The Zac 19 is a pretty strong rim despite being listed as a lightweight. It's a popular budget priced rim for mountain bikers. I got one to replace the rear wheel on my heavy comfort hybrid/errand bike a few years ago after spokes snapped on the original single wall rim, which warped. The Zac 19 was on a budget priced Wheelmaster build, generic loose bearing hub, heavy duty spokes. 700x40 Michelin Protek Cross Max tires, practically bombproof. I've bashed that bike across bombed out pavement and gravel roads while hauling up to 50 lbs of stuff. No problems. I think I've needed to true the rim once to dial out a little wobble.

A zac19 from Weinmann? Would you know your spoke tension on drive side?
Thanks

Ryde doesn't call the ZAC19 rim specifically a "lightweight" rim. It's a part of their "Standard" range, which "varies from lightweight mountainbike rims to heavy duty aluminium rims and almost indestructible stainless steel rims." (The Rival models in that range actually do get called "lightweight" by Ryde.)

The extrusion looks pretty beefy to me, sort of like the Sun CR18, which also shares a very obviously conservative tension rating.

Referring to link already posted above the mfr gives 110kg as max total load for a bike with zac19 rims. OP has stated the tandem in question weighs 470 pounds. Mfr has several other rims with rated capacity of 130kg and at least one with 180kg capacity. Just going by what the people making and selling the rim have to say.

One more time the mfr gives max spoke tension for a zac19 as 1300Nm which is very close to 130kgpf.

My experience is the zac19 is pretty good. I'd not use it for a tandem, not even a light one.

Agree, probably a good idea to use something beefier for a 470 lb tandem.

What?

You have a source for spoke washer use for non eyeletted rims? First i've ever heard of this in 12 years.

Inflated tire spoke tension is the ONLY tension that matters.

The mfr is Weinmann, not Ryde. The mfrs have models with same name, apparently some history between mfrs, though not sure if same rim. The 120kgf I posted was for the inflated wheel. I would have liked a beefier rim, but these are the rims we have, no eyelets, no washers. If the rating is 130 then I will leave at 120 and if the rim does fail, will go with Ryde Andra.

How is not hearing about things a good source of authoritative knowledge?

I know of rim makers who have both suggested nipple washers for non-eyeletted rims, and specified maximum spoke tension WITHOUT tires installed.

Ryde owns the Weinmann brand name.

Well that explains some of the confusion. However, the Ryde website gives specifications for a rim with eyelets (and possibly washers). Our rims have no eyelets/washers, meaning the 130kgf max is not appropriate. So I'm back to thinking that 120kgf is too high.

It’s not uncommon for rims to be sold in a couple different configurations. I think you can find Sun CR18s without eyelets, too, and Mavic made single-eyelet versions of some of their normally double-eyelet rims for OEM use.

Non-eyeletted rims were once simply known as washer rims. Some mfrs supply the washers and some don't. I started building wheels over 50 years ago so some of my knowledge might be regarded as simply historic trivia. But if historic trivia is also good engineering practice maybe use the washers. Guys I learned from and worked with would never even consider building a non-eyeletted rim without using washers. If some funky modern rim were shaped so that washers just wouldn't fit (this is not the case with your rim) the old guys would simply not build that rim.

Take the nearest empty rim and put one spoke and nipple through it. Move the spoke and nipple around, see how much angle the spoke will move through without binding or forcing. Now reassemble with a washer. It's like you just put a gimbal on that nipple. And the washer spreads the load over a larger area of rim. Spoke pullthrough is not very common. On a heavily loaded tandem with a light rim and no washers you can expect pullthrough.

Tandem falls are no fun. If you want to trust your safety and your loved one with a rim that is specifically not rated for even close to the load you carry you are free to do so. Not a good idea.

Thank you for your comments. Our bike was built by a very well known shop and we made it clear that a robust bike was a priority and strong wheels are a critical element. The builder has much experience with comparable loads. I suspect that if rim failure was a problem, the builder would move away from the rim for the sake of having happy customers. Having said that, my suspicion is that the wheels will not last a long time, but it is only a suspicion in the face of the builder's experience. Tandems always press the boundaries of components. 120kgf average tension does not seem too high for a loaded non-dished tandem, but the statements here suggest pullthrough will likely happen and I should reduce tension. When the rims fail, we will switch to ryde andra.