Performance is all about TORQUE
By: James R. Davis
I am constantly amazed at how gullible, or at least willing to be misled, people are when it comes to their transportation. Consider that all automobile manufactures and most, if not all, motorcycle manufacturers, use Horsepower ratings in their ads. Now why is that, do you suppose? Is it because the buyers believe that horsepower is the magic number that boils all the rest down to irrelevance and tells you just what kind of performance your vehicle is capable of?
I would argue that performance is all about torque and has nothing whatever to do with horsepower. I know, I've just stepped into an age old argument. But I'm going to demonstrate my position here so that the argument can go away.
First, I'd like to talk about electricity - something you all know more about than you realize. I want to talk about Volts, Watts, and Amps. Then I'll relate this story to the engine in your motorcycle.
Every outlet in your house has 110 Volts of FORCE sitting there and waiting to be used. But unless you plug something into that outlet that force is not being used to do any work. And because there is no work being done, NO POWER is being consumed. Power? Yep, it's called Watts. Watts is a measurement of how much work is being done over time. Work? Yep, like lighting your house is the work being done when you use 110 Volts of force to push electrons across the filament on a light bulb. A 60 Watt bulb is far brighter than a 30 watt bulb because twice as much current (amps) is being pushed across its filament. More work is being done.
So what does any of that have to do with a motorcycle? Well, it will help you get over the belief that horsepower is what does work for you, for one thing. You see, horsepower is like Watts - it is merely a measurement of how much work is being done over time. But it doesn't cause that work to be done any more than it causes a light bulb to shine. It takes a FORCE to do work. In the case of electricity, that force is measured in and known as Volts. In the case of a motorcycle, the force that does work is known as TORQUE and is measured in units of foot-pounds.
I used the word 'Work' above. It means MOVEMENT resulting from a force. In the case of electricity, it is the amount of electrons moving through your wires and that filament. If there is no movement, then there is no work being performed. Even though the force remains available to do work, if you turn the light switch off the Volts CANNOT DO WORK.
Similarly, Torque (not horsepower) is a force that tries to cause movement. Unlike Volts, however, Torque always tries to cause a rotating movement.
Look at this lug wrench and you will see what I'm getting at.
Torque is measured in foot-pounds. If that wrench was exactly one foot long and it was connected to the hub of your rear wheel, and you stood on the end of it, you would be producing 180 foot pounds (usually written ft. lbs.) of torque. (Assuming, of course, that you weigh 180 pounds.)
If the resistance to movement of that rear wheel happens to be greater than 180 ft. lbs., it would be the same as volts trying to light a bulb with the switch turned off. NO WORK would be accomplished. But if the resistance is not that great then the torque you are applying to that wheel's hub will cause it to ROTATE. In that case, work would be accomplished - and, by the way, ONLY in that case would there be ANY horsepower because horsepower is merely a measurement, like Watts, of work being done.
Can you have horsepower without rotation? Of course. A rocket engine accomplishes work - it involves a force (not torque) that causes movement. Thus, horsepower. But in the case of your motorcycle, virtually all work accomplished is in the form of rotation and that REQUIRES the force called Torque.
About the only linear movement involved in motorcycling is piston motion (up and down) and the movement of your motorcycle along the roadway. In order to translate the up and down motion of your engine's pistons into the movement of your bike across the roadway it is converted to rotations. First, the piston motion becomes a rotation of the engine's crank shaft. That, in turn, rotates gears that rotate yet other gears within your transmission, which causes a drive shaft or chain or belt to rotate your rear wheel. As you already know, rotation is caused by torque, not horsepower.
So what does HP have to do with speed? AS it is only a measurement of how much work is being done over time, HP tells you how fast your motorcycle can go. That is, all the forces that try to move your motorcycle must overcome all the forces that try to stop your motorcycle from moving. Thus, when wind forces and other drag forces add up to the amount of torque you can deliver to your rear wheel, you have reached your top speed. But what HP does NOT tell you is how quickly you can get to that top speed.
Changing speed is called acceleration. The ONLY thing that causes acceleration is FORCE acting on a mass. In other words, your acceleration is determined by how much torque you can produce, not HP.
But surely HP is directly related to torque, right? Of course. HP is calculated as a function of torque and engine RPM. So, if either torque or RPM increases, so does HP. But the relationship is NOT linear! Torque can decrease while RPM increases and the resulting HP can continue to increase. Let's look at a dynamometer chart and see the relationships for clarity.
This chart shows two runs on the dynamometer for the same bike (one is red and the other is blue). The top two lines are Torque and the bottom two are HP.
Notice that the torque curve increases with increases in RPM, flattens out between 3,500 and 4,000 RPM, then decreases at even higher RPM. On the other hand, HP increases as RPM increases.
In fact what you are seeing is that HP is merely a calculated number. It is the torque times the RPM divided by 5252. Note that the HP curve ALWAYS crosses the torque curve at exactly 5252 RPM. In other words, HP is ALWAYS greater than torque at RPM greater than 5252. Notice also that the maximum torque occurs between 3,500 and 4,000 RPM while the maximum HP occurs at around 5,500 RPM. Thus, torque and HP are directly related, but absolutely not linearly.
So performance of a motorcycle is typically measured in how fast you can cover a certain distance (such as a quarter mile). It is tempting to say that the bike that is going the fastest from a dead stop within that 1/4 mile is the highest performing of the two bikes but, in fact, regardless of the maximum speeds reached by those bikes at the end of that distance, it is the bike that crosses the line first that wins.
Why isn't it always true that the bike going the fastest at the end of the race the one that crosses the finish line first? Because they will have different rates of acceleration. The bike that can accelerate the fastest is virtually always the winner.
And you know from experience that the lower the gear you are in, the greater will be your rate of acceleration. That is, if you are in first gear, you can change speeds (accelerate) much more 'easily' (quickly) than you can in fifth or sixth gear. Further, you know that there is a 'sweet spot on your engine's performance where you get the absolute best acceleration as compared to any other RPM.
From the chart above you should recognize that that 'sweet spot' occurs between 3,500 and 4,000 RPM, which as we have already seen is far below where you have maximum HP. In fact, if you look closely at the torque curve you will note that it *IS* the acceleration curve.
Let me make that perfectly clear for you. Let's say that your bike's speed is 35 MPH when the bike is in first gear and the engine is making 3,500 RPM. If you opened the throttle all the way then you could VERY QUICKLY accelerate to 40 MPH because you have max torque available to you there. On the other hand, if your bike was moving at 55 MPH while in first gear and the engine was turning at 5,500 RPM, even though your engine is putting out more HP there than at any other engine speed, it would take you longer to get to 60 MPH than it took you to accelerate from 35 to 40 MPH. That, because you have LESS torque at 5,500 RPM than you do at 3,500 RPM.
Now if you shift gears from first to second your engine RPM will decrease if you continue moving at the same speed. And, as we noted earlier, no matter what the RPM is in second gear, you CANNOT accelerate (normally) as quickly in second gear as you could in first gear. What that is telling you is that your gears act as multipliers of torque. What determines how fast you can accelerate is the amount of torque that reaches your rear wheel and that is determined by your gear ratios. So, for example, if your first gear ratio is 3.5:1 while it is 1:1 in sixth gear, then you can accelerate three and one half times faster in first gear than you can in sixth.
Did you notice that I said that your gears are a torque multiplier, not a HP multiplier? HP is a measurement of what your engine can do in terms of work over time. Torque (a force) is what does work, not HP. Your gears and your engine's ability to develop torque are what determines performance, not HP.
So there remains one more persistent myth (actually a misunderstanding) that needs to be dealt with here before you get the big picture. Many people will tell you that the way to get the fastest speed out of your motorcycle is to always shift at the highest RPM in each gear, not at the highest point of the torque curve. And because there is more HP at the higher RPM points than at the highest torque points, those people believe that performance is clearly HP related instead of torque related.
Though they are correct, more or less, about where to shift gears, it is not for the reasons they think. The reason you would want to shift at higher RPM points instead of at the highest torque point is that even though the engine torque is LOWER at, say, 5,500 RPM than 4,000 RPM, the torque at the rear wheel will be higher there in the lower gear than it will be after the shift, even if the new engine speed in the higher gear is closer to the max torque provided by the engine.
This is a very hard concept to understand but the following charts should help.
The left side of the chart shows the motorcycle's torque curve in first gear (which has a ratio of 3.5:1) while the right side shows it in second gear which has a ratio of 2.4:1. Observe that while the engine max torque is just over 80 ft. lbs. at about 4,000 RPM, because of the multiplying effect of the transmission gears it puts just over 280 ft. lbs. at the rear wheel. Notice also that even though the torque at the rear wheel DECREASES as the engine speed increases from 3,500 to 6,000 RPM, it is always higher than what it would be at the rear wheel after you up shift.
If the gear ratios were closer together than shown then there could be a time where the torque at the rear wheel was greater after the shift than before. So, the trick to maximize speed through your gears is to always shift at the point where the torque at the rear wheel is exactly the same after the shift as before.
Thus, shifting at 5,000 RPM would result in losing the acceleration causing torque shown in red, while shifting at 6,000 RPM results in no such loss. On the other hand, if the gearing was tighter, shifting at 6,000 RPM would have been a bit too late as had you shifted earlier the torque at the rear wheel would have been exactly the same before and after the shift. This should put to bed the argument that you should shift at the highest RPM in order to maximize speed!
Even though it 'looks like' shifting at max HP or max RPM results in max overall acceleration, since acceleration is EXCLUSIVELY about torque, not HP, you now know that in truth, the way you max acceleration is to shift at the points where torque at the rear wheel is equal before and after the shift, and that is determined by the torque curve and the gear ratios.
A couple of more thoughts for you to consider on the subject ...
You RARELY, if ever, get to use the max HP of your engine unless you are into racing while you FREQUENTLY get to use (and enjoy) the maximum torque developed by that engine. Surely you should want to maximize what you use instead of a number?
When you use your maximum HP you are wearing out your engine quickly while when you use your maximum torque you are extending the life of that engine as it relates to normal wear and tear. An important thought, no?
Advertisers and salesmen who insist on talking about how much HP an engine can make simply are playing to a dumbed down audience. HP is easier as a concept than is torque. But now you are an informed owner/buyer. You want to know about torque, not HP, thank you very much.
Copyright © 1992 - 2015 by The Master Strategy Group, all rights reserved.
(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)