Introduction: Ski-bike ( Full Suspension )

Picture of Ski-bike ( Full Suspension )

This Ski-bike was inspired by several commercially available bikes which I lusted after but couldn't afford.

It's the second one that I've made and has only seen 3 days on snow so far.

I took the first one I made to Les Arcs in the French Alps for a week of riding which I learned a great deal from.

This has allowed me to refine the design of this second model and make it significantly lighter, and better to ride.

You might think that weight would be a non-issue since these bikes are carried up on chair-lifts etc rather than pedalled up, but light weight means less sinking in powder, and less of a struggle to lift/carry out of really deep powder when crashes occur ( much more fun than crashing my mountain bike! ).

I have already gathered much of the materials required for model 3, which will be very close in design to this version, with only minor changes to geometry and general design, but I hope to TIG weld the next one.

Step 1: Donor Bike Choice

Picture of Donor Bike Choice

OK this is a very basic mountain bike which is barely worthy of the name, but it has several useful features which lend it to our purpose.

1: Decent 6000 series alloy construction which lends itself to the 'welding' technique which I will use, and surprisingly, low weight.

Only around 20 per-cent of a typical bikes weight comes from the frame, so once we've removed all the cheap and heavy parts and equipment which we wont be using the remaining frame is happily, quite light.

2: Frame design.

The 'interrupted seat tube' design makes it much easier to negotiate chair-lifts by allowing a space for the leading edge of the approaching chair ( while waiting to ascend the lift ) to slot in under the saddle of the bike allowing you to simply rock the bike back as you and it get picked up, with one hand on the saddle of the bike, leaving your other hand free to manage the safety bar.

Once the bar is down ( bike is resting just to one side of your legs ) the saddle can be allowed to tilt back forward to hook over the safety bar leaving both hands free ( handy for taking pictures/adjusting clothing/making inflammatory gestures to friends on the hill )

Step 2: Design and Geometry

Picture of Design and Geometry

The bike is going to undergo substantial changes, so to make sure we preserve the original head angle, and have a workable suspension design it's necessary to work out where everything is going to go, and what we need to manufacture to end up with everything where we need it.

The leverage ratios need to be worked out at this point too, so that you can make the most of the shock you plan to use.

I used photoshop to take the photo of the original bike, 'fade' it out using the transparency slider, then make another 'layer' using the pic of the stripped frame on top at full opacity.

I then simply printed out a few copies to draw directly on to which made working out the dimensions of the new swing-arm etc easier as I had a frame of reference to work from.

Step 3: Shock Mount Positioning, and Swing Arm

Picture of Shock Mount Positioning, and Swing Arm

Once we have a fairly concrete idea where we want everything to be we can take measurements from the design drawings and scale them back up to full size.

To preserve the space under the saddle for the lift chair, the shock mount needs to be moved down from it's original position which requires us to make a new mount.

I used a shock mount cut from another cheap, broken kids MTB suspension frame ( the red piece ).

I left a portion of the frame attached to help cope with the forces from the shock trying to shove the mount forward.

Lacking proper machining/mitering tools, I roughed things out with a small angle grinder, then finished the 'machining' by taping 'Wet & Dry" abrasive paper to a tube and simply rubbing the shock mount on it until I had a perfect fit.

You can see the planned positioning in the first pic', and the final fitted position in the second.

Although it's a very slow method, and leaves a few blisters, this does achieve a great fit and finish, and I employed similar techniques to manufacture the part which became the front ski mount.

Step 4: Positioning the Shock, Swingarm, 2nd Shock Mount and Rear Ski Pivot

Picture of Positioning the Shock, Swingarm, 2nd Shock Mount and Rear Ski Pivot

Once the frame end shock mount position is finalised we can work on the swingarm.

Once again I used a piece of tubing from the donor kids MTB ( silver piece )

I re-purposed the bottom bracket shell of the kids bike as the main pivot, and the main down tube ( flipped upside down ) as the new swingarm.

Why did I bother to make a new one instead of just using the originals?

Since the frame no longer needs to accommodate a wheel/tyre between two chainstays, I could simplify the structure and construction to suit my very limited tool set.

I could also choose a shorter length for the swingarm to avoid the rear ski sticking way out behind.

The underside ( now on top ) also had a convenient flat spot which allowed me to easily position/secure the second shock mount ( also rescued from the kids MTB ).

A front wheel hub can be used as the pivot for the rear ski ( 3rd pic )

I used a specialized 'skraxle' hub as the removable axle provides a quick and convenient way of removing the ski for transport etc.

I removed the spoke flanges with a handheld electric drill fitted with a cheap hole saw.

The shape to accommodate the hub was also cut into the swingarm from either side using the hole-saw.

Step 5: Shock, Foot Platforms, Rear Ski Bracket,

Picture of Shock, Foot Platforms, Rear Ski Bracket,

Here's a close-up of the main pivot and rear shock mount ( pre attachment )

The shock is an old FOX shock which I got cheap off e-bay.

The 'foot platforms' are made from a pair of old 'bear trap' pedals, some tubing to fit through the bottom bracket, and an old downhill through-axle to secure everything.

They can be allowed to turn under your foot, or be locked up tight by tightening the downhill axle, according to preference.

The rear ski bracket is cut from sheet steel by hacksaw and the hole-saw mentioned earlier.

I tried to make it from thick alloy, but lacking proper welding equipment I had to bend the alloy sheet rather than weld pieces together.

Sadly, the lack of resources to anneal the thick alloy meant that it kept cracking when bent, so I went back to thin sheet steel ( which would probably bend/collapse in use, more on that later )

Step 6: Welding?...well, Sort of :-/

Picture of Welding?...well, Sort of :-/

TIG welding is the way to go with alloy bike frames, but since the closest thing I own is a butane torch I had to settle for some Durafix alloy soldering/brazing sticks.

Although I'm reasonably happy with the results, I can't really confidently recommend this solution as I'm doubtful that it provides the strongest possible result.

Why? well the parent alloy has to be heated up uncomfortably close to it's melting point, which will leave heat treated alloy slightly weaker/softer than it was.

Having said that, this prototype has held up to some pretty harsh abuse including hitting a snow fence at speed, and I've seen some remarkable promo vid's of destructive 'testing' on this stuff, so maybe it's fine, I'm still going to rent a TIG rig for the next Ski-bike though, as the Durafix is pretty difficult to use on complex shapes like this, it's a lot harder than brazing steel for instance ( flat pieces like the rear shock mount - 5th pic - are fairly easy though)

BTW, the first pic' shows the forks ( Rock Shox Boxxer world cups ) chosen to provide the greatest length possible ( tall upper crown too ) to reduce the need for a tall 'tower' to connect the fork to the front ski.

My first Ski-bike has shorter forks, a slightly taller front mount, and suffers from a very slight 'pendulum' effect at the front which I wanted to eliminate this time round.

Step 7: Reinforcing the Rear Ski-mount, Painting, Deciding on Exact Height for Front Mount.

Picture of Reinforcing the Rear Ski-mount, Painting, Deciding on Exact Height for Front Mount.

Since the rear ski-mount is thin steel and might be at risk of bending under heavy side loading whilst performing a quick 'hockey-stop' or edging down really steep icy slopes It's best to provide some more reinforcement.

I had to go to a local machine shop to get the fillet brazing done, I hope to TIG weld the next version from alloy.

Painting was done with automotive aerosols ( warm these in a tub of water prior to use, stay well back from the surface you are painting, clean surfaces with isopropyl alcohol or similar prior to painting ) which don't stick to alloy surfaces particularly well so I had the swingarm sand-blasted first to provide an excellent 'key' for the paint.

An alternative is to find a paintshop which does powder coating.

The final shot shows the general lay-out as I calculated the shortest height I could make the front ski-mount whilst preserving the head angle and bottom bracket height I wanted.

This is also an opportunity to check that your chosen ski's ( ski-boards for me this time, 85cm long ) will fit the 'wheel-base' of the bike.

Step 8: Front Ski Mount

Picture of Front Ski Mount

First make certain that you've calculated the height that you want to try, whilst bearing in mind whether you'd like to mount the ski's 'rear of centre' ( where most ski-boards or ski-blades will have their bindings mounted ) or centrally.

I went for a slight rearward position but allowed for a second position by making a second set of mounting holes on the plate, you could allow for even more positions if you're not confident of finding the right spot first time.

Another broken kids MTB provided the tubes which I shaped again with the small angle grinder and finished with the 'Wet & Dry' technique where the abrasive paper is taped down to the flat ( or curved ) surface, and the piece to be shaped is simply rubbed back and forth on increasingly fine grades until the desired shape and finish is achieved.

Step 9: Finishing the Front Mount, Adding 'stabilising Spring'

Picture of Finishing the Front Mount, Adding 'stabilising Spring'

Again, I 'brazed' the front mount.

The small black bolts near the plate attach to another piece of square section tubing inside the bottom of the mount to provide some extra security by bolting to the plate with more flush bolts underneath ( just can't bring myself to trust that brazing unassisted ).

The brazing is then all carried out after all the bolts are secured.

The second pic details a small piece I manufactured to allow the attachment of a 'spring' designed to keep the ski's in a fairly flat position when you catch some air.

Without this the front ski would tilt down in the air and 'submarine' on impact with unpleasant ( but possibly hilarious ) results.

The flat spring ( white Y shaped thing in pic 4 ) is cut from an old broken snowboard.

A bit of advice here:

When cutting up snowboards don't bother with a jig-saw like I did, use the angle grinder, as the P-Tex used in the construction of the snowboard is absolute MURDER on blades.

I finished the shaping with a router mounted upside down in a vice ( dangerous and risky approach, but I took great care and as a result only lost two fingers - WIN ! )

The rear ski has a similar system but much simpler bungee cord approach to hold up the tip of the ski.

Both ski's are set slightly 'tip up' from a neutral flat position, but the flexible nature of the springs mean that even the weight of the bike is almost enough to flatten them whilst stood up.

Step 10: The Finished Article

Picture of The Finished Article

Finish up with the highest rise bars you can find to compensate for the low height of the Ski-bike compared to a conventional bike ( the mounts should be quite a lot lower than the hubs of a convention MTB wheel ).

On the previous bike the shorter single crown forks necessitated the use of a slightly taller mount, and BMX handlebars to reach a decent height.

Use a non quick release seat post clamp to prevent having to fill in those pesky "my ski-bike dropped on to another skier from the chair lift" insurance claim forms.

One day I hope to visit Winter Park Ski Resort in Colorado, as they host a Ski-bike festival which looks like it would be great.

Maybe once I've finished building the next one.... in about ten years.

I hope that this meets at least some of the criteria for an instructable, I'll try to do a better job of documenting/writing the next one!

Comments

big red robot 1234 (author)2017-01-14

how expensive is this project?

Xandaa (author)2015-12-11

Hi!

Great snowbike instructable! Im going to make a similar one this christmas. I was wondering how you worked out the geometry of the new swingarm for the rear ski.

Did it just come with the salvaged part of the small mtb, of did you calculate the movement of the ski vs. the compression of the shockabsorber? What would you think be an ideal ratio between those?

Also, i didnt quite get how you used an old snowboard to make al leafspring to maintain a proper angle for the front ski during jumps. How did you connect this, and woundnt it be easier to put a torsionspring in the hub?

I hope i don't bother you with too many questions, the project looks awesome and i cant wait to get started!

Thanks

gradjevinarko (author)2015-01-03

I like your instructables very much! I am doing a similar build, so my question is are rear skibase too short maybe? How does bike handle? I am still deciding about rear swing arm lenght...

Le Boeuf (author)gradjevinarko2015-01-03

Thanks!
I'm glad you like my project, good luck with yours :-)

Swingarm length is a balancing act between:
Long:
Helps stability, allows rider to move fore and aft on the bike to control centre of gravity rearwards/forwards over big bumps/jumps/deep powder, allows the use of longer ski's), makes the use of tow lifts slightly easier (especially steep ones)

And short:
Easier to swing rear end side to side for speed/direction control (very important!), makes for a more compact bike, makes raising front of bike into 'manual' easier.

BUT!
Long also:
Makes it harder to 'swing' the rear end side to side for speed/direction control.

Short also:
Requires rider to shift position WAY forward while using tow lifts to avoid getting dumped off the back of the bike.

So, like I said, choosing swingarm length is a balancing act. ( also remember that you'll have to work this around leverage ratios ect ).

Don't let me put you off though, this is a lot easier than designing pedal/motor bike suspension where you also have to take account of chain tension feedback.

The bike here uses a 13" centre to centre swingarm, which if translated to a wheeled bike would result in a bike which would want to tip over the rear axle at the slightest provocation, resulting in a very 'light' front end, a problem I'll address in a moment.

With a ski bike we can make use of a short swing arm to place the rear ski more beneath our feet, than behind them, which helps get the rear end moving sideways easily (which we need)
To avoid the front 'tipping up' too easily we can artifically move the virtual rear axle rearwards by introducing a mechanism to restrict the rotation of the rear ski which effectively makes the swingarm/chainstay length extend from the pivot point at the front, all the way to the rear tip of the ski.
I've used a bungee between the front of the rear ski, and the swingarm, this gradually tightens as I raise the front of the bike, which results in the effective chainstay length changing from short while the front ski is weighted, to long when I shift my weight far enough back.
If the bungee is strong enough (currently too weak and stretchy), this helps to prevent the rider flipping over the back when weight is shifted rearwards.

As we needy to be able to shift our weight back and loft the front ski in certain siruations, the rear ski either needs to be fixed (not good), placed far behind C.O.G (not good) or have some sort of limiting mechanism such as a spring, or the bungee I've used.

I think I'm going with a 14" c to c swingarm on the next model, with a stronger bungee as I find the front coming up a little too easily for good control.

Be sure to show us your results, and what you think :-)
(sorry 'bout the length of this post :-/ )

Triggerhappy101 (author)2014-08-19

Great idea!

Are those Rockshox BOXXER forks your using? I've got some of them on my downhill ^_^

Tahquitz (author)2014-08-04

Nice project. Any reason you maintained the original height and didn't bring the COG lower?

Le Boeuf (author)Tahquitz2014-08-05

Hi Tahquitz, I did lower the centre of gravity.

The bottom bracket is lower than it was as a bike.

I've kept it just high enough that at full compression your feet aren't dragging too deep ( an inch or two above the plane of the ski base )

If the shock pressure is left too low the bike wallows in the turns when you try to push through with your legs.

Traversing horizontally across bumpy terrain while the shock pressure is very low also results in your uphill foot dragging and slowing you down.

Neither of these things are an issue when the shock is set up correctly though.

Lowering the general stance also contributes to the need for long forks, and high rise bars.

Tahquitz (author)Le Boeuf2014-08-05

Thanks! I missed the foot dragging angle! I've only skibiked a few times.

Nice work. Looking forward to the next iteration.

veseli (author)2014-08-04

wild idea thanks you

FluffyMallow89 (author)2014-08-03

Wild idea, and I have a feeling it works pretty well.

andrea biffi (author)2014-08-03

WOOW! Impressive! I want to test it!

wyldestyle (author)2014-08-03

superb job, great instructions looks like lots of work but the finished product is great... maybe I rent one for the day next season....

3366carlos (author)2014-08-03

excellent job. I am not a cold weather person, so i prefer regular mountain biking, but your project is outstanding.

Kiteman (author)2014-08-03

This is great - it would be cool if you had shots or video of somebody riding one of your bikes.

tominjose (author)Kiteman2014-08-03

yeah really!! please upload an video

Le Boeuf (author)tominjose2014-08-03

Sorry folks, I don't own a camera suitable for filming ski-biking, so the few video's I do have are appallingly badly shot and in desperately bad visibility :-(

To give an idea of what ski-biking is actually like though there's a few vid's on you-tube.

This one is nicely representative of what it's like at a decent resort with decent weather:

I don't know these guys, and they're riding bolt-on MTB conversion kit's but it's the best video I can find of people who look to be at a similar level of riding that I've experienced myself ( I too can manage small jumps, snaking through trees...and crashing )

Conditions/riding experience in this vid (Ukraine?) looks to be even better than I experienced in the Alps though, and certainly better than most of my experience of Scottish snow!

I long to visit Winter Park!

This one from Durango is much tamer to watch, but they're riding bikes more like my one, and a bit of patience will reward you with a good impression of what people with moderate experience ( a week or two? ) can expect to be doing on a Ski-bike ( it really is that quick to learn, honest )

A man with stones, and broken bones:

crank_girl (author)2014-08-03

Really really very good! :)

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