SkiBike Build - Forks & Geometry

Posted: Sunday, 14 December 2014 by Waynemarlow in Labels: ,
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A primer for skibike front ends

OK lets divide the bike into parts, let’s look at the front first, certainly if we can get the front right then the rear will follow. I have copied a paragraph from Wikipedia to get the “flavour” of what does what at the front. Do look up the whole article and do a bit of “bedtime reading “ as the next little nugget I am going to divulge is extremely important to us.
Trail, or caster, is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the bike) the steering axis intersection with the ground. Most bikes have positive trail.
Trail is often cited as an important determinant of bicycle handling characteristics and is sometimes listed in bicycle manufacturers' geometry data, although Wilson and Papodopoulos argue that mechanical trail may be a more important and informative variable, although they both describe very nearly the same thing.
Trail is a function of steering axis angle, fork offset, and wheel size

Trail - a function of steering axis angle, fork offset and wheel size

So if we look then at a classic ski bike installation, in its simplest form, as drawn above. If you have done your homework, you will realise that the red line is the head angle or rake ( steering axis ) and the green line is the effective centre of the wheel, or in our case the centre of the ski.

So now let’s introduce Wiki’s paragraph on “Mechanical Trail“
Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground. It may also be referred to as normal trail.

Although the scientific understanding of bicycle steering remains incomplete, mechanical trail is certainly one of the most important variables in determining the handling characteristics of a bicycle. A higher mechanical trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable, but skilled and alert riders may have more path control if the mechanical trail is lower.

So if we now take the distance of a typical mountain bike set-up we would have about 30–50 mm of positive mechanical trail (again Wiki has a diagram to show this on varying wheel sizes). That positive mechanical trail is so important to how the bike feels and rides. If we look at the set-up above, which is exactly how most skibike conversions are, you can see that we have little or no mechanical trail. What does that mean? Yep you guessed it, that jittery feeling we so often feel in the handlebars over long flat areas and the almost instant turn when we weight the handlebars.
If you are unsure what I’m saying is correct, then as an experiment, simply turn your forks back to front and turn the handlebars 180 degrees. The fork offset is now to the rear. It does transform your bike into a cruiser with very steady steering, but that feedback you need through the steering has become so dulled as to make the skibike feel a little, well ordinary.

OK another familiar example here. Think of your average supermarket trolley, to make sure all the wheels steer and allow you to push it around the supermarket in an orderly and easy fashion, the wheels are “castered” i.e. the centre of the wheel is always behind the turning point (attachment point) of the frame, to give caster or mechanical trail. Many have the rear wheels with almost no caster or trail to avoid them clashing with your feet. Very often if you push them fast enough and then slightly turn the trolley, you will see the rear wheels “shimmy” and almost become out of control. Is this not dissimilar to many of our skibikes ?

I have long wondered why most manufacturers seem to opt for the long travel (and unnecessarily heavy and expensive) front forks and quite a high front ski attachment, coupled with a very slack head angle. Having ridden just such a skibike, the front feels really dead and not lively, almost like the ski is locked into the snow. Now that’s not a bad thing but, as budding racers we all want that instant turn and slide feeling that one gets on the likes of high powered quad bikes or when we start to throw our cars through turns with a little too much throttle, good fun and what we really should be aspiring to. So look at the drawing below.

Common skibike design - long travel forks and high front ski attachment

Here we have a very slack 68 degree head angle with a 300mm high adaptor. It may get the mechanical trail in the right place but this would not be a very nice feeling front end with a number of problems, principally the ski has moved to the rear and almost certainly would clash with the rear ski unless you move the rear ski back as well. Also the slack head angle would make the bike feel quite ponderous and slow to turn. It would work though but let’s see if we can do better.

Old fashioned trailing link suspension

A long time ago bikes had what were called trailing link suspension.They were the real first suspension units and we could take a lesson from that old design. So then what about...

Lightweight forks, very low ski adaptor, reduced trail

Looking at this, we now have our lightweight 100mm or so suspension, very low ski adaptor to prevent twist and overly weighty adaptors, about 20mm of mechanical trail and that really nice feeling 70 degree head angle. Nice.

Alternatively if you want to keep your existing ski adaptors, some and not all front forks are identical side to side, it’s just the internals which are sided. So simply turn the lower part of the fork 180 degrees so the mounting offset is to the rear. I have tried to do that on the Marzocchi forks I have and it does work well, it’s not for the faint hearted, but if you look on YouTube or some mountain bike forums, most forks are documented as how to over haul them. Do make sure you still have enough room though to prevent the front and rear ski touching each other.

Job done or food for thought? Please leave your comments in the box below or join on the Blog Facebook Group to join the discussion.

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

SkiBike Build - The Right Frame

Posted: Sunday, 7 December 2014 by Waynemarlow in Labels: ,
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Looking at my previous articles and thinking about the nuances of converting a mountain bike frame to a ski bike and the time it takes to convert an existing frame, it’s sort of obvious that one can move things on a bit in a “clean sheet” design. So let’s think about a design that will encompass a bit of the learning curve so far.

Weight

Weight, well lack of it is everything to the day’s enjoyment. Just the lifting of the skibike to and from the car, to managing it on ski lifts, to getting it back to the car after a few too many bevies in the apr├Ęs ski bar; the lighter it is the more you will enjoy your day. Some of the builds we are doing are 7 kg all up, including skis, but if you can get below 10 kg then you will be alright. To put that small 3 kg difference of weight in perspective, pick something that is a bit bulky, say 6, ½ kg bags of sugar, put them in a rucksack and walk, carrying them at mid arm’s length for 15 minutes and you will understand what I mean.

If you can get below 10 kg then you will be alright

Oddly, the heavier skibikes do feel more leisurely once actually on the ski slopes. My first skibike, which is about 11 kg, seems to have a lovely mellow feel about things, it just seems to do everything in not so much of a hurry and I think that is just inertia slowing down all the movements of the skibike. A bit of a conundrum that one, I can equate it to comparing heavy American muscle cars to Euro sports hatches; they both get there equally as fast on the road in different ways, but once you turn into the parking lot that smaller, lighter car has a lot of benefits !

Chairlift Fit

Getting a skibike to fit the chairlifts and the lift system is fundamental to getting up the mountain for a day’s skibiking. Getting clearance over the rear of the skibike to allow the chairlift to pass over it without tangling the chairlift, is there a curve where the bike can sit on the chair with the ski below, if you intend to put the skibike alongside you? Will the seat still fit under the safety bar if you sit with the skibike between your legs as some riders do? Is the top frame low enough to allow the lifts with Perspex canopies, to fully close? Can you take off a ski easily to fit the skibike into one of the small cabin style lifts? So many things to think about here, but with a bit of good design it can be done.

Can you take off a ski easily to fit into one of the small cabin style lifts?

I could name another ten requirements, but for the moment we shall just consider a basic shape that will fit most situations. Unfortunately the standard bike frame, whether as a “hard-tail” or with full suspension, is so compromised by its basic unfriendly ski lift shape, that we need to think laterally a little, but somehow still retain that “bike” feel and look which we so love.


Retain that “bike” feel and look which we so love

So let’s talk about Y shaped ski bikes. Why Y shapes you may ask ? Well that old Trek classic shape of the early 90’s has real benefits as a ski bike. Look at the following shape and you can see the clearances for the ski lift over the rear, that clearance carries forward of the foot pegs to allow the skibike to sit on the seat beside you, the scalloped top tube allows the safety bar to come down into its correct place. This curved Y shape frame fits the lift system well and has enough rider ergonomics to make it a very useful shape to play with as a starting point.

 
Skibike design - why Y shapes, you may well ask?

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