Time To Critically Compare Lugged And Unicrown Forks

This post compares lugged and unicrown forks.


Lugged Fork

Lugged forks are built with the help of a steel sleeve (lug) and a metal-joining process known as brazing.

The tubes of the fork are inserted into the lug. Then, the frame-builder brazes the fork to the lug.

A fork lug

Brazing is a metal-joining process during which two metal pieces are “glued” to each other by melting a metal rod over the points of contact. In most cases, the filler (gluing) materials are brass or silver because they have a lower melting temperature than steel.

via: pixabay.com /brazing is done with a torch and a brass or silver rod/

The main advantage of brazing is that the base material (in this case the fork’s tubes) are never heated to melting temperatures. Consequently, there’s less chance of compromising the integrity of the used materials.

Unicrown Forks

Unicrown forks do not use lugs. Instead, both legs of the fork are TIG-welded to another tube.

The term “unicrown” is used because the crown isn’t an independent piece. It’s formed by the “unification” of the fork’s legs and middle element.

A Unicrown Fork

The Advantages of Lugged Forks

  • Attractive Old School Design

One of the main reasons why people like lugged forks is the appearance of the unit. Lugged forks are elegant and give a classy vintage look to the bike.

Hence why people into retro bikes are often disappointed to see a lugged frame coupled with a unicrown fork.

  • Easier Repairs

Brazed frames and forks are easier to repair because the builder can heat up the joint to melt the brass and then take out the affected tube and replace it.

The Disadvantages of Lugged Forks

  • Pre-determined Width

The casted crowns of lugged forks predetermine the distance between the fork’s blades. As a result, the fork’s clearance is fully dependent on the available lug. If the builder wants to make a fork for wider tires, a new lug will have to be purchased or fabricated.

  • Road Bike Only Steerer Tubes

The lugs are usually made for thin steerer tubes designed for steel road bikes.

Thus, it’s often not possible to build a lugged MTB fork with an oversized steerer tube by relying on stock lugs.

  • Slow Production

Lugged forks are slower to produce because:

  1. A lug has to be fabricated.
  2. Brazing is slower than welding
  • Limited Supply

The low demand for lugged forks and the slow production process naturally results in limited supply. Truth be told, most lugged forks circulating around come from old-school bikes made in the 60s, 70s, and 80s. Of course, a modern lugged fork can still be found, but there aren’t many to choose from.

  • Aluminum Cannot Be Used For Brazing

Aluminum forks cannot be brazed for the following reasons:

  1. The filler brazing rod has a higher melting point than aluminum. The long exposure to heat will irreparably damage the aluminum tubes.

2. There are filler rods designed to melt at lower temperatures so that aluminum can be brazed, but they don’t provide enough strength to build a strong frame.

Consequently, lugged forks are limited to steel only.

The Advantages of Unicrown Forks

  • Simplicity

A unicrown fork is made of three tubes welded together. This is one of the simplest fork designs possible.

  • Design Freedom

Unicrown forks allow the builder to produce a large variety of forks compatible with wheels of all widths without having to purchase a special lug.

The extra width can be achieved by using longer tubes and bending them at a larger angle.

  • Steerer Tubes Of All Sizes

Since there are no lugs pre-determining the thickness of the tubes, the builder can easily make forks using slim or oversize steerer tubes.

  • Lots of Options

The fast manufacturing process and the quick customization represent a strong incentive to produce unicrown forks.

Since most cyclists don’t have a preference in that regard, companies focus on unicrown forks. Consequently, the user can choose among many models.

The Disadvantages of Unicrown Forks

  • Cheap Looking

Unicrown forks are seen on some of the cheapest bikes out there (e.g., supermarket bikes) and thus many people consider them “deprived of uniqueness” in comparison to lugged forks.

FAQ: Are Lugged and Unicrown Forks Interchangeable?

In order for two forks to be interchangeable they have to satisfy the following requirements:

  • Matching steerer tube length
  • Matching steerer tube thickness
  • Approximately the same blade length

If the criteria aren’t met, the following problems will occur:

a. The user won’t be able to secure the fork to the headtube.

b. The fork won’t accept the wheel or will create massive changes to the geometry of the bike.

If the new fork is designed for smaller wheels, the original bike wheel won’t fit.

If the new fork is built for bigger wheels, it will accept the smaller wheel, but the combo will greatly alter the geometry of the bike.

Moreover, if the bike uses rim brakes, the brake pads won’t align with the new position of the rim, and bike will lose its front brake.

But even if all of the criteria above are met, there’s one more problem that the user might face when going from a unicrown to a lugged fork or vice versa.

If you look at the crown area of a unicrown fork, you will see that the distance between the crown race and the bottom of the steerer tube is greater than that of a lugged fork. This distance is known as stack height.

The extra stack height of unicrown forks lifts the front end. If a unicrown fork is replaced by a lugged one, the front end of the bike will drop by as much as 20mm.

This will result in the following changes to the bike’s geometry:

  • Steeper head tube angle.

The head tube angle (HTA) of a bike is the angle formed by the headtube and the ground.

The greater the fork’s stack height, the slacker the head tube angle becomes. Conversely, if the fork’s stack height is reduced, the angle gets steeper.

  • Lower Bottom Bracket

The shorter stack height of a lugged fork will lower the bottom bracket too. A lower bottom bracket increases the bike’s stability but greatly diminishes the bike’s clearance.

Different Fork Rake?

It’s realistic for a lugged fork and unicrown one to have a different offset (rake). This discrepancy will result in additional changes to the bike’s geometry.

The term fork rake refers to the hub’s offset from the steering axis.

The term trail refers to the tire patch behind the extended steering axis.

A shorter rake (offset) brings the axle closer to the steering axis and thus increases the bike’s trail.

Conversely, a long rake (offset) pushes the axle further away from the steering axis and decreases the bike’s trail.

The decreased trail will “sharpen” the bike’s cornering but also make it more unstable when moving in a straight line.

In conclusion

In most cases, it’s best to avoid replacing a lugged fork with a unicrown one and vice versa to prevent incompatibility issues and unpleasant changes to the bike’s geometry.

FAQ: Which one is stronger – lugged or unicrown?

The strength of a fork is not decided by the assembly method.

The parameters determining the strength of a fork are:

  • Design
  • Material
  • Tube Thickness
  • Production quality

Thus, it’s not possible to conclude which one is stronger.

Who Are Lugged Forks For?

The main advantage of lugged forks is their appearance. They shine the most when installed on a bicycle with a classic look. On a modern machine, they’d look out of place.

Who Are Unicrown Forks For?

The unicrown fork is universal. It can be seen on both cheap and high-end bikes. Ultimately, the unicrown fork is for people who are not interested in giving a vintage appearance to their bicycle.

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