The question regarding how much torque and horsepower is more than enough for road riding is being discussed … Yet again. It is a popular and reoccurring topic in the office. There are those who say more is always best, and those who say anything that could put you in jail is more than enough.
Reasonably quickly, this exchange gets to the somewhat unfocused question of whether 150BHP+ motorcycles have any place on the road? Why do we need that much power?
I get what is being asked, but the question seems way too one-dimensional.
The Argument
A Yamaha TZ250J from 1982 developed 57BHP, yet very few of us would think of using it as a road bike. Suzuki’s SV650 produces 70BHP, and whereas you wouldn’t race it against a TZ and expect to win, the SV is the better road bike.
So, is it a wide spread of torque that we are looking for, rather than raw horsepower? Electric motorcycles would have us believe so. They often remind us that maximum torque is delivered instantly and is maintained until the electric motor reaches its maximum RPM.
But, none of these figures tells you if the bike is good on the road. Those old enough to remember Honda’s 550/Four from the 1980s will remember it was an asthmatic, wheezing, wet weekend of an engine – until it got within a thousand RPM of the red line, and then it took off.
Time for some science-type stuff.
Torque or Horsepower
Torque and horsepower are produced when the fuel and air mix is ignited. The energy the explosion produces pushes on the piston, the crankshaft rotates, and we have drive.
If you look this up on the internet, you will be told that:
Energy is defined as the capacity for doing work.
Torque is a rotational force produced by the engine’s crankshaft turning. The more torque an engine has, the greater its ability to perform work.
Work is the result of a force acting over some distance and hence is talked about in foot-pounds. Foot, as in twelve inches and pounds as in weight. It is much more “work” to lift something at arm’s length than to raise it when close to you.
And finally, you would discover – as I have – that Power is how rapidly work is accomplished.
Therefore, you can define Power as the rate of completing work in a set amount of time.
Scottish engineer James Watt defined one [imperial] horsepower as the equivalent of the energy expended by a single horse raising 550 pounds of water one foot in the air from the bottom of a 1,000-foot deep well in 1 second. (33,000 lbs in 1 minute)
From there comes the term Horsepower. It is an arbitrary measurement, but it gives us a universal figure to judge an engine’s performance. Legend has it that Watt used the term to sell his steam engines to a Brewery Owner who wanted a comparison to his best horse … but as normal, I digress.
If you that are wondering – as I was – what the difference between HP and BHP is … HP does not consider frictional losses in power from an engine, while brake horsepower (BHP) does. The difference is marginal, so essentially the two figures can be viewed as the same.
And of course, there is a metric horsepower, which is defined as the energy expended by a metric horse lifting 75Kg, one metre in one second.
I Want More
HP = (TQ X RPM) / 5252 – Where HP is horsepower, TQ is pound-feet, and RPM is how fast the engine is spinning.
Or, in English, Horsepower equals Torque multiplied by RPM and then divided by 5252. If you want to know why it is divided by 5252, there is a brilliant video by Engineering Explained on YouTube that explains it.
If you want more horsepower, an engine needs to generate more torque, operate at a higher RPM, or do both.
The Perfect Engine
If torque and horsepower are interconnected, neither tells you how the bike will ride. I need a magic number that tells me how useful the power is and whether it will be there when needed.
It is often said that Honda’s parallel twin – as found in the Africa Twin, CMX1100 and the NT1100 – performs as if it delivers more than the 92BHP it actually produces.
Overall, Suzuki’s 1050XT V-Strom produces more BHP while the Honda has the edge on torque. Both engines are within a whisker of each other across the rev range.
The shape of the Honda’s torque curve might be why the engine receives so much praise. The convex shape of the torque curve, while the horsepower continues to climb, differs from the Suzuki’s, which peaks at the same 6,300 rpm but fades a little faster.
But I’m chasing a couple of pounds of torque here or the odd horsepower there. There isn’t a substantial difference between them.
So, after all my waffling about measuring Energy, Power and Work, I’m no closer to looking at the dyno sheet for an engine and being able to work out if a motorcycle is going to give me an invigorating or a frustrating ride.
Road Test Time
Perhaps if I look at this problem the other way around – find comparable motorcycles that are fun to ride and then look at the dyno charts – I’ll be able to spot the difference. And with this in mind, we’ve asked those wonderful humans at Triumph if we might borrow one of their Tiger 900 GTs.
The Tiger 900 has a T-plane triple crank and the unusual 1,3,2 firing order. On paper, the Tiger 900 is 17BHP behind the Tracer 9GT yet has only three lb-ft of torque less than the Yamaha.
Therefore, the Yamaha should “eat” the Tiger and be the more invigorating ride. BHP is the figure most manufacturers throw at us when laying out a motorcycle’s statistics, so BHP must be important – right? I’m not sure it is that simple.
The office arguments will still rage. Still, if we get closer to defining the mystical missing number that establishes a motorcycle’s flexibility – rideability – manners, then the road tests will be worth it.
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