Motorcycle Forum / General / Sportbikes / April 2005

There's a lot to know about "motorsickle tahrs", how they work, their
construction and compounding of the rubber that goes into them...

Back around 1966, Avon came up with their new Speedmaster series. The
guy behind the parts counter said I really should try one, because they
used a new "high hysteresis rubber". He might as well have been talking
Martian to me, I knew diddly squat about the compounding of rubber in
the black donuts that protected my rims from wearing out on the
pavement. So the counter guy says, "Oh, yes, you will notice that they
get *hot* from internal molecular friction when you're just riding
straight down the road. This molecular friction absorbs the small bimps
and gives the tire a cushy ride."

So I bought a set of Avon tires to put on my little Yamaha cafe racer.
I believed that motorcycles should have the fattest tires you could get
under the fenders to get the most traction possible. Another rider, who
raced little Ducatis at Willow Springs told me,
"Oh, you will just screw up the tracking of the bike, it won't turn
right."

Another thing I didn't know about was the difference between camber
thrust and slip angles. I was talking to a very knowledgeable guy once
about the possibity of building a 2-wheel drive motorcycle to
share the driving forces between two tires so a motorcycle wouldn't
skid so easily in wet riding conditions...

He said, "Oh, motorcycles corner by using *camber thrust* but cars
corner by using *slip angles*..." I had no idea at all of what he was
talking about...

Bridgestone made a study of what was required from a motorcycle tire in
the mid-1980's, when motorcycle radial tires were first offered.
There is a brochure available online called "Introduction to Motorcycle
Tires". If you want to google for "Bridgestone +tailwork" you can find
it...

In the brochure, Bridgestone explains about how a motorcycle tire gets
most of its cornerinf force from camber thrust, up to a lean angle of
25 years. At 25 degrees, the cornering force is balanced between camber
thrust and slip angles. Then the tire increasingly gets more and more
cornering force from slip angles all the way up to a 55 degree lean
angle...

"Slip angle" means that you are trying to go inn whatever direction you
intend, so you've pointed the tire in that direction, but the
motorcycle doesn't quite go that way, it slips at a small angle...

Like, if you were riding around Willow Springs' 450 foot radius
Turn 2 and you were going 81 mph leaned over at a 45 degree angle, for
every 100 feet you went forward, your motorcycle tire would have to go
above 25 feet towards the inside of the turn in order to go around it.
And, if the tire was slipping a little toward the outside, that would
be "slip angle"...

The Bridgestone brochure showed how their high hysteresis rubber would
distort as each contact patch rolled over pavement irregularities and
how the rubber's springiness would push against the pavement to make
the motorbike corner...

And they explained how a tire continually sacrifices its outer surface,
giving up a layer of dead rubber that couldn't grip any more and
exposing a new layer that still could grip...

Bridgestone explained that the rider could feel a cyclic working of the
rear tire sideways through the seat of his pants. They said that the
frequency was 1 to 3 cycles per second of grip, slough off dead rubber,
and regrip...

Engineers who have studied the suspension systems of motor vehicles say
that a suspension system that moves vertically at a frequency of
around 1 cycle per second makes the driver feel seasick, like he's on a
small boat at sea. Three to five cycles per second gives the feeling
that the vehicle is very controllable and responsive. But that's in the
vertical plane...

When the vertical suspension frequency exceeds 5 cycles per second the
driver or rider is very uncomfortable. The vehicle is actually painful
to ride. It may vibrate on the pavement so badly the rider's eyeballs
are jiggled by the vertical motion. He can't focus his eyes, he sees a
blur...

Now Bridgestone comes along and says that the rear tire is gripping and
releasing about three times per second, making the rear end of the bike
waggle from side to side in a *horizontal plane*...

Then Keith Code mentioned in his book "Twist of the Wrist" that a rider
would feel a certain frequency vibration through the seat of his pants
when he had the right amount of rear tire traction. When the frequency
of this vibration suddenly increased, the seat of the pants feeling
would become suddenly very smooth. Then the tire would lose its grip
and the pavement and the rider would either crash or get a scary ride
until the rear tire started gripping again...

And, all of these things that I said about the rear suspension
frequency and the rear tire are true of the front suspension and the
front tire, except that the front suspension frequency has to be about
half or 2/3rds the frequency of the rear suspension...

The front tire and the front fork springs are a package in this low
suspension frequency problem. The front tire has to have good fresh
rubber in order to vibrate at the right frequency, and it also has to
have the right air pressure to act as an air spring of the right
suspension frequency...

You may read about savvy track riders who know how to set their front
fork "sag" in order to get the best handling. They are setting the
frequency of the front suspension by adjusting the preload on the fork
springs, but they won't know that...

Most riders are probably totally ignorant of the role of suspension
frequency as regards chassis motion. The chassis will "hobbyhorse" over
cyclically repeating bumps (like expansion joints on concrete
freeways). There is a certain critical speed at which the suspension
frequency of the front suspension will interact perfectly with the rear
suspension frequency to stop hobbyhorsing...

Another point about high hysteresis tires. The total dynamic range of
elasticity of the rubber used in high perfomance tires is made wider by
mixing "extender" oils with the rubber. Motorcyclists and car drivers
will say, "Oh, bullshit! There's no oil in my gawddamned
tires!"

But there is oil in the tires. When a rider gets his tires hot on the
race track, the surface layer of rubber is sacrifices and a blue film
of oil rises to the surface. If the rider pushes his tires too hard and
gets them really hot, he might even say, "My tires feel *greasy*, I
don't have the traction I had at the beginning of this practice
session."

The rider may know nothing at all of extender oils in the high
hysteresis rubber, but he still describes the slippery sensation as
"they felt greasy"...

I remember the first Dunlop tire that I ever overheated at the race
track. I was puzzled about the blue film on the tire, which was all
greasy-looking and chewed up at the edges from running too little air
pressure. I wasn't going that fast, but the tire suffered for it...

And then there are neat freaks who wash their bikes and wash their
tires. This washing will remove extender oils from the surface of the
rubber, as will ultraviolet radiation from sitting in the sun and
baking...

Ever notice how a brand new tire smells a little bit like a skunk when
it sits in the sun? That's the smell of butyl mercaptan, a sulfur
compound used in manufacturing synthetic rubber tires. Sulfur helps the
tires to vulcanize, making the rubber stronger...

After a tire has sat in the sun and been washed many times, it doesn't
smell like a skunk anymore...

Then there are "green" race tires. The manufacturer doesn't completely
cure the rubber in race tires, he sends them to the distributors in the
soft "green" condition. The rider gets maximum traction when he heats
them up during the warm up lap, and they cure from the heat of flexing
and slipping sideways...

Riders will talk about "too many heat cycles". A few practice sessions,
some street riding, and a sprint race is more heat than those erstwhile
"green" tires can take. They get hard and they are useless...