Coreless digital servos with stock radio?

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Londoner

c0ckney geezer!!
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Hey all, I'm just about to splash out on a new steer servo, a hs-7955tg to be exact, unless anyone else can give me a better option for around the same price?

But my main question is will a digital/coreless servo work with the stock HPI radio gear without any issues?

Thanks for reading!!
 
I would like to know this as well as I have been looking at the same servos. When someone answers maybe they can help out with the torque rating on the HPI SF-5 versus the hs-7955tg? Not to hi-jack but I seem to be having trouble finding how much torque the HPI servos put out versus the aftermarkets.
 
MikeK1981 said:
I would like to know this as well as I have been looking at the same servos. When someone answers maybe they can help out with the torque rating on the HPI SF-5 versus the hs-7955tg? Not to hi-jack but I seem to be having trouble finding how much torque the HPI servos put out versus the aftermarkets.
Click to expand...

Here are some sf-5 specs i found after googleing it, not sure how accurate they are as it came from another forum.


SPECS: Length: 2.15" 54.5mm
Width: 0.79" 20mm
Height: 1.5" 38.1mm
Weight: 1.64oz 46.5g
Voltage: 4V-6V
Torque: 314 oz/in 8.9kg/cm
Speed: .20 sec/60 degree (6.0V)

And the 7955tg

Control System: +Pulse Width Control 1500usec Neutral
Required Pulse: 4.8-6.0 Volt Peak to Peak Square Wave
Operating Voltage Range: 4.8-6.0 Volts
Operating Temperature Range: -20 to +60 Degree C (-68F to +140F)
Operating Speed (4.8V): 0.19 sec/60° at no load
Operating Speed (6.0V): 0.15 sec/60° at no load
Stall Torque (4.8V): 250oz/in. (18kg.cm)
Stall Torque (6.0V): 333oz/in. (24kg.cm)
Operating Angle: 45 Deg. one side pulse traveling 400usec
360 Modifiable: Yes
Direction: Clockwise/Pulse Traveling 1500 to 1900usec
Idle Current Drain (4.8V): 3mA at stop
Idle Current Drain (6.0V): 3mA at stop
Current Drain (4.8V): 200mA/idle and 2.0 amps at lock/stall
Current Drain (6.0V): 240mA/idle and 3.0 amps at lock/stall
Dead Band Width: 2usec
Motor Type: Coreless Carbon Brush
Potentiometer Drive: 6 Slider Indirect Drive
Bearing Type: Dual Ball Bearing MR106
Gear Type: Titanium Gears
Connector Wire Length: 7" (178mm)
Dimensions: 1.57" x 0.78"x 1.45" (40 x 20 x 37mm)
Weight: 2.29oz (65g)

The 7955 is also coreless and digital which i don't think the sf-5 is either, i don't know? also the hitec is full titanium geared i believe and has 24kg of torque over the sf-2's 8.9, tho the sf-5 is less than half the price on ebay.


PLUH said:
I ran the hitec 5985mg servo for steering and the 5645mg for throttle/brake on my stock radio system for about a month without any problems before switching to a dx3s
Click to expand...

Cheers man, I'm running the 645mg as my steer servo now, its ok but my sf-2 throttle servo is getting really weak now and struggles to open the carb with a throttle return spring fitted, so instead of buying a new throttle servo il upgrade my steer servo and use the 645mg for ny throttle/break.

Just didnt wana splash out to discover it wont work properly with the hpi radio gear, which will be replaced too, but in time.
 
I personally would stay away from digital servos. They suck alot more juice than standard ones.
 
Did I miss something? It says the SF-5 is 314oz/in. and the hs-7955tg is 333oz/in. of torque why the big difference in kg/cm? Maybe it's a typo and it's supposed to be 18.9 instead of 8.9?
 
Fx said:
I personally would stay away from digital servos. They suck alot more juice than standard ones.
Click to expand...

Yea i thought about that too, id read it somewhere before, could you recommend a non digi servo with similar spec? The 7955tg is only 60 something GBP here and most other servos iv looked at with similar spec are way out of my price range, sorry for the noobiness lol but this is my first time buying what i hope is a proper servo, but i don't mind buying a second hump to take out with me.

MikeK1981 said:
Did I miss something? It says the SF-5 is 314oz/in. and the hs-7955tg is 333oz/in. of torque why the big difference in kg/cm? Maybe it's a typo and it's supposed to be 18.9 instead of 8.9?
Click to expand...

Sorry bout that yea it must be a typo, heres the SF-5 spec from the HPI site, its 124 oz/in

HPI SF-5 SERVO (METAL GEAR/8.9kg-cm 6.0V)
Spacer
# 80594
High Torque Servo with Metal Gears and Ball Bearing/Futaba J Connector
Stock steering servo on Savage XL. Standard size and extremely strong. A great steering upgrade for any Savage monster truck!

Speed: (0.20 sec/60° @ 6V) and (0.25 sec/60° @ 4.8V)
Torque: (124 oz-in (8.9 kg-cm) at 6V) and (99 oz-in (7.1 kg-cm) at 4.8V)

Tough metal gears for durability.
 
I ran the 5645 as steering servo almost from day one as my sf-2's gears turned to paste. Ran like that for 9 months and then one day i acted all stupid and drove throug a PUDDLE. Shorly ofterwards my sf-1 got all finiky and died and the 5985 was all my lhs had. Ran a moth like that until i had saved up enough $$$ to rid myself of all the metalic inteference and got my 2.4gHz system.

Running two high powerd didital servos sure draws allot of power but with my hump pack i can go 5 to 6 tanks before a rechrage is required, granted i run a cen 220ml tank
 
Cool, thanks for that Pluh, so i guess id be wise to order a second hump or a higher mah hump with the digi servos, my current one is 1600mah, might see if i can find a half decent 2000+ mah hump cheap somewhere, id only be running the one digi servo tho, the 645mg should be fine on throttle/brake duties.
 
I'm confused, I did the conversion using onlineconversion.com and 8.9kg.cm came out to 314oz/in like the first post says for the SF-5 but that would mean the other servo put out 847oz/in at 24kg.cm even though it says 333oz/in. What am I missing?
 
Londoner said:
Yea i thought about that too, id read it somewhere before, could you recommend a non digi servo with similar spec? The 7955tg is only 60 something GBP here and most other servos iv looked at with similar spec are way out of my price range, sorry for the noobiness lol but this is my first time buying what i hope is a proper servo, but i don't mind buying a second hump to take out with me.
Click to expand...


Here's two that seems like a good option.

JR DS8711 Ultra Torque SX Digital Servo

Specifications:
Type: Digital Ultra Torque
Torque: 347 oz/in @ 4.8V, 403 oz/in @ 6V
Speed: .19 sec/60° @ 4.8V, .15 sec/60° @ 6V
Dimensions (WxLxH): 82" x 1.58" x 1.56"
Weight: 2.36 oz
Gears: Metal alloy


JR Z9100T Digital Ultra Hi-Torque Surface Servo (Metal Gear)

Specifications:
Type: Ultra High-Torque Digital Servo
Torque: 300 oz/in @ 4.8V, 370 oz/in @ 6V
Speed: .18 sec / 60 degrees @ 4.8V, .15 sec / 60 degrees @ 6V
Dimensions: 0.83" x 1.59" x 1.57"
Weight: 2.3 oz
Gears: Metal Gears


Airtronics 94780M MG Digital High Power Servo

Specifications:
Torque: 361 Oz/In (26.0 Kg/Cm) @ 4.8V , 423 Oz/In (30.5 Kg/Cm) @ 6.0V
Speed: 0.19 Sec/60 Deg @ 4.8V, 0.15 Sec/60 Deg @ 6.0V
Dimensions: 1.6 x 0.83 x 1.5 In (406 x 211 x 381 mm)
Weight: 2.33 Oz (66 g)
 
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Fx said:
Here's two that seems like a good option.

JR DS8711 Ultra Torque SX Digital Servo

Specifications:
Type: Digital Ultra Torque
Torque: 347 oz/in @ 4.8V, 403 oz/in @ 6V
Speed: .19 sec/60° @ 4.8V, .15 sec/60° @ 6V
Dimensions (WxLxH): 82" x 1.58" x 1.56"
Weight: 2.36 oz
Gears: Metal alloy


JR Z9100T Digital Ultra Hi-Torque Surface Servo (Metal Gear)

Specifications:
Type: Ultra High-Torque Digital Servo
Torque: 300 oz/in @ 4.8V, 370 oz/in @ 6V
Speed: .18 sec / 60 degrees @ 4.8V, .15 sec / 60 degrees @ 6V
Dimensions: 0.83" x 1.59" x 1.57"
Weight: 2.3 oz
Gears: Metal Gears


Airtronics 94780M MG Digital High Power Servo

Specifications:
Torque: 361 Oz/In (26.0 Kg/Cm) @ 4.8V , 423 Oz/In (30.5 Kg/Cm) @ 6.0V
Speed: 0.19 Sec/60 Deg @ 4.8V, 0.15 Sec/60 Deg @ 6.0V
Dimensions: 1.6 x 0.83 x 1.5 In (406 x 211 x 381 mm)
Weight: 2.33 Oz (66 g)
Click to expand...

Thanks FX, id love to go for one of those but unfortunately to get a high spec servo for a good price here I'm pretty much limited to ebay UK, our LHS and regular online hobby stores are a joke with their pricing, Airtronics and JR seem to be non existent on the bay here, i could get them on ebay US but theyd be well over our tax threshold and id get slapped with a hefty customs charge on that value, PITA customs!. lol arent those all digi servos too?
 
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I wasn't even paying attention. I told you to not get a digital servo then I link you to three digital servos. Talk about bad advice.....:duh: Hold on I'll browse around eBay.
 
Here's the problem with running digital servos with a stock(non-computer) radio this also applies to computer type radios that aren't setup properly. A digital servo will constantly hunt try to go to its commanded position. Here is where the problem comes is the EPA (end point adjustment) needs to be set otherwise your servo will try to keep going even though its limited by a linkage or steering stop eventually burning out the servo. I have a couple robot servos that I use for steering that will hunt when the truck is sitting still.
 
MikeK1981 said:
I'm confused, I did the conversion using onlineconversion.com and 8.9kg.cm came out to 314oz/in like the first post says for the SF-5 but that would mean the other servo put out 847oz/in at 24kg.cm even though it says 333oz/in. What am I missing?
Click to expand...

i used this 1 http://www.unitconversion.org/unit_converter/torque.html

333 oz/in = 23.97859428 kg/cm

314 oz/in = 22.610446258 kg/cm..

please note they note the oz/in and kg/cm as ozf/in and kgf/cm (f stand for force). so the value given by factory is correct.
 
I say get the hs-7955tg you intended on purchasing. If you can get it in the UK then it's just that much easier on you so you don't get nailed by customs. But just to reiterate, a digital servo will draw more power than a standard servo so make sure your using a hump pack that is at least 1600 mAh.
 
clemet said:
Here's the problem with running digital servos with a stock(non-computer) radio this also applies to computer type radios that aren't setup properly. A digital servo will constantly hunt try to go to its commanded position. Here is where the problem comes is the EPA (end point adjustment) needs to be set otherwise your servo will try to keep going even though its limited by a linkage or steering stop eventually burning out the servo. I have a couple robot servos that I use for steering that will hunt when the truck is sitting still.
Click to expand...

Good info I repped you.
 
Fx said:
I wasn't even paying attention. I told you to not get a digital servo then I link you to three digital servos. Talk about bad advice.....:duh: Hold on I'll browse around eBay.
Click to expand...

clemet said:
Here's the problem with running digital servos with a stock(non-computer) radio this also applies to computer type radios that aren't setup properly. A digital servo will constantly hunt try to go to its commanded position. Here is where the problem comes is the EPA (end point adjustment) needs to be set otherwise your servo will try to keep going even though its limited by a linkage or steering stop eventually burning out the servo. I have a couple robot servos that I use for steering that will hunt when the truck is sitting still.
Click to expand...

necromaster said:
i used this 1 http://www.unitconversion.org/unit_converter/torque.html

333 oz/in = 23.97859428 kg/cm

314 oz/in = 22.610446258 kg/cm..

please note they note the oz/in and kg/cm as ozf/in and kgf/cm (f stand for force). so the value given by factory is correct.
Click to expand...

Fx said:
I say get the hs-7955tg you intended on purchasing. If you can get it in the UK then it's just that much easier on you so you don't get nailed by customs. But just to reiterate, a digital servo will draw more power than a standard servo so make sure your using a hump pack that is at least 1600 mAh.
Click to expand...

Ok, thanks a bundle for all the info guys i think what il do is stick with my weak throttle servo a few weeks longer and get a 2.4ghz system at the same time as i get the 7955tg, i cant go wrong that way, ouch this will be an expensive couple of months lol
 
The problem you all are having with conversions is that torque is force * distance. So the units are oz*in and kg*cm. So take the sf-5 servo with 8.9kg*cm of torque is 123oz*in.

http://www.google.com/#hl=en&q=8.9kg*cm+to+oz*in&aq=f&oq=&aqi=&fp=flbC24gbdiA

This holds true for the 7955tg servo with 24kg*cm of torque is 333.3oz*in.

http://www.google.com/#hl=en&q=24kg*cm+to+oz*in&aq=f&oq=&aqi=&fp=flbC24gbdiA

What throws the conversions off is that at towerhobbies, the sf-5 is listed as 8.9kg/cm and 314oz/in, which do not equal each other. The big problem is that different hobby stores list the torque units incorrectly. They should always be force*distance, and not force/distance. So keep that in mind when viewing listed torque statistics for various servos.

http://www.google.com/#hl=en&q=8.9kg/cm+to+oz/in&aq=f&oq=&aqi=&fp=flbC24gbdiA
 
Londoner said:
Hey all, I'm just about to splash out on a new steer servo, a hs-7955tg to be exact, unless anyone else can give me a better option for around the same price?

But my main question is will a digital/coreless servo work with the stock HPI radio gear without any issues?

Thanks for reading!!
Click to expand...

Visionary just got this servo. i went cheaper & got the 5985. the 5985 has 172oz & .15 & is awesome in my 13# savage so i can only imaging the 7955 is better.

Now for the issue i see

if you run this servo without a digital Tx, you will be cranking out "Massive" torque and speed through the entire travel of the servo arm without a means to stop it on its own.

You can set "end points" with a digital Tx so the servo doesn't go to full extension & you don't stress the internals.

i ran a Digital Tx/Rx with analog servos but never the other way around.

Not sure this will or will not kill your servo, but end points are pretty important. & the 7955 is not a cheap servo.
 
Just a thought, but when i used the stock radio I notioced that when i adjusted the steering rate nob my travel on my steering servo would vary. With it fully opened i got full throw on the wheels, too much in fact and the carriers prevented the full travel of the servo. But when i dailed it down, i got less and less throw on the wheels to the point that i barely had any steering at all.

I don't know how this nob affects the servos speed and torque but you can just ajust the steering rate to the point where the carriers nolonger limit the servo's travel thus eliminating stressing the servo. It was my means of limiting the servo's travel.
 
PLUH said:
Just a thought, but when i used the stock radio I notioced that when i adjusted the steering rate nob my travel on my steering servo would vary. With it fully opened i got full throw on the wheels, too much in fact and the carriers prevented the full travel of the servo. But when i dailed it down, i got less and less throw on the wheels to the point that i barely had any steering at all.

I don't know how this nob affects the servos speed and torque but you can just ajust the steering rate to the point where the carriers nolonger limit the servo's travel thus eliminating stressing the servo. It was my means of limiting the servo's travel.
Click to expand...

Yes I do the same thing for my digital servo. I just adjust the steering knob and it reduces the throw so I don't stress out my steering components. Maybe freddy has/had a different transmitter. I know the one I got with my savage x has the knob though.
 
Fx said:
Yes I do the same thing for my digital servo. I just adjust the steering knob and it reduces the throw so I don't stress out my steering components. Maybe freddy has/had a different transmitter. I know the one I got with my savage x has the knob though.
Click to expand...

PLUH said:
I don't know how this nob affects the servos speed and torque but you can just ajust the steering rate to the point where the carriers nolonger limit the servo's travel thus eliminating stressing the servo. It was my means of limiting the servo's travel.
Click to expand...

naw, my Am Tx has that knob also.

i just can't see recommend that as a solution to matching a analog Tx to a digital servo.

is it really an upgrade to buy an expensive high torque digital servo and dial down the power with a AM Tx?

Again, I'm not saying it wont work. just sounds like a waste of $$
 
Yea, your right Freddy, I'm glad i started this thread otherwise i would have gone ahead and bought it and put it on my stock set-up without realising id be at risk of frying it, i had no idea.

And yea its too much money to risk like that, il just keep my eyes open for 2.4ghz system at a good price and when iv got that sorted il order up the 7955 servo.

Trying not to go too off topic but has anybody used an acoms technisport system? I understand theyre far from the best but theyre cheap and theres often very good condition used systems on UK RC forums? And are they a computer radio ie will they work well with the 7955?

I could get one of those alot sooner than if i were to get a spektrum, 3pm or whatever, acoms = 50 odd GBP new 30 odd used, spek or futaba = 200+ GBP
 
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If have seen these systems in my LHS and it feels and looks real cheap so i would stay well clear of them.

A member of RCU got one for his son and was not verry impressed. The range on these systems SUCK, 30 yards on water and not much better on land. Some report that it's range is even worse than some AM systems.

Here's a link to the tread:Acmos Technisport 2.4GHz System

I'm sure you can find a decent second hand system on ebay.

Heres one i found in the UK for 139GPB Spektrum DX3.0

In my case, i had no other option but to run my digital servo on my stock radio system. Really sucks when you don't have spare $$$ to throw around.cornfused
 
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lol Ahh, that would explain why theres so many new looking used acoms systems around then, cheers for that.

Thats the cheapest DX3 iv seen new, thanks if they still have them in a couple of weeks il go for one.

I have seen DX2 systems on ebay quite cheap in the past, i don't know what theyre like but cant be too far off the DX3?
 
Jip, their basically the same, both use Spektrums DSM1 technology and the DX2.0 only has one less channel than the DX3.0. The DX3.0 has a 4 model memory while the DX2.0 only has 2.

IMHO more model memory is always better. On my DX3S i have tree slots assigned to my savage.

1. Is setup for me with everything on full.
2. Is setup for my friends, with the throttle limited to 70%
3. Is setup for my dad, with the trottle limited to 50%

And tree channels gives you a good piece of mind that if you ever deciide to go electric down the road, the third channel is there for reverse.
 
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