Another 4 Link Thread

[Hardline]

More added. Thoughts? Comments? Questions?

I think much of your thread has errors. But because they're confusing topics and I'm not sure exactly what's right I can only pick on the few things I'm pretty sure are wrong. I'm not good at expressing tone, so please know that this is not an attack. It's just the way I discuss things. And you asked.;-)

You wrote: "As long as the tires can maintain traction, the Jeep will not tip."

Your Jeep tip-over thing seems like easy pickin's. The jeep will tip over whenever the green spot gets past a certain point near the outside edge of the downhill tire tread. The only reason it doesn't hang in there all the way to the edge of the tire, is the tire is rubber, and will collapse under too much pressure applied just to the edge.

But the relation to the roll-center is almost non-existent. First, a tangent, a car does not exactly roll about it's roll center. It rolls about the roll axis with is a line passing thru both roll centers. No matter -- that's a nit. How easily the jeep rolls about its RC/RA is more than just the geometry. It's also the spring rate. And limit straps. And bump stops. And while this may affect how the jeep behaves as you start pushing it over, the final roll over always occurs solely as a function of when the CG gets onto and starts to pass the downhill tires.

You're statement that the jeep cannot roll as long as it has traction is clearly wrong. Suppose I nailed the downhill tires to a huge piece of plywood (inifinite traction). Referring back to your statement I quoted above -- You really think the jeep won't roll over anyway once I tip the plywood steep enough?

I kinda want to pick on your TT post too, but I don't understand it well enough yet. Gotta think some more on it. Thanks for the push.

-

 

[EeePee]

I kinda want to pick on your TT post too, but I don't understand it well enough yet. Gotta think some more on it. Thanks for the push.

If you're that into RC then TT is way easy.

 

[Hardline]

If you're that into RC then TT is way easy.

Easy for you maybe. I'm not that smart. I'm still in the ring-gear climbing the pinion camp. But not a 100% believer yet. So I'll think on it.

 

[supermoturd]

I found these articles to be very helpful too:

Truck Suspensions, Four-Link Suspensions, and Suspension Installs - Part 2 - 4 Wheel & Off Road

Suspension Encyclopedia/4Wheel Underground

The first article does a good job of illustrating the geometry of the 4-link crawler suspension (especially that of the roll axis) and even does a pretty good job of presenting the performance gains/losses/tradeoffs associated with the various adjustments. The second article complements the first by providing more in-depth treatment of the various elements of 4-link geometry. Hope this helps and adds to the discussion and knowledge that this thread provides.

Edit: I forget to mention that you will have to click the "Read Full Article" link near the bottom of the page for the first link in order to view the entire article.

 

[Duuuuuuuude]

Exactly the same thing. You forgot one word (okay, two), though.

2. TT is NOT caused by pinion gears TRYING TO climb ring gears.

Thus the resistance.

I've always seen that argument framed like it was the position of the pinion as the cause, as if moving it somewhere else would remove the twisting force.

Any time you transfer force from one point to another , and those two points are not rigidly attached to one another, something is going to move in the direction that force is applied due to any resistance there might be, and limited to the confines of its allowable movement.

 

[Duuuuuuuude]

I think much of your thread has errors. But because they're confusing topics and I'm not sure exactly what's right I can only pick on the few things I'm pretty sure are wrong. I'm not good at expressing tone, so please know that this is not an attack. It's just the way I discuss things. And you asked.;-)



Your Jeep tip-over thing seems like easy pickin's. The jeep will tip over whenever the green spot gets past a certain point near the outside edge of the downhill tire tread. The only reason it doesn't hang in there all the way to the edge of the tire, is the tire is rubber, and will collapse under too much pressure applied just to the edge.

But the relation to the roll-center is almost non-existent. First, a tangent, a car does not exactly roll about it's roll center. It rolls about the roll axis with is a line passing thru both roll centers. No matter -- that's a nit. How easily the jeep rolls about its RC/RA is more than just the geometry. It's also the spring rate. And limit straps. And bump stops. And while this may affect how the jeep behaves as you start pushing it over, the final roll over always occurs solely as a function of when the CG gets onto and starts to pass the downhill tires.

You're statement that the jeep cannot roll as long as it has traction is clearly wrong. Suppose I nailed the downhill tires to a huge piece of plywood (inifinite traction). Referring back to your statement I quoted above -- You really think the jeep won't roll over anyway once I tip the plywood steep enough?

I kinda want to pick on your TT post too, but I don't understand it well enough yet. Gotta think some more on it. Thanks for the push.

-

A tire rolling onto its edge would of course create a pretty good loss of traction, at which point it would slide.

I also pointed out that it was an extreme example and not practically realistic to acheive. It was for demonstrative purposes only.

If you could somehow get your roll centers or roll axis (RA being something that i have not gotten to yet) that high above your center of gravity, then yes, it would not tip. Refer to my pencil experiment.

Also, keep in mind that I was just laying out basics.

 

[Duuuuuuuude]

And I'm not comfortable about the sway bar stuff.

Yes, it's the last bit of tuning to be done.

But, the way it works is thusly: :ror:

While cornering there's a weight shift to the outside from the chassis and body which compresses the outside suspension, the sway bar resists the twisting motion, and therefore lifts the inside tire, the chassis reacts with a weight shift to the inside and etc...

It's the sway bar resisting the torsional twisting movement and doing work to the other tire that makes things happen.

Isn't that just another way of saying what I said? There is nothing you stated that I disagree with.

 

[Duuuuuuuude]

I found these articles to be very helpful too:

Truck Suspensions, Four-Link Suspensions, and Suspension Installs - Part 2 - 4 Wheel & Off Road

Suspension Encyclopedia/4Wheel Underground

The first article does a good job of illustrating the geometry of the 4-link crawler suspension (especially that of the roll axis) and even does a pretty good job of presenting the performance gains/losses/tradeoffs associated with the various adjustments. The second article complements the first by providing more in-depth treatment of the various elements of 4-link geometry. Hope this helps and adds to the discussion and knowledge that this thread provides.

Edit: I forget to mention that you will have to click the "Read Full Article" link near the bottom of the page for the first link in order to view the entire article.

There are many good sources but at some point they all go off into setups or thoughts that either don't apply to us or add to the confusion. What works well for them does not always work well for us. There are differences in designs and practicality that do not always cross over. I didn't add links to anything on purpose.

 

[supermoturd]

There are many good sources but at some point they all go off into setups or thoughts that either don't apply to us or add to the confusion. What works well for them does not always work well for us. There are differences in designs and practicality that do not always cross over. I didn't add links to anything on purpose.

I remember you saying that up front and I made a note of it. It is certainly true and should always be emphasized. However, I always find it helpful to draw from as many sources as possible. You never know when a slight variation in the way that something is described will cause things to "click" for you.

I agree completely that we should refrain from blindly copying and following the trends of the 1:1 community. What we are looking for are the basic underlying geometric and physical principles of suspension mechanics (i.e., the theory.) Once we grasp the theory, we are equipped to apply it in a relevant manner. What I gleaned from those articles was insight regarding how to calculate/determine where things like ICs and Roll-Axes are (not where they "should" be) and how the locations and orientations of these properties affect the handling characteristics of a generic 4-wheeled vehicle.

 

[Hardline]

A tire rolling onto its edge would of course create a pretty good loss of traction, at which point it would slide.

I also pointed out that it was an extreme example and not practically realistic to acheive. It was for demonstrative purposes only.

If you could somehow get your roll centers or roll axis (RA being something that i have not gotten to yet) that high above your center of gravity, then yes, it would not tip. Refer to my pencil experiment.

Also, keep in mind that I was just laying out basics.

I was agreeing with everything you said until I got to the red. No, no, no. I don't care what you do with the roll axis. When the CG passes over the downhill tires it WILL tip. You're not hanging from the roll axis...your being supported by your tires....only.

 

[Hardline]

It's the sway bar resisting the torsional twisting movement and doing work to the other tire that makes things happen.

Not totally sure what you mean by that, but there's no such thing as weight shift during cornering. The bits and pieces of a car weigh what they weigh. What there is is called weight transfer (I know, nit picky again) and the primary duty of anti-sway bars increases the effect of that weight transfer. Adding weight transfer at one end of a car spoils it's traction at that end because the tire grip vs tire weight curve is slightly non-linear, so add weight xfer and the outside tires pick up less additional grip than the inside tires lose (IE net loss of grip). Thus a bigger rear ASB causes the rear to have less grip and promote oversteer (or reduce understeer if you prefer).

If anyone's interested, this is well covered complete with tire grip graphs in Carroll Smith's 'Prepare to Win'. His book series is super, but PTW is the best of them. And no, I don't know why we're discussing this in a crawling thread. Maybe just for completeness.

 

[Hardline]

Back to torque twist. Personally I find this the most interesting area to discuss. Mostly because I don't yet understand it -- so I'm thinking about it.

No theory yet, but observations: Can we all agree (I just checked, again):

Looking from behind, the right front tire lifts in TT (counter-clockwise axle housing rotation)

Also looking from behind, the front drive shaft rotates clockwise. Opposite the rotation of the housing rotation. (bet not everyone realized that ;-) )

And FWIW, the ring gear is on the drivers side, pinion on passenger. For future reference.

Seems to me the axle housing wanting to rotate opposite direction from the drive shaft rotation kinda screws up most power-drill analogies.

 

[Duuuuuuuude]

I was agreeing with everything you said until I got to the red. No, no, no. I don't care what you do with the roll axis. When the CG passes over the downhill tires it WILL tip. You're not hanging from the roll axis...your being supported by your tires....only.

Good point, and very true. I wasn't considering the tire as a pivot point.

However, I can't imagine a circumstance where the tires would be able to maintain grip long enough for that to happen. Unless of course you nail them down. :ror:

 

[Hardline]

Good point, and very true. I wasn't considering the tire as a pivot point.

However, I can't imagine a circumstance where the tires would be able to maintain grip long enough for that to happen. Unless of course you nail them down. :ror:

Happens all the time. A SoCal 4x4 rec area known as the Hammers has multiple jeeps and rock buggies flop every weekend. I could easily flop my jeep on a steep side-hill granite slab (knock on wood). But these are all lifted vehicles. Maybe something low profile would slide before it flopped.

And of course we know we can do it easily side-hilling our RC crawlers. Although now that I think of it, usually one axle lifts off first.

-

 

[Duuuuuuuude]

Happens all the time. A SoCal 4x4 rec area known as the Hammers has multiple jeeps and rock buggies flop every weekend. I could easily flop my jeep on a steep side-hill granite slab (knock on wood). But these are all lifted vehicles. Maybe something low profile would slide before it flopped.

And of course we know we can do it easily side-hilling our RC crawlers. Although now that I think of it, usually one axle lifts off first.

-

Again, my example was an extreme one made for clarity. Lifted off roaders are a poor example of having a high roll center anyway, low slung race vehicles show it much better.

During the warmer and drier months I race RC short course trucks. Our club has a handful of tracks that we rotate to every week, and each presents its own challenges when tuning for them. I don't have the time or will to devise a specific setup for each one, then devise even more setups depending on track conditions.

I have one type of tire I think works best for most tracks, one shock setup, one spring setup, and one weight setup. The only adjustment I make is to my rear roll center. If taken to one extreme it will roll over in a corner, if taken to the other extreme it will slide in a corner.

The roll center is close enough to the center of gravity that I get to play on both sides, you can't say the same about most crawlers.

 

[supermoturd]

From what I've been reading, the Roll Axis can be determined for the front and rear suspension independently. I apologize for the crude drawing, but I hope it illustrates how the roll axis is determined using the intersections of the upper and lower link sets. What I find most fascinating and useful is how the angle of the roll axis relative to horizontal (in side view) affects roll-steer.

What I'd like to see is a discussion on how roll-steer (understeer/oversteer) affects performance in side-hilling since that is where crawlers tend to experience more body roll (as opposed to cornering at speed.)

 

[Hardline]

What do people consider important in side-hilling? Obviously a low CG for the same reasons we discussed with the Jeep example. I would think front tire foams to be important because my rig does everything better with butter-soft foams, except side-hill. Spring rate and shock limiters would factor in to keep the body roll in check. I can see how roll axis has a role there too, but I would think it would be less important than those other factors. Even with very soft springs, I'm not seeing enough body roll to move the CG downhill hardly at all. If the CG doesn't move sideways, I don't see body roll being important.

Typically on a too steep side-hill, my rig starts lifting a rear wheel before things go to hell. That could certainly be geometry related.

 

[supermoturd]

What do people consider important in side-hilling? Obviously a low CG for the same reasons we discussed with the Jeep example. I would think front tire foams to be important because my rig does everything better with butter-soft foams, except side-hill. Spring rate and shock limiters would factor in to keep the body roll in check. I can see how roll axis has a role there too, but I would think it would be less important than those other factors. Even with very soft springs, I'm not seeing enough body roll to move the CG downhill hardly at all. If the CG doesn't move sideways, I don't see body roll being important.

Typically on a too steep side-hill, my rig starts lifting a rear wheel before things go to hell. That could certainly be geometry related.

Those are all good points. I was thinking more along the lines of how roll-steer, particularly roll understeer, could cause the rear axle to yaw in the up hill direction, causing the rear wheels to attempt to climb. Aside from affecting the ability of the rig to maintain a straight course across the grade, this could also cause the rear wheels to lose traction and the back end would then start to side-slip down the hill. Granted, you would have to have a pretty loose suspension and severe roll axis angle to see these effects but still.

 

[Duuuuuuuude]

What do people consider important in side-hilling? Obviously a low CG for the same reasons we discussed with the Jeep example. I would think front tire foams to be important because my rig does everything better with butter-soft foams, except side-hill. Spring rate and shock limiters would factor in to keep the body roll in check. I can see how roll axis has a role there too, but I would think it would be less important than those other factors. Even with very soft springs, I'm not seeing enough body roll to move the CG downhill hardly at all. If the CG doesn't move sideways, I don't see body roll being important.

Typically on a too steep side-hill, my rig starts lifting a rear wheel before things go to hell. That could certainly be geometry related.

The CG we use for reference is the overall CG. Since each axle works independently, they each are subject to their own CG. A front axle with knuckle weights will have a lower CG than a rear axle with no added weights.

 

[mjderstine]

holy nuts. love this stuff. I had talked to Jdub out at ECSC for at least an hour about the setup of my link geometry, and how stock axial setups really start to do funny things on side hills, and up hills. one of the first things we discussed was AS. by creating a mounting point for the rear uppers on the chassis that made the links parallel to the bottom links when looking from the side, i was able to greatly increase the AS of the rig. i tested this by finding a nice big flat rock that had about a 45* angle. pointed the truck straight up the hill i would got WOT before the link adjustment. truck would wheelie and tip over. the back end would noticeably squat under acceleration. the next test was with the higher link mounts on the rear uppers. so, same test. put the truck on the rock, WOT, and the front tires barely lift and it just drives up the hill. i can almost see the back end LIFT under acceleration.

now this succeeded in eliminating squat, and maybe it has too much AS now. but i still have more driving to do with it. one thing i liked it that i instantly got performance enhancements for uphills, side hills, side turn to uphill. it kept the back end more planted, and the truck seemed to tool onto its back much less (like a turtle).

now the trick is this. i need to drive the crap out of it with just this one change. get really used to it, and then change something else and see what happens. say adjust the front anti-squat/squat link geometry. add or remove weight from my tires, etc.etc.

this stuff all depends on your setups too. i appreciate that Duuuuuuuuuude is applying this to a "perfect" test environment. we all remember, or not, physics when we have to calculate a ball falling 10ft and how long it takes to hit the ground. i vividly remember some smartass bringing up friction from the air. (me) in scenarios where you are making a point or explaining you have to make it as simple as possible, and i think we all get that.

i know that in my local area we all have very different ideas about tire foams. some guys love the super soft stuff. some love the dual stage foams, some dont care at all. some cram their wheels full or lead, others keep their truck as light as possible. some like UD/OD, some have a pure stock SCX10 that does just as good as a fully built custom rig. we also have different tires, motors, elecs.

but Duuuuuuude started this about links. and i started rambling. my point is that i made one adjustment to just the rear upper chassis mounting points (there are 8 pairs of mounting points, i changed 1), and it made a HUGE difference in how, i perceived, the truck drove. if you try to make 4 adjustments all at once you will never be able to narrow down what made things worse and what made things better.


sway bars. i don't use them on scalers, but i have made a fully custom built 1/8 SC truck with a live rear axle. the thing was utterly undriveable at speed without a sway bar. the only reason i know that is that it came off once when driving it, and i couldn't turn at any speed but walking speed without it wanting to roll over. with the sway bar i can pretty much peg it left or right on a loose surface and its golden. not sure how many different setups i tried while designing this thing, but infinite might be good.

MP 75 SC Photos by mjderstine | Photobucket



apologies for the semi thread jack. wanted to get that sway bar point across.