This will show you one way to build yourself a Ski Bike (aka Snow Bike, Skibob, Skike, or Skicycle). It is a downhill only machine and if you like bikes a lot, then it will probably keep a stupid grin plastered on your face all the way down whatever crazy hills you choose to take it on. It is easier to ride than it looks and is quite stable at high speeds (not sure about the speeds you could achieve at a ski hill, but its worked great for average hills in Wisconsin).
There are many ways you could build one of these, this is just one way to get you started.

Build one! You won't be sorry.

Step 1: Get a Bike

Find yourself a bike that you think would be suitable. I like to keep it small so that its easier to kick with your legs or do a running start and hop on. Smaller is also generally lighter and easier to carry up a hill. In addition, if you crash, you're closer to the ground. I got this BMX that someone was just throwing away. (Seriously, it just had a blown tire and slightly crooked front wheel, wtf?)

Strip all the unneeded parts off the bike. Namely, brakes, wheels and chain.

Look for a removable link in your chain to make your life easier. If there is none, then use a chain tool to push out a rivet. Or just chop it, whatever works.

Step 2: Remove the Chainwheel

You don't need or want a chainwheel on a ski bike. I don't know if you've ever been damaged by a chainrwheel in a bike crash, but its not fun. This bike has a one piece crank, so this is about as involved as it gets to remove it. If you've got a 3 piece crankset, then you can probably just unbolt the ring(s) with a 5mm hex wrench.

Take the pedals off.

Loosen the jam nut on the non-drive side. Remember, its reverse threaded, so you've got to turn it to the right to loosen it.

Remove the bearing race (cone) the same way and pull out the bearings (hopefully they're in a retainer) and set these aside (keep em clean if you don't want to have to clean and regrease the bearings, or don't if you honestly couldn't care less about the bike. ). Slide the crankarm out through the hole and set on your bench.

Now you have to remove the other bearing race and free the chainwheel. This one is threaded normally. Hopefully you have one of these tools! If not, you can probably make a screwdriver and hammer work, or throw it in the vise and twist it off, be creative.

Put the thing back together minus the chainwheel and tighten it down. Put the crankset back on the bike the way you took it off and adjust the bearings properly. (Note: if you don't want the cranks to move too easily while you're riding and you don't care about the bike, then you can over-tighten the bearings so that it doesn't spin freely.)

You should now have a nice clean looking ski bike frame.

Step 3: Prepare Skis

Pay a visit to your local thrift store and get yourself a cheap pair of old skis. I paid $2 for this pair of Heads.

Decide how long you'd like your skis to be. I've found 24" to work quite well, but try whatever you want. Make a nice deep mark for the cut, it'll make starting the cut straight much easier. I used a steel ruler and this pick to scratch a groove into the ski.

Clamp the ski into a vise or onto a table or something to make the cut. Cutting it this way (versus clamped sideways) makes it much easier to get a nice straight cut.

Get out your trusty hack saw and run the blade gently through the groove until the cut is started nicely. Put some stuff under the tip of the ski to support it when you're near the end of the cut, it'll make it cleaner.

Bonus: See if you can identify the layers of material in the skis by the smell! Mmmm epoxy.....

Step 4: Attach Axles to Bike

For my design, I decided to use 3/8-16 threaded rod for the axles due to cheapness and abundance of compatible hardware. Buy a ton of matching nuts and washers at Fleet Farm if you can (1.25/lb, I got twice as much stuff as I needed for like 85 cents).

Decide how long your axles need to be (allowing space for nuts and washers and pegs or whatever else you might want to attach) and cut the rod to length. You can thread a nut onto the rod and clamp it into the vise. File the ends clean after you make the cut.

Now, attach the axle to your frame. I used 2 nuts and 2 washers per dropout so that they can be tightened against each other to hold the axle in place securely.

Step 5: Cut Wood for Ski Mounts

Get yourself some wood that will fit between the dropouts of your bike (especially the front, which is generally narrower spacing) and be able to rotate freely without catching on anything. Standard 2x4 stud lumber worked for me here. Cut chunks to whatever length you deem appropriate, 6" worked for me. Cut enough so that you can stack 3 or 4 on top of each ski and then one more to bolt over the top of the axle.
NOTE: Axle height definitely affects the handling. I initially went with 3 stacked, but I think I'm going to up it to 4 pretty soon. My first bike's axle was at about 6" off the ground and it handled great. This one, at about 4.75", feels a little off, but not bad.
Sand the edges if you want, paint them if you want, etc...

Step 6: Attach Blocks to Ski

Now its time to screw the blocks onto the skis. First, you'll want to decide how far back you want your blocks. With 24" skis, I decided to put the blocks 6.25" from the back edge of the skis. This is mainly a gut feeling call. However, my design allows for the fore/aft position of the axle to be adjusted after the fact (within about a 4" window) so its not incredibly crucial.

Position the wood nice and straight under the ski and clamp the thing down. Lay out and drill the holes through the skis into the lumber. Now get out your handy countersink bit and countersink those holes! Test fit a screw every once in a while until you get the head just below the surface of the ski. Use a knife to trim off the excess melted plastic around the edges. For this step, I used standard 1.25" drywall screws.

Line up the next block on top of the first and clamp it. Drill holes for 4 screws to fasten the blocks together, making sure not to drill into the same place you put the previous set of screws. Any screw long enough to fasten just the two blocks together securely will work, I used 2.5" drywall screws. Repeat until you've got your 3 or 4 block stack completed.

Step 7: Attach Axle Clamp Block

Now you've got to attach the top block that will clamp the axle in place. This design is a little bit strange, but I decided to go with it because it allows fore/aft adjustment of the ski after you've cut and drilled and screwed everything, but mainly, it allows easily adjustable resistance to pivoting, which is great if you ever plan to depart from the ground on this thing.

You'll need one additional 2x4 block and four 5/16 x 3 lag bolts and washers per axle.

You know the drill..... Clamp the blocks in place and drill appropriately sized holes for the lag bolts. I laid out the holes to be 1" in from the cut edge and 3/4" in from the outer edge of the wood. Unclamp the blocks and re-drill the top block to a size that will allow the lag bolts to be slid easily through the hole.

If required, use a knife to carve out recesses to make room for the inner axle bolts, like I did for my front axle (shown in the pictures).

Install and tighten all of the lag bots, being careful to tighten evenly so that you don't crack the wood. Tighten one by one until the ski pivots with an acceptable level of resistance. If you like, you can over-tighten initially so that the axle gets seated nicely in the wood, and then loosen to taste.

Repeat for the rear axle!

Step 8: Finished Ski Bike!

You're done! Go test ride it if you've got any snow. Make adjustments to the seat and handlebars until it feels right.
To all who may be skeptical of the ski attachment system: I test rode mine the day after I built it on some steep lumpy cross country ski trails and it rode fabulously. The skis did not loosen up or dislodge from their clamped position around a couple fast corners and one fairly good double-up crash. But, if you'd like more security, you can easily cut grooves into the blocks that the axles can rest in and be held more securely.

Afterthoughts:
I might have liked to have cut the skis a little longer, especially after looking at a few commercial ski bikes, all of which seem to run the skis a bit longer and set up so that the tail or the front ski almost touches the tip of the rear.

I did end up raising my blocks from a 3 stack to 4 and I like the change. It handled more predictably and the crankarms didn't hit the ground if you tried to rotate the pedals around, as they did with 3 blocks. My old bike had short kids-length crankarms and it was great, these big ones are a bit awkward.

Things I might change:
I will probably toy with the location of the rear ski, try moving it forward to get a shorter skibase.

It may be a good idea to use threadlocker, lockwashers, or locknuts to hold the axle to the frame, plain nuts and washers have a tendency to come loose, so keep wrenches nearby and check often otherwise.

It's probably a good idea to seal the cut ends of the skis somehow. A layer of silicone or epoxy or something similar should do it, you just want to keep water out so that it doesn't freeze and de-laminate your skis.

I may try replacing the crankset with some form of foot pegs through the bottom bracket. Its what commercial ski bikes are doing and it would probably be easier to kick the bike along or hop on after a running start. The big pedals and cranks just kind of get in the way sometimes.

I would like to make a large padded snowmobile style seat. Allows for more comfortable seated riding, adjustable weight distribution, and will probably look cooler. Many commercial ski bikes do this as well.

I'm considering putting a 26" fork on it to chill out the handling and reduce doubling-up crashes. I'll also undoubtedly monkey with different stems and handlebars.

Other Thoughts:
If you'd like to make one of these to take to a ski area, you might want to make the mounting hardware out of steel or aluminum or something that'll more easily pass safety inspections. Modern kits have torsion springs built into the axles to keep the skis in tip-up position in the air. You'll probably need a leash for the chairlift too.

I'm probably going to make another one of these soon (for experimentation and to have one for friends) using a cheapo full suspension "mountain bike" and longer skis. I'll update on how that works out.

Good luck and happy ski biking!

Step 9: Extra Mods

I finally got around to doing some other stuff to this bike.

I was able to find and fit a 26" fork to this bike in order to reduce its tendency to double up and launch you in tight turns.

I also removed the annoying pedals and made my own foot rests out of steel angle and some 1/8" aluminum plate with 10-24 socket head screws for traction pins.

What an improvement! This thing is way more predictable now and easier to crank through tight turns without fear of getting tossed over the bars, partly due to the longer fork and partly the set back foot rests. Good stuff.