SkiBike Build - Forks & Geometry

Posted: Sunday, 14 December 2014 by Waynemarlow in Labels: ,

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.

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