How's my numbers?

[zerobalance]

The 4 link calc. V4.0 and newer combined 4 link and panhard bar suspension tools.

The best advice I can give on 3 links is don't try to do the cancel out drive torque trick unless you know what you are doing.

I found it. Had to switch to my office PC, don't have excel on my shop computer and it does some funky stuff in sheets. Thanks again.

 

[BentK5]

K5 Blazer. Finally getting links. I'm so tired of getting beat up by the leaf springs. Gonna be primarily a crawler but I'd like to haul ass in between the trails and around the desert and fire roads. Stability would be a huge plus.
I'm not 100% sure on the roll centers and how the oversteer and understeer will affect it. It's a heavy turd and I gave the best guesses I could.

Thanks I appreciate it!

 

[06h3]

The overall cg at ride is an input, it calculates the cgs that it needs from that.

In your case, the pros would be a flatter roll axis.
The general con would be the axle steering through travel. The more triangulated the lowers the more steering.

Kinda to the first part, yes to roll axis, not really to less swing. It is due to the side to side movement caused by the panhard. At ride it looks like this: /\ . But as it moves side to side it looks more like this: /|. Since the one link is getting longer in side view and the other shorter, the axle turns.

I am bringing an old post back up from the dead Treefrog This isnt for me but for a friend building a 4500 car. He has his lowers triangulated in a front 3 link and is concerned about bumpsteer at speed. Does triangulating the lowers cause more bumpsteer even if the trackbar and drag link are equal and flat?

When does inboarding become too much?

 

[Malburg114]

I’ll chime as I’m the one with the front 3 link for a 4500 car.

Numbers are still adjustable to an extent

6.5 inches vertical separation at frame
8.5 at axle

Center of heim to center of heim lower links at axle is roughly 39.5

At the frame is 26.5

I don’t have the panhard or draglink done yet. I have room to move the frame side lower links out maybe an inch or two each way if I redo the subframe some. I can lower the frame side upper link as well some too.

How much will having the frame side lowers triangulated affect bump steer while articulated and while at speed?

Thanks

 

[Treefrog]

K5 Blazer. Finally getting links. I'm so tired of getting beat up by the leaf springs. Gonna be primarily a crawler but I'd like to haul ass in between the trails and around the desert and fire roads. Stability would be a huge plus.
I'm not 100% sure on the roll centers and how the oversteer and understeer will affect it. It's a heavy turd and I gave the best guesses I could.

Thanks I appreciate it!

It looks reasonable for the use case described. The biggest worry I would have is how narrow the rear links are in general. There is evidence suggesting that wider links tend to be more predictable. The next thing that catches my attention is that your up/down travel split will be limiting how fast you will be willing to go on rough roads.

 

[06h3]

what's your thoughts on this? First chart is current state 2nd is projected future state if its ok and not all bad.

Reasoning is the lower links on the frame side bring the suck. They hang too low and are nothing but problematic in terms of clearance. Moving them 2in up, inboard a couple inches and 6in longer gives me better clearance, better roll slope (I think) less anti squat change through travel which seem to be the pros.

The cons I see, and tell me if there are more, is the more triangulated lowers will cause flex steer which sucks on a non steering axle. This would only really occur when one side is near full bump and the other is at full droop right?

Anyways, what are your thoughts.

Current state

Future state?

 

[Treefrog]

what's your thoughts on this? First chart is current state 2nd is projected future state if its ok and not all bad.

Reasoning is the lower links on the frame side bring the suck. They hang too low and are nothing but problematic in terms of clearance. Moving them 2in up, inboard a couple inches and 6in longer gives me better clearance, better roll slope (I think) less anti squat change through travel which seem to be the pros.

The cons I see, and tell me if there are more, is the more triangulated lowers will cause flex steer which sucks on a non steering axle. This would only really occur when one side is near full bump and the other is at full droop right?

Anyways, what are your thoughts.

The 2nd one does not seem to be a downgrade. It will be interesting to hear any handling changes.

3-links make flex steer kinda screwy. But for a 4 link with flat lowers, peak flex steer is usually one side at ride and the other full extended or compressed. But I am guessing that your flex steer assumption will be accurate for your suspension given the x location at the top and bottom of the wheel center curve.

 

[06h3]

The 2nd one does not seem to be a downgrade. It will be interesting to hear any handling changes.

3-links make flex steer kinda screwy. But for a 4 link with flat lowers, peak flex steer is usually one side at ride and the other full extended or compressed. But I am guessing that your flex steer assumption will be accurate for your suspension given the x location at the top and bottom of the wheel center curve.

Sounds good, I’ll give it a go. I wish there was an easy way to calculate flex steer.

An example: currently my max flex steer angle is X and the 2nd iteration max flex steer is “2 degrees more” or whatever it may be

 

[BentK5]

It looks reasonable for the use case described. The biggest worry I would have is how narrow the rear links are in general. There is evidence suggesting that wider links tend to be more predictable. The next thing that catches my attention is that your up/down travel split will be limiting how fast you will be willing to go on rough roads.

Thank you, I appreciate the thoughts. I was wondering about the narrow links. It just fits in there so well. Looks like I'll be carving up more frame. Hard to get more bump and keep the thing low as possible too.
Thanks again!

 

[Treefrog]

Thank you, I appreciate the thoughts. I was wondering about the narrow links. It just fits in there so well. Looks like I'll be carving up more frame. Hard to get more bump and keep the thing low as possible too.
Thanks again!

You should be able to get away with keeping the uppers between the frame rails.

 

[Team_Jake]

I posted my numbers a while back in this thread (on page 8, also posted below). My truck is now built. I want to add sway bars front and rear (the body roll is pretty bad driving without them on the street), but after reading through the sway bar tech thread the consensus I got was that the correct sway bar rate depends a lot on other factors (like roll center, spring rate, weight/bias, up/down travel, etc), so I feel this thread would be the more appropriate place to ask for advise.

Considering my setup, what would be a good sway bar rate to shoot for (both front and rear) and what is the math to determine that?

(I should note that I do have my suspension modeled in Solidworks so designing/building the correct length arms/links shouldn’t be an issue)

Thanks!

 

[Treefrog]

I posted my numbers a while back in this thread (on page 8, also posted below). My truck is now built. I want to add sway bars front and rear (the body roll is pretty bad driving without them on the street), but after reading through the sway bar tech thread the consensus I got was that the correct sway bar rate depends a lot on other factors (like roll center, spring rate, weight/bias, up/down travel, etc), so I feel this thread would be the more appropriate place to ask for advise.

Considering my setup, what would be a good sway bar rate to shoot for (both front and rear) and what is the math to determine that?

(I should note that I do have my suspension modeled in Solidworks so designing/building the correct length arms/links shouldn’t be an issue)

Thanks!

Any other input on how it drives other than body roll?

Can you do the shock sheet in the calculator as well? They are a large factor in body roll.

I'm not good with sway bar rates. Hopefully someone else speaks up.

 

[Team_Jake]

Any other input on how it drives other than body roll?

Can you do the shock sheet in the calculator as well? They are a large factor in body roll.

I'm not good with sway bar rates. Hopefully someone else speaks up.

Here's my shock sheet. Note that the rear is messed up because my suspension is cantilevered, I tried fudging the numbers to act like a trailing arm with the same MR but I couldn't get the dual rate slider stop % correct (there's only a single spring so I don't have a dual rate slide stop)

But after filling out this sheet I realized I forget to set the height of my dual rate slider stop on the front and was set way too high (which I'm sure didn't help my body roll problem).

I've only driven it a couple time since building it, and only on the street. As for the body roll, it never feels unstable, it just leans quite a bit in a turn, and takes a few seconds for it to level out after coming out of a turn. My AS / AD percentages are low compared to a normal build, and I can feel it accelerating and braking (setting the dual rate slider stop where it needs to be should help here as well). It'll take a little getting used to on the street, but I'm sure I'll like it once I get the chance to try it offroad.

 

[06h3]

Here's my shock sheet. Note that the rear is messed up because my suspension is cantilevered, I tried fudging the numbers to act like a trailing arm with the same MR but I couldn't get the dual rate slider stop % correct (there's only a single spring so I don't have a dual rate slide stop)

But after filling out this sheet I realized I forget to set the height of my dual rate slider stop on the front and was set way too high (which I'm sure didn't help my body roll problem).

I've only driven it a couple time since building it, and only on the street. As for the body roll, it never feels unstable, it just leans quite a bit in a turn, and takes a few seconds for it to level out after coming out of a turn. My AS / AD percentages are low compared to a normal build, and I can feel it accelerating and braking (setting the dual rate slider stop where it needs to be should help here as well). It'll take a little getting used to on the street, but I'm sure I'll like it once I get the chance to try it offroad.

Mine did exactly like yours, probably worse because I have crappier numbers compared to yours and a higher COG. It doesn’t feel like it will flop and kill me but it leans and takes a second to recenter.

I have tried multiple setups. First I tried my front bar only of a rate around 40-50. It helped center back quickly but still had too much body roll.

Then I tried rear only. Front disconnected (for science!) around 100 wasn’t enough, 125 was better. I around 158 and it’s good for most stuff.

Then 158 rear and put the front back on (40-50 up front) and it was SOLID. Felt like a race car in the corners.

I haven’t run the front and rear off-road just on road. Off-road I have always preferred rear bar only so I need to play with it more for on and offroad use.

I run a 30in 1in 48 spline Schroeder bar that is machined down to .95 and about a 13 arm (I have 4 holes so I can remember the exact length)

The front is a 36in Currie bar .770 bar also about 12-14in arm.

Before I was linked up front with coilovers and leafs out back. Leaf springs were simple! Self center, no sway bar necessary but my leafs had their own issues of deflection, axle wrap, high speed desert up travel. So I fixed many issues in a trade for some new ones but it should be easy to work through.

 

[Team_Jake]

06h3 that information is super helpful, thank you.

Treefrog I did a bunch of research and it got a little math heavy so I started writing my own python code to calculate sway bar rates, max twist angles, forces, etc. You may find this code useful if you want to incorporate a sway bar sheet in your calculator.

Here’s the code:

#Sway Bar Calculator

#Inputs
sigma = 100 # Max Shear Stress (ksi)
G = 11600 # Shear Modulus (ksi)
L = 40 # Length Tip to Tip (inches)
OD = 1 # Outer diameter (inches)
ID = 0 # Inner diameter (inches), enter 0 if solid bar
A = 16 # Arm Length (inches), center to center
O = 2 # Arm Offset (inches), enter negative number for arms wider at sway bar side
Y = 44 # Distance between shock mounts at axle (inches)
T = 12 # Shock travel (inches)

#Setup
import math
pi = math.pi
L_w = L - 4 # Working Length

#Formulas
theta = 2 * L_w * sigma / (G * OD) # Max Twist Angle (radians)
theta_d = theta * 180 / pi # Convert from radians to degrees
J = pi/32 * (OD ** 4 - ID ** 4 ) # Polar Moment of Inertia (in^4)
tau = 1000 * J * G * theta / L_w # Torque at Max Twist Angle (in-lbs)
F = tau / A # Force at Max Twist Angle (lbs)
K = tau / (theta * A ** 2) # Sway bar rate (lbs/in)
flex = theta * A * Y / (L + 2 * O) # Max articulation (in)
lim = 100 * flex / T # Flex limited from sway bar (as percentage, >100%: sway bar does not limit flex)

#Outputs
print("Max Twist Angle =", round(theta_d,1), "°")
print("Force at Max Twist =", round(F,2), "lbs")
print("Sway Bar Rate =", round(K,1), "lbs/in")
print("Max Articulation (at shocks) =", round(flex,2), "in")
print("Usable Flex =", round(lim,1), "%")

For anyone else wanting to do their own calculations, all you need is a Python IDE (I’ve just been using www.online-python.com, it’s free and there’s no sign up or download), just copy/paste. Mess around with inputs and hit run.

 

[Treefrog]

06h3 that information is super helpful, thank you.

Treefrog I did a bunch of research and it got a little math heavy so I started writing my own python code to calculate sway bar rates, max twist angles, forces, etc. You may find this code useful if you want to incorporate a sway bar sheet in your calculator.

Here’s the code:


For anyone else wanting to do their own calculations, all you need is a Python IDE (I’ve just been using www.online-python.com, it’s free and there’s no sign up or download), just copy/paste. Mess around with inputs and hit run.

I took the calculator part over here: New Version of the 4 Link Calculator

I am going to assume for the time being that the numbers you are working with are what you are considering using. I would consider using a max shear stress lower than the actual maximum for fatigue life. We probably do not need to go as far as using the infinite fatigue life stress. Keep in mind that the rate can also be tuned to some extent through geometry such as the angle between the arms and the links down to the axle.

 

[Criscfer]

Best guess on weight and CG. Haven't even driven it yet, any characteristics you can for-see

 

[Treefrog]

Best guess on weight and CG. Haven't even driven it yet, any characteristics you can for-see

Without details on the intended use, my impression is that there is nothing too offensive. It may be of some benefit for high speeds to get the ICs farther away from the axles.

 

[Criscfer]

Not planning on the extreme rock crawling, but want a capable and comfortable trail rig to drive at a faster pace. Thank you for the review.

 

[busteddodge]

How difficult would it be to add a tab to the four link calculator spreadsheet that shows front and rear axle wheel position when steering lock to lock? Initially, I had a front axle triangulated for link mocked up that would case the tires to crash against the lower links. It might be a great addition to the spreadsheet. I see the complications involved though, regarding not simply overall tire diameter, but wheel backspacing and WMS differences, etc. Maybe it's more complicated than it's worth. Thoughts?

 

[busteddodge]

I posted an initial couple of screenshots here a while ago and got some great feedback. Now I'm at the point in my build where I need to actually mock up these links and I've made some changes based on real measurements taken off of the axles and chassis. Interested in any feedback you guys can offer.

This is an all tube chassis four seater that will be used for east coast slow, technical, off-camber crawling and some throttling up wet muddy hills at times. No high speed desert or trail stuff. Offroad only.

I appreciate all of the expert advice!


chrysler 440
np435 4 speed trans
np241D (2.72:1)
welded kingpin d60
welded 14BFF

22" belly height on 42s

 

[Treefrog]

I posted an initial couple of screenshots here a while ago and got some great feedback. Now I'm at the point in my build where I need to actually mock up these links and I've made some changes based on real measurements taken off of the axles and chassis. Interested in any feedback you guys can offer.

This is an all tube chassis four seater that will be used for east coast slow, technical, off-camber crawling and some throttling up wet muddy hills at times. No high speed desert or trail stuff. Offroad only.

I appreciate all of the expert advice!


chrysler 440
np435 4 speed trans
np241D (2.72:1)
welded kingpin d60
welded 14BFF

22" belly height on 42s

Looks reasonable. May be worth seeing if all of the rear can be moved outboard some.

 

[busteddodge]

Looks reasonable. May be worth seeing if all of the rear can be moved outboard some.

Phew! Man that's a relief. I think I know what I'm doing here just enough to be dangerous. It's a good feeling when someone who looks at these all of the time thinks it's alright.

I originally had the rear lowers way outboard, but moved them more towards the center thinking I'll add rear-steer to the equation at some point. Since there is enough triangulation ( >40 degrees ) what am I trading off here moving them towards the center?

Anything you'd tweak here if you were after a super stable moon buggy style rig?

Thanks!!!

 

[Treefrog]

Phew! Man that's a relief. I think I know what I'm doing here just enough to be dangerous. It's a good feeling when someone who looks at these all of the time thinks it's alright.

I originally had the rear lowers way outboard, but moved them more towards the center thinking I'll add rear-steer to the equation at some point. Since there is enough triangulation ( >40 degrees ) what am I trading off here moving them towards the center?

Anything you'd tweak here if you were after a super stable moon buggy style rig?

Thanks!!!

Ditch the wife and kids?

I believe they typically put the side view ICs 1-2 wheelbases outside the tires, less angle in the lowers, and a very low CG.

Conventional group think wisdom says that spreading the links out side to side helps with stability. From what I have seen, it seems that wider links have the same effect as longer links. They experience less change. If you move all of the link ends out the same amount it will not effect the triangulation.

 

[busteddodge]

Playing around and trying to determine antisquat calculations. In this screenshot, the instant centers for front and rear are both very near the same height as the COG, but the antis are very low. I was expecting antis near 100% in this situation. Front drive bias is set to 50%.

Please excuse the scaling of the chart axis, unless that is causing the lower anti numbers somehow. I increased the X axis to make the instant centers visible.

Why are the anti numbers so low if the instant centers are very near the same heigh of the COG?

 

[Treefrog]

Playing around and trying to determine antisquat calculations. In this screenshot, the instant centers for front and rear are both very near the same height as the COG, but the antis are very low. I was expecting antis near 100% in this situation. Front drive bias is set to 50%.

Please excuse the scaling of the chart axis, unless that is causing the lower anti numbers somehow. I increased the X axis to make the instant centers visible.

Why are the anti numbers so low if the instant centers are very near the same heigh of the COG?

The scaling does not mess with the math. It causes other issues (thanks for being horrible at plotting Excel), but those are a different matter.

The light blue lines show are the lines for 100% antisquat. Since you seem to be targeting just anti's you can use the check boxes at the top to hide the roll center stuff.

The calculation for antisquat uses the height of the green line at wheelbase * the percent of drive torque through that end.

I went into it in more detail here: Linked Suspensions Bible