A4 Single Pivot Suspension System
Posted By admin On 07/04/22
A4 is a linkage driven single pivot suspension design. The added link helps to stiffen the frame, prevents the rear shock from binding and increases pedaling efficiency compared to a traditional single pivot design. Disc brakes for all models. BMW Single Pivot front suspension on E30, E36, E46, Z3, & Z4. (image shows early version of E46 front control arm) Strut: MacPherson design, 'coil over' spring on the strut housing with a large upper rubber-isolated mount.
A4 Single Pivot Suspension Systems
Axle | Part of the driveline. It's a shaft extending from the differential to the spindle and hub. They are more commonly known as half-shafts or axle-shafts. The 'axle' may also refer to the complete front or rear suspension, encompassing the shocks, control arms, bushings, sway bar, half-shafts, spindle, and even inclusive of the brakes. |
Ball Joint | An internal suspension component consisting of the a steel ball encased in a metal, plastic, or rubber isolated cup and with a threaded stud for mounting. The ball is able to rotate within the cup and allow the suspension component to change direction. Ball joints are typically mounted to allow horizontal movement (side to side). They are a common wear item as the internal plastic or rubber isolator breaks down and can no longer contain the ball. |
Bushing | Typically a formed rubber piece used to locate a component to another component. The sway bar is mounted to the body using a rubber block. Rubber is a favorite material (see paragraph above) but is often replaced with a stiffer rubber compound, urethane, Delrin, or even as rotating bearing. |
Camber | The angle of the tire in relation to a flat Earth. BMWs have slight negative camber as a baseline - the top of the tire tilts inward. As the car corners the outside tire will spread flatter to the pavement, increasing its contact patch. It's common to add more negative camber to get more grip on the race track. Too much negative camber causes the tire to ride on its inside shoulder when going straight. |
Camber Arm | A rear suspension component designed to control and/or adjust camber. On the E36/E46/Z4 it's the rear lower wishbone. It's made of sheetmetal and bends easily. It's designed to bend so that in an impact the arm takes all the energy and something else isn't damaged. If you notice one side of the rear is a different camber or ride height check the lower camber arm. |
Camber Plate | A front suspension component designed to control and/or adjust camber. The camber plate often replaces the upper strut mount. More negative camber is desired for track use to improve front grip and reduce understeer. Lowered cars often need the negative camber reduced to correct for tire wear. So not all camber plates are the same. |
Caster | The rearward tilt of the front steering axis centerline, measured in degrees. The Caster is the degrees between a vertical line drawn straight up from the steering pivot point and the angle of the strut. BMWs have non-adjustable positive caster where the strut is angled backwards towards the driver. This increases stability at high speeds. Too much will make steering effort higher. |
Coil-Over | A shock absorber with the spring mounted around the body of the shock. A MacPherson strut is technically a coil over. In the parts world, the term is reserved for performance shock and spring packages that have a threaded spring perch to allow ride height adjustments. |
Control Arm | A suspension component designed to maintain the desired suspension alignment during suspension movement. It will also absorb vibrations and impacts with a bushing. Multiple control arms will do a better job of controlling alignment. |
Damper | Another word for shock absorber or strut. Damper is a catch-all term that can apply to a front strut or rear shock. The damper's purpose is to control the motion of the spring and the rest of the suspension. The shock must be soft enough in compression to provide a comfortable ride but stiff enough in extension to keep the suspension from oscillating and keep the tire planted on the ground. |
Hub | A solid surface for mounting the wheel. It's often used as a reference for the attachment of other components - 'the strut is mounted to the hub'. Used interchangeably with the spindle. |
Hydrobearing | A rubber bushing that holds a thick dampening fluid to help absorb vibrations. Unfortunately, when the fluid is gone the remaining rubber bushing cannot properly dampen the vibration or control suspension motion. A clunk, rattle, or vibration will be felt. Fluid is a great damper of vibration but the bushing is not serviceable and it's easy for the fluid to leak or evaporate. Solid rubber or urethane are excellent alternatives. |
MacPherson Strut | A strut/damper design that acts as another link in the front suspension. This is different than a upper and lower control arm layout with a shock mounted independently from the arms. With a MacPherson design, the strut is an critical piece in the suspension assembly. With the strut mounted directly to the spindle the internal damper moves at nearly 1:1 ratio to the wheel. All BMW 1/2/3/4-series models use a MacPherson front strut. |
Multi-Link | A catch-all term for any suspension that has multiple control arms and attachment points to the chassis. The E9X rear suspension is known as a 5-link layout. |
Radius Arm | Another BMW term for a control arm. It's most often used on front upper control arms. |
Reinforcement | A piece of sheetmetal, shaped to match the pattern and shape of an existing sheetmetal body piece that will add strength and rigidity. Unfortunately, some BMW models require extra reinforcement in suspension locations. |
Shock | A rear suspension damper that slows and controls the coil spring. See 'Damper' above. A shock differs from a strut because the it is not an integral part of the suspension. The control arms, spring, and hub could function without a shock (but you wouldn't want them to). |
Shock Mount (RSM) | A rubber bushing used to mount the rear shock absorber to the chassis. |
Spindle | A central mounting point for multiple suspension components, including the outer control arms and the lower strut mount. It also provides a mounting point for the wheel bearing, hub, and brakes. |
Spring | All BMWs use either a steel coil spring or an airbag. Springs come in different lengths and rates, depending on the model and included options. Ride stiffness most often comes from the spring rate so it's very easy to go too stiff and make the rest of the suspension ineffective. Too stiff and too soft will both negatively affect tire grip and handling but there is always a sweet spot. |
Strut | A front suspension damper that slows and controls the coil spring. See 'Damper' above. A 'strut' differs from a shock because it carries the spring and has some load-bearing function. |
Strut Mount | A rubber bushing encased in steel or aluminum used to mount the front strut to the chassis. |
Subframe | A large steel or tubular structure bolted to the car's chassis and used to locate the rest of the control arms. The front also provides a mounting point for the engine mounts. The rear is also the mounting point for the differential and axle-shafts. |
Subframe Mount | A dense rubber mount pressed into the subframe that provides isolation for the subframe, differential, and control arms. |
Sway Bar (Anti-Roll Bar) | A solid steel bar that counteracts the suspension motion during cornering. It acts as a lever - twisting as the suspension compresses. As one side compresses the bar twists to force the opposite tire to the pavement. The bar is mounted to the body of the car with rubber bushings |
Sway Bar Link | The sway bar is attached to the suspension with a small arm called the link. A ball joint is located on each end and allows the link to rotate and move freely as the sway bar moves. |
Tension Strut | Another BMW term for a control arm. It's most often used on front upper control arms. |
Thrust Arm | Another BMW term for a control arm. It's most often used on front upper control arms. |
Toe | An alignment spec for the angle of the tire in relation to the centerline of the car. Toe is measured at the front/leading edge of the tire on the same axle. If the tires are pointing away from each other the toe is 'out'. When the tires point in towards each other, the toe is 'in'. Front toe adjustment is done via the tie rod. Rear toe is sometimes adjusted from either the trailing arm or a dedicated toe control arm. |
Trailing Arm | The principal control arm in the rear suspension. It's actually a semi-trailing arm because it's forward mounting point is ahead of the rear subframe. But it's shortened to trailing arm. On E36/E46 it's a huge steel piece and includes the rear spindle and hub. On later cars it's now much smaller and the spindle is separate. |
Wheel Bearing | A bearing used at each hub and allows the wheel and brake rotor to spin. The bearing is often pressed into the hub, which is then bolted or pressed to the spindle. |
Wishbone | Another term for a control arm. It's used on the front as the 'lower control arm' and on the rear in multiple points. It's a wishbone because it has three mounting surfaces - one at an end and two at the opposing end. |