Horsepower and Torque

 

Everyone wants more torque and more power, right? Well, the first step to getting there is to understand what the terms actually mean, and many, many people don't.

 

I've heard all kinds of crazy definitions:

 

"torque is what you feel, horsepower is just some obscure mathematical concept"

 

"torque is what accelerates the bike, horsepower is what maintains the speed"

 

"it's not how much torque you have that matters, it's how much total torque AND horsepower you have"

 

"Horsepower is a formula, not a physical sensation, where torque is a sensation!"

 

etc etc. Nice simple definitions that are easy to get your mind around for sure. Unfortunately, they also indicate a severe lack of understanding of what the terms actually mean. It's not difficult to understand torque and power and how they're related, but if you read and believe stuff like this, you'll make it harder than it has to be.

 

Forget about horsepower for a moment, let's define torque and the other ingredient of power, which is rpm.

 

Torque is just twisting force. One ft-lb of torque is literally defined as one pound of force applied at a one foot radius. Well, if torque is twisting force, and twisting force accelerates the bike, what else matters? Isn't torque the only goal?

 

Actually no, and the reason is that torque says nothing about how fast something is moving, only how much force is being applied. But how fast it's moving is enormously important. And, as it turns out, how much force you're able to apply and how fast something is moving are interrelated. It's easy to push hard if you don't have to push fast, and it's easy to push fast if you don't have to push hard.

 

To illustrate this, let's take an example. Say we have a handheld drill motor. It's turning 1000 rpm and it has 2 ft-lbs of torque. If we hook it up to a gear that has 10 teeth on it, and mesh that with another gear that has 10,000 teeth on it, what happens?

 

By doing this we've applied a factor of 1000 gear reduction. Rpm goes down with gear reduction, in this case it'll drop from 1000 rpm to 1 rpm. Likewise, torque goes up with gear reduction. In this case it'll go up from 2 ft-lbs to 2000 ft-lbs.

 

Wow, 2000 ft-lbs of torque from a handheld drill motor. We've really accomplished something, huh? We can move a mountain. But wait ... it's only turning 1 rpm! We have to time it with a calendar.

 

So what we've accomplished is that we've generated a LOT of torque, but unfortunately, we've given up a LOT of rpm. We're pushing very hard (torque) but also very slowly (rpm).

 

What happens if we turn the gearing around, and drive the 10 tooth gear with the 10,000 tooth gear? The small gear will turn at one million rpm! But it'll only have .002 ft-lbs of torque. Touch it with your finger and it'll stop. We're now pushing very fast (rpm), but also not pushing very hard (torque).

 

The basic relationship is that if we gear deeper, the torque goes up and the rpm goes down by equal factors. Likewise if we gear taller, the rpm goes up and the torque goes down by equal factors.

 

Understanding this basic relationship between torque and rpm and gearing is the key. Torque and rpm are interchangeable entities! All you have to do is change the gearing. Want big rpm? Gear it taller. Want big torque? Gear it shorter. Hell, we just showed how we can make 2000 ft-lbs with a handlheld drill motor, but for that matter, we could make a million ft-lbs with a drill motor. All it takes is gear reduction. Making big torque is no big trick if you don't care about speed.

 

So to everyone who thinks torque is a performance metric, well, put simply, torque is simply not a meaningful number for evaluating performance unless you also consider the rpm it's being made at. If the rpm didn't matter, we'd all just gear gear our bikes into the basement, and never take them out of first gear. We have FAR more torque at the rear wheel in first gear than any other gear. The torque and rpm being produced by our engine go through gear reduction in the primary, the transmission (except in 5th), and the final drive. The gear reduction through the transmission is deepest in first gear. You want torque? Just leave your bike in first gear all the time. 

 

But nobody wants to ride around in first gear all the time. Sure, you've got lots of torque at the rear wheel in first, you can probably even pull a wheelie you've got so much torque. But you can't go very fast. Every time you upshift, you give up some rear wheel torque and trade it for some rear wheel rpm. By the time you get to 5th you may be able to go 100mph, but you don't have enough torque left at the rear wheel to pull a wheelie anymore.

 

Unfortunately, there's just no way to upshift and gain more rear wheel rpm without also losing rear wheel torque. So what we really want here, what really will help the performance of the bike, is to make more torque and more rpm at the same time.

 

Well, as it turns out, there's a term for that: it's called "horsepower". Horsepower is literally torque times rpm (divided by 5252, but conceptually you can ignore that part, it's just to scale the number to what Watt's horse could do). The term "horsepower" describes the total combination of torque and rpm, without specifying it's makeup. But for the purposes of evaluating performance, it's makeup doesn't matter. If it's not made of the combination of torque and rpm that we want (and it's not), we just run it through some gearing. That's what gearing is for.

 

In the example above, the drill motor has .38 horsepower: (2 ft-lbs x 1000rpm) / 5252 = .38. After we geared it down to 1 rpm and 2000 ft-lbs, it still has .38 horsepower: (2000 ft-lbs x 1rpm) / 5252 = .38. The gearing didn't change the horsepower, it only changed the mixture of torque and rpm that makes up the horsepower.

 

Let's apply this to the real world in a simple example. Roy Ricer has a 600cc inline 4 that makes 40 ft-lbs at 15000rpm. He's up against Billy Biker with a Buell making 80 ft-lbs at 6000 rpm. Who's going to win?

 

Billy Biker has TWICE as much torque as Roy Ricer. But Roy Ricer's bike is turning two and a half times as many rpm. That means he can gear his bike two and a half times deeper than Billy Biker and still have the same rear wheel speed. Well, if he can gear his bike two and a half times deeper, that multiplies his torque two and a half times more. Two and a half times 40 is 100, and that's gonna put Billy Biker's 80ft-lbs on the trailer. At the rear wheel, which is where it matters, Roy Ricer will have 25% more torque than Billy Biker when geared for the same rear wheel speed.

 

The bikes I love and ride make their power with relatively high torque and low rpm. OHC multis on the other hand tend to make their power with relatively low torque and high rpm. I like the visceral feeling, the sound, the rumble, the ease of pulling away, etc, of a high torque, low rpm V-twin. But let's not kid ourselves. The fact that our power is made of high torque and low rpm doesn't somehow make it stronger than a bike with more power made of  low torque and high rpm. The makeup of the motor's power isn't important for evaluating performance. What matters is more power.

 

One point of confusion, and I see it all the time, is when people look at torque on a dyno sheet and call it rear wheel torque. Seems to make sense, after all, the measurement was made at the rear wheel, it's rear wheel horsepower, must be rear wheel torque, right? I even see this mistake made by veteran motor guys as well as in magazine tech articles. Not unusual at all to see a glowing report of "100 ft-lbs at the rear wheel", for example.

 

Well, let me tell you, if someone really only has 100 ft-lbs at his rear wheel, get a stock Blast and you'll blow him into the weeds. Even in top gear the little Blast has 4.97 of gear reduction between the crank and the back wheel: 1.676 primary times 1.0 top gear times 2.963 final. With 30ft-lbs or so at the crank, that comes out to nearly 150ft-lbs at the back wheel. When you're in first gear, you've got 13.35 of gear reduction between the crank and the rear wheel giving you a whopping 400 ft-lbs!

 

The confusion lies in interpreting the dyno's numbers. It's not showing rear wheel torque, it's showing engine torque as measured at the rear wheel, and that's an important distinction. A Dynojet dyno won't even show torque unless you use the tach pickup, ever wonder why? It's because it needs to understand the gear reduction that lives between the drum and the crankshaft in order to calculate the torque at the crankshaft, which is what it displays. Notice how it plots torque against engine rpm, not rear wheel rpm, and the torque crosses the power at 5252 engine rpm, not rear wheel rpm. That's because it's engine torque, i.e. upstream of the gearing.

 

So now that we know what horsepower is, how do we make more of it? Make more torque and rpm at the same time, that's how! How do we get more torque and rpm from our engines? Well, the engine's torque is fundamentally cylinder pressure and the mechanical advantage it has on the crankshaft (bore and stroke both give it mechanical advantage). Cylinder pressure comes from filling the cylinder as completely as possible, squeezing it tight, and burning it completely. Rpm is how fast everything is happening. As rpm goes up and things happen faster and faster, there's less and less time to fill the cylinder. Hence the torque wants to drop. If torque is dropping faster than rpm is rising, stick a fork in it, it's done, because our total combination of torque and rpm is lower.

 

So what we do is concentrate on filling the cylinder at the rpm of interest. Many people want that cylinder fill at lower rpm so that their horsepower is made of relatively higher torque and lower rpm, we understand that. Cylinder fill is always the goal, we just change the rpm of interest. Reducing losses through the intake and exhaust tract, sizing the components for the mixture of flow and velocity that gives maximum fill, timing the cam events to match, and properly utilizing wave travel effects in the exhaust system are some of the ways we do this.

 

 

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