Digital on T/B - Good idea or bad?

[olds97_lss]

I recently aquired a couple nice servo's for half the cost from my LHS ($60 vs $120 on the package or $150 MFR). They are JR 8550's which are digital. I bought one for a buddy of mine for steering and I bought another just because it was a good deal. Well, as luck would have it, the T/B 590M servo on my aftershock stopped working today... figures.

So, now the only spare I have is the new one I just bought. I've never had digital servos and the best servo I've bought to date was a hitec 985MG which is a standard coreless servo.

So, considering the T/B linkage is setup in such a way that there is constant mild pressure via a spring pressing the carb shut via the servo, the digital servo buzzes a lot. I could just rely on a TRS to pull the carb back, but it works better if I have a mild rubber band as a TRS and use the collars/springs to push the carb shut and hold it shut as well.

I understand that digital servos do not like to be mechanically stalled, but what about a mild push for extended periods of time?

This is my linkage in case my explaination is confusing...

Where it is sitting in the photo is idle or very close too it. The arm would have about a 1mm gap between the yellow thing and the collar if I had thought to turn it on prior to the photo.

Another issue is that whenever my OFNA micro FS blinks, the servo twitches a very little bit. Nothing I can't live with, but again, I'm affraid of damage to my precious servo.

 

[Lessen]

If the servo is buzzing when the TRS is keeping your carb at idle you need to make an adjustment to your "center" on the servo. Such that zero movement is closer in the direction that your TRS is pulling the servo arm. You can try adjusting the servo arm over the splines of the servo but I would imagine that even a single spline adjustment may be too much. Do you run a high end radio? If so this is where I would make a sub-trim adjustment.

 

[olds97_lss]

The rod with the spring/collars on it are putting pressure on the servo when at idle. This helps keep the carb at idle position in conjunction with the TRS. I don't care for using just a TRS to bring the carb back to idle. I like to use the springs/slider method.

 

[Lessen]

Ohhh ok. So your pressure is actually the opposite direction then. I see. That's the exact reason I stopped using the spring/slider on my Maxx. I only use a TRS, but that's me.

well those springs generally aren't very powerful unless it's really cranked down, which by your pictures it isn't. It's hard to know how much strain is on it w/o actually hearing it. Personally, I'd get rid of the slider, save a $100 servo. I suppose that's playing it ultra safe since I don't really know how much strain they can handle w/o burning up. Besides that I would imagine that if it is not making a severe dent in your Rx pack time then it isn't putting a massive amount of strain on it. You could always do a controlled comparison, with or without the slider on it.

 

[Timmahh]

note, digis Buzz all the time, you can Occasionally get them to stop, but its also very commen for them to hum/buzz when still. Good info from Lesson on Sub Trim settings if you have a better radio.
also be sure to correctly set the endpoints on the digital, as overstressing them can cuase early failure. but once its set up and working well. i think you ll love the reponse of a digi T/B servo.
and 60.00 is a STEAL on that servo!! i use the JR Z9000t servo, and love it.

 

[olds97_lss]

I have the MX-3 which has EPA's and whatnot. Trim or subtrim doesn't really matter for what I'm doing. I can get the horn close to home, then adjust the normal trim or linkages to allow the trim to be centered when at idle. I hate using sub trims because I forget they are there and set to anything but 0.

Thanks on the info though.

 

[olds97_lss]

Well, I ran it last weekend and it worked fine, but hummed a lot which tells me it's trying to do something even when it doesn't need to. So, I have 2 985MG's in my revo which are fast, strong, coreless and not digital in it controlling steering and t/b. I pulled the one in it for steering and put the 8550 in it and used the 985MG for t/b on my aftershock. There pretty much the same speed and strength, so it works out well.

Better to be safe than sorry I guess. I did a bit of research and found that it isn't a good idea to run digital where there is constant load. At the very least, it will give it a shorter life and drain batteries. At the worst, it will fry in a very short time span.

 

[Lessen]

Better to be safe than sorry I guess. I did a bit of research and found that it isn't a good idea to run digital where there is constant load.

so just for the sake of conversation...

then what exactly would be the point? If somebody asked me I'd say that both T/B and steering servo's both are being used almost all of the time. Whether you are either bashing or racing your almost always on the throttle anyway. Even during the 6% (hypothesized number) of runtime when you are going straight the steering servo is receiving load through very small amounts of bump steer. I can tell just be pushing down on my Maxx (which has digital servos). When I push it down and let it go the steering servo will buzz until I pick it up (relieving any bump steer and load from the steering servo).

There aren't many R/C theories that I whole hartedly disagree with but I think this may be one of them. I've been running the same 2 Futaba Digi's in either my buggy or T-Maxx for almost 3 years now with no problems. Matter of fact the steering servo took a bath (completely submerged for approx. 10 seconds) in a shallow stream this summer and lived to buzz another day. I also just yesterday replaced the case on the other one because it got pulled apart during a wreck. Servo still works like the day I bought it. My Futaba S9350 and S9451 are the best $200 in upgrades I've spent.

 

[olds97_lss]

I didn't post to prove or disprove it. I just don't want to be the guy to prove it's a bad idea... like the original savage/maxx MS air filter. I had proof of that theory.

 

[Lessen]

I didn't post to prove or disprove it.

I hear ya. I'm just sayin', it doesn't make much sense to me to. I don't think that there is an overwhelming amount of risk as long as you keep the buzzing to a minimum. If it's killing runtime an adjustment definately needs to be made, but I don't think theres anyway small amounts of buzzing is ever going to be completely avoided.

 

[olds97_lss]

Well... I don't really drive the revo. So it should last a long time in that truck

I'll probably break it back out this winter. I always found small blocks easier to run in the winter. Less surface area to cool off so it would stay warm easier.