Hi and welcome,
FIR (Fabbrica Italiana Ruote) M123 is indeed a very old rim. As far as I know, it is relatively thin box double wall construction (so not as strong and rigid as the deeper section double walled rims - and I don't mean the "aero" extra-deep ones).
It was made in 32, and 36 spoke variants, for rim brakes (in the 90s).
I don't know the specific info for this particular rim, but I'll try to give some general guidelines and an "educated guess."
TL/DR (the brief answer):
For a 36-spoke wheel, start with 70 kgf for all the spokes if it's a non-dished wheel (rim brake front wheel or a single speed rear wheel), or 60-90 for the heavily dished rear wheels.
Then, test as explained in the
last ("test") section of this post to find the rim's maximum tension, or just stay at the given tension with some swagged spokes (2-1.8-2 mm) and call it a day. Should be strong enough.
For a 32-spoked wheel, you can go for about 10% higher tension per individual spoke.
For more details with an explanation, read on.
Terms used
To avoid confusion:
- NDS - "Non Drive Side" spokes, i.e. spokes that come from the left hand side of the hub (left hub flange) when looking at the bicycle from the rear towards the front (as you are when riding).
- DS - "Drive Side" spokes, i.e. spokes that are on the right hand side.
- Total spoke tension - aggregate tension of all the spokes in a rim (added). 36 spokes with 100 kgf tension of each would be 36 x 100 kgf = 3600 kgf total tension.
- Individual spoke tension - tension of one spoke.
- Spoke tension - depends on the context, but I will try to be clear.
Maximum total spoke tension for rims - in general
With modern, strong double-walled rims, I have often gone over 3000 kgf total spoke tension (about 75 kgf for NDS, and 110 kf for DS on rear wheels, for example, or even 80-120 if a rim is really stong). Anything over 3500 kgf is really pushing it for most but the strongest rims.
It is wise to have some reserve, and to not use too much tension on any individual spoke - anything over 120 kgf per spoke is not wise, even on rims with steel eyelets. For the front wheels, it is best to stay around 80 kgf on both sides (or 70-85 kgf for the disc brake front wheels).
Old Mavic MA-2 rims were quite light and
not super-stiff and strong double walled rims. They were made for road bicycles. With those rims, anything over 2700 kgf was too much. So the highest you could safely go was about 60 kgf for the NDS, and up to 90 kgf for the DS spokes on the rear wheel.
With such rims especially (but the same goes even for the modern stronger and stiffer rims), using swagged spokes, at least on the left hand side, was helpful to prevent the NDS spokes on rear wheels from coming loose.
A good alternative in terms of durability is using 2-1.8-2 mm swagged spokes (i.e. "double-butted" spokes) for the DS, and 1.8-1.5-1.8 mm spokes for the NDS - with matching nipples (2 mm spoke nipples are too big for the 1.8 mm spokes), but that makes nipple and spoke sourcing a bit more complicated, so I use either single gauge or 2-1.8-2 mm swagged spokes).
FIR M123 look like they are somewhere in between the modern strong double walled rims, and the light old road Mavic MA-2 rims. Which makes sense - they were built for mountain bikes. They should be a bit wider and a bit stonger than the MA-2 rims.
How much that is, I am not sure. But I am quite certain that they are not weaker than the MA-2 rims, and that they could handle a 2700 kgf total spoke tension, or up to 90 kgf for any individual spoke.
How to test yourself?
A good thing about the old rims is they won't get permanently deformed when you do the following:
Build the wheel and keep increasing the tension. Once you get near over the 2000 kgf total tension range, you could start adding tension by 1/4 of a turn only on one side (for example: DS first) and once you've made a full circle (valve is a good starting point) follow it with a
spoke stress-relieve procedure (and then, go to tightening the NDS spokes by 1/4 of a turn, followed by another stress-relieve after youv'e made a full circle).
When the rim is near its maximum total tension, it will go noticeably out of true after you've stress-relieved the spokes (i.e. when you squeeze two pairs of paralel spokes). This tells you that you've reached the maximum.
When that happens, you should back off the spoke tension by 1/2 turn each, and then touch up the wheel just to get it true (adding as little tension as possible if any).
When the rim is not near its maximum tension, it can come very slightly out of true after you stress relieve it, especially when you still haven't reached the optimal spoke tension, but that is minor, just spokes bedding in.
Take care not to add tension in high increments, as that could get the rim to turn into a pretzel shape when you stress relieve it, instead of "just" coming out of true so you can tell it with a truing stand.
Hope I've explained it clearly enough and helped at least a bit.
Relja
