Forks sizes explained?
 

I am searching for a steel replacement for my al fork. The current one is described as a 1 1/8 threaded with 65 CM travel. I've figure it out that that 65 cm is the totality of the length of the fork. ON some new forks i see described an axel to crown measurement (ie 376 mm) and then a steerer measurement (300 MM). Am i correct in assuming these two combined would be close to the original equipment size?

Is this a suspension fork for a mountain bike? If so the "travel", meaning how far the fork legs will move, is 65 mm not 65 cm.

I'm with Hillrider. Forks are rarely sized by the blades except to say what wheels they're for, ie 700c, and sometimes a brake hole to rim distance is offered. Other than that usually only the steerer diameter and length is spec.'d, plus thread length if threaded.

65cm doesn't correspond to fork related dimension, and the word travel implies a suspension fork, so I'd guess there's a typo and it's a 6.5cm, or 65mm (approx 2-1/2") travel fork, probably for 26" mtb, but possibly for a 700c hybrid.

Yep, something is messed up in the specs you read. Besides, 65cm is only 25.5 inches length. That's not anywhere near long enough to be an overall length.

To get back to your fork replacement measurements...... There's two imporant sizes to be concerned with. First is to measure your present steerer length from the upper base of the fork crown to the top of your stem. That, less about 2 mm, is the absolute minimum steer tube length you need if buying a used fork. Do NOT be tempted to go with a shorter one unless you're SURE you will be comfy removing some spacers and leaning forward more. The second one is the crown to dropout measurement. Measure your present fork from the top of the crown where the lower bearing race is pressed on down to the center of the wheel axle. You do not want to deviate more than about 10 to 15 mm from that measurement or funky things will begin to start happening with your steering. Mountain bike frames are designed to work with a particular length of fork leg. The more the travel the longer the leg. But if your frame is not intended to work with that length of leg then the steering geometry will be affected. Typically folks want to go to a longer travel fork. Doing so by more than a small amount will produce a tendency to want to flop into a turn at slow speeds and stand the bike up and a slow to turn effect at higher speeds. To avoid all this you want to stick to the overall travel and length of fork that the frame was designed to handle.

Fork measurements
 

Thanks guys for the explanations. Yes I did goof on my bike measurement..it is 65mm not CM.
I have a "comfort" bike and want to switch out the current fork for a rigid steel one so I can install an electric conversion kit to my front wheel. ( yes the front is where i want it).
BTW is there a way to affix a threadless fork when the current one is threaded??

Yes. You will need a new threadless headset and stem along with the new fork. This conversion is commonly done and any shop can supply the correct parts.

If you want to replace a suspension fork with a rigid one, you have to buy one specifically made for that purpose. Suspension forks are taller to allow for the travel, and the frames are built around that. If you replace your 65mm travel fork with a standard fork, the front of the frame will be lowered by that amount.

You'll need to replace your fork with one what has a similar axle to fork crown distance, with the cantilever brake braze-ons at the right height for your 700c or 26" wheel. Most of these are for threadless headsets, so you'll need to replace the headset and h-bar stem also. Viscious Cycles is one maker of these types of forks and their site describes how to measure and get the right fork.

The travel on a 65mm suspension fork is small enough to not worry about the need for a suspension corrected fork. As long as the fork matches the wheel, the frame geometry changes won't be enough to worry about.

It isn't only a matter of steering geometry.

A standard rigid fork will be roughly 3" shorter axle to crown than a comparable 65mm travel suspension fork. That would drop the entire front end of the bike 3", changing not only the steering geometry by about 4°, but also the handlebar height, which can be an issue for many people.

It'll also lower pedal to ground clearance by 1" or so. And will move the saddle forward with respect to the bottom bracket buy about 1-1/2". So while it the changes may "not be enough to worry about" someone about to make the replace the fork, should be aware of the, possibly unanticipated, consequences. After that it's his choice.

It's 2.5" and you have to take into account the fork sag which is about 1/3 of the fork travel. That makes the effect front end height difference 1.6" and the pedals clearance would be reduced by roughly 0.5". Not a big enough difference to worry about. Since a rigid fork has to have some height for tire clearance anyway, it is likely that nothing would change at all.

To keep the frames front , Headtube, angle the same as the designers planned .
you need a fork that has blades as long as needed to maintain that height..

Fork crown Race ..the surface on the top of the fork that the lower headset bearing race sits upon,
to the center of the wheel axle is what you need to match.

>| ......... |<

Oh yea, and you probably want the brake posts to line up,
so no 559 [mtb] forks with 622 rims. 700-35 wheels

The bike would certainly feel different if the 65mm suspension fork were to be replaced with a low fitted racing fork. But a slightly higher fork such as a cylclocross or touring fork will be very close to what the suspension fork would be when the rider's weight is slightly compressing it during normal riding.

Medusa, if you're planning on an electric hub you'll want a more skookum front fork anyhow to better avoid flexing the legs due to the driving and braking forces. So a heavy duty cyclocross or touring fork or even a slightly heavier duty rigid 29'er fork would be ideal for your application. But again just watch that the fork is no longer than your old one than by maybe 10 to 15 mm's and that it's no shorter than about 20 to 30 mm than your current crown to axle length. The difference here being the allowance for the static sag that would occur when you're in the saddle. And if you can keep the difference closer than these maximums you'll ensure that your bike handling is less affected by the change.

Outside of North Cascadia, FYI .. http://en.wikipedia.org/wiki/Skookum

why don't we have one of those little smiley guys opening a can of worms? *giggle*

I gotcha with that one didn't I.... :D