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You can the entire collection of Safety Tip articles in a 33 Megabyte PDF Portfolio

 All Forums
 Motorcycle Safety
 Contrary Opinions
 Acceleration, horsepower, and torque
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DataDan
Advanced Member
541 Posts
[Mentor]


Central Coast, CA
USA

Yamaha

FJR1300
Peer Review: 1

Posted - 01/25/2011 :  2:20 PM                       Like
A good predictor of a motorcycle's maximum straight-line acceleration is power-to-weight ratio: peak horsepower divided by the machine's full-tank weight plus rider weight. An estimate that uses engine torque output instead will be a poor predictor. If you're comparing two bikes and one factor you're weighing is acceleration (but you don't have reliable test results to compare), the one with the greater power-to-weight ratio will be faster. A difference of 5% probably is not enough to tip the scales on otherwise well-matched candidates, but a difference of 10% probably is.


This subject came up in the recent thread Differences in HP and torque. I was unaware of the topic's history in this forum, but I quickly found Tip 221, Performance is all about TORQUE (Not HP) and a 2007 thread here in Contrary Opinions, Torque causes acceleration. I agree with some of the contrarian views in the latter, but I found that the thread lacked collegiality and specific focus on the points of contention. Thus, I am now revisiting some of those issues.

I do not claim that quarter-mile acceleration is an essential consideration when weighing motorcycle performance. My personal taste runs toward reasonably good maximum acceleration but a torque curve with a wide plateau of thrust. However, from an engineering point of view, there seem to be some misunderstandings that deserve discussion.

These are treacherous waters, but in I plunge...


From Tip 221:

quote:
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.
The Tip also dismisses the significance of power generally:

quote:
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....
In our context, power measures the rate at which a source can supply energy (i.e., do work) to accomplish a task--whether it's a generator supplying electrical energy for illumination or a motorcycle engine supplying mechanical energy for propulsion. A task that demands faster energy delivery requires more power, by definition. Lighting Dodger Stadium demands more watts of electricity than lighting your patio, and accelerating an 800-pound motorcycle from a dead stop up to freeway speed on a short on-ramp demands more horsepower than accelerating through town on a 300-pound scooter.

quote:
...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.
It takes more than just force to do work. It also takes the capacity to apply force in motion. With the proper tool, I can apply 50lb-ft of torque to my motorcycle's crankshaft. But I can't apply it while rotating the shaft at 5000rpm. Even with the help of a Rube Goldberg contraption to drive the engine via an exercise bicycle, and even with Lance Armstrong doing the pedaling, it isn't possible for a human to perform at the same level as the engine itself. That's because a human has a maximum power output of less than 1 horsepower while the engine can produce more than 50 horsepower.


An engine's horsepower isn't just a useless statistic calculated from torque and RPM. It's a measure of its capacity to apply force at speed, and it determines how hard a motorcycle can accelerate.

James R. Davis
Male Administrator
17286 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 01/25/2011 :  2:58 PM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:
It's a measure of its capacity to apply force at speed, and it determines how hard a motorcycle can accelerate.

Is it?

This is a dyno output for a motorcycle that is rated as having 72 HP and redlines at 5,500 RPM.


You will notice that it has 72 HP ONLY when it is at or near its redline. How many people ride their bikes anywhere near redline?

The bike, however, has a very nice (wide) Torque curve topping out at about 82 ft-lbs. You get the best performance out of this bike whenever its engine is turning anywhere from about 3,300 RPM to almost 5,000 RPM - in other words, almost any time the bike is moving, and certainly at other than when the motor is making max HP.

The Torque curve *IS* the acceleration curve. The HP curve is not and has *NOTHING* to do with acceleration.

It is easy to take a HP to weight data sample and draw your conclusion - if you misunderstand what HP is. You could make the SAME conclusion if you charted the Torque to weight instead of using HP and your conclusions would have merit.

Tell me again how important HP is.
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DataDan
Advanced Member
541 Posts
[Mentor]


Central Coast, CA
USA

Yamaha

FJR1300

Peer Review: 4

Posted - 01/26/2011 :  10:52 PM
quote:
Originally posted by James R. Davis
Tell me again how important HP is.

No question about it: A torque curve--along with weight, gear ratios, and rear tire radius--is the best possible information for estimating acceleration. That's what went into this graph:



But comprehensive specifications aren't always available. The best of manufacturers' spec sheets include peak power, peak torque, and road-ready weight, but none of the other data needed to calculate acceleration. Absent those essentials, acceleration cannot be accurately estimated using torque. In that case, the best way to rate motorcycles on estimated maximum acceleration is by comparing power-to-weight ratios.

Power-to-weight is a good surrogate for acceleration performance because at any speed, maximum possible thrust is peak power / speed [1]. Of course, due to the limitations of gearing the engine is seldom right at peak power. But manufacturers select transmission ratios spaced for good performance within a designed speed range, so except in 1st gear, a motorcycle accelerating at its maximum is usually within 5-10% of peak power. In the 919 graph above, the shift from 1st to 2nd takes the engine down to 15% below peak power, but thereafter it is within 5%.

Torque-to-weight, OTOH, is less useful for predicting performance. The most glaring examples of discrepancies occur between sportbikes with small, high-revving engines and big-inch, low-revving cruisers. For example, a Suzuki GSX-R750 and a Harley Road King have about the same torque-to-weight ratio. But the Gixxer's power-to-weight ratio is more than twice that of the Road King, a difference reflected in a quarter-mile ET difference of nearly 3 seconds. (The 2007 GSX-R and 2007 Road King data can be found in the MCN Performance Index PDF linked below.)

The following two graphs [2] show how power-to-weight and torque-to weight ratios differ in their correlation to quarter-mile acceleration:





Tighter clustering on the power-to-weight ratio graph indicates that it is a better variable to use when trying to predict acceleration differences among bikes.


You ask, "How many people ride their bikes anywhere near redline?" Great point. So why should we be concerned about peak power? A rider who has no intention of accelerating at the bike's maximum might still want to compare capabilities when shopping for a new motorcycle. It is the excess of peak performance that makes it possible to ride the vast majority of the time well below redline. That upper 20% of the rev range is there in reserve, and the built-in robustness it represents can extend engine life. In addition, some riders do use a greater part of their motorcycle's capabilities. Mountainous terrain, urban freeways where normal speeds are as high as 80mph, riding two-up, and riding with a lot of cargo all tax the bike's engine.


I contend that the best way to compare the maximum performance of two motorcycles--lacking either actual acceleration data or a torque curve and other key specs--is power divided by weight. But while you and I disagree on the importance of peak horsepower, we are in complete agreement about the importance of the shape of the torque curve. It tells a prospective buyer where he will find best response. Some cruisers have a huge peak of thrust at low revs that quickly tapers off, encouraging short shifting, while sportbikes pack their torque tightly toward the top of the rev band, encouraging high RPM operation. OTOH, the curve you posted (presumably of your GL1500) shows a wide ribbon of thrust through a good portion of the rev range, which makes it possible to ride without lot of attention to gear selection. Like you, that's the kind of engine performance I prefer.


[1] Here's how to calculate thrust from power and speed: Find your bike's peak horsepower, the RPM at which it is reached, and the speed you're traveling at that RPM in some gear (any one you choose). Multiply horsepower by 550 to convert to units of pound-feet per second. Convert speed to feet per second by multiplying MPH by 1.5. Divide converted power by converted speed to get thrust in pounds. That's how much force is being applied at the rear contact patch when you're at full throttle in that gear at that speed.


[2] Data used to construct the graphs is from Motorcycle Consumer News through 2010. At their website, MCN has a downloadable copy of their January 2008 Performance Index (380K PDF), which includes data from 1996 through 2007. Later test results are apparently not yet available in downloadable form. Peak torque and peak power are mulitplied by 500 for convenient scaling and divided by weight + 150lb for the rider.

Edited by - DataDan on 01/26/2011 11:04 PM
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James R. Davis
Male Administrator
17286 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 01/27/2011 :  8:29 AM Follow poster on Twitter  Join poster on Facebook as Friend  
That was an impressive message, Dan. It was filled with facts and illustrations and explanations and dots on all the I's. When I was CEO of a company and asked for a technical briefing, a presentation such as you just made would have marked the presenter for fast-tracking. Bravo!

I'm convinced that a power to weight ratio provides the best insight of how well a particular bike can handle a 1/4 mile race. Your evidence is compelling in that regard.

We had a question from a member which suggested a desire for information about how a bike 'feels' and performs, generally. It did not strike me that he was into racing, nor that he was interested in determining how to select a bike for running a 1/4 mile race.

This site is anything but interested in providing advice about racing or running a 1/4 mile flat out. Still, your presentation was extremely educational for all of our readers, including myself.

A few comments about the dyno chart you presented ...

The acceleration curves presented for each gear are the same curve -- they are the torque curve compressed or stretched as a result of gearing.

That curve shows an engine with a high RPM bias. That is, the torque increases until very near redline. In other words, max acceleration for that bike occurs when the engine is turning very fast. The curve I showed was for a bike with what is called a 'flat' and 'wide' torque curve that provides its rider max acceleration very near the middle of his RPM range. Obviously not a racing bike.

The acceleration curve for first gear also shows something that is typical for racing oriented bikes: the ability to reach 60 MPH while still in first gear. That, because racing bikes are sold with two promotional stats: Max Horsepower (to weight) and 0-60 MPH in so many seconds. When you do not have to shift to reach 60 MPH, you can save a tiny amount of time and you stay at the highest possible torque being delivered to the rear wheel. That is a marketing design consideration. A better behaved bike from the perspective of a casual rider is one in which a 'normal' shift point results in almost the same torque reaching the rear wheel before and after shifting instead of one which feels like you threw 25% or more of the engine away as a result of shifting.

You are a very valuable resource on this site. Thanks for your participation.

[The dyno chart I used was not for a Goldwing. It was, I believe, a 2004 Harley-Davidson Electra Glide.]
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rkfire
Advanced Member
1693 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 01/27/2011 :  5:28 PM   Join poster on Facebook as Friend  
I don't have an argument to which is more important HP or TQ, but when looking at either in a sales pamphlet or a magazine list, I don't think that's enough information. The plotted curve from a dyno shows me what an engine will provide. How many lbs. that torque has to move makes a big difference too though. I think if horsepower means, how much work is done over time, I think that is useful to know. Power to weight ratio, with a broad power curve, and geared to take advantage of that curve is what I want to look for. Maximum HP to me means little, the same as maximum torque, without knowing the rest.

My bike is one of those "tuned" for a wide power band. It's satisfactory for me. Taken from the older sport bikes, it's got less maximum HP and a boost in low end torque. It has more than 70ft/lbs from 3500 to 8500 rpm. That's a nice spread.

I think the sportbikes are getting short changed in this discussion though. A typical 600cc race replica has a torque output of about 45 ft/lbs and on a dyno graph will show about 30 below 5-6000 rpm, then a pretty flat curve to 14-15000 rpm. With it's lower gearing, and light weight, it'll pull harder than the big torquer motored but heavy bikes. I'd argue that a 10,000 rpm range of useful torque is NOT peaky. Yes they'll be revving at 5000 rpm or more, at 60 mph in top gear, but I'm pretty sure that the buyer of such a bike doesn't mind. By the way, I haven't heard 0-60 mph times advertised, or remarked about in magazines or forums. I don't think that's important, and most of the big performance bikes have a top speed more like 80 than 60 mph in first gear. My bike isn't even a performance bike and can go 75 in first.

Never mind a bike like a Hayabusa or ZX14, they have over 100 ft/lbs, and not that high up either.

That Harley dyno chart has a nice fat curve, but it isn't necessarily representative of all big twins. I've seen a lot that showed a peak at about 3000 rpm, but then drop off considerably from that point.

Many of the big cubic inch torquers, also happen to be big heavy engine, taking up considerable acreage as well. That makes the bike bigger, and heavier as well. The buyer wants a low revving, relaxed rpm, so they provide gearing that may result in more like 2800 rpms at 60 mph. There goes the advantage of gearing as a torque multiplier.

I'd suggest a look at Motorcyclist magazine chart, where they show 60-80 mph times in TOP gear for a number of bikes. Maybe 40-70 would have been better, but it's a fair measure of usable torque. There does seem to be a few anomalies, but in general it shows how the bikes performed. I was surprised by a few of them.

http://www.motorcyclistonline.com/p...s/index.html

I'm not an engineer, or even a dyno operator, but it's my understanding from reading several sources, that the popular motorcycle dynamometer Dynojet, is a linear dyno, and actually is measuring HP, then calculating TQ from that result as opposed to other types. If that's true, that's interesting as well. Why wouldn't horsepower be measurable, rather than some "calculated" number?
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DataDan
Advanced Member
541 Posts
[Mentor]


Central Coast, CA
USA

Yamaha

FJR1300

Posted - 01/27/2011 :  7:29 PM
I want to say that I have enjoyed contributing to this discussion, and if members now have a better understanding of the power/torque/acceleration question, that makes it even better.

quote:
Originally posted by rkfire

I'm not an engineer, or even a dyno operator, but it's my understanding from reading several sources, that the popular motorcycle dynamometer Dynojet, is a linear dyno, and actually is measuring HP, then calculating TQ from that result as opposed to other types. If that's true, that's interesting as well. Why wouldn't horsepower be measurable, rather than some "calculated" number?
The Dynojet is called an "inertial" dynamometer because it loads the engine by giving it a massive roller to spin up with the rear wheel (the inertia). What you would find in a manufacturer's R&D lab is a "brake" dynamometer, which loads the engine while it is running at constant speed by using it to drive a pump or generator to which a load can be accurately applied (the brake).

One disadvantage of an inertial dyno is that measurements can be made only when the engine is accelerating, so a "run" goes for only 10 or 15 seconds as the engine revs up to redline. The significant advantage is that they are inexpensive. Dynojet has made a big contribution to motorcycling by offering affordable dynamometers. Thanks to them, magazines are able to dyno the bikes they test, which is itself a "brake" on manufacturers' wild claims. And racers no longer have to rely on the seat of their pants to measure the results of performance tuning efforts.

What is actually being measured on an inertial dyno is rotation speed of the massive roller, and it is acceleration of the roller--the rate at which it is spinning up--that is translated into horsepower.

So far in this thread, we have defined power as the rate at which work is done: force * distance / time. A different definition, which can be proved mathematically identical, is the rate of change of kinetic energy (energy of motion): increase in energy / elapsed time.

In an inertial dyno, power is calculated from the change in rotational energy of the roller over time. Knowing the mass and diameter of the roller, you can calculate power by measuring its change in rotational speed over a short time interval.

So how does the dyno get the torque reading? As James said in Tip 221, power isn't affected by gearing like torque is. A 2:1 speed reduction in the primary gears, for instance, doubles torque but leaves power unchanged (ignoring frictional loss). Therefore, power at the crankshaft will be the same as power measured at the dyno's roller. And since horsepower = torque (in lb-ft) * RPM / 5252, just multiply measured horsepower by 5252 / engine RPM to get torque.
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rkfire
Advanced Member
1693 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 01/27/2011 :  7:43 PM   Join poster on Facebook as Friend  
Thanks for the clarification. I think I meant to say inertial, not linear. Too many browser tabs open at the same time..lol.
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bachman1961
Male Advanced Member
2266 Posts
[Mentor]


colorado springs, co
USA

Honda

CB750 NightHawk

Posted - 01/29/2011 :  12:46 AM

I have a friend that tunes and builds race bikes and seems to have no false hope toward performance realities. He talks of rear wheel hp rather than crank shaft, how altitude affects performance and is frequently at a local shop for dyno testing on bikes he works on.

Is it safe to assume the Dynojet is aimed at replicating the performance of a bike (it's weight) on pavement (load) under straight-line hp testing? I ask because it seems like this would be the 'real-world' number people are looking for and I (again) assume technology is aimed at getting meaningful results and cutting through the 'sizzle' of salesmanship.

I also wonder how many bikes look good on paper with hp and torque graphs until a test rider or racer gets it out on the track to evaluate it.

~brian

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rkfire
Advanced Member
1693 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 01/29/2011 :  9:37 AM   Join poster on Facebook as Friend  
The dyno isn't calculating power to weight etc, so it's not trying to predict 0-60, 1/4 mile etc.

There are online 1/4 mile calculators where you enter all up weight including rider, and horsepower and then get a good ballpark estimate. This is interesting to this discussion. I've never seen a 1/4 mile calculator use torque as the power input. I don't think it would work because you'd need more information.

The number of dynos out there, probably keep the manufacturers relatively honest in what they publish.

The biggest disparity, I think, are the 1/4 mile times published in the magazines vs what the average owner might do if he runs at a drag strip. The mags have a hotshot, lightweight guy that runs any bike very well in a few passes. The average Joe finds out there's more to a 1/4 mile run than whacking the throttle open.

I'd assume every engine today has some weaknesses that could be tuned out, just because they'll be lean in the rpm range that the EPA tests for certification. The tuner or racer will typically look at the air/fuel graph on the printout to see the lower rpms dips and bumps too.

If I recall correctly, Kevin Cameron recently did an article that said in effect that today's computerized, injected, superbikes come from the factory so well sorted that there is less and less power to be had by swapping exhausts and recalibrating fuel.
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Razzoo
Male Senior Member
317 Posts


Phenix City, Al
USA

Triumph

Sprint & HD XLH1200S

Posted - 01/29/2011 :  2:37 PM
quote:
Originally posted by rkfire

The dyno isn't calculating power to weight etc, so it's not trying to predict 0-60, 1/4 mile etc.

The number of dynos out there, probably keep the manufacturers relatively honest in what they publish.

The biggest disparity, I think, are the 1/4 mile times published in the magazines vs what the average owner might do if he runs at a drag strip. The mags have a hotshot, lightweight guy that runs any bike very well in a few passes. The average Joe finds out there's more to a 1/4 mile run than whacking the throttle open.



I like the magazine results as it allows you to compare bike X to bike Y. In the case of my Sprint the magazine tests also proved (to me) that the bike had more acceleration than I would likely ever need, and the bike (in the right hands) could corner quicker than I could.
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bachman1961
Male Advanced Member
2266 Posts
[Mentor]


colorado springs, co
USA

Honda

CB750 NightHawk

Posted - 01/30/2011 :  1:43 AM
quote:
Originally posted by rkfire

The dyno isn't calculating power to weight etc, so it's not trying to predict 0-60, 1/4 mile etc.

There are online 1/4 mile calculators where you enter all up weight including rider, and horsepower and then get a good ballpark estimate. This is interesting to this discussion. I've never seen a 1/4 mile calculator use torque as the power input. I don't think it would work because you'd need more information.

The number of dynos out there, probably keep the manufacturers relatively honest in what they publish.



That does make sense to me. I caught the jest of hp / weight as it related to performance years ago when the car publications added it to the stats. It was a quick way to compare, then I always had fun paring it down to performance per dollar, trying to gauge the better value in those terms. It seemed like the top contenders were $100k or more and you could get darn close performance (minus the status) for a third of the cost.

It sounds like a Dynojet showing a bike to put out 118 hp would be true within reasonable variables outside the building and on a flat surface, provided the spin-up simulated load is designed to meet that criteria (or a specified off-set).
Outside the shop, variables such as wind or drag coefficient and rider weight won't change the horse power of the bike but it will show up in the 0-60 or 1/4 mile runs.

In my case, all the 0-60 times tell me is how much potential the bike has for speed differential in traffic .. seems like certain death unless saving it for the track.

~brian
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CaptCrash
Male Advanced Member
744 Posts
[Mentor]


Nampa, ID
USA

Honda

Phantom

Posted - 01/30/2011 :  9:39 AM
My question here is based solely on technique/teaching issues. When I learned to ride my first bike was a single cylinder 500cc 4 stroke. My first dirt bike was a single cylinder 400cc 2 stroke.

My friends and associates taught me that my 2 stroke "Had no torque so you have to 'ride the pipe' (high RPMs) to make it go". While on my 4 stroke they encouraged "shift quick, everything on this happens down low."

Current curriculum (if memory serves) seems to lean toward "you may need to downshift to pass".

Right now I have 2 bikes, one a 400cc single 4 stroke, and another a 750 v-twin. BOTH respond well to revving them a bit. Meaning, from a dead stop they pull nicely, but at speed, they seem to pull better if I wring them just a bit. I'm not talking about bouncing them off the rev limiter, I'm talking running the v-twin in 4th gear instead of 5th on the crowded interstate so it responds more crisply to throttle inputs.

This is an offshoot, for me, of running inline 4s, where--especially on sportbikes--they pull nicely off the bottom but then, at 7K rpms all hell breaks loose and they take off like a scalded cat.

IF the torque curve is falling off, and the horsepower curve is picking up...how can...I mean...I guess I'm a little lost. The "Sweet spot" on any engine seems to correspond to the horsepower curve.

Am I missing something?


Edited by - CaptCrash on 01/30/2011 10:23 AM
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rkfire
Advanced Member
1693 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 01/30/2011 :  10:54 AM   Join poster on Facebook as Friend  
CaptCrash, I would suggest 2 things.

One is that either being in a lower gear, or downshifting to a lower gear is always going to give the advantage of mechanically mulitplying the torque through the transmission. Your Phantom has about 40 ft/lbs of torque, but it can be multiplied several times, the most times in first gear, the least times in top gear.

The other thing, is if horsepower is a measurement of work (power and time), then your Phantom makes the most HP at about 5,600 rpm. As an example, if you had a 2 cylinder 4 cycle motor, causing one power stroke every revolution, wouldn't you be better off having a power stroke 40 times a second, rather than 20 times a second by revving it twice as fast.

I think of horsepower @ a certain rpm, as the engines limit to efficiently making power.

Here's a graph of your bike: http://www.motorcycle-usa.com/50514...parison.aspx
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Dnc
Standard Member
132 Posts


Edmonton, Alberta
Canada

Honda

CBR250RA

Posted - 01/30/2011 :  11:10 AM
Some more light reading?

http://www.epi-eng.com/piston_engin...d_torque.htm

Maybe if we find enough explanations and examples, we'll find one that 'clicks' for everybody

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James R. Davis
Male Administrator
17286 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 01/30/2011 :  11:14 AM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:
IF the torque curve is falling off, and the horsepower curve is picking up...how can...I mean...I guess I'm a little lost. The "Sweet spot" on any engine seems to correspond to the horsepower curve.

An assumption or two is getting into your way. Your belief that HP curves necessarily continue upward while Torque curves drop off, for example, is misplaced. For example, here is a dyno run on a 2005 Suzuki GSXR1000.



This bike's 'sweet spot' is from just under 8K to almost 11K RPM. You will note that the HP curve falls off dramatically, just as does the Torque curve after 11K. The 'sweet spot' clearly is part of the Torque curve, not the HP curve.

Remember that HP is JUST a calculated value derived from Torque and RPM.
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DataDan
Advanced Member
541 Posts
[Mentor]


Central Coast, CA
USA

Yamaha

FJR1300

Posted - 01/30/2011 :  11:37 AM
quote:
Originally posted by CaptCrash

IF the torque curve is falling off, and the horsepower curve is picking up...how can...I mean...I guess I'm a little lost. The "Sweet spot" on any engine seems to correspond to the horsepower curve.
The sweet spot is the torque peak. In a given gear, the engine will pull strongest there. But when tuned for maximum horsepower, an engine's max torque can be produced at stratospheric revs. See for example this Sport Rider Magazine 600 sportbike dyno chart from 2008. Torque peaks are from 6% to 25% below the power peaks.

To see the experience you describe graphically, scroll back to the 919 acceleration graph earlier in this thread. Imagine that you're rolling along at 50mph in 3rd gear, turning 4500 with redline just short of 10K, and you roll the gas on to wide open. The bike will pull with increasing thrust up to 90mph--the sweet spot--then taper off. If you shift as soon as thrust begins to diminish, you drop down to the 4th-gear curve. If instead, you stay in third up to redline, you'll produce more thrust, even though you're past the sweet spot, because of the gear-ratio advantage of 3rd over 4th.
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CaptCrash
Male Advanced Member
744 Posts
[Mentor]


Nampa, ID
USA

Honda

Phantom

Posted - 01/30/2011 :  11:48 PM
Thanks gang. It's curious to me to see the optical illusion that the HP/Torque charts give--it's easy to interpret it as torque falling off because HP runs up so high...
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rkfire
Advanced Member
1693 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 02/02/2011 :  3:59 PM   Join poster on Facebook as Friend  
quote:
Originally posted by DataDan

I want to say that I have enjoyed contributing to this discussion, and if members now have a better understanding of the power/torque/acceleration question, that makes it even better.

quote:
Originally posted by rkfire

I'm not an engineer, or even a dyno operator, but it's my understanding from reading several sources, that the popular motorcycle dynamometer Dynojet, is a linear dyno, and actually is measuring HP, then calculating TQ from that result as opposed to other types. If that's true, that's interesting as well. Why wouldn't horsepower be measurable, rather than some "calculated" number?
The Dynojet is called an "inertial" dynamometer because it loads the engine by giving it a massive roller to spin up with the rear wheel (the inertia). What you would find in a manufacturer's R&D lab is a "brake" dynamometer, which loads the engine while it is running at constant speed by using it to drive a pump or generator to which a load can be accurately applied (the brake).

One disadvantage of an inertial dyno is that measurements can be made only when the engine is accelerating, so a "run" goes for only 10 or 15 seconds as the engine revs up to redline. The significant advantage is that they are inexpensive. Dynojet has made a big contribution to motorcycling by offering affordable dynamometers. Thanks to them, magazines are able to dyno the bikes they test, which is itself a "brake" on manufacturers' wild claims. And racers no longer have to rely on the seat of their pants to measure the results of performance tuning efforts.

What is actually being measured on an inertial dyno is rotation speed of the massive roller, and it is acceleration of the roller--the rate at which it is spinning up--that is translated into horsepower.

So far in this thread, we have defined power as the rate at which work is done: force * distance / time. A different definition, which can be proved mathematically identical, is the rate of change of kinetic energy (energy of motion): increase in energy / elapsed time.

In an inertial dyno, power is calculated from the change in rotational energy of the roller over time. Knowing the mass and diameter of the roller, you can calculate power by measuring its change in rotational speed over a short time interval.

So how does the dyno get the torque reading? As James said in Tip 221, power isn't affected by gearing like torque is. A 2:1 speed reduction in the primary gears, for instance, doubles torque but leaves power unchanged (ignoring frictional loss). Therefore, power at the crankshaft will be the same as power measured at the dyno's roller. And since horsepower = torque (in lb-ft) * RPM / 5252, just multiply measured horsepower by 5252 / engine RPM to get torque.


DataDan, do the sentences I've highlighted, confirm what I have read that the Dynojet dynamometers actually measure horsepower, and then calculate torque?
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DataDan
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Posted - 02/02/2011 :  4:24 PM
quote:
Originally posted by rkfire

DataDan, do the sentences I've highlighted, confirm what I have read that the Dynojet dynamometers actually measure horsepower, and then calculate torque?
Yes.

Power is measured as the rate of change in kinetic energy of the roller. Then torque at the crank is calculated from power and engine RPM.
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James R. Davis
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Posted - 02/02/2011 :  4:44 PM Follow poster on Twitter  Join poster on Facebook as Friend  
Said differently, ACCELERATION RATE is MEASURED, then HorsePower is CALCULATED, then Torque is CALCULATED.
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wildweathel
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CBR250R

Posted - 04/30/2013 :  11:43 PM

Ah, first post, first experience with the board eating a post...

I don't see torque and power curves as anything different. After all

* = * 5252 ft-lb-RPM / HP

It really bugs me how they're usually graphed, though. I think it adds to the confusion having two different y-axis scales or drawing two completely different engine layouts on the same RPM scale.

100% is engine speed giving peak horsepower - typically there's another 5-20% before redline. Curved gridlines are power. Picked four engines for variety.


(Terrible graphing software can't do labels on the curved lines. They're 9, 16, 25, 36, 49, 64, 81, 100, 121, and 144 horsepower from bottom-left to top-right.)

The two similar bikes are the 500cc Blast and Honda's 250cc sport-commuter. Both are often billed as beginners' bikes, both have about 25 HP. Little engine spins 30% faster, big one pushes 30% stronger - assuming the drive ratios differ by 1.3:1 they should accelerate almost exactly the same. (Or they would if they were the same weight.) A typical graph would hide this fact.

Both these engines, and the 2000cc VW diesel, show the top-end behavior Mr. Davis mentioned.

quote:
a 'normal' shift point results in almost the same torque reaching the rear wheel before and after shifting instead of one which feels like you threw 25% or more of the engine away


You can see this: shifts follow the curved gridlines. Thrust falls off as the engine speeds up (which feels natural) but stays consistent across a shift.

The supersport doesn't. The curve cuts up to the 100HP line, but trace it back and you can almost feel the upshift.

Note what downshifts from 50% or lower do on all the engines. It's a pretty significant difference when both mechanical advantage and improved torque work together.

All of this seems to match experience. I find it much more intuitive than rectilinear grids. Power curves especially all look the same to me - climbing up and up and I guess that's the marketing point.

Can't say one measure is better than the other when they're just different ways of presenting the same data.
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