Horsepower vs Torque

[GuzziMoto]

I loved the CB-1, had a friend who had one. I also had a Suzuki Bandit 400, I still have it but it is not running now. The motor is in parts on the floor and has been for a while. Priorities.

I have owned bikes with a variety of power characteristics. I have had peaky high reving bikes that make more horsepower than torque, mid range bikes that are somewhat even between the two, and low reving bikes that make more torque than horsepower. Due to the nature of the formula to figure out horsepower from torque and rpm, they should all make the same horsepower and torque at 5252 rpm. The way I see it, when a motor makes peak torque well below that number I would call it a torque motor, if it makes peak torque way above that number I would call it a horsepower motor. If it makes peak torque with a thousand rpm of that number on either side I would call it a mid-range motor.

 

And yea, I likes my women and beer like I likes my motorcycles. They all tend to have something to offer........

[Scud]

Due to the nature of the formula to figure out horsepower from torque and rpm, they should all make the same horsepower and torque at 5252 rpm. The way I see it, when a motor makes peak torque well below that number I would call it a torque motor, if it makes peak torque way above that number I would call it a horsepower motor. If it makes peak torque with a thousand rpm of that number on either side I would call it a mid-range motor.

 

It seems we have a useful guideline to inform our taxonomy of Torque, Mid-range, and Horsepower motors. This chart for a Harley Ultra Classic clearly shows a torque motor. The max RPM for the engine is only about 5,700RPM (so no ability to make peak HP at higher revs). Like the diesel truck I referenced earlier, there is more torque than HP at all realistic operating RPMs (but the difference is much greater in the Diesel).

 

It really is impressive that the Guzzi pushrod v-twin can rev as high as it does. (preaching to the choir...)

 

For past-bike comparison purposes, my favorite 4-cylinder, Japanese bike was a Yamaha FJ1100. I think that was probably a mid-range motor. It had power on tap everywhere and it never seemed to end. I recall that one magazine reviewer at the time nick-named it "The Velvet Sledgehammer."

[ScuRoo]

It’s a great little question… harder than you’d expect 

Have a crack at answering it before actually watching the vid

So, what would you say if a spotty kid specifically asked, “Hey mister, why does Horsepower & Torque ALWAYS cross at 5,252 RPM?”

 

As for me - I’d probably repeat my mother’s exasperated answer when I was a kid… “Y’s a crooked letter, & you should know better!”

That’s when I knew the day would go well for me if I shut up… 😏

[LowRyter]

9 hours ago, ScuRoo said:

It’s a great little question… harder than you’d expect 

Have a crack at answering it before actually watching the vid

So, what would you say if a spotty kid specifically asked, “Hey mister, why does Horsepower & Torque ALWAYS cross at 5,252 RPM?”

 

As for me - I’d probably repeat my mother’s exasperated answer when I was a kid… “Y’s a crooked letter, & you should know better!”

That’s when I knew the day would go well for me if I shut up… 😏

I haven't watched the video but just looking at the formula, both HP and TQ seem to be measuring units of power but at different RPM ranges.  TQ under 5252 and HP over that number.  That said, some engines like Harleys, never rev past 5k and others, like race engines, rev so high that the TQ peak is above the 5252.

But when looking at "metric" HP and TQ calculations, there isn't a crossover point.  That was explained somewhere on the board several years ago, not that I understood it.

[Joe]

My Indian Chieftain Limited 116 has 126ftlbs of torque and only 79HP. It will cruise with my wife and I along with all our “stuff” all day over 80 mph effortlessly. The bike weights over 850lbs so it’s not going to win a drag race with a sport bike from a dead stop, but will maintain speed with much less drama. BTW it’s not slow. 

[80CX100]

Iirc there's a saying,,,

HP is what wears parts out,,,torque is what breaks them,,,

[audiomick]

20 hours ago, ScuRoo said:

It’s a great little question… harder than you’d expect ...

 “Hey mister, why does Horsepower & Torque ALWAYS cross at 5,252 RPM?”

Well, I had to watch the video twice to figure out if the guy was talking BS or not, and do a bit of looking, but the answer isn't that hard when you know where the "magic number" 5252 came from.

So where did the number come from? That is a constant that James Watt calculated when he was defining horsepower in conjunction with his developement of the steam engine.

 

This might all seem a bit pedantic, but I'm putting it in on the off chance that it might help someone understand the video better. I had trouble with that because the guy neglected to define his premises at a couple of spots, and to explain why it is 5252, and that this number only applies if you are talking about horsepower against pound/feet (I learnt that as foot/pounds, but anyway...).

 

So, from here

https://en.wikipedia.org/wiki/Horsepower#History

 

Quote

Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute).^[6] The wheel was 12 feet (3.7 m) in radius; therefore, the horse travelled 2.4 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds-force (800 N). So:

${\displaystyle P=\frac{W}{t}=\frac{Fd}{t}=\frac{180\text{lbf}\times 2.4\times 2\pi \times 12\text{ft}}{1\text{min}}=\mathrm{32,572}\frac{\text{ft}\cdot \text{lbf}}{\text{min}}.}$

Watt defined and calculated the horsepower as 32,572 ft⋅lbf/min, which was rounded to an even 33,000 ft⋅lbf/min.^[7]

and in the next section

https://en.wikipedia.org/wiki/Horsepower#Calculating_power

Quote

When torque T is in pound-foot units, rotational speed N is in rpm, the resulting power in horsepower is

${\displaystyle P[\text{hp}]=\frac{T[\text{ft}\cdot \text{lbf}]\times N[\text{rpm}]}{5252}.}$^[16]

The constant 5252 is the rounded value of (33,000 ft⋅lbf/min)/(2π rad/rev).

and there we also see (I've made it red) the definition of the premise that the guy neglected at that point: we're talking about pound-foot and horsepower. The number doesn't apply to Nm and kW.

Edit: I should rather say, as far as I can tell with my limited math, the number most likely won't apply when talking about metric units, as it is part of the definition of hp, and doesn't play a role in the definition of one Watt.

 

The next point at which he neglected to define his premise is where he started talking about 42 at 2,500 revs, and so on. After that he did mention he is assuming a motor with constant torque at any revs. He also doesn't make it clear that the number 42 is "invented" for the sake of the calculation. He say's "at 2,500 rpm you have 42 (units of power)". That 42 sounds like a fixed value, but is not. It is just the number he chose to use for his example. (I suspect he enjoys reading Douglas Adams...)

That should have all been explained before he started in on his example calculation.

 

So, enough nitpicking. I hope that helps somebody...

[ScuRoo]

On 1/8/2023 at 2:33 AM, audiomick said:

So, from here

https://en.wikipedia.org/wiki/Horsepower#History

Wikipedia? Crikey! C’mon man… sourcing info from Wikipedia is about as reliable as listening to the CDC

 

On 1/7/2023 at 11:38 PM, 80CX100 said:

Iirc there's a saying,,,

HP is what wears parts out,,,torque is what breaks them,,,

Succinct

Lovely

[ScuRoo]

On 1/7/2023 at 10:19 PM, Joe said:

My Indian Chieftain Limited 116 has 126ftlbs of torque and only 79HP. It will cruise with my wife and I along with all our “stuff” all day over 80 mph effortlessly. The bike weights over 850lbs so it’s not going to win a drag race with a sport bike from a dead stop, but will maintain speed with much less drama. BTW it’s not slow. 

 

[audiomick]

2 hours ago, ScuRoo said:

Wikipedia? Crikey! C’mon man… sourcing info from Wikipedia...

If you're you're using it as primary and only source, yes, that is questionable.

 

If, however, it is serving as a source of "prefabricated" text to quote which conveniantly says exactly what one wishes to say, it is no longer questionable but rather just very handy.

I am fully aware of the shortcomings and advantages (yes, it does have some) of Wikipedia. I use it to check up on a lot of stuff, often as a starting point, but never as the ultimate authority on anything. Cross-checking sources is the trick, and using your own brain to decide if what you are reading is plausible or not.

The quoted article about James Watt and horsepower is plausible, and is borne up by other sources. I could put in a long list of corroborating links at this point, but I really can't be bothered.

[Guzzimax]

On 1/7/2023 at 2:19 PM, Joe said:

My Indian Chieftain Limited 116 has 126ftlbs of torque and only 79HP. It will cruise with my wife and I along with all our “stuff” all day over 80 mph effortlessly. The bike weights over 850lbs so it’s not going to win a drag race with a sport bike from a dead stop, but will maintain speed with much less drama. BTW it’s not slow. 

My MT01 is 111 ft/lbs at only 3750 rpm, & 89 bhp. A simple old school long stroke air cooled pushrod V twin. It’s 585 lbs, so although not exactly a lightweight, it’s no boat anchor. You ride on an effortless wave of torque, the flattest torque curve I’ve ever seen and by 5000 rpm it’s all over just before the rev limiter cuts in