Power Thrust Curves
Sat, 26/11/2011 - 14:45
Power Thrust Curves:
Captain Scarlet pretty much summed up the challenges and limitations of thrust curve graphs when he said,
"What I really liked was that they based the engine’s performance on actual RWHP measurements"
... Exactly. It is a definitive figure, albeit one based on a specific production model example, but without cloning these verbatim we have to use whatever press example they have been given as a genuine representative yardstick of a production model. It still doesn't allow for a riders weight, how aerodynamic they (or the bike) is, nor whether the rider is sat prone or upright, which will dramatically affect both drag and the overall centre of gravity; so some clear tangibles are intangible to clearly qualify in an almost comedic oxymoron kinda way. For this reason the best compromise is probably to ignore the intangibles and stick with vehicle weight with all fluids including a full tank and its rear wheel horsepower to provide a true power to weight. However even this does not provide absolute clarity regarding straight line performance, even ignoring the rider and vehicle aerodynamics. We still have to think about weight appropriation, manufacturer imposed artificial low gear torque restrictions, RBW throttle not providing WYSIWYG results, geometric design of the bike, tyre width and profiling, in terms of what percentage of that RW HP can actually be applied without loss of traction. Oh, there's another variable, traction-control, which again has different solutions and parameter controls dependent on which manufacturer is bringing the solution to the table!.”
A thrust curve graph usually only displays the maximum potential straight line performance at full throttle.
Maximum straight line performance is only a very small part of biking. Maximum rapid acceleration on many bikes and with many riders is not a prime consideration, nor should it be. Almost any bike has no problems with acceleration on public roads in keeping up with traffic.
To judge a bike’s value just on maximum throttle performance is a mistake.
Good actual riding reports like Kevin’s and so many other generous members of this site have far more value in the real world.
At best thrust curves are an interesting side note to good detailed rider reports.
We can only estimate air drag and its affects. We can minimize this error by comparing bikes of a similar type and style.
JAG
Sun, 27/11/2011 - 19:15
#4
Re: Power Thrust Curves
Thanks jag.
Discounting factors like wind drag, rider weight (as you've pointed out) I suppose the two most likely things to drastically upset the equation will be wheelying and/or rear wheel spin.
Is there an "industry standard" allowance for rear wheel spin used when compiling the curves. Eg 97 per cent or something like that?
As a road rider, additional curves would be useful at, say, 75 per cent throttle. The problem is, how do you define that, especially on different bikes (if we're comparing).
75 per cent throttle could mean twist grip position, throttle butterfly position or some way of defining mixture flow rate.
In terms of what a road rider would feel is good, it would have to be twist grip movement as there's noting else to relate to!
How is all this taken account of and factored in?
Sun, 27/11/2011 - 23:04
#8
Re: Power Thrust Curves
Hi jag
Thanks.
Yes I suppose I do mean wheel slip. Wheel spin is desirable for forward motion! I was thinking of anything that translates to less than 100 per cent transfer of rear axle rpm to the road surface and threfore thrust. Although can not thrust, like torque, also be a static force as well? Further opportunity for confusion!
I find the bit about what happens when we twist the throttle intriguing. If it directly controls compression pressure, why does that not instantly translate to extra torque and therefore extra rpm? I realise it doesn't as I've encountered it countless times, as we all have. But why?
The sensation is of a partial stall.
After many years and countless thousands of miles riding and driving, anticipating he effect becomes second nature, but i still would like to understand it better.
Sun, 27/11/2011 - 23:31
#9
Re: Power Thrust Curves
You guys are on a roll! :-)
It's too bad it's not easier to have some kind of graphic illustration for your discussions. When you start talking about technical stuff, and especially motorcycle dynamics, a picture is worth a thousand words and countless hours of trying to understand them!
Anyway, full throttle, not real world scenario when exiting a turn (at least when picking the bike up) in most cases (unless on my girlfriend's VTR 250).
But what is partial throttle? I have seen in Bike, torque curves at partial throttle openings. That was nice to see. But still, when exiting a turn it's not a fixed partial opening like 30% or 50% or 75%. You pick it up from a few % if you were on "neutral throttle" and you roll it, more or less aggressively eventually all the way to 100% over the course of several second possibly.
And some other food for thought: jag, you list accelerations in g's. That's fine... but that's not the real story as a bike's acceleration is not usually constant. Just as acceleration is the derivative of speed (or the rate of change of speed over time), acceleration can have its own derivative: the rate of change of acceleration over time (called "jerk" in physics, but also known as "jolt", "lurch" or "surge"). This also has a big effect on what sensation the bike gives you when accelerating.
Ninou
Mon, 28/11/2011 - 18:49
#11
Re: Power Thrust Curves
Hello again.
Don't know about on a roll. More like a wing and a prayer in my case!
What I meant was; on powered wheel vehicles, torque translates to thrust, and there can be static torque, so does it follow that there can be static thrust? Or at that point, is it simply referred to as load?
Tue, 29/11/2011 - 03:13
#13
Re: Power Thrust Curves
JAG - Isn't "performance data" more usefull? After all, a vehicle can have all the thrust you might think is sufficient but other factors may have adverse effects.
IMO, we need more detailed performance figures. Not just from a standing start but rolling data for each gear, at given speeds. One or more British mags occasionally provide this, which I find very beneficial.
For instance, Bike magazine provides 40-120mph (in top gear) figures, but IMO it would be even better if they did 60-120mph since some bike are rather tall geared.
However, 40-120 still indicates how flexible an engine is. Most useful if you're looking for a bike which you can simply leave in 6th without having to downshift for passing or hills.
Cheers.
Tue, 29/11/2011 - 03:57
#14
Re: Power Thrust Curves
"Captain Scarlet pretty much summed up the challenges and limitations of thrust curve graphs"
... thanks, Jag, nice to get some recognition at last, lol! ;-D
GT 'top-gear roll-on' tests have been a regular part of UK mag testing for many years. And you're right, it's a good indication of real-world performance, because it takes into account 'all' of the intangibles we mention above, and puts them into a simple formula (movement vs time) that we can all visually compute in our minds. I.e. We might not know what three seconds and a change of 5 lb.ft and 15 hp thrust curve in third between 2 and 7k rpm feels like in our minds; even though this is a very precise measurement of actual performance. But we can all relate to what 40 to 80 mph in sixth gear acceleration might feel like if it take 3 seconds rather than 6. I.e. Like seat heights, it's easy to over-analyse things, but nothing beats sitting on the bike and riding it to know whether it suits our individual geometry and riding preference. E.g. I might open the throttle slower than a young-gun. The bike is still making the same power per throttle revolution, but the user experience is different. Which is better? Because the data certainly isn't highlighting the important bit - the user element and their preference. Still, always nice to gesticulate and expand our noggins (minds) isn't it?
Tue, 29/11/2011 - 07:00
#15
Re: Power Thrust Curves
At the risk of publically outing myself as the uncouth thickie, which I undoubtedly am, I will add that whilst I genuinely do find all of these talks interesting, I can say hand on heart that I have never, ever given a hoot about numbers when selecting a bike when it comes down to it. There is much talk around here just now about wet weight, dry weight, a short wait or a long one but this has never influenced me, not a smidgin, in deciidng to either include or exclude a machine from my consideration. As someone who has spent all his life working in precision engineering, I feel I am pretty technically aware but somehow when it has come to my bike buying decisions then only really two factors come into play at all,
1) does the bike grab my attention on some kind of intangible, emotive way?
2) is it a fun riding experience?
That is it really no more, no less.
I am thinking about this whilst holed up in a hotel in Germany, with too much time on my hands and realising that my stable of bikes would most likely be completely different if I had analysed the purchase in a more analytical way before shelling out. As it is, each of them, all 12 (ridiculous I know but not all work) have a story to tell or mean something to me that can't be plotted on a graph.
I am not taking a pop here at anyone, far from it as this is all really good reading and interesting stuff - keep up the good work chaps.
Tue, 29/11/2011 - 07:03
#16
Re: Power Thrust Curves
now this is interesting when I said holed up in a hotel the post triggered the spam filter
but it does not in this one - we should analyse the algorithms in the filter machanism me thinks.
Tue, 29/11/2011 - 08:42
#17
Re: Power Thrust Curves
Even though I'm very interested in all this stuff, I agree with shuggiemac in that we shouldn't base our bike choice solely on them. We can't. Because no amount of graphs would truly tell us what the thing will FEEL like. I believe they are of some use though as they are bound to give us a good clue. If we HAD to buy " blind" I reckon the best help would be an unbiased, proper road test. An extensive one.
Personally, I just ignore the top gear roll on test. I sincerely believe that a thrust curve would tell me far more. If I want to accelerate fast I wouldn't stay in top anyway. Wouldn't want to. I admit that these graphs and such are only of any use if we can interpret the technical units into meaningful (therefore useful) information. In large part, that comes down to a reference against something we know. For example, we may own a bike which develops 75ft lb torque at 8000 rpm. We would know what that feels like, so it will go some way towards giving us a feel for the new bike we are looking at. Obviously the gearing would make a huge difference, but that's why I'm so interested in the idea of these thrust curves, used along with power/torque curves.
Back in the day, motorcycle mechanics magazine used to give you gear ratios etc. I think they were the first mag to start printing power and torque curves. And they had the formidable John Robinson testing and writing for them.
I'd still want to ride it though. Even the seating position can alter the way a bike feels under acceleration, cornering and braking etc. Then there's the noise...... Etc. etc.
Fri, 02/12/2011 - 08:37
#20
Re: Power Thrust Curves
Sounds a nice thing. Not sure which one is the rebel model. Is that our 250rs or cbx250?
Anyway, I couldn't agree more. I too subscribe to the school which believes HP and riding pleasure are not necessarily the same thing. Often quite the opposite. Big HP and speed will give you the "rush" and that's no bad thing. But there is something very satisfying about riding a willing, small cc machine down a back road. Like a little jack russell. Up for anything. Or even just taking things easy. Appreciating life to the full. A little bit of freedom in an increasingly crowded and "big brother" environment. Roll on springtime!
I think we need a forum subject titled "motorcycling philosophy"
Fri, 02/12/2011 - 12:23
#21
Re: Power Thrust Curves
Honda 250 Rebel:
"motorcycling philosophy forum" or how much money you don't have to spend to enjoy biking.
The picture was taken at some property I have. It's about 58 acres. It's a little hilly so I made a twisty grass track with some straight sections. I can get up to 60 or 70 km/hr with the Rebel as long as the grass is DRY. With a single leading shoe front brake I don't have to worry about too much brake. Love the belt drive. Don't have to lube it.
I think I even got a little air a few times while winding it out in third gear going over the top of one of the hills. I think you can have more fun on a little light bike in the dirt at 70 km/hr that going fast and straight on a highway.
There's something very satisfying about bring back an old bike to life and not spending a fortune to do it. It may still look a little rough around the edges but that's OK. Those scratches and dents tell a story. It's not perfect but then neither are we.
JAG
![user66_pic9_1292416107[1].jpg](http://www.ashonbikes.com/sites/default/files/user66_pic9_1292416107[1].jpg "user66_pic9_1292416107[1].jpg")
Fri, 02/12/2011 - 19:36
#22
Re: Power Thrust Curves
I'm with everybody here. I look at the figures and the graphs and the stats and the tests and then....
I bought the V Strom because it just felt so right when I sat on it. I bought the Guzzi's because they sound nice and shake a lot (and I used to watch a bloke riding around on one and he looked cool).
A good friend has sold his big Beemers and now runs around on a stable of mobylets, keeps a smile on his face.
I conclusion I like the figures/graphs etc... in a theoretical, top trumps, what I would spend a lottery win on, kind of way. Then I actually buy a bike for the way it makes me feel.
Fri, 02/12/2011 - 23:46
#23
Re: Power Thrust Curves
UcR
It's a shame you've seen these graphs in that competitive sense. It's true that it has become fashionable to use them in this way. A set of graphs or curves is just an expression of the machine's performance and specification. That is what it will tell you, not how it actually FEELS.
We can use this in a competitive sense to try and select the fastest machine. But as road riders we should use them to better understand, in he absence of actually riding the thing, how it will perform. To a small degree, how it will feel. As I've said above, we can't help but use this information in a comparative sense, because all information is useless without a datum to compare it to. But it doesn't have to be competitive.
A set of graphs or curves for your v strom or guzzi's (90 degree v twins. Delectable) would tell you quite a lot about them without any need at all for them to compete against another machine.
Mon, 05/12/2011 - 17:45
#25
Re: Power Thrust Curves
2010 BMW S 1000RR VS 2006 Honda Blackbird 1100 RWHP GRAPH
Below is a RWHP graph of a 2006 Honda Blackbird 1100 and a 2010 BMW S 1000RR.
This first graph is a typical RWHP VS RPM graph.
As you can see the Blackbird makes more RWHP between about 5600 RPM to 8600 RPM.
The BMW's high RPM really pays off above about 8800 RPM.
The next graph is a Thrust Curve graph of the same bikes based on the same RWHP.
Tue, 06/12/2011 - 12:10
#26
Re: Power Thrust Curves
BMW 2010 S 1000RR & Honda 2006 Blackbird 1100 Power to Weight Graph
Below is a power to weight graph of the BMW and Honda based on engine RPM.
The Honda still has an advantage from about 6000 to 7000 RPM.
The BMW really takes off of course above 9000 RPM.
NOTE: A 185 pound rider was included in the weight calculations.
This graph shows the affects of weight on straight line acceleration.
Much of the performance or RWHP advantage of the Honda has been lost between about 6000 to 8000 RPM.
Even though the BMW makes less RWHP in the low RPM range, its much lighter weight results in a better power to weight ratio.
Of course the next big factor in straight line performance is gearing.
Fri, 09/12/2011 - 12:33
#27
Re: Power Thrust Curves
How does it feel scale
Below is my interpretation of how the power to weight ratio might feel in terms of response.
The road speeds are less than 100 MPH.
Fri, 09/12/2011 - 12:52
#28
Re: Power Thrust Curves
HP and enjoyment.
I picked up an old Honda Rebel 250 from a friend a few years ago. It was in his garage for many years. It wouldn’t run. He said it was taking up space and wanted to get rid of it. If I wanted it I could have it.
After I cleaned the tank, carb and changing all the fluids and installed a new battery, it started right up.
On a good day it makes about 15 HP. I love how smooth and quiet it idles. Single carbureted engines make life a lot easier. The hotter it gets, the better it runs. The engine tries so hard to please at almost any RPM.
It’s no rocket, but under 50 MPH I like it better than my bigger bike. It’s a real hoot to ride on the back country little roads or in town.
JAG
Couldn't agree more JAG-I love my Multistrada-which being a twin doesn't do terribly well on the 40mph in top gear roll on test's-but it will thrash most bikes coming out of a corner-which is of far more relevance to actual road riding. However, I probably get more pure fun riding around on my 40 year old Ducati 450RT with it's 32bhp, and knobbly tyres...............
John
Fri, 09/12/2011 - 20:23
#29
Re: Power Thrust Curves
Hi jag
Couple of questions to help get my head around this a bit more.
What gear are we in? Top? Is your rwhp estimated from published crankshaft figures and an amount deducted for transmission losses, or from a rolling road test?
To calculate lbs thrust from rwhp you'll have to know rear wheel speed and rolling circle of tyre? Where would one find the data to calculate that? Rolling circle got from tyre aspect ratio and wheel size? But what allowance made for the amount the tyre settles or "squidges"?
Also, not sure if I've got this right, but the calculated thrust is still not the "nett" thrust as other losses will apply, mainly aerodynamic drag, as I believe you have intimated elsewhere.
Wouldn't mind having a crack at doing a set of thrust curves for the new nc700 Honda and comparing it with another 50 bhp machine with a more conventional (for a motorcycle) torque curve. Don't know which yet tho!
I don't think I'd be ready to trade in my r1200r just yet, but I am intrigued by the ultra low rpm at which the new Honda delivers the bulk of it's torque.
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You'll have to forgive my ignorance with this, but I've not seen thrust curves used before.
Am I right in assuming that they are calculated rather than measured? Is the calculation rear wheel torque divided by effective tyre radius? Presumably they're expressed in N or ib's ?
I assume that the idea is to translate a vehicle's power into something meaningful, by taking into account the overall gearing, including the tyre radius?