Setting Coilover Preload Correctly
Coilover preload is the setting most owners either skip or get wrong. It is also the one most people blame for things it does not do. Get it right and the spring stays seated, the travel splits where you want it and the rattles stop. Get it wrong and you chase noises and ride height for weeks.
Understanding Coilovers and Preload
What Are Coilovers?
A coilover puts the coil spring and the damper on one assembly. The threaded body lets you set ride height, and the spring perch lets you set preload. A good coilover kit is the base of any serious performance suspension build. It gives you ride height, damping and preload to work with, and on many kits they are separate adjustments.
Here is the key point. How those settings interact depends on the type of kit you own. Knowing which type you have is the first step.
On a full-length kit, the lower mount threads into the shock body. You set ride height at the lower mount. The spring perch sets preload. BC Racing and Fortune Auto kits work this way. On a perch-only kit, also called a sleeve kit, the perch is the only adjustment. Moving it changes ride height and preload together. You cannot set one without the other.
What Is Coilover Preload?
Preload is how much the spring is compressed when the coilover is fully extended, with no car weight on it. You measure it with the coilover off the car, or with the car on stands and the wheel hanging. Preload equals the spring's free length minus its installed length in that unloaded state.
Zero preload means the spring sits between the perches with no squeeze on it. A few millimeters of preload means the spring is already a little shorter than free length before the car sits down. Once the car is on the ground, its weight compresses the spring much further than any preload does.
The Importance of Preload in Suspension
Keeps the spring seated at full droop. When a wheel drops into a dip or lifts in a corner, the coilover extends. With no preload and no helper spring, the main spring can go loose between the perches. That is where most spring rattle comes from. A helper spring takes up that slack, so kits with one can run the main spring with little or no preload.
Splits your travel. On a full-length kit at a fixed ride height, more preload holds the damper closer to full extension. That leaves more bump travel and less droop travel. Less preload does the opposite. This split is the real job of preload once the spring is seated.
Does not change spring rate. A linear spring has the same rate at any preload. At ride height the car's weight has already compressed the spring past its preload point. So preload does not make the car stiffer or softer over small bumps. Spring rate and damping do that.
Preload vs. Ride Height: The Confusion Most Owners Have
The Key Distinction
On a full-length kit, ride height lives at the lower mount and preload lives at the spring perch. On a perch-only kit, both live at the perch. Owners who slam a perch-only kit by threading the perch down often end up with zero preload. Without a helper spring, the main spring goes loose at droop. The rattle starts.
This is the part that trips up most coilover owners. On a full-length kit, changing preload moves the car too, because the damper now sits at a different point in its stroke. You then bring the height back with the lower mount. That is why the order matters. Set preload first, then set ride height on coilovers at the lower mount.
On a perch-only kit there is no second adjustment. Every turn of the perch changes height and preload together. Need to go lower than the perch allows with sensible preload? The answer is a shorter spring, a helper spring or a different kit.
Factors Influencing Preload Settings
Spring rate. Preload in millimeters does not change how the spring feels at ride height. Rate does matter for force, though. The same millimeters of preload on a stiffer spring push harder against the perches. Stay inside your kit maker's limits.
Ride height and travel. Slammed setups use up bump travel fast. Preload changes where the damper sits in its stroke, so it shifts how much bump and droop travel you keep. The lower the car, the more that split matters.
The kit's own instructions. There is no single correct preload for every car. Start from the number in your kit's manual. Dealer guidance for BC Racing kits puts the preload limit at 5mm. That figure is dealer guidance, not an engineering rule.
How to Set Coilover Preload
Tools Required
Coilover spanner wrenches (included with most kits). Digital calipers or a steel rule. Jack stands, so you can measure with the wheel hanging.
Step-by-Step Adjustment Process
Step 1. Find the spring's free length. Use the spring maker's spec, or measure the spring off the coilover with nothing compressing it. This is your reference.
Step 2. Unload the coilover. Either work with it off the car, or put the car on stands so the wheel hangs and the coilover reaches full extension.
Step 3. Measure the installed spring length in that unloaded state. Measure from the top of the spring to the bottom, where it seats on each perch.
Step 4. Calculate your preload. Preload equals free length minus unloaded installed length. If the free length is 200mm and the unloaded installed length is 197mm, you have 3mm of preload. Compare that with your kit's manual.
Step 5. Thread the spring perch to your target and tighten the locking collar against it. On a full-length kit, lower the car and set ride height at the lower mount. Then get an alignment, because any height change moves camber and toe.
Testing and Fine-Tuning Your Setup
After setting preload, drive over a mix of roads with dips, crests and broken patches. Listen for a rattle or clunk from the spring area. If it comes as the wheel drops, the spring is likely going loose at droop. Check the helper spring first. On kits without one, add a little preload within the kit's limit.
Rattles also come from loose locking collars, top mounts and sway bar links. Check the collars and every mount bolt before you start turning perches. Fix the hardware first, then judge the spring. Our coilover noise guide covers the other usual suspects.
Do not use preload to fix a harsh or floaty ride. If small bumps feel sharp, look at the damping setting and the spring rate. If the car feels loose over crests, start from the kit's manual damping setting and add rebound. Preload is for seating the spring and splitting travel.
Adjusting Based on Feedback
Street Use
Start from the kit's manual setting. If you hear rattles at droop, check the helper spring, then add preload in small steps within the kit's limit. Recheck ride height after every change.
Track Use
Track cars see full travel often. If the car is hitting its bump stops, more preload on a full-length kit gives more bump travel at the same height. You lose droop travel in exchange. Follow the kit's limits on BC Racing, KW or Fortune Auto kits.
Seasonal Changes
On a perch-only kit, raising the car for winter means moving the perch, which changes preload. Recheck preload after any height change. Perch position does not change unsprung weight. That comes from the wheel, tire, brakes, hub and knuckle.
Frequently Asked Questions
What is coilover preload and why is it important?
Preload is how much the spring is compressed with the coilover fully extended and no car weight on it. Correct preload keeps the spring seated at full droop and sets how your travel splits between bump and droop. It does not change the spring rate.
How do I adjust the preload on my coilovers?
Find the spring's free length. Unload the coilover, either off the car or with the wheel hanging. Measure the installed spring length and subtract it from free length. Thread the perch to your target and lock the collar. On a full-length kit, set ride height at the lower mount afterward.
Does more preload make my car stiffer?
No. A linear spring has the same rate at any preload. At ride height the car's weight compresses the spring well past its preload. What changes is where the damper sits in its stroke, so you trade droop travel for bump travel.
How much preload should coilovers have?
The kit design comes first: full-length or perch-only, with or without a helper spring. Then ride height, available travel and how you use the car. Start from the number in your kit's manual. Dealer guidance for BC Racing kits puts the limit at 5mm.
Can I set coilover preload without professional help?
Yes. You need spanner wrenches, calipers or a rule, and jack stands. The math is free length minus unloaded installed length. Just measure with no weight on the coilover. Book an alignment after any ride height change.
What are common mistakes to avoid when setting coilover preload?
Measuring at ride height, which counts the car's weight as preload. Setting ride height first on a full-length kit, then undoing it with the perch. Slamming a perch-only kit until the spring goes loose. Using preload to fix ride comfort, which is a damping and spring rate job.
Why does my coilover rattle over bumps?
Often a spring is going loose at full droop. Check that the helper spring is fitted and seated. On kits without one, a little preload within the kit's limit fixes it. Loose locking collars, top mounts and sway bar links cause rattles too, so check those first.
Preload Settings Across Major Brands
Every brand sets its own preload guidance, so the kit's manual is always your first stop. Dealer guidance for BC Racing kits caps preload at 5mm. BC Racing uses a full-length design, so ride height lives at the lower mount. Swift Springs SPEC-R springs are an upgrade option on most BC Racing fitments. They ship standard on HM and ZR Series kits on most fitments. If you swap springs, measure the new free length and start the preload math again.
Fortune Auto kits are full-length too. Per Fortune Auto's installation instructions, the owner sets ride height at the lower mount, not with the spring perch. KW V1, KW V2 and KW V3 kits ship with KW's own setup instructions, and those are the numbers to follow. The same goes for TEIN, Bilstein and Öhlins. Each one documents its own kits.
The wrong preload setting on any coilover kit shows up as noise, lost travel or a ride height that will not stay put. It is one of the most skipped steps when people learn how to install coilovers at home. If you are unsure about your kit and spring rate combination, call us at 1-800-460-9106. We will help you find the right numbers for your kit.
Keep reading: Our guides to helper and tender springs and spring rates.
Not Sure Which Kit Is Right Before You Start Setting Preload?
Getting preload right starts with having the right kit for your car and how you use it. We carry BC Racing, KW, Fortune Auto, Öhlins, Feal and TEIN, and we know how each one handles ride height and preload.
Call 1-800-460-9106 and tell us what you drive and what you want from it. We will tell you straight which direction to go before you turn a single collar.
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