Sign in to follow this  
Roman

How to: Cut springs like a boss (for coilovers)

Recommended Posts

As a suppliment to Seedy Al's totally sweet home made coilover thread here:

viewtopic.php?f=48&t=26443

Some people have the idea in their heads that as soon as you cut a spring it is going to explode with atomic energy and kill babies and dolphins or something.

But it's just a coiled up piece of wire, that's it!

Cutting a spring changes some of its characteristics, but in a predictable way.

None of the the bad stuff above about cutting springs applies though if you're running coilovers - the shocks you're going to be using are stiffer than standard ones, and having an adjustable bottom spring platform means you can keep just about any spring of the right diameter captive. You've got to get it certed anyway, so theres no exta worries about the WOF man being a dick about it or anything like that.

Early 90s Honda Integras and similar run a double wishbone suspension setup at the front, which has the spring quite far inboard compared to a Mcpherson strut... so it has a much smaller diameter factory front spring to suit.

However, as per every car pretty much ever, the factory spring is quite soft.

How soft though? Well.

Here's what the spring looks like standard, found on the ground at pick a part and purchase for $11:

springc.jpg

Which is the following size:

12mm wire thickness

63mm id coils + 24 = 87mm Outside Diameter

32mm coil spacing

335mm long

10 active coils

Putting this information into here:

http://www.pontiacracing.net/js_coil_spring_rate.htm

Tells me that the factory spring is approx 4.8kgmm. (translated to kg/mm via google)

Which sounds about right for a factory front spring for a car.

4.8kgmm is only good for reinforcing the springs in your mums mattress though.

So we want to find out how the spring rate changes when we cut it down to various lengths. Below is what happens to the spring rate when you cut off x amount of coils:

Standard: = 4.8 kgmm

Cut off 1 coil: = 5.4 kg mm

Cut off 2 coils: = 6 kg mm

Cut off 3 coils: = 6.9 kg mm

Cut off 4 coils: = 8kg mm

Cut off 5 coils: = 9.6kg mm

Cut off 6 coils: = 12 kg mm

Cut off 7 coils: = 16 kg mm

It only costs $22 for a pair of these springs from pick a part, as opposed to paying $200+ for a pair of coil over springs from pretty much anywhere else.

A popular front spring rate for most 80s RWD cars is about 8kg/mm for the front.

So, cutting off approximately 4 coils would be a good place to start in my case.

Taking 5 minutes to work out where a good place to start is, can save you a lot of trouble with having the spring rate being completely wrong because you just cut it to some random length.

As well as having a good understanding of exactly what happens when you cut a spring shorter, or replace it with a longer one.

It starts getting impractical to use this spring with 6-7-8 coils cut off, but I've just included them above to show what happens to the spring rate. The spring rate increases in a linear rate with how much you cut off. If you look above, if you cut the spring exactly in half, (cut 5 coils off) the spring rate doubles. So if you want your spring 10% stiffer, you just have to cut off 10% of the height. The problem with 'makita one way' adjustable springs is that obviously once you cut off too much, you cant undo it. But since they're stupidly cheap anyway, it's no problem to just go get another pair!

A spring can be painted a pretty colour and put in a box and have some brand name on it, but at the end of the day all that matters is the spring rate.

Its just a coiled up piece of springy wire at the end of the day, so in my opinion there's not much point in paying $$$$$ for fancy springs in the above scenario, when with a little bit of thinking you can get exactly what you want for much cheaper, and spend your hard earned $$$ on other bits for your car.

  • Like 1

Share this post


Link to post
Share on other sites
Sign in to follow this