adding suspension

[fjb]

Your question is not an easy one to answer well, but with your engineering background, you might find the references I linked to here useful for helping you figuring out what suspension forks configuration you need.

Separately chance is your mtb has 1-1/8" steerer and 26" wheels, while the hybrid has 1" steerer and 700c wheels. Further they probably have aheadset (or their variants) and threaded headset respectively. You might want to read this and associated links to confirm. You will also have to pick forks that will work with the braking system you are going to use. If still unsure you will probably need to provide a lot more info/spec/photo here or in Knowhow and ask.

Good luck!

Yep. the Mountainf bike has a threadless headset and the hybrid has a threadless one. They are both 26" wheel, 22" frame (saying that, the hybrid is more like 21.5 or 55cm). I was thinking of putting on disc brakes on the front fork (they won't go on the back as it's too narrow as you can imagine)

 

[fjb]

Very good question. Why do you say a pair of trousers? Why do you never see a baby pigeon?

Why do you say a pair of underpants? What does a single underpant look like?

 

[fjb]

much as it will be a fascinating project to convert a bike from rigid forks to suspension the different frame geometries could make it less than pleasant to ride .... and it will probably work out far cheaper overall just to buy a cheap MTB, there are plenty of bargains to be found if you look around.

Where's the fun in that? I was toying with the idea of just buying a mountain bike frame and building it up but i figured the conversion would be a good compromise, besides I haven't got the room for three bikes.