February 20, 2012 - Fun with springs.
The new 750 lb front springs are stiff enough that they don't compress much under the weight of the car, so that means they go loose at full droop. So I decided to start playing with secondary springs again. I've done this in the past, but I had new parts and new info.
The secondary springs available through AFCO start at 300 lbs. A couple of years back, I tried this very setup but found that it was tricky to set the damping and there was too much initial roll. After talking to AFCO, I discovered that they had an extra part that would have helped: a stop that takes the secondary spring out of play after a certain point.
Here's how it works: when both springs are active, you get a lower overall spring rate until the softer secondary binds up. So you adjust the stops so that the spacer between the primary and secondary spring can't move any lower when the car is at rest. So when you're moving in compression, only the big main spring is active. Once the wheel starts to drop below the normal ride height (or wherever else you set that stop) both springs come in to play. Clever.
But also designed for cars with much different suspension than a Miata. With my stiff 6" primaries and 4" secondary springs, I simply couldn't get enough adjustment to drop the car down below rally heights. I might have been in luck with tubular front upper control arms, but I don't have any of those.
So I moved on to a more traditional secondary spring. For a lot of coilovers, that means a super-soft spring of 5-10 lb rate that is fully bound at standard ride height. That keeps the main spring from rattling around at full droop, but it's not strong enough to help extend the suspension when you're actually driving the car - especially if your rebound damping is set for a 750 lb primary. I wanted something closer to 100 lb.
Luckily, I had a set of those on hand - the purple springs in the picture. They were very slightly too small in diameter to fit over the perches on the AFCOs, but a bit of noisy die grinder time took care of that. Voila, 100 lb secondary springs that are in bind at rest.
In the rear, I'm running 550 lb springs without secondaries. I may add some later, we'll see what needs to be done for ride height. 750/550 is more rear spring bias than we usually run on Miatas, but it's actually about the same as I ran during the race. I'll also run with very little (or no) rear sway bar so I maximize my drive off the corners and this spring rate will cut down the understeer. Turn-in understeer will be managed by compression damping, I had it too high during the race.
Fun stuff! Although I am getting a bit sick of pulling the front shocks out.
tags: suspension |
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