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> Are you sure? Yes. It's pretty confusing, I did the same thing and came up with the same answer as you. Then I typed a sentence about "even on fully indepen

> Are you sure? Yes. It's pretty confusing, I did the same thing and came up with the same answer as you. Then I typed a sentence about "even on fully independent multi-link rear suspensions" before going off into the weeds. If the suspension were rigid, it seems like it should do the opposite, climbing upwards, but there is more going on in the car than the assumptions we typically make with a model. A rear suspension is of course not rigid. It is, in short, a vertical damped spring mounted on a lever attached to the frame in front of it's axis. The axle/wheel hub is attached to the bottom of this damped spring at the end of the lever. During braking, the rear will deflect rearwards and also be drawn towards the centerline of the vehicle following the arc of a circle drawn on the ground. The suspension will control the forces through the trailing arm, directing them towards the rear along a second arc drawn on the side of the vehicle, centered on the rotational vertex where the trailing arm is bolted to the car in front of its axle. The sway bar will control both sides to the same level which will be the apogee of the circle, closest to the rear bumper. You can test this by pulling your parking brake below 5mph. If this doesn't work, call your mechanic :) >You can't easily compare because the rear wheel easily skids with a fraction of the braking force the front wheel can take. That is because braking the front bike wheel causes the lever to apply an upward force on the rear axle, pulling the front wheel rearwards, causing absurd instability and poor handling. This also happens to the rear wheel, it wants to hop upwards in response to the brake forces. That's why the proportional brakes exist in highway vehicles, to stabilize the braking forces into the most efficient braking scenario. If the front wheel were mounted in a reverse mounted trailing arm suspension (This doesn't exist, so I don't know what else to call it.) it would drop the front of the bike as happens to a car instead of flipping you off the bike. The side effects of this are atrocious understeer and a nonsensical center of gravity. The reason the wheels move like this is to control for a characteristic called caster. Bikes have negative caster, if you let go of the handlebars at low speed, they will flop to one side or another at random, causing you to crash. Cars have positive caster which is the most stable configuration for the steering to operate. This is also why your steering wheel returns to center when you let go of the wheel while moving forward, and it is why the braking forces can be stabilized in this manner.

exporectomy 5y ago
I see. It's specifically an effect of the trailing arm type of suspension. Damping, the wheel's rotational inertia, the sway bar and lateral movement of the wheel aren't needed to explain this, are they?
- cartoonworld 5y ago
  No, I suppose the sway bar can be ignored for the conceptualization, the only thing is cars have 4-wheels and they all affect each other. If you want to simplify it, you gotta invent a 2-wheeled conveyance. The shocks and sway do a lot of work. If that satisfies you, don't read the rest:) Some vehicles don't have swaybars, or instead they might have a "solid rear" rear subframe to which both knuckles are attached, and moves as one large unit. They always have shock absorbers though. I'll just keep considering only the rear axle here. I can tell you are away of, but do note, that even without a trailing arm configuration but something else, the wheels will still always want to go back and towards the centerline of the vehicle during braking. The suspension geometry will determine how this is controlled, usually the move is to go down. I've never seen one that goes up unless something is broken, usually the shocks. People normally call this excessive movement "nose dive". It happens because a functional shock absorber momentarily control the climb, allowing the suspension to control the wheel characteristics. Without it, the forces will yank the wheel out of configuration. In most vehicles, without the sway bar controlling both sides of the suspension, or the shock absorber damping, you will experience many varieties of reduced braking and handling performance unless both wheels on the axle are reacting in exactly the same way, which is very unlikely. If the spring is undamped, aside from the "nose dive", it will start to oscillate in an uncontrolled manner during braking, causing hopping. This will cause the wheels to both not brake well and also lock up. This will still affect the solid rear, causing wonky camber. If one wheel locks up, the ABS will kick in. This is normally fine, and increases performance in all categories, but only if the wheels are in contact with the ground and exhibiting correct camber. If they aren't and are instead hopping because of the undamped spring, the ABS won't be able to control much, it will in fact suck. If the sway bar is absent/broken/ the linkage or bushings damaged, and one wheel has different handling or braking performance than the other wheel on the axle, (such as in all the time or during hopping or ABS engagement) the ride height will change as the wheel reacts along the second circle, causing body roll which will alter camber and other factors on every other wheel, especially the opposing wheel, which will experience much worse performance in every category. All this stuff will show up on the tire. You can often tell whats wrong just by glancing at the tire tread.

exporectomy 5y ago

I see. It's specifically an effect of the trailing arm type of suspension. Damping, the wheel's rotational inertia, the sway bar and lateral movement of the wheel aren't needed to explain this, are they?

cartoonworld 5y ago
No, I suppose the sway bar can be ignored for the conceptualization, the only thing is cars have 4-wheels and they all affect each other. If you want to simplify it, you gotta invent a 2-wheeled conveyance. The shocks and sway do a lot of work. If that satisfies you, don't read the rest:) Some vehicles don't have swaybars, or instead they might have a "solid rear" rear subframe to which both knuckles are attached, and moves as one large unit. They always have shock absorbers though. I'll just keep considering only the rear axle here. I can tell you are away of, but do note, that even without a trailing arm configuration but something else, the wheels will still always want to go back and towards the centerline of the vehicle during braking. The suspension geometry will determine how this is controlled, usually the move is to go down. I've never seen one that goes up unless something is broken, usually the shocks. People normally call this excessive movement "nose dive". It happens because a functional shock absorber momentarily control the climb, allowing the suspension to control the wheel characteristics. Without it, the forces will yank the wheel out of configuration. In most vehicles, without the sway bar controlling both sides of the suspension, or the shock absorber damping, you will experience many varieties of reduced braking and handling performance unless both wheels on the axle are reacting in exactly the same way, which is very unlikely. If the spring is undamped, aside from the "nose dive", it will start to oscillate in an uncontrolled manner during braking, causing hopping. This will cause the wheels to both not brake well and also lock up. This will still affect the solid rear, causing wonky camber. If one wheel locks up, the ABS will kick in. This is normally fine, and increases performance in all categories, but only if the wheels are in contact with the ground and exhibiting correct camber. If they aren't and are instead hopping because of the undamped spring, the ABS won't be able to control much, it will in fact suck. If the sway bar is absent/broken/ the linkage or bushings damaged, and one wheel has different handling or braking performance than the other wheel on the axle, (such as in all the time or during hopping or ABS engagement) the ride height will change as the wheel reacts along the second circle, causing body roll which will alter camber and other factors on every other wheel, especially the opposing wheel, which will experience much worse performance in every category. All this stuff will show up on the tire. You can often tell whats wrong just by glancing at the tire tread.