I can get a single pivot caliper deep enough, but the performance is inadequate. Does anyone know of a dual pivot caliper that's at least 90mm deep? Or another solution? It's for the front of a cargo bike where a disc brake or roller brake is not possible. I've done a lot of Google searches without result. Thanks.
Even a dual-pivot brake probably isn't going to solve the problem you found with a single-pivot brake: after a certain point, those long brake arms are flexing and absorbing the energy you're trying to put into the pads. This is why you generally don't see caliper brakes on fat-tire bikes. I did a quick check, and even Tektro's long-reach calipers aren't long enough for your application.
Cantilever brakes would be the more orthodox solution to this problem. They're widely available and a known quantity.
Thanks for all the input and advice.
I've found an Alhonga U brake sold as suitable for folding bikes, it is a self contained centre pull almost like a Delta brake, in alloy with 110 mm max drop. Not easy to find a Google image of it, but here's a link https://www.bicyclepartswholesale.com.au/product/4459-brake-alhonga-front-brake-arm-alloy-for-folding-bikes-drop-110mm .
That's what I'm going with, I'll let the forum know how it works.
Another possible is a 91mm deep Altenburger/Point/Alhonga Synchron which is an early design dual pivot. Reputation for being very flexible , possibly little better than many single pivots.
I could buy a new fork with disc mounts but it's very expensive. I can't readily get any fork mods done around here, frame builders are rather thin on the ground here in Western Australia :(
Henry - I ended up using a long reach brake caliper and had to create a lower mount point for it.
For me, this was caused by changing from 27" (630mm) wheels down to 700c (622mm) wheel.
So I made a dropper plate to provide a lower mounting point. In theory I should have used two plates, one in front and one behind the brake bridge.
Steel would have been a better material than aluminium as pictured, but it was thick and its what I had.
Also, its a rear wheel so there are far lower peaks in braking effort on this end of the bike.
The pictured calipers are Tektro R559, and even with my mount plate, they only just reached the rims.
Personally, for a front wheel, I'd recommend you find a better fork with disk mounts, and a wheel with a brake rotor.