What I've learned while I lived in Amsterdam is that you want to invest ~30% of the value of your bike to purchase a bike lock. Basically make it not worth the effort to break the lock.
Also second-hand and rusty ones are better than new ones. Some Dutch cities have a free-of-charge engraving mechanism where you basically engrave a serial number on a bike to make it easier to track down stolen bikes, as well as to know that you're not actually buying a stolen bike.
Also, you'll be responsible if you're caught with a stolen bike, which is a nice deterrent to actually use a reliable source when purchasing a bike. Shops act as a middlemen between you and the person selling the bike, and they're responsible for making sure that the bike you're purchasing from them isn't stolen. On the downside, that drives the cost up a bit, but on the bright side, you usually get some months of warranty period in which they'll repair your bike free of charge in case anything happens to it.
> What I've learned while I lived in Amsterdam is that you want to invest ~30% of the value of your bike to purchase a bike lock.
That advice predates the battery-powered angle grinder, which many bike thieves in Western Europe now use. Even the toughest, most expensive U-lock can be broken in about a minute with an angle grinder.
Having two locks can also be a good tactic, especially two locks in a different style e.g. a d-lock and a cable lock. The theory being that a thief would need to have two tools to steal your bike which means it's likely more hassle than other bikes that'll only have been secured with a single lock.
Founder of a bicycle insurer here and I can tell you that the only way to deter people is by parking in secure private or public spaces (never, ever semi-public) and by making your bike less attractive than the one next to it.
2 locks is indeed the standard in big cities. Also make sure they are sold secure gold rated or Art 3. Anything else is a waste of money. There are a few new lock types which are supposed to work better against angle grinders like Tex-lock or LitLok.
Because the average bike price is going up it also becomes interesting to steal parts. A decent handlebar + shifters are easy to sell individually. Companys like Hexlox or Pinheads are working on locks to tackle that.
Where would you install them on the bike where they couldn't be removed easily? I've thought you could place one in the frame, but that would be difficult to do aftermarket and the GPS and WWAN antennas would need to exit it somewhere.
Shaft were the seat affixes, maybe use the frame as the antenna - dunno, was more interested in what is out there already in a way that is respected enough for insurance companies to make a difference. That's the true test of a security product - how much of a discount does your insurance get for having it ;).
A cable lock can be cut with a dozen pocket-sized tools. They're not worth the money if you think theft is remotely an issue; D-locks are far more secure.
I challenged a guy I saw fiddling with one of the locks on a motorbike with its alarm sounding, by asking if he'd mind unlocking one of the other locks to show it was his bike. Which he promptly did - his alarmed disc brake lock had jammed.
Of course, my confidence to do so was buoyed by the fact there were two other bikers parking at the same time as me, it was in a busy public area, and I was wearing a bunch of leather and kevlar.
Also second-hand and rusty ones are better than new ones. Some Dutch cities have a free-of-charge engraving mechanism where you basically engrave a serial number on a bike to make it easier to track down stolen bikes, as well as to know that you're not actually buying a stolen bike.
Also, you'll be responsible if you're caught with a stolen bike, which is a nice deterrent to actually use a reliable source when purchasing a bike. Shops act as a middlemen between you and the person selling the bike, and they're responsible for making sure that the bike you're purchasing from them isn't stolen. On the downside, that drives the cost up a bit, but on the bright side, you usually get some months of warranty period in which they'll repair your bike free of charge in case anything happens to it.