Hi !
Does someone know what will be the effect of installing a 700c road bike fork on a MTB frame ?
(Handling, change of geometry etc.)

You'll have a 69er!
26 in back and 29 in front.
Thinking about the same thing for myself, but am too lazy.
Handling changes probably depend more on the height of the fork (axle to crown) and perhaps shape of the fork (rake) than the wheel size.

I have thought about doing that to my mountain bike. The rear will easily fit a 700c road wheel and tire (probably a 35mm easily- I ran a 27x1 1/4" wheel/tire in it for a few days once while waiting for a new wheel to arrive). I have a 1" headtube (old rock hopper, see sig). So, if I were to put a road wheelset on it it would be very nice for touring, off-roading, whatever. But, then again, its fine as it is.

brakes won't work, unless you're putting a 700c wheel on as well.

If you're going 69'er and your frame was originally designed for a front suspension, using a 700c fork will somewhat give you a suspension correct front end height so there shouldn't be much change in frame geometry. Handling should be just fine.

Yeah, I guess that's the general direction you want to go, but suspension is really overdone on a lot of bikes. I'd say it will work about right for an older 2.5" travelnsuspension, but some of these goofy 4" jobs, I dunno.

At any rate, the 700c fork will move it the right way, and even if it lowers the front, that's probably fine for road use

What if he's replacing a rigid 26" fork with a 700c fork?

And what's the idea behind a 69er anyway?

oh boy. I just ordered a 26 inch LHT fork to replace my suspension fork that kept bob bob bobing along. Sounds like I need to get the shop to change the size.

A 700 LHT fork is 390mm axle to crown, and a 26 fork is 376mm axle to crown. That looks like 14mm difference. 1 inch is 25mm, so it looks less than an inch difference. If you are replacing a suspension fork that would usually have some preload sag of an inch or more depending on riders weight, it would seem like staying with the 26 fork would work, if you are trying to replace a 26 suspension fork with a 26 solid. What have I over looked?

There have been some awfully good mountain bikes over the years that have the same sized wheels on both ends , so I'm not going to say a 69er is a great thing necessarily. But the idea behind it would be along the same lines as front-suspension only. Relative to smaller wheels, bigger wheels make obstacles smaller, pure and simple. The disadvantages of a big wheel are increased weight and, all other things being equal, a weaker wheel. Whether a bike is less responsive in the handling department because of bigger wheels depends as much on the bike's geometry as the wheel size, in my opinion. I've ridden a 29er off road enough now to be convinced that a well designed 29er handles just fine, even on tight, technical singletrack.

But getting back to the 69er concept, the idea is that it's more important for the front wheel to make mincemeat of obstacles, and where the front wheel can go the rear wheel will follow, even if the rear wheel's a smaller wheel (as on a 69er), or even if it doesn't have the benefit of rear suspension (as on a hardtail). Whether or not you're better off to have a bigger wheel on both ends, or to have suspension on both ends, would be another argument altogether (I don't mean this to start a hardtail vs. FS war, however ). With a 69er, you'd retain whatever benefits a 26" wheel has on the rear of the bike, and in the place you need it most (particularly the element of strength). And you'd get the benefit of an obstacle-eating big wheel where you need it most, which is up front. In my opinion, another advantage of a 69er would be that the geometry would be inherently slack, which makes for more stability. This would be an advantage, of course, only if stability over responsiveness is a priority.

If you look at the design of off-road motorcycles, you'll see smaller diameter, super beefy rear wheels, and larger diameter, not-quite-as-beefy front wheels. I suspect the 69er concept was born there (Cannondale saw to this with a 24"/26" bike back in the '80's in fact).........Sorry to the OP for going OT, I just enjoy talking mtb's sometimes -