Motorcycle at max lateral acceleration

[SixFootSix]

In the case where ground clearence isn't the limiting factor;

Since motorycles are steered using countersteering, a motorcycle cornering at constant max g can achieve any decrease of lean angle (bringing the bike up) by turning harder towards the inside of the turn momentarily, which is not possible since the tires are already at the limit of adhesion. So if you are cornering at max lateral acceleration, how do you straighten the bike up?

Attempt to solution: I think it has got to with body position but so far anything I managed to think of could be used corner faster in the first place anyway.

 

[jbriggs444]

So if you are cornering at max lateral acceleration, how do you straighten the bike up?

I could swear I've seen this exact question posted here previously.

If you can find a place to dump angular momentum (for instance, wind-milling your arms or revving the engine appropriately) you could, theoretically, right the bike slightly and pull out.

Realistically, I imagine that one only tries to get close to maximum lateral acceleration, saving a little bit for control. One measure of skill is how close to the edge of lost control one can reliably come.

I remember watching one road race (with cars, not motorcycles). The cameras were following the leader and one could see that upon approaching a hard corner, lapped cars would brake first while the leader's car was passing them at full speed. His advantage was the ability to come closer to that particular edge.

 

[A.T.]

So if you are cornering at max lateral acceleration, how do you straighten the bike up?

Or you assuming this actually happens and trying to understand how? Or are you merely asking if it is theoretically possible?

 

[berkeman]

I could swear I've seen this exact question posted here previously.

Maybe this short thread from a few months ago...?

https://www.physicsforums.com/threa...-faster-if-the-rider-puts-a-knee-down.972994/

 

[A.T.]

Maybe this short thread from a few months ago...?

https://www.physicsforums.com/threa...-faster-if-the-rider-puts-a-knee-down.972994/

No. There was exactly the same question a few weeks ago, that was deleted on request of the OP, who wanted to post a clearer version. But I still don't quite get the motivation for the question.

 

[SixFootSix]

I could swear I've seen this exact question posted here previously.

If you can find a place to dump angular momentum (for instance, wind-milling your arms or revving the engine appropriately) you could, theoretically, right the bike slightly and pull out.

Realistically, I imagine that one only tries to get close to maximum lateral acceleration, saving a little bit for control. One measure of skill is how close to the edge of lost control one can reliably come.

I remember watching one road race (with cars, not motorcycles). The cameras were following the leader and one could see that upon approaching a hard corner, lapped cars would brake first while the leader's car was passing them at full speed. His advantage was the ability to come closer to that particular edge.

Would throwing your upper body more "in" work as well (or down towards the ground since these usually take place at over 60 degrees of lean)? That is what many of the top guys in MotoGP are doing as I watch their onboard runs. Some even lift their outside leg if they already slid a bit and panicked. My problem is I am not able to figure out the direction of the resultant force between the contact patches and the ground. It doesn't seem possible to me that the tire can push "in" or "down" which are the obvious beneficial directions. My reasoning is that almost all of the force applied by the rider is through the seat (the outside leg. is sometimes up in the air and the inside foot is not on top of the peg, more infront of it so you can see the peg when looking from behind.) so if he leans to the right he will push to the left on the seat and i would assume this would cause the tire want to go left (to the "outside of the turn") and overwhelming the tires. But pretty much all of the top riders in the MotoGP do this right after the apex of the turn. It is really impressive how close they get to the edge so often and even "crash" a bit then save it. I would definitely recommend watching it if one is into that sort of thing.

Or you assuming this actually happens and trying to understand how? Or are you merely asking if it is theoretically possible?

I think it happens in top levels of racing pretty frequently, espically MotoGP during the last laps of a race or qualification laps. I just can't exactly get the physics of it.

 

[jbriggs444]

I doubt that it happens at all. Reality departs from the convenient theoretical model and the imagined sharp cutoff at "maximum lateral acceleration" likely turns out to be more complicated than that.

Theoretically, if you did a momentarily increased inward/downward lean, you'd pushing the tires slightly into the ground, gaining a momentary bit of downforce and a momentary increase to maximum lateral acceleration. As I suggested in post #2, that is a place where you could dump some angular momentum.

But that's pure theory. I do not drive bikes for a living. Or for fun. The moment the tires bite again, you will have to be on top of your game to avoid over-controlling and flipping over the opposite way.

 

[A.T.]

I think it happens in top levels of racing pretty frequently,

And this is based on what?

 

[SixFootSix]

I doubt that it happens at all. Reality departs from the convenient theoretical model and the imagined sharp cutoff at "maximum lateral acceleration" likely turns out to be more complicated than that.

Sure maybe they are not right at the limit mid corner but anything they can do to increase the grip even if momentarily so they can pick their bikes up, and I agree that probably doesn't mean that if they didn't do what they did with their body they fall, but if you could increase your tires grip for a moment, doing so while trying to pick up the bike would probably be the best time.

Theoretically, if you did a momentarily increased inward/downward lean, you'd pushing the tires slightly into the ground, gaining a momentary bit of downforce and a momentary increase to maximum lateral acceleration. As I suggested in post #2, that is a place where you could dump some angular momentum.

This is the bit where I would love to have some insight. As I said in my previous post, in my mind that inward lean would cause a decrease a downforce on the tire. Probably because I am missing something.

The moment the tires bite again, you will have to be on top of your game to avoid over-controlling and flipping over the opposite way.

That bit is probably the best bit to watch for most people I know. But at least riders have very good gear so serious injuries are rare.

 

[jbriggs444]

As I said in my previous post, in my mind that inward lean would cause a decrease a downforce on the tire. Probably because I am missing something

You flex at the hips to increase lean. Torque on your upper body is into the lean. Torque on the bike is out of the lean. Head goes in/down. Hips go out/up. Tires go down. Increased downforce.

Note that this is not about pushing on the pavement with your knee. That does decrease downforce. And its not about shoving your entire body down toward the pavement. That also decreases downforce. It is about dumping angular momentum into rotation of your upper body one way and the bike the other.

All pure theory-crafting. You're not going to get a lot of oomph out of a weenie little move like this.

I did tightrope walking as a kid. This is exactly how you balance on a tightrope. And exactly why you windmill your arms to avoid falling off a cliff. It doesn't buy you much. But with skill, it does not have to.

 

[SixFootSix]

You flex at the hips to increase lean. Torque on your upper body is into the lean. Torque on the bike is out of the lean. Head goes in/down. Hips go out/up. Tires go down. Increased downforce.

Note that this is not about pushing on the pavement with your knee. That does decrease downforce. And its not about shoving your entire body down toward the pavement. That also decreases downforce. It is about dumping angular momentum into rotation of your upper body one way and the bike the other.

All pure theory-crafting. You're not going to get a lot of oomph out of a weenie little move like this.

I did tightrope walking as a kid. This is exactly how you balance on a tightrope. And exactly why you windmill your arms to avoid falling off a cliff. It doesn't buy you much. But with skill, it does not have to.

Yes I agree pushing on the ground with knee or any other body part would definitely decrease the downforce on the tires and is a very bad idea indeed.

The thing I am struggling with is when I imagine a rider and motorcycle system in space from rear and the rider pushes with his bottom the left (leaning to the right at the waist) what my mind shows to me is rider going off to the right (translating) and spinning counter-clockwise while motorcycle is translating to the left and spinning clockwise and it is very hard for me to imagine if the initial movement of the lower ends of the tires are to the left or to the right. I have an easier time with a tightrope walker with a pole in his hands because that is pure torque with no translating. I feel like I miss something but I don't know what it is.

Thank you very much for this very engaging conversation.

 

[SixFootSix]

I am also curious if it makes difference that the rider is "tucked in" to the bike, close to the gas tank before he leans with his body.

 

[jbriggs444]

Remember that we are trying to explain an effect that may not exist.

 

[A.T.]

Remember that we are trying to explain an effect that may not exist.

So we still haven't seen any actual data supporting the premise of the OPs question?

 

[berkeman]

I haven't bothered to follow the thread, other than posting the link to the old thread which had lots of good info in it. What exactly is the OP's question?

 

[SixFootSix]

Does anyone know a free program to simulate a free body's movement in space with internal forces?

 

[FactChecker]

The motion of a motorcycle on the ground has a lot of forces that are dependent on relative velocities (shock absorbers) and on opposing other forces (e.g. friction on ground constraints). That is not the same as the motion of an unconstrained body in open space. In general, one must model the basic forces of the suspension, tire forces on the ground, spring forces, shock absorbers, stearing motion of the front tire, etc., so it is very specific to the vehicle.

 

[berkeman]

Does anyone know a free program to simulate a free body's movement in space with internal forces?

There's a simulation software package advertised at the end of this tutorial video...

(apologies for the short 15-second commercial at the start of the video)

https://motorsport.tv/embed/YPf9hsn...7VznNYQ-q345_0OoRzrcPvc-qpd6g0vsS0vaSglk_IlH4

 

[Chris S]

Summary:: Since you can not countersteer, how do you decrease the lean angle?

In the case where ground clearence isn't the limiting factor;

Since motorycles are steered using countersteering, a motorcycle cornering at constant max g can achieve any decrease of lean angle (bringing the bike up) by turning harder towards the inside of the turn momentarily, which is not possible since the tires are already at the limit of adhesion. So if you are cornering at max lateral acceleration, how do you straighten the bike up?

Attempt to solution: I think it has got to with body position but so far anything I managed to think of could be used corner faster in the first place anyway.

The bike can be straightened back up without counter-steering, so the bike will not need to counter-steer past the maximum adhesion point. Turns can be initiated and completed merely by leaning, which changes the center of gravity, which causes the bike to start tilting in the desired direction.

Regarding your follow up question, asking whether a rider tucked close to the gas tank makes a difference, it does because a tucked rider is lowering the center of gravity, which will cause leaning and counter-steering to have more of an effect on the shift of the center of gravity that initiates the tilting for starting turns and ending turns.

 

[John Archer]

As A championship winning motorcycle roadracer, and former motorcycle racing instructor, I will give you what I think is the answer.

This is based on real world vs. theoretical, but there may be some (poor) theory as well.
When cornering at the maximum, you will have some throttle already applied to the rear tire in a best case scenario.

The rear tire is normally much larger, and carries more of the cornering load. Ideally, you are back on the throttle BEFORE getting to the apex.

Leaving the corner, you are going to increase throttle (drive out of the corner). On a motorcycle, this gives the effect of "lifting" the front wheel into the corner (imagine what takes place during a wheelie, now imagine that same movement with the bike on it's side).
Also, remember that at maximum lateral acceleration, the tires ARE sliding somewhat, in fact, a very slight amount. The nearly constant "sliding" effectively cleans the tire, giving a fresh layer of virgin rubber. This is not like drifting, but a very minor amount of sliding (this works much differently in the rain).

So exiting the corner, we are adding power to the rear of the bike, and yes, turning slightly into the corner to pick the bike up. On a very powerful bike, you can actually pick the front tire completely off the ground while still leaned over!

This adding of throttle changes what maximum lateral acceleration can be. The more vertical the bike becomes, the more throttle you can feed in. The tire can only take so much, the force accelerating the tire takes some available traction from lateral acceleration for longitudinal acceleration (?), so even though you aren't cornering as hard, the tire is still at it's limit.

Now, seeing that we already know that maximum lateral acceleration does involve an amount of sliding, the act of steering into the corner simply changes the amount of sliding, and the change in angle of the front tire on the road means that the lateral acceleration as far as the front wheel is concerned, has simply changed direction slightly.

The amount needed to pick the bike up is incredibly small. most people can't see it without knowing what to look for.

NOTE: Your body position, quite normally with modern motorcycle racers, is designed to keep the bike as upright as possible. At maximum cornering speeds, your knee is quite normally planted on the road surface (sometimes your elbow as well). This can also be used to both help get the bike out of the corner, and prevent a crash if the front wheel starts to slide too much (pushing the front, eventually becoming a lowside).

When this occurs, the rider simply pushes down on the road as hard as possible (the knee is normally tucked in somewhat to reduce cornering drag...and pricy knee sliders ;) extending his/her knee outward (radially?). This keeps the bike on its tires. While that slide is occurring, the bike is slowing dramatically. Once it gets to a speed slow enough for the tire to regain traction, it does. An excessive example your lap time, but saves a crash.

There are some dramatic examples of this on YouTube, but most higher end racers will tell you that they will normally save a front and slide with their knee several times per track session, if not per lap. Those are nearly impossible to see, and these don't sacrifice much, if any time.

Short version, at maximum lateral acceleration, you ARE already sliding (even if an imperceptible amount) the slight change in steering input needed to exit the corner only changes that slide slightly.

A bike WANTS to go straight, the rider is normally turning the bar away from the corner (counter steering) to maintain the lean angle. Reversing that effort, and giving the bike some more throttle gets you out of the corner.

MOST crashes exiting a corner are due to the rear sliding too much, not the front. The front end crashes are quite normally due to the rider using the brakes too far into the corner.

In the rain, the way a bike breaks traction is quite different, the "safe" window of sliding is dramatically reduced. In this case, the rider simply is not normally at the absolute traction limit (as it is too hard to feel how much sliding is going on, and as mentioned, the window leaves little room for error).
even in this case, simply ending the input needed to maintain lean angle is enough to pick the bike up smoothly. In motorcycle roadracing, you need to be smooth to be fast. In the rain, you need to be smooth to survive.

 

[John Archer]

The bike can be straightened back up without counter-steering, so the bike will not need to counter-steer past the maximum adhesion point. Turns can be initiated and completed merely by leaning, which changes the center of gravity, which causes the bike to start tilting in the desired direction.

Regarding your follow up question, asking whether a rider tucked close to the gas tank makes a difference, it does because a tucked rider is lowering the center of gravity, which will cause leaning and counter-steering to have more of an effect on the shift of the center of gravity that initiates the tilting for starting turns and ending turns.

Body position does almost nothing to steer the bike...especially at speed. Tests have been done with motorcycles that have an extra set of frame mounted bars. Using those bars, the bike will only make the slightest change in direction.
Anything resembling quick steering at anything above a walking pace is done almost exclusively through counter steering.

 

[berkeman]

Body position does almost nothing to steer the bike...especially at speed. Tests have been done with motorcycles that have an extra set of frame mounted bars. Using those bars, the bike will only make the slightest change in direction.
Anything resembling quick steering at anything above a walking pace is done almost exclusively through counter steering.

That's not really true, so I do need to respond. Body steering works well, although for very quick chicane changes of direction, countersteering in addition to body steering helps.

Keith's "No BS Bike" was a valid and interesting experiment, but as far as I know, neither Reg nor Jason has tried riding it. It would be interesting if they had.

For me, body steering was a revelation when I first started doing Reg's CLASS racetrack schools. I just could not get the hang of countersteering through the Corkscrew at Laguna Seca, and then I started trusting my bike and working on body steering more, and it just started to flow. I also had some very close calls on the street and in the twisties prior to that using only countersteering, because it's a natural reaction for your arm muscles to tense up when you overcook a corner entry or when a cage snap-lane-changes into you on your commutes, countersteering is of limited usefulness in those situations, IMO.

One of the weirdest steering experiences I ever had was climbing the hill at Laguna Seca between turns 6 and 7 at about a buck with a very strong left-right cross-wind cutting across the track. I actually had to use reverse countersteering to stay pointed straight in that crosswind. Such a strange feeling... I was fine with only body steering at freeway speeds with strong crosswinds (even in the rain and going under underpasses), but that crosswind at a buck needed some extra persuading to keep my sportbike going mostly straight up the hill.

I don't want to get into a big countersteering-versus-bodysteering debate on the PF, especially in response to an OP who has no racetrack miles under their belt. But let's just say that they both can be used to steer a sportbike at racetrack speeds, and each has its place.

Nice that you raced so successfully. Was it AFM?

 

[rcgldr]

a bike wants to go straight

At high speeds (around 100 mph or 160 kph), gyroscopic reactions dominate over self-stability and at these speeds a bike "wants" to hold the current lean angle and keep turning (as opposed to wanting to straighten up and go straight). The rider has to exert considerable counter-steering effort to straighten up a bike. An extreme example of this is at Daytona, where 2 banked turns and the banked straight in between are used. Riders exit onto the straight nearly horizontal due to the banking, at over 180 mph. Most state they have to clamp their knees onto the tank in order to generate enough counter steering effort to get the bikes to straighten up at a reasonable rate (and not end up in the infield).

 

[John Archer]

That's not really true, so I do need to respond. Body steering works well, although for very quick chicane changes of direction, countersteering in addition to body steering helps.

Keith's "No BS Bike" was a valid and interesting experiment, but as far as I know, neither Reg nor Jason has tried riding it. It would be interesting if they had.

For me, body steering was a revelation when I first started doing Reg's CLASS racetrack schools. I just could not get the hang of countersteering through the Corkscrew at Laguna Seca, and then I started trusting my bike and working on body steering more, and it just started to flow. I also had some very close calls on the street and in the twisties prior to that using only countersteering, because it's a natural reaction for your arm muscles to tense up when you overcook a corner entry or when a cage snap-lane-changes into you on your commutes, countersteering is of limited usefulness in those situations, IMO.

One of the weirdest steering experiences I ever had was climbing the hill at Laguna Seca on the kink between turns 6 and 7 at about a buck with a very strong left-right cross-wind cutting across the track. I actually had to use reverse countersteering to stay pointed through the kink in that crosswind. Such a strange feeling... I was fine with only body steering at freeway speeds with strong crosswinds (even in the rain and going under underpasses), but that crosswind at a buck needed some extra persuading to keep my sportbike going mostly straight up the hill.

I don't want to get into a big countersteering-versus-bodysteering debate on the PF, especially in response to an OP who has no racetrack miles under their belt. But let's just say that they both can be used to steer a sportbike at racetrack speeds, and each has its place.

Nice that you raced so successfully. Was it AFM?

Body possition plays a role, but I don't think nearly as much as counter steering... grab your frame on the front straight sometime (warm up lap) and see how quickly you can change direction. The faster you can change direction, the better lines you can make.

I'll agree to disagree on the amount each plays, they both play a role.

I raced AMA CCS

 

[John Archer]

At high speeds (around 100 mph or 160 kph), gyroscopic reactions dominate over self-stability and at these speeds a bike "wants" to hold the current lean angle and keep turning (as opposed to wanting to straighten up and go straight). The rider has to exert considerable counter-steering effort to straighten up a bike. An extreme example of this is at Daytona, where 2 banked turns and the banked straight in between are used. Riders exit onto the straight nearly horizontal due to the banking, at over 180 mph. Most state they have to clamp their knees onto the tank in order to generate enough counter steering effort to get the bikes to straighten up at a reasonable rate (and not end up in the infield).

Banking adds a whole new element to the equation. I've seen riders thrown off at high speeds (partial highside) , and the bike (sans rider) stood up perfectly vertical...right into the hay bales :-O

 

[FactChecker]

Leaving the corner, you are going to increase throttle (drive out of the corner). On a motorcycle, this gives the effect of "lifting" the front wheel into the corner (imagine what takes place during a wheelie, now imagine that same movement with the bike on it's side).

It sounds like this may do more at that time to turn the front end into the turn than anything else.

 

[rcgldr]

I've seen riders thrown off at high speeds (partial highside) , and the bike (sans rider) stood up perfectly vertical...right into the hay bales

It would be rare for the bike to continue to be going 100+ mph in such a circumstance. The main exception would be Ilse of Man TT (high speed, no banking), but there is no run off, except for the country side, but generally the bikes just crash there. Willow Springs turn 8 would be a better example since it has some run off, but due to the speed there (130+mph), I assume most riders are a bit more cautious and high sides would be rare there.

I'll have to search for the article, but I recall something like 30 lb ft of counter-steering effort is needed to straighten up out of the banked turn at 180+ mph at Daytona, which I don't think is related to the banking, but just the fact that the nearly horizontal position coming out of the corner means that nearly 90 degrees of lean angle have to be undone to straighten up.

In mathematical models with infinitely thin tires, at a high enough speed, the bike goes into "capsize" mode, where it tends to fall inwards at a very slow rate. With real bikes, the contact patch is off center and provides a small amount of torque that would tend to straighten up the bike, but the overall effect results in a response so slow that it is imperceptible.

 

[John Archer]

While they have since added a chicane there, my experience was with the "Kink" at Road America. (After the Carousel and before Thunder Valley and Canada corner).
When the bike hit the hay bales, we they literally had to stop bikes on the track, as one could not see where the track ended and the grass began :-O
Of course, the steering effort would be different with bikes that have different steering geometry. I personally have not raced at Daytona, so I can't speak from experience there.

 

[rcgldr]

Daytona

I found a copy of an old article, 1996 by Gordon Jennings for Motorcyclist magazine, talking about Daytona and also the Bonneville type speed bikes. He mentioned having to clamp the tank with his knees to apply enough countersteering torque to straighten up on the banked straight. I haven't found the article that stated it was about 30 lb ft of steering torque for the bikes running at Daytona at that time of the article. Reducing trail will reduce steering effort at low speeds, but at high speed, the issue is gyroscopic reaction in response to countersteering torque, and the effort increases as speed increases in order to change lean angle at the same rate.