How to upgrade suspension without compromising comfort

Maybe the single most tortured descriptor of cars in our world is “dual duty.” It’s typically used to describe a car that’s intended for both street and track, but in reality is typically optimized for one and rendered nearly unusable for the other.

And yeah, that kind of makes sense. After all, the keys to success through the Carl’s Jr. drive-thru are far different from those for the kink at Road America, yet we all continue to chase the dual-duty dragon–or at least pay it lip service while we clunk along potholed roads pretending we’re enjoying our commute.

 

Someone Else Noticed, Too

Look back on our history of project cars and you’ll see more than a few examples of “We’re going to leave this one fully streetable for a while” followed one issue later by “ZOMG WE CAN GET 315s ON IT IF WE JUST CARVE OUT MOST OF THE TRUNK.”

And who can blame us? We live in Florida, where the roads are fairly decent and the lure of quicker lap times is frequently stronger than the desire for comfort on the way to the mini mall.

And we’re not the only ones who feel the frustration of these competing scope creeps. Anyone who tries to compete in their daily has recognized this struggle as well. 

“So much of motorsports, even at the amateur level, is a place where individuals express themselves through their cars, but one of the downsides of that is it can easily turn into an arms race,” notes the SCCA’s Heyward Wagner. “And while that arms race can actually be the appeal for some folks–and we completely respect that–it can also be a barrier to entry, or at least a source of confusion and intimidation.” 

A few years ago, Wagner and the SCCA, appreciating the natural scope creep in so many competitive endeavors, proposed a solution in the form of the SCCA’s new Time Trials program. 

“The SCCA has long had the equivalent of a lowercase t time trial program,” he continues, “but there were some definite barriers to entry, and the class structure was mostly based on the road racing side of things. So it was really serving a market between licensing and classing that was already being served by the road racing side, since the requirements were nearly the same.”

The response, with the new-age Time Trials program, was to give it a more market-focused approach. Driver requirements lined up better with existing track clubs. Prep levels were designed around more common modifications to the cars currently seen in the community, which were already powerful and highly capable. The Sport division was maybe the ultimate expression of this concept.

 

Enter Our 435i

And that’s where we come in. When conceiving the focus for our BMW 435i project, we really wanted to build a dual-duty car–and to mean it this time. So when we looked over the rule set and prep guidelines for the Sport division of the SCCA’s Time Trials program, we saw that our BMW fit nicely into the Sport 3 class.

“Sport is really designed around a car you can drive to the track,” Wagner says. “And not just one you can drive to the track, but one you can prep for the track with a knowledge and resource set that’s possessed by a large number of individuals in the scene.” 

What that means in non-marketing speak is that the cars should be easy to build, both regarding parts acquisition and installation, while the limited prep shouldn’t completely ruin them for street duty or put them in jeopardy of being illegal in certain jurisdictions.

“To limit the ‘arms race’ nature of some classes, the Sport division allows certain performance replacement parts but also mandates that those upgrades be readily available and not overly complicated,” Wagner continues. In practice, this means certain components like dampers can be replaced, but those replacements can only have a single available adjustment and must be readily available through common retail channels to all competitors. This limits measures like custom valving or bespoke parts, keeping costs down and complications to a minimum.

“We have a Tire Rack rule for the Sport classes that’s really more of a euphemistic guideline than a hard and fast rule,” Wagner explains. “Basically, if an allowed replacement part is available from [series sponsor] Tire Rack, it’s legal.” 

This doesn’t mean parts have to be sourced from Tire Rack, but the company’s broad stocking strategy provides a handy reference for what’s allowed. And if a part isn’t available from Tire Rack but meets all the other requirements and is available to all competitors through common outlets, it’s probably already listed in the rule book as legal, or legality is just an email to the SCCA away.

This “off the shelf” mentality for the Sport division keeps prep costs in check and drivability to a maximum. Allowed modifications include replacement of springs and dampers, installation of camber plates, and the addition or substitution of anti-roll bars. ECU tunes and cat-back exhausts are also allowed. Wheels are free, but a maximum tire width is specified for each class, and no cutting or flaring of fenders is allowed. 

Brakes and safety modifications are slightly more liberal, although neither are explicitly required to be modified outside of OEM standards. For brakes, upgrades and size changes can be made at will, provided the replacement parts are at least as large as the OEM bits (no subbing in tiny brakes to cut weight). And safety gear–including a one-piece, SFI-rated bucket driver’s seat–can be added, but it must be done as a system. In other words, a racing seat requires at least a four-point roll bar and must be used in conjunction with proper harnesses. Interior bits may be removed to accommodate safety gear. 

 

Let’s Rip Apart Our BMW but Keep It Streetable

Armed with these prep guidelines, we knew we couldn’t completely reengineer our F32–not that we even wanted to. The basic manners and balance of the car were solid, although as we saw from our first baseline track laps, the limits were frightfully low. 

Part of this lack of capability was due to its 540 treadwear tires, but the complete lack of negative camber at the front certainly wasn’t helping. The car would simply overwhelm the outside edge of the front tires and give up grip with little force.

Our initial session in the 435i showed promise. While the BMW (blue trace) lagged behind the GRCorolla (red trace) in the corners, the acceleration zones were a dead heat, with the BMW occasionally winning. Negative camber would be a priority, though, as just a few baseline laps absolutely lunched the outside edges of our front tires.

There was good news in our first track test, though. Looking through some of the data from our VBox HD Lite, we saw that the 435i produced ample thrust–often equal to or better than the 300-horsepower Toyota GRCorolla, one of the cars we’ll have to face in the Sport 3 class. So it was nice to know that if we could get the chassis working a little better, we weren’t giving up anything in the acceleration department, at least in stock-versus-stock trim.

[Toyota calls our bluff and builds a giant-slayer from a Corolla]

For chassis prep, we decided to go to the limit of the rules, which in this case was not particularly high. It meant choosing parts that worked well together was important to preserving overall balance. Negative camber was a top priority, as was lowering the center of gravity–although without the ability to correct roll centers or roll steer via moving the ball joints or the tie rod attachment points, going too low could easily cause more problems than it solved. Time to head into the shop. 

Photography Credit: J.G. Pasterjak

1. Our choice for springs and dampers was a time-tested combo of Koni struts and shocks paired with Eibach Pro-Kit springs. The Eibach Pro-Kit increases the wire diameter by about 8%. Upsizing wire diameter raises spring rate given no other variables of the coil are changed, but Eibach adds about three-quarters of a coil to the front springs and half a coil to the rears. It also reduces the free length. 

Photography Credit: J.G. Pasterjak

2. Increasing the number of coils in a given spring is a rate reducer, while free length has little to no effect on rate. The overall result here is springs that have only a very slightly higher rate than stock, but they drop the rear of the car about 5/8 of an inch and the front by around ¾ of an inch–enough to lower the center of mass, but also not too much to knock the tie rods and lower control arms into uncomfortable positions to articulate from.

Photography Credit: J.G. Pasterjak

3. If you’ve ever installed front struts on a BMW (or a VW, or many Hondas), you know what an utter pain in the butt it can be, and our 435i was no different. The front struts are held to the knuckle via a clamp, and that knuckle must be unbolted with the strut lifted high enough so it can be freed. It’s a real chore, and we went through the entire catalog of handy tools sold specifically to make this job easier. (Spoiler alert: They didn’t.)

The handy tools specified for such a job are designed to spread the clamping part of the knuckle so the strut can be easily lifted from the top while the suspension is pushed downward–or at least that’s how it works on YouTube. But watch enough videos and you’ll notice strategic cuts right where things seem to get stuck, which is at the “driving the suspension downward” part. The reality is that even the nicest BMWs with 70,000 miles of use and 10 years of age have collected a lot of road grit and corrosion, and separating the strut from the knuckle is nowhere near as easy as it looks on TV.

Photography Credit: J.G. Pasterjak

4. We tried the cam-style tool that’s supposed to slip into the clamp’s gap and then, with a quarter turn, spread it open. It was mediocre. 

Photography Credit: J.G. Pasterjak

5. We also tried the little screw spreader that uses a screw wedge to spread the clamp. It was also unhelpful. Neither really opened up the knuckle clamp far enough to allow it to fully release the stock strut. And lest you think, “I’ll just whack the knuckle with a hammer,” remember that knuckle is aluminum and expensive. A few blows with a soft-face hammer or a couple of light taps with a metal hammer might help loosen a frozen joint, but heavy blows with a hard hammer intending to move the assembly are a recipe for cracks.

Photography Credit: J.G. Pasterjak

6. In the end, our method for removing the stock struts was to insert a wedge-shaped chisel in the top of the clamp. Driving down the chisel not only spread the clamp, but also applied driving force in the direction we wanted the knuckle to go. Even so, this is not the ideal way to remove these things, as the chisel chews up the inside edge of the top of the clamp a bit. For many owners doing this job for the first time, though, it might be the only choice.

Photography Credit: J.G. Pasterjak

7. Another handy hint is to loosen all the nearby suspension arms supporting the assembly. There’s a lot of tension in the lower control arm bushings and the trailing arm bushings, and taking a turn or two out of the fasteners can allow them to swing a bit more freely. Removing the tie rod end from the knuckle can also give you a little more room to work, but this isn’t entirely necessary for the process (although it will seem like it is the first time you do this). 

Photography Credit: J.G. Pasterjak

8. Koni does you a solid with handy tabs to slip into the gap on the knuckle clamp, ensuring the struts are properly clocked into position, as well as index holes so you can reuse the OEM rubber spring isolators in the correct orientation. You’ll also probably freak out when you take the front Konis out of the box. They both look the same. But fear not: The right- and left-front struts actually are the same. On the driver’s side, the mounting tab for the anti-roll bar end link protrudes toward the back of the car, while on the passenger side the tab is toward the front. It’s the same as the OEM arrangement, and the Koni replacements cleanly slide in while offering external rebound adjustment via the top-mounted screws.

\​​​​​​Photography Credit: J.G. Pasterjak

9. In the rear, shock installation is much easier. Four E12 bolts fasten the top shock mount to the chassis, while a large bolt with an 18mm head secures the bottom eye. Realigning the lower control arm to the knuckle can be a bit of a pain since, again, there’s a lot of tension in the bushings. Loosening the control arm bolts where they fasten to the chassis and arming yourself with a nice pry bar or two plus a floor jack to compress the suspension upward into position will make the job easier. So will some patience.

Photography Credit: J.G. Pasterjak

10. Installing the rear springs requires removing the control arm connection from the rear hub so the arm can be lowered a bit, but once it is, the Eibach springs easily slide into position, cradled by the stock locating rubbers.

Photography Credit: J.G. Pasterjak

11. The final piece of our suspension puzzle in the current phase of prep was a set of camber plates from Vorshlag. In a way, this was the scariest part of the operation from a “dual duty” perspective, because installing camber plates frequently means sacrificing ride quality or introducing that familiar metal-on-metal clunk at every bump and each turn of the wheel. 

Photography Credit: J.G. Pasterjak

12. But the Vorshlag plates were kind of a revelation, and according to the company’s founder, Terry Fair, this is very much by design: “We have a database with thousands of entries to know what the shaft specs and spring specs are for our applications, and each set of plates is specced out for the customer’s exact configuration. In the case of your Konis, they have shorter than usual threaded areas on the shaft. And it’s also a priority to make sure that the top mounting point is located vertically at the right spot so we don’t steal any travel. So our solution is some custom-machined, long-reach top nuts, which are a royal pain in the ass to make, but it’s the only way to ensure we’re putting everything at the exact right spot, using the fewest amount of parts necessary, and that those parts are robust enough to stand up to street use.” 

Photography Credit: J.G. Pasterjak

13. “We also use huge 19mm sphericals and completely sealed radial ball bearings instead of open Torrington bearings,” Fair continues. He says that this makes for maintenance-free camber plates that are suitable for street or track. And so far, so good. In fact, they’re the quietest, most OEM-like camber plates we’ve ever experienced. No clunking, no rattling, no weird steering feel, just factory-like performance with the added benefit of easily adjustable camber.

Photography Credit: J.G. Pasterjak

14. Based on some suggestions from our friends at BimmerWorld, after we reassembled and properly torqued our suspension, we set front camber to -3.5° (which should be a huge improvement over the 0° and +.1° it was showing from the non-adjustable stock configuration) and front toe to 1/8-inch total toe-out. In the rear, BMW provides nice leeway for adjustment from the factory, so we set camber to -3° along with 1/8-inch total toe-in. While these may not be our ultimate settings, they give us a workable baseline to start from.

Photography Credit: Photos by Tradd

15. We’ll have track performance numbers and data to compare our baseline setup to soon enough, but the road comparison isn’t actually much of a contrast–in a very good way. Road manners are just as composed and quiet as they were during our original trip down from that used car dealer in Cincinnati, with maybe just a bit more urgency from the chassis and a bit more steering feedback due to the slightly stiffer Eibach springs. But there’s zero harshness, and all the ride quality remains.

Our initial seat-of-the-pants track impressions reflect the road composure but add a level of grip and responsiveness that was completely absent in stock form. Steering feel­—a longtime weak spot of this generation of BMW coupes and sedans—is greatly improved as well. Even a quick glance of our on-track photos from the SCCA’s Time Trials event at Virginia International Raceway shows the heavily loaded outside-front tire planted more vertically, maintaining that all-important contact patch interface.

And even though the Pro-Kit lowered the chassis slightly, the car still easily clears any driveway or curb we encounter and rolls on our trailer without the use of extension ramps. These are all factors that contribute to the usefulness of the car as a driving tool rather than just a track toy, so knowing we haven’t negatively impacted the comfort and usefulness of the car while adding topnotch damping, better chassis control and cg optimization, and enough negative camber to keep our contact patches on the road under high lateral load is a huge dose of mission accomplished for our first round of work on our new project.

Now we get to lock up the Sawzall in the tool cabinet and enjoy our ride for a while.

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