A Long Overdue Upgrade

From the moment the Z3 hit the ground after I finished the conversion I knew that I was going to have to upgrade the suspension. The car's front springs were already tired, and before I added 485 lbs worth of batteries and equipment, the car was already riding 3/4 of an inch lower than stock.  After it gained all that weight, it was riding an additional 3/4" too low.  So now, both the front and the back were 1.5" too low.

I tried to reduce that 1.5" drop by adding some rubber bushings in between the lower coils of the springs.  That brought the ride height up by 3/4".  It was still a bit low, but the car looked good.  The problem was that the trouble was deeper than just ride height. The springs were never meant to carry the additional weight I'd added to the car.  When I went over a speed bump at just the right speed, the front end of the car would dip too much as I was coming down, and the car would bottom out.  The springs were simply too squishy.

So, knowing all of this, why did it take me so long to finally address this problem?  When people asked, I'd blame it on the cost of the components: all the suspension components add up to $1,500 or so.  But truthfully, it was fear.  I've never worked on any aspect of suspension systems before and as far as I was concerned the car's suspension is made up of springs, swing arms and shock absorbers all held together with  black magic and voodoo.  It seems silly really, after all, I did take a perfectly good car apart and turn it into an EV.  What could be so difficult about suspension?  Clearly this was not a rational fear.

Doing research into the subject, I came across a recommendation for a company called TC Kline Racing that specializes in BMW and Porsche suspension systems.  I contacted them and told them what I'd done, explaining that my goal was to get the car back to a normal ride height with properly rated springs.  I'd kind of anticipated this before I'd started the conversion, so I'd had each corner weighed both before and after the conversion.  That allowed me to tell them exactly how much weight each corner gained.  They informed me that the original springs were 325# rated springs, and I would need to upgrade to their 400# springs.  Those heavier springs would get me the ride height I wanted and the suspension would have the proper firmness.  The end result would be the ride should have the right feel and handle better.

The full kit included the 400# springs for both the front and back, Koni coil over struts for the front, shocks for the back and a threaded barrel and collar system that would allow me to adjust the ride height on the back of the car.  One of the claims they made was that the system weighed less than stock, which would reduce the car's unsprung weight and further improve handling.  So when everything arrived, I weighed it and it came in at 41 lbs.

My dad agreed to come over and help me with the project, which I had hoped would take only a day to complete.  <shakes head> Oh... silly boy.  I'll spare you the brutal details, but it took us about 2 hours to get the first strut done.  Getting it off the car wasn't too bad, but we needed to compress the spring so that we could get the top plate off since it needed to be re-used.  That top plate is what holds the strut assembly to the tower.  Compressing the spring was scary.  We had the right tool, but we felt like we were working with a live hand grenade that could go off at any minute.

The second strut went a bit quicker, and after 3.5 hours of work we had both new struts on the car with everything torqued down to spec.  We weren't going to set the car on the ground until I had the back end complete, so I didn't know whether the ride height had been improved or not.  So we moved our attention to the back end.

Getting the old springs off was equally as terrifying, but we got it done and took the old shocks off.  When I went to put them on, it was clear that the larger central shaft of the new shock would not fit through the stock bushing/mount.  A quick call to TC Kline and he said "You needed to order the proper new ones as the old stock ones won't fit."  You don't say!  Since I'd ordered exactly the parts they'd told me to he took responsibility for leaving them out of the package and had them rush shipped to me.  That meant a two day hiatus waiting for the parts.

When the parts arrived, I got the new shocks on the car and the new springs.  I set the car on the ground drove it down the street and back so that it would settle to it's final ride height.  I had to make a few adjustments in the back to get the right height, but the barrel/collar system made it easy to do.  I ended up setting the front struts so as to push the car up as high as the adjustment would go, and it was still 1/2" lower than stock.  I was a bit concerned, thinking that Kline's recommendations had been off.  But thinking back, the real problem was not the ride height, it was the fact that the springs were too soft.  So I decided to take the car out for a spin and put them to the test.

I was astounded at how perfect the car handled.  The car handled speed bumps perfectly, no bottoming out, and if felt like I'd remembered it did when I first bought the car.  Actually no, it felt better.  I really put it through it's paces taking a few corners near the house at speed and found that it handled better than it ever had.  There's a particular corner nearby that has a nasty bump right at the apex.  Previously when I hit that, the front would squirm a bit and the back end would kick out resulting in a relatively unpleasant feeling.  But this time through, the car took the bump like it was nothing at all.  I was so surprised an after market system could be that much better than the stock BMW suspension.  But to be fair, the stock system was 13 years old and had 149,000 miles on it.

I weighed all the parts that came off the car and they totaled 50 lbs.  A savings of 9 lbs, some of which is considered unsprung weight.  TC explained to me that a spring's weight is considered half sprung and half unsprung.  Makes sense.  Incidentally, I spoke with TC personally on the phone 2 or 3 times.  He was very personable and helpful and he took the time to educate me.

I guess the lesson learned here is that if you're converting a car, don't wait to have the suspension addressed to handle the new weight.  If I'm fortunate enough to convert another car, I'll be incorporating that cost into the initial build cost estimates.

When it was all said and done, the car rides 1/2" lower than stock, and looks great.  But more importantly, the springs are rated correctly for the new weight of the car.  Why did I wait so long?