SkiBike Build - Steer From The Rear

Posted: Saturday, 24 January 2015 by Waynemarlow in Labels: ,
1

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.

1 comments:

  1. Hi Wane

    Sounds a great project and I’d be really keen to do likewise

    I assume the “surfboard construction” would be hot wiring a polystyrene core and then coating with carbon & epoxy…is that correct?

    It might be good if the components can be made at the same machine shop; then they’d have a good pattern that could be re-used

    Looking forward to the next article in the series!

    Kind regards,

    Peter