There is often a lot of confusion around tender springs and helper springs and the two are often mistaken for one another. Although they have a similar appearance and sit in the same place as each other they do in fact do very different jobs.
A helper spring is a short, very thin, flat wire spring that sits above or below the main spring. The main purpose of the helper is to keep the main spring engaged with its upper and lower seat at all times through full suspension travel. In a scenario where a wheel is lifted from the floor, the damper extends fully and experiences full droop. This is a common situation on a FWD car lifting an inside rear wheel when cornering hard or on a RWD car lifting an inside front in the same circumstances. It also occurs during pit stops where the car is lifted off the ground for a tyre change.
When the damper is fully extended, the main spring can unload completely and become free floating. When the spring is free floating it unseats from its locating rings. When the car drops back to the ground, compressing the damper and re-engaging the spring, it doesn’t always locate and can sit crooked on the damper. This will affect the ride height on the unseated corner as the corner will now sit slightly higher than before, affecting the mass balance of the car. When the damper begins working again, moving through its motions over bumps or during cornering, the spring will likely re-seat. During this brief moment, as the spring re-seats, there will be a momentary lapse of support from the main spring causing a brief moment of instability. Despite this being a small period of time, it is enough to unsettle a car at high speed, fully committed through a corner and will cause an uneasy sensation for the driver.
The helper spring can be installed to prevent this situation from occurring. It achieves this by extending once the main spring becomes unloaded due to the damper extending. It has just enough of a spring rate to withstand and support the mass of the spring. The helper spring pushes against the main spring enough to keep it located on its upper and lower perch. When the damper compresses, the helper spring fully compresses and becomes a solid base for the main spring to operate upon, not affecting the main spring rate at all.
If you are in the situation where you are trying to lower your car and cannot go any further due to the locking collars being at the bottom of their adjustment, you might be tempted to remove your helper spring in order to drop the car further by the compressed stack height of the helper. In this scenario you should not remove the helper spring. Instead you should contact your coilover provider and ask for a shorter spring of the same spring rate. This will allow you to move the locking collars back up by the difference in the new springs length and begin adjusting again. That way you don’t lose the benefit of the helper spring and still have room for adjustment when it comes to corner weighting etc.
Some of you might have coilovers installed that have main springs that remain under compression even on full droop, without the use of helper springs. If each spring has some pre-load even on full droop then the need for helper springs is not required. Some lowering springs also come with helper sections built into them. These sections are obvious like in the image below where there are a series of grouped coils at the bottom of the spring. It’s normal for these coils to compress and sit on top of each other when the car is sat on the ground. They are there in order to achieve the desired ride height of the vehicle whilst providing the ideal spring rate and keeping the OE damper loaded even under full droop conditions.
Tender springs are the most commonly mis-understood of the two and are often confused with helper springs. However, their role is much different; a tender spring is a stiffer spring than a helper spring and does not fully compress at ride height but acts as part of a dual rate spring system. Under low loads, such as normal road driving, the tender spring is used to provide a softer spring rate than if only the main spring was used. Upon compression such as cornering situations, the tender spring closes up becoming inactive, allowing the main spring rate to take over and increase the cornering stiffness of the vehicle and provide a more firm, stable cornering platform.
A tender spring must be fitted in series with a main coil spring. It is most often made of flat wire so that once fully compressed it makes a stable consistent platform for the main spring to operate upon. When choosing the right tender spring for your application, it is worth remembering that the rate you choose will not be the rate present when it is in use. This is because the tender spring is stacked in series with the main spring so is not acting upon a solid base. Therefore, a spring in series equation must be used to calculate the relative spring rate of the tender spring when it is mounted to the car.
Using the example figures of a main spring rate of 120N/mm and a tender spring rate of 30N/mm, the equation to calculate the relative rate of the tender spring is:
Therefore with our figures this becomes:
As you can see the relative rate is less than the spring rate of the tender spring. Therefore, when you are deciding which tender spring rate to use it is worth running this equation so you can choose a slightly stiffer tender spring that provides the correct stiffness once mounted in series with your main spring rate. Once the tender spring becomes fully compressed the main spring rate acts at its actual rate.
There are two main uses for tender springs. The first is for road use and bumpy circuits such as the Nurburgring Nordschleife or Sebring. With the tender spring installed, the soft overall rate absorbs all of the lumps and bumps on the road surface allowing the tyre to stay in contact with the ground instead of being bumped off the surface if only a stiff spring was being used. This keeps the car compliant and maintains high levels of grip through bumpy sections or over potholes etc. Then once the car begins to corner, the tender spring is fully compressed quickly and the main spring rate takes over to provide much better cornering stiffness, keeping the car flat and controllable through the corner.
Some racers don’t like using tender springs and would rather put up with the bumpy ride over rough sections of circuit. This is because when you initiate a turn into the corner the first sensation the driver feels is the quick transition of the tender spring closing up. In reality this is usually within a second and is over with very quickly, but it can leave the driver with an inconsistent feeling and can reduce confidence in the set up as it makes the car feel initially uneasy before the mains spring takes over. It can feel at its worst in a chicane section where the car is changing from one direction to another very quickly, compressing both tender springs in turn. This is why many high level motorsports opt for a single linear spring rate main spring, often with a helper.
Another popular use for tender springs is on cars with high downforce aero packages. This allows them to run very stiff main springs for when the downforce is in full effect but still allows the car to be softer and more compliant in slow, low downforce sections of the circuit. Once the tender springs have fully closed at maximum downforce, the main springs help to control pitch, roll and downforce to the tyre. High downforce cars have multiple different techniques and components that they use to transfer the massive amounts of downforce to the tyres. We will cover these in more depth in a future article.