Why does a bike turn

[MajBach]

...cause it behaves like a carrot.

Or so Lee Parks would have you believe.

So many years ago when I was new here, I got caught up in a rather spirited discussion about why a bike leans. I got royally trounced on (and maybe deserving so) when I voiced my *erroneous* opinion.

Anyhow, the other day, I was reading Total Control by Lee Parks (trying to learn more about the science of suspension) and came across a passage about turning that after days I still cannot come to grips with. He states on page 28 "...the outside edge of the contact patch has a larger circumference than the inside edge of the contact patch. " He compares this to a Styrofoam laid on its side and pushed forward. The cup will turn due to the smaller diameter of the base of the cup.

I'm not even going to elaborate on why I think that is way wrong. But, I've been wrong before. Just wanted to hear what you guys think.

 

[Uncle Hud]

Well, I think the way Lee Parks thinks, too. As you lean over, the tire geometry initiates the turn. (Darksiders should be able to provide some insight here.) Not to fear, sir, I have been way wrong before, too, and this may be another instance.

If you're going really slow, you can turn using the handlebars while staying upright. <---- That's the way cars do it; by aligning the front tires along the desired travel path.

 

[NTXFJR]

Some of us know enough about physics and formulas to be dangerous, and some of us know little at all, I'm in that category me thinks. It seems to me that Lee is using an analogy incorrectly to arrive at a desired conclusion. For instance, a motorcycle tire has a round left to right profile, if properly inflated, it is only slightly flattened under weight onto the road surface. The contact patch location on the front tire moves to the side the bike is leaning toward, which induces more rearward force on the inside fork and less on the outside fork. I think that's how a bike can be steered simply by leaning it with no hands on the bars. That's also why instructors will say don't brake in the corners because the forward shift of the bike's cg shifts that contact patch making the bike want to stand up and therefore harder to control. If I follow what Hud is saying, a bike will change direction if you turn the front wheel in the direction you want to go just like a car, the difference is that because of such a narrow center of gravity, or single side to side balance point, a bike at speed will fall to the opposite direction of which it's being turned unless it is leaned proportionately to compensate.

 

[gapiob]

To get our licence to drive a moto, we have many theoriques item to know. One of those item, it's the physics of the bike.
Globaly they are 2 parts :
- under 15 Mph, you turn you're handlebar

- above 15 Mph, the centrifugal force keeps turning. It is therefore appropriate to move the bike's center of gravity is outside the axis of the two wheels (place or is exercised centrifugal force). This will have the effect "to attract the bike" to be the new center of gravity has to seek to rebalance.
The tire shape just keeps a contact surface that is almost always the same whatever the angel of motorcycle

 

[wil780]

Anyhow, the other day, I was reading Total Control by Lee Parks (trying to learn more about the science of suspension) and came across a passage about turning that after days I still cannot come to grips with. He states on page 28 "...the outside edge of the contact patch has a larger circumference than the inside edge of the contact patch. " He compares this to a Styrofoam laid on its side and pushed forward. The cup will turn due to the smaller diameter of the base of the cup.

I find it useful to distinguish between turning force and steering force.

Turning force is what forces the front wheel to diverge away from a straight line onto a curving path. It arises because the wheel is pointed slightly to one side of the current path, so the contact patch slips sideways slightly and produces a sideways force, the turning force.

Steering force is what turns the handlebars to deflect the front tire to one side or the other, away from where it is currently pointed.

What Lee Parks describes is often called camber thrust. It you think about it, you should be able to see that it is a pure steering force. It causes the front wheel to steer to the same side the bike is leaned towards. But it cannot turn the bike directly. It is only one of many steering forces. The balance of those steering forces is what determines whether the front tire slips sideways slightly, and which direction it slips, and ultimately which direction the bike turns. (Incidentally, it is a very powerful steering force. If you're leaned over 30 degrees and camber thrust were the only steering force, it would steer the front wheel to the side far enough to try to follow a circle with a radius of only a couple feet. Imagine how controllable the bike would be at 60 mph if that were the only steering force. lt would yank the bars right out of your hands.)

Fortunately, trail produces a steering force in the opposite direction to camber thrust. In a well set-up bike these two main steering forces balance each other, as do all the other minor steering forces, and the bike tracks around a turn without needing any steering force from the rider once it's leaned over.

This is one of the reasons a bike seems to corner in a more pleasing way than a car - it seems to go around corners naturally, whereas a car needs steering force applied to the steering wheel continuously. It's just a illusion though - a bike hides the counteracting steering forces from you. Even though I know the front tire is scrubbing sideways just as it does in a car to generate turning force, it still feels better.

 

[radiummadman]

I love it when all of you smart guys add your info. One can learn a lot by listening(or reading) as ol' Yogi Berra used to say.

thanks

 

[ionbeam]

Let me add to the contusion confusion.

Next time you are out riding and moving faster than a walking speed gently push on the left handle bar and observe 1) which way did the bike lean & 2) which way did the bike turn 3) does the bike turn because of the handle bar steers the bike in that direction?

Next time you are out riding and moving faster than a walking speed gently pull on the right handle bar and observe 1) which way did the bike lean & 2) which way did the bike turn 3) does the bike turn because of the handle bar steers the bike in that direction?

Cogitate.

 

[PhilJet09]

Cogitate.

There's a word not often used on a motorcycle forum, or anywhere else. I'll have to remember to use it more.

 

[Fred W]

Actually, you turn a bike the same way at any speed, it is just more pronounced at higher speeds.

You turn the bike by getting it leaned in the direction you want it to go. You do this by steering the front wheel in the opposite direction you want to go. Once the bike has some lean angle the front wheel wants to turn itself in the direction that it is leaned due to the steering angle trail force and gyroscopic force of the tilting wheel (try tilting a horizontally spinning gyroscope some time). That is why you have to keep pushing on the inside bar (or pulling on ionbeam's outside bar) to stay in the turn.

Even at slow speeds, if the wheel didn't turn itself (or you didn't help it) back into the turn after the initial counter-steering, the bike would just fall on its side. You could demonstrate that by locking the wheel straight forward on a bicycle. You can get it to lean one way or the other by shifting the weight but it won't turn and you will fall on your side.

 

[Donal]

Interesting article here it focusses on hanging off but does a good job of explaining steering/ counter steering IMO

 

[BanjoBoy]

...cause it behaves like a carrot.

Or so Lee Parks would have you believe.
So many years ago when I was new here, I got caught up in a rather spirited discussion about why a bike leans. I got royally trounced on (and maybe deserving so) when I voiced my *erroneous* opinion.

Anyhow, the other day, I was reading Total Control by Lee Parks (trying to learn more about the science of suspension) and came across a passage about turning that after days I still cannot come to grips with. He states on page 28 "...the outside edge of the contact patch has a larger circumference than the inside edge of the contact patch. " He compares this to a Styrofoam laid on its side and pushed forward. The cup will turn due to the smaller diameter of the base of the cup.

I'm not even going to elaborate on why I think that is way wrong. But, I've been wrong before. Just wanted to hear what you guys think.

Personally, I don't try to delve into how a bike works, I really don't care. I try to relax, feel wut the bike wants, feel wut the bike does, 'n let it do it's thing. Long before Lee Parks there wuz this feller named Kieth Code who pumped out these books. He's a riding guru. But my friend (Who had no clue as to how a bike worked.) raced against Kieth in the AMA and regularly used to spank his ***. Code would show up w/ all this fancy ****; big "Dianetics" transporter, a lavish canopy, 'n a who crew to attend his needs. My friend would show up in a broken down van, 'n a crew of hoodlums who hid in the back to get in the track for free, then this guy would beat the mighty Kieth Code, whilst have'in no clue as to how his bike worked.

On top of that, all the best racers seem to have the worst form; just watch Johnathan Rea and Tom Sykes race; Sykes has much better form, yet Rea is almost always quicker.

I have no clue as to how my anatomy works when I walk, I just walk. Same w/ ride'in; if you want to learn how the bike works, go ride!

 

[PhilJet09]

Yea, but then what we cogitate over?

 

[SLK50]

No one has yet mentioned precession ( torque free vs torque induced ), Euler angles or nutation.

Gotta love physics.

 

[N4HHE]

Some of us know enough about physics and formulas to be dangerous, and some of us know little at all, I'm in that category me thinks. It seems to me that Lee is using an analogy incorrectly to arrive at a desired conclusion. For instance, a motorcycle tire has a round left to right profile, if properly inflated, it is only slightly flattened under weight onto the road surface. The contact patch location on the front tire moves to the side the bike is leaning toward, which induces more rearward force on the inside fork and less on the outside fork. I think that's how a bike can be steered simply by leaning it with no hands on the bars. \

If what you say is true then a slight imbalance in fork spring rates left/right will cause a motorcycle to "pull" to one side when upright like an automobile with poor wheel alignment. However, we know that is not what happens. Dirtbikers commonly mix spring rates they have on hand to get a rate they do not. Some motorcycles have only one spring in one fork leg. Many do compression damping in one leg, rebound in the other.

 

[gapiob]

And coriolis force!

 

[N4HHE]

Anyhow, the other day, I was reading Total Control by Lee Parks (trying to learn more about the science of suspension) and came across a passage about turning that after days I still cannot come to grips with. He states on page 28 "...the outside edge of the contact patch has a larger circumference than the inside edge of the contact patch. " He compares this to a Styrofoam laid on its side and pushed forward. The cup will turn due to the smaller diameter of the base of the cup.

I don't doubt that plays a role, but I do doubt that is the only force in play. Front end caster (rake) affects steering on motorcycles and automobiles.

The longer outer vs inner path is the cause of tire scalloping. I can imagine how some tread patterns can be less susceptible but can't imagine how any motorcycle tire can be immune. 40 years ago the ribbed front tire which was in vogue had the same problem.

We experience the above inside/outside thing differently with different tire selections. Currently my OE Bridgestone at 3500 miles with 38 PSI tries to fall into slow speed turns when cold but not so much 5-10 miles later. Have seen lengthy discussions in other forums as to whether a tire is "neutral".

 

[Uncle Hud]

ionbeam posted: Let me add to the contusion confusion.
Next time you are out riding and moving faster than a walking speed gently push on the left handle bar and observe 1) which way did the bike lean & 2) which way did the bike turn 3) does the bike turn because of the handle bar steers the bike in that direction?

Next time you are out riding and moving faster than a walking speed gently pull on the right handle bar and observe 1) which way did the bike lean & 2) which way did the bike turn 3) does the bike turn because of the handle bar steers the bike in that direction?

Cogitate.

Been a LONG time since I actually paid attention to how the bike is steered; it's instinctive now, or at least ingrained deeply into my subconscious.

Your suggestion, Professor, is as timely as it is refreshing: I shall be commuting homeward through midtown Atlanta this afternoon, and have ample opportunity to ride at speed and at a crawl. Experiential data will be forthcoming, and (with luck) unaccompanied by a contusion.

 

[Fred W]

The reason we think so little about the physics of steering is because it has no importance whatsoever in the outcome. Most of us physically learned how to ride a 2 wheeler before we even learned to add and subtract, so clearly an understanding of "how" it happens is not very important. Once you have learned through muscle memory what inputs are required, and what the outcome will be, there is no good reason to waste neurons deliberating over the physics that occur in between, unless of course you just enjoy cogitating.

 

[ionbeam]

Sadly, I have a predilection for cogitating. It has afflicted me most of my life.

Counter steering is counter intuitive but it's what I endeavor to do all the time but I really use it when going fast on twisty roads. Riding 2 up most of the time means that I have a lot of mass to get moving to change directions and counter steering gives good leverage on the handle bars. While vacantly cogitating while tour riding I probably share the steering load between both hands but when going fast I tend to counter steer using one hand to initiate the turns.