Tom Suddard
Tom Suddard Digital Experience Director
2/14/18 1:33 p.m.
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What springs do we run on our race-prepped turbo Miata? Simple: The stock ones. Before our trip to Daytona (where, yes, we blew up the car after three laps), we swapped the eBay coil-over conversion kit for a set of cut stock Miata springs in order to meet ChampCar rules. Unfortunately, we'd cut the springs in a hurry, and it showed: Our car's ride height was too high, and the rear was even higher. Even worse, one rear spring was longer than the other, making that side of the car sit nearly an inch higher than the other. Why is all this bad? We covered corner weights in-depth a few years ago, but here's the Cliff's Notes version of why our Miata needed some adjustments: 

 

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GameboyRMH
GameboyRMH MegaDork
2/14/18 1:47 p.m.

Corner-balancing by trimming cut springs, now that's grassroots! Also, dem Dial-Ins...cool

Dusterbd13
Dusterbd13 MegaDork
2/14/18 2:03 p.m.

So, how much did you cut for how much drop/stiffness increase?

Tom Suddard
Tom Suddard Digital Experience Director
2/14/18 2:34 p.m.

In the third photo, you can see how many coils we cut off this time. That's in addition to the last round of spring cutting, where we removed 2 coils in the front and 2 in the rear. So about 2.25 coils in the front and 2.75 in the rear total.   

How much drop and stiffness? Honestly, we eyeballed it based on past experience with other Miatas. We'd like to get the car lower, but the stock springs just aren't stiff enough to keep it off the bump stops at any lower ride height. 

 

mazdeuce - Seth
mazdeuce - Seth Mod Squad
2/14/18 3:34 p.m.

I need scales......

Ransom
Ransom PowerDork
2/14/18 3:51 p.m.

You lost me on bullet point five. It sounds like you're jumping back and forth between talking about shortening springs and just compressing them. I don't think the observation about rate rising as you compress a spring is accurate, or related to the thing you're discussing? This sentence is the root of my confusion: "It's much more than twice as hard to compress a spring two inches than it is to compress a spring one inch."

On the one hand I feel like I'm being pedantic. On the other, this seems central to the whole thing.

Tom Suddard
Tom Suddard Digital Experience Director
2/14/18 5:08 p.m.

Yeah, it's been a long day, and my explanation wasn't super clear. I edited it to clarify. My point is true, though: If you stand on a spring, it won't go all the way down to the point of coil bind, it will compress a little bit, and to push it down further you'd need to ask a friend to stand on it with you. 

If you really want to dig into the math, here's a pretty decent article:

https://www.eatondetroitspring.com/cutting-coil-springs-calculations/

ProDarwin
ProDarwin PowerDork
2/14/18 9:21 p.m.
NickD
NickD UltraDork
2/15/18 7:44 a.m.

 

Ahhhh, I rocked the cut springs for a while, because, surprise, no one makes lowering springs for a first-generation Acura Legend. 

Robbie
Robbie PowerDork
2/15/18 8:55 a.m.

My Saab truck rocks cut springs (in front). Didn't do the corner weight trick though!

Very nice.

Dusterbd13
Dusterbd13 MegaDork
2/15/18 10:09 a.m.

Ive cut and driven many, many springs over the years. I see nothing wrong with it. 

Hungary Bill
Hungary Bill UberDork
2/15/18 11:27 a.m.

What sort of an effect does the increase in spring rate have on the dampers?  Would an adjustable Koni set up for the stock spring rate be able to compensate?  What about a non-adjustable unit like a Bilstein?

airwerks
airwerks Reader
2/15/18 12:08 p.m.
Hungary Bill said:

What sort of an effect does the increase in spring rate have on the dampers?  Would an adjustable Koni set up for the stock spring rate be able to compensate?  What about a non-adjustable unit like a Bilstein?

I did cut springs and Koni adjustibles on my 94 swift gt, loved how that car handled with an upsized addco rear bar. Nothing wrong with springs cut when done right.

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