I Bet You're Doing it Wrong!
I hope you've stumbled upon this post at just the right time. A big, fancy box of coilovers has arrived at your door, and now you're wondering, 'How the hell do I set these up properly?' The internet is full of information, but much of it is only half-right or flat-out wrong. Who can you trust, especially for something as seemingly simple as setting ride height? Well, fear not! As a car enthusiast, engineer, and damper expert, I understand the struggle of trying to find trustworthy sources amidst a sea of conflicting advice. That's precisely why I've written this blog post, which focuses specifically on spring preload and ride height adjustment. In my research, I was surprised by the amount of misinformation coming even from the coilover suppliers themselves. This problem is exacerbated by the reduced available stroke that many coilovers come with, especially those designed for extreme lowering. So, let's dive in and clear up the confusion once and for all using basic data and calculations for verification.
The focus of this post is on coilovers that offer independent spring preload and ride height adjustment with the use of any linear racing springs on the market. If your coilovers only have a threaded spring seat adjuster and stiffer OE style springs, the problem is much easier and won't be covered here. I also want to highlight the variables that you should be thinking about throughout this process. These are:
- Of your coilover's total available stroke, how much of it do you want to be allocated to jounce travel at ride height? (This really is the bit everyone is missing)
- Of your spring's usable linear travel, how much (if any) of it will need to be taken up by preload and how much is left for all the damper travel you have? Essentially, will you coil-bind with the appropriate preload factored in?
First, let's review what I believe, based on a reasonable amount of internet searching, is the most common advice you'll find on how to set up coilovers. The gist of it is that, no matter what spring you selected or were provided, you apply 5-10mm of preload or just enough to keep the spring from moving around at full rebound then set your full rebound damper length to lock in this preload. You can find this in the BC Racing written instructions as well as here, here, comes so close but not quite, here, I think this "Dr." gets it but doesn't realize it and designed around it for some good shock travel reasons, and finally this is completely fucking wrong.
The same info seems to be presented endlessly and it all misses a key tuning parameter available to you when you have independent preload and ride height adjustability. That is of course tuning how much of your total stroke is used for jounce/bump/compression and rebound/extension at your ride height position.
So what are they doing? You can use the pic below to help visualize it. By putting 5mm of preload (left strut in pic example) in at the full droop position regardless of spring rate and then put the vehicle on the ground the wheel will compress an amount equal to ("Unsprung mass" / "Wheel rate") - "Preload travel".
What is my made up preload travel term? As I'm sure you're aware, when you preload a spring 5mm, the spring exerts a force and you have to impart an external force greater than that preload force to compress the spring any further. "Preload travel" is the amount of spring preload compression MEASURED AT THE WHEEL. I'm subtracting this from the equation above because the mass of the vehicle has to overcome this preloaded wheel before the mass starts to move the wheel up relative to the chassis. The solution to the above equation is now your available rebound travel AT THE WHEEL from ride height position and this is only tunable with independent preload adjustment.
At this point all the "experts" say you're good, just thread the body in or out to fix your ride height or thread your spring perch up or down with rear decoupled suspensions (different spring and damper motion ratios) like the BMW.
So you're ride height is set, maybe you even went through corner weighing, but how much friggin bump and rebound travel do you have? When I started this journey with my set of BC coils, I realized I had no friggin clue. I'm a professional racing and active damper designer. This is wildly unacceptable and I couldn't believe the companies peddling these products have nothing to say on the topic, as if they don't know...
Can you imagine an example where a set of coilovers was installed without any thought about this, the ride is harsher than the customer wanted, they complain like hell all over the internet, maybe even swap springs to a softer rate only to find its the same or worse? Countless examples... truly incalculable. Well when you have little to no preload at full droop you're more likely than not to have far less bump travel and far more rebound travel at ride height than a proper setup would which is not ideal for street driven cars.
Here's an actual example based on the rear BMW F30 and I'll go over the setup calculator in more depth on Youtube so please subscribe to get notified if you enjoy or benefit from this content. I am also working on building it into the website with the help of ChatGPT so you can use it for free whenever you need.
|BMW F30 Rear||OEM||BC Racing Bad||BC Racing Good|
|Spring Rate (lbs/in)||426||672||672|
|Spring preload (in)||1.391||0.195 (5mm)||0.499|
|Total shock travel (in)||6.157||5.118||5.118|
|% of total stroke for bump @ RH||55||46||55|
|Bump travel @ Ride Height (RH) (in)||3.386||2.379||2.815|
|Rebound travel @ RH (in)||2.771||2.739||2.303|
Well hooooly shiza. Who wants more rebound travel than bump travel on a lowered street car?????????????????????????????????????????????? Did you realize what you were getting when you followed their instructions? That's a 0.436" reduction in bump travel from where the car could and should be and you can tune in more bump travel if you want (city folk)! The situation is even worse on the front where you have a higher mass, maybe a lower wheel rate, and a motion ratio closer to 1.
Seriously, what a fuck up at the industry level. With these highly adjustable dampers, the extra preload AFFECTS NOTHING ELSE (almost). It just places your rod correctly in the damper at RH and the threaded damper body can handle achieving desired ride height as usual. Hell, look at the OEM preload amount. We all know how dangerous OEM springs are to remove without a spring compressor. That is because they preload the shit out of soft springs to get the wheel rate they want at the ride height and bump/rebound travel they want. The only difference is they're designing everything as a non-adjustable package to work together so they don't need spring preload adjustability to get the bump and rebound travel where they want it.
Here is a post I found by Andre, the owner of HPAcademy from down unda...
So what is affected by adding more preload to your coilover besides the pain in the ass it can be to install correctly? Well I've now found out with my BC kit. I intentionally bought their default suggested springs to assess the ride my future customers have knowing I'll be swapping to different rates I want later. The supplied springs in the rear are 200mm (7.87") long and 12kg/mm. The total spring travel I would need to accommodate with 0.499" of preload plus full bump travel is 4.080". BC doesn't provide data on their springs so I have to go off of what is typical and usually the spring travel limit is around 55% of its overall length. That would mean this spring has 4.33" of usable travel. This is too close for comfort with a cheap-loose-tolerance-no-data-provided Taiwan/China spring. The 55% is a highly variable estimate and I would still be operating near coil bind which is a higher stress in the spring than necessary. An actual 8" spring with travel data would be better though Swift states at this spring rate and 8" length you have 3.6" of travel! So a 9" or 10" is probably required and since the stock springs are around 12" long I should be able to accommodate that.
So there you have it. Everyone is installing their coilovers wrong and its not just clickbait. I honestly can sit here imagining so many happy faces of people reading this, adding preload and resetting ride height to an otherwise unchanged vehicle, and completely changing how harsh their ride is. This simple change will:
- Get you away from riding your bumps stops.
- Give you the opportunity to lengthen your bump stops for less progressive stiffness curves and reduced harshness.
- Reduces your rod friction and rod force at RH because more of your rod is outside of the damper seal at RH so internal pressures will be lower.
- May allow you to go to softer spring rates for better ride and grip.
- Allows your damper to do its job with more bump travel before bump stops take over!
Thanks for your attention. If you enjoyed this and got some value out of it please let me know and follow me through whichever medium you prefer to see more in the future. The calculator will be available at some point but it needs to be cleaned up to make it more user friendly.