I'm gradually upgrading an old Gary Fisher that has 80 forks on it. The plan is to upgrade all the bits, (v brakes to disk etc), and then buy a new frame to finish.

However what would happen to the handling if it suddenly had say 120 forks placed on it?

Apart from it feeling like a shopping trolley, you'd probably find that the front and back ends of the bike would have something of a falling out and go their separate ways, i.e. the increased leverage of the longer fork would snap the head tube off the bike.

it would have less weight on the front causing,
front wheel lifting during climbing,
understeer,, the front wheel washing out in corners,
more stiction in the forks due to the increased angle

plus points more weight on the back wheel
better traction,

as a ball park figure 10mm extra fork length loses 1 deg of head angle

Can upset the geometry where it doesn't handle well, but it might handle as a pogo stick.

Also some frames could crack the head tube welds due to more stress.

I used a U-turn with my zaskar 80-100mm (90 sweet spot) but I have wound it to 115 to see what it was like-aweful! but it was designed around 85mm and 10%.

Ooooh I know this one – I have the scars to prove it. It'll absolutely rock on the downhills and corner like it's on rails when the trail points downwards but forget to weight the front end on the flatter twisty stuff and it'll be front wheel washout city 😉 . Climbs, hmmm . . . you can ride climbs??

I have done this with a bike designed for an 80 mm fork – I put pikes on it. Many frames of this era were built with steep head angles anyway so going up a bit will be fine. so if it was steep by modern standards going up a chunk on the travel will only make it slack by moderen standards
My experience – with the pikes at 95 mm its still a bit twitchy and sharp steering, at 110- 120 its fine – feels a bit slack tho and the weight is starting to shift backwards a bit. At 140 mm it is totally wheelietastic – impossible to ride uphill as the weight has shifted so far back. Good on bumpy downhill tho. I don't find the slow steering an issue but the lack of weight on the front can be. I have to r3emeber to concentrate on getting the weight back over the front

So long as you don't start going mad on big drops and stuff I wouldn't worry about the extra stress on the fork – you are only talking about a 10-15% longer lever – less once sag and sus movement is taken into effect. Mine has been fine for a couple of thousand miles

Depends entirely on the frame, people like to rattle out the "It'll ruin the handling" line but tell that to Cotic, you can run a Soul at 100mm or 140mm and it's ace, or a BFe at 160mm. Lots of bikes actually work better with added fork, the GT Idrive 5 for instance loves a 140mm.

Course, you might snap it and be killed.

It would probably be OK if you run the forks with a lot of sag as this'll bring it back down to normal geometry. run them with a lot of sag, rebound quite slow, should be fine. If you were putting 160's i'd be worried and in general forks that are too long will make it handle like a barge on the corners.

Many thanks for the replies guys. I'll have to finish the upgrades to the brakes first then look around. I guess I could always go new frame with solid forks and then move to suspension later on.

Don't forget that you'll be raising the whole bike including the cross bar, so it'll be a bit like a bigger bike. Given that older bikes tended to have higher cross bars at the seat end this could make it a bit gate-like.