A fellow wrote inquiring about a suitable servo arm for a 1/10th scale model. Only fly in the ointment being . . . he didn't specify which servo, or which model, he just said 1/10th! This could mean a Traxxas TRX4, an Axial RBX10 Ryft or SCX10 Pro, or any of *several* buggies. After all, 1/10th is the most popular class on the planet!
So I asked for more info and he quickly responded with more questions and *still* didn't say which rig, or which servo he was interested in ordering!
So lacking enough info regarding for what servo *or* which rig, meant he kind of left me in the position of responding to an open ended question - otherwise known as cruising blind. And here's the thing; responding to these types of queries often results in a long winded answer. This, because as an engineer I learned when the boss asks open ended questions, you'd better cover all the bases!
Of course, this also means his relatively simple question has turned into fodder for an askJOHN, which you may find useful if you're also wondering the same thing. Especially if the thought occurs; why do we produce so many different servos and arms suitable for 1/10th scale rigs?
Well here goes with his initial query and his follow on questions . . .
Q. Before I order, which servo arm do I need for a standard size 1/10th scale servo?
A. I'm sorry, but there's no such thing as a standard size servo for 1/10th scale. Reason is;
- servos don't know in what they're being installed,
- we produce more than a dozen standard-class servos,
- complicating things, there are 2 different splines available.
- and all these servos are suitable for installation in 1/10th scale.
. . . so I need more information, please. He quickly responded with;
Q2. I just need to know, which servo is best? Also, how do I decide which arm to use?
A2. As usual, the answer to what's best is . . . it depends. Like what's best for a 12 y/o dinking around in the backyard, and what's best for a grown ass man blasting around in the boonies with his buddies, or what's best for a kid, but one competing may all be the same, or they may all be different. So I dunno how to answer without more information.
Anyway, before discussing available standard class servos (before we address his question about an arm), let's first address the 2 different available splines to see if this matters to you - these are;
. . . important because the latter is significantly larger than the former, and this alone may inform your decision. Thus, allow me to share a photo that demonstrates the difference because a picture really is worth more than a thousand words!
- Adopted from our quarter-class servos, the gnarly 8mm spline dwarfs the 6mm spline
Note; both are standard size servos (20x40mm footprint), the difference in spline size is readily apparent. Bigger is better for applications where surviving to fight another day may be crucial.
Anyway, I'd bet a milkshake the fellow who wrote me is quite likely more interested in selecting from amongst our standard size servos with 25T spline (DLS, BLS0, and BLS1), but who am I to say? Since it's his decision (meaning not really any of my business), let me mention this; our BLS2 servos (8mm spline) are the most expensive we produce.
And no, not making a judgement about someone's wallet, just explaining so you can decide what's best for you. Basically, what makes the BLS2 series somewhat pricey are features which, differentiate them from everything else available (whether from us, or our competition).
What features? Basically, they're more ruggedly built. Larger bearing, larger gears, larger internal shafts, and immediately obvious from the outside, the 8mm spline shaft. This is what immediately tips you off that BLS2 servos are not ordinary because that sucker is huge!
However, since they all have a 20x40mm footprint, this means they're all also well suited for 1/10th rigs, and the guy didn't specify. So this photo shows you internal differences between one of our BLS2 and a very well regarded competitor's offering, the Savox SB-2290SG servo.
. . . as to why do I show you this picture? Basically, if we don't nobody else will. This is because our competitors don't usually show you the guts of their servos. You see only the outside, not the inside. That, and maybe you hear guys touting them, but you don't know these folks, or if their criteria is the same as yours. Since you can't cast your own eyeballs Mk II on what's inside, we show you. And as we all know, the only thing that really counts is what's inside!
Anyway, straight up, BLS2 servos are usually the domain of professionals. Pros for whom spline shaft failure is a big deal because it means not finishing. This is because in competition, a DNF (did not finish) is to be avoided at all costs because you can't possibly win if you don't finish!
And look, you also must take into consideration how racers may travel hundreds if not thousands of miles to compete. Means they'll do *everything* in their power to avoid not finishing. This includes using our BLS2 servos (despite their greater cost). And better strength and thus, reliability is down to things like the gnarly spline and more more rugged internals. Bigger really is better because it's what makes them more durable. Of course, if this isn't an important consideration for *you*, then just select from amongst our 25T spline servos and save yourself a few bucks - this becomes an easy peasy decision!
By the way, since I mentioned it, if you're curious what I mean about spline shaft failure mode (and since we use the best steel on the planet, this is useful information for everybody because *all* 6mm shafts may fail in this manner - regardless of brand). Means it's probably worth your time to review this brief article, where we detail the repair process of a pair of DS505 servos:
Note: the DS505BLHV is far and away our MOST popular servo for 1/10th scale rigs. Also, clicking this servo-link opens a new tab on your browser (so you don't lose your place).
Major point being, all of the servos series mentioned above have a standard class footprint of 20x40mm, which is what's used in 1/10th scale rigs. And I'm very sorry for the long winded answer *but* had you inquired about a specific servo, then I could respond with certainty about both the servo and the servo arm. But right now, I don't know which servo you're considering so here's a link to all of ProModeler's different standard-class servos.
And allow me to warn you - don't give into temptation - and select from amongst DLS or BLS0 servos to save money and instead, stick to either BLS1 or BLS2.
Why? It's because the former utilize hybrid case construction, which simply isn't as rugged, whilst the latter are all alloy. Both use steel gears so this is purely a consideration of the case construction in regard to durability.
This photo shows the insides of a BLS2 servo;
- Note the difference in the output shaft and also the gear it's part of for both servos
This next photo shows you the guts of a DLS (and BLS0) type servo. Focus on how there are o-rings beneath the Allen bolt heads. And more o-rings at the case sections (X2), plus another o-ring at the output shaft spline. There are 13 of them in total.
The servo also has a center section which is CNC-machined from a solid billet of 6061-T6 aircraft aluminum. The center has cooling fins machined expressly to help shed heat when the servo is working hard. Cheaply made servos use an extrusion and don't have the fins, either.
Also, these servos all have rugged all-steel gears. Plus there are also hardpoints made of brass (insert molded into the case sections - these are the shiny bits you can see in the case top). These are what reinforce the glass-filled nylon case sections where the gear shafts are fitted.
- DLS and BLS0 are for light duty applications - crawlers and bashers should look elsewhere
But it's this last part, the plastic case components, which principally see me guiding you away from the servos with this type of construction (DLS and BLS0) and toward a BLS1 or BLS2 servos, instead. Simply put, it's because the plastic can break.
Next thing to consider is servo height. Within the standard class (20x40mm footprint), there are also low profile servos available. We only make the one, the DS490BLHV because anything outputting less torque would just be a few bucks cheaper and it seems pointless to make a 350oz-in servo instead of a 490oz-in if the savings amount to 5 bucks, take my meaning?
And in the same vein, it seemed stupid to make it with the 6mm shaft so for our only low profile offering, it's a BLS2 with the gnarly internals and the big ass 15T spline. This is our low profile DS490 beside the DS635 . . . both are standard class.
- DS635BLHV on the left versus DS490BLHV low profile - the DS490 is great for axle mounting
By the way, if you're wondering what the model numbers actually means, here's the code;
- DS = Digital Servo
- xxx = torque rating in oz-in
- BL = Brushless motor
HV = High Voltage (to 8.4V)
And note, if you don't know diddly about servo motors, within this article we cut the different types open (iron core, coreless, and brushless) using a lathe expressly to show you what makes them different.
About RC servo motors
All manufactures use these three types so this is good useful stuff regardless of what brand you favor.
So to the question, which arm do I use? As I said . . . it depends, and that's true, so let's also see some of them installed side-by-side. This, to see what we may glean from their appearance, which may make one better for you than another.
Note; they all use M3 mounting hardware so the commonly available balls fit. The first two (left to right) are the ones with mounts at 15mm, 20mm, and 25mm). The outermost mounting point is 25mm, hence the 25 in the model number PDRS25. One is 15T and the other is 25T, so this is the -15T and -25T.
The two on the right are fork type arms (shielded linkage with aluminum alloy protecting top and bottom of the linkage. These are expressly marketed to Traxxas Sledge and UDR but may find a home on axle mounted servos, also where protection from rocks is important. Point being, you need to think around corners and avoid being boxed into just one use.
From left-to-right, these are;
- The two fork arms on the right are great when axle mounted as they protect the linkage
Wrapping things up
Finally, while I'm almost certain you'd be pleased with either a PDRS25-15T servo arm or the PDRS25-25T (15T and 25T respectively) because these are the two arms most often selected for 1/10th scale rigs, I've included the others on the off chance one of the fork arms will suit you. That, and I've mention specific applications (Sledge and UDR) since just because that's what they're marketed to doesn't limit them to use on those rigs, only, capsice?
Last thing, don't be put off by the slender appearance of the PDRS25-25T because in common with all the arms in our lineup, it's made of 7075-T6 alloy instead of 6061-T6 (7075 is a lot stronger than 6061), so that seemingly slender arm is MUCH stronger than the chinesium arms you're getting for cheap off Amazon. Appearances can be deceptive.
OK, I lied, this is the last thing. You may be wondering if you should fork over $100 for a DS505BLHV when for 60 bucks you can buy a 45Kg-cm servo off Amazon, and it has steel gears and an alloy case. In fact, we're asked this so much we bought one to take apart and show you the difference because as in all things in life, you get what you pay for.
Review this brief article and your questions on this score will be answered in full:
Anyway, if there are any errors or omission in this article, just reach out and tell me, please, because these are living documents meaning I can revisit and edit to make it better. After all, only God is perfect. The rest of us are doing the best we can.
Call 407-302-3361, or email: info@promodeler.com to reach me directly.
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John