I'm refurbing an old Cannondale ST (touring) frame, but do not have the original fork.
The Frame is 25" size, and has a 72° head tube angle.

Dimensions for the original, fork 1 and fork 2 are:

Fork.......Axle to crown...Rake....Trail
Original.....385mm..........55mm...55mm
Fork !.......403mm..........50mm...58mm
Fork 2.......400mm.........44mm....64mm

Question is: are either of these acceptable, or will the increase in trail make the bike demonstrate unsatisfactory handling characteristics for loaded touring??
I cannot afford a custom fork, and I have not as of yet located an original.
Can fork blades be "adjusted" safely to achieve the original rake, or am I worrying too much about it?

The axle to crown numbers make me wonder about your trail numbers. A fork that is longer will decrease the head tube angle and increase the trail

If the head tube angle is really 72 degrees, the difference between fork 1 and the original would probably be unnoticeable. fork 2 might be a little stiff.

I would look at this with reference to an original fork, which should be around somewhere for someone to measure. Then I would draw this out on paper or CAD of some sort. It is really quite revealing to swap forks and see what happens to the head angle even with rake the same and only length from axle to the crown race seat changing. It was more change than I would have thought.

I understand the geometry and the changes that result form the axle to crown length increases.
Trail dimensions were obtained using Bikecad.ca software online, filling in the variables as I listed.

I do have another 25" complete Cannondale bike, but I do not want to purchase the other forks for test purposes.

I am still curious s to how much trail is too much?

You mention that this a touring frame.
If you're planning on any front loaded touring I'd opt for the least trail possible, down in the 50's.
Longer trail with a front load results in a pretty sluggish response.
If not front loaded, even 64mm is within reason IMO.

Good threat on effect of trail for loaded bikes here; https://www.bikeforums.net/showthread...nneur-Geometry

It is certaily possible to carefully "adjust" the rake of a steel fork if you find that the available forks dont offer the rake you need .

The reason for short trail on a touring fork is that long trail increases wheel flop, which becomes problematic with a heavy load over the front wheel. The larger, higher and further forward the load, the worse the wheel flop.

The reason for this is easy to see - the trail defines the distance between the contact point and the neutral balance point of the wheel; as it increases the balance point follows the curve of the wheel and rises. The greater the vertical distance, the more energy to be gained by flopping. The greater the load and the further the load is above the axis, the more energy to be gained from flopping*.

Obviously the problem goes away if we can reduce or remove the amount of energy to be gained when the wheel flops. The traditional method of achieving this was to reduce the trail to nearly zero. Since there's so much inertia with a loaded front end, the thing stayed stable at speed as long as it was loaded. That last bit's important - with no load a bike with zero trail is a beast.

Another plan of attack is to absorb the energy gain somewhere else, like a spring or a damper. This can be as simple as a piece of bungee cord tied between your downtube and the rear end of the front rack. Simple and terribly inelegant but effective. You can buy a more elegant demountable spring version of this from the likes of Rivendell.

Another way is to reduce or reverse the vertical distance to the load - the problem is worst with handle bar bags, there exist special front racks which if used correctly can position the load so it rises when the wheel flops, thus enhancing stability and rendering the whole trail argument moot. "Used correctly" means keeping the centre of mass of the load on the rack back and low.

* The actual energy gain is due to a reduction in the vertical displacement of the centre of mass of the whole bike when the wheel flops. Since the locus of the centre of mass also changes as the wheel turns, this is not as simple as I have made it sound

Good input from all so far.
As I am concerned about the axle to crown length changing the head tube angle, I did some measurement and calculation.
The rear axle to crown race steering axis (radius of travel for the delta in blade length) is 987.4mm. That makes for a circle of 6204mm.
A 18mm arc (fork 1; 15mm for fork 2) along this circle results in a head tube angle delta of 1.04° for fork 1, and .84° for fork 2.. Correct me if I'm not doing this correctly-my math is a bit rusty.
Factoring an other 1° (it will not accept more precise dimensions) into the CAD changes things a bit.
Fork 1 now has a trail of 64mm for fork 1, and 71mm for fork 2. Bummer.
Looks like fork 3, a hi-ten unit that is 385mm axle to crown race and 50mm rake (58mm trail) will have to be the one.
It is a threaded steerer, and I was hoping for threadless. It also lacks cantilever mounts and mid-blade rack mounts.
Unless I want to "tweak" blade rake that is............

I just wanted to make sure since you didn't indicate that you understood the angle changing.
when I said fork 2 might be a little stiff, I meant that the steering would feel a little stiff. This could be disconcerting on that bike, particularly with a front load. Fork 1 should have very similar feel/handling to the original.

Thanks, unterhausen. However, in light of the 1°change in steering axis due to longer blade length, I think I'm inclined to go with the cheapie fork outlined above. That one actually will have 58mm trail.
I hate to give up the cantilevers and rack mounts,tho. Is it improper to ask custom builders to change the rake of a fork built by someone else,say, Surly??

So long as this is a steel fork (not Alu or carbon) adjusting the rake of the fork is not terribly difficult, give it a try yourself, perhaps practice first with a junk fork.
You should probably use fork #1 since it will need the lease amount of rake added. Shouldnt need much more than a bench vice and some wood blocks drilled out to clamp and hold the fork blades. Go slow in tiny steps and measure frequently.

Thanks, GrayJay. I do have some old steel forks around. I'll give it a go.
The "adjustment" will need to be of the order of 10 to 15mm of rake. Is that doable??

you can ask, but it's going to be hard to find someone that will do it. At least from the experience of people that have asked about that on this forum

Hopefully, if anyone knows someone that will re-rake forks, they'll speak up.
Cantilever and rack mount bosses complicate things a little.

Looking at the numbers you presented, there were some holes in them and I had to make some assumptions:
Wheelbase: 43”. I used the same wheelbase as my Hollands touring bike. It is big, and it could easily be longer than yours. That would obviously throw off the numbers.
Tire size (radius): 350mm. I assumed 700x35c, which is a size I use, but it too could be bigger than you plan to use.
With the above built in errors, here are numbers I get:
Trail:
Original: 55.9mm. That is longer than I now use on my touring bikes. I have found high trail produces very heavy steering with a full load. I get much better handling with 46-47mm.
Fork 1: 67.3mm. This is with a decrease in head angle of .94 degrees due to the longer fork, but assuming the 43” (1092mm) wheelbase. This is much longer than I prefer.
Fork 2: 72.6mm. This is with a .79 degree decrease in head angle due to longer fork. This is much more than I prefer.
What can you do about it? One thing you might want to consider is having the fork offset increased in any of the three forks. I have had several forks reshaped and I am very pleased with the results. Bending the fork will increase the head angle slightly. That angle change can probably only be estimated, but when I did it, it was between .3 and .5 degrees, depending on how far the fork was bent. Using my sweetspot of 46mm of trail, for the original fork, bending to an offset of 62.6mm would work, for fork 1, 67.7mm and for fork 2, 65.6mm.
These are of course estimates, based on my preferences and my assumptions. They could be way off for your particular case, but they may provide some help in sorting these things out.

Thanks, ClemY, Your numbers are not too different from mine. Per the published specs, the original wheelbase is 42.7" Wheel and tire size assumptions are correct.
The original fork actually had a rake of 2.13", or 54mm. Trail was 2.15", or 54.6mm.

Thanks,too, unterhausen. If the canti and rack bosses complicate reshaping the forks, I'm just as well off to go with the hi-ten fork that only lacks 5mm of trail from being a virtual match for the original, but lacks mid-blade mounts and has only 1 set of dropout fender bosses (same as forks 1 and 2.) I could make that one work.
If I find I like the 25" frame better than the 27" 'Dale I've been riding, I could eventually spring for a custom-built fork-unless I find an original 25" or 27" frame fork without a frame!

Do you have the original fork? If you do, and it is in good shape, it can be bent, if you desire, even though it has brake pegs. The forks I had bent all have brake pegs. If you have a "new" fork, for me, dropout bosses are more important than mid-blade mounts. I find I prefer a full front rack because I put a nice bag on it. And with the reduction in trail, having high mounted panniers works just fine. The low trail bikes handle much better than the old high trail setup.

I have an original fork on another,complete original bike. I don't want to mess with that one. No cantilevers on this one, but it does have mid-blade rack mounts.
I bought another frame without a fork, intending to use a carbon cyclocross fork until I got curious about this trail situation.
I also ride a 27" ST400 with the original fork. I do have a handlebar pack on it, but it is not too loaded. It handles fine so far, and should get better when I use the paniers on my Blackburn low-rider rack.

I didn't see you say who did this for you? This question comes up occasionally, and it would be nice to know if there is someone that will do it.

You'd think that it would be easier to find the specs on replacement forks.

Just for completeness, do you want threadless or threaded? And is it 1 1/8"?

Igleheart did my most recent fork bend for me. He also will make a fork or modify one for you.

https://igleheartcustomframesandforks.com/

Thanks for the link, ClemY !

64 mm is getting up there, as trail numbers tend to be between about 40 and 60 for a 700C road bike, generally, with the shorter end being generally better for a front load situation. 50 mm is sort of a good 'average' if you don't know what the bike will be used for.

The difference between 55 and 58 mm is not much (about 3 mm or so, I guess!), but 64 may not work well when loaded, as others have indicated.

Yes, it is probably very possible to add some rake to that #1 option fork, which would make it more like the original.

On one Trek I had a framebuilder reduce rake to increase trail. On trying to get the change reversed on the same fork, I couldn't get anyone to do it. Looking for a mod on another fork, I struck out completely. If Igleheart will do it for you, more power to you. At the time I was inquiring, nobody had suggested them.

As far as how much trail is too much, it's interesting to look at a survey of bike geometry on Dave Mann's "The Bike Geometry Project." I think you'll find very few frames offerring more trail than 62 mm, but I don't know why.

I got a local framebuilder to build me a fork with high offset (low trail) for a very reasonable price. PM and I'll send you his info. Fork is nice, but it took a long time.

The less expensive way is of course to have a fork bent, if it can be bent to provide the end result you want. If it has been bent once, many builders may be reluctant to do it again in the opposite direction.

The much more expensive route would be a new fork. Unfortunately, some builders don’t do forks alone, and some don’t build forksat all, they buy them. If you can find someone to do it, expect a price of $300+.

Thank Road Fan. I do not have IM privileges as of yet. Perhaps you could IM me,please??
Thanks

I had one made for a lot less than that. Yes, most of the established builders charge $300 and more.