I Can a cornering motorcycle go faster if the rider puts a knee down?

  • Thread starter koodawg
  • Start date

russ_watters

Mentor
18,524
4,753
Yes, center of gravity has much to do with cutting corners.
When you are turning, the torque caused by two opposing forces (the horizontal component of the weight acting on the center of gravity and the horizontal component of the reacting force to the tires this being not vertical but on the axis joining tip of the tires to CoG as in the case of grip) must be minimized not for the sake of turning itself (always 0 since the bike is in balance) but for the sake of stability.
And the way to do this is by lowering the CoG.
While it is true that a bike is more stable with a lower center of gravity, that has nothing to do with this discussion. This discussion is about how you generate traction to "hold" the corner. For the purpose of this discussion, the resulting force vector is a straight line through the center of gravity and tire contact patches (or, rather, a point on the curve between the tires) and the net torque is zero. There is no net torque when you are properly balanced and if you are fighting a torque, then you are doing something wrong (and probably in the process of falling).
 

berkeman

Mentor
54,964
5,191
At Laguna Seca, turn 1 is a high speed slight kink, no one is hanging off there. From the start/finish line, the riders don't hang off until the very tight turn 2.
You might be right. I remember watching the Superbike races there many years ago, and I thought I remembered them getting knees down in Turn 1. But looking at Google Images now, it does seem like they do have a knee out a bit, but are not hanging all the way off. You do lift a bit going through Turn 1 (at least I do, but I only got up to about 120mph through that turn at my track days), so staying tucked in through the turn isn't super-important. Braking for Turn 2 starts pretty much as you are leaving Turn 1.

BTW, funny story about those short red marker rods on the left side of Turn 1 (they separate the track from the pit exit lane). The first year we went to watch the Superbike races there, during the first practice several riders kept grazing the rods with their helmets (the rods were taller at first). You can imagine how distracting that is at 140+mph. Before the next practice, the track workers went out and cut all of the rods down to that lower height. LOL :smile:


244726
 

rcgldr

Homework Helper
8,577
472
Yes, center of gravity has much to do with cutting corners. When you are turning, the torque caused by two opposing forces (the horizontal component of the weight acting on the center of gravity and the horizontal component of the reacting force to the tires this being not vertical but on the axis joining tip of the tires to CoG as in the case of grip) must be minimized not for the sake of turning itself (always 0 since the bike is in balance) but for the sake of stability. And the way to do this is by lowering the CoG.
The lower the center of mass, the greater the amount of lateral force required to change lean angle at a certain rate. The guys who run torpedo bikes for top speed runs at Bonneville often have to add balast to the top of the frame to raise the center of mass from being too low. If the center of mass of the bike is too low, it takes too much lateral force to get the bike to lean.

For "flickability", the idea is to have a reasonably high center of mass on a bike and to have much of the mass located near the front to back line of the center of mass to reduce angular inertia. Some superbikes rotate the engine backwards (which requires internal gearing to "unreverse"), in order to reduce the rotating angular momentum of engine and tires.
 
Last edited:
The lower the center of mass, the greater the amount of lateral force required to change lean angle at a certain rate. The guys who run torpedo bikes for top speed runs at Bonneville often have to add balast to the top of the frame to raise the center of mass from being too low. If the center of mass of the bike is too low, it takes too much lateral force to get the bike to lean.

For "flickability", the idea is to have a reasonably high center of mass on a bike and to have much of the mass located near the front to back line of the center of mass to reduce angular momentum. Some superbikes rotate the engine backwards (which requires internal gearing to "unreverse"), in order to reduce the rotating angular momentum of engine and tires.
I really have no idea what are you talking about here. I googled Bonneville and found no racetrack, and what type of superbikes are you refering to?
One must notice that you are progressing to lean over ( lowering the CoG ) not at once but gradually from the entrance to the apex of the curve. The tilt of the bike generates angular momentum too, and since this must be conserved any further diminishing of the distance between CoG and point of touch of the wheels generates more "flickability" as you put it.
 

berkeman

Mentor
54,964
5,191
really have no idea what are you talking about here. I googled Bonneville and found no racetrack, and what type of superbikes are you refering to?
They go very fast and (mostly) straight at the Bonneville salt flats. The "racetrack" is a straight line painted on the salt. If you've ever seen the classic movie "On Any Sunday", you'll remember a pretty comical failure to steer one of these "torpedo bikes" at low speeds... :smile:


244738
 

rcgldr

Homework Helper
8,577
472
One must notice that you are progressing to lean over ( lowering the CoG ) not at once but gradually from the entrance to the apex of the curve.
On a long (time wise) curve, a lot of time is spent at max lean angle.

The tilt of the bike generates angular momentum too, and since this must be conserved
Angular momentum is only conserved if you include the reaction by the earth.

I googled Bonneville and found no racetrack, and what type of superbikes are you refering to?
They are called streamliners. Current record was set a few years ago, 376 mph average speed on a flying mile (peak speed 394 mph), two runs (both directions):

 
@rcgldr
The situation is as follows:

1.Assuming that the radius of the curve remains constant throughout the turn, the motorcycle enters the curve.

2.Assuming unlimited grip of the tires, the lean angle is limited only by the frame of the motorcycle and the body of the rider of course.

3.The angular momentum of the system motorcycle-rider is constant throughout the bend, but the speed is not! The rider by jutting out his knee and bringing his whole body closer to the center of the curve and by lowering his center of gravity thus and bringing it closer to the center of the curve can manipulate his speed and ( yes! ) increase it at the apex of the curve ( no throttle ) which he could not do otherwise by increasing the lean angle because there is a physical limit to it as explained in 2.

As of the streamliners you are talking about, I know nothing about their geometry so I can't comment.

Thank you all for the beautiful discussion.
 

jbriggs444

Science Advisor
Homework Helper
7,219
2,416
The rider by jutting out his knee and bringing his whole body closer to the center of the curve and by lowering his center of gravity thus and bringing it closer to the center of the curve can manipulate his speed and ( yes! ) increase it at the apex of the curve ( no throttle ) which he could not do otherwise by increasing the lean angle because there is a physical limit to it as explained in 2.
Won't work.

Once you have fixed the radius of the curve, the mass of the bike+rider, the distance of the center of gravity from the wheel track and the angular momentum of the assembly as it rounds the curve, there are no more free parameters. The lean angle is locked in. You cannot stick a knee out one way without sticking something else out the other way -- else you blow the lean angle and fall over.
 
Won't work.

Once you have fixed the radius of the curve, the mass of the bike+rider, the distance of the center of gravity from the wheel track and the angular momentum of the assembly as it rounds the curve, there are no more free parameters. The lean angle is locked in. You cannot stick a knee out one way without sticking something else out the other way -- else you blow the lean angle and fall over.
Look again at the picture post #18. The bike can be in a complete upright position and still negotiate the curve!

The torque of a weight put away at a distance from the wheels ( the knee and position of the body of the rider away from the saddle ) generates the extra centripetal force needed for the increased velocity at the apex of the curve.

As I have explained before, if the lean angle of the bike weren't limited by the frame of the motorcycle there wouldn't be any need for this and the turning speed would be as big as you like provided unlimited traction of course.
 

jbriggs444

Science Advisor
Homework Helper
7,219
2,416
Look again at the picture post #18. The bike can be in a complete upright position and still negotiate the curve!
The effective lean angle of that bike (the angle from the tire track to the center of gravity) is not vertical. And it is not even a two-wheeled vehicle. It does not count. Nor do I see a knee extended.
The torque of a weight put away at a distance from the wheels ( the knee and position of the body of the rider away from the saddle ) generates the extra centripetal force needed for the increased velocity at the apex of the curve.
It does not generate any centripetal force at all. It is responsible for a torque due to gravity that can counter the torque from the couple of friction and centrifugal force. Increasing this can allow an extra centripetal force.
As I have explained before, if the lean angle of the bike weren't limited by the frame of the motorcycle there wouldn't be any need for this and the turning speed would be as big as you like provided unlimited traction of course.
The specifications for your claim involved a fixed angular momentum. You cannot increase turning speed by increasing lean angle if you are holding angular momentum fixed. As I already pointed out and as you have failed to grasp.
 
The specifications for your claim involved a fixed angular momentum. You cannot increase turning speed by increasing lean angle if you are holding angular momentum fixed. As I already pointed out and as you have failed to grasp.
What failure?
220px-Cup_of_Russia_2010_-_Yuko_Kawaguti_%282%29.jpg
More weight towards the center of rotation, more speed.

You are right about the picture, the lean angle is not vertical, is not 0, is negative and away from the curve, this more support of my claim.
 
And it is not even a two-wheeled vehicle. It does not count. Nor do I see a knee extended.
Yes, for the moment the third wheel doesn't touch the surface of the track the tricycle becomes a bicycle!!!
 

jbriggs444

Science Advisor
Homework Helper
7,219
2,416
What failure?View attachment 244751More weight towards the center of rotation, more speed.

You are right about the picture, the lean angle is not vertical, is not 0, is negative and away from the curve, this more support of my claim.
Read the specifications for my counter-claim again.
 
@jbriggs444 You will never see a competitive racetrack rider in a very tight bend sticking out only his kneecap to feel the road as Google claims. He always leans his whole body towards the inside of the curve, literally dismounting from the saddle. It's a center of gravity thing. Those who are putting their knee down only to feel the road, simply don't win races!
 

rcgldr

Homework Helper
8,577
472
1.Assuming that the radius of the curve remains constant throughout the turn, the motorcycle enters the curve.

2.Assuming unlimited grip of the tires, the lean angle is limited only by the frame of the motorcycle and the body of the rider of course.

3.The angular momentum of the system motorcycle-rider is constant throughout the bend, but the speed is not! The rider by jutting out his knee and bringing his whole body closer to the center of the curve and by lowering his center of gravity thus and bringing it closer to the center of the curve can manipulate his speed and ( yes! ) increase it at the apex of the curve ( no throttle ) which he could not do otherwise by increasing the lean angle because there is a physical limit to it as explained in 2.
#2 is unrealistic, there's a limit to the grip. If the tires have a circular profile, then contact patch area remains the same regardless of the bikes lean angle. Some tires have a taller profile (similar to a parabola), where the contact area is largest at some specific lean angle.

#3 - the "closer to center of curve" effect is minimal. Other than hair pin turns, most turns have a radius of more than 100 ft (100 foot radius translates into ~ 40 mph 1 g turn). The inwards component of center of mass shift is only a few inches, as the rider's weight is 1/3rd or less than the total weight of bike and rider, and the riders's body shifting mostly moves the center of mass with respect to the contact patches radially, barely changing the distance from center of mass to contact patches.
 
Last edited:

berkeman

Mentor
54,964
5,191
Apparently it also helps you go faster if you use your elbow as a feeler... :smile:


244871
 
I race bikes, so I have some perspective on this. Here's start of race at Sears Point Sonoma Raceway.

port=download&id=18MBKfVBnfk-zlb0PZv72wmd8VQ6JDhZ6.png


While lots of ideas were covered relating to "knee out" physics those may more effect rather than cause. Also some are completely unrelated at all. That being 3-wheel side-cars vs. 2-wheel bike. With 3-wheels, it corners like car and requires turning steering into corner. The monkey hanging off side is to countreact lateral weight-transfer that's trying to tip it over. Moto on other hand only uses steering to initiate leaning and once desired angle is reached, steering is aimed straight ahead again. Centripetal force pushing bike into circular-path is generated by tyre's camber-thrust.

Here's an overview of cornering angle, it's dictated by angle between effective COG of bike+rider and ground.
port=download&id=1ewgBt2aoLJzf4dIRgpaJh-E4cfCPwEq0.jpg


Note that distance (height) between effective COG and contact patch makes no difference. In steady-state cornering once lean-angle has been set, tyres ony notice that there's certain vertical-force from rider's weight+bike's weight, and lateral centripetal force from cornering. Rider can be sitting on 10-ft high-chair above bike, and it will still corner exactly at same speed as another bike with rider tucked in at same lean-angle. Obviously if lean-angle is 45-degrees, vertical and horizontal forces are identical and bike corners at 1G. Here's table of lean-angles and resultant cornering-G.

port=download&id=1hH5K_3IIle5OnNqxlX5R-MAeUfr2mIwZ.jpg


Notice that function is not linear? Well, up to 45-degrees, it's fairly linear, and increase even faster than that. Going from 45-degree to 60-degree lean generates double cornering force. Here's roughly lean-angle maximums for various types of tyres.

port=download&id=1DqBCvK3q69zmVFMhxH054VTuUsfivlVF.jpg


Different rim-wdiths, casing constructions and rubber compounds allow professional race-bikes to achieve extreme lean-angles and cornering speeds. Race-tyres only has to last about 100-miles, so their super-sticky rubber-compounds have incredible grip.

port=download&id=12iuQKeXuvfgLqe4QtGNa9_9jC7blBK8y.jpg


Now, why is there lean-angle limit and what happens if you go over???

port=download&id=1iOsmJ_fopD7AtrUNR7s5wSglRr6JvdHQ.jpg


You roll off edge of tyre and crash!!! Street tyres tend to have flatter profile and wont handle too much angle before rolling off. DOT-R tyres, soft-compound street-legal tyres tend to have more triangular profile so that when its leaned over, it has larger and flatter contact on ground. THIS is why racers stick out their knee, as an angle-feeler gauge to compliment the excellent angle and G-force measuring device known as ears.

You see street-riders street-riders trying to out-macho each other by seeing who has least amount of "chicken strips" on their tyres. These are those unused stripes on each side of tyre. And they brag about "getting their knee down", which is supposedly some indicator of skill.

For racers, it's exact opposite. We try to keep tyres off edge as much as possible to not go over and crash. Knees are used to gauge how close you are to edge, as such, it's only done at maximum lean-angle. In slow/medium-speed corners, where I may actually tip out knee tiny bit and if there's slight rub or kiss, then I pull it back in, once I know how much lean I have. For high-speed corners where I'm not even gonna use fulll-lean (i.e. turn-1 @ Laguna Seca), the knee stays tucked tightly in for aerodynamics. Sticking knee out at those speeds, causes an immediate 3-6mph reduction in speed.

So how can you corner even faster once you've leaned up to tyre's edge? By recognizing that effective COG is combined system of bike AND rider.

port=download&id=1bOv4BA_pojQGpXkh3eMtcke0lZjanDRz.jpg


By shifting entire body inside of bikes centre-line, you've effectively moved COG towards inside of bike. This bike+rider system at 39-degrees lean, has an effective lean-angle similar to previous picture of 52-degrees. It can corner at same speeds for less lean-angle! This gives you overhead to lean bike+rider more to limit of 55-degrees while not rolling off edge of tyre at 50-degrees like before. Here's some examples of moving entire body towards inside to keep bike as upright as possible for more cornering speed.

port=download&id=1zcQ0OjXzGe4dYqRTGjMnqRFanVmSJ5B7.jpg

port=download&id=1vpcmT8OYVVEeUtWVSuEJ8U5NgFQsMzjV.jpg

uc?export=download&id=0B2u9SQWpJMCTZ2hCZnNIQ2o3N2s.jpg

port=download&id=1x746P7pepL_GszeCtVC_2tMOJKVsqf2P.png

port=download&id=1bHPx369CopcgSiEBPr9IB8ELTCBE4gl6.jpg


Another benefit of hanging off inside and keeping bike more upright, is you can use more throttle coming out of corners for higher top-speed down next straight. So really, knees touching is not cause of higher-speeds, it's just tool to allow rider to know how close they are to edge of tire and maximum lean-angle.
 

cmb

686
4
Excuse me if I missed any subtlety above which says the same as this, but, no, putting your knee down (resulting in a downward force, and thus friction forces on the knee pad) does nothing for cornering. Quite the opposite, it removes some of the vertical normal reaction so reduces the vertical load on the tyre edge, thus lowers its grip and ability to apply a centripetal force.

I used to ride bikes and never really understood the reason people would do this for cornering purposes on the road. Wanna-be road racers seemed to be aping racing riding, which is pointless.

I had a range of bikes from very large 1200cc stuff to two stroke race bikes, and there was really no issue in cranking the things over until bits of the bike started scraping along the ground. Road bikes tend to have foot peg sliders which are spherical headed bolts in the end of the foot pegs, and my right one tended to get a bit of abuse. (We drive on the left here, so taking left hand bends fast is just dangerous and silly, because the only way you are going to go if you get it a bit wrong is straight into the oncoming traffic. At least right hand bends you can be a bit safer if there is a ploughed field to your left!)

The 250 two stroke I had was flat out a road legal race bike. It had no pegs and would in theory have happily scraped its chain on the floor before worse things happened. Anyway, the steering was extremely limited, only 15 degrees or so on the handle bars so the only way to get tight cornering was fast and deep! Forget a slow speed U turn, race bikes don't really do that. This steering was, however, extremely sensitive. The bike weighed about 100kg and the lightness and tall castor angle meant that you could literally lean your head to one side of the bike and it'd start turning in that direction! So there are a host of other reasons racers do the knee out thing, one is for sure the comfort thing and to be able to gauge where the bike's dangling bits are relative to the tarmac, but at high speed sticking a knee out also slows the bike down considerably. You'll see race guys popping up like meerkats as they approach corners, along with their inside knees popping out. If you are at a serious speed, this slows you down and pulls you into the corner without even thinking about steering. As they enter the corner, there's not much point pulling their knee back in because the next thing they do is move even more over on the bike and also hang off it, along with their knee!

So, does sticking a knee out in a race help them win? Yes, there are reasons why it might help (though it doesn't help every rider's style). Does it help on a road bike? No, just makes you look cool!
 
It was mentioned earlier that putting weight on knee would jack up bike, lowering vertical-loading on tyres and reduce grip. While this is true, you'd have to have super-human strength to push your knee out that hard.

What's more common is some hard non-yielding parts will touch down, such as kickstand bracket, or exhaust or footpegs. When these parts touch, further leaning WILL jack up bike on that point and lift rear tyre and cause crash. So I've cut off kickstand bracket on my bike, installed exhaust that sits higher and use folding pegs.

Having some fun at Laguna Seca (3 videos in playlist).
 

berkeman

Mentor
54,964
5,191
Having some fun at Laguna Seca (3 videos in playlist).
Great stuff! The bigger bikes have the power to pass you in the straights, but you have the braking and cornering skills to pass them right back. Awesome :smile:

Was that at a track day? Keigwins or a different group?
 
the last video is mine and it was track-day practice with Pacifc Track Time.


Unfortunately, Keigwins is no more. Done in and bankrupted by idiot rider who crashed and sued them and Laguna Seca. However, it has been revived as Carters at the Track with previous GM as new owner. :)
 

rcgldr

Homework Helper
8,577
472
Moto on other hand only uses steering to initiate leaning and once desired angle is reached, steering is aimed straight ahead again. Centripetal force pushing bike into circular-path is generated by tyre's camber-thrust.
Steering relative to a bike's path is always inwards. Both front and rear tire are angled slightly inwards of the bikes actual path, due to flexing at the contact patch, known as "slip angle". Flexing at the rear tire contact patch (slip angle) coexists with the overall bike's frame being oriented slightly inwards of the bikes path. While in a turn, the steering relative to the bike's frame may be inwards, straight ahead, or slightly outwards (relative to the frame), depending on the amount of inwards orientation of the bike's frame due to the rear tire contact patch flexing under load.

The contact patch on a tire flexes in response to a side load. Camber thrust is related to the net linear centripetal flexing of the contact patch. I'm not aware of a term related to the net twisting of the contact patch. Slip angle is related to the total (both linear and twisting) amount of flex of the contact patch, the angle between the actual path and the geometric path if there was no flex.

Sometime the term camber thrust is mis-used to describe an effect similar to a cone rolling on its side will roll in a circular path, but a two cone vehicle with one cone in front of the other cone, with parallel (no steering) axis, will travel in a (nearly) straight line, with a lot of skidding of the cones surfaces.
 

sophiecentaur

Science Advisor
Gold Member
23,049
3,657
One thing that doesn’t seem to have been mentioned is that a small drag force on the knee could produce a torque (about a vertical axis). Depending on where this force acts relative to the CM of bike and rider, it could alter the forces on the tyres and tend to steer in or out of the curve.
With M/C combination racing, a brake applied to the third wheel could have a similar effect ( different for right and left hand turns, of course). If anyone knows about this they could put me right about whether or not it’s used in racing. (IMO it is the nuttiest form of racing ever invented!)
 

Want to reply to this thread?

"Can a cornering motorcycle go faster if the rider puts a knee down?" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top