SkiBike Build - Steer From The Rear

Posted: Saturday, 24 January 2015 by Waynemarlow in Labels: ,

OK what about the rear of the bike?

Well in theory if my little primer on skibike forks and geometry is correct, then thinking about it, if we measure the distance from where the front steering axis line touches the snow to where the centre of the rear ski touches the snow, we should have a quite a distance of mechanical trail. In theory if we get the ergonomics right that fit our physique and the design also fits the ski lifts, most designs will simply trail the front.

Well that’s just got to be too easy? Sadly there’s whole lot of other influences at the back end that also have to be taken into consideration. Rider weight, weight distribution between the front and rear, can the rear end handle the sideways force and not flex too much, can the very rear arm keep the ski on its edge without twisting and flattening off the ski, can it keep the ski alignment between the front and rear ski, gosh there’s a lot going on down there on that rear arm.

There’s a lot going on down there on that rear arm

At this point I can’t stress just how much the front geometry affects the feel of the whole skibike, the rear effectively just follows the front.

So first, lets the raise the big issue, suspension or no suspension. Well I’m sort of ambivalent now having built both, based on the same fundamental design. Snow slopes are actually pretty smooth and to have suspension for the sake of suspension is probably not the reason why we do or do not need it.

Certainly the hard tails I have found are lighter and much more direct on the snow, any minimal weight movements across the pegs, seems to have real influence on where you go. The hard tail I built, to begin with, even as an advanced rider, I found a handful, with the slightest of movement, the bike would be off in that direction. It was actually a joy to ride once I got used to it, weighed 6.5 kilos and certainly a bike for advanced riders. One can easily fit a seat suspension post if you need a minimal soaking up of the bumps on the long flats, but in truth, if you stand up your legs should do the same, after all skiers don’t have mechanical suspension and they survive.

Skiers don’t have mechanical suspension and they survive

So why then do we need rear suspension? Well there is certainly an elegance about their aesthetic looks, the design I last made has been described as “SkiBike Porn” by a couple of people whilst I have been on the slopes with it, they have never ridden it, just purely on its looks. Most aircraft designers tend to say, “if it looks right, it will work right” and I have to subscribe to that, but there has to be some mechanical reasons for a bike to need suspension, not just for the “bling“ as it does put on about 2 kilos of weight to the frame.

"If it looks right, it will work right”

The very first skibike we built was a modified hard-tail mountain bike frame, and it did the job, but after 3 days of use, the owners “derrière” was so chafed and saddle sore, that he had to retire to the bar. So was that because we were learning or is there a bit more to this suspension thing? Yup, even though the slopes are relatively flat, it’s our changing of our body weight to steer the bike that does the damage. Every time we un-weight the bike to make it turn downhill and then re-weight the inside peg, our hips rotate and if you are a sitter, the “derrière” takes a bit of scuffing or if you stand up, the inside of your thighs will scuff on the saddle. I end up by the end of a week’s riding having little bruises on the inside of my thighs from the point of the saddle but then I do waggle the rear of the skibike, just like a skier short swinging down the slopes.

So chafed and saddle sore, that he had to retire to the bar

So then, what’s that got to do with rear suspension? Well that un-weighting and weighting of the skibike together with a hip rotation means that the skibike needs to have an up and down motion (let's assume the snow surface is good) to be able transfer the ski from one edge to the other edge. On the hard tail any body weight motion is directly transferred to the frame. Whereas the skibike with suspension, the rear suspension allows the bike to rise and fall, controlled by the suspension damping rate and suspension spring rate. In simple terms, that up and down weighting is slowed down giving a much mellower feel that seems to suit both the body movements we can make and how that affects the ski on the snow surface.

So then, for me, a good rear suspension is a desirable thing. But we should think not along mountain bike lines, but a simple lever dampening down our body movements to a point where the ski can transition from edge to edge at a controlled rate that suits the terrain. We seem to find a relatively high suspension spring rate with an overly damped shock absorber seems to work best and that then leads us to ask why we need long travel. I put a tell tail on my shock and seem to be only using 50 - 75mm, but then I’m not doing large jumps in the ski park more, cross country general skibiking.

But please don’t forget that the rear is only half the story and you do need to have the front suspension set up to match the rear.

Let’s build an 8 kg full suspension skibike

OK, so let’s build an 8 kg full suspension skibike. Over the next few months I’m going to publish a build from scratch, carbon bike of the same shape and form of the one below. We’ll make the plan available for you to get printed and we can then get started. It is a very very nice skibike to ride and own. You will need a donor Ali frame and some components made at a machine shop, learnt knowledge of carbon composite construction (there are so many sites now on the internet with detailed videos and books a plenty) which I’ll try and keep to the basic surfboard type of construction.

So lets get going then....

Looking for parts to build a skibike? Or perhaps one ready to ride away? See our Parts For Sale page here.

SkiBike Tour 2014-15 - Good News & Bad News

Posted: Saturday, 17 January 2015 by Mark Kinnon in Labels: , , ,

So there's good news and there's bad news...

I will start with the bad news....

To the best of my knowledge, this is proving to be the worst ski season since the Blog started in 2010. This Christmas many slopes had no snow at all at and it wasn't just the low altitude ski stations that were affected. Even in famous resorts like Morzine, the lifts remained shut and guests were sent by bus to neighbouring Avoriaz, at which few slopes were open and tickets were rationed to avoid over crowding, I can only guess how hellish that must have been.

Then on New Years Eve snow arrived in such quantity that many were stranded on change-over day for hours on end, with motorways and access roads grid locked. The huge dump promptly melted in the subsequent heat wave, leaving pistes bare or icy and off-piste non existent.

An awful lot of fresh snow is now needed in order to build a firm, cold base before the strengthening sunlight drives warmer temperatures in February. If this doesn't happen, the season will be a total write-off. At the time of writing it looks like mother nature is providing a much needed top up, but will it be enough? For these reasons I am delaying the start of my 2014-15 tour, which really annoys me, as I was particularly looking forward to rides with both novice riders and the usual suspects.

So now to the good news....

Wayne has written a couple of cracking new skibike design articles and I have finally got round to producing one myself on modifying the Alpine Skibikes conversion kit to give more stable and progressive handling. These topics remain one of the most popular according to the statistics and I will be publishing them to fill in the gaps caused by the lack of touring posts.

Alpine Skibikes conversion kit - modified for better control

A lot of new members have joined the group, including a few from the UK with planned holiday trips. There has been renewed interest in Scotland as a skibiking destination, perhaps due to the poor snow cover in regular European resorts. I will need to update the situation regarding skibike access, as I suspect things have moved on since my last visit back in 2010.

Two exciting indoor skibiking developments are in the pipeline.

I have been contacted by a UK centre that wants to make skibikes and snowscoots much more of a mainstream activity, needless to say, once all the health & safety and liability issues have been ironed out.
I have also discovered a local company that has been running weekend trips to Bottrop, Germany from London in the UK. Working with them would really open up the possibility of many more "Introduction to Skibiking" outings in 2015. This would be a fantastic opportunity for skibike virgins  to finally take the plunge and break their skibiking duck.

"Introduction to Skibiking" outings in 2015 - could be on the cards

Also for 2015, we have a shiny new European Skibike Association with a bias towards modern era freestyle skibike riding, this is a radical departure from the stuffy, traditional skibob clubs and associations. I expressed a little too much enthusiasm about this prospect and have been drafted in as a board member, mostly to look after the web site. Some cooperation between the European skibike manufacturers would really add to the momentum, so far only SledgeHammer Skibikes are involved, BullSkate, Firem VS and Avalanche Skibikes, please take note.

This website is all about the gathering together of like minded individuals to ride the slopes of both Europe and America. I am hoping there will be record breaking snowfall in the forthcoming weeks and I will have the opportunity to get in some quality riding in late February.
Till then, keep the faith.

DIY Skibike Build - Modifying Alpine Skibikes Conversion Kit

Posted: Saturday, 10 January 2015 by Mark Kinnon in Labels: ,

When the Alpine Skibikes adapter kit system was first introduced, there had never been a similar product available that could so easily transform your much loved mountain bike into a skibike.
The emerging freestyle skibike scene was heavily focussed on all mountain riding in Colorado powder and freestyle stunts in the snow park. Controlled carving on European, rock hard, groomed snow just wasn't on the agenda.

Alpine Skibikes conversion kit - unique when first introduced

The Alpine Skibikes design uses a clever concentric spring design that allows a great deal of tilt of the skis, whilst still returning them to a neutral position should you find yourself airborne. It is a neat  engineering solution, that reflects inventor Matt Hanson's aviation background.
In my experience of using these adapters, for many weeks of riding, spread over quite a few seasons, I began to appreciate that there was quite a fine balance point beyond which the front ski would tuck under and you would find yourself abruptly thrown over the handlebars. This is something that a motor cyclist would call a "high sider" or I might call the "superman" ejection.

Alpine Skibikes conversion kit - a clever concentric spring design

I was eager to find a solution to this aspect of the design, eventually I saw another Alpine Skibikes user had created a solution. It was based on adding a bicycle crank to the front bracket, attached to the end of the crank was a rubber strap which was looped around the brake arch on the front forks. This is very much a simplified version of the system used on the Lenzsport skibikes, which are noted for their stability and ease of control.

I wanted to make something similar, so my first task was to hunt through my garage scrap pile for a suitable crank piece. I found some nice box section aluminium tubing, originally part of a now defunct photographic stand. In retrospect, it is hugely over engineered, but I would rather this than have parts fail on you half way down the mountain.

Scrap pile surprise - box section aluminium tubing

Finding the right type of rubber strapping proved to required a fair bit of searching which eventually lead to Matlock brand cargo straps. These are designed to secure the curtain sides of heavy goods vehicles; any item designed to withstand that level of vibration, across a gamut of temperatures, whilst under constant tension should be up to the task.
With all the parts gathered, I spent a couple of hours mocking up the arrangement I had in mind, playing around with locations and working out the correct length of the crank. I intentionally made the bar slightly longer than needed, then drilled a selection of mounting holes to allow for plenty of adjustment for bungee length and tension.

The "dry run" - mocking up the arrangement I had in mind

The trickiest part of the process, for me, is accurately marking out and drilling the pilot holes, after that you just have to go slow and steady enlarging them to the appropriate size. You can do this by working your way up through drill sizes or use a step drill which saves considerable time swapping drill bits.

The tricky bit - marking out and drilling of the pilot holes

I never had the opportunity to test the system on snow before the start of the 2013-14 season, but I was delighted to find that the whole system worked perfectly from the start and needed little modification. The straps create a very nice progressive increase in resistance, having little effect on the flat, but saving you from going past the point of no return mid-carve and having the front ski suddenly tuck under.

The straps create a very nice progressive increase in resistance

Carving ability was taken to a whole new level by allowing you ride with a much more aggressive riding position, weight well forwards, putting downward pressure on the handlebars. It is also easier to put the skibike beyond regular tail slide drifting and into a totally sideways slide, much more like the classic skier's hockey stop. The increase in confidence created, meant I was much more likely to stand up on the foot pegs, riding in the classic "pegger" stance, rather than seated, as had been my previous dominant riding style.

Fellow Blog writer, Wayne Richards suggested that the effect was purely psychological, a case of "Emperor's New Skibike" perhaps? Towards the end of the season and after quite a lot of riding, the curved hooks on the end of the strap began to suffer from metal fatigue and straightened out enough for them to pop out on a number of occasions. As soon as this occurred, the result was to return the skibike to it's old handling characteristics, which usually resulted in a fall. As this happened on the first couple of occasions "blind" without me realising what was going on, it proved that the effect is tangible and not just perceived.

Modified Alpine Skibikes conversion - carving ability improved

For this season I have reversed the position of the rubber straps with the mounting hooks on the outside and secured them permanently with a nut, bolt and washer combination. One downside is that this makes the mounting of skis less convenient than it used to be.
In the past I would transport two skibike frames in the boot of my car and keep the Alpine Skibikes brackets mounted to the skis permanently. This made for a quick and convenient method to setting up two skibikes, which could be transported within the confines of my sub-compact car's boot. With the adapters permanently mounted to the frame, I am either going to have to up-scale my vehicle or mount a bike rack to the rear.

I am going to have to up-scale my vehicle

Even though I have sacrificed the ability to quickly mount and dismount the ski adapters, I still feel it is worth the addition effort, as this; easy, cheap and quick modification makes such a difference to the skibike's handling.
Another future modification that would be a real game changer, would be some form of quick release plate, such as those used on professional camera tripods, so that you could pop on and off skis with a quick flick of a lever.

Interestingly within days of making my modifications, a second generation of Alpine Skibikes conversion kits were launched with a similar, but greatly finessed iteration of the same layout.

Looking for parts to build a skibike? Or perhaps one ready to ride away? See our Parts For Sale page here.