YT Jeffsy Core 4 review

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The YT Jeffsy Core 4 is a bike that arrives in a box that is not quite perfect out of the box. BUT… there is loads of potential here.

  • Brand: YT Industries
  • Product: Jeffsy Core 4
  • From: YT Industries
  • Price: £4,999
  • Tested by: Benji for 3 months


  • Great feeling frame and wheels
  • Geometry generally decent
  • Suspension settings make a massive difference


  • Head angle is a bit steep
  • Dropper insertion not as deep as rivals
  • Suspension settings make a massive difference

The Jeffsy is YT’s bread and butter 150mm ish travel trail bike. The Izzo has less travel and sits in what is now pretty much known as the downcountry category. The Capra has more travel than the Jeffsy (170mm) and lies within the enduro class of mountain bike.

Having said that, I think they must be a few people out there who are torn between the Capra and the Jeffsy; the Capra has more travel and a slightly slacker head angle but the Jeffsy has longer reach and better standover. Anyways, we’re not comparing two YTs against each other here. We’re reviewing the YT Jeffsy.

As a trail bike, I’d argue that the Jeffsy has to be a bit more all-round capable than an enduro bike. Specifically as regards uphills and traverses. Enduro bikes seemingly are based around smooth access/fireroad winching interspersed with steep gradient technical descents. Trail bikes need to be able to be adept at more types of ascending. Fireroad, loose shale, roots, rocks, mud, tarmac, concrete, grass… anything really.

And trail bikes should be able to eek fun out of shallow gradient terrain. The sort of trails that meander along, curving this way and that. Also, they should be able to mould to the riders mood or level of fitness/freshness; trail bikes should be able to be all bouncy puppy dog – pinging and popping off features – when the rider is feeling fine, trail bikes should also be forgiving of error and comfortable when the rider is feeling rather less-than-rad.

The Core 4 Jeffsy here has a full carbon frame. There is an alloy Jeffsy available. And also a fancier, lighter, more expensiver high modulus carbon Jeffsy. On this regular carbon Jeffsy Core 4 there’s storage in the down tube, acessed wvia a pleasingly snug and snappy latched hatch. And yep, there’s room for a water bottle. So long as it’s not a big ol’ 750ml jobber.

This is the third incarnation of the Jeffsy and as such there have been some tweakings since V2.0 in terms of geometry and suspension kinematic. Oh, and it’s not now UDH, so is SRAM Transmission compatible (as you can see).

There is a flipchip on the rear shock mount that changes the geometry a tad (0.6° angles, +/-8mm BB) but there’s no way this bike will ever get put into the High/Steep mode. Why? The seat angle and reach figures are fine in Low/Slack mode. It’s the head angle that is key here. There’s not going to be anyone who rides the Jeffsy in Low/Slack mode (which is how it ships) who will think “Hmm… I wish this head angle was a bit steeper”. More about head angle later, when it comes to how the bike rides.

Head angle aside, I think the YT gang has done a pretty decent job on geometry. But I don’t think the bike is entirely ‘there’ yet. As with almost all mountain bikes still, the chain stays are on the short side (especially for L-XXL riders). And the head tubes could be a bit taller (again, for L-XXL sizes). But, credit where credit’s due, the reach numbers are good, the BB height isn’t too low [sic], the seat angle is decently steep and the seat tube length/standover is acceptable.

Dropper insertion isn’t quite as far as I’d like (need) but it was a relatively simple task to get hold of travel reduction kit (essentially a few spacers) and reduce the travel by about 10mm to get my perch where I wanted. The YT Postman is basically a rebadged SDG Tellis post by the way.

In terms of rear suspension, although the Jeffsy is ostensibly just A.N.Other 4-bar, it rides very differently compared to other 4-bar bikes that we’ve been testing amidst the Jeffsy’s test period. Despite all the usual talk of anti-squat and progression, I’m very much of the opinion that rear shock tune (and set-up) is vastly more significant to how bikes behave on the trail.

In terms of shock tune and adjustment, I’m here to give the YT and RockShox colab a hearty pat on the back. The Jeffsy is one of the most genuinely adjustable full suspension bikes I’ve encountered in recent times. It’s not that the dials don’t do anything on other bikes, it’s just that usually I’m running all of the taps fully open.

Indeed, on the first couple of rides on the Jeffsy, out of sheer habit, I did run the rear shock damping circuits fully open. And the bike was a soft mess. Which could be useful I suppose when faced with a route consisting entirely of ripple roots. But for regular riding, a free-diving back just robs you of control and propulsion. It can also makes the back end ‘sink’ – and the front wheel lift or wander about – when climbing up steeper stuff.

With two or three clicks (from fully open) of low speed compression dialled in the Jeffsy felt like a different bike. More feel, more dig-in traction, improved handling accuracy… more fun. There may only be five low speed compression settings on the RockShox Super Deluxe rear shock but they make a difference. To stop things feeling overly harsh or dead, it was fine to run the rebound on the faster side of things.

Ultimately, if you have a bit of understanding about yer shock’s blue and red twiddlers, you can get the Jeffsy to be a high-speed rock-solid punchy ‘big BMX’, or a slow motion super supple root hoover, or something in between.

On technical traverses and all-out descents I pretty much knew how I liked the bike to feel (middling LSC and fast rebound) so dialled that in and left it be. It was when climbing that I did the most on-the-fly damping twiddlings. Basically, when I was occasionally faced with a bit of scrabbly, loose and generally unpredictable uphill tekkers, I’d reach down and open up the LSC (it’s a big, clicky, glove-friendly dial) and appreciate the reduction in wheelspin traction spit and enjoy the grip. It was as supple as a coil shock (albeit still missing the midstroke support that coil has).

The impressive climbing chops of the Jeffsy is also clearly enabled by the steep seat angle and healthy reach of the frame geometry. I say ‘healthy reach’ but I probably should also point out that I’d recommend erring on the side of larger frame sizes if you’re looking at YT’s recommended rider height chart. I’d say YT are around 10mm shy on reach compared to other more ‘progressive geometry’ brands.

All in all, the Jeffsy does a fine job of being an all-day trail bike. The down tube storage I ended up using like I do on other bikes that offer it. Namely, using it as a place to stow a layer of clothing and some sort of modest non-smashable snack. It’s no substitute for a backpack but down tube storage can definitely mean the death of your bumbag.

Once I’d got to grips with the rear shock setting, as mentioned, the Jeffsy was excellently adept at a variety of climbing. In terms of shallower terrain, whether that be open rolling hills or manicured trail centre woodland, the Jeffsy felt entirely at home and unfazed.

I feel a quick word or two about the frame and wheelset is in order. The Jeffsy is one of the nicer (calmer? forgiving?) feeling carbon framees I’ve ridden of late. And the Crankbrothers wheelset no doubt comes into play also. Would I go as far as to say the frame and wheels felt flexy? Do you know, I might. I could couch the terms in less controversial language (“give”? “compliance”?) but then again, I’m not going to. I’d quite like to reclaim the pejorative term ‘flex’. Flex can be a good thing. In fact, it usually is.

On steeper descents, I must confess to finding the Jeffsy a bit frustrating. And yes, I’m going to be boring about head angles again. The suspension can handle whatever you throw at it once it’s dialled in (not too much sag and plenty of LSC) the progression of the rear suspension makes it feel like you have more than 145mm travel. The frame and wheel flex can handle whatever you throw at it. The rear-wheel-to-cockpit dimension can handle whatever you throw at it. What can’t handle steeper and faster stuff? The 65° head angle.

There were numerous times when the front end nearly had me exiting via the gift shop. Which, contrary to what some folk may call ‘engaging’ or ‘responsive’ handling, is just not-very-fun. It’s actually very frustrating. 65° on a 150mm travel front end. We’re getting similar head angles on World Cup XC bikes these days.

Is the head angle the killer blow for the Jeffsy? No. But you will need to buy an angle adjust headset. With a head angle of 64° or shallower, the Jeffsy would absolutely rip downhill. And I really don’t think it wouldn’t impair the bike’s manners elsewhere (the 50mm stem could always be swapped for a shorter one if you felt like adding some ‘response’ back).

And while you’re at it, I’d strongly recommend getting some different handlebars. The Renthal bars are plenty comfy for a 35mm diameter bar but they lack rearward sweep (further adding to front end nerves) as well as lacking sufficient rise (for L-XXL sizes at least). I shall stop talking about swapping out parts now, because it gets people’s backs up. Needless to say, there is even better bike here, with some relatively modest extra spend.

As this is spec-is-King mail order mountain biker, I’ll quickly delve into the finishing kit.

The RockShox Lyrik Ultimate is a decent fork. I would say that it dynamically runs a bit deep into its travel (adding air just makes it harsh and prevents full access of travel) this deep-running would be fine if the front end wasn’t already a tad nervy. Again, this wouldn’t be an issue once an angle adjust headset is installed (sorry!)

The DHF and DHR tyre combo is perfectly fine. I still really rate the DHF as a front tyre, although it would be nice to have a MaxxGrip version instead of the skittish MaxxTerra flavour specced here. The DHR II is the AK47 of rear tyres.

The GX AXS Transmission worked flawlessly. I’m totally onboard with why it is ‘slow’; read my SRAM GX Eagle Transmission review for more deets. Kudos to YT for speccing the ‘old’ rocker style shifter. It’s just better than the new Pod shifters in many ways (not least the fact you can use your index finger to shift if you want to). The SRAM Code RSC brakes worked great too.

The SDG Bel Air 3.0 saddle was fine. The YT Postman didn’t go into the frame far enough for me when set to its full 200mm of drop so needed the aforementioned shim kit tweak to get things sorted. Once shimmed, the post worked just fine.

The cockpit wasn’t my cup of tea. They so rarely are. The 50mm stem just feels oddly long to me these days. The bars were just an odd combo of sweeps. And the ODI grips are just a bit too minimal for my paws.

As touched upon earlier, the Crankbrothers Synthesis enduro Alloy wheelset is the secret star of this Core 4 in my opinion. Great hoops.


The YT Jeffsy Core 4 is a bike that arrives in a box that is not quite perfect out of the box. BUT… there is loads of potential with this beguiling bicycle. And some folk may well be fine with the relatively steepish head angle. In my opinion, trail bikes shouldn’t just be limited to climbing and contouring. Trail bikes should be able to get down the same sort of stuff as enduro bikes, just with a bit less velocity and a bit more care. Putting that niggle to one side, the Jeffsy has a great feeling chassis, genuinely versatile suspension and a build kit that impresses for the price. If you’re prepared to put the work in (and the upgrade spend) once the bike arrives, there’s not many bikes as enjoyable as the Jeffsy.

YT Jeffsy Core 4 specification

  • Frame // YT Carbon, 145mm
  • Shock // RockShox Super Deluxe Ultimate RC2T, 210x55mm
  • Fork // RockShox Lyrik Ultimate Charger 3, 150mm
  • Wheels // Crankbrothers Synthesis Enduro Alloy
  • Front Tyre // Maxxis Minion DHF 29×2.5in 3C MaxxTerra EXO+
  • Rear Tyre // Maxxis Minion DHR II 29×2.4in 3C MaxxTerra EXO+
  • Chainset // SRAM GX Eagle, 170mm, 32T
  • Brakes // SRAM Code RSC, 200/200mm
  • Drivetrain // SRAM GX Eagle AXS Transmission, Rocker Paddle shifter, 10-52T
  • Stem // Renthal Apex 35, 50mm
  • Handlebars // Renthal AL2014, 780x30mm, 35mm, 7° x 5°
  • Grips // ODI Elite Motion V2.1 Lock-on
  • Seat Post // YT Postman V2, 31.6mm, 200mm
  • Saddle // SDG Bel Air 3.0
  • Weight // 15.8kg

Geometry of our size XL

  • Head angle // 65°
  • Effective seat angle // 78.3°
  • Seat tube length // 460mm
  • Head tube length // 124mm
  • Chainstay // 442mm
  • Wheelbase // 1,266mm
  • Effective top tube // 627mm
  • BB height // 33mm BB drop
  • Reach // 495mm

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Review Info

Brand: YT Industries
Product: Jeffsy Core 4
From: YT Industries
Price: £4,999
Tested: by Benji for 3 months

Orange Switch 6er. Stif Squatcher. Schwalbe Magic Mary Purple Addix front. Maxxis DHR II 3C MaxxTerra rear. Coil fan. Ebikes are not evil. I have been a writer for nigh on 20 years, a photographer for 25 years and a mountain biker for 30 years. I have written countless magazine and website features and route guides for the UK mountain bike press, most notably for the esteemed and highly regarded Singletrackworld. Although I am a Lancastrian, I freely admit that West Yorkshire is my favourite place to ride. Rarely a week goes by without me riding and exploring the South Pennines.

More posts from Ben

Viewing 3 posts - 1 through 3 (of 3 total)
  • YT Jeffsy Core 4 review
  • arkike
    Free Member

    I can’t believe you said that this bike’s kit impressed for the price of £5000! It sounds like a sales person.
    I like YT frame design but it seems that they are unable to make a CF light bike.
    I was in the market last year and when you compare with others direct sales brands you find out that there are even aluminium enduro bikes lighter than this bike.
    Most people who prefer to pay the extra cash for a carbon bike would expect a light bike. So, this carbon trail bike 16kg weight for £5000just seems unsaleable

    Full Member

    I’m more surprised at the 65 degree HA being singled out as being too steep to ride it downhill with any joy 🤔

    Surely there’s a combo of HA/reach/stack going on there that’s an issue for the reviewer?

    Full Member

    Oh, and it’s not UDH, so is SRAM Transmission compatible

    I’d hoped this might have been double checked after publishing on a Sunday.

    The weight is also surprising. My Bird comes in less than that and a mates Transition Sentinel is ~1.5kg lighter in a lower spec.

Viewing 3 posts - 1 through 3 (of 3 total)

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