For the total beginners out there just know that on a nitro rig you will need either 2 or 4 servos (1 throttle, 1 or 2 steering and 1 for reverse depending on the kit). For electrics you generally one need 1 or 2 for steering depending on the rig.
Your standard servo for a 1/10 or 1/8 scale RC will be approx. 40mm long x 20mm wide x 38mm tall. The length and the width may vary a couple mm but not much. The height how ever can very probably anywhere from approx 35 to 45mm. The height really doesn't matter as long as the kit that you're putting it into doesn't have a real confined space. The majority of the kits I've seen can accommodate a 45mm tall servo but measure it to make sure before you buy if it looks like there could potentially be an issue. Some kits it obvious that the height doesn't matter. Others not so much. i recommend Savox brand servos if you can get them. For the money their quality is hard to beat.
There are two other factors to take into consideration when buying a servo (for steering...I'll mention throttle servos separately at the bottom of this post). Torque and speed. torque is represented in oz-in here in the US or kg-cm for the non-oddball parts of the world. Speed is the time in a faction of a second for the servo horn to travel 60 degrees. A common servo for a light 2wd 1/10 scale RC might have specs that look something like whats below. This is the specs on a Savox SC-1258TG.
Dimensions(mm): 40.3x20.2x37.2
Weight(g): 52.4
Speed(@4.8V sec/60): .10
Torque(@4.8V oz-in): 133.3
Speed(@6.0V sec/60): .08
Torque(@6.0V oz-in): 166.6
Gear: Titanium & Aluminum
Bearing: 2BB
Case: Aluminum
25 Tooth Spline
Frequency: 250-333hz
Pulse Width Frequency: 1520
Servos will always have their specs listed at 2 different voltages. Normal servos will be a 4.8V and 6V for 4 or 5 cell NiMH battery packs respectively. High Voltage or HV servos will list 6V and 7.4 V. HV servos are designed to be used either with a 6V NiMH pack or a 2 cell LiPo (also seen written as 2S LiPo and LiPo stands for lithium polymer) which puts out 7.4V nominal voltage. HV servos are most commonly seen in electrics where the kit itself is already running on a 2S LiPo so you dont have to use a
BEC (or Battery Elimination Circuit...a device that takes X volts and steps it down to Y volts...in this example 7.4V down to 6V) however some guys like powering their nitros with 2S LiPos for increased run times. Heres an example of some similar specs from a Hitec servo (Hitec is another highly trusted name brand for servos). This one in particular is from the HS-7985MG and it shows not only the oz-in rating but the kg-cm rating as well.
Motor Type: Coreless
Bearing Type: Dual Ball Bearing
Speed (4.8V/6.0V): 0.16 / 0.13
Torque oz./in. (4.8V/6.0V): 144 / 172
Torque kg./cm. (4.8V/6.0V): 10.4 / 12.4
Size in Inches: 1.57 x 0.78 x 1.45
Size in Millimeters: 39.88 x 19.81 x 36.83
Weight ounces: 2.18
Weight grams: 61.80
Probably the last thing you want to know is "How much torque/speed do I need?" Speed isnt all that important unless you are racing but generally the faster the better. The size of the kit will affect whether or not you think a servo is fast enough. This is gonna be a personal preference more than anything. For torque there is some personal preference that plays into it but the heavier the kit the more torque you will want. The problem is of course the more torque you want the more speed you have to give up. Dont get me wrong, you can find servos that are plenty fast in a given torque range (if you're willing to shell out the money for it) but generally as a servo gains torque it will loose speed. The chart below is a rough idea of about what torque range you should look at for each class of RC. This list is strictly my own personal opinion and others opinions may very but it will give you a place to start. Also remember that a good rule of thumb is that it never hurts to have as much torque as you can get as long as you get the speed you are looking for
Ranges are approx...
1/10 scale light 2WD: 133oz-in/9.5kg-cm to 180oz-in/14.5kg-cm
1/10 scale heavy 2WD: 166oz-in/13kg-cm to 220oz-in/15.8kg-cm
1/10 scale light 4WD: 166oz-in/13kg-cm to 220oz-in/15.8kg-cm
1/10 scale heavy 4WD: 180oz-in/14.5kg-cm to 300oz-in/21.6 kg-cm
1/8 scale light 2WD: 166oz-in/13kg-cm to 220oz-in/15.8kg-cm
1/8 scale heavy 2WD: 180oz-in/14.5kg-cm to 280oz-in/20.1 kg-cm
1/8 scale light 4WD: 180oz-in/14.5kg-cm to 300oz-in/21.6 kg-cm
1/8 Scale heavy 4WD 300oz-in/21.6kg-cm to 500+oz-in/36+kg-cm
These are just rough guidelines. The weight of a kit can vary a fair amount in each of these categories and again personal preference will play a part in your future decisions but these are probably good ranges to look at. There are a handful of kits that will not fit into the above categories. On-road and crawlers come to mind. On-road will probably fit into the 1/10 scale light 2WD category regardless of what it is just because on road has less resistance than off road and therefore wont need as much torque. Crawlers will fall into the 1/8 scale heavy 4WD category regardless of what they are because of the demand that rock crawling puts on a servo.
Throttle servos dont need to be very beefy in any rig. Anything in the 60-133oz-in/4.3-9.5kg-cm range is fine. i look for speed more than torque for my throttle servos but again if you arent racing speeds not all that important.
Trusted commonly used servo brand names to look for:
Savox
Hitec
Futaba
JR
Airtronics
One last segment of servo info. Spline count. The spline is the part that has teeth on it that the servo horn attaches to.
Spline count is the number of teeth that the spline has on it. You need to know the spline count for whatever servo you have if you plan on using a servo horn other than the ones that came with the servo. The horn will need to have the same spline count as the servo.
23 splines: KO, Airtronics/Sanwa, JR, ACOMS, Solar, EXI.
24 splines: HiTec, RCD, Apollo (plus some Tower/Hobbico as HiTec makes them)
25 splines: Futaba (newer)
, Savox, (plus some Tower/Hobbico as Futaba makes them)
, traxxas 2075 digital waterproof servo, HPI, HobbyKing-BlueBird, some TowerPro [mg995 & mg996r], RC4WD
Additional info on spline counts for Hitec and Futaba.
Futaba Servo Splines
Hitec Servo Splines
Even more servo info
The difference between digital and analog servos.
(The difference between digital and analog isnt really
that important and analog servos arent even used that
much these days but I thought Id throw it in for
completeness.)
I hope this gives you some insight into servos.
P.S. If any one has any other servo info they would like to add to this please let me know and I'll be happy to add it to the post. trying to make it as comprehensive as possible. Also if anyone disagrees or thinks my torque ranges for various kits needs to be tweaked please let me know and I'll try to tweak them into something that is commonly accepted by everyone.
