I'm building up a new 16" Inbred, which replaced an 18" GT frame that I always felt was too big for me (5'7" tall, 30" inside leg).

I need pointers for bike setup advice as I don't want to get it totally wrong from the outset. There's a few sites I saw after a quick search but none that inspired me.

I've suffered before with backache & other stuff from bad riding posture & never really feeling properly balanced on my previous bike no matter how I set it up, so this is my last chance to get it sorted before I get fed up & give up!
And I haven't got £125 to throw at it like the bloke from the other thread…

What kind of riding will it be used for?

Good question!
Just general xc bits & bobs. Usually ride a mix of woodland & moorland stuff.

How much shorter in the top tube is the inbred? Use your old setup and this to get an idea of where to tweek.
I ran a 16" inbred SS with a 50mm stem and it felt good for me, although i could have ran a 70mm. Im 5'10".

Andy Pruitts Medical Guide For Cyclists… brilliant for bike fit and problem diagnosis…

A google gives some basic info..

HTH

I'm obsesses with bike set up,far more important than tyres(heresy on here),or even the type of bike you ride.
I'm fortunate to have a really good and helpful LBS that have set-up my bikes.Result;no back ache,or knee pains.

Tough to say when youve not given dims of either bike or stated what you felt was wrong.

start using what you've got and experiment.

2nd for

Andy Pruitts Medical Guide For Cyclists

covers all the basic extremely well

A shorter stem is a must, I'll be putting 110mm travel Magura Ronins on it, so may not go too short as they could be a smidge longer than it shoudl have ideally (possibly).
Currently i've had to put a 100mm stem on the GT to keep the front end down & get some weight over it, but i've also got the saddle quite far back & post with rear mounting – any further forward and things feel sort of cramped and/or "sit up & beg".
I've previously played with headset spacers, although this is is the last thing to try with the current stem on old bike before I change over for good.

The problem is I don't have much seatpost out (just below the T of TITEC lettering) and that's about as much as i'm happy with. Knees are almost correctly bent but any more and the frame just feels like a big old gate even though it's only a medium.

The front of the GT sometimes feels sort of twitchy, a bit too easy to tuck in when cornering, etc, but other times feels a bit like it's wandering.
Naybe the forks are crud & are to blame?! 😆 Not so likely though, they're supposed to be decent & i've got XT QR hubs on there.

I'll have a look at the Andy Pruitt's guide i think and just google a bit more.
Kinda hoping the frame + a suitable stem will all be good and it'll be job done.
We shall see…