Centering Servo
 

(PART 2 posted in post #18 )


So, on the weekend, I was having some fun, repeatedly trying a challenging line with my RTR Ascender. My fun ended when I lost steering, as a result of a slipped / stripped servo horn.

Fortunately, that is the one part Vaterra was kind enough to include two spares of. It's as if the plastic horns are a fail safe, like a shear pin in machinery.

Anyways, I went to replace the horn. I made sure to power things up, and turn the servo full left, full right, then return to centre before mounting the horn. I found that true centre is located between the splines. Is this common? While I can adjust the trim on the radio, so that straight is straight, it does not adjust the end points for the servo, so now I can turn slightly tighter in one direction vs the other.

I don't think it's a big enough discrepancy to really be a problem, but I suppose more than anything, I'm curious as to why this happened, since my trim was dead centred before. Is this a common happening? A quick boogie around the backyard suggests the servo is otherwise working fine. The servo output has some play in it, though I never payed any attention to what it was like before.

I understand alloy servo horns, and the servo are popular candidates for first upgrades on these rigs, but I'm not in a rush to spend any money replacing something that still works.

Any insights would be appreciated.

https://uploads.tapatalk-cdn.com/201...aa4bf1ceee.jpg

https://uploads.tapatalk-cdn.com/201...f26187866d.jpg

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Re: Centering Servo
 

I've found that's it's pretty common that when you're changing servo horns or servos the center point is just slightly different from unit to unit. It's annoying, but normal and common.

When the horn induces the change, I've always just assumed it's because of slight differences from mold to mold; when it's the servo I tend to assume it's just because the pot gets oriented a little different each time they put one together.

I try to minimize the amount of trim use on the transmitter by altering the push-rod length before dialing in trim, thin washers to lengthen it or sandpaper/grinder/razor knife to shorten it.

Re: Centering Servo
 

Thanks for the reply. I kind of figured that perhaps it was a bit of an inevitability that the splines would not line up perfect every time...


Adjusting the push rod length sounds like a reasonable solution that did not really cross my mind.

Re: Centering Servo
 

Just curious, are you sure the 2 other included servo horns are the same? Generally, if a kit includes multiples, it's because they're all different spline counts, >ie: 23, 24, 25t. With a plastic horn on a plastic output gear, it might be possible that you were able to force on the wrong tooth count, and that may be the cause of your steering now being off center?

Centering Servo
 

Winner!

That’s exactly what I did!

I’m such a noob, I just assumed the tooth count was standard, and the additional horns I had were spares. I did in fact force a 25T horn on, when the one I pulled of was marked 23.

I was a little surprised by having to use the screw to force the horn on, but just assumed “it must be right.”

Thanks for the helpful post.

Edit.

I went to the LHS and picked up a metal servo horn, which will definitely outlast the stock servo. I think my issue with the original horn was caused by the screw that attaches the horn coming loose.

Anyway, I still needed a touch of trim with the new horn, but not as much as with the wrong horn jammed on.

https://uploads.tapatalk-cdn.com/201...e128301177.jpg

This whole experience has really drawn my attention to the servo, and I have definitely now noticed it’s torque limits. For now, I’ll just consider it a sense of realism though. It’s like when you put 37s on a Jeep Wrangler with stock steering components, and go rock crawling. You’re going to find situations where you can’t turn the wheel.

Re: Centering Servo
 

Once/if you upgrade your radio, you'll find most feature a sub-trim setting whereby you can set your initial servo centering to compensate for spline/servo variations with your primary trim settings zeroed. Then your primary trim starts from true center.

Re: Centering Servo
 

Some things come to mind...
1. Stock servo is junk. A DS3218 from china is $12. Consider ordering a couple of them - they don't last unless you're willing to spend $100 on a servo.
2. A metal horn - you did fine there. The china servos come with nice alloy horns.
3. You don't need to mess with your draglink - I never have with any of mine, ever.
4. You can adjust the end points and throw by putting the radio into setup mode - instructions in the manual.
5. It's common to just get close, that's what the fine-tune adjust knob is FOR. Don't feel bad about using it - no setup will EVER be absolutely in the middle.
6. Along with a new servo the HW1080 ESC is about the best $40 you can spend on a brushed crawler. It transforms drivability.

That's all I got at the moment. ;-)

Re: Centering Servo
 

I'm learning as I go here with all of this stuff.

1. I've read the stock servo is junk countless times. But, I find it interesting to come to my own conclusions, and try to get the most of the parts I already payed for. Is the DS3218 plug and play with the stock RTR ESC, or would both parts need to be upgraded for the servo to function correctly?

2. I paid too much for it, but supporting a local business is not the worst thing you can do with your money I suppose.

3. I have no immediate intentions to adjust the drag link length. It's always an option though.

4. I did not know that.

5. I'm ok with using the fine trim to make slight adjustments. An off centre knob will not keep me awake at night.

6. I'll keep the HW1080 in mind if I'm looking for an upgrade. I have not yet found a reason to want to upgrade the ESC though. Can you elaborate on what driveability improvements I would see?

Thanks for the helpful post.

Re: Centering Servo
 

Always. :)


You can run the stock steering servo till it dies...most of us do. It will either just quit working or the shaft gets so loose your wheels wobble. Another common malady is simply a loose keeper screw - the horn gets loose. If you over-tighten it can strip the brass shaft. If you locktite it you may never get it back out - BTDT. LOL I usually use a very tiny amount of blue threadlock on them, and a hex screw, not a phillips - you can get it very tight that way.



The DS3218 isn't the end-all for servos, but they're so cheap and work well we use them by the dozens. Good thread on it here - same title...'cheap servo...' ;-)


There's also a good thread on the HobbyWing speed controller. Goes into lots of details about what it is and what it does. If you're running 2s you're probly still in beginner mode and that's fine. For more speed and performance 3S really wakes them up and then all the more reason for the updated controller - let's you fine tune 15 different settings for start punch, drag brake, reverse percentage, etc. check it out. I like it so well in a rig I built I bought 3 more - all my rigs have them now, and a spare. They're the shiznit. "thumbsup"


EDIT: Oh, and the stock servo is pretty weak, so if you find your wheels not turning in the rocks - stuck 'till you move forward and release tension on the tires, that's the time you may come to the conclusion you need something with more turning power. The stock is about 130oz/in - a measure of how much turning strength it has. The DS3218 is double that, depending on the voltage you give it. Some of the comp guys are running 400 and 500 oz/inch servos, just as a comparison.
Bigger tires and wheel weight will exacerbate the problem. When you get tired of your rig flipping over backwards on accents you may go looking for some added weight forward - be it axle or wheel. "thumbsup"

Re: Centering Servo
 

I'm doing my best to protect my credit card from too much harm too soon! I'm glad I got the purchase of my truck in when I did, because recent developments in my personal finances mean that my tiny truck will likely be a sensible budget build, that might evolve somewhat slowly. There are plenty of back-flips in my future!


It is indeed very easy to exceed the torque limits of the stock servo. All you need to do is drive up beside a wall, and the steering will not typically be able to push the vehicle away from the wall if the tires are on a grippy surface. But, even with that "handicap" the truck's capabilities still really impress me.

Re: Centering Servo
 

Going to jump in here briefly- I hadn't driven my truck since the day I got it because my wife's dog destroyed 3 of the 4 tires the day after I bought it when nobody was home, and the replacements just got here yesterday. When I powered everything up to drive the truck, the wheels turned full left and stayed there. What I found out was that, even though it was only driven for maybe an hour in snow, the stock servo horn was slipping on the servo itself. I was able to get the tires re-centered but it slipped several more times, and rather than fight with it I'm just going to go ahead and replace the servo and horn. I knew they were bad, but I didn't expect it to to be THAT bad...
I was going to order the DS3218 online but I'd like to mess around with this thing over the weekend, so I may just go to my "local" shop (nothing within 40 miles of me) and pick up a new servo and horn.

Re: Centering Servo
 

It's quite possible the dog chewing blew out your servo horn splines as well as the tires. Put your tiny-truck on a shelf or counter. :lmao:


Your LHS may have a ADS-15 servo from ARRMA, they're not too bad, reasonable cost in comparison. Or even a Savox 0231 which is also not too bad. Both should be at or under $40. We cringe a bit cuz if you can wait for china post the DS3218 is better than both and about $12. But alas, if you want some running time the other two are a possible solution.



Someone else may be able to comment on offerings from Hitek - I can't cuz I always thought they were over-priced, but they have a good reputation.



If you get anything over about 200oz/in you run the risk of browning out your ESC as they don't have enough amps offered up to power everything. Likely mentioned prior, the HobbyWing HW1080 speed controller is a great $40 upgrade to the whole operation, and includes a 3ah BEC built in which will power up to about 300oz/in servo, plus a whole plethora of other neato settings with a programming card included.



If your servo is still operational you might even just get another plastic servo horn - your LHS may feel sorry enough for you to give you one. ;-)
I'm pretty sure the stock servo has a 23 or 24 spline shaft.



You can use the 25spline horn out of your kit with an upgraded servo. They're still not very good, so an alloy long-reach one is a good option. The DS3218 comes with one - saves you about $15 your LHS will want for a decent name-brand alloy servo horn.

Re: Centering Servo
 

That idiot dog broke into our bedroom (where she knows she's not supposed to be) and got the truck. Even if it had been up on a counter she probably would have went after it and probably did more damage when it fell off the counter, so I guess it's a live and learn experience.
The horn that's installed is the 23T, not sure if that's correct or not. I've been messing with it trying to make sure the steering is centered, and the center seems to wander as well as the throw is different from right to left so hoping that a new servo will correct those issues.

Re: Centering Servo
 

I stopped at my local store this weekend and picked up an aluminum servo horn and I'm back in business. Still not totally satisfied with the stock servo, it doesn't seem to be consistent in how far it turn to each side but at least I can keep driving and learning for the time.


https://i40.photobucket.com/albums/e...psz4j9carv.jpg

Re: Centering Servo
 

You can set your end points by putting your radio in setup mode. Instructions in the manual. Ya, RDFM. LOL
Just be careful if you push them too far out you run the risk of breaking the axle cv's.

Re: Centering Servo
 

If the steering throw is inconsistent, that's probably not a setup issue though, is it?

Re: Centering Servo
 

Hahaha yes I did RTFM but I was going to give it a little while to see if maybe it's just me or if it actually turns tighter one way than the other.

Re: Centering Servo
 

PART 2

So, the RTR servo reached the end of its life last weekend.

I replaced it with the economical DS3218. As my luck would have it, the centre point for the servo horn is exactly between the two available spline positions. Ironically, the horn sits perfect in the rotated 180degree position.

I installed it, and but the controller into the programming mode to set the end points. I could not get it to travel far enough in the one direction. To account for this, I moved the steering link into the outermost hole on the servo horn, to increase the throw. I then re-set the endpoints. So, I can steer sharply, and equally in each direction now. The only problem is, I don't have enough adjustment range in the steering trim to get it to drive straight. It pulls right very slightly when driving forward.

From reviewing the earlier responses in this thread, it looks my options are to consider it good enough, adjust the length of the steering link, or upgrade my transmitter. Is that correct?

Re: Centering Servo
 

Move the horn one spline over and re-do the setup - end points, center, etc.
Make sure you power up your system and center up the steering trim, then attach horn, then make endpoint and centering adjustments.

We all go thru it. ;-)

The stock setup can be a bit fussy to adjust endpoints - we've fiddled with them for a time before finally getting it. The better radios make this process like child's play.

Centering Servo
 

That’s what I did. Connected servo. Powered on. Set trim to centre. Went to mount servo horn, and found the two available positions were (360/25)/2 = 7.2 degrees left or right of centre.

This image illustrates.

https://uploads.tapatalk-cdn.com/201...fc82191e16.jpg

I can set endpoints, but that does not seem to move centre to the midpoint between those end points.

I'm using the setup per the manual. I don't understand the note about changing the travel. It says if you change the travel, you must re-bind, but where does it say how to change the travel?