 |
Trick (trik) -noun: a clever or ingenious device or expedient; adroit technique
R/C Tips and Tricks: Savage Gearing and Top Speed
"How fast does it go?"
That's the question everyone asks, right? Whether it's the neighbor kid, your folks, or the people on the forums, everyone wants to know how fast our R/C vehicles can go. Speed is subject to a LOT of variables, such as friction within the vehicle (bearings, gears, dogbones, etc.), friction with the surrounding environment (wind drag, tire-to-surface friction), and of course variables from the components installed on the vehicle such as the tune of the engine, engine power, tire size, gearing, etc.Well, I've got some theory for you, provided you're a Savage driver. Below is a table listing many clutch bell/spur gear combinations and the top speed theoretically achievable using stock tires and an engine singing along at 30,000 rpm. Can your engine hit that rpm in high gear? Who knows? It depends on MANY factors, but this should get you in the ballpark. I'll also provide the equation so you can plug in your own rpm, gearing, and tire diameter for various combinations. There are three top speed colums shown. The first is for the original 2-speed tranny found in most Savages. The second is for the HPI 3-speed transmission #87218 and #87220. The third is for the X 3-speed transmission which utilizes different tooth counts on the high-speed gear for more top end. This gear set is available separately as HPI #77065.
You may notice very little difference in the top speed potential between the 2-speed and regular 3-speed. That's because of the gearing HPI selected for the 3-speed. Despite HPI's claims to the contrary, there is less than a 1% difference in the gear ratio of high-gear between those two transmissions, which results in less than a 1% difference in the potential top speeds, all else being equal. Fortunately, the high-speed gear set now available bumps up that speed potential by nearly 15% over the 2-speed.
Disclaimer! As I mentioned above, the top speed a vehicle can achieve is depends on many things but it comes down to the balance point between the engine's power and rpm vs. resistance. If you want to go faster, do whatever you can to increase power and rpm while reducing resistance.
Resistance comes from friction, and there are many forms. Wind resistance is a major one, so the smoother the body and the narrower the tires, the better your machine will slice through the air. A big wing to add downforce is good for control, but you're adding wind resistance and turning it into downward pressure, which adds more friction between the tires and the road. Only use as much downforce as you really need, if any. If your vehicle is stable without wings or spoilers at high speeds, go with it. Also, use the narrowest tires you can. They'll cut through the air better and have a smaller contact patch on the pavement. Use bearings in good condition to minimize drag there, and make sure your gear mesh is properly set. Too tight will add friction and thus more heat. You can also adjust your alignment to zero toe to further reduce friction.
Speed Calculation
The equation may look complicated, but it's quite simple. Just punch into your calculator the numbers requested and provided using the multiplication or division commands where appropriate and you're set. The first one is a general equation that works for any vehicle without a transmission (buggies, truggies, HPI NMT, etc.), the second is for a two-speed Savage, the third is for a standard 3-speed Savage, and the fourth is for a 3-speed Savage with the high-speed third gear (HS). All Savage equations are for high-gear only. Contact me if you want additional equations or if you have any questions.General: RPM x bell/spur x pinion/ring gear x tire circumference in inches x 0.000947 = speed in miles-per-hour
Savage 2-Speed: RPM x bell / spur x 0.000186 x tire circumference in inches = speed in miles-per-hour
Savage 3-Speed: RPM x bell / spur x 0.000187 x tire circumference in inches = speed in miles-per-hour
Savage 3-Speed HS: RPM x bell / spur x 0.000214 x tire circumference in inches = speed in miles-per-hour
Typical Savage Speed Examples
This table shows some typical Savage gearing setups and the speeds you could potentially achieve, assuming a couple things. First, we're using a 6" tall tire. The original Savage tires were about 6-5/16" tall, but Dirt Bonz are more like 5-5/8" tall, so we're using 6" as an average height. Also, we're using stock diffs and 30,000rpm as the max engine speed. Some engines will go higher, but no-load speeds are much higher than what an engine can spin while pushing a heavy truck down the road. Use these numbers for comparison purposes and for theory, not as a guarantee of a certain speed. If you swap to CEN ring and pinion gears with no other changes, your speed will increase by 14.5%. So find your gearing combo and transmission type to get your 30,000rpm speed, and if you have CEN diffs, multiply that speed by 1.145.
| Note | Spur | Bell | Ratio | 2-Speed Top Speed* | 3-Speed Top Speed* | HS 3-Speed Top Speed** |
|---|---|---|---|---|---|---|
| - | 52 | 12 | 4.33 | 24.3 MPH | 24.4 MPH | 27.9 MPH |
| - | 52 | 13 | 4.00 | 26.3 MPH | 26.4 MPH | 30.2 MPH |
| - | 52 | 14 | 3.71 | 28.4 MPH | 28.4 MPH | 32.5 MPH |
| - | 52 | 15 | 3.47 | 30.4 MPH | 30.5 MPH | 34.9 MPH |
| - | 52 | 16 | 3.25 | 32.4 MPH | 32.5 MPH | 37.2 MPH |
| A | 52 | 17 | 3.06 | 34.4 MPH | 34.5 MPH | 39.5 MPH |
| A | 52 | 18 | 2.89 | 36.5 MPH | 36.6 MPH | 41.8 MPH |
| A | 52 | 19 | 2.74 | 38.5 MPH | 38.6 MPH | 44.1 MPH |
| A | 52 | 20 | 2.60 | 40.5 MPH | 40.6 MPH | 46.5 MPH |
| B | 49 | 12 | 4.08 | 25.8 MPH | 25.9 MPH | 29.6 MPH |
| - | 49 | 13 | 3.77 | 28.0 MPH | 28.0 MPH | 32.1 MPH |
| - | 49 | 14 | 3.50 | 30.1 MPH | 30.2 MPH | 34.5 MPH |
| - | 49 | 15 | 3.27 | 32.3 MPH | 32.3 MPH | 37.0 MPH |
| - | 49 | 16 | 3.06 | 34.4 MPH | 34.5 MPH | 39.5 MPH |
| - | 49 | 17 | 2.88 | 36.6 MPH | 36.6 MPH | 41.9 MPH |
| - | 49 | 18 | 2.72 | 38.7 MPH | 38.8 MPH | 44.4 MPH |
| A | 49 | 19 | 2.58 | 40.9 MPH | 41.0 MPH | 46.9 MPH |
| A | 49 | 20 | 2.45 | 43.0 MPH | 43.1 MPH | 49.3 MPH |
| B | 47 | 12 | 3.92 | 26.9 MPH | 27.0 MPH | 30.8 MPH |
| B | 47 | 13 | 3.62 | 29.1 MPH | 29.2 MPH | 33.4 MPH |
| B | 47 | 14 | 3.36 | 31.4 MPH | 31.5 MPH | 36.0 MPH |
| - | 47 | 15 | 3.13 | 33.6 MPH | 33.7 MPH | 38.6 MPH |
| - | 47 | 16 | 2.94 | 35.9 MPH | 36.0 MPH | 41.1 MPH |
| - | 47 | 17 | 2.76 | 38.1 MPH | 38.2 MPH | 43.7 MPH |
| - | 47 | 18 | 2.61 | 40.3 MPH | 40.5 MPH | 46.3 MPH |
| - | 47 | 19 | 2.47 | 42.6 MPH | 42.7 MPH | 48.8 MPH |
| - | 47 | 20 | 2.35 | 42.6 MPH | 42.7 MPH | 48.8 MPH |
NOTES:
A - Likely will not fit Savage without mods (gear combo is too large)
B - Likely will not fit Savage without mods (gear combo is too small)
* - "Top Speed" is controlled by many things, as stated above, thus these speeds are intended for comparison purposes to help with clutch bell and spur gear changes, and also to compare the potential top speed difference between the available Savage transmissions.
Feedback is always welcome, please click the "Contact Me!" link if you'd like to leave some.
Copyright © 2007 Bruce Johnson