The 135i is by no means a slouch in stock form, but some work had to be done to tap its full potential. As a goal, I intended to made the car pleasurable to drive quickly as well as competitive, but still maintain the reliability and comfort of the stock car. To do this, I relied heavily on the BMW parts bin. Below is my journey, in the order taken, to get to where I am…
Ditching the stock run-flats should be first on your list. I replaced them with Dunlop Direzza Star Specs due to their price combined with their track record as extremely grippy tires in both wet and dry. A plus-one size upgrade is a must, resulting in 225mm fronts and 255mm rears. The first thing noticed with these tires was a more comfortable ride – the runflats can be truly jarring over rough pavement. The sidewalls of the Star Specs, though stiff, soak up the pavement imperfections well. The new tires were also quieter than the stock runflats at first, but once broken in they began exhibiting a distinct hum, particularly at highway speeds. So, how did they perform? Admirably. They had far more grip than the stock tires, and also communicated what they were doing much better. Their response is excellent as well, although it’s hard to beat the stock runflats’ response with their ultra-stiff (read: solid) sidewalls. The breakaway characteristics are very linear, making for a great tire to learn the limits of this car with. With full treads in the wet, they are extremely competant – moreso than many of the other top-crop street tires. They handle cool temperatures well, too, something some of the other top tires have real trouble with. Their one Achilles heel seems to be extremely hot temperatures, where they can tend to get greasy and fall off quicker than some tires. I’m happy to report their wear rate is excellent considering their use, as I have 6 track days and 5 autocrosses on my current set and they still have significant tread left. Since they are directional but symmetric, they can be rotated on the rims, although they seem to be wearing very evenly with my current setup and alignment.
A few track days into my learning, I started getting fast enough that brake fade was becoming an issue. Usually, this would start to occur during the later sessions, or when two sessions were close together. I had no desire to install an overly expensive big brake kit, and the BMW Performance brakes are just repainted 135i calipers with cross-drilled discs – a big no-no for serious track use (search “cross-drilled” and “cracks”). The problem with fade stems from the fact the front discs are not very thick in throat area, and cause the pads and fluid to overheat easily. A set of upgraded pads and some better fluid is a must, but this is one area where BMW parts aren’t going to fit the bill. I went with ATE Typ 200 fluid, as it is similar in color to the factory fluid, but the same chemical makeup as their excellent “Super Blue” fluid minus the dye. The dye in their Super Blue formula is known to stain plastic over time, something I wasn’t interested in. For pads, I started with a mild track pad; the Hawk HP+. This pad compound proved better at resisting fade, with a temperature range still capable of driving to the track with. Note that I re-used the factory “shims” on the back of the brake pads, since the Hawk pads definitely were sized to use them (contrary to what their rep claims). Also, extreme heat seems to be causing some issues with the ceramic piston “caps” crumbling. To avoid this, I used Titanium shims behind the front pads and bent back the dust shield to gain some additional cooling. Cooling ducts are probably the best solution here, and are something I may look into in the future if I am still having trouble. Overall, the HP+ pads are good, but still don’t provide enough braking force at low/moderate pedal force – it seems like I really have to dig into the pedal to get them to bite hard. A set of more aggressive Hawk DTC pads will be in my near future…
Front Sway Bar
Now that I have sticky tires, the soft stock suspension becomes even more exaggerated. I feel like I can touch the ground with my shoulders with the amount of body roll generated! So the first thing I tried was a stiffer front sway bar. I went with the BMW Performance Suspension sway bar, since I had already purchased the kit (more on this later). I also picked up the BMW Motorsport Group N bushings to use with it, which are stiffer than those supplied in the kit. A couple things to note here; this sway bar is the same diameter as stock, however it utilizes an extra “S” bend on each side in order to create more effective stiffness. Also, you will hear a million “internet engineers” tell you how a front sway bar increases understeer, one item we definitely want to reduce. In an otherwise perfect suspension design, this would be entirely true. However, the 1 series utilizes a MacPherson strut design up front. Though simple, this design has problems maintaining negative camber during turns (i.e. dynamic camber) due to the body roll. The larger sway bar works to reduce this roll, allowing the suspension to maintain its geometry better. The effect this has on negative camber is greater than the effect of load transfer, and we end up reducing understeer. And boy did the sway bar stop roll! The timer showed that my autocross times went down around half a second in comparison to my usual competitors. Part of this gain may also be from the added control the FSB gives the driver – with the front staying much flatter, you spend less time fighting to stay in your seat and more time comfortably and precisely driving the car!
Next up I installed the springs/dampers that came with the BMW Performance Suspension kit. The springs are progressive rate, and offer a slight amount of lowering (10mm for those with the sport suspension, 25mm for those without) as well as being stiffer. Although the absolute rates aren’t published, BMW claims “48% stiffer than standard suspension and 34% stiffer than sport suspension”. During the install, you can knock out the “strut alignment pins” and pull the struts inward a bit, gaining a small amount of negative camber (a few tenths). The drive home after installing this kit was disappointing, as I barely noticed any difference. I think much of the roll reduction from the kit was accomplished with the front sway bar alone. Since most people install the full kit at once, they don’t have a chance to observe the gains separately. The shocks seem to still have too soft of a rebound, and the progressive rate springs require too much compression before the stiffer rates become effective. For some people, this might be perfect, as it keeps the ride very compliant. However, I was looking for sharper handling even when just cruising back roads, which it would appear this setup isn’t the best at (a linear spring setup with some adjustable shocks would probably have been a better fit for me, but again I was after a reliable and hassle free setup). But once you ratchet it up to 10/10ths, it becomes apparent BMW did their homework, as the balance with this setup is really nice. There is a very distinct feel to it at the track, where you can feel the weight transfer and react to it. I can’t tell you how much fun the “Bus Stop” at Watkins Glen was with this setup – it was almost rhythmic the way the weight transferred through this chicane. The car is still understeering pretty heavily, so the best approach is still slow-in-fast-out, and this suspension works well to that technique.
Front Control Arms
With a ton of understeer still around, it’s time to address the lack of negative camber. My car came with .2 degrees of front negative camber, and after the suspension install, it was still only around .4 degrees. The front control arms from the M3 offer plenty of advantages that help here; they are a direct bolt-in, they come with stiffer bushings and ball joints, they are made of lighter and better-shaped aluminum, they increase the front track width, and they add around .5-.6 degrees negative camber. Plus they’re inexpensive and easy to install. With these installed, I felt a noticeable increase in front grip. I’ve still got understeer, but less than before. Also, the steering response feels sharper – a result of the stiffer bushings preventing a lot of deflection felt with the mushy stock bushings. For reference, with my setup at this point I now have 1.1 degrees negative camber.
Rear Subframe Bushings
Since new, I’ve noticed that the rear of this car has quite a bit of slop side-to-side when cornering. The result is a feeling of delay in what the rear does, as well as some unstability over rough pavement. To cure this, I had the M3 rear subframe bushings installed. Looking at the stock bushings, it’s easy to see why there was so much slop – there’s large gaps in them to create a “compliant” ride. The M3 bushings eliminate these gaps, and made quite a difference in the “feel” of the car, although I can’t say for sure if it translated to better lap times. What it definitely did do is provide a more direct connection between the steering wheel, the rear tires and your right foot, which combined with my next modification ended up a big winner…
Since the subframe was being dropped to install the bushings, I decided to have a Quaife LSD installed. It’s far more expensive than most diffs I’ve purchased, mostly due to the extra labor upfront required in order to machine off a ring gear from a previous core diff. An extra special thanks to BMW for welding the ring gear to the diff! The results made this worth the money though. I never really had a huge problem spinning the inside tire, even on tight corners, as I had trained my right foot well. But I had to retrain it all over again after the LSD install. Drifting out wide and want to tighten the corner? Screw lifting, instead apply more power! Yes, as the inside tire tries to (over)spin, power is instead routed to the outside tire. If the outside tire is spinning faster (and gripping) than the inside tire, the end result is that the car rotates tighter into the turn. It’s like stability control, only instead of using the brakes to change wheel speed, you use the throttle. This made tight corners a breeze, and combined with a higher rear tire pressure to aid in rotation, had a tremendous effect on the balance of the car – the understeer is all but gone unless you do something really bone-headed. The only downside to the diff is a slight bit of a “thump” when letting the clutch out between lower gears. The thump also occasionally occurs when stationary and moving the shift lever into first gear. I’m told by the experts this shouldn’t have any adverse effect on the diff. Also, for a short time the diff felt like it was “binding”, particularly when making tight turns at intersections, but this has subsided now that the diff is broken-in.
I still feel as if the rear spring rate is too soft, as it twists and bobs in its best attempt to follow the front when cornering. In theory, a larger rear sway bar may help, but I was a bit worried about creating too much oversteer. So I went with the smaller of the M3 rear sway bars from the coupe/sedan instead of the convertible. Although listed as 20mm, it turns out this was mislabeled by various vendors, as they are actually a measured 22.5mm. The stock sway bar is a pathetic 12mm – it’s entirely useful as a bowflex… seriously. Even with the far larger bar, my oversteer worries were unfounded. The bar flattened the rear out, and still made it entirely predictable and controllable. In fact, it feels even more in touch with driver inputs now, as the rear is now a part of the turn-in process instead of just lazily following along with the front. Recently I participated in a wet autocross after installing this bar. The results spoke for themselves – 1st in class, 2nd in PAX, and 3rd fastest time of the day. This was on a day filled with Subarus and Evos (just like any other autocross!). The balance felt amazing – neutral with mild hints of beneficial oversteer. I’d have no reservations recommending this modification, particularly if you already have an LSD.
In current form, I’m very happy with the balance and feel of my car. I’ve taken it from a tight-but-sloppy luxury coupe to a competitive and fun sports coupe. There’s only one nasty characteristic the car still exhibits; heavy bump steer. Take this car through a corner with a raised expansion joint and the steering seems to jump in the direction of the turn, while the car does a strange side-step. I suspect this may be inherent to the suspension design, and without changing factory mounting points it won’t be easy to cure.
Otherwise, the car is great. I’ve got plenty of seconds to gain through driving skill, so I’ll focus on that next. If I had to choose a modification next, it would be a set of camber plates. I’m sure many would advise those earlier on in my upgrade path, but remember I was looking for off-the-shelf BMW parts (a goal which I succeeded). After that, I’d probably need bigger wheels to fit the larger tires the camber plates can allow for. Once I’ve thoroughly learned and exceeded the potential of the current suspension, a set of adjustable coil-overs would be the natural progression, but I’m still a long way off.
I have to say, this car has been my favorite to develop over the course of the past few seasons, as it seems to have a very large potential, and responded well to each modification I have performed. It also gives me a sense of satisfaction that I was able to accomplish my goal using all BMW parts, a testament to their engineering.
In summary, the balance is just sublime now, and the car feels near perfect – responding to your every input and feeding you inputs of its own along the way.
True BMW Joy.