The ultimate front suspension geometry guide for BMW E30/E36/E46 by MRT Engineering, page 1
Introduction
In this section we take a deep dive to BMW E30/E36/E46 front suspension in its eternity including common and uncommon swaps. Since all these models share a similar design most components are interchangeable as is or with minor changes, but as seen later, the outcome may not be anywhere near optimal.
We will also talk about how to make these swap parts work better with recommendations of MRT Engineering parts and products.
We will be using our generated 3D model assemblies of 3D scanned components, which dimensions are backed up by precise manual measurements and/or based on factory blueprints. Even though precise measurements are used, the tolerance of a complete assembly grows by adding multiple components, thus we mostly use rounded numbers.
In numeric tables, such as bump steer or camber curve tables, we will use more decimals to highlight the changes. 100mm suspension sweep (±50mm) is used for demonstration.
In all assemblies we will be using a chassis at same, fixed ride height. We often hear or read about description of "control arms being in horizontal orientation", this roughly converted to 150mm measured from ground to steering rack mounts' bottom plane.
As best assumption wheel size is based on OE tyre size of the spindle/knuckle used in the assembly.
It should be noted that no flexibility of bushings/top mounts/joints/tires etc. is taken on account so the numbers may not represent real-life values perfectly, but can be used for reference undoubtedly.
If you're not familiar with the geometry terminology, please read our basics first HERE.
Chapter 1: E30 front suspension
1.1 Baseline
We start from the oldest, in this case E30 chassis, which is probably the most common platform for various suspension swaps using parts from E36 and E46.
Before going "all-out" let's see how a stock E30 non-M geometry looks like when lowered to meet our 150mm ride height.
E30 non-M front suspension (pdf)
As seen on the pictures (and more clearly on pdf format) caster trail and scrub radius are pretty neutral as you may expect on a factory car.
Camber and caster angles are slightly off the factory values due to lowering.
What you cannot see or measure by hand is roll-center, which has come down a significant amount, still above ground, and causes more body roll.
Please pay attention to tie rod - LCA height difference, on a stock setup inner and outer ends have almost equal distance and we commonly hear about setting the control arm and tie rod parallel for neutral bump steer. Later on this chapter we'll see that it can be a helpful reference, but front end geometry is much more complex than that.
Once we had made the assembly we then performed 100mm (±50mm) suspension sweep to create bump steer and camber curves for reference:
E30 non-M front suspension bump steer - camber chart (pdf)
It is obvious that bump steer is present in this setup, toe is presented per wheel so total toe would be double the value. Toe in degrees may not sound much, but if we convert it to length, toe change of 0.45° at wheel diameter would be as much as 4.73mm! When we go up the other way, 0.3° toe change equals to 3.17mm. (As a rough conversion you can multiple degrees by 10 to give reference value in millimetres.)
Camber curve shape is inconsistent since control arm orientation goes from pointing upwards to downwards.
Next let's take a look at E30 M3 front end geometry which is usually considered as the best handling factory setup. Since parts are getting rare and super expensive this may not be the first swap option nowadays.
E30 M3 front suspension (pdf)
Changes on a E30 M3 front suspension compared to a non-M car are marginal yet very efficient.
Since this setup uses eccentric front control arm bushing (FCAB), caster is increased.
Due to caster change caster trail has been increased too which will effect on steering self centering and camber change on cornering.
M3 strut has a greater length from wheel axis to control arm pivot (spindle height) which has a positive effect on roll-center, at same ride height, use of this strut results in 27mm higher roll-center than the non-M counterpart.
Scrub radius change is mainly due to a lower wheel offset.
We then performed the same 100mm (±50mm) sweep for suspension to create the curves. If you paid attention to tie rod to LCA height differences or parallelism, what would you think that 3mm difference makes on bump steer?
E30 M3 front suspension bump steer - camber chart (pdf)
After multiple checks and trials we just cannot make it worse, with this lowered height and all solid joints the bump steer geometry seems to be spot on with very minimal changes.
Camber curve has more ideal shape than the non-M due to increased spindle height, M3 seemingly has more room for lowering and works better in these conditions.
Let's consider this as "the good baseline" and see how can we make things better (or worse).