Coil-over spring rate vs spring load

[dontlifttoshift]

So I am replacing a set of Aldan's with Afco's on an off topic car. It is a Heidt's superride on a 34 chevy. Front end weight is approximately 1300#.

The afco is a 3" stroke model (Aldan was 3.5") so I am married to a 7" tall spring. I would like the rate to 350 lbs per inch but I need to support about 700 lbs per spring.

So essentially I need to compress the spring 2" to support 700#. If this is correct can I run an 8" spring and install it with a strut compressor to assemble the coil overs? That would give me 1" of preload initally and 1" more as the suspension settles in to ride height and give me my desired spring load. Or is that a really bad idea?

[Bryce]

If its a coilover you can just spin the nut down and preload the spring.

[dontlifttoshift]

Thanks Bryce but its not that easy. The afco shock will only take a 7" spring and that is causing my problem.

So is the relationship between rate and load as simple as I stated it above?

[dontlifttoshift]

I guess what I am saying is this - If I use an 8" spring and compress it 2" I have an installed height of 6" If I use a 7" spring and compress it 2" I have a 5" installed height. Assuming both springs are made with the same wire diameter and the same number of free coils (which they should be, I think) the 6" intalled height is farther away from coil bind. Being that I am working within a very small window for installed height I thought it would be better to use the 8" spring. the Afco will not take a taller spring than 7" without precompressing it so here I am.

[U_SAY_GO]

You need to take into account mechanical leverage. It's really unlikely that you are 1:1.

I would not preload the shock if it were me. That will give you very little droop and will make the car perform like you are running a lot of rebound restriction in the shock. If you have a travel constraint then there's not too much you can do about it other than run the suspension real stiff to minimize roll.

700lbs in the front is a lot of spring, especially if the total car weight is 2800 or less. I would guess you will end up closer to 400-500.

If you really want to do the math, you will need the length of the lower control arm and the location of the coil over mounting point relative to the ball joint. Technically you should also factor in the angle of the shock, but I haven't seen that to be a huge factor.

If it were me I would run a 6" 450lb. Hypercoil and see what happens. Just make sure you run bump rubbers on the shock to prevent damage if you bottom them out. With a spring that short you will probably go into coil behind before you run out of travel anyway.

[U_SAY_GO]

I just re-read your post and realized you want to run a 350lb. spring. That sounds like a good rate, but I think you will find you have less than 2" of compression to support the weight of the car.

[dontlifttoshift]

I would not preload the shock if it were me. That will give you very little droop and will make the car perform like you are running a lot of rebound restriction in the shock. If you have a travel constraint then there's not too much you can do about it other than run the suspension real stiff to minimize roll.

I would still have 1" of shock travel to the rebound side.

700lbs in the front is a lot of spring, especially if the total car weight is 2800 or less. I would guess you will end up closer to 400-500.

700 is way too much spring rate

If it were me I would run a 6" 450lb. Hypercoil and see what happens. Just make sure you run bump rubbers on the shock to prevent damage if you bottom them out. With a spring that short you will probably go into coil behind before you run out of travel anyway.

I am trying to avoid coil bind. The aldans that were on the car run a 9" spring, same number of coils and wire diameter as the 7" spring but because it starts taller I have much more room before coil bind with the aldan shocks. The 7" spring is nearly bound by the time I reach ride height.

[DynoDon]

So the AFCO's come with a spring that will coil bind before the shock runs out of travel? Sounds like the wrong spring for the shock application

[dontlifttoshift]

The spring will bind before the shock because I have to turn up the adjuster so far to get ride height.

[U_SAY_GO]

Usually the diameter of the wire used to make the spring varies with the rate of the spring and is consistent regardless of length. So along the same lines of thinking, you should have less winds for a shorter spring of the same rate compared to a longer spring.

Sounds like you have a pretty good understanding of what you're up against. The only other input I can think of would be to look to using a heavier sway bar to increase your wheel rate without comprising the ride too much. That obviously won't help with straight line compression but it will help control compression from roll which is typically where you get full compression from anyway.

I have worked on GrandAm cup cars for several years and we only see about 3-4" of compression on those cars, so you will probably be fine especially on a lightweight car. If you aren't already aware...you can get bump rubbers in different rates, just like springs, and they are a good way of controlling that last inch of movement before you go into bind.

[DynoDon]

The spring will bind before the shock because I have to turn up the adjuster so far to get ride height.

Interesting if you have to run it up that far and yet the 8" is too long. Would you possibly be better off with a 400 lb. spring in the 7" length?

[dontlifttoshift]

If I up the rate 50 pounds that only gives me 50 more pounds of load. If I use the 8" spring and preload it onto the shock I get 350 pounds of load and maintain the same spring rate. I think. I found this at Hyperco. Read the third one down. taller spring with a lower rate equals better ride quality it seems. Now how to get that spring on the shock.
http://hypercoils.com/TechTips/?TipNO= 7

Link is bad just click on tech tips

[DynoDon]

It is true that a taller spring with a lower rate can equal a better ride as long as there is sufficient travel vs rate available.
Mercedes has used this theory for years - long travel, long springs. It's why you can hit substantial dips or speed bumps in a 600 S class sedan and it soaks then up like a Baja race truck, surprising the crap out of passengers

[DynoDon]

If I up the rate 50 pounds that only gives me 50 more pounds of load.

At the required 700 lbs of ride height it would give you less preload on the spring than the 350 lb springs. Instead of 2" it would probably be more like 1.5". Would also help with bottoming if that is an issue.

[dontlifttoshift]

Bottoming is not an issue. This was just going to be swap out junk shocks for good ones. Initially i wanted to use the springs that were on the car but no go, so we supplied the new springs but I did not take into account spring load. So here we are. This is all an effort to improve ride quality so I am not willing to up the spring rate.

Thanks for everyones input so far, lets keep it coming. Has anyone done this, compress a spring to assemble the coil over?

[DynoDon]

Bottoming is not an issue. This was just going to be swap out junk shocks for good ones. Initially i wanted to use the springs that were on the car but no go, so we supplied the new springs but I did not take into account spring load. So here we are. This is all an effort to improve ride quality so I am not willing to up the spring rate.

Thanks for everyones input so far, lets keep it coming. Has anyone done this, compress a spring to assemble the coil over?

I understand what you are going for now.

As for compressing them to install on a coil over - have done it many times on everything from Off Road Trucks to motorcycles. No problems

[U_SAY_GO]

So you will end up with 350lbs. of stored energy in that spring at idle. The only real down side of that is: A. Like I said earlier, limiting the negative travel will make the car behave like you increased the shock rebound. (you can tune around that, but solid rear axles don't like a lot of rebound and you will have a hard time keeping a good contact patch in the rear) B. When street driving you may hear and feel the shock hit it's extension limit because the spring will still be pushing when you reach your extension limit. The bump rubbers inside the shock will determine how violent that is.

So we've circled around and had some good conversation that basically resulted in confirming you had a good plan before you ever posted the question!

I have also run springs with pre-load but never on a street car. I would like to hear how it works out for you.

[dontlifttoshift]

Now I am confused.

So you will end up with 350lbs. of stored energy in that spring at idle. The only real down side of that is: A. Like I said earlier, limiting the negative travel will make the car behave like you increased the shock rebound. (you can tune around that, but solid rear axles don't like a lot of rebound and you will have a hard time keeping a good contact patch in the rear) B. When street driving you may hear and feel the shock hit it's extension limit because the spring will still be pushing when you reach your extension limit. The bump rubbers inside the shock will determine how violent that is.

How is that any different than the original situation where the adjuster had been turned up 1"?

So we've circled around and had some good conversation that basically resulted in confirming you had a good plan before you ever posted the question!

I like it when no one tells me I am crazy, sometimes you just need to hear someone else say "that makes sense"

I have also run springs with pre-load but never on a street car. I would like to hear how it works out for you.

I bet you have. Any time you run the coilover adjuster up past where it makes contact with the spring it puts pre load on it, correct? I will post here if it works or not, but at this point I have no reason to believe that it won't.

[ArtosDracon]

RE Post #1: You should be fine, but if you're going to get crazy on it you'll want to consider a limiting strap to keep the inside shock from maxing out and dammaging itself.

[dhutton]

I used an chassisworks coilover spring compressor to install longer springs on the coilovers on my G-Bar. The end result was catastrophic failure of the shock when I hit an abrupt dip in the road. The high preloaded spring had a lot of stored energy and it literally blew the shock apart. Chassisworks supplied instructions for calculating the correct spring rate. You can read more here:

https://www.pro-touring.com/forum/sh...ock&highlight=