Gearing: Horsepower's Better Half
Posted: Wed Oct 17, 2012 7:19 pm
Not completely devoted to scooters, but a good read for the aspiring gear heads out there.
When it comes to performance, everyone’s familiar with the all-mighty horsepower rating; but horsepower is only half of the acceleration story. The other half is gearing, and it works like this: The engine makes a specific amount of horsepower at the crankshaft. That’s nice and all, but how it’s delivered to the rear wheel is dependent upon the transmission and final drive gears. By “how” it’s delivered, I mean there’s a trade off between top speed and acceleration that can be biased in either direction by changing gear ratios. For example; 1st gear provides the lowest top speed, but the highest acceleration rate. Inversely; top gear provides the highest top speed, but the lowest acceleration rate. In layman’s terms: Low gears are strong but slow; high gears are weak but fast; but each uses the same amount of horsepower.
“Gear ratio” refers to the rate that one gear is turning VS the rate another gear is turning. We could compare rotations of the crankshaft, gears in the transmission, final drive gears, etc. It’s really just a relative measurement. So, how exactly do lower gear ratios make more torque than higher ones? Well, there’s a couple of things to look at. Glance at the animation below for visual reference. Say the small front gear is the input gear, and the large rear gear is the output gear. For starters, a small gear has more leverage from its center than a large one, and a large gear has more leverage from its outside than a smaller one, and the ratio between them creates a torque-multiplication effect. Another thing to note is that it also adds RPM’s at any given speed to increase power (up to peak horsepower at least, but I’ll cover shift points below).
What about the transmission? How do you accelerate the hardest? Let's look at this chart of torque per gear below (for reference, this is from a car). The 1st column of data is engine torque alone. The stars denote peak torque and peak horsepower as reference points. The other columns are measuring torque output from the transmission. Notice the gear ratios are torque multipliers for the engine. To make things perfectly even, you'd need a gear ratio of 1:1 (hence why any gear with a ratio of 1:1 or higher is considered "overdrive"). The reason 1st gear has a slower top speed but a heck of a lot more acceleration is because it has so much lower of a gear ratio. Low gears have the torque to get a vehicle moving from a stop; high gears can afford to sacrifice torque to hold a higher cruising speed.
The most critical performance observation to note though is that any RPM, including redline, produces more power at the rear wheel in any particular gear than upshifting to a higher gear. Now, this happens to be the case for this particular vehicle, but the optimal shift point for acceleration is whenever upshifting yields more horsepower than staying in the current gear since horsepower, in a way, has gearing already factored into the eqaution. Therefore, for any given road speed, peak horsepower is the maximum amount of pull for the vehicle. In the case of a CVT, the variator holding the engine speed at this RPM will continuously provide the strongest acceleration.
(I have made corrections to this post as it's been discussed, so if the comments in the thread seem to not make sense, that's my fault and not the members of MB.)
When it comes to performance, everyone’s familiar with the all-mighty horsepower rating; but horsepower is only half of the acceleration story. The other half is gearing, and it works like this: The engine makes a specific amount of horsepower at the crankshaft. That’s nice and all, but how it’s delivered to the rear wheel is dependent upon the transmission and final drive gears. By “how” it’s delivered, I mean there’s a trade off between top speed and acceleration that can be biased in either direction by changing gear ratios. For example; 1st gear provides the lowest top speed, but the highest acceleration rate. Inversely; top gear provides the highest top speed, but the lowest acceleration rate. In layman’s terms: Low gears are strong but slow; high gears are weak but fast; but each uses the same amount of horsepower.
“Gear ratio” refers to the rate that one gear is turning VS the rate another gear is turning. We could compare rotations of the crankshaft, gears in the transmission, final drive gears, etc. It’s really just a relative measurement. So, how exactly do lower gear ratios make more torque than higher ones? Well, there’s a couple of things to look at. Glance at the animation below for visual reference. Say the small front gear is the input gear, and the large rear gear is the output gear. For starters, a small gear has more leverage from its center than a large one, and a large gear has more leverage from its outside than a smaller one, and the ratio between them creates a torque-multiplication effect. Another thing to note is that it also adds RPM’s at any given speed to increase power (up to peak horsepower at least, but I’ll cover shift points below).
What about the transmission? How do you accelerate the hardest? Let's look at this chart of torque per gear below (for reference, this is from a car). The 1st column of data is engine torque alone. The stars denote peak torque and peak horsepower as reference points. The other columns are measuring torque output from the transmission. Notice the gear ratios are torque multipliers for the engine. To make things perfectly even, you'd need a gear ratio of 1:1 (hence why any gear with a ratio of 1:1 or higher is considered "overdrive"). The reason 1st gear has a slower top speed but a heck of a lot more acceleration is because it has so much lower of a gear ratio. Low gears have the torque to get a vehicle moving from a stop; high gears can afford to sacrifice torque to hold a higher cruising speed.
Code: Select all
Engine Transmission output torque (ft-lb):
Torque 1st 2nd 3rd 4th 5th
RPM (ft-lb) 3.54 2.13 1.36 1.03 0.72 <- gear ratio
---- ------- ---- ---- ---- ---- ----
1000 50 177 107 68 52 36
1500 65 230 138 88 67 47
2000 80 283 170 109 82 58
2500 92 326 196 125 95 66
3000 104 368 222 141 107 75
3500 114 404 243 155 117 82
4000 120 425 256 163 124 86
4500 125 443 266 170 129 90
5000 130 460 277 177 134 94
5500* 133 471 283 181 137 96
6000 130 460 277 177 134 94
6500* 122 432 260 166 126 88
7000 110 389 234 150 113 79
(I have made corrections to this post as it's been discussed, so if the comments in the thread seem to not make sense, that's my fault and not the members of MB.)
