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The History of U-Locks – Classics to New Innovations

U-locks (otherwise known as D-locks) are one of the oldest types of bike-specific lock and probably the most popular style of bike lock on the market. 

In this article, we’ll take a deep dive into the history of U-locks as well as looking at how U-locks are likely to evolve moving forward.

History of U-Locks - Early U Lock Designs

In 1972, Stan Kaplan invented what’s thought to be the earliest U-lock and had the original design patented in the same year. 

Sheldon Brown, the famous American bike mechanic, published a short interview in 1988 with Stan Kaplan, as they had previously worked together in a bike workshop.

Stan described how a customer had reported that their bike had been stolen in 1971 and gave him the idea of using a chunky aluminium U-shaped shackle for securing bikes. 

At the time, the only popular bike locks were cheap, flimsy chain and cable locks, and heavy-duty chains, and padlocks.

Realising there was a gap in the market for innovation, Stan then set to work designing the most secure lock possible, opting for a hardened steel shackle and protective padlock housing.

When out for a walk, he came across several vending machines with bulky housing fitted around the padlock, which rounded the first design off nicely, and the product was ready for patenting.

Soon after the launch of Stan’s lock, MIT began researching and developing their own bike lock technology. 

Ray Seakan, who ran the initial MIT bike lock project, later left and founded Citadel, quickly becoming Kryptonite’s main competitor.  

Over the next few years, Kryptonite and Citadel fought for market share, making tweaks and changes to their designs.

In 1978, Michael Zane (now the sole owner of Kryptonite after buying Stan Kaplan out) produced the first frame mounting mechanism for U-locks, which was a popular addition to their portfolio.

Citadel went on to be absorbed by ABUS, the European lock brand, who remain Kryptonite’s main competitor today. 

Materials and Manufacturing Advances

So, as we found out above, hardened steel was chosen as the material of choice from the birth of the U-lock. 

Citadel were the first company to introduce a rounded hardened steel shackle which was 1/2 an inch thick and provided fantastic resistance to bolt cutters. 

Shortly after, Kryptonite also began producing round, hardened steel bike locks, which have been the norm ever since. 

It wasn’t until more recently that companies like ABUS and Seatylock started to produce locks with square and triangular shackles, such as the ABUS Extreme 59 and the Seatylock Mason.  

These shapes offer impressive resistance to bolt cutters compared to circular shackles since they naturally fall out of the jaws of the bolt cutters. 

LITELOK X1 barronium shackle close-up
The LITELOK X1 uses a ceramic alloy, which contains thousands of fine ceramic particles, ready to chew up grinder blades

Different hardened steel alloys remain the most popular material of choice, but more recently, with the introduction of anti-grinder locks, we’ve seen locks utilising a wider variety of materials. 

The Altor SAF recently became the first lock to utilise an anti-grinder aluminum shell encapsulating a hardened steel shackle. 

Then followed the Hiplok D1000, which uses Ferosafe technology to provide impressive anti-grinder resistance. Ferosafe is a hardened ceramic alloy. 

Shortly after the D1000 launched, the LITELOK X series hit the market with their own anti-grinder tech, Barronium.

Barronium is a ceramic alloy that’s fused to the outside of a hardened steel shackle to shred grinding discs. 

Key Developments with Locking Mechanisms

To begin with, U-locks used padlocks to secure their shackle in place.

Kryptonite’s third version of their U-lock featured an integrated tubular locking mechanism, quickly becoming the norm.

Citadel also used a tubular cylinder on the first lock, which pushed a ball bearing against the shackle, securing it in place until unlocked. 

Since the 70s, we’ve seen multiple locking mechanisms used with U-locks. We’ve seen wafer, disc detainer, slider and combination locking mechanisms.

Five bike lock keys from different locking mechanisms, tubular, wafer, slider, disc detainer and Abloy
A range of different cylinder types are available on today's U-locks, some offering better security than others

More recently, Assa Abloy locking mechanisms have become the mechanism of choice on high-security locks designed to thwart all theft attempts. 

When producing this article, the most common locking mechanism U-locks use is undoubtedly the Disc Detainer.

Disc Detainer mechanisms typically offer good resistance to picking and manipulation and require specialist tooling that’s not commonly available to be picked. 

With the introduction of Smart bike locks, we’ve also seen electronic locking mechanisms on U-locks such as the ABUS SmartX 770A

This lock is Bluetooth-operated and is opened and closed using the ABUS app on your mobile phone.

High Tech U Locks

The ABUS SmartX 770A was the first smart U-lock on the market to be produced at scale by a reputable company. 

As described above, the 770A uses a Bluetooth locking mechanism, which unlocks when it senses your phone within the vicinity. 

This smart U-lock also uses a 110dB anti-theft alarm to startle thieves and attract the attention of other passers-by alongside a 13mm hardened steel square shackle. 

The ABUS 770A SmartX out of its packaging
There are no keys supplied with the ABUS 770A SmartX and an On/Off switch sits where the keyhole would on a conventional bike lock

However, as it stands, there are several flaws with smart bike locks like this.

Firstly, the 770A requires charging to keep it functioning, meaning the lock could run out of battery when locked to your bike, leaving you in a difficult situation.

Secondly, despite being “weather resistant”, several users have complained of faults with electronic locks when left outside. 

Finally, the fact that your phone is required to operate the lock is a problem. Phones constantly run out of battery, so you’ll be walking home if yours does when you’re using the 770A. 

Read more about the 770A here.

The Fight Against the Modern Bicycle Thief

As e-bikes become increasingly common, the bicycle security industry must adapt and offer adequate protection for these increasingly expensive bicycles. 

For anyone interested, we recently published a short guide on how to buy a bike lock for an e-bike.

As we’ve covered, companies like Altor, LITELOK, and Hiplok have all released U-locks that offer anti-grinder technology. 

The bike thief’s weapon of choice in cities and other urban areas is typically a pair of bolt cutters or an angle grinder. 

Thieves who target high-end bikes will typically use an angle grinder, hence the introduction of “anti-grinder” or “grinder-proof” U-locks

Three different U-Locks with anti-grinder technology
Skunklock, LITELOK X1 & Altor SAF

The fight against bike thieves is a battle of cat and mouse.

Since any lock can be defeated with the right tools, it’s only a matter of time before a lock’s vulnerabilities are learned and exploited by thieves. 

For this reason, bike lock manufacturers must stay up-to-date with the latest attack methods so that their locks remain relevant and resistant. 

The Skunklock Begins the Fight Back

The Skunklock was the first U-lock on the market designed to fight back against thieves. 

This lock contains a pressurised “vomit-inducing” compound that disperses into the surrounding area if a thief cuts into the shackle. 

You can read more about the Skunklock in our detailed review.

Current & Future Trends in U Lock Design

At the time of publishing this article, Anti-grinder technology is all the craze within the bicycle security industry. 

ABUS are due to release their version of an anti-grinder bike lock, the ABUS Super Extreme 2500. 

The anti-grinder trend will stick with us for as long as thieves are using grinders to steal our bikes, and it’s still early days for this technology, so we’re bound to see some improvements over the coming years. 

Anti-grinder tech to one side, there’s another market area with stiff competition regarding U-locks. 

the best lightweight bike lock - Master Lock 8278 weighed on scales

Companies like MasterLock and Seatylock have been fighting to provide the lightest Sold Secure Diamond-rated U-lock on the market. 

Currently, the Master Lock 8278 EURDPRO (shown above) holds the top spot, but with future innovation, we’re bound to see other contenders coming for this crown.

Recent Updates:

Author of This Post:
Picture of James Grear (Lead Editor)
James Grear (Lead Editor)

Understanding how devastating it is to have a bike stolen, I've researched & immersed myself in the world of bicycle security since 2013.

I then built BikeLockWiki in 2019 to share everything I'd learned with the worldwide cycling community so that cyclists can improve their bike security skills and make informed decisions when purchasing new products and services.

Learn More about Me & BikeLockWiki here.

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