How much shorter can/should a fork be?
 

I'm looking to get a new fork and front wheel. There are two forks I have my eye on; one is essentially the exact same crown-to-axle length as my current fork (400 mm), and the other is 20 mm shorter (almost an inch). I'd slightly prefer the shorter one, but would that have a noticeable effect on the steering and fit of my bike? I mostly like the bike the way it is, so I don't want it to feel radically different.

time for you to google and spend the time figuring out trail and rake and all that fun stuff, stuff I've never taken the time to figure out cuz I never had to

but pretty important, you dont want to spend money and time and end up with an half assed job with a weird handling bike.
wish I could help. I'd be wary also of answers that you get, and educate yourself as much as possible and talk to the few folks who really know this stuff (which must be tricky to know who really knows what they are talking about and have actual experience)

As above, look up the calculation of the dimension 'trail'. If the 'trail' is changed significantly, the handling will change significantly. For this calculation you need to know the head angle of your bike and the length and 'rake' of the fork.

Generally speaking, a shorter fork will make the head tube angle steeper and this will reduce trail, making the bike quicker handling/less stable. If the shorter fork has less rake then this will lessen the effect of the steeper head angle.

My gut feeling is that a 20mm shorter fork with similar rake will noticeably change the handling of the bike. It might be perfectly acceptable, or it might not.

BH, while general recommendations w will be of help, I know from personal experience that rider preference and or individual riding experience and bike handling abilities will muddy the waters. I'm comfortable with a quick handling bike, to a limit, but I have swapped bikes with friends a few times and I found their bikes to steer like molasses, would drive me crazy, yet they found my bike way too nervous feeling and didn't like it (whereas to me that bike is stable at 8 or 80kph)

so in otherwards,I suspect unless you can ride different bikes with different geometry and compare, and you can be confident in measuring calculating all this stuff accurately, it will be tough to know what you will be comfortable with.

I do know that from experience that on the same bike, different tire sizes can change how a bike feels, so good luck with this.
tricky as you can't just order a fork and return it.... plus you have to put headset stuff on etc.

again, I'd be very wary of suggestions, and would want to get more than one cooberating answer to be sure, from very reliable sources.
seems to me, to be safe would be to replicate what's there.... given specific rider preferences.

You don’t need to look at the trail numbers to realize that dropping your front end by 2 cm will make your bike feel different. Doesn’t matter if the trail is exactly the same. You’re really changing the relationship of your body position to the behavior of the bike at this point. That’s not necessarily bad, maybe you’d prefer how it feels with the shorter fork.. If you’re happy with the fit and handling as-is and want them to remain the same, go with the fork that’s closer in length to the current one. If you want some surprise and excitement, go for the shorter fork.

Most people would change to longer forf if any change is desired. Especially on an older steep head angle bike. Few people want to go steeper.....

It also would feel like riding downhill unless you can add more bar height.

Either way, it would be a significant change. Maybe install a smaller wheel to try out.

What's the bike? Most touring bikes are more sluggish than they need to be anyway. It might be an improvement.

"Most people would change to a longer fork" sounds like a pretty general statement to me, based heavily on personal opinion. I certainly don't want sluggish handling and extra wheel flop(as in the wheel tries to flop to the side more) on my bike. That's the direction you go by adding a longer fork...

I believe it was determined along the way that every 20mm changes the head angle by 1 degree. You may or may not notice the difference, but it is good to note that it will happen.

A fork that's 2 cm shorter might be designed for use with a racing bike, which would likely mean greatly reduced tire clearance.

The seat tube angle will also get steeper with a shorter fork, which may mean you'll need a seatpost with more setback to compensate.

One option is to draw your bike in a program like Rattle CAD, Bike CAD, or even Sketchup. You don't need to get fancy. A simple line drawing would work. After drawing the bike as is, swap out for the shorter fork and measure the difference. Only you'll know if that difference is good or bad.

A 2cm shorter fork will result in approximately a 2 degree steeper head tube and seat tube angle. Also BB height will decrease by a bit less than 1 cm. If the subject bike already has a low BB height, then you will likely experience occasional pedal strike if you pedal through fast, leaning turns.

You can probably still ride the bike this way if you simply coast through any leaning turns.

You can measure your frame tube angles accurately with an angle-measuring app on your phone (with same size tires and bike on flat surface). I recommend "Angle Meter Pro" app - it will measure to 0.1 degrees. Using this tool, you can accurately assess before and after condition.

My usual advice is stay within 1cm of original axle-to-crown distance for fork swap and the effects will be fairly insignificant.

seeker (and others)--arm chair spectator here ----along with fork length, wouldnt also the shape at the bottom of the fork change things also, ie the wheelbase and angles?
I'm thinking a straight fork vs one with a forward curve at the end, wouldnt both have the same technical " length" , but behave diff because of where the front wheel axle will be?

Fork length and fork offset (rake) are considered driving dimensions. The designer determines both dimensions and the builder builds to those dimensions. It doesn't matter if the fork blades are curved or straight so long as they produce the right length and offset. However if two forks have the same length but different offsets, then yes, they will make the steering feel differently. That steering behavior is measured as trail and wheel flop factor. There's plenty of info on the Web about those topics.

thanks for the general explanation.
I guess in relation to what this fellow is asking, one of my bikes, my more tough, expedition type tourer, was originally a sort of mtb design , surly troll, and even in its dropbar config, and loaded a good amount, I love it's steering characteristics. The designers did a great job with it handling reasonably quickly, but is nice and stable at any speed.
my point here is that if I were to change a fork, I wouldn't want its steering and handling to change, as it's clearly one of the factors i enjoy riding this bike, so I hope this fellow can retain how his bike feels after the forkchange.

What about compensating with a smaller rear wheel?

Advantage: center of gravity is lower and more standover clearance.
Disadvantage: BB and pedals may hit obstacles.

No good.
Post up your frame model, and the lads will post a dozen or more fork choices for you.

Shorter is bottom of the barrel bad,

How about 20 mm more fork?

17 for bad idea - 0 for yes go for the shorter fork. hmmm