The ultimate front suspension geometry guide for BMW E30/E36/E46 by MRT Engineering, page 2
Chapter 1: E30 front suspension
1.2 Geometry correction
Before jumping into the world of interesting 5-lug swaps, let's take a look if we could improve that E30 non-M suspension and make it perform like an M3 or even better.
Some hints were given previously in the context.
Since most changes effect on everything and not just on one particular property, let's start with the simplest, most economical upgrade and add OE eccentric FCAB as on the M3 or rather use our adjustable FCAB to be able to adjust the suspension model for a different outcome. Eccentricity on a OE part is approximately 10mm compared to a standard concentric one.
Every 1mm adjustment (shorter) will add caster approximately by 0.1°, affect wheel position (increase wheelbase) by 1mm and add caster trail by 0.5mm. As a side effect adjustment will also tilt the strut assembly which will change the tie rod to LCA height ratio which may not be always wanted.
Here's E30 non-M geometry with MRT-20009-1 FCAB adjusted 10mm shorter:
E30 non-M front suspension with MRT-20009-1 (pdf)
As we'd expect with 10mm adjustment, caster is now increased from 9° to 10°, caster trail is increased from 10mm to 15mm and wheel position went from 55mm to 63mm (by the way we use control arm inner joint on the subframe as a reference point). We can also see that this change raised tie rod outer end by 2mm while the geometry seen from the front view remain about the same as before.
Next comes the interesting part again, how does this affect on bump steer and camber curves:
Tie rod height clearly has effect on bump steer, this time the change actually made the curve better, decreasing the difference by a whole 0.22° compared to a stock suspension. Camber curve follows the stock path by a minor variation.
Next popular single component is our bump steer adjustable tie rod kit. While being aluminum alloy and providing significant weight savings over the stock steel tie rods, these will allow fine tuning of bump steer ie. height adjustment of the tie rod end. We went through all 3 positions to define which works the best. As geometry (other than tie rod end height) won't change, we will jump straight to bump steer curves:
As seen on the pictures relatively small changes can even invert the curve direction. If you use the kit with stock suspension setup you'd likely choose middle or highest position to minimize bump steer, with added caster either middle position or lowest position will provide the best results. Clear improvements over the stock setup in any case.
Now it's time to move on to next, less known issue, which is roll-center. If you read the front suspension basics, it is easier to understand why lowering and control arm orientation highly effects this geometry.
With E30 and E36 LCA width roll-center to lowering ratio is roughly 2:1, every 1mm of lowering affects over 2mm on roll-center (lowers the height). Increased spindle height (from wheel axis to control arm outer joint) improves roll-center at same rate.
We have two different style roll-center correction kits available, first we take a look at the more popular roll-center / bump steer / angle -kit and how it will change geometry:
E30 non-M front suspension with MRT-20003-1 (pdf)
As expected with increased spindle height, roll-center has been improved by a bunch and is now 90mm above the ground. But not only roll-center has been effected, why so? Since steering axis inclination exists, for both KPI and caster, other parameters will follow along the axis. This slightly effects wheel position and caster as well as width and scrub radius. First two can be easily restored by adding our adjustable FCAB while width and scrub radius could be increased with wheel spacers if you're looking for E30 M3 numbers.
As we found out earlier, that higher tie rod position actually works better with E30 non-M struts, you may have noticed there's now 4mm difference compared to all stock setup. And here's the curves:
E30 non-M front suspension with MRT-20003-1 bump steer - camber chart (pdf)
Again with simple changes and optimization geometry has been improved a lot, at lower ride heights bump steer is minimal as the kits have been developed especially for the lowered cars.
Camber curve now looks smooth, as it should, thanks to restored control arm orientation.
Where the previous kit allowed use of stock tie rod, it had to have mounting point moved away from stock location which in addition makes steering ratio quicker and also adds steering lock due to shorter lever ratio, this wasn't preferred by everyone. We then developed our second roll-center / bump steer correction -kit that was made without the steering ratio / angle -change. Slightly different approach had to be used due to several design limitations and naturally as the stock tie rod can't be spaced down safely, we included our bump steer adjustable tie rods with extended hardware with these kits:
Changes are similar, but smaller compared to what we saw on the previous comparison.
As the tie rods have 3-position adjustment we run all settings to create the curves:
As with tie rods alone, with all three setting results are similar, the adjustment range can invert the curve direction. The sweet spot is somewhere between the middle and lowest setting with stock suspension, with added caster we'd lean towards the lowest setting.
Even the reduced correction length on these kits is enough to smooth out the camber curve, but camber gain is fraction of a degree less.
For now this is all we have to offer you about making your E30 a better handling track car. Hopefully this covers the most common questions and gives direction for choosing parts and making initial settings.
You're always welcome to suggest any additions or notify about errors in the context.
In the next chapter we take a look at E30 5-lug swaps using common and less common E36 and E46 spindles as well as other related components.