I was recently looking into the topic of suspension spring preload and sag, and it seems there is very little information on the internet about these topics. There is, in fact, quite a bit of misinformation on the topic.
What is preload, anyway?
When a spring is “preloaded” it is compressed from its natural “free” length. Consider an imaginary spring. When you stand on it, at your own specific body weight, the spring compresses to half its original length. Now imagine you put that spring in a vise and squeeze it until it was half its original length. You then tie a rope around it to hold it at that length and release it from the vise. If you stand on that spring, with the rope holding it at half its normal length, it will not compress at all. It will feel like you are standing on a solid block.
Now go get that guy from work who seems to eat nothing but cheeseburgers. Anybody heavier than yourself will do. If you can coax him up onto onto the spring, watch as the spring compresses. For the heavier person, it acts as a real spring and compresses, whereas you were unable to compress it by standing on it.
What you have done is preloaded that spring to your weight. Any weight equal to or less than yours will not compress the spring, but any weight (or force) greater than yours will compress the spring.
Preloading makes springs harder, right?
The popular misconception is that preloading a spring increases the spring rate. This is not true. Because the preloaded spring does not compress when a load is applied it behaves similarly to a spring with a higher rate - hence the confusion. While a preloaded spring and a spring with a higher spring rate may compress to the same height under the same load, it is the addition of more load which differentiates the two springs.
For example, a 200 lb/in spring without preload is compared to a 100 lb/in spring that is preloaded such that adding a 50 lb weight compresses both springs equally. For the non-preloaded spring, the compression is easy to calculate as ¼ inch (50 lb divided by 200 lb/in equals ¼). Adding a second 50 lb weight to both springs, the 200 lb/in spring compresses another ¼ inch, but the 100 lb/in spring compresses ½ inch (50 lb divided by 100 lb/in). While they seemed similar with only 50 lb added, adding more weight clearly shows the difference.
Does this matter for cars?
Preload is not something the car community thinks much about, because for most of us it is not something we can even adjust. Standard springs on standard shocks have no means of adjusting preload. The spring is compressed enough to fit it on the shock or strut, and that’s the preload you’re stuck with. Expensive coilover suspensions do have the ability to adjust preload, but it is mostly neglected.
Motorcyclists are more concerned with spring preload. Their concern stems from the fact that 99% of the motorcycles produced have adjustable spring preload from the factory. Why do motorcycles have adjustable spring preload while cars to not?
Well, simply put - when this guy gets on his bike he is changing a 400-pound vehicle into an 800-pound vehicle, doubling the weight on those springs. Consider that when that large gentleman gets into his Cadillac, he is changing a 3800-pound vehicle into a 4200-pound vehicle - a paltry 11% increase, divided amongst four springs.
How is preload adjusted?
On a motorcycle or a coilover-equipped car, preload is adjusted by turning a threaded collar which squeezes the spring. You can see it on this motorcycle shock, adjusted about 1/3 into its available preload range.
Why would we want to adjust preload, anyway?
Here’s another area where the internet seems confused. Most guides on setting preload involve taking a “sag” measurement, in which you note the height of your suspension with and empty vehicle (usually a motorcyle) and a loaded vehicle (with a rider), then adjusting the preload until the difference between suspension heights is at the specified figure. The specified figure is usually given in terms of “X” millimeters of difference, and nobody ever asks just WHERE that particular measurement comes from.
Here’s an example to illustrate the benefit of setting preload (or sag). Consider an unloaded spring/shock with no preload. It has about 14 centimetres of travel before it bottoms out.
Now we put a heavy load on it - we lower the vehicle off the jackstands, fill up a full tank of fuel, some luggage in the trunk, and a few friends in the passenger seats. That weight compresses the spring and the piston in the shock absorber moves way, way down into the shock, leaving only a little over four centimetres of our total 14 centimetres until the shock bottoms out.
Hit a nice bump (or take flight over a jump) and that shock will bottom out, turn solid, transfer all that force into the bump stop and frame, and potentially break something. Also, those friends in the passenger seats will have their spines compressed rather uncomfortably.
What if we jack the car back up and crank that preload adjuster (pictured in red) to maximum? The spring is now much more compressed, but the shock piston is still in the same resting position as in the non-preloaded configuration.
Lowering the car back onto its wheels (with your gas, luggage, and friends still inside) reveals the true benefit of setting preload. That 4cm of suspension travel jumps up to nearly 8cm! It’s about halfway into its range, which allows the car to move its wheels a long way into compression or extension before running out of travel. The car will sit a little higher off the ground than it did before.
(Note that the numbers used in this example are completely made up - this is a fictional spring that reacts to preload in a fictional but theoretically-sound manner)
By setting sag to a predetermined amount, you are placing the suspension into a position where it has an optimal amount of travel to compress and extend, which differs based on the type and use of the vehicle. When this is properly set, the ride height of the vehicle will be in its proper setting.
Who needs to worry about preload, then?
Spring preload is mostly applicable to off-road racers, be they on two or four wheels. The suspension travel they require necessitates placing the suspension into the proper range of adjustment to prevent bottoming out (or topping out). This isn’t something we have to worry too much about on tarmac. In fact, many tarmac-based coilover suspension manufacturers will recommend simply setting the spring preload just tight enough to hold the spring in place.
Wait… what?
In conclusion…
Spring preload does NOT:
What is preload, anyway?
When a spring is “preloaded” it is compressed from its natural “free” length. Consider an imaginary spring. When you stand on it, at your own specific body weight, the spring compresses to half its original length. Now imagine you put that spring in a vise and squeeze it until it was half its original length. You then tie a rope around it to hold it at that length and release it from the vise. If you stand on that spring, with the rope holding it at half its normal length, it will not compress at all. It will feel like you are standing on a solid block.
Now go get that guy from work who seems to eat nothing but cheeseburgers. Anybody heavier than yourself will do. If you can coax him up onto onto the spring, watch as the spring compresses. For the heavier person, it acts as a real spring and compresses, whereas you were unable to compress it by standing on it.
What you have done is preloaded that spring to your weight. Any weight equal to or less than yours will not compress the spring, but any weight (or force) greater than yours will compress the spring.
Preloading makes springs harder, right?
The popular misconception is that preloading a spring increases the spring rate. This is not true. Because the preloaded spring does not compress when a load is applied it behaves similarly to a spring with a higher rate - hence the confusion. While a preloaded spring and a spring with a higher spring rate may compress to the same height under the same load, it is the addition of more load which differentiates the two springs.
For example, a 200 lb/in spring without preload is compared to a 100 lb/in spring that is preloaded such that adding a 50 lb weight compresses both springs equally. For the non-preloaded spring, the compression is easy to calculate as ¼ inch (50 lb divided by 200 lb/in equals ¼). Adding a second 50 lb weight to both springs, the 200 lb/in spring compresses another ¼ inch, but the 100 lb/in spring compresses ½ inch (50 lb divided by 100 lb/in). While they seemed similar with only 50 lb added, adding more weight clearly shows the difference.
Does this matter for cars?
Preload is not something the car community thinks much about, because for most of us it is not something we can even adjust. Standard springs on standard shocks have no means of adjusting preload. The spring is compressed enough to fit it on the shock or strut, and that’s the preload you’re stuck with. Expensive coilover suspensions do have the ability to adjust preload, but it is mostly neglected.
Motorcyclists are more concerned with spring preload. Their concern stems from the fact that 99% of the motorcycles produced have adjustable spring preload from the factory. Why do motorcycles have adjustable spring preload while cars to not?
Well, simply put - when this guy gets on his bike he is changing a 400-pound vehicle into an 800-pound vehicle, doubling the weight on those springs. Consider that when that large gentleman gets into his Cadillac, he is changing a 3800-pound vehicle into a 4200-pound vehicle - a paltry 11% increase, divided amongst four springs.
How is preload adjusted?
On a motorcycle or a coilover-equipped car, preload is adjusted by turning a threaded collar which squeezes the spring. You can see it on this motorcycle shock, adjusted about 1/3 into its available preload range.
Why would we want to adjust preload, anyway?
Here’s another area where the internet seems confused. Most guides on setting preload involve taking a “sag” measurement, in which you note the height of your suspension with and empty vehicle (usually a motorcyle) and a loaded vehicle (with a rider), then adjusting the preload until the difference between suspension heights is at the specified figure. The specified figure is usually given in terms of “X” millimeters of difference, and nobody ever asks just WHERE that particular measurement comes from.
Here’s an example to illustrate the benefit of setting preload (or sag). Consider an unloaded spring/shock with no preload. It has about 14 centimetres of travel before it bottoms out.
Now we put a heavy load on it - we lower the vehicle off the jackstands, fill up a full tank of fuel, some luggage in the trunk, and a few friends in the passenger seats. That weight compresses the spring and the piston in the shock absorber moves way, way down into the shock, leaving only a little over four centimetres of our total 14 centimetres until the shock bottoms out.
Hit a nice bump (or take flight over a jump) and that shock will bottom out, turn solid, transfer all that force into the bump stop and frame, and potentially break something. Also, those friends in the passenger seats will have their spines compressed rather uncomfortably.
What if we jack the car back up and crank that preload adjuster (pictured in red) to maximum? The spring is now much more compressed, but the shock piston is still in the same resting position as in the non-preloaded configuration.
Lowering the car back onto its wheels (with your gas, luggage, and friends still inside) reveals the true benefit of setting preload. That 4cm of suspension travel jumps up to nearly 8cm! It’s about halfway into its range, which allows the car to move its wheels a long way into compression or extension before running out of travel. The car will sit a little higher off the ground than it did before.
(Note that the numbers used in this example are completely made up - this is a fictional spring that reacts to preload in a fictional but theoretically-sound manner)
By setting sag to a predetermined amount, you are placing the suspension into a position where it has an optimal amount of travel to compress and extend, which differs based on the type and use of the vehicle. When this is properly set, the ride height of the vehicle will be in its proper setting.
Who needs to worry about preload, then?
Spring preload is mostly applicable to off-road racers, be they on two or four wheels. The suspension travel they require necessitates placing the suspension into the proper range of adjustment to prevent bottoming out (or topping out). This isn’t something we have to worry too much about on tarmac. In fact, many tarmac-based coilover suspension manufacturers will recommend simply setting the spring preload just tight enough to hold the spring in place.
Wait… what?
In conclusion…
Spring preload does NOT:
- change how “hard” your spring is
- change “loaded” suspension travel and, as a secondary effect, alters ride height
You should teach.
ReplyDeleteExcellent , easy and very thorough.