Tagged: GEAR RATIOS
- April 6, 2021 at 8:15 PM #16284BarkingSpyderParticipant
Gear ratio *terminology* can be confusing.
One thing to remember is that gears are actually round-Levers, increasing the work that the power source can effectively do.
HIGH/TALL/BIG – “High” gear ratios have Smaller numbers (e.g. 2.67) and promote higher rear-wheel RPMs and faster top-end Speed (cm/sec, Ft/sec, Miles/hr, Kilometers/hr etc.) Note that the highest rear-wheel RPM and horizontal speed depends on a) the run-out time needed to allow the motor to overcome standing inertia, b) which is sped up by a higher torque to overcome the starting “lag”.
The ratio is always relative of the DRIVER gear compared to the DRIVEN gear. On a slot-car the Driver is the Pinion (off the motor shaft) and the Driven is the crown or spur (on the axle). On a bicycle the Driver is the front Chain-ring (crank) and the Driven is the rear Cog (axle).
Braking will have a slower response with a High gear ratio; because the inertia from the motor’s Rotor/Armature, (and the car’s weight) will keep the armature at the same rotational speed, preventing the Arm from spinning fast enough to act like a generator and thus “fight” the can magnets [When you release your controller throttle lever, you short the coils together in the Armature, turning the motor into a Generator, where the rotor coils reverse their magnetic Pole (No/So) and try to spin the motor in Reverse to what it was turning when the poles were powered].
NOTE: The closer the gear ratio gets to 1:1, the rear wheel RPM gets closer to matching the Motor-shaft RPM. For a 2.67 ratio, from a 9 tooth pinion and 24 tooth crown/spur/drive gear; the motor shaft has to turn 24 times, for the rear axle to turn 9 times; e.g. every one turn of the motor turns the rear axle 0.37 times (i.e. 1/2.67)
LOW/SHORT/SMALL – “Low” gear ratios have Bigger numbers (e.g. think a 4.10 in a 1×1 muscle car, or a 3.11 in a slot car) and promote Quicker Acceleration, yet slower top-end speed. Quicker acceleration results from the ability of the motor to turn easier (less load and less inertia) and hence requires less torque to overcome starting inertia. The quicker acceleration aids faster recovery after braking into a corner. Braking has a smoother response, and grows quickly, with a Low gear ratio.
Just like down-shifting in a 1×1 car, (hence “loading” the engine against the compression of the pistons and flywheel inertia) the “advantage” is reversed, where the larger Crown/Spur now spins the smaller pinion (hence Armature) faster, hence being a stronger Generator thus fighting harder against the can-magnets.
NOTE: The wider the gear ratio gets away from 1:1, the rear wheel RPM gets slower relative to the motor-shaft RPM. For a 3.11 ratio, from a 9 tooth pinion and 28 tooth crown/spur/drive gear; the motor shaft has to turn 28 times, for the rear axle to turn 9 times; e.g. every one turn of the motor shaft the rear axle turns 0.32 times (i.e. 1/3.11), 0.05 times LESS than a 9/24 combination (2.67 ratio).
For a 21,500 RPM motor this difference is significant! The 24/9 (2.67) setup results in a rear wheel speed of 8,052rpm vs. 6,913rpm using a 28/9 (3.11) setup, over a 1,000rpm Delta!
WHY THE INVERSE TERMINOLOGY?
Believe it or not, it goes back to High-wheeler bicycles! A large-diameter, and hence “taller” front wheel resulted in faster top-speeds (but Slower starting acceleration) (naturally the longer (leverage) and stronger (power) legs of the taller riders added a distinct advantage over smaller riders!). When “safety” bicycles came along, combinations of large front drive gears and small rear “cogs” were needed to provide the equivalent leverage of the large (direct-drive) front wheels.
The impact on terminology, was that a larger ratio of the “driven” rear (cog) to the “driver” front (chain-ring) gears was called a TALL or LARGE “Chain”, (actually Gear-Inches) to reflect the equivalent leverage of a taller front wheel.
So a 52/12 chain-ring to cog ratio (every turn of the front gear turns the rear wheel 4.33 times) is appropriately viewed as a “Large” gear since it provides a higher Leverage, and hence a “large chain” resulting in faster top-speeds, but with a cost of more Power (torque and watts) needed from the legs.
A 30/38 chain-ring to cog ratio (every turn of the front gear turns the rear wheel 0.79 times) is viewed as a “Low” gear since the rear Driven gear (and axle) turns a Lower amount of rotations compared to the Driver front gear (in this case the 30z Ring, like a slot car pinion). In vintage bicycle technology this is called a “Small chain” that eases the engine (leg) load; enabling easier climbs but with a Cost of slow top-speed.
NOTE: Most gear ratios on a bicycle are Low, because compared to proportionately more efficient electric motors and even (moderately Inefficient) combustion engines, the power-to-weight ratio is orders-of-magnitude smaller!
In a 1×1 race car, in addition to the net gear-ratio (at the drive-yoke) of the transmission, the Rear-end ratio (in the differential) is faster and less expensive to change to suit track conditions and desired performance. Pick up any drag-racing magazine and you will read about changing the rear-end for various track conditions. 3.xx ratios are High (faster in the Traps at 1,320 ft) while 4.xx ratios are Low (faster off the Tree and for the first 900 feet). In this case the Pinions (driveR) typically stay the same while the Ring (driveN, e.g. crown or spur) is changeable.
NASCAR oval cars will use ratios in the mid-3.xxs to reach bank speeds over 200 mph and high-3.yys to low 4.zzs on the occasional road course (e.g. COTA) to accelerate out of corners and quickly reach Peak speed on short straights. Short-track and off-road cars stay in the Low gear-ratio ranges.
1×1 car transmissions have all but their “top” gear combinations as “low” gears, allowing the engine to spin fast and exercise it’s torque at low throttle, and grow in a 3k to 6k rpm “Power-Band”, using its inertia (added by the flywheel and compression) to ease the load as the turn ratio gets smaller in difference from the flywheel to the transmission’s drive-yoke. This also allows the common use of the compression and inertia from the flywheel, crankshaft and transmission to Slow the cars rear wheels, as the gears are dropped and throttle backed off.
RULES OF THUMB
LOW = Bigger number; less rotation of the rear axle compared to the motor shaft. Quicker acceleration from a stand-still; slower top-end-speed. Faster Braking, esp, in corners.
HIGH = Smaller number; more (or closer) rotation of the rear axle compared to the motor shaft. Slower acceleration from a stand-still; Faster top-end-speed. Slower Braking, esp, in corners.
GLOSSARY: (the following are often incorrectly used in place of each other -LOL)
Quick = greater rate-of-change, as in Acceleration.
Fast = greater distance traveled per a given time period, as in Speed.
Speed = rate of *distance* traveled vs. a given unit-of-time (usually a large unit, as in minutes or hours).
Acceleration = the *rate-of-change* between two given location/points, per a unit of time (usually a small unit, as in a factor of seconds)
"I cant drive 55!" Sammy
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