The Yost Autosport BMW M4: Part 2 – Controlling the Chassis

by Mike Bonanni

photos by Garrett Wade & Eddie Long

A proper race car is nothing without control and control comes in the form of a number of components that help one another work to their maximum efficiency. Tires of course are a huge part of control but just putting sticky tires on an otherwise stock car wont do you as much good as you think. You need a proper suspension setup to match.  A proper suspension setup revolves around the spring & damper (aka shock) combination. They have to be properly rated to your specific race car in order to maximize efficiency. Once you have that taken care of you need a proper alignment to make sure that you are using all of the tire you have available. Once you have that you have a car that has enough control to drive at its maximum speed, but you’re not done there, you still have to slow down for that next corner you’re now carrying a bunch of speed into. For that you need good brakes, but in order to utilize your good brakes you need those good tires and good alignment, and the cycle continues on and on.

The point is you really don’t have a proper race car until you have the complete package.


For our M4 we chose the all-new DR series coilover from BC Racing. Their new customizable DR series features digressive valving which changes the way it acts depending on the speed in which it is compressed. Long story short, these are fantastic for bumpy tracks, driving over apex curbs, etc. We chose custom spring rates and the addition of helper springs.

The complete package of course, starts with a great set of coilovers. We have been using BC Racing Suspension since the beginning of Yost Autosport in 2013 mainly because they are the best and most customer friendly on providing a service that you really need if you’re trying to build a proper race car; customized coilovers. BC Racing is fantastic at working with you to spec out and build a custom set of their coilovers for your specific needs and that can result in a much better outcome than buying the highest-end off-the-shelf coilovers that bolt-up, but aren’t really made for your car specifically. We started in 2013 with BC Racing’s double-adjustable ER series coilovers. They allowed for separate adjustment of compression and rebound meaning you can adjust how the car reacts as the suspension compresses versus how it acts as the suspension decompresses. In 2014 we proved that their build quality and their ER Series coilovers specifically could stand up to the ultimate test as we completed 25 Hours of continuous racing at the N.A.S.A. Pro Racing 25 Hours of Thunderhill.

For 2015 BC Racing released an all-new triple-adjustable coilover known as the ZR series. This did the same thing as the ER except now we had the ability to independently adjust the compression two-ways; high speed, and low speed. That meant we were now able to adjust how the car transfers weight under low speed events like braking, turning, and accelerating separately from how the car reacts during high speed events like driving over bumps or curbs. We put nearly 40 hours of track and race time on our set of ZR series coilovers and needless to say we had them dialed in to our specific needs, but when we looked at the direction we were heading with our adjustments we realized we were gravitating more toward a digressive style setup.

Up until this year most of BC Racing’s suspension products have used linear valving in their shocks meaning given a constant setting they would react the same throughout the stroke. A digressive style is different being that it can change the way it acts depending on conditions. To put it simply a digressive valved shock will bleed off more energy the higher the speed of compression. To put it even simpler, a digressive shock will soak up heavy impacts from bumpy surfaces and driving over curbs a lot more effectively without disturbing the car. With a triple-adjustable linear valved coilover like the ZR series we were finding ourselves trying to mimic those characteristics through tuning, as many of the tracks we run at are bumpy.

Thankfully for us, BC Racing has just released a new DR series digressive valved coilover. These are a single-adjustable coilover, which may sound odd to most of you who think that more adjustment means a better race car but bear with me. With the customization services on offer from BC Racing we were able to work with them to create a set of single adjustable DR series coilovers that out-perform our previous triple-adjustibles without the need to make many external adjustments. This is a new venture in technology for BC Racing that this car will be a major part of development for but expect to see even more options with digressive valving from BC in the future!


For ease of replacement and ride height adjustment we opted for a custom true coilover setup in the rear moving the rear spring from the spring perch to the shock itself. Notice the custom machined top-hat from BC Racing to make it all work. Doing this on your own car will change your wheel rate so make sure you adjust your spring rate accordingly.

Aside from the new style valving we also did something a little different by switching from a divorced spring & shock setup to a true coilover rear meaning the rear shock goes through the center of the spring as opposed to them being separate as the car is set up from the factory. There’s not really a performance advantage to doing this but it certainly makes things a lot simpler when adjusting ride height, corner balancing, and making quick replacements in the event of damage during a race.

Under the hood, our shock adjustments for the front are easily made at the top of the shock itself, but in the rear the top adjusters on the shocks are not so easy to get to. To remedy that we used some BC Racing adjustment extenders to bring that adjustment knob up and to the rear quarter panel Lexan windows so now our rear shocks can be adjusted quickly from outside the car. This allows us to adjust all four shocks easily during a pit stop, which is beneficial for situations where changing track conditions like rain are occurring.

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