Feb 14, 2018 update to the Mazda Miata project car

Project Turbo Miata: Cutting Springs

Photos by Wayne Presley

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 from Treasure Coast Miata 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:

  1. An uneven cross weight percentage makes a car hard to drive. This is because the car will handle differently in right-hand turns than it does in left-hand turns.

  2. A tall ride height means a high center of mass. This means that the center mass exerts more leverage on the roll axis of the car, leading to dynamic weight transfer and geometry issues. Load transfer is bad because in a perfect world, all four tires would be contributing equally to the cornering force. That’s not possible in the real world, but lots of load transfer in a corner can overload the outside tires and underutilize the inside tires, reducing grip.

  3. A tall ride height also causes more body roll, thanks to the higher center of mass. Body roll is bad because it uses up suspension travel and camber in a corner. This makes the car slower because the tire can no longer operate at its optimum camber angle, and any bumps encountered could cause the car to bottom out, which overloads the tire and makes the car handle unpredictably.

  4. A taller rear ride height can tilt the car’s roll axis forward, creating greatly different load transfers at the front of the car vs the rear of the car. This can be used as a tuning tool, but in a naturally well-balanced car like a Miata, big deviations from the stock roll axis angle can lead to trouble.This means less load transfers to the rear wheels than it normally would, which causes oversteer that slows us down.

  5. Springs get stiffer as they get shorter. Why? Because removing coils means what’s left needs to compress further for the same amount of suspension compresion. In a spring with four coils that is compressed one inch, each coil will compress one quarter of an inch. If two coils are cut off of that spring, each remaining coil must now compress half an inch, meaning a stiffer spring rate. Stiff springs benefit our car by reducing suspension motion that leads to dynamic camber change and geometry issues

With the technical explanation out of the way, we faced a question: How do you corner-weight a car that doesn’t have an adjustable suspension? Simple: Time. Wayne Presley, our overworked crew chief, spent an evening in the garage with a set of scales and a cut-off wheel. The process goes like this: Weigh the car, identify the corner that needs a lower ride height, remove that spring, cut it a little shorter, then reinstall and weigh the car again. After repeating this cycle a few times, our car was better than ever: A perfect 50.0% cross weight and a race-appropriate ride height with the stiffer springs and lower center of mass.

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Comments

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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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