Too much of a good thing

[Mrowka 493]

When I built my latest Javelin, I didn't want to mess around. A lot of time was spent agonizing over differential fluids, tires, paint for the driver,  and micrometers. 

Originally, I was going to install a strong BL motor and ESC, but I ended up using a HW 17.5T BL that didn't make it into another project.

After a lot of research, the steering servo is a Hitec HS-7955TG. Not only is it fast *and* powerful, not only does it have titanium gears, it does not require high voltage. No brownouts, wicked fast, robust, and enough torque to make even the most recalcitrant buggy listen up. 

At least that was the plan with the original idea I had for the motor. If a powerful servo is good, a more powerful servo is better and too much speed and power is just right. Right?

Because the inside of a Javelin is very cramped, and because the nice thing about a Kyosho kit is that if you follow the instructions to the letter, everything fits and aligns perfectly, I used the Kyosho servo arm supplied in the kit.

Big mistake. This servo is fast and powerful, so fast and powerful that I keep breaking or stripping out parts in my steering mechanism, especially the servo arms.

This servo was designed to convince a fast, heavy, runaway buggy that doesn't want to turn left that it's going to turn left, right now and no talking back.

And it works. When it works, the steering is a sight to behold. The problem is that this servo will also find whatever failure points and weak spots are in your steering mechanism. You may not know about them, but you will.

I've installed a metal servo arm now. It was the only thing I could fit, and even then a lot of Dremel was required. We'll see how it goes, or whether that just means that I find new failure points that I didn't know I had.

 

[Jonathon Gillham 4605]

That servo sounds ideal for racers...

Did you adjust the endpoints on the steering? I have to restrict steering throw on some cars, otherwise the servo appears to twist the chassis when it goes beyond the max steering angle, and this was with a Savox 1258TG with a lot less torque. I've seen some people running some crazy powerful and fast servos (Futaba 9373SV) in their 10th cars without problems, but they need setting up right. Probably doesn't help now though since you've already made the alloy arm fit.

[MadInventor 3884]

Like when I found out that gearing my War Rig to do 30mph was probably a mistake, especially as the 2400Watt 6S brushless boat motor would pull that speed without a problem. Very, very twitchy on the steering made it terrifying to drive at that speed with 2 trailers behind it, I particularly did not want a high speed roll over with trailers made  with 1 off 3d printed parts. I didn't roll it, but then managed to twice at 18mph (New top speed after I rebuilt the transfer case with additional gears to slow it up). Still, it's all fun

[Juggular 4961]

hmm... I guess that makes sense.  A shoulder cannot lift 1000 pounds even if a robot arm could.  It is only as strong as where it's attached to.  We see that with an aftermarket shock tower withstanding impact, but the plastic chassis can't.  I'm sorry that you have gone through that experience. Thank you for letting us know, though. Collectively, we learned something.  Maybe Jonathon's suggestion of limiting the throw would work?  

 

[Mrowka 493]

FWIW, I did limit the throw. I intend to limit it some more, even though the steering works great ("works a treat" for non-US English speakers) when it works.

Seriously, the crisp steering is a glorious sight, when everything is working as it should. That's the worst part - it's awesome and I don't want to give that up, not if I don't have to. Otherwise, I'd replace with a spare 3004 and call it good.

And yes, this is more a servo for the full-on racing set.

[Mrowka 493]

Update, for anyone who cares.

Ran the buggy again yesterday evening. Managed to loosen the screw that holds the steering pivot ball to the servo arm. Twice.

Took the buggy home. Found out that the little black steering pivot ball looks and feels like it's made of plastic, but it is magnetic and therefore made of metal.

Threadlock, it is, then! That and a slightly longer screw.

I tested the setup this morning. Worked flawlessly. You have to be careful on grass, since this steering is so quick that the car can flip.

I also noticed that my servo arm is not quite vertical. Probably one or maybe two teeth off. I really don't want to "fix" it, now that everything is finally working right for now.