For example putting a 190mm on a 200mm frame? Thx Martin

the full scientific answer is 'it depends'

10mm.?

it depends is a good answer but expand on that for the op

I did that on my Kona Howler to slacken it a touch and lower the BB. Makes it a nice mini DH bike, well, better than it was anyway

Taking into account the knowledge displayed above I've bought a 200mm 🙂

slacker head and seat angles and lower BB height I'd have thought. I found a few people on american forums talking about it when I looked at getting a SC Bullit (for which it would seem to be A Good Thing) but i suppose it depends on the bike. I have ridden some FS bikes that were plenty low and slack enough already!

It could just relax things and work well or it could cause swingarm to come crashing into the mainframe with terrible consequences. I'd say that it depends on the frame/size/design

ok, you're quite right Knottie8, I should have been a bit more useful. It's
as Onzadog says. A few mm in the shocks is usually many more at the wheels or steering head or bb.

Best idea is as you've decided, fennerhorne, get the same length as the designers intended unless you're really sure.

I've an old-style bullit and tried it with a (bit) shorter air shock – made sure the compressed length was no shorter though (as onza suggests)

Rode pretty well I think but ended up back on the original coil shock with plenty of sag

I've tried both 200mm and 190mm shocks in my Enduro (which is supposed to have 195mm) and can't say I've noticed a difference due to the length.

I have seen a Trek Liquid with a scary dent in the frame where the
new shock let it hinge too far

it's all about the relationship between shock length (eye to eye) and stroke length and whether the resulting compressed length means that the swingarm/ stays are going to foul on the frame at the seattube/ pivots/ linkages
I'm replacing an air shock with a coil and although the shocks are the same length the stroke is different so I'm putting a bump-stop on the axle to compensate
even though this should work in theory, I'm still crossing my fingers 😕

Good luck bakes – that crossed my mind, too, as 've got a coil Vanilla upfront, but coil shocks seem a whole other world rider weight springs, weird lengths and strokes, and I don't have any idea what a bump stop is. The frame's a Stumpjumper btw and I figured anything would be an improvement on the 165mm Triad it turned out to have on it – an interesingly slack head angle and pedals hitting the tarmac alerted me to the fact that something was wrong 🙂

You could have a "stuckdown" shock. Caused by a combination of a soft seal compound used by Fox for some manufacturers, and temperatures low enough to let the main seal contract allowing the pressure in the pos and neg chambers to equalise.
An air sleeve service will rectify this and if you rotate the seal when doing it, it'll give you extended life out of the original seals. but the best course of action is to buy a new seal kit for about £15 – £20 which will be made from a harder compound and stop it happening again.

If the Stumpy has a 165 x 38mm shock fitted as standard, a 200 x 50mm shock won't fit, or work very well. And that's not even accounting for the shock's comression and rebound tuning. Spesh get specific valving for their suspension geometry.

New seals will be cheaper and better if the bike has the correct shock fitted in the 1st instance.

Please excuse a double post, some tech problems with my machine.