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The YT Jeffsy Mk3 Core 4 is a capable all-mountain bike which, thanks to its spec, has a bias towards descending but is equally able to climb. While the spec is hard to argue with and the value for money is excellent, it's worth paying attention to the frame sizing and geometry to get the ride you want. The bike could be improved further with a longer dropper post and a slacker head angle but YT has done a good job continuing to polish the already excellent Jeffsy. 

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YT Industries Jeffsy Mk3 Core 4  - Technical details 

The long-standing Jeffsy has received an update and is now in its third iteration. Serving duties as the all-mountain offering, the Jeffsy sits between the enduro-orientated Capra and the short-travel Izzo. With 150mm travel up front and 145mm out back, the Jeffsy aims to be a capable trail bike that balances efficient climbing and fast descending. 

YT's direct-to-customer model has vastly improved over the years and, with the presence of the YT Mill in the UK, there is no longer the fear of having to deal with an overseas headquarters if there is an issue. YT has a great set-up experience with the bike being delivered mostly built and providing all the tools you need to finish the assembly.  

YT’s spec model structure starts with the Core 1 base model and climbs the top-of-the-range Core 5 spec. For the Jeffsy, the aluminium framed Core 1 model retails at £2,500 and a slightly higher specced alloy framed Core 2 comes in at £3,000. Core 3, 4 and 5 models feature a carbon frame with gradually increasing specs and prices, which are £4,000, £5,000 and £6,000 respectively. 

The Mk3 Jeffsy has received some new features in addition to the already well-sorted offering. Following the recent trends of in-frame storage, YT has joined the group chat and now offers a neat solution with the carbon-framed Jeffsy. It features a lever for access and a bag for organised storage. More storage is on offer with some mounts underneath the top tube for a spare tube or pump - YT has already got this covered with the Crankbrothers SOS storage that comes as standard.

YT_Jeffsy_mk3_C4_11.jpg, by Ty Rutherford

The new Jeffsy retains the V4L (Virtual Four Link) suspension layout but it has been tweaked with a 105 per cent anti-squat at 30 per cent sag that’s said to drop away quickly as it dives past the sag point and that should minimise pedal kickback. This update intends to increase acceleration and traction. A little more progression has been dialled in towards the end-stroke.

A flip-chip in the lower shock mount adjusts geometry slightly from the stock Low position. The High position will steepen the head tube and seat tube angles by 0.6 degrees and raise the bottom bracket by 8mm. Speaking of the bottom bracket, YT has used a press-fit variant here.

YT_Jeffsy_mk3_C4_6.jpg, by Ty Rutherford

Internal cable routing through the front and rear triangle is used, giving a clean look and the frame is equipped with SRAM's Universal Derailleur Hanger (or UDH) and is compatible with the brand's latest T-Type drivetrains. 

Frame protection has been well considered as usual with a chunky down-tube protector to fend off rock strikes. The chain stay gets a similar treatment keeping the carbon safe and the bike quiet.

YT_Jeffsy_mk3_C4_9.jpg, by Ty Rutherford

Rounding off the features, YT includes a side-loading bottle cage with the bike and there’s room for the brand's very own Thirstmaster 6000, which is a 620ml bottle. 

YT offers the Jeffsy in five sizes ranging from S through to XXL, using the size guide this caters for riders from 153cm (5ft) to 203cm (6ft 8). At 6ft (183cm) I fit right in between the Large and XL sizes where I could ride either. With the trail bike intentions of the Jeffsy, I decided to go with the size large opting for a fun and nimble bike which is arguably a little small over the XL which would have been more stable but potentially a little too big for me on the tight technical trails. 

The reach measurement is my go-to number when figuring out how a bike will fit me, with the Jeffsy measuring 435mm for size S and increasing in 20mm increments up to 515mm for the XL. Although the reach has increased by 5mm for the new model, the bikes are still a little conservative compared to other brands – this shows YT’s intention for the Jeffsy to be a playful trail bike. 

The new model receives a head angle that's a degree slacker, now sitting at 65-degrees in the low position which should help the bike feel a little more stable at speed than the previous generation. The new model has also grown by 2mm in the chain stay with S-L running 437mm and the XL-XXL growing to 442mm – that’ll add a little more stability although I’m not sure we mere mortals would notice the 2mm…

The seat tube angle is still on the money hovering around 77-78-degrees effective in low depending on the frame size. This will give a good balance between a comfortable position without too much weight on your hands but forward enough to be efficient on the pedals and stable while climbing.

YT Industries Jeffsy Mk3 Core 4 - Componentry

The Core 4 pictured here represents good spec for the money, YT has done a great job of providing a bike that is ready to charge out of the box. Drivetrain duties are handled with SRAM GX T-Type set-up, a well-proven option with plenty of range to climb and descend. 

YT_Jeffsy_mk3_C4_3.jpg, by Ty Rutherford

SRAM also takes care of stopping duties with the powerful Code RSC brakes, sporting 200mm discs front and rear. YT has kitted its own branded dropper, the “Postman” offering size-specific lengths – the Large pictured here gets 170mm of drop. Renthal bars and stem are a high-quality choice, a nice touch on an OEM spec. 

YT_Jeffsy_mk3_C4_5.jpg, by Ty Rutherford

RockShox handles the suspension with the ever-popular 150mm Lyrik fork and Super Deluxe rear shock in the top-tier Ultimate spec offering plenty of tuneability. The rear shock also features a climb switch to help with climbing efficiency.  

The Crankbrothers Synthesis alloy wheels are a welcomed choice offering an excellent balance of weight and strength. Along with the Maxxis Minion DHF (Exo+) and DHR (DoubleDown) tyres, this is a reliable and confidence-inspiring wheelset for trails. 

YT_Jeffsy_mk3_C4_2.jpg, by Ty Rutherford

A nice little bonus is the Crankbrothers SOS tube storage which includes a tyre lever and tyre plug tool. 

YT Industries Jeffsy Mk3 Core 4 - Setup

YT has put a lot of thought into the setup process to get you up and rolling as quickly as possible. The bike comes mostly assembled out of the box and includes their “Me first” tool kit to get you going. The tool kit includes an 8mm Allen key for the pedals, shock pump, torque driver and handheld pump – everything for your set-up pleasure. 

Jeffsy_Tool_Kit, by Ty Rutherford

An online guide is available on YT’s website for suspension setup. After entering your weight and bike details, you’ll get step-by-step instructions on pressures and clicks for the fork and rear shock. This is super handy to get you close to a dialled set-up but of course, these are only a guide to get you started on a fresh bike.  

For my setup, I raised the stem and fitted some fatter grips based on personal preference. After the initial ride, I found the setup to be a little soft so I added some pressure to the fork and shock and adjusted the rebound accordingly. I’m usually a fan of RockShox suspension for its ease of set-up but it took me a couple of rides to feel at home with the new Lyrik and Super Deluxe. Nevertheless, after a handful of rides, I got the set-up dialled in, feeling poppy whilst retaining traction.

YT Industries Jeffsy Mk3 Core 4 - Performance 

Let's start with the climbing. A trail bike needs to be handy on the ascents and more than just winching up fire roads. There needs to be an element of agility and efficiency that you don’t get from a harder-hitting enduro bike. I’m happy to report that the Jeffsy strikes the balance as a capable climber mostly thanks to some well-sorted geometry, the seat angle and reach numbers make the Jeffsy a comfortable place to sit.

The short chainstay does hamper efforts when the going gets steep, pitching the rider’s weight a little too far towards the rear axle causing the front end to lift. This can be remedied by utilising the lock-out on the rear shock reducing the sag and sitting higher in the travel. Around the sag point, the suspension is quite active which equates to excellent traction for technical climbing however it does sap some efficiency. I found myself using the lock-out more often than not on the smooth fire road climbs. 

YT_Jeffsy_mk3_C4_17.jpg, by Ty Rutherford

With chunky tyres and the powerful Code brakes, it's clear the Jeffsy is aimed more for descending. As with any new bike, it took a couple of rides to get the Jeffsy dialled into my preferences but once there I found it to be a capable trail bike. The four-bar layout is commonplace amongst many brands however YT has managed to do a great job pairing the rear shock with it. There was a very noticeable range of adjustment with the Super Deluxe which completely changes the characteristics of the bike – running it harder with less rebound damping results in a more playful platform and softer with more rebound damping gives a more compliant trail bike. 

YT_Jeffsy_mk3_C4_13.jpg, by Ty Rutherford

With the reach being on the smaller side, this bike lands on the playful end of the spectrum which suits its trail bike intentions. The Jeffsy is easy to move around on the trail and encourages finding little trail gaps, it's a real hoot which rewards a dynamic riding style. Cornering is a real strong point for this bike, the short chainstays, low standover and shorter reach mean this bike can turn even with its full 29er set-up.

YT_Jeffsy_mk3_C4_14.jpg, by Ty Rutherford

When the trail opens up and things get faster I find the bike starts to lack stability, the geometry that makes this bike corner well also makes it feel skittish at speed. The 65-degree head angle has a strong part to play in this, at slow speeds it provides reactive steering however at speed I’d like something a little slacker. On the steeps this is also prevalent, matched with the low bar height I didn’t feel as comfortable as I would have liked. This lack of confidence was compounded by a seat that didn’t have enough drop. At six feet, I’m probably on the taller end of people riding a size Large however I found 170mm of seatpost drop not enough and had a lot of room for more. 

YT_Jeffsy_mk3_C4_12.jpg, by Ty Rutherford

The Rockshox suspension, in Ultimate flavour, is hard to argue with providing some confidence-inspiring traction and support and I never felt I was asking too much from them. The Code RSC brakes are downhill-level stoppers with more than enough power for a trail bike, they dare you to brake a little later than normal and charge that bit harder. The Crankbrothers wheels are worth a mention too, they’re not overly stiff giving plenty of traction and trail dampening, a really smart choice you don’t often see on OEM specs.

YT_Jeffsy_mk3_C4_15.jpg, by Ty Rutherford

YT Industries Jeffsy Mk3 Core 4 - Verdict

Retailing at £5,000, the Core 4 represents solid spec for the money with top-end suspension, brakes and GX T-Type. Canyon is another direct-to-market brand that directly competes with YT, its Spectral CF 9 comes with the same Rockshox suspension and SRAM brakes, T-Type transmission and in-frame storage. From a component standpoint, I’d argue the YT is marginally better thanks to the wheelset but it's more of a preference. The Canyon’s geometry is slacker and longer than the YT and this is where the real difference is, the Canyon will have a more stable nature but less nimble on the trail. Retailing at £4,800 there is very little between the Spectral and the Jeffsy.

Another option from a direct-to-consumer brand is the Orbea Occam SL, this is a 140mm offering but is aimed less at rowdy descending and more at speedy climbing. For £4.900 you won’t get wireless shifting, you’ll get less adjustable suspension and geometry that is less focussed towards descending prowess. 

YT Industries continues to polish the all-mountain capabilities of the Jeffsy and the MK3 proves to be a solid offering. The spec build across the range has been very cleverly chosen and at least on the Core 4, it didn’t leave me wanting to change anything. The set-up is smooth thanks to YT’s provided toolkit and their online suspension guide gets you rolling on some baseline settings with minimal faff.

As an all-mountain bike, the Jeffsy remains a capable climber thanks to a neutral seated position although the shorter chain stays do hamper this a little. With top-tier suspension and powerful brakes, the Jeffsy comes alive on the descents with the geometry being on the playful end of the spectrum rather than the stable end. Of course, there are some gripes mainly around the seatpost length and head angle which have been improved for the MK3 – it's just not enough in my opinion. Overall, the Jeffsy proves to be one of the best value-for-money offerings on the market. 

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