Servo mounted vertically to chassis in a Wraith

[new2rocks]

I've been running the VP CMS/panhard kit in my Wraith since I built it earlier this year but had some problems recently. In the course of solving those problems, I came up with an interesting solution that I hadn't seen before (which isn't to say that someone hadn't thought of it before), so I thought I'd share.

The first problem I started noticing was the VP CMS bracket starting to slide a bit along the lower frame rails after 15+ hours of fairly aggressive running. Shortly after that, I noticed that the wheels didn't seem to want to turn all the way to the right when the rig wasn't moving. They would move a little bit, and then the body would begin rolling to its left. Once I began moving, the body would settle back down and the wheels would turn.

I tried tightening the CMS clamps on the lower frame rails, but that didn't completely eliminate the slop, and it didn't really help the steering/rolling issue. In addition to the slop with the CMS bracket, I also noticed some flex in the front of the Wraith chassis when I turned the wheels. I then tried the STRC CMS bracket, which uses set screws on all four rails (as opposed to set screws on the top and clamps on the bottom), and the RC4WD aluminum front bumper supports. That completely eliminated slop in the CMS bracket and flex in the front of the chassis, but it still didn't solve the rolling issue. Even playing around with the location of the bracket moving it slightly fore and aft to adjust geometry didn't help. After eliminating the slop and flex didn't work, I decided that the primary culprit must have been the horizontal orientation of the servo on both CMS brackets, which was leading the body to roll when the servo pushed on the tires and the tires provided too much resistance to turning.

The solution I came up with fixed the problem completely by putting the servo in a vertical orientation. To make this work, I pushed the CMS bracket as far forward on the rails as I could and added simple servo brackets (stock would work, but I used aluminum) to the back of the CMS bracket with the servo screw holes facing up. I had to dremel out the inside of the front bumper supports just a hair to make room for the servo case. After that, the servo screws right into the brackets from above with the same length drag link as before. The added benefit of this setup is that it makes water less likely to want to find its way up into the servo than when the servo was mounted horizontally (though I recently switched to a waterproof HH servo, so it shouldn't be a problem for me either way at this point).

Here are some pix (pardon the dirt from today's run and the temporarily improvised magnet for the hood latch):








Let me know what you think!

[TwistedMetal]

That's a good idea. Makes it look more realistic first of all. Not sure how it cured the body roll problem though. Does look good though.

[new2rocks]

Thanks. My take on the roll issue was that the horizontal axis of rotation of the servo horn made it easier for the body to roll when the tires provided resistance to turning. The servo basically continued its movement by rolling the chassis along the pinion axis. With the vertical orientation, when there's resistance from the tires, it doesn't get translated by the servo into rolling the chassis along the pinion axis, because the pinion is upright. Anyway, it's worked well and was a fun little experiment.

[TwistedMetal]

Makes sense in that concept. I was already considering doing it that way anyway since that's how I did my scx10

[KBR97]

What is stopping the mount/servo from sliding on the frame?

[new2rocks]

4 grub screws anchor into the frame at each corner of the STRC bracket. It doesn't budge and actually adds strength to the front of the chassis (which had started to flex with the VP bracket that I was using before).

[Eric0424]

All this trouble just to get the servo off the axle. Seems a bit much for something that's purely esthetic, but I'd be lying if I said I haven't put more effort and expense into something I felt just looked "better".

Don't care for the set/grub screws on servo mount though.

[new2rocks]

All this trouble for a bunch of tiny little trucks... Having tried two different CMS brackets, I find the 4 grub screw design much better than the VP system of two grub screws at the upper corners and two clamps at the bottom corners. The clamps at the lower corners end up coming loose and sliding when the servo is working. That hasn't happened with the STRC bracket (nor will it).

[Eric0424]

Originally Posted by new2rocks

All this trouble for a bunch of tiny little trucks...

Yeah, right....

I was thinking about spacers on the frame and bumper posts to position the servo mount, but that may be more trouble than it's worth and might not work too well on the curved sections.

The clamp style could possibly hold a little better if the inside of the clamp were scuffed a little, or if it had a slight rise/ridge in the center to dig into the plastic when tightened down.

[wouterb]

I guess you figured this out already; but from the pics it looks like the panhard might be hitting the top of the servo axle at full compression. I assume to much thought went into this to let that happen.

[new2rocks]

Originally Posted by wouterb

I guess you figured this out already; but from the pics it looks like the panhard might be hitting the top of the servo axle at full compression. I assume to much thought went into this to let that happen.

I assume you mean the servo horn, and the answer is that it clears. By microns, but it clears. And that's with an Axial servo horn, which is rather bulky. If I switched to something with a lower profile (like Racer's Edge), it would have a bit more clearance.

Too much thought...you give me too much credit.

[TwistedMetal]

Hey n2r, any way that you can get me a close measurement from the bottom frame bar to the panhard hole?

[new2rocks]

Center of frame bar to center of Panhard mounting hole is 12mm.

[TwistedMetal]

Sweet. Thanks man!