Savage FLUX: The JUMBO-Servo Conversion

Last edited by offroad-CULT on Sat Oct 24, 2009 3:53 am; edited 1 time in total

First of all, the Savage Flux together with the latest upgrades (namely the bulletproof diff gear set) is truely an outstanding monster truck, in almost any way.
However, as the power increased over the last few years, I observed that competent steering mechanisms got more and more neglected - in fact, they just remained in the 30km/h territory of the mid 90's monster trucks!
And the Flux, as sturdy as it may be, is no exception here: There is a weak steering servo, joined to an even weaker servo saver saver that gives in at around 60-70Ncm of torque applied to the wheels. Not only once did I notice its front tires pointing in almost the opposite direction of my thumb on the stick-type transmitter.
Inadequate steering is difficult to notice due to its insideous nature: When driving slowly, steering may seem sufficient with a nice and tight turing circle, but once we get up to speed (and we're talking FLUX speed here, right?), the built in caster (and the Savage has quite pronounced caster to increase its rough terrain handling) puts a serious load on the steering that simply flattens the stock steering mechanism altogether.

Sometimes in May I decided to do something against this, and I quickly realised, that standard-sized servos may have impressive torque figures, but they are just not tough enough: Broken gear pins and cases are not too uncommon for high performance servos in demanding situations. And since I didn't like the nitro-style rc-box in the Flux either, I thought, I'd better start from scratch and go the whole hog:

HiTec's HS-5745MG is a nice big-scale servo, that dwarfs the Savage stock equippment with its 161 grams, 3mm gear pins, 3 ball bearings and 5mm thick 48dp full metal gears. At 240Ncm, it may not be much stronger than the best of the standard sized servos, but its built to last, and at only 60 Euros (here in Austria...) it's cheaper than the Ace-RC/Thunder Tiger DS1015 in the picture above. Additionally, the case is completely sealed with o-rings, so the HS-5745MG is dust- and splashproof.

Putting such a big servo into a stock savage may seem to be a difficult task, but once the rc-box is removed, you can see just how much unnecessary clutter it adds to the FLUX.

To install the Jumbo-Servo, you'll need a pair of chassis braces. Mine are from GPM (SAV1016) but you can also use HPI's own motor plate braces from the Savage X.
These have an m4 thread on their sides, so you'll need to drill the the front 3mm hole in the TVPs to 4,1mm. Additionally, you need another 4,1mm hole that is exactly 83,5mm behind the first one (see the picture for reference)

On the braces themselves, the top-bottem holes have m3 threads. They need to be drilled to 4mm for the front brace (since the steering posts are threaded) and the rear brace needs an m4 thread (drill to 3,4mm, then tap the larger threads)
With the braces spaced 83,5mm apart, the rear brace will almost touch the gear box.

You can see, that I already installed a new servo-link, made of standard 1/8th scale componenents. From rod end to rod end, it should be approximately 55mm long. Fine tuning comes last, so make sure that you use an adjustable turnbuckle.
You can also see, that I used GPMs servo-saver-less (we won't need it anyways) steering. The Jumbo-Conversion also works with the stock steering, but make sure that the servo-saver is locked.

Now that we have a sturdy base to work on, it's time to come up with a servo/receiver plate that can take the new hardware. Mine version above is a simple 2,6mm thick carbon plate, loosely cut to these dimensions
The slots in the place let you mount the servo in the upper position (for the stock steering) or in the lower position with the servo tabs below (as in the picture below)

You can now bolt the new rc-plate right into the chassis:

For the front, you'll use M4x16mm screws that thread into the steering posts, and in the rear, M4x10mm screws will do.
The new antenna mount is in fact an old HPI Sprint GTI servo mount, bolted to the TVP where the stock radio box has its rear mounts.

Now, you can adjust the length of the steering link, so that the servo horn and the steering arm are parallel to each other, and the servo horn is 90? to the TVPs when the tires are straight.
Note that the HPI servo horn in the picture will not hold up to the power of the jumbo servo. Mine stripped on the first run. However, HiTec supplies carbon reinforced plastic horns with the servo, and they work flawlessly. You could even go with an alloy servoarm.
The servo linkage seems close to the outdrive-cup, but in reality, there are a few mm in between, so there's no chance of the servo accidentially catching the outdrive.

You can add a small rc-box for the reciever, or (with those tiny 2,4GHz receivers) you could even bolt the BL-Controller right beside the servo to bring some weight to the front.

With this new, "FLUX-worthy" steering upgrade, the Savage's high speed handling and turning ability is increased dramatically (in fact, I had to turn down the dual rate setting)
The tires now turn effortlessly even when standing on grass, and driving on it, as well as any other medium to high grip surface really is a whole new experience!

At first, I've been a little concerned using the jumbo servo without a servo server. But such a big servo is virtually undestructable in an 1/8th scale, and the tie rods also hold up just as well.
I'm driving my Jumbo-converted Flux since May, and I haven't had any problems since - just a truck, that now is even more fun to bash around at high speed! Here's the original thread

Congratulations - Very original and outside-the-box thinking. Well written and illustrated too - a very good presentation.

Very impressive, though the stock servo is plenty for my needs Laughing

Very interesting.

I never felt stock gearing was lacking, although it is obvious that wheels don't turn well when standing still. But that does not worry me since I bought this truck for driving it.

Now that does not mean that things could be a whole lot better with this modification.
Yet I don't feel such a pain with stock steering that I would actually need to experience this setup before deciding to upgrade.

nice write up. are you useing a bigger battery? does the the bigger servo need more power?

LaMe.

Quite a response here, thanks so far for the comments and opinions!

pdcsq, when assessing steering torque, one must differentiate between low speed and high speed handling. Low speed handling is, of course adequate as I've written. But looking at high speed handling, the suspension's caster angle places a significant load on the steering so that we face the same situation, when the truck is standing still and trying to steer: Positive caster shifts the steering-angle backwards which creates leverage for the centrifugal force to work on and against the servo. The centrifugal force in turn, (simplified) increases exponentially with the corner speed, so you'll eventually reach a point where it completely annihilates any steering input, if the tires don't lose their grip beforehand. This however is never the case with most higher grip surfaces or while steering under heavy breaking - both situations, where the stock steering clearly works sub par.

The mod is really worth it, if you're looking for a more responsive and consistent, "buggy-like" steering.

ralphie, the HS-5745MG draws the same current as similarily specced standard sized servos (e.g. HPI's own optional high torque servo)
The Blur controller has a switching BEC and hence no problems with the servo under full load/stall.

arisking, care to elaborate a bit on your juicy, if now a bit toned down comment? Smile

Good mod, I love the precision of the cutting. Did you get the parts cut for you on a machine?

I love this solution suits the savage alot better then small high torque servos. I have noticed on my last few goes out that when at high speed I turn a corner if goes but then the wheel stop turning as it becomes to much for servo to take (I have a Hi-tec, high torque).

Just wich I could do this on my Nitro. Might see if there is space in the cen radio box to fit the bottom of the jumbo servo in while still have space for the radio gear.

Anyway nice work, real clean looking

Good stuff. With the flux, there is plenty of room up front now compared to nitro, so there's virtually no reason not to go big!

Nice write-up, I've added it to the FAQ!

interesting mod. i cant say a large and heavy power hungry servo is something id put in my truck, but it is different.

rock crawlers seem to be the hardest on servos as they run no servo saver and and are vulnerable were they are mounted. the hitec 7750 ti gear servo is about the toughest, never heard of a failure yet. as most guys say, this servo can push the rocks out of the way at 403oz/in at 6v, the servo is also rated at 486oz/in at 7.4 volts.

Nicely done. I am considering doing something similar. Not for the steering because I already swapped the stock servo with a 20kg/cm Sav?x servo. But I have a datalogger and some additional electronics in the receiverbox. The receiverbox is too small for all the stuff I put in there. Your mod seems to create a lot of room.

deejay, of course you could even lengthen the rc-plate all the way to the front gear box, so that you gain another 20x74mm of free space for additional equipment. The stock nitro-rc-box really is a big, useless chunk of wasted space with the FLUX.

I didn't do this for the sole purpose of making the steering and the front gearbox more maintenance-friendly, in case you're wondering.

Hmm, I might do the same. But I think I will make a box to protect the electronics. The only drawback is that I will have to find a way to mount the servo. Since is is much smaller than the jumbo I have to cut a smaller hole.

I'm certainly not debating the mod, but the stock HPI SF-5 servo is actually rated at a slightly higher torque than the Hitec HS-5745MG. I know Hitec makes highr quality servos and being digital makes a difference, but what am I missing here?

ozzynichols, are you sure?
Because Hitec lists the HS-5745MG as beeing twice as strong (besides it being noticably quicker)

Besides that, a standard-sized servo, though there are some of the same torque, will just crack or break its gears in a setup like that. You don't only want more steering power, but beefier parts as well. With the servo, it's the same as with everything else on this truck - why do so many people convert to CEN/HPI-bulletproof diff gears...? Wink

Yeah you're right, I was looking at the Tower Hobbies Posted specs, not the one on the official Hitec website. Guess I'll know better from know on than to get my info from anywhere besides the primary source.

I was told that replacing the steering crank post with an alloy one or at least replace the bushings with a bearing did a lot to improve the steering experience. Do you have the same experience? I am inclined to believe it because I now have a 20kg/cm Savox servo and sometimes it still isn't steering correctly. Maybe the servo is pushing the crack all the way and the movement isn't transported to the wheels because of the give in the post.

I have a 7955TG in my savage and it is rated at 333 oz at 6v whereas the 5745MG is only rated at 250 oz, the 7955 also has titanium gears so is there any benefit I am missing in going to a larger servo over the something like the 7955?

http://www.hitecrcd.com/servos/show?name=HS-5745MG
http://www.hitecrcd.com/servos/show?name=HS-7955TG

Sorry, just battery sucking is all I would be worried about. I'll stick with my Ace.


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