Shortening front forks.

Professional racers use a lot of upper body strength to countersteer
their racers down to maximum lean angle in a minimum amount of time.

This is called "flicking it into the corner" and it requires a lot of
confidence in available tire grip.

One racer ruefully remarked, "I flicked the bitch and the bitch
flicked me!" after he tried this on cold tires.

You'll see professional racers sticking a knee out in the direction of
the curve they are about to negotiate, but they have to put their
weight on the outside footpeg to accomplish this.

A Finnish GP racer named Saarinen started hanging his knee out to keep
the motorcycle from sliding out back in the early 1970's and the young
Kenny Roberts started knee dragging in the same period.

Roberts was wearing out his rear tires from spinning them under power
while his mentor the Austrailian Kel Carrauthers was wearing out his
front tires from using the to steer the bike with countersteering.

Gary Nixon rode his racers like Mike Hailwood, his body was always
straight in line with the motorcycle, at whatever lean angle. This
technique was called "ear 'oling" which was to say, "Get yer ear 'ole
as close to the ground as you can."

I used to ride with an "ear 'oler" and it made me nauseous to watch
him leaning over to maximum angles without stick a knee out to catch a
slide.

Nowadays the tall, long-armed ben Spies has been seen dragging his
elbows as lean angles reach 60 degrees from the vertical in MotoGP and
World Superbike races.

Motorcycle tire engineers were challenged by the ultra-high speeds
encountered in qualifying on the Daytona Speedway in the early 1970's.
Qualifying involved taking a lap around the tri-oval that NASCAR stock
cars raced on, it did NOT involve diving down into the infield and
negotiating those tight corners at all.

More than one rider made the mistake of failing to peel off for Turn 1
at the start of the actual race, they would continue to head straight
for the banking where they'd qualified.

Anyway, as speeds surpassed 150 mph and reach 175 mph (Agostini
crashed at 175 in 1973) the rear tires would get very hot and they
would lose chunks of rubber.

So rear tires in particular were designed to be wider, with less rain
grooving (or none at all, in the case of a slick tire). The wider
tires had more mass so it would take longer for them to heat up to the
point where the extender oil in the tread would start coming to the
surface and making the tire greasy.

In the early 1980's the tire engineers started designing smaller
diameter front tires as well. The riders needed their motorcycles to
flick in and out of the haybale chicane at the end of the back
straight at Daytona.

Smaller tires worked better for flicking and the engineers would make
them even smaller and narrower if the front tire's rubber could
contain the heat of cornering and braking.

And then it's necessary to consider what the amateur racer or
sportrider is likely to do to a machine that has been set up and
equipped according to the recommendation of experts.

Back in 1986, it was a very common practice for a newbie racer to
install a rear tire that was over an inch wider than the original
equipment tire.

The resulting machine would be harder to turn and the front tire would
have to work harder to deal with the unbalanced traction (front to
rear) while cornering.

The rear tire would have extra traction, and the newbie racer would
lean the machine into a corner, and add a little power, which would
widen his line around the curve.

So he would countersteer a little more to tighten his line. This would
slow the machine down.

So he would add a little more power and countersteer a little more and
add a little power and countersteer a little more until the front tire
suddenly washed out and he crashed.

Then he wonder why he crashed. It was hard to convince a newbie that a
motorcycle that came with a 140mm wide rear tire wasn't going to
behave the same way with a 170mm rear tire on it.

 

Well, an Earles fork with rear suspension units is not all that bad an
idea, if you're interested in suspension compliance.

The shock absorber shafts are small diameter, so there's little
stiction compared to a strut type fork.

Problem with an Earles fork is that there is a lot of mass in front of
the steering pivot, so the front end will waggle and hunt on rough
pavement.

I rode a friend's 250cc Greeves MX-5 in the Mojave desert a few times.
It had too much weight on the front tire and the front end waggled and
hunted in the dry sandwashes.

I couldn't go fast enough to get the front tire up on top of the sand,
so the Greeves wallowed along like a pig.

Another Earles fork-equipped machine in those days was the popular
Sachs 125.

Problem was that the front brake drum was anchored to the Earles fork
swing arm, and if the brake was applied it affected suspension
compliance.

Not good, having the brake make the front tire skid early.

The next motorcycle my friend bought bought was a pretty Ossa Stiletto
with conventional forks and much less weight on the front tire. It
handled more like a Yamaha Enduro model like I was riding.

But, by that time, the Japanese had caught onto lightweight
motocrossers with long travel suspensions and those machines could
skitter across the whoop-de-doos like they weren't even there...

 

OK, I watched a couple of episodes on chopper building on Sky - rear tyres
about the same width as diameter and front tyres more normal.
Never did see what happened on corners as the obviously couldn't corner ;-)

My personal experience is on much narrower and around 1" difference in
width.
Don't forget that "proper" racing motorcycle tyres are close to triangular
in profile for more grip when heeled over.
Yes, have experienced front wheel heading for the fence under the influence
of power in corners - assumed it was time to back off!
Ordinary road tyres won't grip successfully at racing lean angles, but
normally
there are foot rests, mufflers etc to keep one more upright.
I used to push those limits until one occassion when I had a pillion
passenger
with big boots and minimal brain - we went around the corner in a series of
bumps, straight lines and leaps..

 

Err, most of the mass is _behind_ the pivot!

Sounds like there was insufficient trail.

Too true - the front of the bike rises under hard braking as the links
roll around the pivots. Conventional forks sink under hard braking
transfering
weight from rear to front - back wheel locks

Hmmm, Euro off-roaders were ahead of the Japanese, although they tended
to be higher priced specialist bikes. (1960s-70s) Japs eventually made
them cheaper and within a decade just as good.
I wonder if the US experience came from demanding the smaller front
wheels?

 

That's USKrusty for you: thinks he knows, but doesn't.

 

The point of the Earles fork, which was invented _after_ the telescopic
fork is a) it's lighter, b) it gets over the telescopic fork problem of
binding under braking.
Older enthusiasts will remember BMW staying with Earles style forks for
years after the market demanded telescopics - they knew what they were
about.
Eventually they were forced by fashion to go to teles.

(I'm not getting into discussions about who knows what - we all know
different things and there's always something to be learned)

Regards,
Greg.P.

 

So true. And then they nicked the Hossack design (or maybe bought it?)
and everything was nice again.

 

Depends on where the spring/shock units are located, which depends on
whether it's a trailing link or a leading link set up.

The inertia of masses disposed in front of, or behind the pivot is
helpful in maintaining stability *while the front tire is firmly
planted on the pavement*.

Once grip is compromised and the fork starts oscillating, the inertia
of the masses will work against a return to stability.

That's debatable. I have no doubt that the strut portion of an link
type front suspension can be made lighter than round tubes if it's a
pressed metal welded structure or if it's made from carbon fiber like
Britten used.

b) it gets over the telescopic fork problem of binding under braking.

Yes, the link type fork does away with stiction, but having the link
motion restricted by application of the front brake is as bad or worse
than stiction.

Don't get me wrong. I love the idea of a link-type front suspension,
especially what BMW has done with the latest multi-link front end.

But if *you* want to graft a modern motorcycle front suspension onto
your scooter, you're most probably going to use telescopic struts from
a mass-produced motorcycle after acquirng the parts from a breaker.

I know what it's like to have a machine shop full of lathes and mills
and grinders, but whatever you produce in your shop is going to be one-
off and, if you break it, your motorscooter will be out of commission
until you make a new part.

 

Earles is "leading link".
Trailing link only exists on the odd motorscooter like Vespas. They dive
like hell on braking!

Sure, I like to keep mine right there!;-)
(Apparently I don't always do that but ...)

Huhh, why would the wheel start to oscillate when it's clear of the
ground???

No, my bad ... The _sprung weight_ is lighter on an Earles fork than
with telescopic forks. Sprung vs unsprung weight is the vital factor.

Err, the telescopic fork has the forces horizontally rearward during
braking.
The rake angle divides that force two forces, a) along the forks. b)
rearward.
depending on the fort angle. The rearward force binds the forks, the upward
force compresses the forks and causes dive. That dive would be much worse
if
the rearward binding force did not exist.
The Earles fork takes all the force rearward. An additional force is
rotational
around the axle - transfered vertically by the link arms to the rigid part
of the
forks and to the frame. The lifting force is approximately countered by
weight
transfer of the bike itself. (no binding)

Yes, exactly - provided I can find some that can be
reconfigured/shortened.

Certainly. However, the scooter is intended as a testbed for the motor and
as such won't travel very far. My intention is to make it so that it won't
break!