Tested : Pete’s YT Jeffsy Core 4 Review.

Having made all the changes Pete would have made to the outgoing model, how does the YT Jeffsy Core 4 stack up in the real world?

Pete was a big fan of the previous model of the YT Jeffsy, and the latest edition addresses all of that bike’s shortcomings. First impressions were good, very good. So how has the bike performed?

Photos by Pete Scullion.

Key features:

  • Rockshox Lyrik Ultimate 150mm fork
  • Rockshox Super Deluxe shock
  • SRAM GX AXS T-type 12-speed drive
  • SRAM Code RSC brakes
  • Crankbrothers Synthesis Enduro alloy wheels
  • YT Postman dropper
  • £4,999.00 RRP
  • YT-Industries.com


The Jeffsy is available in S, M, L, XL and XXL sizes.

Reach on the M, seen here, is 455mm with a seat tube of 410mm. Head angle is 65 degrees with a seat tube angle of 77.5 degrees with the flip chip in Low. Chainstays are 437mm on the smaller of the three sizes with the wheelbase on the M of 1214mm.

The moment I swung my leg over the Jeffsy this time around, it felt familiar. Even without getting the suspensions setup nailed on in the car park like I did with the Fox-equipped Core 3 I tested last year, I was excited to see if the bike retained the fizz whilst improving where I felt it needed it.

With the seat tube clipped, head angle and kinematic tweaked, plus the addition of SRAM Code brakes in favour of the G2s of the previous test bike, plus the addition of EXO+ casing tyres and my wish list was complete.

After taking the time to full appreciate the bright red paint job, the setup was the usual spring and damper tweaking, plus lever throw and tyre pressure. Off we went. With fairly fast tyres and both in the MaxxTerra flavour, the Jeffsy was whizzing up the climbs. A little bit less active in climbing helped this feeling too.

Fox Boa Leaderboard 2024

Despite the Lyrik Ultimate not quite feeling as supple off the top as the 36s that were so good on the last Jeffsy, they held up very well in the mid stroke and deep in the travel. I think the lower speed of the wet winter might be more of an issue than the damping as whenever the speed increases, the forks feel mint. I have been a big fan of the Super Deluxe air unit and it’s no different here. Combined with the linkage it offers a well-rounded feeling throughout.

As soon as you point the bike down the hill, the added control offered by the Code brakes make the Jeffsy an even better descender. The G2s were overwhelmed far too easily, even with HSC rotors fitted. With the bigger SRAM anchors, you can attack a trail harder, knowing you’ll be able to keep the bike in check.

Whilst the faster rubber does lend itself well to getting back to the top, grip, howwver especially when you’ve some dirt to cut through the find it, is the trade off. On steeper trails the back wheel does lock up a fair bit and the Minion on the front does clog too.

Something like the Jeffsy, say a 145mm 29er, does feel like the sweet spot, especially when they’re this good. Nimble enough for a bit of classic Scottish tree slalom, big enough to take on a hill day and efficient enough to help you do both on the same day.

I think the Jeffsy has joined the realms of the bikes I would spend my own money on. The most recent raft of upgrades make it a cracking bike on pretty much any terrain and it’ll put in a good showing almost anywhere.

What do we think?

YT have taken a very good Jeffsy recipe and updated it to make an even tastier proposition. The fast tyres hit the design spec of covering the ground quicker but if you value high grip, they’ll be the first thing you change. Value for money is high too and you’ll be buying yourself veritable rocketship from the get-go.

We love:

  • Confidence-inspiring ride
  • Great value
  • That colour

Could do better:

  • Fast tyres are not grippy tyres

You can check out the YT Industries Jeffsy Core 4 over on their website here.