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

[Quasimodo]

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?

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.

 

[Quasimodo]

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]

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.

 

[Quasimodo]

@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!

 

[Quasimodo]

Here's another article in support of my claims by the same author: https://www.ridinginthezone.com/body-position-tips-2/

 

[rcgldr]

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.

 

[berkeman]

Apparently it also helps you go faster if you use your elbow as a feeler...

 

[DannoXYZ]

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

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.

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.

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.

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.

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

You roll off edge of tyre and crash! Street tyres tend to have flatter profile and won't 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 going to 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.

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.

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]

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!

 

[DannoXYZ]

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]

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

Was that at a track day? Keigwins or a different group?

 

[DannoXYZ]

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. :)

 

[berkeman]

track-day practice

Nice riding! Gotta love that "pass on the outside" track day rule

 

[rcgldr]

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]

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!)

 

[rcgldr]

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.

Any net "inwards" torque, would reduce the lateral force on the front tire and increase the lateral load on the rear tire, and the rear tire lateral force is already compromised since the rear tire is generating a forward force as well.

 

[sophiecentaur]

Any net "inwards" torque,

'Inwards or outwards' effect on direction would depend on where the horizontal component of force on the knee lies relative to the CM. I would think that the net effect would be either to alter the angle of the line of the the wheels into or out of the curve. The angle of the bike and the handlebars as it goes round a corner is chosen by the rider to be optimum and that optimum would be different if the knee were used (perhaps marginally).
One thing that would be interesting to know is where the CM of bike and rider lies. If it's above the knee pivot point then the lateral front wheel load would be greater and if its below then the front wheel load would be less.
I realize that any reduction of the vertical forces between road and tyres could affect the cornering force so the knee contact would reduce it but that force would not be very great (or the rider's knee would suffer).
I have never been to 'Speedway" / Dirt track(?) but the riders (iirc from films) actually have their inside foot dragging on the ground. Is that just to avoid falling over on the slippery surface or for another reason?

 

[DannoXYZ]

There IS a lot of squirming of contact patch when cornering because radius of tyre at edge is smaller than towards centre. When cornering, outside edge of tyre is forced to travel longer distance while centre is forced through smaller distance. Somewhere in middle is neutral. As I pick bike up coming out of corner, I can see RPMs of engine decrease as contact patch moves towards larger diameter centre of tyre.

Knee puck is hard plastic, nylon I think. Negligible amount of fricition when touching. Cornering speed doesn't change whether knee is touching or not, it's just a gauge. My lap-times do not change when I have speedometer cable attached or not.

On dirt track, foot on ground is for balance (think tripod). When sliding back end around, rider's body and leg touching is used to aim bike along with throttle.

 

[DannoXYZ]

academic discussions need numbers to understand various magnitudes of forces involved and their contribution to entire system.

bike = 315 lbs (48/52% F/R distribution)
rider = 180 lbs with full gear (helmet, suit, air vest, gloves, boots)
speed through T1 & T8 @ Thunderhill = 105mph
cornering G = 1.5g
load on knee-puck = 1-2 lbs max
effective grip of tyres > 1 (friction+mechanical grip)
effective grip of puck < 0.05

then more accurate model can be developed.

 

[berkeman]

As I pick bike up coming out of corner, I can see RPMs of engine decrease as contact patch moves towards larger diameter centre of tyre.

I'm guessing that you hear it. I've never had the guts to look down at my instruments anywhere except maybe mid-straight...

 

[DannoXYZ]

"net inwards torque" will always occur due to knee puck being inside of tyres' contact patch around curve. However, there are two rotations to consider here:

1. rotation of bike+rider around centre of curve
2. rotation of bike+rider around knee-puck contact (think moon rotating around Earth)

Now, let's take theoretical instance where rider uses super-sticky knee-puck that may actually rival grip of tyres, say... freshly-chewed bubble gum. As mentioned earlier, any vertical-loading of knee-puck sufficient to generate friction will take away from loading on tyres, will reduce grip and cornering centripetal force and cause to go on bigger curve (radius) around turn.

Now , if we have sufficiently sticky puck, which only drags in straight line BTW, it can force bike+rider to rotate around puck's contact patch (not around centre of turn). The effect of reducing tyre grip is bike takes straighter increased radius path through turn. With sticky-puck, bike+rider will carve even straighter line out of curve. CM will dictate whether they leave rider first or bike first out of curve. But there's no way the puck can generate additional centripetal force to carve a tighter line around curve.

 

[DannoXYZ]

I'm guessing that you hear it. I've never had the guts to look down at my instruments anywhere except maybe mid-straight...

heh, heh... yeah many riders have crashed into T1 @ TH by missing their braking markers while staring at their clocks to see if they broke +180mph! I actually use digital Trailtech Vapour unit with bar-graph tach (speedo disconnected). It's mounted higher than stock and I can see it in field-of-vision when tucked it. Also have yellow shift-light in corner programmed for 13 500 RPMs so I can see it without looking.

It's shown in this video where I follow my buddy Carlos around for couple laps to get video for his coach. In session 2, I get to demo Aprilia RSV4 ! Woohoo!

 

[DannoXYZ]

Need to clarify the dirt stuff.

For motocross and supermoto stuff, the foot down is for balance. When cornering over uneven surfaces, front tyre can slide after coming off bump mid-corner. The hanging foot then 'steps' down to keep rider upright, then throttle and steering keeps bike up and moving. Sometimes, if it's tight corner, it may take a couple steps, or 'walking' the bike through corner so you and bike doesn't fall over.

In flat-tracking on watered-down dirt track, steel-toed boot is used for balancing the bike's front & rear tracking. Since throttle is held fully-open all time for speed (ideally), rear tyre wants to step out sideways in corners. This is balanced by turning front tyre to outside of corner to slide front-end at roughly same angle as rear. By using steel-toed boot sliding on ground, the rider can unweight bike to adjust its lean angle to shift rear tyre's balance between cornering vs forward drive. Rider can also slide forwards or backwards on seat in order to adjust front & rear slide balance. All this while keeping throttle pinned wide open!

 

[sophiecentaur]

@DannoXYZ
Thanks for that information. The practicalities need to be known in order to apply the right Physics in this sort of thing. (The numbers count - as they say)

 

[berkeman]

For motocross and supermoto stuff, the foot down is for balance.

Fun quiz question -- Why do Moto GP riders sometimes put their leg out under heavy braking into corners?

https://www.redbull.com/us-en/the-history-of-motogp-leg-dangle

 

[sophiecentaur]

Fun quiz question -- Why do Moto GP riders sometimes put their leg out under heavy braking into corners?

https://www.redbull.com/us-en/the-history-of-motogp-leg-dangle
View attachment 245454

Answer: Naked Fear!

 

[DannoXYZ]

hahahahahhahhhh!

I like this one:

Others have suggested that its enthusiastic embrace is more the result of a copycat club, the clown prince of Grand Prix racing demonstrating his massive global influence via some sort of meta joke — the racing equivalent to convincing the world the proper way to eat a Snickers is with a knife and fork.

Rossi, the eternal jokester, is trying to see how outrageous he can make his copycats dance!

 

[berkeman]

Rossi, the eternal jokester, is trying to see how outrageous he can make his copycats dance!

LOL, yeah. The first time I saw him do it on TV during a race, it looked like he was doing it just to keep his legs relaxed. He did it at a few places on the track during breaking, and I thought to myself, "Okay, looks kind of dorkey, but if it helps to keep you relaxed whatever." I guess there may be (or may not be) more to it...

 

[DannoXYZ]

New Olympic sport: Synchronized Leg Dangling!

 

[bobob]

Summary: Does the rider benefit from putting a knee down by being able to take the corner faster?

I remember this problem from my freshman physics class ~38 years ago. As I recall it was explained to us how a motorcycle rider could actually take a corner faster by putting his inside knee down on the ground. We see this in motorcycle racing where the riders have a knee pad that let's them place their knee on the ground, effectively sliding it along the pavement as they lean into turns..

...
So the idea is that, if the rider puts his inside knee down on the ground while in the turn, that corresponding amount of weight is removed from the apparent centrifugal force. So there's a little less centrifugal force so that allow the rider to go a little bit faster then when his knee was not sliding on the ground.

That is not quite how that works. The reason that riders put tgheir knees down (or possibly a little short of actually touching the ground) is that they use their body position to gauge their lean angle so they don't exceed the leangle possible for the bike. The knee actually doesn't function to assist in the turn apart from it's position contributing to getting the rest of your body into the right position. In order to get the maximum lean the rider actually shifts his entire body weight by moving somewhat back in the seat, which places a bit more weight over the rear tire with his body hanging slightly off the bike over the knee that is down with his chest and head pointed in the direction of the turn. Then the key thing to pulling off the turn while leaned way over is to countersteer, i.e., the front wheel is turned in the opposite direction the bike is turning. If you watch a video of motogp riders and you can see a bike from the front, you will notice that front wheel is NOT turned in the direction of the turn.

The onnly real purpose the knee serves beyond gauging the amount of lean and helping to obtain the right body position is to try to save your ass using brute force to keep the bike from falling over if you misjudge the lean angle.Actually, if you just look at the photos above, you can see that the front wheel is not pointed in the direction of the turn.

 

[Elroch]

For sure not the high speed banked turns like Daytona. I've read that the g-forces in those banked turns are high enough that the riders have to rest their helmets on a small pad on the tank, because they cannot hold their heads up against those strong g-forces. Yikes!

This cannot be strictly true. The sideways component of the force is of the order of 1.5 g, because it is limited by the friction of the tyres on the road. See https://www.f1technical.net/forum/viewtopic.php?t=20627 for data for F1. The support would make it a lot more comfortable and less tiring on the neck muscles.

 

[berkeman]

This cannot be strictly true. The sideways component of the force is of the order of 1.5 g, because it is limited by the friction of the tyres on the road. See https://www.f1technical.net/forum/viewtopic.php?t=20627 for data for F1. The support would make it a lot more comfortable and less tiring on the neck muscles.

Did you take the banking into account? That is what makes the difference...

 

[Elroch]

Did you take the banking into account? That is what makes the difference...

Short answer: not correctly! Long answer leaves me skeptical for a different reason.
For a car, in the frame where the road is horizontal, you can think of the component of the centrifugal force that is perpendicular to the road as being an increment k * g to gravity so g is replaced by g * (1 + k), and forces are scaled up with this pseudogravity in a fictitious frame where the road is horizontal.

So sideways acceleration experienced by the head of a car driver can be as high as g * (1 + k) * mu, where mu is the coefficient of friction. This is definitely relevant to neck strain for car drivers on banked tracks.

But for a bike, the vertical axis of the bike needs to be aligned with the total acceleration in a frame of reference that is rigidly attached to the bike. Otherwise it falls over - it only has stability in one direction, not two like the car. So the additional force is solely in the vertical axis of the bike's frame of reference (the vertical direction of its frame). My first thought was that this mean no lateral neck strain, but the rider's head is pointing forwards, so his head is being pulled forwards from his point of view, and is subject to that enhanced gravity, g * (1 + k).
I really can't see where the rider can rest his head (see image), so I think he just has to build his neck muscles if this is a problem.

 

[DannoXYZ]

Note that friction is not what generates traction forces on tyres. This was resolved decades ago when friction-model would only allow dragsters to reach 200mph maximum. We now know that's not true.

For example, F=Nu is higher for rubber+steel than it is for rubber+asphalt. Yet going around corners on asphalt has much more grip than sliding over sewer-lids.

 

[DannoXYZ]

So sideways acceleration experienced by the head of a car driver can be as high as g * (1 + k) * mu, where mu is the coefficient of friction. This is definitely relevant to neck strain for car drivers on banked tracks.

But for a bike, the vertical axis of the bike needs to be aligned with the total acceleration in a frame of reference that is rigidly attached to the bike. Otherwise it falls over - it only has stability in one direction, not two like the car.

Don't forget that there's actually two forces on cornering bike. Downwards gravity and lateral centripetal force.

This is similar to if you hung an apple from rear-view mirror of car via string. String would have 1G when car goes in straight line. Under 1G of cornering, string would experience 2G total. Note that apple and string are no longer vertical, but hanging at 45-degrees, aiming towards outside of turn.

Now, hang that same apple from bike's... rear-view mirror. It too would have 1G of load when bike goes in straight line. Under 1G of cornering, string would experience 2G of load. This is supported by monitoring suspension-travel and under steady-state cornering of +1G, suspension-compression is much higher than during upright-riding. On many street-bikes, suspension is completely compressed and locked at full-travel. Thus street-bikes taken to track must have upgraded suspension with much stiffer springs.