There is something I occasionally see written about tender springs that has always bothered me.
“With a lower spring rate, the suspension has less force pushing the wheel outward, so less load remains on the tire.”
Isn’t that actually the opposite of what happens?
Rather, when the front and rear spring rates are different, the distribution of lateral load transfer changes. So, if you lower the rear spring rate, more load should remain on the rear inside tire than on the front inside tire.
So why does the opposite explanation keep showing up?
After thinking about it more carefully, I realized there is actually a reason why someone might come to that conclusion.
Tender springs are sold by various manufacturers, and they go by various names, including tender springs and helper springs.
There may be subtle differences in what each term means, but I’ll simply use tender spring throughout this article.
The tender springs I have are roughly 60 N/mm and 40 N/mm, but you can find all sorts of rates if you look around—200 N/mm, 10 N/mm, and so on.
I’d love to have something around 700 N/mm with a working travel of about 10 mm… It would probably have to be ridiculously thick, though. Haha.
(I looked into it, and apparently you can get something close to that by using Belleville washers. The world is full of surprises!)
When lateral load transfer occurs and the car rolls, the front and rear spring rates determine the front-to-rear distribution of roll stiffness.
As a result, lateral load transfer is distributed between the front and rear axles in different proportions.
If the rear spring rate is lower than the front, the rear has less roll stiffness, so less lateral load transfer occurs at the rear than at the front.
As a result, more load remains on the rear inside tire than on the front inside tire.
The rear inside tire ends up carrying more load than the front inside tire.
This tends to produce understeer.
In other words, a softer rear spring means more load remains on the rear inside tire.
If you’ve driven on track and experimented with tender springs, you’ve probably felt this yourself.
So, what does the spring rate of a tender spring actually mean?
When you use a tender spring together with the main spring, the overall spring rate becomes lower.
That’s because you now have two springs contributing to the whole “boing-boing” springy effect.
The math for the combined spring rate is dead simple: just remember multiply, then divide by the sum.
For example, with a 100 N/mm main spring and a 60 N/mm tender spring:
(100 × 60) ÷ (100 + 60)
And with a 150 N/mm main spring and a 100 N/mm tender spring:
(150 × 100) ÷ (150 + 100)
See? Dead simple.
In the second example, the result is about 60 N/mm.
When I first learned about tender springs, I remember thinking, “Wait, it gets that much softer!?” But that’s exactly what happens.
My Roadster has roughly 120 N/mm springs in front and 100 N/mm springs in the rear.
Let’s calculate what happens if I add a 60 N/mm tender spring to the rear, compared with a 40 N/mm tender spring.
(100 × 60) ÷ (100 + 60) = about 38 N/mm
(100 × 40) ÷ (100 + 40) = about 29 N/mm
The 40 N/mm tender spring gives a lower combined rate than the 60 N/mm spring.
At 29 N/mm, that’s getting pretty close to the rate of a stock spring, isn’t it?
I looked it up, and the rear spring rate of an NB8C RS is about 22 N/mm. Not all that different, really.
When you brake before entering a corner, the rear suspension extends due to pitch. Then, as you turn into the corner and the car rolls, the rear inside suspension extends even further.
In other words, the load carried by the rear inside spring continues to decrease.
Once that load drops below the point where the tender spring comes out of coil bind, the main spring and tender spring begin working in series.
As a result, the combined spring rate becomes lower.
The important question is when this transition occurs during cornering.
For example, the 40 N/mm tender spring I have comes out of coil bind when the spring load drops below roughly 1,500 N, while the 60 N/mm version does so at roughly 2,500 N.
That difference of about 1,000 N corresponds to roughly 10 mm of wheel travel with a single 100 N/mm spring.
The 60 N/mm tender spring begins working in series once the spring load falls below about 2,500 N, giving a combined rate of about 38 N/mm.
The 40 N/mm spring, on the other hand, doesn’t begin working in series until the spring load falls below about 1,500 N, at which point the combined rate becomes about 29 N/mm.
When the tender spring starts working has a significant influence on how the car feels, so this difference matters.
In this example, the 60 N/mm spring causes the combined spring rate to drop earlier, so the car tends to develop understeer sooner.
With the 40 N/mm spring, the combined rate stays higher for longer, so the car tends to develop understeer later.
If this is how someone defines the “effect” of a tender spring, then I can understand why they might say:
“With a lower spring rate, the suspension has less force pushing the wheel outward, so less load remains on the tire.”
Strictly speaking, however, the effect of a tender spring is proportional to the difference in wheel travel compared with a single-rate spring—in other words, the difference in how much the suspension actually moves.
From that perspective, the lower the spring rate, the greater the effect of the tender spring.
Most commercially available tender springs have a fixed working length, which makes suspension setup much easier.
As a result, the load required for the tender spring to come out of coil bind increases with spring rate.
That means a higher-rate tender spring begins working later, even though a lower-rate tender spring produces a larger change in combined spring rate once it does begin working.
It’s easy to mix up these two completely different effects.
If 60 N/mm and 40 N/mm tender springs with the same release load were available, the 40 N/mm spring should produce the greater effect.
Although, to be fair, the difference between 38 N/mm and 29 N/mm isn’t exactly enormous.
So, every time I came across that explanation, it left me with a nagging feeling that something wasn’t quite right.
Now it finally makes sense.
I think I’ll sleep well tonight.