? For the coilover crowd II

3587 Views 31 Replies 8 Participants Last post by sleepsontoilet, Jan 10, 2011

Ken White

Discussion Starter · Jan 2, 2011

Lets take the ? for the coilover crowd one step farther since I am getting ready to start my build. Here are some of my thoughts so feel free to add your thoughts, or correct me if any of my assumptions are wrong.

Get the TJ weighed when it is loaded for trail riding to get the correct F/R weight bias. Then subtract the axle and tire weights from the measured front/rear TJ weights to get an estimate of the sprung weight.

Estimate the stock front axle at about 265 lb and rear at about 225 lb. The tire and wheel combination is estimated at somewhere between 125-150 lbs each.

Using the estimated sprung weight, and the desired up/down travel of the coilover, calculate the effective spring rate that achieves the loaded down travel.

Since the force applied to the two springs is exactly the same, the effective spring rate is found from the following equation:

Effective_Rate = Rate1 x Rate2 / (Rate1 + Rate2)

Application of this equation leaves endless options all of which achieve the desired value – which generally is not a good thing.

If the equation is analyzed, the options can be limited based on the 2 rates provided by the coilover, the effective rate and the final rate.

The final rate which is reserved for some portion of the up travel is determined by the bottom firmer spring and the effective rate as calculated above. So these values become important when trying to tune the shock.

It is interesting to note that the delta between the effective and final rate is minimized when the individual spring rates are the same and that this delta increases non-linearly as the delta between the individual spring rate difference increases.

If the final rate and effective rate are known, which they should be, a better equation that calculates the softer spring rate is given below:

Rate2 = Effective_Rate x Rate1 / (Rate1 – Effective_Rate)

Round the rate 2 value up from ideal calculated value to allow some room for height adjustment as needed.

Now all of these calculations assume the shock is traveling in a completely vertical motion, or some simple trigonometry will need to be applied to the force vector to adjust for these values.

Please post up your rates, how you calculated them, and if you are happy with the results. Or anything else that you think is relevent.

Thanks,

Ken

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Mouse

· #2 · Jan 3, 2011

Looks like your doing the homework necessary to find a good starting rate. My method was a little simpler - I called the guys at Poly Performance and gave them a wheel-by-wheel weight and shock measurement with the first set of springs to determine the optimal setup. I'm currently removing and redistributing some weight since I first set it up and will need to address the spring rates again.

Since I know some of those guys on a first name basis and visited the shop many times I have developed a great relationship with them. I would think they would be willing to give you a second opinion on any spring rates you pick.

Ken White

Discussion Starter · #3 · Jan 3, 2011

Mouse, thanks for the advice and Poly is a great asset. Can you post your sprung weights, spring rates/lengths, and up/down travel? I am just going through my spring length calculations and will post me findings soon. Thanks again,

Ken

Mouse

· #4 · Jan 3, 2011

Ken White said:
Mouse, thanks for the advice and Poly is a great asset. Can you post your sprung weights, spring rates/lengths, and up/down travel? I am just going through my spring length calculations and will post me findings soon. Thanks again,

Ken

Its all disassembled at the moment. I probably won't be ready for new measurements for a minimum of a couple of weeks. I'll see what I can do then.

Ken White

Discussion Starter · #5 · Jan 4, 2011

Mouse said:
Ken White said:

Mouse, thanks for the advice and Poly is a great asset. Can you post your sprung weights, spring rates/lengths, and up/down travel? I am just going through my spring length calculations and will post me findings soon. Thanks again,

Ken

Click to expand...

Its all disassembled at the moment. I probably won't be ready for new measurements for a minimum of a couple of weeks. I'll see what I can do then.

Thanks Mouse. I am in no rush. If you have any of your old numbers they would be good too. Thanks again,

Ken

bnine

· #6 · Jan 6, 2011

I honestly did mine with pretty much all trial and error. I didnt scale prior to tearing down, so I mimic'd what a friend was running and went from there.

Since you are so good with numbers Ken, I wonder if while you are researching this stuff, you could look into suspension frequency and lay that out in laymens terms for us.

Most the coil over articles, racing tech, and vendor sites talk about, but its just one of those subjects I've always had a hard time getting my head around.

Good to see your build coming together.

Mouse

· #7 · Jan 6, 2011

bnine said:
I honestly did mine with pretty much all trial and error. I didnt scale prior to tearing down, so I mimic'd what a friend was running and went from there.

Since you are so good with numbers Ken, I wonder if while you are researching this stuff, you could look into suspension frequency and lay that out in laymens terms for us.

Most the coil over articles, racing tech, and vendor sites talk about, but its just one of those subjects I've always had a hard time getting my head around.

Good to see your build coming together.

x2

Ken White

Discussion Starter · #8 · Jan 6, 2011

bnine said:
I honestly did mine with pretty much all trial and error. I didnt scale prior to tearing down, so I mimic'd what a friend was running and went from there.

Since you are so good with numbers Ken, I wonder if while you are researching this stuff, you could look into suspension frequency and lay that out in laymens terms for us.

Most the coil over articles, racing tech, and vendor sites talk about, but its just one of those subjects I've always had a hard time getting my head around.

Good to see your build coming together.

bnine, are you looking for a basic overview on how spring/mass/damper 2nd order systems behave? Then maybe how it relates to the coilover application?

I haven't really looked into the suspension frequency aspect of my design since my rig is going to primarily be a low speed, trailer queen and not a higher speed daily driver or race type rig.

I can go over the basic 2nd order concepts if you want and can then research the high speed design and discuss that too if you want.

Let me know what you'rs looking for.

ken

bnine

· #9 · Jan 6, 2011

Basics I have covered. I understand the combined rates, and transition to final rate and such.

Its the wheel cycling on the high speed stuff that the racers express in Hertz frequency that throws me for a bit of a loop.

Im not sure, but I think because of our heavy unsprung weights, getting proper frequencies may be more difficult then other applications, but like I said, my understanding of the concept is murky at best.

Ken White

Discussion Starter · #10 · Jan 8, 2011

The natural frequency response of a spring/mass system is vital for a few reasons.

In terms of ride comfort, the human body prefers a system motion that has a natural frequency from about 1 to 1.5 Hz, which correlates to 1 - 1.5 up/down cycles per second.

As the natural frequency increases and approaches 2 Hz and greater, the human body starts to experience discomfort and the ride is perceived as harsh.

As the natural frequency decreases below 1 Hz to a range between 0.5 - 0.8 Hz, the human body experiences symptoms of motion sickness.

If the coil spring and vehicle mass are considered a lumped (single) set of parameters, then a simple mathematical model can describe their natural frequency behavior.

The ideal mass (kg) and spring (N/m) devices both store and utilize kinetic and potential energy in forms opposite of each other.

This means there exists a single frequency (rad/s) where both devices exchange all of their energy, equal magnitudes, and this is called the natural or resonant frequency.

If there are no losses at this particular frequency, these devices would oscillate in a sinusoidal motion forever, without any added energy, as long as the spring stays linear.

When the system is modeled mathematically, the natural frequency (symbol wn) is determined to be a function of the mass (symbol m) and spring rate (k).

wn = (k / m)^0.5

Writing this equation in its base units, and converting between metric and English units, allows this equation to be rewritten in terms of the natural frequency in Hz and spring compression (symbol x) in inches.

fn (Hz) = 3.127 / (x)^0.5

If any energy is added to the system (bumps) at the natural frequency, it will grow very quickly and become non-linear in real systems.

If energy is added to the system that is not at the natural frequency, it has less and less effect as it becomes farther away.

So if the natural frequency is designed above 1.5 Hz, it will get excited much more easily by road bumps, which means more energy transferred, thereby making control of the spring with damping (shocks) much more difficult and a lot more of the bumps vertical acceleration will be felt through the seat.

If you remove the shock from the vehicle, and bounce a corner up and down at a rate that cause the most motion, this will be the systems natural frequency.

Sorry for the long winded explanation, but this is as short of an explanation that I could come up with...

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Ken White

Discussion Starter · #11 · Jan 8, 2011

Oh, I forgot to address damping basics.

Damping, or adding a shock, reduces the natural frequency to a lower frequency which is called the damped frequency.

Shocks are valved for behavior in the rebound and compression stroke.

As a general rule, rock crawling is valved light in compression and heavy in rebound.

As a general rule, racing or high speed operation is valved heavy in compression and light in rebound...

doodoob

· #12 · Jan 8, 2011

Sounds like you covered it all. I went the easy route and just installed the GenRight Legend Extreme kit.

jimhat

· #13 · Jan 9, 2011

Compare buying and tuning ORI struts vs coilovers and coils (and rebuying coils as it's very hard to get it right the first time) air bumps/cans and swaybars

ORI's are all 3 of those in one thing.

JerryC

· #14 · Jan 9, 2011

Ken White said:
...Writing this equation in its base units, and converting between metric and English units, allows this equation to be rewritten in terms of the natural frequency in Hz and spring compression (symbol x) in inches.

fn (Hz) = 3.127 / (x)^0.5...

Gawd, I love it when Ken talks dirty.

jimhat said:

Compare buying and tuning ORI struts vs coilovers and coils (and rebuying coils as it's very hard to get it right the first time) air bumps/cans and swaybars

ORI's are all 3 of those in one thing

Click to expand...

Click to expand...

Care to elucidate some more?

Ken White

Discussion Starter · #15 · Jan 9, 2011

JerryC said:
Care to elucidate some more?

Hi JerryC...

Are you asking jimhat to give more details about the ORI struts?

Or me, about the things I am trying to figure out about my build?

Ken

jimhat

· #16 · Jan 9, 2011

Coilovers = need to be valved for each rig and for what you wheel. Then there is buying the coils, hardly anyone gets it right the first time around for weights. Also you would need a tripple rate kit to get best ride. Then there is bump stops, some use a stock type bump but most go with air bumps and air cans. Dont forget a suck down winch for those steep climb when they unload. After all that you need a sway bar that will support on road and the flex offroad of coilovers without breaking.

After buying all that and the time to install, troubleshoot, re-valve ect your into that for a nice bit of money

ORI struts = first off I will say they are not for everyone. But most rec wheelers they are perfect. Have a bump stop built into the struts. They are side sensing and 90% that run them have not needed a swaybar. Also they are charged top and bottom so they will not unload on climbs like an air shock or coilover. Much more simple setup. TJ owners (depending on rig weight) usually put around 100 psi in the bottom then fill the top to ride height. Then just adjust pressure from there for the type of driving and wheeling they do.

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Ken White

Discussion Starter · #17 · Jan 9, 2011

jimhat said:
...Also you would need a tripple rate kit to get best ride...

ORI struts = first off I will say they are not for everyone. But most rec wheelers they are perfect. Have a bump stop built into the struts. They are side sensing and 90% that run them have not needed a swaybar. Also they are charged top and bottom so they will not unload on climbs like an air shock or coilover. Much more simple setup. TJ owners (depending on rig weight) usually put around 100 psi in the bottom then fill the top to ride height. Then just adjust pressure from there for the type of driving and wheeling they do.

jimhat, can you please explain (give me an example of) how the triple rate set-up does this? I am having a very hard time figuring out how that very low rate 3rd spring adds any real benefit.

I understand the concept of the triple-rate set-up, I just can't seem to have the math show me that its there...

It sounds like the ORI set-up is a sophisticated (dual-rate) version of an air-shock that is very easy for the end user to adjust.

Thanks,

Ken

JerryC

· #18 · Jan 9, 2011

Ken White said:
Hi JerryC...

Are you asking jimhat to give more details about the ORI struts?

Or me, about the things I am trying to figure out about my build?

Ken

Hello Ken:

I was asking jimhat for more details on the ORI struts. I've seen some discussions online about them, but I have yet to see any detailed install pics or discussions on a TJ application.

And I am following your figuring...

Jerry

Ken White

Discussion Starter · #19 · Jan 9, 2011

JerryC said:
Hello Ken:

I was asking jimhat for more details on the ORI struts. I've seen some discussions online about them, but I have yet to see any detailed install pics or discussions on a TJ application.

And I am following your figuring...

Jerry

Hopefully jimhat is running them and can give us some of his experiences.

8)

jimhat

· #20 · Jan 10, 2011

Really you deleted my post?

Wjw

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