Why Cutting Springs Is Okay (Kinda) February 01 2019
I've heard internet experts and suspension salespeople alike say that cutting springs is a terrible idea. Almost all of us have come across the guy on the internet who aggressively tries to convince everyone that cut springs will cause you to crash your car into a group of disabled children holding golden retriever puppies.
I, on the other hand have a suspicion that cutting springs a modest amount can yield the benefits of a lower ride height and stiffer spring rate. Why would I have the temerity to question the authority and omniscience of the internet experts?
1) I changed from factory springs to performance aftermarket springs on my BMW E30 318iS. When comparing the factory and performance springs, all the dimensions were virtually identical, except the performance springs were slightly shorter, as if they were just a cut version of the factory springs. I'll admit that it is possible different metallurgy can account for some unseen changes, but it's also possible that it was just a shorter version of the factory spring. Either way, it is inconclusive, but it raises some doubt on the assertion that cut springs will cause endangered baby panda bears to die.
2) In an undergraduate course called Machine Component Design, we learned how to calculate spring rates of coil springs. Cutting a coil spring should decrease its length and increase the spring rate. Both of these are good things, and it is exactly what an aftermarket spring provides.
3) While watching some videos featuring the "Drift King" Keiichi Tsuchiya, he mentioned cutting springs in his teenage years to improve performance. If spring cutting has been used with some success, maybe it can be okay.
Let's compare some aftermarket springs to cut springs. We began by finding data on aftermarket springs. We looked at the front spring rates and how much drop they provide over stock. We tried to find options in a broad spectrum, from mild street springs to full race springs.
Next, we needed to see what happens to spring rate and ride height when we cut the springs. Instead of actually cutting springs and measuring spring rates and ride height drop, we just used a mathematical model. Springs are very well understood and equations have already been created to accurately describe spring characteristics. We use the equation below along with some measurements we took from our BMW E36 M3.
Where k is the spring constant, d is the diameter of the coil wire, G is Young's Modulus of the material, D is the diameter of the coil, and N is the number of coils in the spring. I'll spare you all the math that follows, but the main point is that springs have already been modeled mathematically, and there shouldn't be much debate over what happens to the spring rate when you cut springs.
I also used the F=kx spring deflection equation to determine how much drop in ride height would result. Below is the chart that resulted. The blue data is from springs that exist on the market. This shows that generally, you want an asymptotically stiffer spring as ride height drops. This makes sense, because if your suspension is bottomed out, there is no upward travel left and you would need an infinitely stiff spring to keep the suspension from compressing any more. The red data points are theoretical spring rates that you would get from cutting the factory spring.
What this tells us that cutting springs drops the ride height and stiffens the spring, but the stiffening effect is not as much as typically is desired. You can get some acceptable results with a mild cut, but as the cut gets more aggressive, the stiffness becomes further and further from ideal.
Other problems with cutting springs:
1) You can go too low. If the springs are cut excessively, the spring coils can bottom out on themselves. This is called coil binding and is bad in a number of ways. First, when the spring bottoms out, the spring rate suddenly shoots way up, leading to unpredictable handling dynamics and severe impact loading to the chassis and control arms. Secondly, the spring coils are grinding against themselves and will destroy themselves over time.
2) You can cut springs incorrectly. Using a plasma cutter or oxyacetylene torch to cut springs will destroy the temper of the metal and will make the spring too soft. You also shouldn't cut progressive springs or springs with square ends. Said another way, you shouldn't cut your springs unless they are perfectly helical.
3) If you cut your springs excessively without changing the shocks, you will have poor handling dynamics. The system will become underdamped. The suspension will bounce up and down, which is bad for traction and bad for comfort. This is true for any aggressive spring that is not paired with an appropriately aggressive shock.
There are other problems with cutting springs excessively, but they are not limited to cut springs, but rather any aggressive lowering.
You can have a situation where the spring reaches it's full extended position before the suspension has reached its full length of travel. In other words, the spring now isn't long enough to push the suspension down all the way. The suspension droop is limited, and this can result in the inside wheel(s) lifting during cornering, or the wheels losing contact with the road over bumps, jumps and drops. This may upset the car in unpredictable ways.
Lastly, the suspension geometry can suffer if dropped too low. The instantaneous roll center can rise without the use of some sort of roll center correction measures. The camber and toe can also reach unfavorable conditions depending on the control arm geometry. The nuances of suspension geometry are beyond the scope of this article, but the key point is that lowering a car excessively can negatively impact performance.
I haven't personally cut springs on any of my cars, but I can see its benefits as long as it is done in moderation. I think that extreme cutting has made some people erroneously conclude that cutting springs in any amount is bad, but I don't think this is true. On our chart comparing cut springs to aftermarket springs, there is an aftermarket spring with a 1" drop that is very similar in spring rate to a cut spring. And let's not fail to consider the possibility of cutting aftermarket lowering springs. If you had an aftermarket setup that was still a little too soft and a little too high, you could tweak it by trimming the springs just a little. Maybe your ride height is uneven and trimming springs can level the car. This is what tuning is all about - making small changes to optimize performance.
So don't believe everything the internet experts tell you, but also don't be an idiot and cut your springs so aggressively that you go into a skid and sending a bus full of noble laureates and philanthropists swerving off the side of a cliff. Cut a little at a time. You can always cut more, but you can't add back spring.