12-14-2013, 02:36 PM | #41 | |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread 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. Quote:
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. - Last edited by Hardline; 12-14-2013 at 02:41 PM. | |
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12-14-2013, 02:41 PM | #42 |
MODERATOR™ Join Date: Jul 2004 Location: Ohio
Posts: 18,928
| Re: Another 4 Link Thread |
12-14-2013, 02:52 PM | #43 |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread |
12-14-2013, 03:12 PM | #44 |
Quarry Creeper Join Date: Nov 2013 Location: Huntsville
Posts: 222
| Re: Another 4 Link Thread
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. |
12-14-2013, 03:42 PM | #45 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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. Last edited by Duuuuuuuude; 12-14-2013 at 04:09 PM. | |
12-14-2013, 03:49 PM | #46 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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. Last edited by Duuuuuuuude; 12-14-2013 at 04:07 PM. | |
12-14-2013, 03:53 PM | #47 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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12-14-2013, 03:59 PM | #48 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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12-14-2013, 04:39 PM | #49 | |
Quarry Creeper Join Date: Nov 2013 Location: Huntsville
Posts: 222
| Re: Another 4 Link Thread Quote:
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. | |
12-14-2013, 04:45 PM | #50 | |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread Quote:
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12-14-2013, 05:06 PM | #51 | |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread Quote:
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. | |
12-14-2013, 05:28 PM | #52 |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread
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. |
12-14-2013, 05:35 PM | #53 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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. | |
12-14-2013, 06:43 PM | #54 | |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread Quote:
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. - Last edited by Hardline; 12-14-2013 at 06:46 PM. | |
12-14-2013, 07:38 PM | #55 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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. | |
12-14-2013, 09:28 PM | #56 |
Quarry Creeper Join Date: Nov 2013 Location: Huntsville
Posts: 222
| Re: Another 4 Link Thread
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.) Last edited by supermoturd; 12-15-2013 at 09:25 PM. |
12-15-2013, 09:56 AM | #57 |
Rock Crawler Join Date: Oct 2011 Location: Great Outdoors
Posts: 651
| Re: Another 4 Link Thread
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. |
12-15-2013, 10:34 AM | #58 | |
Quarry Creeper Join Date: Nov 2013 Location: Huntsville
Posts: 222
| Re: Another 4 Link Thread Quote:
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12-15-2013, 11:00 AM | #59 | |
Suck it up! Join Date: Mar 2008 Location: Arkansas
Posts: 11,652
| Re: Another 4 Link Thread Quote:
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. | |
12-16-2013, 01:08 PM | #60 |
SuperShafty.com Join Date: Jan 2008 Location: Visalia CA
Posts: 2,902
| Re: Another 4 Link Thread
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. Last edited by mjderstine; 12-16-2013 at 01:11 PM. |
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