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I have a new bike, not very expensive... but still doesnt want it to disappear the next day and I was looking at different locks and came across lock that have an alarm which triggers a siren. Not sure how good they are. Would anyone recommend ?

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    I think a strong U-Lock (Abus, Kryptonite) is still your best bet. Make sure you use it correctly (e.g. don’t just lock the front wheel to something).
    – Michael
    Commented Dec 10, 2021 at 10:49
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    Note that in our FAQ, we do say that we consider product recommendations off topic.
    – Weiwen Ng
    Commented Dec 10, 2021 at 13:24
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    WHOOO- BAM! Twaaaang! crunch! What alarm? Commented Dec 10, 2021 at 15:08
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    @Michael The point was a cable lock requires different tools to defeat than a U-lock, whereas a chain can be defeated with the same tools as a U-lock. And a good cable isn't that easy to unravel... Commented Dec 10, 2021 at 16:00
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    Comments are for clarification of the question, not extended discussion.
    – mattnz
    Commented Dec 10, 2021 at 21:18

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I'm leaving all the general anti-theft advice aside and try to focus on the alarm side.

Basically I believe there are two different types. One with some loud audible system on the lock/bike itself, and another type that sends an alarm to your phone to alert you about something dodgy.

The first type (audible alarm) is not likely to help much. A good lock has to be cut with power tools, so this makes a lot of noise already. If that noise doesn't alert people, then the alarm won't make a difference. And once the lock is cut, the thief leaves the lock with the alarm behind and escapes with the bike, so from that point onwards the alarm is pointless. It would perhaps make more sense if the alarm was separate from the lock and is fixed to the bike, as a rider with a howling bike might attract some attention or get worried and dump the bike after a few metres, so you can recover it. But I don't think any of the locks work that way.

It can be potentially useful if you are still nearby (eg. in a cafe or a shop or at home) and can get to the bike within a few minutes when you hear the alarm and actually stop the thief. But in a noisy environment you may not hear it if you're inside.

Here the other type, that sends an alarm to your phone, is potentially more useful. Also with this alarm the thief will not know that you have been alerted.

Some of these systems include a GPS tracker so you can find out where the bike is when you arrive too late. This seems more useful. I installed a GPS tracker once, but in the end the hassle of keeping it charged and arming/disarming it was too much, so I removed it in the end.

Also, what triggers the alarm? I think it's usually movement, but bikes can be moved inadvertently for many reasons, as people bump against them. So you'll likely get more false alarms than real alarms. Even car alarms go off a lot although cars are much bigger and people don't touch them.

Finally, some alarms (especially the phone alert and/or GPS trackers) are expensive (and you may need a subscription to use them), so you have to weigh the risks. If the risk in your area is such that a bike with a good lock has a chance of getting stolen once in five years, but you buy a new one every five years anyway, then it's not really worth spending additional money for alarms beyond a good lock. If you are in a really high risk area with an expensive bike, then a GPS tracker might be a good idea.

I had my bike stolen from my garden shed 3 years ago, and an alarm might have woken me up, but being more careful with locking the shed (which I had forgotten to do on that particular day) would've helped even more. But in the end I was almost happy about the theft as it made me buy a new bike, which I had started thinking about anyway as the old one wasn't so great anymore.

An update on the Trackers - thanks to comment from @200_success: A new alternative is the Apple AirTag or the Samsung SmartTag (possibly similar systems from other companies exist). These are small tags with Low Energy Bluetooth technology that the smartphone can locate. As they are small and the battery (CR2032, ubiquitous battery in various items like fitness trackes or kitchen scales) is supposed to last for a year, you can hide one (or several) on your bike somewhere.

If the tag is not within reach of your own phone, then it uses the network of other Apple/Samsung users to locate it. It seems (see e.g. this Test (in German)) that it takes only a few minutes to locate a tag when it is in a reasonably busy place (they left their tag in a car that had an accident and were able to locate it in the workshop and the paintshop). This seems a promising way to locate a stolen bike.

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  • An alternative to GPS trackers these days is an Apple AirTag. It won't phone home as reliably as a GPS tracker, but it's cheaper and uses less battery. You could hide one or two of them on your bike in plain sight by using some clever mounts. Commented Dec 13, 2021 at 9:30
  • @200_success Thanks, I didn't know (even though I'm a fairly technical person), this is indeed useful and I added a paragraph.
    – uUnwY
    Commented Dec 13, 2021 at 10:52
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Sirens are utterly useless - consider how many people ignore a car or property alarm.

They simply can't be loud enough to be a deterrent on a bike and remain small. Also they need battery, will be flat the one time they're needed, and can be defeated by yanking a wire, smashing the speaker, or simply muffling it with a hand or cloth.

  1. Your best option is to bring the bike inside, such that it doesn't need to be locked.
  2. The second best is to lock it outside in a place it cannot be seen.
  3. Locking it in sight of casual passers-by requires a decent lock, removing all easily-stolen items (lights, water bottles, tools) and to secure the saddle and wheels. Ideally you'd have two locks and secure all of it to something immovable.
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    From my gut feeling I agree that audible alarms are mostly useless and everyone just ignores them. But are there any statistics on the matter? A properly designed one should use none or very little battery and should be hard to muffle or destroy.
    – Michael
    Commented Dec 11, 2021 at 13:34
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Your approach of loud sound is not feasible.

The typical way locks are broken are very loud. One foolproof way for the thief is to carry an electric battery-powered angle grinder. It will make a very loud sound when operated.

Another way is to carry a ramset gun. This may not be easily available on all continents, but at least in United States where everyone can buy an assault rifle from a supermarket, firearms that are intended to drive nails into concrete are readily available and will easily break most locks. As a firearm, it is very loud too. Someone will think the bicycle thief committed a murder!

Since most common ways to steal bikes make a loud sound, why do you think a second different loud sound would benefit you in any manner?

About the only way that is not loud, but is somewhat slower, is to use a hydraulic jack. This can be made harder by using the smallest U-lock that you can buy and lock the bike in such a manner that the "U" of the U-lock is as full of stuff such as bike tubes or tires as possible, so the thief can't fit a hydraulic jack here.

Picking a lock is obviously possible. On door locks, there are locks available that are so hard to pick that even LockPickingLawyer on YouTube can't pick them (newest Abloy locks), but on bike locks the best locks available are disc detainer cores with no special anti-pick features, such as the Kryptonite Evolution U-locks. Nevertheless, even these Kryptonite locks require special tools (a disc detainer pick) and special skills to use the said tools.

If you want to prevent a thief cutting or breaking your lock with a hydraulic jack, buy a Skunklock. It contains a chemical inside the lock that will evaporate when being destructively opened, and is so annoying the thief will probably run away after encountering the chemical. Even Skunklock won't prevent lockpicking, though.

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