I have a very roadbike akin cyclocross bike. Would it be unrideable if I put a 80-100mm suspension fork on it, to use it for a few leisure MTB rides in summer? Mostly gravel roads of different quality, and easy trails, no hardcore stuff.

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Bikes: Raceur, Beateur

I've been riding what I'd classify as easy trails with my cross bike (standard setup, rigid fork) without any problems. If it's mostly fire roads I think you should be fine.

The frames geometry will remain the same if the axle to crown height of a suspension fork is the same as the rigid that you have already. This is very unlikely though. Looking at a few, if you are going from a road fork you could be adding as much as 120mm to the fork length, from a cross for more like 90mm. I think this would have a very noticeable effect on the handling of the bike, if you could borrow a fork from someone to try it might be worth it.

But as the poster above mentioned, it's possible to ride a standard cross set up off road with no problems whatsoever!

Yes, I've been doing lots of MTB with a rigid fork in the 90s, and with a suspension in the early 2000s. I know if I have an MTB(-ish bike) I'll be using it more, also on trails, but probably not enough to justify a whole bike. Riding the winter beater with a flat bar and suspension fork sounds like a budget solution, unless people think it won't work out.

Hmm maybe I could run a 27.5" front wheel to compensate for the added rise, although that might look a bit weird.

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Bikes: Raceur, Beateur