Photography Credit: Tradd Slayton
The dual-duty ethos has always been a guiding force for our BMW 435i project. And, dear reader, know that the struggle is real when it comes to maintaining this balance.
The lure of the stopwatch is also real, and we have a few trailers available, so we always know that the Sawzall is primed and ready to drop those lap times the way God intended: with obscenely large tires and a fully gutted interior.
But it’s so nice on the road, gang. Even with some bolt-on suspension upgrades, it’s still a perfectly civilized street companion suitable for local errands or long-range cruising.
So when we were devising its track configuration, we decided that it would be worth a small amount of effort to have a more idealized setup for each of its primary functions. Our goal was one hour. Sixty minutes to prep for a track weekend, and another 60 once we got back home to put the car back into street mode. Here’s what we decided were our priorities.
Given the highly limited nature of the available prep allowances in the SCCA’s Time Trials Sport division, our chassis was fairly mildly prepped. We were also running on 200tw tires–a set of 275/35R18 Bridgestone RE-71RS tires to be exact–which are theoretically fully street capable, although they may experience accelerated wear in comparison to a more typical set of street tires with a higher treadwear rating.
[200-treadwear tire test | Bridgestone Potenza RE-71RS vs. Falken Azenis RT660]
So the main focus of our switchover would be on two areas, one of which would also give us a good chance for other pre-and post-track inspections.
Our first switch would be the seat.
Photography Credit: J.G. Pasterjak
Our OMP HTE-R race bucket is spectacular on track and has even comfortably held our butts for 150-plus-mile trips to track tests, but the OEM BMW sport seat is far more suitable for mundane errands and far easier to climb into and out of with a $7 venti vanilla sweet cream cold brew in one hand.
Pulling the stock seat starts with sliding it fully forward and backward on its tracks to expose the torx-headed fasteners securing it to the floor. You’ll need to leave the large connector between the seat and the car in place while you do this so the power seat has the power to slide.
But once that seat is free, you’ll want to disconnect the battery before disconnecting the junction between the seat and the car. Most likely nothing bad would happen, but in general BMW safety systems don’t like to be disconnected while under power, so a quick trip to the trunk with a 10mm wrench before you pull that seat plug only adds a minute or two to the process.
Once the stock seat is out, the BimmerWorld seat mounts bolt to the floor and the seat can be bolted on top of them. In this case, the Macht Schnell sub belt bracket from BimmerWorld is a huge benefit, as it not only provides a safe and secure mounting for the sub straps of our Lifeline harness, but it also keeps the seat mounts properly aligned with each other, minimizing changeover time.
Photography Credit: J.G. Pasterjak
We also broke out the white paint pen and made a few index marks where various assemblies meet to expedite alignment and ensure we always reinstall things in the same spots.
Once the seat is in, we move on to the brakes, substituting in a set of fully capable track pads. One of the most common questions we get is, “What are the best brake pads I can use on the street but also on track that don’t make any noise or dust and last a long time and are cheap and also pay contingency?”
Really wish that was a joke.
The real answer is that the demands of street pads and track pads are different so the approach–if you’re going to be optimized for either–needs to be different. Otherwise, you’re always going to have to decide which compromise you’re most willing to live with. We discussed this in-depth with Mitchell Boom from Hawk Brakes several months ago, and that conversation stuck with us enough to put those sentiments into action on this project.
Our BMW 435i’s M brake package also makes simple pad swaps a legitimate option, as the pads can be pulled from the multi-pot calipers and replaced in a matter of minutes with minimal tools. So our plan was really coming together here.
We perused the Hawk lineup and decided on the HPS 5.0 compound for street use and the DTC-60 compound for track. Bloom confirmed these were solid choices, so we added a set of each to our cart.
Photography Credit: J.G. Pasterjak
The HPS 5.0s are tuned for heavier and/or more powerful cars. Our 435i puts only 300 horsepower to the ground, but it does weigh in above 3600 pounds, so the additional friction and thermal capacity are a bonus here. Compared to Hawk’s standard HPS compound, the 5.0s have about 20% more coefficient of friction across their temperature range, and do not begin to fall off notably until well past 600º F. So these are street pads that could survive a recon lap or two, or even a light track session if they needed to, while still being quiet, clean and seemingly easy on rotors.
The DTC-60 track compound is designed with a very flat friction response over a wide range of temperatures–the friction varies by barely 6% from ambient temperatures to well past 1500º F–but likes just a bit of heat to get into its ideal range. From 300 to 1300º, friction varies by barely 2%, and the response and release back these numbers up. Hawk also claims that the DTC-60s are easy on rotors when operated in their ideal temperature window north of 400º, which should be no problem for our nearly two-ton BMW as it takes to the track with a driver.
And, yes, you could certainly operate the DTC-60s on the road if you wanted to drive the car to the track. The produce more friction at ambient temperature than the HPS 5.0s, so you’ll never have to worry about emergency braking performance. The tradeoff, of course, is more dust, more noise (although with the anti-squeal shims in place, it’s not horrible), and accelerated rotor wear. Still, for use in transit to the track, they’re entirely realistic.
Swapping pads on this BMW is a piece of cake. Once the wheels are off, you’ll need to pull the pins holding the pads in the calipers, remove the spring clips, retract the pistons, drop in the new pads and replace the spring and the pins and motor on.
Photography Credit: J.G. Pasterjak
A couple of specialized tools make the job easier, including a specific punch to remove the retaining pins. The proper punch should have a cupped end so it doesn’t slip off the rounded ends of the pins and mar the calipers–you can see evidence on one of our calipers that an improper punch had been used in the past. We bought a tool for about $8 that was actually made from an old bolt, and it does the job just fine.
You’ll also need something to retract the opposed pistons simultaneously, and while this is something that can be improvised, if you’re going to do this a lot, we recommend getting the proper tool. In this case, that means a set of expanding spreaders from Lisle that we bought about $60 for online. They did the job in seconds with no fuss and were more than worth the price of admission.
So what complications are we creating by switching out our pads each track weekend, and what best practices can we use to keep those complications to a minimum? Well, Mitchell Bloom says our complications are minor, and our best practices are simple to adopt.
“In a perfect world,” he says, “you’d be re-bedding pads each time you swapped over.
“Like, you’d hit the rotors with some 100-grit and scuff them up, then re-bed the pads before you went hard on them. But, in the real world, you might not always do that, and maybe long term it will slightly accelerate wear of the pads or rotors or both, chances are you’re not even going to notice the reduction outside of normal wear.”
He did have some concrete recommendations for pad swapping, though. “You can actually move the pads around to compensate for wear patterns,” he explains, “but once they’re worn in, you should always have them meeting the rotors in the same direction. So whatever position they’re in, the leading edge should always be the leading edge and the trailing edge should always be the trailing edge when it comes to direction of rotor rotation.”
Once those pads get heat cycled a few times, the granular structures align along the rotational paths where the pads interface with the rotors. Reversing that direction once the pads have heat cycled can accelerate wear until they become reset in their new orientation.
We deal with this with the same paint pen we used for marking our seat bolts. A dab of white paint goes on the leading edge of the pads whenever they come out.
In total, while we were shooting for a sub-60-minute changeover, we managed to do our swap in as little as 42 minutes. And since the wheels are off for the brake pad change, it’s also a great opportunity for a general nut-and-bolt under the car and around the major suspension joints. Overall, 42 minutes seems like an extremely reasonable changeover given the increase in safety, comfort and performance in each of our chosen venues.
