Another 4 Link Thread

[Duuuuuuuude]

Warning: I will keep this as short as possible, but its still going to be long. :shock:




The web is full of charts, diagrams, calculators, discussions, and terminology concerning suspension setups. There is much to be learned, but a good deal of it either has nothing to do with rc crawlers, or is extremely vague and confusing. While the calculations and physics are the same, the goals and end results are typically very different between the vehicles. Drag cars, monster trucks, street rods, even some 1:1 offroad rigs all have varying reasons for their setups that are not always things we should concern ourselves with, but sometimes the methods they use are.

I spent much of the year away from crawling and focusing on short course racing. At some point I dove headfirst into suspension tech and learned quite a bit. The biggest thing that I learned was how to correlate that tech between different platforms and designs. It removed much of the confusion I had before when studying pictures and diagrams.

I will tell you now that this may be a long read. I want to keep everything as simple and short as possible, but going into depth is unavoidable at times. I do not know everything, nor claim to, but I feel that sharing a basic grasp of what is going on will help everyone. Everything I am sharing deals directly with rc crawlers, and will reference the 1:1 world rarely and only as examples.

I am going to do a basic rundown of what will be covered, then expand on each point with a separate post. Again, this will only be in relation to rc crawlers. I don't care how real trucks are set up because these aren't real trucks.

All discussion is welcome, as well as corrections if I get something wrong. I do ask that all discussions be based upon the expanded posts as I post them and not ahead of time.



Terminology!



Squat (S) and Anti-squat (AS)

The biggest topic of discussion here at RCC is antisquat. What is antisquat? To understand antisquat (AS) you have to know what squat (S) is. S is the motion of the rear of the vehicle under acceleration. As the vehicle accelerates forward, its weight is pulled back and causes the rear to drop, or "squat". AS is the resistance to such motion.


Torque Twist (TT)

Something that quite possibly ties for most discussed here is TT. TT is the force of motion induced between two points that are not rigidly tied together. The first and originating point being the one producing the force, and the second being the one receiving it. Very commonly found with shaft driven rigs.

Line of Force (LF)

I bet this is one that most haven't considered, or probably heard of before. The LF is the angle and direction that the suspension applies force through the chassis. LF goes hand in hand with S/AS values.


Center of Gravity (CG)

CG is the exact balance point, both horizontaly and vertically, of the vehicle. This is the point that gravity pulls from. A high CG will be top heavy and easy to tip, a low CG will be the opposite.


Ride Height (RH)

Simply put, the distance between the ground and the bottom of the chassis.


Roll Center (RC)

RC is the point that the chassis pivots or rolls on. A low RC has more chassis roll, a high RC has less. The axles also have their own individual RC's.


Instant Center (IC)

IC is the point in space where the upper and lower suspension links meet. It is used to determine AS values, the LF value, and the RC value.

 

[Duuuuuuuude]

Squat & Antisquat

We all want to climb steeper and roll over less, and it seems the answer is in antisquat, right?

Many people reference drag cars when discussing AS, because a well tuned drag chassis will launch hard and not flip over. That is in fact a product of high levels of AS. We want traction, and we want to not flip over, so that seems like a good setup to pull from, right? Wrong. Drag cars have different goals, operate in different environments, and are tuned to do specific things that are different than our crawlers.

A drag car suspension does not focus on turning, driving off camber, or driving on uneven surfaces. Its suspension also puts things that are important to us in bad places. We will not at all focus or refer to drag car suspensions again.

How do you determine AS? First you have to know a few other things, namely Center of Gravity and Instant Center.


Center of Gravity (CG)

This is the exact point of horizontal and vertical balance of the vehicle. If you know where this point it, you could balance the whole thing on the point of a needle. CG is very important to know when setting up your suspension, because you have to know where you are before go somewhere else.

How to determine CG? There are a few different ways, and it all depends on how exact you want to be. The method I use is to hang the rig from each end and draw a line from where it is hung to the floor. It isn't 100% accurate, but it gets darn close. When doing this method it is best to find a way (zip ties, tape) to secure the suspension at its ride height position and not let it hang at full droop. Depending on how much travel you have, a fully extended suspension can change the point that the CG falls.

You can draw this line quickly by placing tape on the side of the rig and drawing the line by hand, or the more involved method of taking pictures and imposing one over the other with a drawing program (photoshop, gimp, etc) on the computer to create a composite picture.

Basically you hang the rig from one end, making sure the string is visible and you are looking at it dead on its side, taking a picture, then doing it again from the other end.

Here is my old Fastback II. The yellow lines cross at the CG.

Now we need to know where Instant Center is.


Instant Center

IC is the point in space where the upper and lower suspension links meet on either end of the vehicle. It is used to determine AS values, the LF value, and the RC value.

IC is relatively easy to find, and keep in mind that all calculations from now on will be done with the chassis and axle shaft centers at ride height. This is very important! If they are not at ride height you will not have a solid base to reference all of your other point to, and they will all be worthless.

First thing: measure chassis ride height and axle shaft centers. Remove the front and rear wheels from one side, and find something secure to rest the axle on to maintain your measurements. Remove the body if it obscures the link attachment points on the chassis.

Next, take a good clear picture with the rig centered in the frame. It is not a bad idea to leave some empty space ahead and behind. You don't want a close up glamour shot. Take that picture and either print it out or open it in some sort of program or app that will let you draw on it.

On that picture, draw a line through the mounting points of the lower and upper links (starting at the axle end) until they meet. Where they meet is the IC of that axle.


Here's my Wraith. The red/yellow circle is the CG.

To find the S/AS value, you need to draw a few more lines. One on each axle centerline, one from the CG to the front axle centerline, and one diagonally from the bottom of the rear axle line to where the front axle line and CG line intersect. Go ahead and carry that line out to the edge of the pic. You should have something like this (in blue)...

That diagonal blue line is called the Neutral Line (NL). It is exactly what it describes, the neutral point between S and AS. Where your IC falls in relation to that line determines how much AS you have, and where things get a bit confusing, because that line also represents 100% AS. Weird, I know, but that is how it is done. If your IC falls above the NL, AS is over 100%, and if it falls below the NL, AS is less than 100%.

Your IC is also the point where the suspension links apply force to the chassis. But there is a little bit more to it, in the form of Line of Force (LF). I will get to that shortly.

Looking at the pic of the Wraith, we can see that we have more than 100%, and that the IC is right under the CG. I'm going to add some more lines to see just exactly how much AS I have. It turns out I have 125%.

Line of Force (LF)

LF is the angle and direction that the IC applies force, and it is that force that causes the chassis to lift. It is something that should always be considered when chassis tuning, and why you should not rely on AS alone. It is entirely possible to have desirable AS but a horrible LF.

To find the LF, draw a line through the bottom of the rear axle line and through the IS. Note that the force comes from underneath and up through the hood.

LF is commonly referred to in 1:1 cars as Percent of Rise (PR). That percentage is determined by where the LF crosses under the CG line. A higher PR means more lift, a lower PR means less. A higher PR also means that more force will be initially transmitted to the rear tires, a lower PR means less force. That is how weight transfer is manipulated under acceleration.

I mentioned earlier that you can have decent AS but poor LF, and here is an example. We'll pretend we have more link mounting points available on the Wraith chassis, which are not too unlike some others we've seen in the past. We're also going to pretend we've added some separation at the axle end. The end result is a fairly acceptable AS but a less than desirable LF.

IC and CG

Where your IC falls in relation to the CG will determine how the chassis and suspension will react, and is partly demonstrated by the LF. An IC behind the CG will lift the rear of the vehicle, an IC ahead of the CG will lift the front. But remember, we are lifting at angles, not straight up and down. Having your IC ahead of the CG will mean that the force will be applied forward as much or more as it will be applied up. Putting your IC in such a position means that you likely will be running close to or less than 100% AS, or below the Neutral Line. This is not a bad thing.

Here is an example of a less than 100% AS setup. Notice how the LF has changed.

Note: all of these points and measurements will change when ride height or vehicle weight changes. If you raise or lower your chassis, or add weight in some form, your suspension characteristics will be different. That also means that as the vehicle is traveling everything is changing. But this isn't about keeping things in check, its about a baseline to build and drive from.

 

[Duuuuuuuude]

Torque Twist!


Oh boy...where to begin with this one...how about I start with what Torque Twist (TT) isn't...

1. TT is NOT caused by the motor spinning.
2. TT is NOT caused by pinion gears climbing ring gears.
3. TT is NOT caused by bad suspension geometry.
4. TT is NOT cause by particular axle gear placement.
5. TT is NOT caused by motor orientation in the chassis.


TT is simply resistance to a rotational force. You are applying torque to something that is trying not to rotate. If the applied torque is greater than the resistance, rotational movement is achieved.

Lets take this guy for example.

Here we have a happy carpenter drilling a hole in a block of wood. The drill is providing rotational force through the drill bit and into the wood, and since there is a great amount of resistance to that force within the block of wood, he is holding it steady with his hand, providing further resistance and keeping the block from spinning around and knocking everything off of the work bench and quite possibly clocking himself in the jewels.

How does this relate to crawlers? Specifically shaft driven crawlers? Easy. Imagine the carpenter is the chassis, his arm is the suspension, the drill is the motor/transmission, the drill bit is the driveshaft, the block of wood is the axle, the workbench is the ground.

The carpenter is firmly holding the drill so that it does not rotate when powered on, much like the chassis holds the motor and transmission.

The drill bit transfers the rotational force, much like a driveshaft does.

The block of wood resists that rotational force, much like an axle does.

The workbench aids in the resistance of that rotational force, much like the ground does.

The carpenters arm aids in the resistance of that force, much like a crawler suspension does.


So, if he were drilling into something softer there would be a reduction in resistance. The same is said of gear ratios. A higher split in ratio equates to less resistance to rotation. Why? Because a higher split means that it is easier to transmit force from one gear to another.

The optional Axial UD gear set provides a 2.57:1 reduction ratio. The Losi worm gear set provides a 25:1 reduction ratio. It is easy to see why the LCC does not suffer from TT as Axial based rigs do. The reason they can have such a low split is because of the worm gear design. It would be completely impractical to have a traditional ring and pinion gear set with that amount of reduction because the ring would have to be incredibly tall or the pinion unusably small.

All of that reduction comes at a price though, and that is speed. An average Axial comp crawler can run a traditional 35t motor and have decent torque and wheelspeed. The Losi, because of its huge amount of reduction, needs a much faster motor to achieve usable wheelspeed.

Are you sure the Losi doesn't have TT because the pinion is on top of the ring gear and not on the side? Yes I am. Why? Because the top is still a side, the pinion is still applying force, and the ring gear is still trying to resist it. Don't believe me? Find a way to lock the front wheels in place. Mash the gas and see what happens to the axle. If you don't burn your motor up, you will see evidence of TT, its just much harder to achieve because of the gear reduction.


Back to the effects of TT in a crawler. :mrgreen:

So we've got a transmission bolted into a chassis that is loosely connected to an axle. That transmission is trying to transmit rotational force to that axle through the driveshaft. The axle is trying to resist that force via gearing, the effort it takes to turn the wheels, and the friction the wheels have against the ground.

Somewhere something has to give. If it is the axle, it will lift a wheel. If it is the chassis, it will twist to one side. Sometimes it is both. You can combat wheel lift by adding weight to the axle or wheel. You can combat chassis twist by adding preload to one shock or build it into the suspension geometry. Sometimes fixing one will also fix the other.

But remember, adding weight changes your CG, which changes your IC, which changes your AS, which changes your LF.

Why does it lift one wheel? And why is TT not related to the pinion climbing the ring gear? It lifts one wheel because the axle is trying to rotate around the thing that is applying force, which is the pinion gear. Whether the pinion is "climbing" or trying to move up or trying to move down doesn't matter. It is simply because the pinion applies force and the ring resists it.

Preloading shocks is another way, but if taken too far can make for an unbalanced rig that can be sketchy or unpredictable to drive. Having rock hard springs and super thick oil will certainly keep the chassis in check, as well as the axle, but the effects it will have when navigating the rocks will be quite bad. If the suspension isn't allowed to work it will drive poorly.

So running more preload, or heavier springs or oil on one corner is a bad thing? No. Remember that tuning is little tweaks here and there. If you make one small change and it improves things, great. Leave it. If it makes things worse, change it back. If you have to keep making little changes in the same spot, resulting in a huge change in that one spot (and assuming you weren't waaaaaay out of whack to begin with), something else is off base and should be addressed instead.


So, how should you go about fighting TT? Gearing is a very effective way. We learned that a higher split means less resistance, and less resistance means less twist. You can also run split ratios front and back. While putting a lower split gear set in the front axle would seem to make things worse (which it can), it is the combination of front and rear ratios that make it work.

A split gear set, with the front axle spinning faster than the rear axle, makes the chassis stretch out, and in essence preloads the suspension. Since the front is always having to pull, it will naturally want to pull itself into the ground. As long as it has the traction to do so, TT will be minimized. If it loses traction, the TT will come back.

Another way to fight TT is through suspension geometry. Running your rear upper links in different positions side-to-side on the chassis will also add a bit of suspension preload because each side is operating at a different level of IC, AS, and LF. Strategically positioning your IC will also fight TT because of the way it applies accelerational force back through the chassis. If the chassis is wanting to twist, and the axle is wanting to lift, and the IC is in a place as such that it can push back against those movements, TT will be reduced. But remember, more is not better. Running one side aimed at the sky and another aimed at the ground will be horrible.


Why don't MOA's have TT?

Easy. Because the thing providing the rotational force is solidly attached to the thing receiving it. If you wanted to construct a shaft driven crawler that did not suffer from any TT at all, you would have to build one where the differentials and transmission were attached to the same structure, much like a touring car or truggy.

 

[Duuuuuuuude]

Roll Center (RC)


When discussing axles, RC is the point in space that is the center of the chassis' movement around the axle.

Back to the Wraith. The yellow line represents the upper links, where that line terminates behind the rig is where the two upper links would meet in space if they were that long. The red line represents the Roll Axis (RA), which is parallel to the lower links. Where the yellow line crosses the centerline of the axle is the RC.

Here's a pic of where the two upper links would meet.

If the picture was taken with the axle in the center of the frame, the RC would lay right on the vertical axle centerline like in the pic above. Where they meet determines the amount of axle steer is induced when the axle articulates.

Note: this example is for 4 linked suspensions only. 3 link/panhard or even 4 link/panhard suspensions will have roll centers in different places. I will get to those later.

RC is important to consider when cornering or attempting a sidehill section of a crawling course. The height and angle of this will determine whether you will roll over or keep moving forward.

When you are taking a corner at speed, or driving across an incline, the CG of the vehicle will move to the outside or lower point of the vehicle. The RC determines the point that the CG will swing from.

This is a pretty simple diagram for RC. RC is the black circle, CG is the green dot.

If you have a low RC, and a sideways force is applied to the vehicle (gravity when driving off-camber) the CG will pass over it and you will tip. If you have a high RC, the CG will try to slip under it.

Here's another example.

The green dot represents the CG, the red dot represents RC. When the CG moves, it does so in an arc around the RC.

Example 1 shows a low RC, and a Jeep on an incline that has brought the CG to a point where it is moving to the side and its only way to go is down, which causes the Jeep to tip.

Example 2 shows a high RC, and another Jeep an another incline. This time the CG has moved to a point where it can only go up. As long as the tires can maintain traction, the Jeep will not tip.

Example 1 is fairly realistic, Example 2 is pretty extreme, though not impossible to achieve. It would require either very steep upper link angles, or very poor triangulation. Either of those would wreak havoc on all other points of drivability.

If you want another, example, try an experiment. Take a pencil and balance it on its side on your finger. Once you find its CG, mark it. Try holding it, this time straight up and down. Hold it above and below the mark. When you hold it above the mark, it wants to stay upright. When you hold it below the mark, it want to tip. Where you hold it represents the RC.


Roll Axis (RA)

RA is the combination of the RC of both axles. It is the plane that the chassis rotates around. Basically it is a line drawn from the RC of the front axle to the RC of the rear. It may be flat or angled, depending on where these point are at. Assuming you have identical axles and identical mounting points for your links, your RA will be flat across the chassis.

 

[Duuuuuuuude]

 

[Duuuuuuuude]

Added to post #2

 

[Duuuuuuuude]

More added. Thoughts? Comments? Questions?

 

[B94Cast]

Great information - I think I need to re-read it a few time to absorb it better - but if you modified your top link point to where you IC theoretically met in front of the vehicle - then your LF would be to the point that AS would be fairly impressive. I would imagine that would cause crazy amounts of axle rotation and subsequently limit travel - right? So the trade off on that would be excellent AS, but at cost of suspension travel?

 

[Duuuuuuuude]

Torque Twist added to post #3

Great information - I think I need to re-read it a few time to absorb it better - but if you modified your top link point to where you IC theoretically met in front of the vehicle - then your LF would be to the point that AS would be fairly impressive. I would imagine that would cause crazy amounts of axle rotation and subsequently limit travel - right? So the trade off on that would be excellent AS, but at cost of suspension travel?

Re-reading is almost mandatory. There is a lot to absorb. Don't feel bad if you have to keep coming back, that is the whole point of this thread.

Remember, you have to focus on more than AS. It is not the be-all, end-all of adjustments. Chassis tuning is a give-and-take process. You have to give something up in one place to gain something else in another.

 

[B94Cast]

Makes sense on the give and take - will keep digging and reading and look forward to the next posts.

 

[Duuuuuuuude]

Added Roll Center and Axis to post #4.


Its late, I'm sleepy, I hope I got it right. If not, let me know.

 

[Colgout]

appreciate your effort. i think i am gonna print it out and keep it handy...

thank you.

 

[imthatguy]

Someone sticky this thread. A wealth of relevant information.

Thank you sir.

 

[ROLANDROCKSHOP]

Nice job Duuuuuuuude!

"thumbsup"


yes,STICKY this please!!

 

[Full_Throttle]

Wow... just read through the whole thing, and, like suggested, I'll be reading it again. Thanks for spending the time posting this. I already have a better understanding of the intricacies around suspension geometry. This is very helpful stuff for us non-technically savvy guys. "thumbsup"

 

[Szczerba]

Ooooh, that Duuuuuuuude with his so eloquently put terminology in reguards to suspension geometry.

Tons of great information here and definitely a great read. "thumbsup"

Thanks for posting.

 

[Duuuuuuuude]

Thanks guys, I'm glad its making sense. Anything else that needs to be covered or expanded upon? Questions? Corrections?

 

[weck]

for determining CG, you say to hang it from the end. what exactly are you hanging it from? I can't tell from the pic and I have never done it this way and was going to try it. Thanks for the awesome write up. On my second reading through right now to see if I can understand more..."thumbsup"

 

[Duuuuuuuude]

for determining CG, you say to hang it from the end. what exactly are you hanging it from? I can't tell from the pic and I have never done it this way and was going to try it. Thanks for the awesome write up. On my second reading through right now to see if I can understand more..."thumbsup"

I hang mine from the chassis ends. There is no specific point, just pick a spot and tie on to it.

 

[Szczerba]

for determining CG, you say to hang it from the end. what exactly are you hanging it from? I can't tell from the pic and I have never done it this way and was going to try it. Thanks for the awesome write up. On my second reading through right now to see if I can understand more..."thumbsup"

Kinda sorta shown here.

http://www.rccrawler.com/forum/axial-ax-10-scorpion/353094-helheddeds-comp-shafty-7.html#post3488827

There is some suspension chatter on pages 5 and 6 of the above linked thread too but Duuuuuuuude has it covered in this thread.