I am not a physicist, but help me through this. I have a lot of questions.

I think I understand the basic of the torque ratings of a servo - how much it could lift at a certain distance from the output shaft (fulcrum). Is that rating then the breaking point of the servo?

For instance if it was 160oz-in rated servo, then with a 1 inch servo horn it could lift 10lbs.. Torque = Distance from fulcrum X Force applied, so at 2 inches it would be half the torque.

So I was thinking about a servo in a car, lets use the same one I described above. The servo would put out 10lbs of force through the linkages to the axle, but with a larger diameter wheel acting as a lever on that axle it would cut that force down depending on the size of the wheel. Lets say with a 1/8 scale wheel it would be by half. So does that mean that in a 1/8 scale buggy with the servo above, the front wheel hitting something with more than 5lbs (1/2 of the 10lbs at the servo) of force it would strip/break the servo, or at least push the servo motor backwards against it's force?

I also am not sure how much the servo saver can absorb either, or how much force is need to activate it. 10lbs in the example above does not seem like much force to engage a servo saver. Does the sever 'bottom out' eventually and continue to transmit the force, or once the saver is engaged no force goes to the servo? And if the saver is so effective, how do servos get stripped? Should the saver be 'tuned' to the spec of the servo?

I know many run with 170-250 oz-in servos successfully. So I'm sure my numbers are pretty error ridden, but is the idea is on the mark? An RC10B44.1 is less than 1/2 the weight of a RC8Be.

At the end of the day, are 400 oz-in servos what 1/8 scale cars should be starting with?

I am not a physicist, but help me through this. I have a lot of questions.

Me neither, but I am an engineer (applied physicist?) with considerable educational and real world experience with servo systems (non RC), in addition to 20+ years of RC. Allow me a shot.

I think I understand the basic of the torque ratings of a servo - how much it could lift at a certain distance from the output shaft (fulcrum). Is that rating then the breaking point of the servo?

Essentially yes. It’s the stall torque, the maximum torque the servo can generate at the output shaft. (not breaking as in breaking gears though)

For instance if it was 160oz-in rated servo, then with a 1 inch servo horn it could lift 10lbs.. Torque = Distance from fulcrum X Force applied, so at 2 inches it would be half the torque (force).

On the right track, except the last word should be force, not torque. Typo I suspect.

So I was thinking about a servo in a car, lets use the same one I described above. The servo would put out 10lbs of force through the linkages to the axle, but with a larger diameter wheel acting as a lever on that axle it would cut that force down depending on the size of the wheel. Lets say with a 1/8 scale wheel it would be by half. So does that mean that in a 1/8 scale buggy with the servo above, the front wheel hitting something with more than 5lbs (1/2 of the 10lbs at the servo) of force it would strip/break the servo, or at least push the servo motor backwards against it's force?

Still on the general right track (without getting into every step of the steering mechanism, but you’ve got the overall concept).

I also am not sure how much the servo saver can absorb either, or how much force is need to activate it. 10lbs in the example above does not seem like much force to engage a servo saver. Does the sever 'bottom out' eventually and continue to transmit the force, or once the saver is engaged no force goes to the servo?

All of the servo savers I’m aware of are spring loaded, still transmit force even when working. The force will even continue to increase some, just at a much slower rate. Never really looked at bottoming out, but I suspect it would be possible to bottom many in an extreme situation, in which case they would loose all effectiveness and transmit full force.

And if the saver is so effective, how do servos get stripped?

Well, they are not that effective obviously. In a practical sense the saver has to be tight enough to stay put in all but the more extreme situations in order to provide stable steering through cornering and ruts and such. Thus a lot of dynamic impact shock still gets transmitted to the servos even with a saver.

Servo (& RC) life tip: Sometimes when a RC gets jammed into the pipe the wheels will be forced over and stuck. When this happens it is a very good idea to turn the wheel in that direction to reduce loads until the marshal helps ya out. Besides being hard on the servo, a stalled condition also puts a lot of load on the servo power source (receiver battery or the BEC function of the ESC, the wiring, etc), not good.

Should the saver be 'tuned' to the spec of the servo?

Ideally yes, that would make sense. Something I’ve done when upgrading servos, not a scientific adjustment though, more gut feel and experience with the RC.

I know many run with 170-250 oz-in servos successfully. So I'm sure my numbers are pretty error ridden, but is the idea is on the mark? An RC10B44.1 is less than 1/2 the weight of a RC8Be.

Yes. The required servo torque varies by vehicle type, with several factors coming into play: The overall weight, the tire size, even the drive (2wd/4wd). A 1/10 stadium truck (bigger tires) or a 4wd buggy (drive induced loads in addition to steering) need more torque than a 1/10 buggy, 1/8 buggy needs even more, etc.


At the end of the day, are 400 oz-in servos what 1/8 scale cars should be starting with?

I think most recommend at least 200oz-in for a 1/8 buggy, perhaps 250 or better for a truggy. One of the most well regarded 1/8 servos is a Hitec 7955, is rated for 250 to 333oz-in depending on operating voltage. More should be fine, as long as you don’t sacrifice too much speed to get it (torque and speed are always tradeoffs with each other).

Hope this helps some.

Thanks Dave for helping me out with understanding this stuff. Helpful people like you make this forum what it is. Much appreciated.

Good to know that I am on the right track here.

jason