Twin Hammers Steering and Sway issues

[Sethmetal]

This is my 2nd go at the twin hammers. I thought I figured out all the tricks. But two issues persist. Maybe you guys know some things I don't.

Configuration;

Stock battery and ESC
Stock tires
Stock gearing
Traxxas 12T 550 brushed


Issue 1: Steering

I threw on the Irie bellcrank plate and shock tower expecting this to remedy the shot steering. But, I still have issues with it pulling a little left or right.

I adjusted tie rods and transmitter settings. But it will still pull a lil left and then after some driving switch to a right pull, for instance.

There is a little bit of play in bellcrank assembly. Maybe needs to be tightened.

Stock servo, horn and linkage. I mounted linkage to rear of horn and the other end of linkage goes to top of bellcrank arm. It rubs the plate a tiny bit, but seems to move freely.

The screws that mount the tie rods were a bit long and rubbing plate, so I put washers under tie rod to pull them away.

Do I need to toe in my tires? I'm aiming for straight on.

Is this caused by my rear locker?

Crappy stock servo not resetting to zero?


Issue 2: Sway

I'm running stock rear shocks and in front I have some cheap shocks until my gmade 85xd show up.

Heaviest swaybar front and rear.

When I had the cantilever shocks up front it was tighter, so I know my cheap front shocks are soft. These are causing me to lift a front tire under acceleration even with sway bars!

But even before the shock switch, I was still getting a lot of lean in back.

I'm guessing it's just the flexy rear geometry.

I'm gonna try the GCM rear link mount. But after that what can I do?

I guess I could mount the shocks upright more?

Make a sway bar that actually goes to the axle tube.

Or I could run heavier spring or shock weight. But, I want this thing to have usable simple suspension with no sway.



I really appreciate your ideas.




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[lonleycreeper]

Issue 1:
Definitely tighten the bell crank until it moves freely, but doesn't bind.
Could be a shot servo.
Rubbing concerns me, none is better.
I run 0°toe on mine.
Not the locker.

Issue 2:
Throw a medium bar up front and put some heavy diff oil in the front10,000 or so.
Crank the spring collars down if you want the rear stiffer.
Standing the shocks up will effectively raise the spring rate.
And you nailed it, flexy rear geometry.

 

[smog]

Intuitively and based upon my comprehension of what you have written, I also have to wonder if your steering servo is going.

I don't run any sway bars on either of my Twin Hammers. Both are undoubtedly prone to roll overs at moderate speed. My kit Hammers has the GCM rear axle truss. The geometrical change it provides does without question help to stabilize the vehicle somewhat. I have not had any issue with the rig leaning in any direction however. Certainly not like the Slickrock has with it's 2 link rear suspension.

 

[meatmonkey]

Being a shaft driven solid rear axle R/C your never going to be able to eliminate that "wheel pull" sensation when goosing it at WOT from a dead stop on a grippy surface. Proper suspension tuning will help, but ulitimately adjusting driving style ever so slightly is what keeps it under control. Adding some torque limiting through the ESC, if possible, helps as well.

Sounds to me like you stock servo is going out. Either the gears are loose/sloppy, or the potentiometer is worn.

It really bugged me that the rear sway bar twisted the trailing links significantly before the bar would load. This results in about 20* of axle swing before any roll stiffness from the sway bar is introduced. Not exactly helpful for taming torque twist.

I installed some O-rings intended for the rear diff of my HPI blitz on either side of the trailing arm's front pivot ball at the chassis. These act as wobble stoppers. I also replaced the lower ball cups on the sway bar links with plain traxxas 1942 3mm rod ends because they sit closer to the link and reduce the leverage the bar applies to the trailing arm. Plus, I couldn't keep the stock ballnuts tight. The rod end allows me to secure it with a 3mm nylock (and jamb nut because I'm paranoid).

Taking most of the rotational slop out of the trailing arms forced the sway bar to be much more effective. This resulted in better straight line stability at speed and less visible torque twist upon hard acceleration.

I personally can't sacrifice the up-travel for the anti-squat values provided by the GCM truss, so I just run the upper links in the lower chasis hole.

The rear locker makes it want to go straight.

I run about 0 toe, perhaps just a touch of toe out when on the bench so that when it all pulls tight at WOT it's pretty much 0.

 

[Sethmetal]

Great ideas!

I kinda figured I'd hear that the geometry makes it flexy. I think that's pretty apparent.

I'm glad Neat monkey pointed out the lower arm rotation. Not only does that make sway bar less effective, but the fact that the sway bar meets the arm halfway sacrifices its effectiveness.

I will have to try that o ring trick to stabilize.

As for the steering servo, I suppose it's days were numbered from the beginning, but I'll tighten the bellcrank and see if that helps.

As for the GCM truss, it only loses clearance because it hits the battery, right? I'm planning on relocating battery when I install it.

Cranking the rear spring down ads firmness, by it also changes ride height making it more prone to rolling in turns. Especially since the firmness doesn't allow it to absorb as easily.

But, I suppose a good deal of the steering and sway is due to the torque of this 550 can. It's pretty awesome in this little truck.

Didn't really want to go brushless to avoid clogging and costs. I had a 4pole 3800 castle and it was more than this thing could handle. Ha.




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[Sethmetal]

Thought I'd report back and thank MeatMonkey for the bushing suggestion.

Solid idea. Will test it today.

The GCM truss actually lost very little if any up travel. It really is minimal.
Plus, I added longer shocks with it, giving me more droop, so if anything there is more travel now.

Running a body like I have you realize the detriment of the Twin Hammers design. It's mostly up travel, so it for my purposes, it actually helps a bit to limit the up travel a bit.

As for the steering, I believe it to be the servo not centering well.i also switched the end links and servo horn (plus some grinding on the new rod ends). Now there is no rubbing.

I'm gonna kill this servo, and then maybe drop in a Tower Pro 995 or I'm open to budget minded solutions you guys might have.

Anyway, thank you all for the guidance. I feel like I got the direction I needed. You guys rock!!!

If you'd like to follow my build....

http://www.rccrawler.com/forum/showthread.php?t=549654

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[joe007]

I think the stabilizer bars do any justice to our trucks. I feel if you twin shock the rear of our trucks it will take the body sway out of our truck on speed runs. Also when you rock crawl the body will flex and roll with the terrain where the stabilizer bars hinders the suspension.

 

[89redranger]

The trick is in finding a sway bar that does control twist, sway, and body roll without limiting the car too far. I have been running the medium bar for a while, but have since switched to one I made fron 0.062" music wire. Just a touch lighter than the medium and significantly stiffer than the soft bar. While you're right that these cars don't "need" them, the use of sway bars allows both softer springs AND lighter dampening, which in turn allows the wheels to follow the choppy stuff at speed much better. Springs and shocks have a job, tune them to do that job. Tune the swaybar to do it's job independently of that. Just my $0.02.

 

[89redranger]

Btw, for the guys running o-rings on your trailing arm pivots, as much as we all love how much this tightens up the sway bar it also fatigues it when severely crossed up. This is due to the fact that music wire, like anything else, has it's own limits. With the stock floppy trailing arms it allows the arms to twist before the music wire reaches it's point of fatigue at the ends of travel. When you take this roll out of the equation, the arms get pushed past their limits, fatigue, and evetually the bar will break.

I'm not saying don't do this, just keep spares handy or buy some music wire to make your own sway bars as needed. Tightening up the trailing arms has been one of the best mods I've personally done so far.

 

[Sethmetal]

I run heavy bars front and rear with o rings.

It's great.

It controls body away, eats up the bumps and it still flexes a hell of a lot more than I'd expect.

I actually want to find a firmer bar than the stock heavy bar.

As for music wire. I'm not sure what this is.

I know of guitar strings, as a guitarist. There's also piano wire. Is it one of these?

If so, remember music wire has a metal content for its sound not torsional strength require in a swat bar. I bet vaterra uses spring steel. Possibly hear treated.


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[89redranger]

Music wire can be bought at most hobby shops, and has little to actually do with music in this instance. Most often used by airplane and boat guys for pushroads and whatnot. It is just spring steel sold in varied diameters, metric and standard. Works great, and seems to both hold up better and retain less memory than the Vaterra bars. Maybe you need to pick up some of a heavier gauge and play around with making your own as well?

 

[meatmonkey]

Interesting pic of the broken sway bar. Is that a stock or homemade jobbie? Would figure it would break at the inner bend where it was stressed when bent into shape.

I've been running mine with 0-rings for wobble stoppers and the soft bar for almost a year now without breakage. I know that if I fully droop out the rear suspension, then use my hand to only lift one side tire that once it gets about 2/3 through the upstroke the other side will start to follow because of the sway bar. Seems to me that this isn't overloading the bar?

I only use the O-rings at the forward pivot ball. This allows the rear pivot to easily accommodate the misalignment during flex travel. I bet if you used them at both ends you wouldn't even need a sway bar. Might need large washers to fully capture the O-rings.

No idea what that would do to the axle side link mounts that would have to incur extra rotational stress. But, if your looking for more roll stiffness it may be an option to look into.


Sorry, I have no suggestions for cheap servos. I've found that servo replacement is kind of a pain, and I've broken 1 Hitec 7954, and 2 Hitec 7950's, which are reminders of that. I don't see a cheap servo lasting long in a Twin hammers, at least not with me driving it.

Some of this may be due to the decreased leverage ratio from the Erie bellcrank. It's not a flaw, just a necessity due to the limited space to work within. It does require a stronger servo to keep the same torque at the wheels though. Keep in mind that there is no servo saver, and the TH is capable of higher speeds than most scalers. Both of these factors increase stress on the servo.

 

[89redranger]

That is the stock bar that broke. Still running my "home-made jobbie." The break is right at the 90* bend where the bar goes through the chassis mount. Right where the arms movement starts to transition into torsional load. When I bent my new bar I bent these corners with a slight radius, versus Vaterra's sharp 90* bend. And yes, that movement you see at about 2/3 of your articulation is about the limit of the spring steel, after which is just fatigue. If I had to speculate, your soft bar hasn't fatigued badly because of it's small diameter.

I'm only using o-rings at the forward pivots as well.

 

[Sethmetal]

I have o rings all around and the heavy large diameter bar.


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[Sethmetal]

Here's flex with o rings at all lower link ends. Rear bar disconnected for pic

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[eMoon]

What size o-rings are you guys using

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[Sethmetal]

I forget. I just got stuff from Home Depot in the faucet parts section. Very small. Diameter is just barely bigger than the ball joint.


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[89redranger]

AX10 shock o-rings. AXA1201.

 

[Sethmetal]

I bought a bunch of 3/16ID and 5/32ID

In the pic, you can see the smaller 5/32ID o rings I used the 3/16 are too big.
But I threw out all the 5/32 packages.

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[Natedog]

Being a shaft driven solid rear axle R/C your never going to be able to eliminate that "wheel pull" sensation when goosing it at WOT from a dead stop on a grippy surface. Proper suspension tuning will help, but ulitimately adjusting driving style ever so slightly is what keeps it under control. Adding some torque limiting through the ESC, if possible, helps as well.

Sounds to me like you stock servo is going out. Either the gears are loose/sloppy, or the potentiometer is worn.

It really bugged me that the rear sway bar twisted the trailing links significantly before the bar would load. This results in about 20* of axle swing before any roll stiffness from the sway bar is introduced. Not exactly helpful for taming torque twist.

I installed some O-rings intended for the rear diff of my HPI blitz on either side of the trailing arm's front pivot ball at the chassis. These act as wobble stoppers. I also replaced the lower ball cups on the sway bar links with plain traxxas 1942 3mm rod ends because they sit closer to the link and reduce the leverage the bar applies to the trailing arm. Plus, I couldn't keep the stock ballnuts tight. The rod end allows me to secure it with a 3mm nylock (and jamb nut because I'm paranoid).

Taking most of the rotational slop out of the trailing arms forced the sway bar to be much more effective. This resulted in better straight line stability at speed and less visible torque twist upon hard acceleration.

I personally can't sacrifice the up-travel for the anti-squat values provided by the GCM truss, so I just run the upper links in the lower chasis hole.

The rear locker makes it want to go straight.

I run about 0 toe, perhaps just a touch of toe out when on the bench so that when it all pulls tight at WOT it's pretty much 0.

X2 and remove the washers under your tie-rod screws, buy or cut your screws shorter as needed.