This article is a small study about choice or correction of spring rate, and common misconception. The actual reason was the experience of the first drive with my fully “upgraded” 600-based Abarth. It felt like shit!! 

The front was bouncing like a young wild rabbit. Acceleration while cornering initiated loads of understeer. I always implied that expensive parts were an instant upgrade to the car. It was just that moment; I realized that all the upgrades resulted in a sooped car. If you read this and you do not agree, please feel free to respond, I am always open for discussion.

Okay I replaced the front leaf spring suspension by independent swing arms, installed coilovers and added an anti-roll bar. In the rear, I installed lower springs and stiffer shocks. At that moment, I thought it must handle like a F1. So why did it not?

The biggest problem is understeer. Sometimes when I drove over a bump, it felt like there was no contact between tire and road at all. Relatively the rear is quite heavy compared to the front, with a typical ratio of 40:60. We can fill the tank and put extra sandbags in the front, but total vehicle weight will increase and we don’t want that in our racy Fiats:D.

There is a whole list of adaptions to decrease understeer, from tyre pressure to aerodynamics. Also dependant to slow-fast corners, out of corner acceleration etc. However, all these measures are just plasters on a big wound. With this big wound, I meant my base setup ;).

What should a base setup look like then? Because of the heavy rear, we need a lower spring rate in the front, higher in the rear. I leave the shocks damping rate out of this discussion, but adjustable sitfness is preferred.

So what do I have? I bought coilovers, made for the 600 Abarth. I assumed the spring rate would have been matched to this car. Wrong thought. The spring has approximately four active coils, a wire diameter of 10 mm and internal diameter of 62 mm. A simple spring calculation shows a spring rate of +/-70 N/mm or 400 lbs/inch. For comparison, I installed coilovers for my BMW E46 trackday car that had a spring rate of 70 N/mm in the front. Okay that is not full race, but still pretty stiff for a 1400Kgs car. I did not test the actual value of the spring rate, but I can imagine there can be a 10% deviation. A spring rate of 70 is still far too much for our small lightweight Fiat.

As I stated before, the rear must be stiffer, because most weight is in the rear (I know there is more than that, about suspension geometry, but we are still looking for a base. This is just a simplification). Then I measured the rear springs and estimated their spring rate. (ID: 125mm, wirediam: 10mm, 3 active coils).  The spring rate will be around 20 N/mm] which is around 120 lbs/inch. This is a lot lower than the front, but a lot better matched to the car. It also feels better when you give the car a push downwards (static), but I think it can be stiffer for racing.

Out of these estimations, I conclude our base setup is way off. I think a good starting point will be around 150lbs springs in the front and 250lbs springs in the rear. I ordered these parts and I hoped this will make an instant improvement to the handling, but that was just a common misconception. If you do the same or comparable upgrades, realize you will change the car entirely. Review the setup; make basic calculations. To be continued…