If i were to swap the current fork with a modern fork made for a 700c tire would it drastically effect the geometery? would there be any down side to it? I know that 700c wheels are smaller I am assuming the forks are also shrter which would tip the front end down and it would probably have less rake?
The bike has a nice classic look now, are you sure you want to pimp it up with a "modern" fork?
A little more information would be nice, I assume you now have 27" wheels and are considering 700mm wheels and a fork with a threadless steerer? When changing wheel sizes the first thing to consider is if the brake pads can be adjusted to fit the new rim or if you will also need new brake calipers.
The fork leg length does have some effect on handling but the difference between the length of a 27 fork and a 700 fork shouldn't make a huge difference, leg lengths vary from make to make even within the 700 size forks. The fork's rake may make a more noticeable difference and rake is not necessarily related to leg curvature because rake on straigter legged forks is controlled at the crown by the angle between the steer tube and fork leg. A straight fork can have as much or more rake than a curved fork. Steering response is effected mostly by trail, and trail is a function of head tube angle, wheel size, and fork rake. A change in fork leg length effectively changes head tube angle. If you shorten the legs you lower the front of the bike the steepening the HTA and shortening trail. If that fits your case you could compensate by using a fork with a shorter the rake which in turn will increase trail.
Here is a link to a rake and trail calculator I like to use to compare forks. It does not allow for an independent calculation of a fork leg length change but that could be done by tweaking the HTA input. The calculator is set up to run using inch or cm parameters. You can make the same calculations using basic trigomometry.
Thanks for the comprehensive response Al. I don't see the link however. I have to qualify modern fork. I meant a fork that would take a 700 wheel. I'm still going to go with a steel fork but I can't find one with the curved legs. Most forks these days have the straight legs and I didn't know how much that would affect things. I actually wouldn't mind something a little snappier and with tighter steering.
I've seen some steel forks with threaded steerers I'm running 700 wheels right now track in the back so brakes aren't a problem.
I don't know, must be the Alzheimer's. If you want to use this calculator, go metric, clear the variable that you want answered, use cm values for rake, trail, and wheel diameter input. When buying forks the rake number is usually given in mm.