Rear Steering - 3-Link vs 4-Link and Chassis vs Axle Mounted Servo

[Panther6834]

I'm in the process of building my new C3 rig with 4WS. This vehicle is being built on a rail-based chassis based loosely based off the SCX10-II design, which came with a 3-link w/ panhard setup for the front, and 4-link for the rear. The front servo will be chassis-mounted (servo winch mounted side-by-side to the steering servo), using the included 3-Link setup. However...it's the rear I'm inquiring about.

As I mentioned, the chassis comes with a rear 4-link setup. I see three possibile ways to set this up - 1) chassis-mounted servo & 4-link (my preferred configuration), 2) chassis-mounted servo & 3-link w/ panhard (although, I'd have to figure out how to attach a panhard mount to the rear of the chassis), and 3) axle-mounted servo & 4-link (my "fallback" option, and as it is on my 'old' C3 rig).

The 'problem' I'm having is this - in regards to my primary choice...chassis-mounted servo & 4-link...I've been told two completely different things by two different people. One said it's easily doable, and will work without any problems. I'd just have to remove the rear half of the rear chassis brace, and come up with a way to chassis-mount the servo (which I've already figured out how to do). The other said it won't work, and that my only options are options 2 & 3.

Both of these people have been involved in competitive RC crawling for many years (one would think, because of this, both should be quite knowledgeable), plus they know each other. However, once again, they are saying completely opposite things. Obviously, I'm "confused", as both CAN'T be "true"...either 'option 1' will work, or it won't. This is where input from others that more knowledgeable than I am would really help, and would be greatly appreciated.


~ More peace, love, and kindness would make the world a much better place

[GA Dawg]

It might be physically possible to set it up with a 4 link and a CMS, but your steering geometry will be off. Your wheels will turn in unexpected directions as the rear suspension cycles.

[Almighty Malach]

Leave it 4 link and AMS. You want the benefits of a 4 link in the rear instead of the goofy 3 link geo that comes with a 3 link /panhard set up.

[Panther6834]

Sorry for this (probably) noob question, but, AMS? CMS?


~ More peace, love, and kindness would make the world a much better place

[GA Dawg]

Originally Posted by Panther6834

Sorry for this (probably) noob question, but, AMS? CMS?

Chassis mounted servo and axle mounted servo.

[Topher Builds]

I wouldn’t want to contradict anything Ikaika says, dude is one of the best there are in this realm. His podiums prove that. He’s quite literally the champ.

Keep in mind the info I’m giving you is theoretical, I’ve never tried to run a 4 link cms rear steer myself but it wouldn’t be the option I personally would choose if I were to do it. I’d either 4 link ams or find a way to get a panhard back there if I really wanted to rear cms.

[Panther6834]

Originally Posted by GA Dawg

Chassis mounted servo and axle mounted servo.

Thanks...got it now.

Originally Posted by Topher Builds

I wouldn’t want to contradict anything Ikaika says, dude is one of the best there are in this realm. His podiums prove that. He’s quite literally the champ.

Keep in mind the info I’m giving you is theoretical, I’ve never tried to run a 4 link cms rear steer myself but it wouldn’t be the option I personally would choose if I were to do it. I’d either 4 link ams or find a way to get a panhard back there if I really wanted to rear cms.

Well...as you said in the PM, 4-link & CMS is worth trying. Best case scenario, it's a success (even if there's a bit of 'bump steer').

Worse case scenario, it's a "fail" (possibly miserably), and I have to put it back to AMS...but, on the 'plus' side, as the rear brace needs to be modified exactly the same, it will have already been done. Should be a simple matter of reinstalling the AMS part into the axle, and replacing the servo/bumper mount with a standard rear bumper mount.

Either way, was just going for some "insight"...especially from anyone who might have tried all three (or, at least, any two of the) options.


~ More peace, love, and kindness would make the world a much better place

[GA Dawg]

Is there a reason you prefer to mount the servo on the chassis? The only reason it is done on the front of most vehicles is for scale appearance. Rear steer isn't scale so you may as well go for the perfect steering geometry and mount the servo on the axle.

[Panther6834]

Originally Posted by GA Dawg

Is there a reason you prefer to mount the servo on the chassis? The only reason it is done on the front of most vehicles is for scale appearance. Rear steer isn't scale so you may as well go for the perfect steering geometry and mount the servo on the axle.

Worth the front, the chassis brace has side-by-side servo mounts (steering & winch). Even though the previous iteration of my C3 rig had AMS front/rear, it also didn't have a winch. Wanting to include a winch this time around, I have to do CMS on the front. So, I thought, visually, it might be best to go also CMS on the rear. Is just hoping that I wouldn't run into any problems while still using 4-link.

At this point, I'm thinking I'm on the verge of giving up on that idea, and just sticking with AMS for the rear. As I've learned from other's input, CMS w/ 3-link & panhard won't provide enough rear articulation...and CMS w/ 4-link is starting to sound less & less 'reasonable' (even though I still want to give it a try).


~ More peace, love, and kindness would make the world a much better place

[ferp420]

theres a thread somewere on rcc i think its called off the deep end or something like that but the guy goes in to how he built his 4ws crawler with 3 link front and rear he was able to use the geomitry of the 3 link to cancel his torque twist he sead somthing about it being better than a moa rig becauze no clod stall
might be worth looking in too

[Panther6834]

Originally Posted by ferp420

theres a thread somewere on rcc i think its called off the deep end or something like that but the guy goes in to how he built his 4ws crawler with 3 link front and rear he was able to use the geomitry of the 3 link to cancel his torque twist he sead somthing about it being better than a moa rig becauze no clod stall
might be worth looking in too

That's Erik's (aka D_Lux Fab) rig. Only "problem" is his rig isn't 4WS...it's traditional front-only steering. Plus, the panhards on that rig are 100% custom...after all, Erik is a master at custom fabrication (and, his Dravtech shocks are outstanding, to boot)...I don't have the machinery to do anything like that. Best I'd be able to do is buy something, and slightly modify it (if needed). The highest 'tech' tooling/machinery equipment I have would be a cordless drill, a cordless screwdriver, and a Dremel.


~ More peace, love, and kindness would make the world a much better place

[Honeyman108]

Axle mounted servo on your application will be best and there are options for many axle types as far as 4 wheel steer since I have 3 rigs 4ws and a new TRX4 4ws project im starting with a platform like TRX4 you can mount the servo on chassis mirrored to the front with panhard and 3 links but for your C3 im not sure how that would work but axle mounted works really well and Shapeways.com have a lot of options.... See the pics of this set up for a 4WS on TRX4 SWB using x2 Samix TRX4 Shock tower/ Panhard and the servos lay belly side using the Side Servo mounts. BUZZZZZZ.....

[ferp420]

Originally Posted by Panther6834

That's Erik's (aka D_Lux Fab) rig. Only "problem" is his rig isn't 4WS...it's traditional front-only steering. Plus, the panhards on that rig are 100% custom...after all, Erik is a master at custom fabrication (and, his Dravtech shocks are outstanding, to boot)...I don't have the machinery to do anything like that. Best I'd be able to do is buy something, and slightly modify it (if needed). The highest 'tech' tooling/machinery equipment I have would be a cordless drill, a cordless screwdriver, and a Dremel.


~ More peace, love, and kindness would make the world a much better place

your right its only front steer but the link and panhard set up is the same it actualy makes more sence in 4ws mode in my opinion

[Panther6834]

Originally Posted by ferp420

your right its only front steer but the link and panhard set up is the same it actualy makes more sence in 4ws mode in my opinion

Actually, as GA Dawg pointed out, the panhard in the rear on 4WS would create "screwy" problems. After thinking it over now, and actually visualizing everything in my head (it's where I get some of my best wish done...lol), I think I've decided to stick with the 4-link & AMS in the rear.

However, when it comes time for building a C2 rig (once I've had the chance to go over all the SORCA C2 rules), I just might give Erik's 3-link w/ panhard & CMS front & rear a try. While that rig is quite a ways away, I did order the body today, but only because it's discontinued, and I happened to find someone having a couple in-stock. As for the C3 body, I'm still searching.


~ More peace, love, and kindness would make the world a much better place

[ferp420]

the rear 3 link wouldent be any different than the front 3 link exsact same set up if it works in the front it will work in the rear just the same at that point there isent realy a front or back other than how the body is mounted but anyway its something differnt just thought maybe it would be somethihg to look at but sounds like you looked at already so build on and enjoy

[KenB925]

Originally Posted by ferp420

the rear 3 link wouldent be any different than the front 3 link exsact same set up if it works in the front it will work in the rear just the same at that point there isent realy a front or back other than how the body is mounted but anyway its something differnt just thought maybe it would be somethihg to look at but sounds like you looked at already so build on and enjoy

agreed, I've ordered parts to do just that. I can't imagine it being a big problem.

I'm joust going to replicate the front system on the back.

I was initially planning on going axle mounted, but then I would have had to design and print a bracket (TRX4), and this will be cleaner looking I think.

[KenB925]

I got my 4 wheel steer done. I used all traxxas parts, I still have the stock transmitter and everything works.

I basically replicated the front suspension on the back, I started with a TRX4 Land Rover, the suspension links were all too long for the new rear steering axle, I ordered a set of suspension link for the 12.3" wheel base and those worked perfect. I did have unscrew the ends on the third link a couple of turns to get the length correct.

I used a Bastens Quad steer module, a Servo signal reverser, and a Traxxas Servo Y adapter.

I set it up so that the switch on top of the controller is the only thing that is changed. Now all the way back is still open diffs, the first click locks both front and rear diffs, and the second click activates the different steering modes. You toggle it between second an first click to advance the modes.

It turned out great, fun little project.

[Panther6834]

Originally Posted by KenB925

I got my 4 wheel steer done. I used all traxxas parts, I still have the stock transmitter and everything works.

I basically replicated the front suspension on the back, I started with a TRX4 Land Rover, the suspension links were all too long for the new rear steering axle, I ordered a set of suspension link for the 12.3" wheel base and those worked perfect. I did have unscrew the ends on the third link a couple of turns to get the length correct.

I used a Bastens Quad steer module, a Servo signal reverser, and a Traxxas Servo Y adapter.

I set it up so that the switch on top of the controller is the only thing that is changed. Now all the way back is still open diffs, the first click locks both front and rear diffs, and the second click activates the different steering modes. You toggle it between second an first click to advance the modes.

It turned out great, fun little project.

Sounds cool. For the 4WS, what 'modes' do you end up with using the components you installed? For example, with the Futaba 7PX & 4PM transmitters, you can toggle between front only --› front/rear opposite (super-tight turn) --› front/rear same (crab walk) --› rear only.


~ More peace, love, and kindness would make the world a much better place

[KenB925]

Originally Posted by Panther6834

Sounds cool. For the 4WS, what 'modes' do you end up with using the components you installed? For example, with the Futaba 7PX & 4PM transmitters, you can toggle between front only --› front/rear opposite (super-tight turn) --› front/rear same (crab walk) --› rear only.


~ More peace, love, and kindness would make the world a much better place

With this set up it does the same 4 modes you listed.