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Back
to school. If there was ever a subject destined to make kids squirm, it was going to be geometry -which is perhaps why people shy away from the expression when it relates to motorcycle handling.
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| There are many aspects of a bike's shape that affect its behaviour, but the one that is most easily identifiable is the geometry of the forks. We inherited it from the bicycle and it is this that sets a Harley-Davidson apart from a Yamaha R1, a cruiser against a trail bike, a tourer against a sportster. Without straying from the special interest area of these pages, the differences can be see quite graphically between the two rising stars of the Motor Company's ranges: the V-Rod and the Firebolt. The V-Rod's forks are kicked out further than any production motorcycle before it, while the Firebolt's are tucked in tighter  than
any other bike its size has so far dared to attempt.
But what are the implications beyond the cosmetic? Well, it's really quite simple, in its basic form. The relationship between the frame and front wheel is governed by a number of factors, and because the front wheel is the steering wheel of a motorcycle, the handling of the bike is radically affected.
The
rake is NOT the angle of the forks, although in a lot - if not the
vast majority - of cases they happen to be the same, but Harley
have been playing. Not often known for ground-breaking engineering,
Harley-Davidson are more adventurous than most in playing with the
fork yokes to such an extent that the apparent rake, seen in the
more easily spotted fork legs, is not always as it appears. The
first time we were really aware of it was the 4-speed FXWG which
shared the common FL frame of the shovelhead Electra Glide, complete
with its 30-degree rake, and they built a Small differences had been seen before but the odd half a degree or so can be explained by the height of the back end off the ground compared to the front: if the back end is lower, and the front end higher, the whole bike's natural horizontal line slopes up at the front, thereby making the natural vertical line lean backwards, affecting the rake. To see that at its most visible, dig out a copy of an early-seventies bike magazine and check out the effect of sticking twelve-inch overs into an unmodified frame: it makes a British vertical twin look like the back two pots off a Honda V4 and makes you worry for the poor oil pump that has to scavenge its crucial juices from a completely different place than Edward, Bert or Val ever anticipated.
In short, the rake of a frame is a contributing factor that helps determine the speed of response of the steering: steep = quick steering, laid back = slow steering. Trail A
final example of the respective advantages of playing with the trail
of a motorcycle comes from the days when motorcycles were cheap
transport, and sidecars were good family transport. A useful trick
on a bike with leading axle forks was to ride the bike solo during
the week with the axle leading, and reverse them at weekends when
you attached the sidecar for a run out in the country. In short, the trail of a bike determines the lightness of the steering: short trail, light steering. ERRR
… What? If you're not, go to the next subheading. We all know - even if unknowingly - about castors, They are all around us: under furniture and the curse of tired shopping trolleys. A conventional castor has a shaft that pivots - like a steering head except vertical - and an offset which puts the wheel behind the pivot: net result being that the wheel follows the pivot (or at least it does as long as the pivot is rotating freely, and the plates that hold the wheel haven't been wrecked by the unexpected weight of the car park stunt team on the disabled ramps). It looks different but it is identical, just rotated through to vertical.
But we've got handlebars for steering, so what's the problem? This is where the geometry comes in. If you put a single castor with zero rake and zero trail at the front on a shopping trolley and attached a "T"-bar to it, you could make sure the wheel was facing in the right direction and resolve the problem … until you pushed the trolley ahead while you got something off the shelf at which point it would have no directional stability and would tend to steer in a direction determined by the weight loading. Put the trail back into the equation and it would tend to steer straight ahead. Every inch behind the pivot that the wheel was located would increase the tendency for the trolley to steer a straight path with or without input, but conversely it would make it harder to steer it in any other direction. It would also increase the side loading on the pivot point and any of the components that comprise the assembly. It is actually possible to produce a bike with zero trail or negative trail but you wouldn't be advised to do it on the grounds of stability as it would have as strong a tendency to turn as it would to go straight ahead if you took your hands off the bars. Very interesting but it would give you much more to do than is absolutely necessary in keeping the bloody thing in a straight line, and at even thirty miles an hour on a 600lb motorcycle, you'd be a braver man than I. You could go further and actually give yourself a negative trail. This would be the same as putting the wheel in front of the pivot on the shopping trolley and would have the same effect. As soon as you pushed the bike or trolley forwards, the wheel would attempt to follow the pivot. If you never needed to take your hands of the bars, and you had very good steering lock stops and fancied arms like Arnold Schwarzenegger without paying out for a gym membership, it might be exciting for a time and would probably look great, but it wouldn't behave well. Tony Foale's tests showed that a motorbike can survive with a very radical, almost vertical steering head retaining a standard trail, which provided better handling … but it looked 'orrible and a complete redesign of the motorized bicycle would have been required to get it past a very conservative market. Yokes
& Forklegs
Yokes are the way Harley prefers to play with geometry and they do it well. What started with a few extra degrees for the Shovelhead Wide Glide is continuing through the V-Rod which increases the already radical 34-degrees frame rake with an additional 4 degrees to give 38-degrees, and the visual impact is amazingl. You too can play with yoke angles quite easily because they are available off-the-shelf either with in-built rake - which is the more common - or adjustable yokes from the likes of Tolle with - check this out - between zero (normal) and seventeen degrees (!!) of rake - although the more raked out the yokes, the greater propensity for getting a zero or negative trail. Less
obvious is the touring frame which combines a
The
Springer fork harks back to a time when the world knew lttle of
damping, and everyone was pleased just to have something absorbing
the worst of the bumps. It had rivals, in the form of the Castle
forks fitted to Brough among others, and the Girder forks fitted
to just about everything else, but they all were superceded by the
arrival of damping. Damping came in oil form, in a tube and that
tube became the fork leg. In their enthusiasm for the past, and especially considering their reinvention of the hardtail frame's lines in the 1984 Softail, Harley-Davidson reintroduced the Springer forks in 1990, but they were spingers with a difference: they had damping. Yippeee. For
those unfamiliar with Springers - and as Harley withdrew them from
the UK market in 1998 there could be a few - they comprise a pair
of rigid, narrow gauge fork legs that run elegantly from the top
yoke, Springer geometry works exactly the same way as any other forks, although steep-looking Springers are actually a lot more laid back than appearances suggest with the leading axle configuration of the rocker set up. There are three things you need to know about new-generation Springers: they are bastards to clean; they are bloody heavy; and they work better than they have a right to. Extremes Extreme
rakes suffer from the laws of gravity. It's not that they'll bend
- or at least they shouldn't - more that they won't slide and absorb
the bumps as they should. In
Practice
Keeping a shallow custom bike angle but foreshortening the trail gives you the straight line stability and aesthetic lines, but offers easier steering: Street Custom. Keeping a tight rake but increasing the trail will keep tight steering but the increased steering effort will aid stability - Sports Tourer. It goes without saying that this is only part of the whole handling equation: there's weight, unsprung weight, springing and damping before you leave the front end and decide on hardtail or suspension, wheelbase, ground clearance, tyres etc. But then no-one said it would be easy. If
this has whetted your appetite for knowing more, I recommend that
you take a glance at Tony Foale's articles that he has reproduced
on-line, and you'll find them, complete with massive detail of practical
experiments on: http://www.ctv.es/USERS/softtech/motos/Articles/ |
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Rake
that
there are aftermarket frames out there which have a bigger than
normal headstock and provide the ability to change the angle of
the steering head as it passes through, by use of eccentric bearing
housings.
The
additional trail made the outfit much more stable and easier to
keep on the straight and narrow, at the expense of heavy steering
- although steering on such devices is usually heavily damped so
you'd probably not notice. It was a wonderful cheat until twin-leading
shoe front brakes came out because you couldn't quickly reverse
the front wheel and maintain good braking (TLS shoes become twin
trailing shoes when run backwards, as anyone who's ridden a Russian
motorcycle backwards - or a three-speed and reverse-equipped big
twin - will tell you), and now we've got discs, there are no longer
significant numbers of sidecars.
With
a shopping trolley - which is as good an example as I can give because
everything is visible - the rake is zero degrees: bolt upright.
If the wheel was mounted directly underneath the pivot it would
have a trail of zero millimeters (or inches, funnily enough). If
you pushed such a trolley forwards, there is every chance that the
wheel would be at ninety degrees to the direction of travel and
you'd go nowhere fast - the wheel would have no inclination to straighten
up to aid forward progress. Put the wheel an inch behind the pivot
and it will swing into line behind the pivot and move forwards easily
- and all without the need for steering.
The
current tendency in sportsbike terms is to have as straight a line
through the centres of the steering head and forklegs. This keeps
the trail short, making the steering lively. Because the fork legs
are likely to be parallel with the steering head, and because the
wheel spindle of such machines passes through the bottom of the
yoke, you can get the minimum rake with the least radical appearance.
The downside is that the steering has a limited lock. You can cheat,
of course. You could reverse-rake the yokes so that the forks were
at a steeper angle than the headstock, or you could reverse a leading
axle fork slider to give a trailing axle and bring the wheel spindle
behind the fork leg - the aforementioned, time-honoured trick of
the sidecar fraternity to give outfits greater straightline stability
- you don't need steering on an outfit beyond ten miles an hour
because that is almost entirely passed over to judicious use of
the throttle and brakes. 
It
is far more common to see the back end of a sports bike kicked up
in the air to reduce the rake because messing about with a couple
of well-placed shims is much cheaper than hacking at the frame with
a gas axe and protractor, and CNC lathes and lumps of billet don't
make experimentation within the budgets of the masses.
In
the cruiser market, and equally true of chops, greater stability
is preferred and so raked yokes are a quick and easy way to kick
the front end out further. Or you could triangulate the yokes more
than standard, placing the fork tubes forward of the steering head,
increasing the trail that way, or using a leading axle fork slider
to plant the wheel spindle ahead of the fork leg - a common trick
when US custom versions of the big Jap superbikes of the late seventies
were in vogue.
28-degree
rake with the most bizarre yokes and gooseneck frame. The gooseneck
is essential because the yokes are reversed. That is to say the
headstock is in front of the fork legs, although you can't see it
as it is all tucked away beneath the plasticwork of the Electra,
or the nacelle of the Road Kings. Without the gooseneck, the fork
legs would clash with the tank, and the fact that touring Harleys
have Wide Glide forks is fortunate to further aid the full lock
of the steering - although it is still less than most others in
the respective ranges as anyone who's had to manhandle one around
will testify. In the Electra's case, it is a trade-off between making
it light enough to turn lock-to-lock with the minimum effort, or
giving it a greater lock that fewer people could wrestle with.
Fork
Types
And
that would be the end of the story except BMW brought our the telelever
- which divorces the sliding of the stanchion and slider from the
springing, reducing weight and endowing it with good anti-dive properties
to boot (which I can't draw so you'll have to look at a BeeEm if
you're interested) - and Harley dragged out an old friend from retirement:
the Springer.
Damping
prevented the spring that had so generously absorbed the bump in
the road from rebounding with almost as much vigour and so was considered
- quite rightly - to be a good thing. Girders did adopt hydraulic
damping for a time, with the Girdraulic forks championed by Phil
Vincent, but the writing was on the wall.
This
is called stiction and is the point at which the sideloading forces
on the fork bushes are greater than the forces of the road bumps
so the sliders don't slide. You will also be aware that the shallower
the rake, and the longer the forks, and the greater the leverage
stresses on the headstock will be so it needs to be proportionally
stronger to cope with it. Just for a laugh, draw a cartoon chop
with massive rake and then draw lines corresponding to the rake
and the vertical line intersecting the wheel spindle: perhaps the
ideal bike for a latterday Roman given their historic passion for
the straight and narrow, but God help you if you find a corner.
Rod
look (38-degrees of fork angle) with your Custom Softail (34). It
would enable you play round the back lanes in the morning on your
SuperGlide getting the most out of the tight Dyna chassis, and then
switch it for an afternoon's run back over sweeping roads in an
altogether less hurried manner: Jeckyll and Hyde. You won't realise
the true nature of the extra 4 degrees because it isn't true rake
even if it looks like it, but the upside is that it will be generally
lighter to steer because the fake rake will be accompanied by a
reduced trail.