How on earth do I work out what weight of coil I need? 205 by 65 trunion mount air shock currently fitted on a 140mm travel bike, tried a calculator and got lots on 60/40 distribution etc.

Start with a calculator and then trial and error.

There is an element of trial and error also usually. TFTuned calculator is an ok starting point but there can be some quite big discrepancies

https://j-techsuspension.co.uk/pages/spring-calc

I used this, then i paid them £5 to work it out for me. They then give you a £5 credit if you buy the spring from them.

I think I used the tf tuned calculator for my sentinel and ended up way out. But on the Bird Aether I had it was only one weight out I think.

It also depends on the shock imo – some of them have heavier damping as standard than others. I think my Cane Creek Kitsuma is lighter damped and so I’ve ended up using a significantly heavier spring than tf tuned’s calculator suggested. But people of a similar weight / same bike as me with an Ohlins or EXT shock have got away with a lighter spring.

Tftuned calculator

Mention the bike, the shock and your weight in your post and listen to what others say from here?

My bike came with a 500 which seemed a bit too soggy, tf and j tech calculators both said to go even lower. In the end I went for a spindex 500-550, 520 is nice for local stuff and have the ability to add a bit more for dyfi and the alps etc

Not all 500lb springs from different manufacturers are the same either, so bear this in mind if picking up a range.

Nukeproof springs are a reasonable weight and suit a lot of shocks with the supplied shins.

I stuffed my info into the TFtuned calculator and bought one spring either side of its recommendation as well as the weight it suggested. That way if I loose or gain weight or want to tune it to perform a certain way I can. Plus I got to play about with lots of springs.