I'm thinking of replacing my fork on an '08 Six13. I ride a smaller frame, so the rake is 50mm, most of the forks i'm looking at have a smaller rake of 43-45mm. I measured from the end of the tire to the downtube of the frame, and there should be plenty of room.

Are there any adverse affects from using a fork with a slightly smaller rake?

Riding no-handed is harder with a shallower rake fork. -ie twitchier(?)

I thought it was the other way around. Less rake is more trail and more stable.

how much of a difference would a rake of under 1cm make (probably 1/2cm?)

Maybe I'm thinking it wrong. Which gives a shorter wheelbase?

less rake or offset leads to more trail, which leads to more stability and less responsiveness.

5mm shorter rake is probably less noticeable than 5mm longer. You'll 5mm more of toe overlap to deal with if that's an issue now.

The difference will be 5-7mm (depending on the fork I get). I'm guessing it should be too noticeable.

I believe this is correct. Only drawback is toe overlap... can easily get use to this. Stability is kinda important. I have 45mm rake on my caad, wish it was 43 - i use short stem (80mm) so less twitchiness is welcome... however, i did improve bike handling...

Give it a few hundred miles... if too twitchy, replace imo

There are two things at work here, trail and wheelbase.

Trail will increase with less rake, given a head tube angle that slopes back (i.e. 73 deg).

Once you hit 90 degrees, rake makes for negative trail. Think shopping cart wheels - the wheel, pointing forward (backwards) is a 90 degree head tube angle with perhaps 1/2" rake. You push the cart forward and the wheel spins to its "correct" direction, which would be 1/2" negative rake.

Which brings to point, if you have a negative head tube angle, you need negative rake.

Wheelbase - a shorter rake will give you a shorter wheelbase. Wheelbase doesn't necessarily make it more or less stable. However, a longer wheelbase make the bike slower to steer. Think tandem versus BMX bike.

Shorter rake will make the wheelbase shorter. It will increase trail and therefore stability; this means the bike will change direction slower. It will increase toeclip overlap.

cdr

OK. I think I've got it. Less offset <> less angle. Less offset is more trail is more stable.

Too much trail can lead to a bike that won't initiate turns at speed or easily get pulled back out of turns.

My old school bomber mountain bike is that way but Lord is it fun in a straight line!

More trail increases the lever arm that keeps the front wheel straight and hence the force required to turn the bike. Turning the bar also lowers your center of gravity slightly. More trail increases the effect and hence the force necessary to straighten the bars back up. Less offset is also a shorter wheelbase. A shorter wheelbase can improve low speed maneuverability.

Overall, I liked what changing the fork rake from 45mm to 43mm did for my road bike. I think a 5mm change would've been too much though.

There are just too many factors to be covered in a sentence or two. Check out the chapter on frame geometry and bike stability in Zinn's Cycling Primer. https://www.velonews.com/media/Block40.pdf There's also this https://velonews.com/article/9314 with some additional comments on shimmy.

If you decide to try it, leave the steerer cut 15mm too long for a while so you can sell it easier if you want to switch back. Taking the loss would be tough, but it would be worse if the fork were left with a steerer too short to fit on anything else.