Does cutting a spring reduce or increase it's spring rate? (+ PHOTOS!!!)

[cheekychimp]

... or does the rate remain constant?

And what if the spring is progressive rate rather than linear? What happens if you cut coils from the bottom of the spring and or from the top?

Little bit of background to this. Did an MR Bilstein suspension conversion on the FWD GTi in the UK. Rear ride height was obviously higher than with the AWD because the rear shocks on the FWD mount on top of the solid axle cross member. I have tried to reduce this by using lowering springs to attain the 30 mm desired drop at the rear. The springs still aren't compressing to anything like that drop (I really can't see or measure any drop at all!) and I think this is because the spring rate is still far too high for the lighter car (no rear diff, no rear driveshaft section, plus further overall reduction in weight with no transfer case front driveshaft etc).

I understand the issues with dropping a car too low or cutting lowering springs to get more than 40 mm of drop and reduced shock absorber lifespan. Remember this is not the case here. I haven't achieved any drop at this stage as the spring rate is still too high for the weight of the car. There are other options to reduce the ride height by modifying the top mount location, axle mount location and the spring perch.

The issue however is that I currently am getting zero to minimal rear shock travel and zero damping affect from the strut piston. I need to reduce the spring rate AND drop the car so that I engage some piston travel and get some damping effect. The spring is an Eibach pro kit spring (already arguably the softest spring that I can find) and it is progressive.

Best/Worst case scenarios if I remove 3-4 of the dead coils at the bottom of the spring?

I know I can revalve the struts but I don't want to do that and even if I do it is pointless unless the spring is allowing the damper to engage.

Paul.

 

[Whoodoo]

Cutting a spring will increase the spring rate. It seems counter intuitive, but think of it this way: If you stretched the coil out so it was straight, then tried to bend it, the longer the piece is, the easier it is to bend.

For a more technical insight, the spring rate is equal to the following:

modulus of steel [lb/in^2] x wire diameter^4 [in^4]
__________________________________________________ = spring rate
8 x number of coils x coil diameter^3 [in^3]


As for a progressive spring, I would assume in order to maintain the lowest possible spring rate you would want to cut as few coils as possible. This means cutting the ones that aren't as tightly packed together, but I'm not absolutely sure on that one.

 

[Barnes]

Cutting the end of the spring increases the rate. This is because you are changing the geometry of the spring. That's why you end up cutting more off is so you can get the car low enough.

 

[cheekychimp]

So is lowering the perch as Curtis is suggesting, a better alternative?

 

[Barnes]

Actually, I might have to take back what I said. In Whoodoo's equation the term 'number of coils' is actually 'number of active coils'. An active coil is a coil that is acting as as pring. A non-active coil is really just sitting there. Depending on how the spring is finished at the ends, the number of actice coils will be different than the total number of coils. Our springs are finished with squared ends. Cutting a our springs gives you a plain end. Turns out springs with squared ends effectively have 2 fewer coils, or 2 fewer active coils. Thus, active coils (Na) is equal to total coils (Nt) minus 2. [Na=Nt-2] A plain end finished spring's total number of coils is equal to the active coils. [Na=Nt] So if you cut 2 coils off a stock spring (squared end), you should end up with the same number of effective coils, and thus the same rate.

This is where it gets a little confusing. The above holds true for a spring that is finished the same way on each end. But 99% of the time, when we cut springs, we don't cut BOTH sides of the spring. We only cut one side of the spring. Thus, one side has a squared end, and the other has a plain end. My assumption is that in this case, your active number of coils now becomes Na=Nt-1. This is because in the original equation, EACH sqaured end causes you to lose an active coil. Since we are only cutting off ONE squared end you are gaining back ONE active coil. Of course, this is now offset by the fact that you have removed a coil (more or less depending how you cut it)

Now, remember from Whoodoo's equation the following is true.
If Na (active coils) goes UP, spring rate goes DOWN.
If Na (active coils) goes DOWN, spring rate goes UP.

So this means that theoretically if you cut one coil off the spring (only top, or only bottom), you should have the exact same rate as the stock spring.If you cut MORE than one coil off the top or bottom, you will RAISE the spring rate. This is because now you have surpassed the offset, and now you have fewer active coils. If you cut less than one whole coil, you will LOWER the spring rate.

Example: Let's say that a stock spring has 5 coils. And both ends are squared ends. So we get the following:

Na=Nt-2
Na=5-2
Na=3

Now lets cut ONE coil off the end, change one end to plain, leaving the other end squared. Notice we now only have 4 total coils
Na=Nt-1
Na=4-1
Na=3

As you can see, it's the same active number of coils, and thus the same spring rate.

Now let's say we cut off TWO coils. So one end is plain, leaving the other squared. Now we have 3 total coils.
Na=Nt-1
Na=3-1
Na=2

In this situation, we now have fewer actice coils than stock, and thus a higher rate.

Now let's say we cut HALF a coil. We now reduced our total number of coils by a half giving us 4.5 coils. At the same time we changed the end of the spring from squared to plain.
Na=Nt-1
Na=4.5-1
Na=3.5

In this last example, the number of effective coils went UP, thus LOWERING the spring rate.

 

[Barnes]

EDIT: Actually, it depends how many coils you cut off. See post below.
Quoting Barnes:
Cutting the end of the spring increases the rate. This is because you are changing the geometry of the spring. That's why you end up cutting more off is so you can get the car low enough.

 

[Barnes]

Quoting cheekychimp:
So is lowering the perch as Curtis is suggesting, a better alternative?



If you can do this, it is always recommended. It is much better to keep ride height, and spring rate as two seperate things. ALSO! The spring perches on our cars are designed to work with stock springs that have squared ends. (OR any off the shelf car spring really.) Cutting a stock spring will give you a plain end, and it doesn't fit that nicely with the stock spring perch.

 

[cheekychimp]

I don't dispute that but the Eibach Pro Kit springs have at least 5-6 non active coils at their base so I could at least keep the bottom flat in the perch. I was also led to believe that cutting these dead coils has no affect on the spring rate because only the active coils compress since the others are already bound. Changing the spring perch will cost a lot of money which is not the idea on this car. I may take a chance and try cutting the lowering spring and use the stock spring at a later stage in a modified perch if I really need to do that.

 

[curtis]

Man Get me the perches sent over and tell me the lenght you need added. Its not hard. I could actually do the ones here and then you send those to me to put on your others. I've actually spent more time with Pm's about it than it will take for me to do it.

 

[cheekychimp]

Man, maybe I will later but I need the car. I'm only here for 10 days and need a quick fix. The shipping is what will take the time. Let's see how this pans out and if it doesn't work, I'll throw the red springs back on and have you do the perches later. A 60mm drop would be ideal allowing me to throw the front lowering springs on as well.

 

[Specter]

I had the same kind of problem. I got GAB shock absorbers but they were made for the vr4, and i had my fwd galant. Due to the difference in fitting on the rear sub-frame as you said, my galant was way way up after fitting those shocks. No suspension travel on the back, so what i did was, i cut the springs to my desired height. Turns out i had to cut too many coils to get it to the stock ride height. The shock absorber shaft got in the middle of its working space but there was again no suspension travel. The setting on the shock is at the softest. It rides very hard from the rear of the car. I do not know what to do next. The only thing which would work in my case is to modify the top place where the bolts mount on the body, otherwise i need to get some other shocks.

 

[cheekychimp]

Well to be honest that isn't really a bad way to do it. I have spent a LOT of time thinking about this and whilst modifying the perches or using some other lowering springs or cutting the existing ones might lower the car, modifying the top mounts retains all the factory functionality of the shock whilst dropping the ride height.

If you have stock springs you could consider modifying the lower perches as suggested. This however only really works satisfactorily if modifying the perches drops the car low enough whilst retaining enough tension in the spring to match the correct spring rate for the FWD car. I think it is very likely possible but doing it correctly would require doing a LOT more calculations.

I don't disagree with the maths presented in this thread but I am still struggling to find an answer as to whether cutting dead coils i.e. those that are already non-active and binding at the bottom of the spring actually has any affect on the spring rate of active coils that remain uncut. An analogy was given earlier of thinking of springs unwound and that a longer piece of metal is easier to bend than a short one. If however the coils are inactive and bound, it is like having 2-3 feet of that unwound metal stuck in a hole in the wall so that you are only trying to bend the section that is left over. I still maintain that this would leave the spring rate essentially the same as the progressive rate works from the top down. But it is possible that I am missing something here.

I intend now to cut my rear lowering springs in the hope that it will not make the ride any worse than it is now. I doubt the ride will get any better, but if it ends up no worse than it is now, I can live with it. If I am proven wrong (and believe me I will man up if this makes things worse) then I will either have Curtis modify the perches or raise (i.e. extend) the rear upper mounts on the strut towers at a later date and put a set of stock springs back in.

Paul

 

[Specter]

Well if your thinking about the effects of cutting the coils, i would mention what i did with my front shocks. I had to change my front shocks and the ones i got as replacement were around 1.5 inch taller then the old ones from the base of the shock (the compressed chamber itself). They were stock but they came from some galant I do not know but the shocks are pretty solid. So naturally the car sat 1.5 inch higher then before.

I did cut the springs. But i had the same thought in mind at that time as to cut the dead coil or the active coil. The springs were also stock. I ended up cutting the active coil. But in my case it was only one coil for the drop i required. Apparently the decision was good, because the front suspension travel feels good and i got my required drop.

This was the case where i did not require a LOT of drop, only 1.5 inches. Therefore cutting one coil from the active part of the spring did not have much effect on the effectiveness of the setup. I will mention it did have some. The front of the car is just a tad bit stiffer then before, but it hardly makes a difference. If i cut the dead springs, i would have needed to cut at least 2 coils from the dead part of the spring. But in both cases the important point is that the shock absorber shaft would be sitting the same place assuming i got exactly the same drop in both situations. So the dampening effect should not be very different on part of the shock absorber, but on part of the spring rate, i have no idea what would be the result if i went to cut the dead coils.

If your going to cut the springs, I think you should work out on how much drop you want, then check out how much coils you would need to cut to achieve that drop, both on the active part and on the dead part. Also work out where would your shock absorber shaft sit after the drop. You could do a combination of both too, cutting dead and active coils. All in all, i think a spring should have balance between the active and dead coils, too much of one and less of other would not work.

 

[cheekychimp]

Well here's an update. For the record this isn't meant in any way to suggest that I think cutting springs is a good thing because I don't. Nor is it meant to suggest that I think any of the information I was given above is wrong because again I don't, although I do think there are very likely more variables here at play than were taken into account.

I ended up having a total of three coils cut off from the bottom of the Eibach Pro-Kit rear springs. This made them slightly too short to fill the gap between the top hat and perches and they are now held in place by the weight of the car and secured at the bottom with cable ties. Not ideal, but dropping the perches by the desired amount at the rear would have had a similar effect since the springs would not have expanded sufficiently to fill the gap.

The net effect of this was to give the car an essentially stock ride height (which gives you some idea of how ridiculously high it was riding at the back before) but it has clearly both reduced the spring strength and engaged the piston giving some damping and a smoother ride. Make no mistake it is still very firm both front and rear and getting it perfect in my opinion would require a lot more work (raising the strut top mounts to compensate for the higher lower mounts, purchasing custom springs matched to the weight of the car as well as having the Bilsteins revalved) and I am just not ready to spend that much money on this car.

Bottom line in my opinion having done this is that it is not a budget suspension option for the FWD. The rear shocks are clearly much too long for this application requiring substantial modification to gain a no lower than stock stance. For what I have into this setup, I could likely have bought a decent set of coilovers. That said, whilst the ride is firm, it is not uncomfortable and now that it is done I am sure this will give me many years of reliable service since the shocks are going to receive very light duty with the lighter weight that they are dealing with on a day to day basis. And whilst I may be ostracized for cutting up expensive lowering springs, it was the least complicated solution for me and allowed me to utilize the rears for a useful purpose. I have kept the front lowering springs to use on my car in Hong Kong to even out the ride height when I do a similar Bilstein conversion. So, all in all, I consider it money well spent since I was only able to purchase the springs as a set of four anyway.

Knowing what I know now, If I had to do it all again, I would probably just buy four stock shocks from the dealer and throw the Tein lowering springs on but as we all know, that is not a cheap option either.

 

[Specter]

Great! Yes the rear shocks are far too long for the FWD. I would like to see some before and after pictures. The after pictures would do too if you do not have the before ones.
Thanks!

 

[cheekychimp]

Specter,

I wish now that I had taken some pictures of the car in it's various stages, just to give people an idea of how high the back sits with the Bilstein struts on stock springs. At the time to be honest I felt like a complete fool and was so embarrassed about how it turned out that taking photos to post up was the farthest thing from my mind. So I'm sorry but I don't have any of the 'monster truck' before pictures.

However, I will definitely take some pictures of how it sits now, which looks stock or close to stock. It is going to stay as it sits for now but when it needs new tyres, I will likely throw on some 205/65/15s like I have on my VR4 in Hong Kong which should close the fender to tyre gap even further. I don't have a camera here in the UK on this trip but I'll take some photos with my phone or borrow my friend's camera and post up pics next week if that helps you at all.

Another thing that came to mind was that I use to have some GAB rally struts on my VR4 and those were really high even for the VR4. I think those gave me a 3.5-4 inch tyre to fender gap all the way around and those were stiff as hell. So stiff in fact that I ran the car without strut bars for a while and I still got zero body roll. I never really understood why they were called rally struts though because in my opinion they were way too stiff for anything other than Tarmac. If your GAB struts were the rally ones, I can forsee you having a real problem because the springs and valving will likely be even stiffer than the Bilsteins have.

One possible solution that might work well is a set of shocks designed for the 1G cars (Talons, Lasers, Eclipses etc). If I recall from other threads, when people used 1G struts in the Galant the fronts sat okay but the rears were always too low. Maybe you could find some nice coil overs or struts for a 1G quite cheaply that would work for the FWD Galant application.

 

[cheekychimp]

Okay for those that don't remember this is how I got the car last year.





It is quite a nice stance but it was very low, too low for ramps of any sort and had I am sure contributed to the blown shocks which were compressed over the recommended 40 mm

After much messing about, this is how she now sits with EVO IX FQ360 Bilsteins with less than 1000 miles on them.









A lot higher obviously but I think it looks okay. And since I didn't modify the chassis I can go back to the previous setup by purchasing stock struts if I want to as I kept the Tein lowering springs.

 

[curtis]

I like it.... maybe get the next size taller tire like Mark put on the ski patrol vr4 and you'll be fine.

 

[Specter]

Yes i have those GAB rally struts on my galant and yes they are very stiff even though they are set on the softest setting on the strut. But as you said no body roll on them at all that is why i am still running them. I think your galant is a tad bit higher but it looks fine. Curtis is right taller tyres will help. Taller tyres will reduce the fender-tyre gap but they will increase the ride height further. I think if the car is riding fine, the dampness of the struts is comfortable then the car is good, you have loads of road clearance, peace at mind. How many people do normally sit in your car? Does the suspension sit down little bit if lets say you have 4 passengers in the car?

 

[cheekychimp]

Yeah the car rides better the more passengers it has in it. To be honest that's another reason I prefer it. On holidays it spends a lot of time 5-up sometimes with luggage and it drives great like that.