Servos seems a little of an odd subject but for their size they, and there associated parts, do seem to cause an inordinate number of problems! So its time to examine some of those and but some of the problems to bed.
First up is the amount of slop in the Tamiya servo runs, particularly in the Euro trucks. Every link has around 0.2mm of play and there are 6 in a standard euro truck plus the down link and the servo saver, and then there are two more in the cross chassis link. That is a lot of possible slop.
First up is the standard servo saver, it tilts and looks generally insecure, it comes with a plastic spring and one of those sloppy ball links.
Next along the line is another ball link to the transfer arm. The shaft as supplied is a sloppy fit in the transfer case bearings and replacing bearings with ball ones is not going to help. There are commercially available CNC machined units with much better tolerances but...
So it is possible to reduce all that mess with some work. Replace the servo saver with one that has metal springs ( Tamiya pro, Kimborough etc etc), just hold that thought.
Next up would be replacing all the Tamiya ball links with something better and there are a couple of options, one is a ball and cap and the other is a captive olive set up.
Replacing the rod ends with ball cups is probably the cheapest but they tend to have a fair bit of flex so may not be the best in the long run
My preference is for captive olives and replacing the ball with a 3mm screw and nut. These tend to be more robust and although a little larger than ball cups are in my opinion better, they are available in metal or plastic. The plastic seem to be more than man enough for the job
The standard Futaba style S3003 servo is what Tamiya recommend for trucks, unfortunately there are some really cheap and nasty "copies" out there and the copies don't perform anywhere near as well as the original and will jitter, not centre correctly and my burn out, so if you are using these, please ensure that they are original. The only way to ensure that is to buy from a reputable model retailer. The S3003 is a great servo for lightly loaded trucks on most surfaces , there are of course other choices from similar manufacturers.
Some surfaces and loads require a little more oomf from a servo so a good choice is to go for a heavy duty servo with metal gears ( there is no point in having extra torque if the gears give up), and again there are some good ones and some that you would not want to use, an example is the tower pro MG995 which the aero guys labelled as dangerous as it does not reliably repeat and the dead band is awful. The 995 suffered from cheap copies, some with smaller motors in them and whilst the MG996 fixed a lot of problems cheap copies seem to have got relabelled to 996. So beware.
Good digital servos give much better centreing and repeatability so it would seem that that is the way to go.
If you use higher torque servos then if they limit or are put under high load they draw a lot of current and many esc's ( and I include Tamiya MFU, Servonaut, Beier and hobbywing in that) have Battery eliminator circuits that will grey out and cause glitches, MFU restarts etc. This will aslo happen if you remove the servo saver! You can solve that by using a UBEC ( universal battery eliminator circuit) to supply the servo but that can just move the problem up the line.
So what to do? Well The best option has to be to reduce the number of links to a minimum. That would mean mounting the steering servo at the front of the truck and that can present a few problems in itself, the good news is that the US truck servo mounts are on the universal sprue in every kit!
We cannot give an exact guide as all servos differ in size and it will depend on whether you have the tamiya motor/ gearbox ( and whether you have that locked in a particular gear) or whether you have an alternative motor gearbox. What we can say is that the servo linkage needs to be as parallel to the axle as possible or that the angle from the axle pivot to the servo link attachment point is the same as that on the servo, having said that any angle will lead to problems of unequal throw on either side of the steering (unless the servo is central and you use one link to each wheel and no cross bar). In general the servo saver arms are too short but Active Hobby Products from Japan make a metal upgrade set for the arm and cap of the Tamiya servo saver.
Once you have everything in place it is essential to check a few things:
- check the throw on the servo and make sure you have equal steering on both sides.
- check that the servo does not limit and travel end, or that the servo saver is loaded.
You can do both of these by using the adjustments on your transmitter to adjust the end stops and the throw on each side. Of course that does assume you have a half decent transmitter/receiver set up. If you dont do this or if you dont do this and chuck the servo saver then when the servo limits it will draw more current which can lead to two things:
- servo fails with burnt out circuitry
- bec in the esc or mfu etc fails - oops that could be expensive.
We hope that this has given an insight into the possible problems and solutions attached with steering, linkages, servo savers and the servos themselves. Please feel free to mail us if there is anything you think we could add.
I asked some people to show me their solutions for the front mounted steering servo and this was the result.
This is not my favourite but I included it as the it shows the initial arm at right angles. Of course there is no motor in the way. ( Karl Eastwood)
This one comes close but the servo arm is offset, that may be to account for the slight angle in the servo rod though ( Mark Ling)
This one is, in my opinion, perfect. The servo mount came from ebay but you can see the principle. (Pete Gale)
Getting the steering to work correctly will enhance the driveability of your truck and give you a better experience.