Most street tires do not provide enough traction for a car to flip over forwards. Cars will even skid sideways before rolling on most pavement surfaces. Vehicles with a high center of mass and a short wheelbase can do endos, like dirt bikes. I could hop the rear wheel of my mountain bike off the ground by purposely putting my weight way forward and hitting the front brake.
Another vehicle that is prone to nose overs is tail dragger airplanes. The main wheels have to be just slightly forward of the airplane’s center of mass. If their location is too far forward, the plane won’t nose over, however it becomes so directionally unstable while taxiing that it goes into an uncontrollable turn and does what is known as a “ground loop”.
…and sport bikes/racing motorcycles. My former standard motorcycle, a Nighthawk 750, never did this, even in emergency braking, although I considered it might be possible.
Now that all new cars are required to have ABS, it’s even less likely to happen to any four-wheeled cage.
A front endo is pretty much physically impossible on any normal car or truck. Too much wheelbase, too low a CG and not enough traction.
The reverse is a “wheelie” where the engine lifts the front wheels off the ground. The new Dodge Challanger SRT Demon will lift the front tires a small bit but it has 840 hp and special high traction tires.
Before ABS systems and disc brakes automobile brakes were engineered to reduce lock up and make use of the traction on all 4 wheels by matching the size of the drums and the diameter of wheel cylinders to the traction available and the engineers were somewhat successful. Front brake drums on large top of the line cars were larger than the rear with larger diameter wheel cylinders.
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No argument that the endo thing isn’t a common occurrence, if it ever happens. For the sake of argument, what if the brakes did stick to the road with the same fictional force front to rear? There remains a difference of where that frictional stopping force is applied w/respect to the center of mass of the moving vehicle. With the front brakes only applied, the center of mass tends to rotate up, with the rotation center at the front wheels. With the rear brakes applied the center of mass rotates down, with the rotation center at the rear wheels. That’s how it seems in my pinhead way of thinking anyway.
Having practiced panic stops in motorcycle safety class, I can tell you what happens. The rear tire skids until the vehicle stops. When we repeatedly do the exercise where we practice panic stops, we experiment with applying different pressure to the front brake and the rear brake. You can learn a lot in that process, but the most important thing you learn is not to release the rear brake until you come to a complete stop.
On a sport bike, you end up doing an endo. If the front end is raked out, like on a cruiser, the rear end cannot lift up. The cruiser more closely approximates the geometry of a car.
The thing is, that front wheel cannot “stick” like what happened in that recent Batman movie with the semi truck. The friction created by the front wheel cannot create the same jolt as the cable that Batman attached to the front of the truck that the Joker was driving.
Decceleration no matter which end it comes from causes the CG to do the same thing, compress the front and raise the rear. It doesn’t rotate up or down depending on which end that deccel comes from.
That said, the suspension design tends to make the front resist compressing - called brake anti dive - and the rear is designed to resist the rise - called brake anti-lift. So in practical use it does matter which end is braking.
Overall, brake systems are designed, by regulation, so the rears don’t lock before the fronts. Locked rears give nondirectional stability, locked fronts just slide in the direction the car was going when they locked.
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What @Mustangman said. Weight shifts forward, rear end up, front end down, regardless of which end does the braking. Much more with front brakes because they can exert much more stopping force.
Many years ago when dealing with 3/4 ton chassis cabs with roach coach beds on them became a problem due to the front-rear bias of the brakes. When the brakes were applied on wet pavement or even pavement with loose sand or gravel on it the front wheels would lock and if the wheel was turned the truck would skid straight ahead then suddenly dart in the direction of the turn. I experimented with wheel cylinder diameters and proportioning valves from 1 ton chassis and found a satisfactory combination testing them on a nearby gravel drive and a grassy lot. One truck had an especially large and heavy bed and a 1 ton axle with larger drums was needed to handle it.
FWIW, life was so much simpler years ago when litigation wasn’t such an issue. But I had the owner and driver take the trucks for a brutal road test and sign off on the work referencing the original issues and never faced any comebacks. As a matter of fact the company owner and many of the drivers were regular customers.
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That 3/4 ton truck was designed for worse-case braking which is un-loaded. Add all that roach-coach hardware and now the brakes are front-biased, as your customers found. Before ABS, you had to do that or add some load-compensating pressure regulator in the rear brake line. Where the chassis up-fitter that installed the roach coach stuff failed, you succeeded!
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So I had to look up Roach Coach, Food truck or mini restaurant on wheels. I find it interesting the action of helium balloons in a car, come to a stop the balloons move backward, turn right they move to the right, air pressure change I guess.
It’s the Special Theory of Relativity. The deceleration imposes what is equivalent to an added gravitational field. Braking is the same as tipping the rear of the car up, which would, of course, cause the balloons to go to the rear.
It’s the air in the car moving forward in the car when you come to a stop that pushes the helium balloon backwards.
True, but that’s b/c of the bobble head doll effect. the body isn’t bolted to the wheels, only attached with springs. Any deceleration at the wheels front or back will make the body pitch forward and down, which raises the rear end. That’s a separate effect from what I was posting about above.
As long as it come from the tire contact, this is true. However, if you had a parachute fixed to a point on the car above the center of mass, it would do the opposite.
Extreme acceleration on a motorcycle makes it do a wheelie. A strong gust of wind on a sailboat going downwind can result in a “pitchpole” capsize, lifting the stern out of the water, sort of a reverse wheelie. Hobie 14 catamarans were particularly prone to pitchpole.
Also, when running downwind, you can feel the rudder push and pull as the sail’s center of area moves from one side of the boat to the other on rolling waves.
As I said above I think that depends on the type of motorcycle. On a cruiser or a chopper with a raked front end, I’ve never seen one pop a wheelie. I’ve also never seen a cruiser or a chopper do an Endo.
I only mention this because I think the weight distribution in a plane on a straight line of a slightly raked cruiser would resemble the geometry of a car stopping in a straight line more than that of a sport or racing motorcycle.
Well, I suppose drag bikes are long and low to the ground for a reason.
Also, the army’s M-16 rifle has the sights sitting way above the barrel on tall stilts so the barrel can be nearly inline with the stock and close to the gun’s center of mass, resulting in a gun that pushes nearly straight back instead of having the muzzle climb up in the air when firing full auto. It’s the same principle.
Agree with your examples but cars (non-dragsters anyway) get their primary dynamic forces from the tires touching the ground below about 60 mph. Aero comes into play, in small doses above that.
I was referring to this comments, as I believe you are as well. The answer is No, the center of mass (CG) doesn’t tend to rotate UP, it just rotates. The force at the ground and the reaction at the CG create a torque. The magnitude of that torque is the decceleration times the mass times the height of the CG. That torque pitches the body and the angle of pitch depends on the magnitude of the torque and the stiffness of the front and rear springs. I think you are thinking the suspension goes stiff (locks up) when brake torque is applied and the body rotates about the front wheels raising the CG, it does not do that.
Again, no, as it appears you again are thinking the suspension goes stiff and the body rotates about the rear tires forcing the CG down. It does not do that. As an example, if the brake torque is the same while braking front only or rear only, the pitch angle created is the same since the torque generated is the same. The calculation is the same whether is comes from the front or the rear.