I'm currently running an old (1996) Specialized M2 Stumpy frame which originally came with 80mm travel Judy's, I've since upgraded these to Pace 100mm RC38LTs. The increase in travel didn't give a significant change to the geometry, just enough to slacken things off slightly from it's hardcore XC breeding.

My question is this:
How long forks do you think I could get away with and what would be the limiting factor? Would it be the headtube strength or the sloppy handling that would be the limiting factor. I don't want to go crazy, maybe fit some 120 or 130mm forks, but should I really be looking at a new frame?

I would say the bigger risk is the type of riding that 130mm forks will encourage!

If you think you need more than 100mm travel, you need a new frame.

That, and it'll likely handle like a barge until the head tube fails! Are you sure they were 80mm originally? A 1996 would surely have had 63mm?

You won't know if strength is a limiting factor until it breaks.

As for handling it sounds like you could take more OK. Modern bikes are slacker in any event.

I used to have the same frame and I also had 100mm forks on it. They handled well downhill but the front wheel tended to lift on steep granny ring climbs.

TBH I don't think longer than 100mm will handle well – I don't think the bike will climb at all well. And I'd also be worried that the frame would fail. You've got a lightweight XC frame – stick with lightweight XC forks.

You can always shove your seat forward.

Whether strength is a limiting factor depends on too many factors.

Are you sure they were 80mm originally? A 1996 would surely have had 63mm?

Actually, you could well be right there.

Oh well, it looks like I'll be sticking to the 100m forks, and will have to add another steed to the garage for a longer travel more hardcore HT – what a bummer!