Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Mechanized82 on 04/12/2019 15:32:38
-
I'm going to do my best to clarify my question here. I understand the basics that a motorcycle needs to lean in order to counter the centrifugal (centripetal? i mix those up often) force from turning, otherwise it would just fall over toward the outside of a turn. In motorcycle racing, riders hang off very far to the inside of a turn, and this act reduces the lean angle required in order for the bike to turn, effectively allowing them to then increase the speed and lean the bike over to its maximum lean angle before losing grip. In other words there is a maximum speed/lean angle that bike can go before losing grip. Leaning to the outside will increase lean angle, which needs to be countered by slower speeds in order to maintain grip while hanging on the inside will decrease lean angle which can then increases the maximum speed before loss of traction. So here are my questions, sorry, there are more than just the title of the thread.
1. Does a heavier or taller rider have a greater effect on the lean angle of a motorcycle?
I'm assuming that the rider is a moment arm applying a torque to the overall equation, and that the heavier or taller the rider the more torque added.
2. What has a greater effect? Leaning away from the bike or lower on the bike? Meaning.. leaning my head toward 3 o'clock on a right hand turn, vs leaning toward 5 o'clock (this is lower, but not as far away, because you're basically attached to the side of the bike)
I asked a guy with a phd in physics about this question and we deduced that there are 2 forces applied to a motorcycle in a turn, Gravity and centrifugal force, and that your body has an effect on each force depending on how far away and how low you are. Long story short he said he thinks that being lower on the inside is more important than further away, especially during a fast turn where centrifugal force is greater than gravity, but he didn't sound very sure, so here I am. :D
-
I'm going to do my best to clarify my question here. I understand the basics that a motorcycle needs to lean in order to counter the centrifugal (centripetal? i mix those up often) force from turning, otherwise it would just fall over toward the outside of a turn. In motorcycle racing, riders hang off very far to the inside of a turn, and this act reduces the lean angle required in order for the bike to turn, effectively allowing them to then increase the speed and lean the bike over to its maximum lean angle before losing grip.
This is done because if the rider were to maintain a straight line with his bike, the sides of the bike would touch the track and the wheels would be lifted. The bike can only lean so far, and that's no enough to move their center of gravity as far over as is needed to turn at those speeds. So the rider does the extra work that the bike cannot.
In other words there is a maximum speed/lean angle that bike can go before losing grip.
Max angle yes, speed no. I mean there certainly is a max speed for a bike before it loses its grip, but those bikes are not sufficiently powerful to reach those speeds. Aerodynamic stability is usually lost before loss of traction. There are bike categories in the Bonneville events.
1. Does a heavier or taller rider have a greater effect on the lean angle of a motorcycle?
Not if his center of gravity is in line with the bike. The angle is then purely a function of speed and radius of the turn. But these guys don't stay in line with their bikes.
I'm assuming that the rider is a moment arm applying a torque to the overall equation, and that the heavier or taller the rider the more torque added.
Not really a torque thing, it is a center of gravity thing. If a mouse leaned to the inside of a turn, it isn't going to move the center of gravity of the bike/rider much. The bike can only turn so fast. But if a much heavier rider (one that outweighs the bike say) leans far over and gets as low as possible, that makes a significant change in the center of gravity of the bike/rider pair, and the turn can be taken faster.
But one the straight sections, the bike with lard boy is also going to have more trouble accelerating. A fat rider can only get his center of gravity down so far.
2. What has a greater effect? Leaning away from the bike or lower on the bike?
You want to move the center of gravity as low and to the side as possible. Taking the bike angle as defining 12 oclock, I'd say go for 3 oclock (down towards the track) and not outward (more like 1 oclock). The latter would help, but the g forces will just not allow one to move that way.
I asked a guy with a phd in physics about this question and we deduced that there are 2 forces applied to a motorcycle in a turn, Gravity and centrifugal force, and that your body has an effect on each force depending on how far away and how low you are.
The gravity is constant (1g), and the centripetal force (track force accelerating the bike to the right) is going to be tan(θ) where θ is the angle made by line connecting the center of gravity to the track contact point, relative to a vertical line. The faster you go, the larger this lean angle is. Speed is limited by how far over you can push this center of gravity.
(https://qph.fs.quoracdn.net/main-qimg-48d8cf770ff07bc2eee547592f382e34.webp)
This guy's bike is leaning nearly 60° but the CoG is perhaps a bit left of that 'R' for a net lean of more like 70°. He's making no effort to raise himself up to move that CoG outward. Doing so would obviously just create wind drag for one thing.
Long story short he said he thinks that being lower on the inside is more important than further away
If the angle is large, then lowering yourself by a cm has more effect on the angle than does moving further away 1 cm, so the mathematics bears this out.
Also, it is really difficult to move yourself further away. You don't see these guys trying to stand on their bikes like the horse jockeys do.
-
I'm going to add that I'm a race track coach and while I love the topic of physics, I'm not good at it. I do my best to use the proper physics terminology, feel free to correct me if I do. With that said, much of what I relate here is from personal experience, I just have this quest for knowledge in order to make me a better teacher on the track
Max angle yes, speed no. I mean there certainly is a max speed for a bike before it loses its grip, but those bikes are not sufficiently powerful to reach those speeds.
Aerodynamic stability is usually lost before loss of traction. There are bike categories in the Bonneville events.
We are talking about a corner so when speed increases the amount of sheering force where the rubber connects to the road increases due to centrifugal force pushing the tire sideways on the road, as well as lean angle... lean angle also reduces grip because the contact patch (rubber) shrinks due to less weight no the tire. An upright bike has the largest contact patch because gravity combined with the rider and bike's weight "flattens" the tire a bit giving more grip. As you turn a bike over this contact patch shrinks. I don't know much about the physics here but I assure you of these facts we teach at the track. 1. Speed and lean angle go together, the faster you go the more the bike leans over. so lets say 1 bike taking the same corner at 60mph has 40 degrees of lean angle, while one taking the turn at 85mph needs 55 degrees of lean angle. 2. When someone hangs off it reduces lean angle, and when we get that reduction we just go faster to achieve faster lap times. This means in the previous example, the second rider now hangs off which reduces the angle to 45 again at 85mph... then he increases his speed to 110mph and lean angle back up to 55 degrees 3. In a corner there is definitely a maximum speed that a turn can be taken until the tires lose grip. Depends on the tires and surface grip of course, friction.
Not if his center of gravity is in line with the bike. The angle is then purely a function of speed and radius of the turn. But these guys don't stay in line with their bikes.
That is my question though... So.. for simplicity sake, lets say I hang off a bike 12 inches and I weigh 155lbs and it reduces the lean angle from 55 to 45 degrees. I'm assuming a 255lb rider who hangs off the bike 12 inches will be able to reduce the lean angle maybe to 40, or 35 degrees because his weight has a larger effect on the CoG. Maybe I'm wrong, but I've definitely noticed on the track that big guys do not need to hang off the bike at all while smaller riders like myself have to do what you pictured above to move the same speed around the same turn. I think you answered my question below, I'm just responding in order lol.
Not really a torque thing, it is a center of gravity thing. If a mouse leaned to the inside of a turn, it isn't going to move the center of gravity of the bike/rider much. The bike can only turn so fast. But if a much heavier rider (one that outweighs the bike say) leans far over and gets as low as possible, that makes a significant change in the center of gravity of the bike/rider pair, and the turn can be taken faster.
But one the straight sections, the bike with lard boy is also going to have more trouble accelerating. A fat rider can only get his center of gravity down so far.
I think I saw it in terms of torque because of how my friend explained it. The CoG is the torque in this equation so the larger rider is able to effect CoG to a greater extent which changes the torque. he said there is torque because where the tire meets the road is the pivot point and from there up to the CoG is your lever. And that CoG has a toque force due to gravity cuz the CoG in a turn is no longer over top of the tires, its off to the side, acting like a lever on the bike. And also the centrifugal force pushing the bike to the outside of the turn which is a horizontal force... so the lower you get, in order to lower the CoG, the less leverage you have pushing against the bike horizontally.
I get the gist of the rest of your post. The forum will not let me link to an outside picture but do a google search for "valentino rossi hang off" and you'll see some pictures of him hanging far off to the side rather than further down and hugging the side of the bike like the pic you posted.
There are some people trying to say that this method is better but it doesn't sound like the physics agrees.
-
We are talking about a corner so when speed increases the amount of sheering force where the rubber connects to the road increases due to centrifugal force pushing the tire sideways on the road, as well as lean angle.
Fair enough. Angle is a function of speed and turn radius, so the answer to your question is yes: there's a limit before the grip is lost.
lean angle also reduces grip because the contact patch (rubber) shrinks due to less weight no the tire.
This can't be. Gravity is no different, so the weight (on a level track at least) is constant. If the turn is banked then the centrifugal force adds force (weight) to the tire/track interface.
An upright bike has the largest contact patch because gravity combined with the rider and bike's weight "flattens" the tire a bit giving more grip. As you turn a bike over this contact patch shrinks.
Perhaps so. Don't know the properties of those rounded tires and how the contact area is affected by riding the side of the tire rather than the center face.
I don't know much about the physics here but I assure you of these facts we teach at the track. 1. Speed and lean angle go together, the faster you go the more the bike leans over. so lets say 1 bike taking the same corner at 60mph has 40 degrees of lean angle, while one taking the turn at 85mph needs 55 degrees of lean angle.
And this would be true regardless of weight or height or the rider leaning or anything. It would be true of a unicycle, but not of a segway (side-by-side tires). The rider leaning changes the orientation of the bike, but not the lean angle (center of gravity to contact with track).
That is my question though... So.. for simplicity sake, lets say I hang off a bike 12 inches and I weigh 155lbs and it reduces the lean angle from 55 to 45 degrees.
OK, you're measuring the angle of the bike, and I'm measuring the angle of the whole system, which is fixed for a given speed. Yes, the bike is effectively what is being unnaturally rotated when the rider twists it out from underneath him to reduce the bike angle.
I'm assuming a 255lb rider who hangs off the bike 12 inches will be able to reduce the lean angle maybe to 40, or 35 degrees because his weight has a larger effect on the CoG.
Yup. Bike weight is sort of uniform, but rider weight is not. That said, a thinner guy can get his CoG closer to the ground.
Maybe I'm wrong, but I've definitely noticed on the track that big guys do not need to hang off the bike at all while smaller riders like myself have to do what you pictured above to move the same speed around the same turn.
If the rider stays above the bike, then their respective weights should make no difference. I don't see how the heavier guy could take the turn any faster. Maybe the extra weight makes it less likely to lose grip despite the added lateral force of accelerating the extra mass around the turn.
I think I saw it in terms of torque because of how my friend explained it. The CoG is the torque in this equation so the larger rider is able to effect CoG to a greater extent which changes the torque. he said there is torque because where the tire meets the road is the pivot point and from there up to the CoG is your lever.
But the force from the CoG (leaning or not) is straight at the pivot point, so there's no torque at all. Sorry. Not getting the torque way of looking at it. Torque causes rotation, and rotation (tipping) is disaster for a bike on a turn.
And that CoG has a toque force due to gravity cuz the CoG in a turn is no longer over top of the tires, its off to the side, acting like a lever on the bike.
OK, I see that. I would think of it as vector addition, as opposed to the torque of gravity (weight * sin θ) balance by the torque of the centripetal force (weight * cos θ).
I get the gist of the rest of your post. The forum will not let me link to an outside picture but do a google search for "valentino rossi hang off" and you'll see some pictures of him hanging far off to the side rather than further down and hugging the side of the bike like the pic you posted.
Looks like more air drag might result.