Monday, June 24, 2013

Dual steering servos for E-Revo VXL

Installing a second steering servo on the Traxxas Mini Revo (1/16) VXL is an easy and fairly inexpensive upgrade (about $30 in parts). The E-Revo VXL has already second servo mounting position on the chassis - it is just closed off with a blank cover.
Parts needed - Traxxas 2080 Mini Servo, Traxxas #7043 steering arm kit, link ends and some screws.
There are a few advantages to dual servo steering - the steering is more precise and it holds on better at high speeds, the extra torque provided by the second servo allows you to run much bigger tires, which could otherwise overload  and possibly damage a single servo. Additionally, this setup decreases the wear-and-tear of the servos as the steering load is evenly distributed across both servos.
The main challenge with mounting a second servo is to make sure that both servos are mechanically linked to the bell crank steering arms in such a way that their center (zero) positions (radio: 0 trim, 0 sub-trim) will match perfectly - otherwise they will work against each other!
If their center position is not exactly the same, when linked with the standard, fixed, non-adjustable links, they will "fight" for the center position "ownership", each pulling its side of the bell crank to its own center (zero) and this will cause a premature failure from the constant load (not to mention the increased electrical current).
Some drivers, who use aftermarket 3-channel radios (like the Spektrum DX3S) suggest the use of the AUX channel for the second servo control and then enable the steering (STR) channel mix to the AUX. Doing so brings some serious limitations due to the poorly designed firmware in DX3S - the steering trim on DX3S does not affect the mixed AUX channel - same thing goes for the sub-trim adjustment! If trim is added to the steering channel, AUX will still stay fixed on its own position causing servo binding. Mixing works only during steering wheel commands but not with any of the trims adjustments. AUX channel has its own servo trim function but using it means that one should adjust the steering trim first until the car drives straight with only main servo (STR) linked to the bell crank arm, then adjust AUX trim until spacing between the steering arm and the second servo horn is exactly as the fixed linked and then install the link between that servo and steering arm. This is a severe limitation - no steering trim will be possible on-the-fly without removing a mechanical linkage from one of the servos and going through this procedure again. It turns trimming of the steering into a rather slow and painful process - forget about quick trims when suspension or wheel alignment is adjusted. In addition, the "fighting servos" condition will still exist if the car (ESC) is powered on with no radio turned on to apply trims.

The other way (better IMHO) to do it is to drive both servos from the same steering channel - the original Traxxas receiver has two ports for CH1 or if using a Spektrum receiver one can use a Y-split for the steering channel. Then, the difference between servo's centers is compensated by using an adjustable length mechanical link - turnbuckle or regular screw type adjustable link will do the job. Things are never simple tho - the spacing between the steering arm and the servo horn is very small and a custom adjustable link must be fabricated to fit.

I just used two plastic toe link ends (GPM toe links), trimmed them short enough so their combined length when put back-to-back is just a tiny bit less than the original Traxxas steering link and used a M3 screw as a push rod between them (screw head removed of course)
This approach worked perfectly. I connected both servos to the STR channel on my Spektrum receiver and set the radio to 0 trim, 0 sub trim. Using the original Traxxas fixed link, I linked the servo with the best 90-degrees-servo-horn-to-body position to the steering arm and then connected the other servo with the adjustable link so there was no binding (no buzzing or humming from any of the servos). Using steering sub-trim,  I made the bell crank arm exactly perpendicular to chassis.
I also set the radio for servo travel adjustment of 116% and it seems to give me a little more steering range.
That's all there is to it - steering trim is now available as usual. Adjustable link is not needed if you are really lucky to have both servos matching their center positions, but this is very unlikely due to the way the splines are made in the servo horn - normally, the servo will take the horn exactly at 90 deg. on one side but it will be off when placed 180 degrees from that position (which is actually required for dual servo setup).
I installed metal servo horns but they have exactly the same splines as the plastic ones.
Another mod  I made to the steering system is to replace the plastic bushings in the bell crank with real bearings - Traxxas #5114 - 5mm x 8mm x 2.5mm / sealed which made movement absolutely smooth.

1 comment:

christopher bair said...

what are the exact parts and where can i find them becuses i would love to do this