# What makes a chassis move?

## Main Question or Discussion Point

I understand the basis behind the idea that a rotating wheel driven by an engine will generate torque and the static friction of the ground will push opposite to its direction forcing the tire to go forward. However, I cant seem to understand how the wheel can drag the chassis with it. Does the middle of the tire, which will follow a linear path if the tire rotates and drives along, push the wishbone? Is it the cv axle that push anything?because on a bicycle, when the tire rolls in front, the center of it, which follows a linear path, pushes the bike chassis. How does it work for a FWD car with suspension then?

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Simon Bridge
Homework Helper
If the wheel did not drag the chassis behind it - what would happen?

You seem to be able to follow what happens with a bicycle - it is exactly the same with a car with suspension except that the suspension springs change length during acceleration and braking. The wheels are attached to the chassis - how they are attached changes the characteristic of the motion.

jack action
Gold Member
The wheel pushes the axle, the axle pushes the bearing, the bearing pushes the suspension (upright & wishbone), the suspension pushes the chassis.

If there are no suspension, then the bearing pushes the chassis directly.

On your (non-powered) bicycle front wheel example, it's the opposite:

The chassis pushes the bearing, the bearing pushes the axle and the axle pushes the wheel. The tire-road friction causes the front wheel to roll as it moves forward.

On some non-powered wheel designs, the axle is fixed and the bearing is inside the wheel, such that:

The chassis pushes the axle, the axle pushes the bearing and the bearing pushes the wheel.

Cars dont have solid straight axles but cv axles (they bend). So my take is, if we take them off and turn the wheels with our hands instead of the engine, the hub/bearing assembly will push the knuckles which will push the wishbones and then the chassis.
Logical?

jack action
Gold Member
Cars dont have solid straight axles but cv axles (they bend). So my take is, if we take them off and turn the wheels with our hands instead of the engine, the hub/bearing assembly will push the knuckles which will push the wishbones and then the chassis.
Logical?
Not only logical, that is what happen and what I described. The part of the CV axle that goes within the hub/bearing assembly is what I refer as the "axle". No matter the design, there must be a part of the axle that is connected to the wheel (to allow rotation) and another part of the axle that is connected to a bearing (to support the lateral forces); CV joints or solid axle.

Here is a solid live axle:

The axle (23) pushes on the bearing (20), the bearing (20) pushes on the differential casing which is actually part of the suspension.

Here is one for an independent suspension:

The axle (green) pushes on the bearings (orange), the bearings (orange) push the upright (yellow) that is part of the suspension.

Here is a more complex one for a front live axle that incorporate steering and a locking/unlocking feature:

The "rotating" axle (21) goes through the "fixed" axle (60) and connects to the disc brake (67) via the locking/unlocking mechanism (70-76). The disc brake (67) pushes on the bearings (66 & 69) which in turn push on the "fixed" axle (60). The "fixed" axle is solidly linked to the differential casing and steering mechanism.