Spring Rates

[edin]

This post is for everyone who is/was confused as to why cutting coils off springs can increase the spring rate. I was of the impression that the spring rate was purely a function of material, number of coils per some distance, etc. which is why I was confused when people cut a few coils off and say that it's a higher spring rate. Maybe this is common knowledge, but it sure wasn't to me, so it's worth sharing. Had a long conversation with a co-worker today regarding suspension and learned a lot. Hopefully this is accurate information, feel free to add.

 

When a spring compresses, the spring force comes not only from the coils compressing and getting closer together, but also from the actual spring material in torsional loading. So when you cut coils, you're shortening the length of the coils (obviously), but the cut spring is also more torsionally stiff. Makes sense, right? Take a ruler at 12 inches and twist it along its axis, then do the same at 8 inches. Harder, right?

 

I thought this was interesting. Again, I hope it's correct!

[patra_is_here]

sounds right to me

[wayno]

A longer spring has more spring area, so for it to move compress 4 inches, lets say each pass of the coil pass(one time around) only has to move a quarter inch, but if you cut a couple coils off, now for it to compress 4 inches, it may have to compress a half of an inch, which is harder to do.

There is a point though, where you can cut off to much, the spring may be harder to compress, but there is not enough spring to hold the weight.

[Ranman72]

all of these posts above are correct

[datzenmike]

This is the formula I use to calculate my spring rate.

 

11,250,000 X wire diameter⁴
--------------------------------------------------------------------
8 X number of active coils X mean coil diameter³

 

 

11.250,000 X 0.5"4

____________________ =

 

8 X 6.5 X 5.0625"³

 

 

11,250,000 X .0625

_________________ =

8 X 6.5 X  129.74

 

 

 

703,125

-------- =

6746.8

 

104.21 lb/in

 

 

My stock 710 spring works out to about 104 lb/in

 

I increased the rate to 140 by removing 1 3/4 coils.

[KlassicMotion]

Mike, how much difference did you feel between the two?

 

I hear of people using 200+ with their coilovers.... Is there different math when using the coilovers, or are they just getting a harsher ride?

 

I ask, because I might be shopping for a set of coilovers soon, I suspect most people just like a harsher ride than I do, so I was going to look for springs in the 150-175 range....

[TenafterFive]

Personally. I have 200lb with an L series and I think it's perfect for the street. For the rear. I took a 500# 16" spring. Cut it in half and fine tuned the ride by cutting a lil bit till I got the height I wanted. I figure its about 1100#. I love the way my car rides. Super low and not harsh or bouncy

[KlassicMotion]

Personally. I have 200lb with an L series and I think it's perfect for the street. For the rear. I took a 500# 16" spring. Cut it in half and fine tuned the ride by cutting a lil bit till I got the height I wanted. I figure its about 1100#. I love the way my car rides. Super low and not harsh or bouncy

 

Stock diameter rear springs in the stock location, or coilovers?

[TenafterFive]

Stock spring location. Working on rear coilovers. 1100# on a Coilover is pretty stiff. :)

[datzenmike]

1,100 on a rear spring on a 510 'feels' like 289 pounds because the spring is between the axle and the front mount. That's still pretty stiff. 1,200 isn't unheard of.

 

Mike, how much difference did you feel between the two?

 

I hear of people using 200+ with their coilovers.... Is there different math when using the coilovers, or are they just getting a harsher ride?

 

I ask, because I might be shopping for a set of coilovers soon, I suspect most people just like a harsher ride than I do, so I was going to look for springs in the 150-175 range....

.

I liked it a lot but now may try stiffer yet as I have a spare set of 710 coils. 175 sounds ok and can always increase right? I cut one coil off my 'wifes' car so she wouldn't get all 'what's wrong with the car???" and used thinner ATF in the damper where I used 20W. 

[KlassicMotion]

Personally. I have 200lb with an L series and I think it's perfect for the street. For the rear. I took a 500# 16" spring. Cut it in half and fine tuned the ride by cutting a lil bit till I got the height I wanted. I figure its about 1100#. I love the way my car rides. Super low and not harsh or bouncy

 

What strut inserts and rear shocks are you running?

 

I just ordered my coilovers from T3.  After talking to Gabe for 20 minutes on the phone, I bought the 8" 200# springs.  (On a side note, they will let you exchange your springs for different rates, for up to a year or more, pending they trust that you haven't put many miles on them!)  Anyways, I mentioned I'm going to be running the 91-95 MR2 front KYB AGX inserts, and he told me, they only use those inserts on 350# springs and greater, because of the valving on them.  The only inserts he could recommend, aside from the discontinued Tokico Illuminas, were the KYB GR2 (which I've read on here, guys are saying they blow out pretty easily with 200-250# springs ??), or the very expensive Koni's (Yellows I believe he said...).