Thread: Understanding Spring Rates

Originally Posted by joel0407

Originally Posted by Bush65

I'm continually bemused by people who think of only the spring rate when selecting springs or the typical recommendations that others make, along the lines of, "use xxx lbs springs"

The spring rate is important, but should not be used in isolation.

What you have done, with those calculations is make a far better comparison between different springs, but you still don't have the full picture.

Fill us in mate.

I cant seem to find many specs of springs. All I get is standard and heavy duty. I was just lucky the guy up here has a sort of cheat book. It listed other brands, it wasn't just a Dobinsons book

Happy Days

When I wrote that I was rushed and skipped the first part of your original post.

The first two sentences were not aimed at you, but mearly my general feelings toward most posts on suspension springs, as a context for the third sentence. That was meant as an endorsement of your logical approach and in particular the steps you had taken, along the lines of:

1. measure the spring length at ride height
2. calculate the spring deflection, i.e. (spring free length - spring length at ride height)
3. calculate the force required to compress the spring by that deflection, i.e (deflection x spring rate)
4. for the alternative spring candidates, determine the deflection that would result using the force at step (3), i.e. (force / spring rate)
5. for the alternative spring candidates, determine what spring length at ride height would result if they were installed, i.e. (free length - deflection)
6. calculate the change in ride height from the spring lengths at ride heights, i.e. (length at step (5) - length at step (1)

My reference to not having the full picture came from some possible concerns about the reference data used for your existing spring specifications, and there are more issues than spring rate and ride height.


Firstly my concerns with your existing springs. When I posted I was just going from the second part of your original post, and thought you had used the spring rate of 190 lb/in because the guy said that is what you had. My thoughts were, "what has he based that on".


I was, and still have a concern about whether you used a "specified" free length, rather than the actual free length as they exist now. Since the springs were installed they have probably settled down resulting in a lower free length than given in the specifications.


There still remains a fair bit to fill in the full picture and mostly it will be based on what you want the suspension to ride like, and how it is used.


Suspension springs absorb energy from bumps when compressed and that energy is fully recovered when they extend to the original length. The energy absorbed/restored isn't affected by the velocity that the compress/extend.



However energy can be affected by velocity, e.g. bumps create 'kinetic energy' (KE), i.e. {1/2 (mass x velocity)}. The velocity for KE is a result of the vehicle speed and the height and length of the bump.


Shock absorbers dissipate kinetic energy into heat. A shock absorber doesn't care about whether it does that during the bump stroke, the rebound stroke, or both. Unlike springs, their performance is a function of velocity.



The total energy from a bump has to be absorbed by the spring and shock absorber on the bump stroke, or else the axle will hit the bump stop, which proceeds to absorb most of the remaining energy. We want springs that absorb the energy from normal use before the axle hits the bump stop hard, if not the ride will be bad and structural/mechanical damage can result.



From (Energy = Force x Distance) the energy absorbed by a linear rate spring is {1/2 (spring rate x bump compression^2)}.


The spring force resulting from the bump (felt by the occupants and the structure) is {spring force at ride height + ((spring rate x bump compression)}.


It should be obvious now, that hitting the same bump at the same vehicle speed, a spring with a lower rate will require more 'bump travel' to absorb the energy than a firmer spring. Furthermore the bump will result in less force applied to the vehicle structure and occupants if the spring rate is lower.


For the same conditions, if the suspension ride height is increased, the spring rate can be lower, resulting in a more comfortable ride on a rough track. This is providing it rarely hits the bump stops.


What you showed is that, if the load on the springs is increased, by for example adding a winch, the pre-existing ride height can be restored by increasing the free length of the springs, increasing the spring rate, or a combination of free length and spring rate. As I said in my first post

The spring rate is important, but should not be used in isolation.

You could have 'filled in the picture' a bit further when comparing the springs by calculating the spring force when the spring is compressed to the height where the axle hits the bump stops.


You could also calculate the energy absorbed by the springs when the axle has hit the bump stops.another good way to see this is to plot the lines for 'spring force' vs 'compression travel' on paper (or spreadsheet) and compare the area below the line, which happens to equal the energy absorbed.


Going back to spring force at bump stop contact, divide this value by the spring force at ride height this will give you the 'G-force'. This is somewhat useful as a 'measure' of 'ride'. Approximately 2g is a reasonably hard hit, but it is a subjective number, and depends how you drive and how rough the tracks are.


This calculation for excessively stiff springs will give an impractical 'G-force'. Which means that the vehicle weight and 'pitching inertia' will never compress the springs to the bump stops. What you will have is a pig of a vehicle that rides terrible.


A quick word on 'pitching inertia'. This is increased when weight is added further away from the 'centre of gravity'. So adding the weight of a winch and/or bullbar at the extreme front of the vehicle increase the pitching inertia much more than the same weight added close to the centre. Stiffer springs are required to combat higher pitching forces generated by rough tracks.


I won't bother to touch on body roll resistance, except that with soft springs, increasing the bump damping of the shock absorbers can help, but it is usually a job for anti-roll bars.


If your aim is achieving the best ride on rough tracks, and the best suspension articulation:

• increase the suspension ride height a bit (for greater bump travel)
• use the longest and softest springs that will do the job, with only occasional, moderate impact with the bump stops
• increase the bump dampening of the shockies a little, combined with a similar reduction in rebound dampening

The longer springs will push the axle down further during rebound, and being lower rate, the rebound damping needs to be lower, to allow the axle to rebound before the next bump. This is important because you want to maximise available bump travel when you hit the next bump so that its energy can be absorbed without hitting the bump stops. With less rebound damping, higher bump damping is required to dissipate the same total energy from a bump-rebound cycle.


That is about all I have time for.

Going back to spring force at bump stop contact, divide this value by the spring force at ride height this will give you the 'G-force'. This is somewhat useful as a 'measure' of 'ride'. Approximately 2g is a reasonably hard hit, but it is a subjective number, and depends how you drive and how rough the tracks are.

Just a further comment about what I said there.

When the axle has little articulation, i.e. weight is reasonably evenly shared by both springs, it takes a good hit to compress the spring enough to create a dynamic force equivalent to 2g.

However crawling on a badly rutted track, or a track with large rocks, etc. that induce nearly maximum articulation, or one wheel is lifted, then the spring on the bump side is carrying all or most of the weight. In that situation you need a bit of bump travel remaining to absorb extra dynamic force.

If it is an uphill grade, the load on the front suspension springs will have been reduced, increasing the available bump travel, but the load on the rear springs has increased, reducing the available bump travel.

This is reversed on a decent.

Suspension springs that are too stiff will limit articulation if it takes more than the vehicle weight at that end to compress the springs enough for the axle to hit the bump stop.

Dougal mentioned natural frequency in an earlier post. This in another value to check. It relates to pitching and comfort.

For rough 4wd tracks that limit speed to <25 km/h, the desirable natural frequency is about 0.75 Hz at the front and 0.93 Hz at the rear. This corresponds to a spring rate of 57.53 lb/in for every 1000 lbf on each front spring and 88.46 lb/in for every 1000 lbf on the rear springs.

For 4wd tracks that limit speed to <50 km/h, the desirable natural frequency is about 1.10 Hz at the front and 1.375 Hz at the rear. This corresponds to a spring rate of 123.75 lb/in for every 1000 lbf on each front spring and 193.36 lb/in for every 1000 lbf on the rear springs.

For general on and off road, the desirable natural frequency is about 1.35 Hz at the front and 1.688 Hz at the rear. This corresponds to a spring rate of 186.40 lb/in for every 1000 lbf on each front spring and 291.42 lb/in for every 1000 lbf on the rear springs.

Those rule of thumb figures give a natural frequency of the rear springs about 125% of the natural frequency of the front springs. The ideal difference reduces for longer wheel base and for increased speed.

For a particular spring rate, the natural frequency reduces as the sprung mass increase.

Many will have experiences choppy uncomfortable ride in an unloaded vehicle with stiff springs (high natural frequency). Then when load is added the ride improves.

They may have also experienced vehicles that are comfortable when unloaded, but wallow about uncomfortably, like the shock absorbers are stuffed when loaded, because the loaded natural frequency is too low.

Edit: The natural frequency of a spring, often called suspension frequency is:

f = 3.1269 x square root (K/W) cycles per second (Hz)

where 'K' is spring rate in lbs/in

and 'W' is weight in lbf

Hey Bush65,

Thanks for you detailed response.

I'm reading through but I suspect I was/am on the right track.

My bump stops are getting hit pretty hard as I only replaced them less than 4 - 5 months ago, when I installed the shocks.

My shocks are Bilstien 7100 with 360/80 dampening. The next step up was 400/100. I got all the information for the shocks from others on here. I read someone was running one softer 275/60??? or something like that and he had said they were far too soft. Others had recommended the 400/100 for the rears. No one specifically said the 400/100 would be too hard for the front just no one said the 360/80 was too soft.

So as I'm pretty sure I have the shock dampening right the only other option is increase spring rates and/or increase the distance to the bump stops so the dampener has more distance to absorb more energy and the greater distance the spring has to compress the more rebound force it will have.


One thing I possibly should have tried before ordering the new springs is using spring spacers. As I have raised the bump stops by 20mm, I could have added 20mm spring spacers without having to worry about coil bind. This is going to be my next move if the heavier springs dont achieve what I need.

I was very happy with the ride, on road, as it was. The ACE keeps it handling very well in corners and ride was very smooth. I suppose that very smooth ride is part of the problem, it's too smooth.

I notice you dont mention anything about unsprung weight but thinking about it I guess this has more to do with shock dampening than springs. Something I noticed as is, it's very good at keeping the wheels in contact with the ground. Even on corrugated corners, there's none of that sideways movement you get when the wheel start bouncing of the ground. Apart from the usuall power slide.

Happy Days.

I find this pretty impressive. Espesiaclly the part about the 30 sec point.

BILSTEIN 9100 SUSPENSION TUNNING BY OFFROADPROJECTS & TD5 INSIDE - YouTube

Happy Days.

John.

Just so I can get it straight in my head. (sorry for jumping into your thread Joel, hope you don't mind) I have a stock standard D90, they ride pretty stiff. Are you saying that I could soften this and get a much better ride just from changing the shocks?

Originally Posted by Bush65

For rough 4wd tracks that limit speed to <25 km/h, the desirable natural frequency is about 0.75 Hz at the front and 0.93 Hz at the rear. This corresponds to a spring rate of 57.53 lb/in for every 1000 lbf on each front spring and 88.46 lb/in for every 1000 lbf on the rear springs.

For 4wd tracks that limit speed to <50 km/h, the desirable natural frequency is about 1.10 Hz at the front and 1.375 Hz at the rear. This corresponds to a spring rate of 123.75 lb/in for every 1000 lbf on each front spring and 193.36 lb/in for every 1000 lbf on the rear springs.

For general on and off road, the desirable natural frequency is about 1.35 Hz at the front and 1.688 Hz at the rear. This corresponds to a spring rate of 186.40 lb/in for every 1000 lbf on each front spring and 291.42 lb/in for every 1000 lbf on the rear springs.

Thanks for these values John, but the Hz values you've got for general on and off road seem quite high to me. Where did you get those from?

I measured my rangie front end years ago (4BD1T, ~1170kg total weight on the ground, 180lb/in springs) and got ~1.1Hz.
Calculates out to 1.15Hz.
I'm quite happy with the ride of the springs and certainly wouldn't want to go any firmer for general use.

To go to 1.35 Hz front I'd need to use ~250lb/in springs and that's getting silly. I had 220 lb/in springs in the front once and the ride from those was terrible with 40psi in the tyres. It would have been okay with bigger and softer rubber.

I do need to do better with the dampers. I'm currently running Koni's for an LC80 and while the length/stroke are ideal the valving is all wrong. Too much rebound damping, too much low speed compression and not enough high speed compression. A mate has some LC80 front springs I need to measure up to see what spring-rate they were designed around.

Hello Joel,

I'm from the P38 fraternity and I'm nowhere close to a suspension guru, but having read your posts, I believe you might have made the wrong choice going for the harder springs.

This is just my opinion, and I'm on bags so have no idea about springs, but if it were me, I would have gone up in spring length rather than up in spring rate. I think you might find the firm springs uncomfortable on bad roads.

An extract from your post above;
"So as I'm pretty sure I have the shock dampening right the only other option is increase spring rates and/or increase the distance to the bump stops so the dampener has more distance to absorb more energy and the greater distance the spring has to compress the more rebound force it will have."

You are correct, more distance for the shock absorber to do its thing. By only increasing the spring rate you would be putting more compression duty on the springs rather than the dampening duty of the shocks.

As for the bags in the rear, yes the spring will have a more progressive rate as it compresses (that's air for ya!), but the real benefit of air springs is that the spring rate at a given height will always feel the same (it does increase on paper, but due to weight difference when loaded up, it is not noticed).

Bags are the bomb!

I hope I didn't stuff up what I was trying to say there! And I'm not having a go at ya either just putting my opinion up!

Cheers
Keithy

Originally Posted by Dougal

Thanks for these values John, but the Hz values you've got for general on and off road seem quite high to me. Where did you get those from?

I measured my rangie front end years ago (4BD1T, ~1170kg total weight on the ground, 180lb/in springs) and got ~1.1Hz.
Calculates out to 1.15Hz.
I'm quite happy with the ride of the springs and certainly wouldn't want to go any firmer for general use.

To go to 1.35 Hz front I'd need to use ~250lb/in springs and that's getting silly. I had 220 lb/in springs in the front once and the ride from those was terrible with 40psi in the tyres. It would have been okay with bigger and softer rubber.

I do need to do better with the dampers. I'm currently running Koni's for an LC80 and while the length/stroke are ideal the valving is all wrong. Too much rebound damping, too much low speed compression and not enough high speed compression. A mate has some LC80 front springs I need to measure up to see what spring-rate they were designed around.

Running my numbers is giving me the same impression.

@ 1140kg front axle (lets assume 50/50 split over wheels) that would be (1140x2.2046)/2 =1256.622. 1256.622 lbf @ 184.40 lb/in per 1000 lbf would give me 234.23 lb/in springs.

Im currently on king 210 lb/in and have just last week gone down and orderd a 190 lb/in pair using my current measurements. Im trying to reduce some of the harsh front end that is worsend by my spring lift and 80 series shocks.

Edit: Dougal, my Koni info is telling me 80 series bump valving is 1000nm/sec and The LR spec Koni is 600nm/sec. Hopefully that means something to you?

reducing my tyre pressures from around 38 down to 32 has helped a little (clipped the very peak of the "wave" so to speak, but only the very tip)

John mentioned pitch, I have yet to read up on this, but it always comes into my mind when driving on concrete roads, especially the end of Burmuda St and the M3 at Burleigh. The buggers got the joins in it at just the right distance to make me feel like im riding a pogo stick.