Who's Afraid of Body Steering?
CityBike © 2002
Welcome fellow seekers! Judging by the emails we received on CityBike's Colossus MK VII, it seems our investigation of Keith Code's No BS bike in the Dec. '01 issue stirred up quite a hornet's nest.
Not everyone agreed with my views on the subject and that's OK by me. Participation is the name of the game on this one.
Some of you seemed to think that I was poo-pooing the concept of body steering.
Not so! We wanted to report on Keith's bike and so the article focused on countersteering as a consequence.
Countersteering does a good job of making a bike lean, but saying that's all there is to turning is kind of like saying that chewing is the only thing to digestion (maybe because when something goes wrong with either, things get messy?).
Be that as it may, it's important to understand the forces that actually make the bike turn. The bottom line is that the whole point of leaning, steering or any other chassis manipulation we may do is to allow the tire to react with the road in a way that produces a force in toward the center of the curve.
There are primarily two forces at work here: slip angle and camber thrust.
Slip angle is the difference between the direction the tire is pointed and the path that the motorcycle is actually traveling and is the result of deflecting the front wheel from its straight ahead position as you do when making a u-turn, for example, or when driving a car.
The reason it's called slip angle is because your tire is never really as hooked up as you might suppose. We're not going to cover slides at least not like the ones our favorite racers dazzle us with but the fact is that in the real world we're all sliding to some extent when we ride.
Our good friend, Mark Pfenning at Avon Tires of North America, had this to say:
“The concept of rubber hysteresis, the physical contact of the rubber molding itself into the road surface, is almost impossible to quantify, however, at steady state on the throttle, say neutral throttle, you're going down the road at about 92 to 95% efficiency”
That doesn't mean you're spinning up the tire 5 to 8%. It means that internal friction within the tire materials and mechanical motion, such as squirm, create energy losses through heat and noise that reduce the transmission of power to the ground by that amount.
Even so, that's a surprising figure. Even more so are test results that show that a fast street rider may have as much as 20% slippage going on while experiencing excellent traction.
Some of the increase is due to slip angle, which ends up as a lateral force that causes your bike to change direction.
“That 20% number is pretty commonly accepted, explained Mark, and it makes sense because there's only so much you can do, short of having the rubber so soft and a pavement so coarse, that every molecule of the rubber is interacting with that...there's got to be some slippage”
While slip angle works fine at low speeds, as speed builds so does centrifugal force and if you don't add some lean angle to balance this out, I'm sure you'll see that you would high side into oblivion.
Lean angle not only maintains a bike's balance, it pulls double duty by producing the major player in cornering, camber thrust, camber being merely the term for a tire's angle from the vertical.
To find out how it works we asked Avon's Motorcycle Design Manager, Pete McNally, to weigh in on the subject. He explains: “The lateral tire force due to the tire camber angle is known as camber thrust or force. At a given corner speed when leaning the bike over, the inside edge of the tire will want to travel slower than the outside edge as it is turning on a smaller radius. This is similar to pushing a cone shaped object over a table.”
To see how that behaves, find a tapered object, like a drinking cup, and roll it across a flat surface. Since the rigid cup has to spin at the same rpm at both ends, the larger circumference must travel farther than the smaller one so it must always turn in the direction of the smaller end.
Tape two cups together at their large ends and you approximate the shape of a tire profile.
If you tip it off center either way, it turns in that direction due to its camber thrust and notice that it turns quite sharply even though its 'profile' is much less pronounced than any motorcycle tire.
“This is where tire compound, profile and construction come into play, Pete continued. Tires with more triangular shapes can give more camber force as the footprint increases as we lean. In simple terms, the more pointy a tire’s profile, the quicker it will steer”
You can see that a 'pointy' profile, when it's leaning, will have a wider contact patch with a greater difference between its inner and outer radii, than a similar sized tire with a round profile will. And this probably accounts for this profile being popular in racing use.
It doesn't seem like the difference in tire section should amount to much, however, Mark offers this example: “If you've ever talked to anyone who's road raced, Brainerd turn one is this balls-out, top gear corner. Now, it's not a corner because the speed doesn't change much, but what happens is that guys are maxed out in top gear, they heel in and what happens is that the rpm goes way up because the effective diameter of the tire is reduced because they're almost at full lean. And it's really amazing because what you have to do is gear for the track set up and essentially go back and build an overdrive capacity into the gearing because of it”
There does, however, seem to be a fly in the ointment.
If you consider the angle the 'cone' makes with the road it looks as though the bike should turn around an impossibly small radius. Pete explains that “A bike, however, would not corner as tight as a cone due to centrifugal force throwing the bike outwards and hence would travel along a larger radius”
Centrifugal force is really a pseudo-force since it is the result of inertia resisting your attempt to move the bike from a straight path, but its effects are real enough.
By leaning the bike into the turn you create a balance between this force and gravity that keeps the bike from falling over, as we noted earlier.
The combined result of these forces act down the bike's centerline to the tire contact patch, then outward from the direction of the turn counteracting camber thrust. It's the same force that causes the tire to slide out when it has exceeded the available grip.
Since the force due to camber thrust is constant for a given lean angle and centrifugal force increases with speed, this explains why your turn radius increases as your speed goes up.
It turns out that the physics of cornering are unequivocally expressed in mathematical terms, so that the lean angle for any given bike/rider combination is proportional to the square of the speed and inversely proportional to the radius of the curve.
If you want the exact equations involved, just send a bottle of Jose Cuervo and a SASE to me C/O CityBike World HQ, otherwise just think about the relationship of these variables to each other.
It's helpful to consider one of them as a constant and see what happens to the other two.
If you increase your lean angle, for example, your speed through a constant radius turn will have to increase accordingly or, if the speed remains constant, the radius of the turn will decrease and that's exactly what we experience when we ride.
Of course, in the real world you don't hold any of these variables constant as you corner.
The important thing is to grasp how they relate to each other and use that to understand your bike's behavior.
For example, if you hold a constant lean angle and increase the bike's speed, the radius of the turn will increase as well and that's what happens when you gas your bike out of a turn.
Why is that interesting? I'd say because it shows how throttle alone can affect your bike's line. So, for the sake of argument, we can definitely say that there is such a thing as throttle steering.
All well and good, but what does this have to do with body steering?
Dave Bromfield, who wrote the companion piece to the No BS article had this to say about one of Keith Code's training drills:” body English does allow you to fine-tune lean angle and line in a more stable way than sawing at the handlebars. Code recognizes this explicitly at Level 3 of his course. An on-track exercise has you go through the corner and, just after you hit the apex, you're instructed to drop your chest to the gas tank. Dropping your CG in this fashion immediately tightens up the line”
Why should this be so? It so happens that a bike with a low CG must corner at a larger lean angle than one with a higher CG, this being mainly an effect of the contact patch offset created by the large tires used today. So if our bike's lean angle increases while maintaining a constant speed, the corner radius must decrease just as the formula predicts.
And, hello, isn't that body steering?
Isn't it interesting that Keith includes such a drill in his curriculum even though he discounts the concept?
Keith's latest foray on the subject is a contrivance he terms the Body Alignment Device (BAD bike).
It consists of a yoke that clamps the rider's butt onto the seat so as to prevent any sideways movement...an anti-bodysteer device, so to speak.
He claims that it trains his students not to 'counter-lean' in turns just as a bad passenger or timid rider might do. He's quick to point out that, 'Passengers aren't actually steering the bike, they are causing it to become unstable.'
It's at this point where the semantic nature of the debate becomes clear.
It's just as easy to argue that a bike traveling along in a steady state must be destabilized before it can change direction.
The only question then is, does the rider or passenger input destabilize the bike in such a way that it will cause it to take the desired path?
Any number of rider inputs can be inappropriate and the counter-leaning passenger is only one. Clearly, the counter-leaning passenger will “cause the bike to run wide in turn”' as Keith acknowledges, but doesn't that imply that leaning into a turn will tighten the line as his Level 3 training drill bears out?
This seems to be a case of selective observation.
Perhaps Keith should call his latest creation the More BS bike?
Looking at what we've discovered so far, it's interesting to note that no amount of throttle jockeying or dropping your CG (straight down, at least) will do a damn thing to turn your bike while it's straight up.
These techniques only work once you've got some lean happening.
So to address Keith's No BS bike from a different perspective then, you'd have to say that since you can't countersteer it, it's difficult to get the bike into an attitude where body or throttle steering really begins to manifest itself.
Keith may have proven the value of countersteering, but he hasn't disproved body steering either.
However, trying to quantify the exact effect of body weight shift is next to impossible because a rider can move about in an infinite number of ways.
All we can say is that shifting your weight, laterally for instance, can induce some lean, but since you're working against the gyroscopic stability of the spinning wheels it's not a technique that's as aggressive as countersteering.
Well, that's just what we want while we're at the limits of adhesion, isn't it?
As Reg Pridmore pointed out: “Often times, I see riders who have been taught countersteering exclusively, and use it to the bad extreme. I have watched them push the bike right out from underneath themselves, literally”
What we want then is a subtler, gentler method of adjusting our lean angle and body steering appears to be just the ticket. If we can change our line ever so slightly with relatively large body weight inputs, we've found a method to control it with precision.
And body steering also seems to offer a way to control slip angle as well as lean.
It's interesting to examine photos of road racers who are deep in a turn and not sliding the rear, a la Garry McCoy, and if you do, you'll see that the front wheel is often pointed into the turn a few degrees.
Camber thrust is doing the lion's share of the work in getting the bike to turn while the rider is fine tuning the turn radius with slip angle, but that begs the question: If the front wheel is spinning and is therefore subject to the laws of gyroscopic precession, then what input makes it steer into the turn?
If you simply turn the bars into the corner, by all rights the bike should then countersteer out of the turn.
We covered precession in the No BS article, but perhaps a recap is in order.
Because of rotational inertia, a gyro (i.e. your bike's wheel) does not respond to applied forces as a stationary object would.
Instead, the wheel will respond at a point 90-degrees in the direction of rotation from the point where the force was applied.
The best way to visualize this is to spin a bicycle wheel and see what happens. If you turn the wheel as you would with a handlebar input, you'll see that it precesses (countersteers) away from the direction you turned it, as we just noted.
However, if you simply tilt it as if you're body steering, you find that it does an interesting thing.
The wheel turns in the same direction as it has been tilted, i.e. when you tilt it to the right, the wheel also turns to the right. The implication is that when you increase lean angle solely through body steering you not only increase camber thrust, precession throws in a bonus slip angle as well.
Both together will tighten up your line and so it seems that weight shifting is a method whereby you can directly steer the front wheel to some degree.
We may examine other aspects of chassis dynamics in the future, but as far as the No BS 'controversy' is concerned, I'm done with it.
Factory engineers and tire designers have to know this stuff, but we do not. It doesn't take a physics degree to ride well. Still, it's fun to ponder the complexities of motorcycles because their challenging nature is what makes them so attractive to us in the first place.
What was it JFK said about going to the Moon? 'We choose to do these things not because they're easy, but because they're hard.' Or something like that, I think.
So go out and ride and if some of this enhances your experience, then great! If not, go screw yourself. Just don't forget to send the Cuervo!
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