Thread: Understanding Spring Rates

Originally Posted by Dougal

Great info, thanks.
Do you have any more detail on that? Specific models or shaft speeds?

http://www.aulro.com/afvb/modified-z...absorbers.html

post #4

so, sort of apples for apples as they are both 82 series. And as in that thread, I can not tell you any more than those figures. I got them from Toperformance.

Originally Posted by rick130

I was going to ask the same thing (and I'm sure Sergio has told me in the past too)
Bilsteins stated forces are at silly high shaft speeds (IMO) for general use.

I've re-shimmed the foot valves in my Koni's to reduce the low speed bump harshness, worked a treat (after wrecking the foot valves I had, which were two steps down in bump valving from the stock TLC settings)

note: these are the same 82 series Koni's for Toyota LC 80 series that I run.

Rick, did changing the shims only affect bump? or does it have a side affect of rebound?

Bump only.

I found the other new set of foot valves I have too, but cant recall what spec they are as the bags have deteriorated......

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.

Dougal, the rule of thumb for suspension frequency in Hz came from the attached pdf file. The figures I gave for corresponding spring rate per 1000 lbf were my calcs. I will re-check, tomorrow maybe when I have more spare time.

However at no time was 1.35 Hz given for the front. 1.35 Hz is only for the rear springs when front springs are 1.1 Hz. The suspension frequency at the rear should be somewhat greater than the front.

Edit: sorry, in my rush, searching for the original source of my info, then somehow I read the wrong bit. Indeed the original 1.35 Hz front with 1.69 Hz at rear was correct. (end edit)

Originally Posted by uninformed

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 @ 193.36 lb/in per 1000 lbf would give me 242.98 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.

Serg, I'm not sure what you are describing with the pogo stick analogy. If you understand the following, you might be able to describe the issue in another way.

Suspension frequency figures provide a measurable method to compare ride characteristics of vehicles with widely different weight, for similar use/terrain.

The reciprocal of frequency (1/Hz) gives the period in seconds.

Simplistically, as this applies to suspension, the period is the time (sec) for the suspension to return to the normal ride height when it is disturbed by a bump. For this discussion call that period t1

Depending upon vehicle speed and wheel base, the rear wheels hit the same bump some short time after the front wheels. Call that period t2

Ideally, to make the pitching more comfortable, it is desirable for the rear suspension to return to the normal ride height at the same time as the front wheels.

This means that the period for the rear suspension t3 = (t1 - t2).

The the ideal rear suspension frequency, to 'balance' the pitching is 1/t3 Hz.

Clearly, the suspension frequency at the front needs to be somewhat less than for the rear.

If they are not reasonably 'balanced' then you will have pitching when the front wheels hit the bump and then pitching again when the rear wheels hit the bump.

Suspension frequency is not the only consideration, but it is part of having the 'full picture' and particularly when you have found what is good for your use and what the same ride characteristics from a different vehicle.

John, in your post #13, you say

"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."

That is where Dougal and I are reading 1.35Hz for the front. I based my calcs on the wrong lb/in per 1000 lbf. I re did them with the correct 186.4 lb/in per 1000 lbf and it still gives me 234 lb/in springs.

Pogo stick. Well i'll try again. It feels like bouncing up and down, but I would say not in unison, but slightly seesawed. i.e. front/rear/front/rear/front/rear.

Currently at 210 lb/in front and 250 lb/in rear springs I'm at 167 lb/in per 1000 lbf front and 244 lb/in per 1000 lbf at rear.

Looking at your info, rear springs are about 1.25x the front for frequency, or about 1.53-1.55x the front spring rate per 1000 lbf on the springs. Given my current figures, im quite close and will be al most on the money when the 190 fronts go in. I guess then it comes down to my shocks and their valving?

I must be much closer than LR as their rears where 320 lb/in in my truck. Not sure of the fronts, looks like possibly 154 lb/in, but with a little less weight over them due to my bar and winch.

Originally Posted by debruiser

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?

I haven't said that and I'm sorry if it came out that way. The thread is about spring rates, and my advice for a 4x4 that sees much off road use, on rough tracks or when good articulation is wanted, is to use the longest and softest springs that will do the job.

With soft springs the rebound valving needs to be lighter, which means the shockie may have to absorb more of the energy on the bump stroke. That also helps with body roll to some extent.

If your shockies are valved to hard, for what they need to do, then changing that will improve your suspension.

Tuning shockies is more difficult than selecting springs. It is made easier if the correct springs are chosen. And very hard to tune shockies to fix issues caused by poor spring choice.

I don't consider myself qualified to tune shockies, or give any more asvise than the general stuff above,but I am working on improving my knowledge.

My own shockies are pretty fancy and reasonably easy to adjust just about every parameter. And I have a smart software package that gives more information about just about every performance aspect, much more than I can understand and properly deal with at this stage.

Originally Posted by uninformed

John, in your post #13, you say

"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."

That is where Dougal and I are reading 1.35Hz for the front. I based my calcs on the wrong lb/in per 1000 lbf. I re did them with the correct 186.4 lb/in per 1000 lbf and it still gives me 234 lb/in springs.

Pogo stick. Well i'll try again. It feels like bouncing up and down, but I would say not in unison, but slightly seesawed. i.e. front/rear/front/rear/front/rear.

Currently at 210 lb/in front and 250 lb/in rear springs I'm at 167 lb/in per 1000 lbf front and 244 lb/in per 1000 lbf at rear.

Looking at your info, rear springs are about 1.25x the front for frequency, or about 1.53-1.55x the front spring rate per 1000 lbf on the springs. Given my current figures, im quite close and will be al most on the money when the 190 fronts go in. I guess then it comes down to my shocks and their valving?

I must be much closer than LR as their rears where 320 lb/in in my truck. Not sure of the fronts, looks like possibly 154 lb/in, but with a little less weight over them due to my bar and winch.

Serg, thanks for pointing that out, I stick by what I said in post #13, and have gone back and edited the reply I made to Dougal, to correct my bad fubar. Yesterday was chaos, I apologise to you and Dougal, sorry.

Taking back up the comments out 1.35 Hz producing springs that are too stiff. Do not base the calculations on the weight measured at the wheels, that will definitely produce springs that are too stiff.

To caculate the natural frequency of a spring, like any flexural structure (or member) e.g. a bridge, you need to know the mass supported by the spring (or structure). The axle assembly, wheels and tyres, plus half of the suspension arms, must be excluded as the springs do not support their mass.

If you go back to early in my second post, I showed that it is easy to determine the force and mass supported by the springs (spring force = rate x deflection). If you don't know the specified spring rate it is easy to calculate, the free length and installed length of the spring is relatively easy to measure.

Your further comment about the pogo stick effect

feels like bouncing up and down, but I would say not in unison

is what can be reduced by selecting the right balance between the frequency of front and rear springs.

If it does turn out that the 'rule of thumb' for the target frequency produces too high (or low) a spring rate, that is no reason to ignore the balance between the front to rear frequency.

Edit: it is more usual to choose springs for a desired ride height than frequency. However when you have choices in both the free length and spring rate of springs you can find the best spring that achieve the required ride height with the least compromise for use (off road and on road performance) and comfort.

Thanks for clarifying that John.

Serge, I find springs that are too firm give the feeling of riding on top of the suspension rather than in it. Each bump is felt as a definite event rather than just contributing to vehicle motion.

This is best known in many circles as "riding like a toyota"*. The japanese appear have a completely different suspension tuning process to the europeans and it gives a very different result.

*except for the 100 series, which ride like a boat.

When I inherited my rangie it had 240 lb/in springs in the front and 330 lb/in springs (+2 height also) in the rear.
It was bloody awful in ride, vicious handling at the limit and average traction off-road. I suspect the guy ordering the springs screwed up. It was supposed to be HD in the rear and HD+1 in the front.
Not HD in the front and SHD+2 in the rear.

I was pretty happy with 180lb/in front and back (the rear leveller I think contributes some additional spring rate over bumps) but currently have 240lb/in which I put in for some heavy towing and haven't yet taken back out.
My rear konis (30-1312) are very good when the load matches the springs.
My front konis (30-1426- LC80 fitment) are overdamped but are far better than the koni that were in it prior (84-1124 Rally Spec twin tube).

With my 4BD1T the front and rear axle weights are 1170 and 1130kg with half a tank of fuel. Should be almost equal with a full tank.