Shortening front forks.

It strikes me that you are concerned with how to get the best performance
from existing suspension systems, rather than how to build the best/ most
practical suspension. (practical vs theory) That's fair enough.

One's theoretical motorcycle and rider must cope with both raised bumps
and dips in the surface. My theory says the suspension needs to be set
mid-point in it's travel with the bike normally laden at rest.
The standard of the road surface will dictate whether one wants that rest
point to be above or below the half way point. The length of movement,
vehicle speed and spring rate determines the spring design.
Preload is something to be considered when attempting to improve
an existing suspension. With zero unsprung weight (unachievable)
one would need (almost) no spring preload.
One needs the wheel to remain in direct, weighted contact with the
road surface, but the unsprung to sprung weight ratio in conjunction with
the change in spring pressure with the extension due to that movement
affects the rate at which the suspension will move. (also of course the
rate at which the bike and rider move vertically)
Coil springs are effectively torsion springs which have (almost) no
rebounce. (naturally damped) Of course, the total suspension design etc
means that dampers are required.

I remember riding on an old concrete slab road where each slab was slightly
tilted. I watched the front wheel rise and fall over each slab and settle
a tiny fraction higher each time (somewhat higher speed than legally
acceptable)
Eventually the suspension hit the top stop and I was forced to back off.
Watching the suspension rebound was also interesting!

 

Back in the late 1960's, most of the motorcycles known as
"scramblers" and "enduros" which were available to the off-road
enthusiasts in America had 19-inch front wheels.

We were amazed by the improvement in ride and precise steering offered
by the
21-inch front wheels that European motocrossers favored.

The outside diameter of the tire on the 21-inch wheel really wasn't
larger than the
outside diameter of the 19-inch wheel, but Honda caught on to the
possibility of larger diameter wheels and equipped one model with 23-
inch wheels.

That only lasted one or two model years on their XR models in the
1970's...

Don't parents give their kids rocking horse down in NZ?

When your front wheel hits a bump, it compresses the spring and
compression damping and the ratio of sprung to unsprung weight play a
role in limiting the upward movement of the handlebars.

But the handlebars are connected to the chassis and a certain moment
is transferred through the springs and rear dampers to the rear wheel
*before it hits the same bump that deflected the front wheel.

Then the rear wheel hit the same bump a fraction of a second later and
the process is reversed as the bump is transmitted back through the
chassis to the front wheel and this chassis rocking is called "hobby-
horsing".

Hydraulic dampers cannot work if there is very little motion of the
fork or shock.

In order to eliminate hobby-horsing on California freeways, one has to
carefully adjust front spring sag so it's about twice what you'd use
on the rear springs.
..

 

No, it's not magic, the optimum tire size and weight distributions of
modern sport bikes have been arrived at by trial and error and the
perceptions by experienced riders as to what feels right to them.

We saw sportbike manufacturers adopt 16-inch front wheels in the early
1980's to speed up steering response. But motorcycles equipped with 16-
inch front wheels would not recover from a front tire washout, the
riders could expect to crash.

The 17-inch front wheel has been universally adopted as the standard
for sportbikes.

But some professional GP riders like 16.5-inch wheels for the quicker
steering.

 

Most sportriders have no interest in designing a motorcycle, they just
want to get their machine's suspension system adjusted so they can
ride with their friends.

When they read the recommendations of the motorcycle magazines, which
are made by expert riders after riding around a race track that they
know very well, the recommendations will be for a race track set up
which will not be very comfortable on California's rough freeways,
where each pavement section is set at
a slightly different angle than the next.

Even in cases where the freeway was paved in one continuous pour of
concrete, which is brushed to enhance traction before it dries, the
pavement will have height irregularities.

When attempting to ride on freeways so paved with a race track
suspension set up, I encountered eyeball jiggling so bad that I
couldn't focus on distant objects.

When I backed off spring preload to the point where half the travel
was used up in sag, suddenly turning into a tight corner resulted in
severe out tracking as weight transferred to the front tire.

Race track set ups usually recommend that front spring sag be
somewhere between 25% and 33% of the total travel. I wound up with
about 40% sag as the best compromise.

Also, the Bridgestone BT-020 sport touring tire that I was using was
about one inch larger in diameter due to a taller sidewall.

I changed to a tire which more closely resembled the favored sporty
sport tire
profile and diameter to eliminate the out tracking, which was always
noticeable at
any speed.

Your front suspension was packing down because the fork oil was too
thick.

Damper rod type forks are very sensitive to fork oil weight, that's
why cartridge forks with their stacks of flexible washers to act as a
variable orifice were invented.

And the Gold Valve Emulators offered by Race Tech are a much cheaper
alternative to buying a whole new cartridge fork to replace a damper
rod fork.

Of couse the ultimate fork would be an Ohlins, with damping that's
adjustable at
both high stroking speeds and low stroking speeds.

Nevertheless, a hydraulic damping system cannot damp motion very well
if the
suspension units aren't moving quite a bit.

Then traction reverts back to the tire contact patch and the damping
caused molecular friction (aka "hysteresis") in the rubber has to do
all the work of absorbing those tiny bumps that may only be 1/4 of an
inch high.

And larger diameter tires will do a much better job than a small
diameter tire in such cases.

But, consider a go kart chassis which has no suspension at all. The go
kart driver expects to be turning constantly and he *wants* his rear
tires to be "loose" so he can turn rapidly.

Mini-bikes often have tires that are about the size of go-kart tires
and some of the smaller scooters have tires that aren't much bigger,
so they will feel "loose" at rather low speeds, compared to the speeds
that I ride my sportbike at on the highway.

 

I thought that was your 'thing'. No, come on, this being a tech forum,
we really need to know what 'moderate sized wheels' are. What diameter?
Because obviously we don't want to be unsafe, do we?

 

Utter, utter nonsense.

 

Actually, the 16" wheel problem was only a problem on bikes that had not
been designed for such a front wheel. Manufacturers just shoved them
onto bikes with little or no other chassis changes - Suzuki did, Guzzi
did... I'll bet you never rode a Guzzi T5 or Mark Four Le Mans.

On bikes designed for such wheels, they worked. GPZ900R, anyone?

And your ignorance means you don't appear to know about the
all-conquering 1990s sports bike, that came with a 16" front wheel: the
original Fireblade.

 

Ah, so we're talking about six-inch diameter wheels, now?

Rather a different kettle of fish. Not 'moderate' sized wheels, which is
what you first banged on about. So you're moving the goalposts. It
happened quicker than I expected.

 

Of course, but one needs to understand how the system of wheels,suspension
and geometry work if one wants to make real improvements.

I'm in the situation of a near blank drawing board with only rake and
wheel size fixed. Admittedly, the original suspension worked exceptionally
well, but I'm uncomfortable with it's strength. (or apparent lack of
strength for the use I intend for it)

I'm not really sure what you mean by "out-tracking"?
(perhaps the infamous separation of two cultures by language

Yes, that sounds about right.

Bear in mind that that stretch of road is the only concrete slab road
I know of in New Zealand. I never felt the need to change damping rates
elsewhere in NZ.

I'm stuck with the original wheel diameter of 12" plus 3.50 - 19"
overall diameter. That factor is fixed for authenticity.

The cart has two driven tyres on a rigid axle the ground and generally
a much smoother surface than I drive on.

As I say, my wheel diameter is fixed - I need the suspension to do the
work.

 

We went with the Europeans, but I always thought the US market was
poking itself in the eye demanding the smaller front wheels.

I certainly fell on my head from one of those - nowdays I think they
are becoming PC incorrect ;-)

It's just the different terminology - plus of course you yanks have tended
to have heavier and softer sprung m/cs which would be more prone to such
antics.

Different road conditions here

 

Of course. But what you perceive may be different to my perception
thereof. Also, I'm aiming at a much lighter bike that that which you
might consider a "normal" sports bike.

(With only one leg, I can't pick up a dropped 700lb bike. 300-400lb
would be my absolute maximum)

 

You Kiwis still keep left on the roads, don't you?

Imagine you're riding on a curvy road in the hills and you're trying
to see around a
hill that blocks your view of oncoming traffic, so you're riding near
the center dividing line to see whatever is coming at you sooner.

So you see the lorry coming around the bend and he's taking up more
than his share of the road and you countersteer by pushing the left
hand bar forward.

If the scooter initially out-tracks, the chassis follows the direction
of the front wheel, which is pointed towards the lorry for a
terrifying split second, before the
scooter begins to turn left.

Initial out-tracking is the scary part, and initial weight transfer
makes the out-tracking worse.

There can also be a persistant tendancy to out-track if the front and
rear tires are of different widths and profiles or if the front ride
heigth is too high or the rear ride height is too low.

In the case of persistent out-tracking you'll need to maintain
pressure on the handle bar to keep the scooter from widening it's line
in a curve.

An equally annoying condition is when the motorcycle doesn't out-track
in the least, but tends to fall into the inside of the curve.

Sport riders have to learn to time their countersteering input exactly
so they don't have to countersteer oppositely to keep the motorcycle
from falling into the corner, with a resulting widening of the arc,
followed by yet another input to have another attempt at establishing
the correct line.

Riders may not realize that their problem is front/rear ride height or
a problem with tire profiles and they may insist that they have their
front/rear sag set up correctly and won't want to change it.

That type of rider may also be unwilling to slide the fork stanchion
tubes up and down in the yokes.

So he continues to have to make mid-corner changes in his lines.
That's a safety problem on the street, and is annoying to racers who
are trying to pass him on the track.

There was an engineer at Michelin who came up with a set of formulae
for tire profiles and diameters and coning angles and how everything
related to rack and trail and wheelbase.

I found his name on the interweb once but have never run across his
formulae.

That's still a fair bit larger than the tire diameter I had envisioned
when you first mentioned that you wanted motorcycle forks on a
scooter.

I suggest that you go to some of the tire manufacturer's web pages and
see what they have available in that size.

A quick way to get a list of the tire manufacturer's pages is to go to
www.mawonline.com and click on "tires".

This will take you to a list of tire manufacturers and there is an URL
leading to each manufacturer.

With a smaller diameter tire, the suspension is going to have to work
harder.

A soft rubber compound on the front tire will help the suspension to
deal with the
surface roughness by absorbing the smallest bumps.

 

<snip>

As you've never actually ridden a scooter, let alone on (for you) the
'wrong' side of the road, where does this advice come from?

 

Are you aware that all the manufacturers have been building very light
sportbikes
since the mid-1990's?

A 1993 Honda CBR900RR weighs about 350 pounds without fluids and a
battery.

The Yamaha YZF-1000R1 that came out around 2000 also weighs about
350~375 dry weight.

What annoys me about the lightweight sportbikes is that they typically
have a
33-inch seat height.

I'm riding around on a 550-pound FZR1000 with a plywood seat so my
feet will reach the ground and keep me from falling over if I get
experience vertigo while stopped for a traffic light.

Go to alt.motorcycle.sportbike and start a thread titled "Ping Bob
Nixon".

He has no right leg and still rides a Suzuki SV650. Ask him how he
copes, and he will tell you.

Some of the riders on alt.motorcycle.sportbike would like to hear
about your association with John Britten, too.

 

Okay, you're closer to it than I am. I just assumed that using
non-leading forks would move the wheel rearward so the rake would have
to change. Photos seemed to suggest same, but my eyes aren't too hot,
so maybe I was wrong to think about rake.

 

Still???
You yanks still drive on the wrong side, don't you?

Hmmm, seems I always rode the wrong sorts of bikes - toss your weight the
way you want to go and the bike goes that way.
The technique probably requires that the rider has his feet under his/her
centre of gravity. It always worked for me!

Silly design for motorbikes!

That's why I'm interested in the particular model - plus there's only
a partial dip between steering head and seat rather than the usual "U"
spring frame design that comes from a flat floor.
It's a very rigid scooter. (one might say overengineered)

 

The "driving side of the road" doesn't make much difference to the
handling,
other than when one goes against the flow, when it makes a considerable
difference!
As to road surfaces, it's hard to make absolute comparisons because not
all US roads are the same and not all NZ roads are the same
However, we have half the population density of the US and the country
is generally lumpy.
Where I live is geographically near a sub-join between the Pacific and
Australian techtonic plates with the two crunching upwards.
In addition the region is made up of recent volcanic activity so the soil
is very unstable - the roads are always moving. (slowly) Result paved but
uneven surfaces, hills and valleys and regular 45-50mph bends on 60mph
roads.

 

Rake seems adequate. Triple clamps are easy to mill to set the trail.
(advantages of having CNC mill in workshop)
In fact, the axle and head would have (exactly) the same relationship
whether leading link or telescopic were used. The leading link design
appears to have been used by the designers so they could use "parts-
bin" rear suspension units rather than making new telescopic forks.
As my motor design will have vastly more power and stopping distances
have become more critical in the 50 odd years since the original design,
I think the forks strength might be suspect.

Greg.P.

 

North Island or South Island? BrianNZ is always ranting and raving
about drinking in the pubs and hooning around near New Plymouth over
on rec.motorcycles.

 

I'm in the far North (AKA Northland) All dairying and pot-growing.
New Plymouth is waaay south of here on the sides and base of a volcano
that is the spitting image of Mt.Fujiama. (ie much flatter)