sheathe cables as they enter the holes in the frame. Those little details go a long way in keeping things muted, and I'd love to see them adopted by other brands pursuing silent nirvana. Even with a day's worth of goodies in the downtube, things were silent, which is more than I can say about a lot of other in-frame storage systems I've used.

The pinnacle of testing the Jeffsy was taking the bike with me on a trip to Chile, where I got to ride the old EWS course at Parvazo, high in the Andes above Santiago. This area is famous for the rocky conditions and "anti-grip" soil, and proved to be quite a good challenge for bike and rider. Distractingly beautiful scenery aside, I definitely had some loose moments, but was impressed at how well Jeffsy handled things. It wouldn't be my choice if I were going back to race, but I had a great time careening down the mountain regardless.

How does it compare?

The geometry between these two all-mountain 29ers is impressively close, with only a few millimeters and fractions of degrees separating the two. That said, their character on trail is quite different, with unique traits to each that may bias them towards certain riders. Both fit that jack-of-all-trades bill, but they get there via different avenues.

The Jeffsy biases more towards the descent and traction when compared to the Hightower, which feels like a much more efficient bike. The latter generates speed a bit better in pumping terrain, where the Jeffsy can hold speed better through rougher chop.

Both bikes feel well-acclimated to a plethora of terrain, but for those who plan on getting into looser, more challenging territory, I might edge towards the Jeffsy. When it comes to value, the choice is much easier. YT's pricing is still highly competitive, even while other direct-to-consumer brands have edged up towards traditional shop-model pricing. Comparing similar build kits, like the Jeffsy Core 3 and Hightower S, the YT sports slightly nicer parts, a $1,600 lower price, and only loses on overall weight by about a pound.

Which Model is the Best Value?

Though I found the Core 4 build to have an impressive overall value, for my money I might opt for the Core 3. With a price difference of $1,200, the most significant gain in the higher model tier is the GX Transmission drivetrain, though surely there are people who are going to prefer the mechanical option. Code R brakes are lesser than the RSCs, but you can upgrade those with the twelve hundred clams you've saved.

Luckily, regardless of the price tier you end up opting for, certain constants remain true. You'll get excellent tires front and rear, a long-travel dropper that works for your given size, powerful brakes, and a wide-range 12-speed drivetrain. Those details are all key to me, and very impressive considering just how well-priced each tier of the Jeffsy is.

Technical Report

Crankbrothers Synthesis Enduro Alloy Wheelset: This spec choice was a surprise highlight for me. I didn't think much about the choice initially, but after trying the bike with some stiffer wheels and going back to stock I realized the benefit of utilizing the relatively forgiving Synthesis wheels. The carbon Jeffsy frame is quite stiff, and having a bit of give in the system is key to providing good grip and a comfortable ride feel. I didn't hate things with a stiffer wheelset, but you definitely have to push a bit harder to get things to hook up the same way.

YT Postman V2: I tested the Jeffsy with a V1 post mounted, simply due to the 200mm variant of the V2 not being available yet. I'll have a standalone review of that new design in due time, as their new saddle clamp design is interesting. It works similarly to the AXS post, where you tighten the saddle rails via two bolts on the sides of the head, as opposed to the fiddly undercarriage method most posts use.

SRAM Dub Pressfit BB: Despite the groans that are sure to ensue as a result of a new bike using a pressfit bottom bracket, I can attest to this one proving to be completely trouble-free. With proper frame manufacturing standards and tight tolerances, pressfit shells really shouldn't pose any issues, and I'm happy to say that's the case here. Dry dusty powder, absolutely wet slop, and temperate in-between all reared their heads over the past few months, so it seems safe to say that the weather test was thorough.

Avid Centerline Rotors: Centerlines get the job done - we've been more than okay with them for years - but now that SRAM's new HS2s have hit the market, the older thinner variants just don't seem to bite the same way. After the first few rides, I ended up swapping the Centerlines for the equivalent HS2 size, and enjoyed better power and heat control. I wouldn't call this a critical upgrade, but it definitely takes the power of the Codes up a notch.

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Pinkbike's Take

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