The ultimate front suspension geometry guide for BMW E30/E36/E46 by MRT Engineering, page 3
Chapter 1: E30 front suspension
1.3 Suspension swaps
In the previous chapter we went through some popular geometry correction approaches using a stock E30 non-M strut assembly, now it's time to cover common (and less common) knuckle / spindle swaps, also known as 5-lug swaps. The 5-lug conversion can be also made with relatively simple 5-lug adapter kit to avoid struggle with front suspension geometry. If you're not familiar with this topic let's write some background information about it first.
E30 is a very popular platform to be converted to a track, drift or any sort of race car. High horsepower numbers have been always easy to achieve, but stopping power and aftermarket brake solutions were dragging behind for a long time. E30 front coilover upgrades are more demanding and generally require welding a new strut tube onto the spindle since it is a one-piece unit from the factory, whereas newer E36 and E46 units have a separate strut instead. 4-lug wheel selection wasn't that great either since BMW and other makers started preferring 5-lug wheels, some builders even had issues with wheels staying attached with 4 bolts only.
All this lead people to start experimenting with E36 front suspension parts which were easy and cheap to acquire and were considered as an upgrade as is. However, there are many known geometry related problems, so let's start digging.
Here's E30 non-M with E36 non-M spindle:
E30 non-M with E36 non-M spindle front suspension geometry (pdf)
There's no doubt that this setup is not going to work. Wheel position is 35mm off (too far behind), caster trail has increased 37mm, tie rod outer end position is 15mm higher and camber angle is positive to name a few. Due to wrong tire size roll-center has gone even worse and scrub radius has also inverted from positive to negative, for sure these latter values could be easily addressed by using different tire and wheel specs or spacers, but we have more fundamental issues to solve first.
To save you from browsing a dozen of pictures of non-working setups, we'll just leave a list of pdf links here that will improve the wheel position a little and worsen other values:
E30 non-M front suspension with E36 non-M spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E36 M3 3.0 spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E36 M3 3.2 spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E46 non-M spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E46 M3 spindle and eccentric FCAB, front suspension geometry (pdf)
If you compared the assemblies side-by-side you may have noticed that E36 M3 spindles gave more ideal wheel position as well as tie rod end height mainly because they're designed for same caster range as E30. Also a E46 M3 spindle would have been close.
The wheel position is still the main issue, using OE parts there's only one way to improve it, which is E36 M3 3.2 front control arm. We picked the best assembly from the previous comparisons, which happens to be E36 M3 3.2 as well and this is the geometry we end up with:
E30 non-M with E36 M3 3.2 spindle and LCA and eccentric FCAB front suspension geometry (pdf)
Finally we're getting closer to correct wheel position, only 5mm off. Caster is already at 11° and caster trail is 39mm higher so we don't probably want to extend the arm any further.
This control arm is made to add caster and alter wheel position on a E36 M3 3.2, but it is also slightly narrower and will reduce camber as well as KPI.
BTW. Did you know that by adding shims between the strut and the spindle you can increase camber without changing the KPI?
This is the setup that has been considered as the best 5-lug swap on the internet since forever (after E30 M3 parts obviously), so let's perform the 100mm suspension sweep to create bump steer and camber curves:
After comparing these curves to original baseline geometry, what do you think? Bump steer is worse at 0.9° vs 0.75° as original, and camber gain has split in half, not to speak of any of those issues mentioned above. Since there's relatively simple solutions to improve it further, we will cover it next.
Only modification made here was to reduce caster by adjusting the top mounts for couple of degrees (move the top mount forward). Obviously you'd need adjustable top mounts that are capable of doing so, which requires approximately 20mm offset from the center.
E30 non-M with E36 M3 3.2 spindle and LCA and eccentric FCAB, reduced caster, front suspension geometry (pdf)
Now that's where the wheel should sit, in the middle of the arch. Caster reduction brought the caster value back to 9° and reduced caster trail by 13mm, which is quite significant change. Still, if we compare to original E30 M3 caster trail geometry at 25mm or E30 non-M geometry at 10mm, it is still much higher.
Well how did this affect bump steer or camber curve, previously we have seen positive results where tie rod to LCA inner and outer heights are close to equal, tie rod outer end sitting just few millimeters higher:
E30 non-M with E36 M3 3.2 spindle and LCA and eccentric FCAB, reduced caster, bump steer - camber chart (pdf)
Even this time changes are as expected, bump steer curve flattened and it is already reasonably good. Camber curve is still acting weird because of the control arm orientation (pointing upwards).
At this point we could say that the baseline geometry for this setup is nearly finished and we could start adding aftermarket upgrades for roll-center correction.
NOTE: If you haven't got the baseline right, correction parts will not fix the fundamental issues!
But, we still have one neat little trick to improve bump steer with OE parts. As you may know, E36 and E46 steering racks will bolt-on to E30 subframe, only the mounting slot is taller on the subframe so you will need to use spacers to fill up the space. With these spacers you can adjust bump steer as inner end height will affect it as well. In this case we need to mount the spacers below the rack to lift the centerline higher:
E30 non-M front suspension with E36 M3 3.2 spindle and LCA and eccentric FCAB, E36 steering rack with 9mm spacers, caster reduced, bump steer - camber chart (pdf)
And this is as good as bump steer can get, we tried with several different heights and the change only shifted the curve position, difference between upper and lower value stayed the same.
As a reminder these results would be only valid with this exact setup, if for some reason you'd like to run other spindles the geometry would follow on same ratio as on the comparison we made before:
E30 non-M front suspension with E36 non-M spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E36 M3 3.0 spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E36 M3 3.2 spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E46 non-M spindle and eccentric FCAB, front suspension geometry (pdf)
E30 non-M front suspension with E46 M3 spindle and eccentric FCAB, front suspension geometry (pdf)
As a bonus let's run one more assembly for shits and giggles with a E36 non-M spindle, where wheel position would have been fixed to same 55mm value with extra offset FCABs or custom control arms without any further optimization:
E30 non-M front suspension with E36 non-M spindle, wheel position fixed, front suspension geometry (pdf)
If you paid attention to previous context you may already guess the outcome, here's related bump steer and camber curves:
E30 non-M front suspension with E36 non-M spindle, wheel position fixed, bump steer - camber chart (pdf)
Wiggle wiggle! Unfortunately we run out of chart for bump steer.
Since we know there are a lot of people running these budget setups, we have one extremely useful product for you: MRT-20006-1W
So let's run the same assembly one more time with these bump steer adjustable tie rods and see what we can achieve:
E30 non-M front suspension with E36 non-M spindle, wheel position fixed, MRT-20006-1W, bump steer chart (pdf)
Worth consideration? Now you also know what lead us to add this variation of the product.
Hopefully you learned something new about "E30 5-lug swaps".
There would be a million more comparisons to do, but after you've digested the content for a few times you should be easier to conclude what works and what doesn't as well as how to make simple and effective improvements. If you feel like the spindle/knuckle swaps are too much work, please consider our E30 5-lug adapter kit instead.
You're always welcome to contact us if you need help with your setup or recommendations of MRT Engineering parts and products.
Thank you for reading! Next time we will continue with E36 chassis.