How Berk built this astonishing 3.9kg road bike

Slovenian carbon specialists build one of the world's lightest road bikes (and it can be ridden every day)

3.9kg Berk road bike in studio

Berk has built one of the world’s lightest road bikes that, unlike many featherweight bikes, gives up little in terms of usability. The Slovenian brand has exclusively shared details with BikeRadar on how it created this mind-blowing 3.9kg road bike.


The bike was intended as an in-house project to test the limits of what Berk can produce. The result is a sub-4kg bike that can be confidently ridden daily. 

That last point is the key one. According to Berk, it would be relatively easy to produce a considerably lighter bike but the brand’s mission is to build truly “lightweight yet reliable parts”. 

Berk also wanted to illustrate that, while the rest of the industry has been focussed on advancements in aero and disc-brake technology, there are still significant advancements to be made, and public interest, in lightweight technology. 

We can’t help but agree. Aero may be fast but there’s little that stirs our weight-weenie loins quite as much as the sight of a true featherweight build. Anyone who’s tuned in to BikeRadar’s Hill Climb Diaries will know as much.

How Berk built a 631g frame

Berk is best known for its lightweight saddles and integrated saddle/seatpost combinations. However, the brand also has extensive experience producing custom framesets. 

This particular frameset began life three years ago when it was built for Berk employee Aleš Arnež.

The frame features an integrated one-piece seat mast and saddle combo. This is absolutely fixed at the time of manufacture, so you’ll definitely want to know your saddle height before committing to one of these. 

The total weight for the frame is an astonishing 631g. 

Rear view of 3.9kg Berk road bike including integrated seat mast and saddle combo
The frame, seat mast and saddle form one whole unit.

According to Berk, the frame is constructed with Toray’s highest modulus MJ-series carbon fibre in combination with the Toray T1000 carbon more commonly seen on top-end bikes.

Berk also used Innegra in some areas – a specialist lightweight composite material that is lighter than Kevlar and is commonly used to provide additional durability in carbon frames. 

As if 631g wasn’t light enough, Berk claims it would be possible to go lighter still, producing a sub-500g frame that would be “safe to be used daily and completely reliable”. 

To produce this, the frame would need to be constructed entirely from the aforementioned MJ-series carbon.

Custom one-piece cockpit on 3.9kg on Berk bike
The cockpit is custom-made by Berk.

However, Berk does not wish to go down this route because it would be too expensive and believes the market for such a frame would be extremely limited. 

The fork is a THM Scapula, which is already one of the lightest aftermarket forks on the market at 275g. Berk has sanded the fork back to the absolute limit, bringing its weight down to just 228.8g.

The rider limit for the frame as pictured is 85kg.

Crazy customised components for a crazy-light build

Making an ultra-light frameset is only part of the process when building a sub-4kg bike. 

Arnež is also an expert component ‘tuner’ (which is a polite way of saying he really knows his way around a Dremel) and he has put his skills to good use with this build. 

To start, the bike is built around a highly-modified SRAM Red 11-speed groupset

The rear derailleur has been slimmed down, with the cage replaced with a custom skeletal carbon option. Even the jockey wheels have been replaced with ultra-light carbon options. The shifters have received a similar treatment.

Moving forward, the bike features a modified Tiso Altore front derailleur. This shuttered Italian brand’s ultralight shifting components have attained a near cult-like following in the weight weenie scene. 

This particular example has been modified with carbon cages to bring its weight down to a paltry 49g. The derailleur mount comes in at just 7.4g.

Tiso Altore derailleur on 3.9kg Berk bike
The Tiso Altore derailleur has been customised to bring it down to a truly feathery weight.

(For those that are interested in niche derailleur history, Disraeli Gears is, as always, the go-to resource for all things shifty.)

The cassette comes courtesy of SeqLite. Very little information is available about the brand online and it doesn’t seem to be in business anymore. However, we can tell you that this 11-speed cassette weighs a feathery 111.7g. 

For comparison, a Rotor 11-28 UNO cassette – which is the lightest cassette we’ve ever had in the BikeRadar office – weighs 135g.

24g may seem trivial to most of us, but these sort of marginal savings are catnip among the weight-weenie elite. 

THM Clavicula cranks with carbon chainrings mounted to 3.9kg Berk road bike
An older set of THM Clavicula cranks are used on this build.

Powering all of this is a set of older THM Clavicula cranks. A set of unidentified carbon chainrings, which weigh just 76.9g, adorn these cranks. This is not a drivetrain you will want to run dirty.

A PYC chain (213.2g) rounds out the drivetrain. 

The custom one-piece cockpit comes courtesy of Berk itself. This weighs in at 203.6g. Like the one-piece saddle/seatpost combos, these are available to purchase from Berk on special request. 

For comparison, the Canyon CP20 one-piece cockpit seen on its very lightest builds weighs 270g. 

Ciamillo Gravitas brakes on 3.9kg Berk road bike
Ciamillo’s Gravitas brakes are some of the lightest brakes ever made.

Braking is taken care of by a set of Ciamillo Gravitas brakes. These are among the very lightest brakes in the world, coming in at 156.8g for the pair.

A set of unidentified Reynolds rims are laced to Tune hubs to make an 898.5g wheelset, according to the Berk scales.

It’s hard to believe, but there are actually lighter wheels out there. The Extralite CarboClimb A25T is the lightest option we’re aware of at 780g for the pair. 

However, we appreciate that Berk has specced something a little (only a little, though) more sensible in the name of day-to-day usability.

Speedplay pedals on 3.9kg Berk road bike
The bike gives up little in terms of practicality, and we like that.

Glued to these are a set of Tufo tyres. The exact model and size is not disclosed but, at a claimed 317.5g for the pair, they’re most likely the Tufo Elite Jet.  

As a fun aside, according to the stats provided by Berk, the air in these tyres weighs 14g. Yes, you read that right – Berk has weighed the air. Remarkable stuff. 

All shifting and braking is controlled via Powercordz. These funky Zylon cables are claimed to be twice as strong as Kevlar and, more critically here, they are also said to be 75 per cent lighter than steel. 

Overall, what strikes us about this build is that, while the spec definitely leans hard into the weight weenie spectrum, it gives up little in terms of practicality.

It still uses standard shifters (although they have, of course, received drillium treatment), it used unmodified Speedplay pedals, other components haven’t been modified within an inch of their life and it still has a double crankset. 

Below is a full list of the components and their weights as supplied by Berk. Where possible, we have identified each component.

If you have any clues as to what the unidentified components are, do let us know in the comments.

3.9kg Berk road bike full specs with weights

  • Custom Berk frame (including seat mast and saddle): 631.1g
  • Custom THM Scapula fork: 228.8g
  • Expander plug: 3.9g
  • Top cap screw: 4.8g
  • Headset: 39.5g
  • Custom 3D-printed spacer: 2.1g
  • Modified SRAM Red 11-speed shifters: 255.8g
  • SeqLite cassette: 111.7g
  • Cables: 46g
  • Powercordz internal cables: 8.5g
  • THM Clavicula crankset: 331.7g
  • Unidentified carbon chainrings: 76.9g
  • Bottom bracket: 71.4g
  • Modified SRAM Red 11-speed rear derailleur: 121.9g
  • Modified Tiso Altore front derailleur: 49g
  • Front derailleur mount: 7.4g
  • Rear derailleur hanger: 15.6g
  • PYC SP1101 chain: 213.2g
  • Speedplay Ultra Light Action Titanium pedals: 133.8g
  • Ciamillo Gravitas brakes: 156.8g
  • Custom one-piece Berk cockpit: 203.6g
  • Handlebar tape: 5.2g
  • Tune/Reynolds wheelset: 898.5g
  • Tufo Elite Jet tyres: 317.5g
  • Quick-release: 26.6g
  • Air: 14g
  • Screws: 8g
  • Fabric cageless bottle stubs: 3g
  • Total weight: 3,986.3g

Many thanks to Berk for sharing these photos and taking the time to talk us through how it created this amazing bike. 


What would you do to make the bike even lighter? Would you go 1x? Or does the idea of riding such a light bike fill you with horror? Can you help us identify some of the unknown components? As always, leave your thoughts in the comments.