[wdt3]

To all,

From time to time, I have seen posts by riders who have replaced the regular, curved motorcycle tires with some type of car tire that I assume has a relatively flat tread surface. I don't see how this works at all. What happens to the tire's tread surface when the bike leans. Doesn't the tire roll up onto the edge?

This started me thinking - why does leaning a bike over (at speed of course) onto the curved sides of the tread surface, cause the bike to travel in a curved path?

I have been attempting to observe whether or not, in a turn with some speed, I am causing the bike to follow a curved path by ever so slightly maintaining a deflection of the front wheel in the direction of the turn (with the handle bars) or is the lean, which rolls the tires over onto the curved side of the tread area, somehow causing the curved track.

If the curve is being maintained by the aforementioned deflection of the front, steering wheel, it is imperceptable.

When we lean a mmoving bike into a turn, the patch of tread that is in contact with the road shifts from the center of the tire out toward the edge of the tread area. Something else happens - the effective diameter of the wheel is decreased. Right? The bike is now riding on a patch of rubber that is some distance to the left or right of center, which puts it downhill from the center of the tire, hence the distance from the wheel's axle to the point where the rubber meats the road, literally, is decreased.

The dynamics of a moving motorcycle are really fascinating to me. Have any of you folks ever wondered about this? Can anyone explain the physics behind the fact that a speeding motorcycle (on standard, curved-tread tires) will cease to travel in a straight line and begin a curved track when leaned in the direction of the turn while the steering wheel is kept perfectly straight?

Bill T.

 

[Waldo]

The steering is never kept "perfectly straight" is the answer. Even when you countersteer, the bike will make the turn with the bars in the direction of the turn. You initiate the turn with pressure in the opposite direction and the bike takes over from there.

One reason the turn signals cancel.

And, yes, the tires are "effectively" of a smaller diameter. This causes "side band wear" which will show up more on the left side in right driving countries such as the US. It's roughly 20º of horizontal. E3 tires for the wing show this type of wear rather quickly, even tho the wing has a short trail.

http://www.rattlebars.com/valkfaq/tirewear

 

[INTHEWIND]

Bill,
This will explain your curiosity...

http://www.rider-ed.com/tips/motorcyclestability.htm

later..Randy

 

[MotorcycleBoy]

In the Northern Hemisphere you lean right to turn right and lean left to turn left. If you go south of the equator do you lean left to turn right and lean right to turn left???? :?

 

[Lucky Phil]

Because (for you religious folks) God said so. For me, the voices in my head tells me that it'll work. :lol:

 

[wdt3]

In the Northern Hemisphere you lean right to turn right and lean left to turn left. If you go south of the equator do you lean left to turn right and lean right to turn left???? :?

I have heard that. A good friend of mine took a long road trip that actually carried him across the equator. He wrecked in the first turn he attempted after crossing into the Southern hemisphere.

 

[wdt3]

Inthewind,

Thanks for the link on steering. It is a very good explaination of the physics.

The article reads this:

Once the bike is leaned over to give the turn radius you want, ease pressure on the bar. Motorcycle steering geometry (primarily trail again), tire profile and other factors tend to keep the bike stable in the turn. Depending on a number of factors, the motorcycle may even track through the turn with no subsequent steering input (ie it may require NO steering force in the turn to keep the bike stable through the corner).

Apparently, there are other factors that keep the bike in a curved track in a stable manner, with tire profile being one. Back to the thing that got me going down this path - wouldn't car tires on a bike screw up the turning physics?

Bill T.

 

[wdt3]

Because (for you religious folks) God said so. For me, the voices in my head tells me that it'll work. :lol:

The thing that worries me about you guys with voices in your head is that some of those voices are saying that car tires on a bike are a good thing.

Bill T.

 

[kg5ie]

Here is a cool experiment my high school physics teacher used to illustrate this. Using a bicycle tire with the tube filled with water to give the tire some weight, he added some "pegs" (type used by BMX bikers for tricks) to the ends of the axle and added bicycle grips. In effect, he had a front tire with a type of handle bars. We used a lazy susan device, that is a wooden platform with ball bearings that allows the top to rotate.

He put a student on the platform, handed them the wheel. told them to hold it out at arms length, and spun the wheel with his hands (slowly). He told them to turn right. Of course at low speed, the student did just that. Like any two wheeler at low speed the student pulled on the rigt peg, pushed on the left and the whole assembly turned, but not the student. Sample of low speed turn on a bike.

Next he used a high speed grinder with a rubber wheel attached to spin the bicycle tire up to about 60 miles per hour. Now we had gyroscopic force with the weight of the water spinning with the tire. He told the student to turn left. Not understanding counter steer and the gyroscopic force, the student pushed on the right, pulled on the left. Of course the top of the wheel leaned RIGHT, the wheel turned right, and the student took off spinning to the right. After just a few seconds of playing with the thing, the student was able to figure out that he had to "counter steer" to turn the way he wanted to go. It also let us understand gyroscopic force.

Never did know if my teacher rode a motorcycle.....I was to young to care.

 

[Cliflyri]

Re: For all you closet physicists - why does a motorcycle tu

Can anyone explain the physics behind the fact that a speeding motorcycle (on standard, curved-tread tires) will cease to travel in a straight line and begin a curved track when leaned in the direction of the turn while the steering wheel is kept perfectly straight?

Bill T.

There are several factors, one being camber thrust - the fact that motorcycle tires roll out an arc, much the same way a rolling cone does. Countersteering is to initiate the lean, camber thrust and steering deflection (varies with speed) then take care of the actual turning. The diameter of the tires at the middle is larger than along the sidewall, thus while leaning the tires roll out an arc. Just take a cone and roll it along the ground to see the effect.

Chris

 

[topster]

Simple, because it can.....................

 

[Waldo]

Countersteering is almost entirely inertial. When you "push right," the
front tire "tracks out" to the left -- while the center of gravity tries to
maintain a straight line. This forces the upper part of the bike to lean
in the opposite direction. Unlike gyroscopic forces, this works at ANY
speed -- and it doesn't require filling your tires with water.
[/size]

The above is a good plain (plane) explanation. When countersteer causes the "track out" the bike pitches opposite and the bars then, most times imperceptibly to the rider, the bars return beyond center to the direction of the turn. This is more noticeable in a bike with a long trail which is hard to "pitch back" and the steering feels heavy. With the wing, the trial is short so the "pitch back" is less noticeable and the bike is more responsive to rider input. One cannot make a turn on a bike without handlebars, tho many claim that "leaning" only will make a turn.... not so.

A good article to read on this is found here: http://www.popularmechanics.com/automot ... 68501.html

And another: http://en.wikipedia.org/wiki/Countersteering

This pic shows how the trail causes this and is illustrative of how this causes excessive tire wear on the front tires on a bike with a long trail.

 

[MotorcycleBoy]

Extreme left side tire wear????????? I looks like the right side has been worn to the extreme also. Maybe it was just way past time for that tire to be replaced.

 

[Waldo]

Extreme left side tire wear????????? I looks like the right side has been worn to the extreme also. Maybe it was just way past time for that tire to be replaced.

Indeed. Both sides wear at around 20º off horizontal, but the left side more than the right in right driving countries. It was WAY past time for that tire for sure. Not my tire tho.... a pic collected when I wrote the tire wear article.

BTW: Your hemisphere lean question - only if riding in a toilet! :lol:

 

[Cliflyri]

The rider would have to be strong enough to brute-force the bike from leaning hard left to hard right -- in a matter of a second or two.

Perhaps why Kenny Roberts in his book on motorcycle roadracing talks of working out for upper body strength and applying so much force to the handlebars that he would actually bend the handlebars out of shape. If I remember, he even talks of his pit crew using hammers to whack handlebars back into shape for him.

 

[Cliflyri]

Perhaps why Kenny Roberts in his book on motorcycle roadracing talks
of working out for upper body strength and applying so much force to
the handlebars that he would actually bend the handlebars out of shape.

That must be why Hal sells so many replacement handlebars. ...huh?

...and why the parking lot at the gym is always full of Wings,

LK

:lol: With that logic I now understand why you dont' think the front wheel of a motorcycle exhibits gyroscopic principles. :lol:

Take it in jest - it's not worth debating

 

[Uncle Al]

Read the article and you will see.
http://www.msgroup.org/TIP048.html

 

[Fred H.]

Ahh Grasshopper, you ask an honorable question. Why does motorcycle turn just cause it is leaned over?

The answer lies in the relationship between the circle and the plane. Forget about motorcycles for a minute and draw a picture of a circle rolling on a plane at 90 degrees vertical. Let the circle paint a line on the plane as it rolls along it's path. It will paint a straight line.

Now tip the circle so it is 45 degrees to the plane and rotate the circle and let it paint a line. It will now paint an arc on the plane. The more it is tipped, the tighter the arc it will paint.

I believe there is a calculus proof for this as well.

 

[Rastoff]

Nice question. I haven't read the links so, I'm sure this will seem naive...

There are many forces at work here. While gyroscopic precesion may not be the main driving force here, it certainly can't be counted out. Even so, it's not what makes the turn. Then, we have to consider the different diameters and widths of the tires. Then there is the steering.

I believe that if the tires were the same diameter and width and were perfectly aligned, i.e. the steering was held straight, the bike would still travel in a straight line even if leaned over. Therefore, I believe that in a turn we actually steer the bike by altering the alignment of the front wheel and holding it there.

I just couldn't help but put in my $.02. I return you now to those with more intelligence than me....

 

[Waldo]

Ahh Grasshopper, you ask an honorable question. Why does motorcycle turn just cause it is leaned over?

The answer lies in the relationship between the circle and the plane. Forget about motorcycles for a minute and draw a picture of a circle rolling on a plane at 90 degrees vertical. Let the circle paint a line on the plane as it rolls along it's path. It will paint a straight line.

Now tip the circle so it is 45 degrees to the plane and rotate the circle and let it paint a line. It will now paint an arc on the plane. The more it is tipped, the tighter the arc it will paint.

I believe there is a calculus proof for this as well.

I'd like to see that proof. Bikes will not turn unless the bars are turned in the direction of the turn. Put an axle in a poker chip, ink the edge and run it across a sheet of paper. Whatever angle you choose, it will draw a straight line. My dear wife draws straight lines with chalk on material for quilting with a chalk WHEEL at an angle.

Even if what you say is true - in some relativistic sense or nonsense - it is not relevant to this discussion.

Check this link - http://www.popularmechanics.com/automot ... 68501.html