it started with "it pulls 100MPH at only 1/2 throttle"

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And ask a GT750 owner to compare experiences with a H2 owner.
Chalk and cheese in the bikes weights and engine torque and flywheel weight depts.
 
acotrel said:
Rohan - how about an honest answer - how many times have you done 100 MPH on a motorcycle ?
Click to expand...

Tried it with most of the big bikes I've had.
850 C'do, Beemers, Guzzi, Suzook.
Too many roos out in the country to risk it, and too many noddy drivers in the city...
 
Dances with Shrapnel said:
From Tuning for Speed, Second Edition, Phil Irving suggested 2% for rolling resistance as "a good average figure to employ". For perspective, from recollection, heavy off-highway equipment usually uses about 3% rolling resistance (Caterpillar Handbook). Rolling resistance factors in tire/wheel properties as well as of rolling surface properties be they of concrete, asphalt, well compacted rock/earth, etc.
Click to expand...

I have not read Irving's book, or done any study on the topic of rolling resistance. I am just applying an engineer's "guts" to surmise the power consumption of a tire at speed.

A fixed percentage (% of weight on wheel?) does not consider the effects of tire distortion that occurs with increased speed. If you have ever seen high speed films of tires at high speeds, it will make you queasy to ride fast on a mc again! Centrifugal force distorts the rearward side of the tire in a bulbous lump. This requires energy and results in tire heating. The distortion is zero at slow speeds and increases with increasing speed (increased centrifugal force), so a fixed amount cannot account for the energy being dissipated in the tire. And that is for the front tire! The rear has additional distortion besides that of centrifugal force, because the tire patch exerts a tangential force (which increases with increased power requirements) on the tire as well.

Perhaps Irving was considering only the rolling friction force, and either ignored tire distortion or considered it somewhere else. I chose to lump friction and tire hysteresis as one term, and surmised that it could be represented by a function of the form:

RR = Crr x V^y ............ my "guts" says y is between 1 and 2. Crr is a coefficient of rolling resistance that makes the function work with proper numeric value and units of measure.

As I said above, I am not an expert in power dissipated in tires. Lots of engineering data must be in the archives on this topic, and I will certainly defer to data obtained by tire engineers. I do wish to stress that some speed related function must be included in the calculation of rolling resistance. And, of course, factors such as road surface, tire construction (radial or bias), inflation pressure, and loading (weight), and perhaps other factors should we think hard enough, all affect Crr.

Slick
 
All these cruisers with big wide tires, especially the front one, would likely haven a much higher rolling resistance than the slim tires used in Mr Irvings day...
 
C'mon guys, calculating hp is basic junior school stuff.
Have we all forgotten that much.
 
As long as “we” are trying to ‘splain the squared and cubed functions, couldn’t resist getting my oar in the water too, to take a stab at it from a very simple standpoint. I believe some/most of the confusion arises from statements that might be correct as far as they go, but yet were incomplete in light of the larger context. For example, a citation was posted in the other related thread stating “The air resistance increases with the square of speed, so at 200km/h you need 4x as much power as at 100km/h” which is perfectly true, but incomplete, as it addresses only air resistance. The correct statement would read “The air resistance increases with the square of speed, so at 200km/h you need 4x as much power as at 100km/h to overcome only the air resistance.” Additional power above and beyond the 4X is required to perform the work in ½ the amount of time.

At this stage there appears to be no disagreement that air resistance increases as the square of speed, so if you double speed you quadruple the drag (2^2 = 4). However, if you are now going twice as fast, the work to achieve this has to be done in ½ the amount of time, which in addition to overcoming air drag, requires doubling the HP. HP is a linear function of torque, i.e., doubling engine speed doubles HP if torque remains constant. Thus if the work is done in ½ the amount of time the HP has to double. So you have the quadruple factor required to overcome air resistance (4X), but you also have to have 2X the power do the work in ½ the amount of time, so multiplying these factors together provides the total power increase required to double speed which is - 2 x 4 = 8, or restated, the total power required to double speed increases as the cube of the change in speed - 2^3 = 8.

For example –

If speed doubles, the power required goes up by 2^3 = 8.

If speed goes up by 1.5, the power required goes up by 1.5^3 = 3.38

If speed goes up by 1.25, the power required goes up by 1.25^3 = 1.95

Let’s translate this last example into the real world, e.g., consider some imaginary Norton that goes 100 mph on 30 HP and we now want to go 125 mph. How much HP is required to attain this higher speed? Well, speed went up by 1.25 (125/100), so we cube the change in speed (1.25^3) to get 1.95, and multiply the original HP by this result (30 * 1.95) to get 58.5 HP required to increase speed from 100 mph to 125 mph, (HP almost doubled to increase speed by only 25%).

Clear as mud? I hope not.
 
Perhaps with a bike with a commando motor, when you raise the overall gearing high enough you end up in the same situation as with a two-stroke as far as torque is concerned ? However if you raise the overall gearing with a 4 speed box, first gear becomes much higher and unless you have close ratios, acceleration is much slower. With a six speed close box in a high geared commando-engined bike, you probably go better towards the ends of the straights than with four speed wide ratios ? The heavy crank is slower to spin up and more difficult to stall in a head-wind.
So the question is about whether higher top speeds is about horsepower or torque, given that situations such as the Bonneville Salt Flats are pretty rare.
 
so how does all of this apply to the original or other thread?

what was the main point/argument/disagreement? just cube vs square?

estimated top speed? power required? both?

was this part of it? based on a given speed vs throttle markings?
"it pulls 100MPH at only 1/2 throttle"
Click to expand...

WZ507 said:
As long as “we” are trying to ‘splain the squared and cubed functions, couldn’t resist getting my oar in the water too, to take a stab at it from a very simple standpoint. I believe some/most of the confusion arises from statements that might be correct as far as they go, but yet were incomplete in light of the larger context. For example, a citation was posted in the other related thread stating “The air resistance increases with the square of speed, so at 200km/h you need 4x as much power as at 100km/h” which is perfectly true, but incomplete, as it addresses only air resistance. The correct statement would read “The air resistance increases with the square of speed, so at 200km/h you need 4x as much power as at 100km/h to overcome only the air resistance.” Additional power above and beyond the 4X is required to perform the work in ½ the amount of time.

At this stage there appears to be no disagreement that air resistance increases as the square of speed, so if you double speed you quadruple the drag (2^2 = 4). However, if you are now going twice as fast, the work to achieve this has to be done in ½ the amount of time, which in addition to overcoming air drag, requires doubling the HP. HP is a linear function of torque, i.e., doubling engine speed doubles HP if torque remains constant. Thus if the work is done in ½ the amount of time the HP has to double. So you have the quadruple factor required to overcome air resistance (4X), but you also have to have 2X the power do the work in ½ the amount of time, so multiplying these factors together provides the total power increase required to double speed which is - 2 x 4 = 8, or restated, the total power required to double speed increases as the cube of the change in speed - 2^3 = 8.

For example –

If speed doubles, the power required goes up by 2^3 = 8.

If speed goes up by 1.5, the power required goes up by 1.5^3 = 3.38

If speed goes up by 1.25, the power required goes up by 1.25^3 = 1.95

Let’s translate this last example into the real world, e.g., consider some imaginary Norton that goes 100 mph on 30 HP and we now want to go 125 mph. How much HP is required to attain this higher speed? Well, speed went up by 1.25 (125/100), so we cube the change in speed (1.25^3) to get 1.95, and multiply the original HP by this result (30 * 1.95) to get 58.5 HP required to increase speed from 100 mph to 125 mph, (HP almost doubled to increase speed by only 25%).

Clear as mud? I hope not.
Click to expand...
 
84ok said:
so how does all of this apply to the original or other thread?
Click to expand...
It doesn't !
This is just a nice stroll, in the garden of horsepower.

84ok said:
"it pulls 100MPH at only 1/2 throttle"
Click to expand...
Click to expand...

I suggested that 100 mph requires ~30 hp, which is (neatly) half of the claimed 60 bhp for a showroom C'do.
Before this was violently refuted ....
But proved ~correct, exactly as per "Tuning for Speed".

OK, so it does pertain to that other half throttle, and a mentioned terminal speed of ~130 mph.
Explained above as that 58.5 hp
Which seems a little low, but never mind. RWHP ?

A nice clear explanation above too, BTW.
But it is pretty clear in Tuning for Speed.
They were writing for an audience often with little schooling too, so it had to be clear.
 
84ok said:
what was the main point/argument/disagreement? just cube vs square?
Click to expand...
Pretty much.
Dances with Shrapnel said:
Rohan said:
Lets not beat around the bush, air resistance rises with the cube of the speed.
THE huge factor in top speed.

A minor point, but good to quote accurately...
Click to expand...

Again, .............no Rohan.

Air resistance rises with the square of the speed, ............power rises with the cube of the speed.

A major point, but good to quote accurately... :lol:
Click to expand...

then it morphed into confusing an example of wind drag power requirements for a car with power required to overcome wind drag and other rolling resistances of a motorcycle based on some commonly quoted numbers. This is where we compare Orangutans to Oranges a bit. I clearly pointed it out in the reference to Phil Irving - Tuning for Speed where he clearly explains the power required to overcome wind drag is directly proportional to the cube of the velocity; Irving went on to present a graph illustrating the total power required (b.h.p) at the wheel to overcome wind drag. He factored in a reasonably good estimate of rolling resistance. The Orangutans and Oranges were clearly presented in the Phil Irving text. Unfortunately, this went over one bloke's head but I think he may be on board now.

Rohan said:
Dances with Shrapnel said:
https://en.wikipedia.org/wiki/Drag_(physics)
Click to expand...

For motorcycle applications, there is something notably lacking in that wiki discussion.
At speeds below ~40 mph, air resistance is fairly minimal for motorcycles.

This means common motorcycle data points don't really fit those equations.
For the average motorcycles (whatever they are),
it is commonly quoted that ~10 hp is needed to do 60 mph,and ~32 hp to do 100 mph.

Compare those numbers to their car example there,
wiki said:
A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW)
Click to expand...
Click to expand...

So as for relevance, it forced a few down a path of understanding and enlightenment. The actual power required at a given speed can be described by a polynomial which factors in power losses from the crankshaft through the drive train and also factors in drag forces at speed as well as rolling resistances.

Motorcycle Tuning Chasis by John Robinson has a nice presentation on the empirical analysis of total drag and presents hp drag in the form of aV + bV^2 + cV^3
 
i guess that puts us here, interesting you both cite phil irving, tuning for speed, tho there are also additional references

one sez tho
I suggested that 100 mph requires ~30 hp, which is (neatly) half of the claimed 60 bhp for a showroom C'do.
Before this was violently refuted ....
But proved ~correct, exactly as per "Tuning for Speed".
Click to expand...

along with the below


Rohan said:
84ok said:
so how does all of this apply to the original or other thread?
Click to expand...
It doesn't !
This is just a nice stroll, in the garden of horsepower.

84ok said:
"it pulls 100MPH at only 1/2 throttle"
Click to expand...
Click to expand...

I suggested that 100 mph requires ~30 hp, which is (neatly) half of the claimed 60 bhp for a showroom C'do.
Before this was violently refuted ....
But proved ~correct, exactly as per "Tuning for Speed".

OK, so it does pertain to that other half throttle, and a mentioned terminal speed of ~130 mph.
Explained above as that 58.5 hp
Which seems a little low, but never mind. RWHP ?

A nice clear explanation above too, BTW.
But it is pretty clear in Tuning for Speed.
They were writing for an audience often with little schooling too, so it had to be clear.
Click to expand...

so the "violently refuted" is about?

Rohan wrote:
air resistance rises with the cube of the speed.
Click to expand...
 
texasSlick said:
Dances with Shrapnel said:
From Tuning for Speed, Second Edition, Phil Irving suggested 2% for rolling resistance as "a good average figure to employ". For perspective, from recollection, heavy off-highway equipment usually uses about 3% rolling resistance (Caterpillar Handbook). Rolling resistance factors in tire/wheel properties as well as of rolling surface properties be they of concrete, asphalt, well compacted rock/earth, etc.
Click to expand...

I have not read Irving's book, or done any study on the topic of rolling resistance. I am just applying an engineer's "guts" to surmise the power consumption of a tire at speed.

A fixed percentage (% of weight on wheel?) does not consider the effects of tire distortion that occurs with increased speed. If you have ever seen high speed films of tires at high speeds, it will make you queasy to ride fast on a mc again! Centrifugal force distorts the rearward side of the tire in a bulbous lump. This requires energy and results in tire heating. The distortion is zero at slow speeds and increases with increasing speed (increased centrifugal force), so a fixed amount cannot account for the energy being dissipated in the tire. And that is for the front tire! The rear has additional distortion besides that of centrifugal force, because the tire patch exerts a tangential force (which increases with increased power requirements) on the tire as well.

Perhaps Irving was considering only the rolling friction force, and either ignored tire distortion or considered it somewhere else. I chose to lump friction and tire hysteresis as one term, and surmised that it could be represented by a function of the form:

RR = Crr x V^y ............ my "guts" says y is between 1 and 2. Crr is a coefficient of rolling resistance that makes the function work with proper numeric value and units of measure.

As I said above, I am not an expert in power dissipated in tires. Lots of engineering data must be in the archives on this topic, and I will certainly defer to data obtained by tire engineers. I do wish to stress that some speed related function must be included in the calculation of rolling resistance. And, of course, factors such as road surface, tire construction (radial or bias), inflation pressure, and loading (weight), and perhaps other factors should we think hard enough, all affect Crr.

Slick
Click to expand...

Based on what I have read, Phil Irving's 2% rolling resistance is a good estimate, though, in practice, the rolling resistance does vary some with velocity and is certainly dependent upon road surface conditions, tire type, inflation. In the book The Racing Motorcycle - A technical guide for construction by John Bradley, he presents a rather detailed discussion on rolling resistance (he calls it rolling drag), the factors and a few equations. Notable is his presentation of two equations; one for below 150 ft/s and one for above 150 ft/second. He goes on to say (paraphrasing here) tire rolling resistance is, from a practical sense, linear. According to the author, standing waves begin to occur when one reaches the tire speed rating at which point rolling resistance rises dramatically. For us mortals, this is a non-issue; or at least it should be.

Furthermore, if we take say a Commando with a fully ladened weight of 620 lbs and use 2% rolling resistance this works out to 12.4 lbs drag. At 60 mph this drag equates to (12.4 * 88fps)/550 = 1.98 hp. At 120 mph this drag equates to (12.4* 176 fps)/550 = 3.97 HP. In reality, there is some proportionality to speed but from what I have read, it is not significant for most speeds we are dealing with and the tire rolling resistance is a relatively small component of overall drag at speed. On the other hand, a land speed record would need to address the speed aspect. As another side note, most if not all of the power lost to the tire rolling resistance must be dissipated from the tire as heat - something to think about.
 
initial supporting argument/rational was

Rohan said:
Indeed.

Something to watch out for here, and where some of the confusion is creeping in, is that there are 2 rather different sets of equations involved here.
The straightforward power= blah blah ones,
and the differential power= blah blah blah ones.
The differential ones are the RATE OF CHANGE (per per per) (and so involve another per), so squared becomes cubed. ie per second per second per second, as distinct from just per second per second.
Say like - what is the difference in power required if we go from that 50 to 100 mph.

If that gross simplification makes any sense whatsoever.
You have to have an inkling of maths and differential equations to see them...

I'm not going to defend that air resistance cubed comment, that was incorrect in the context I mentioned.
It should have been squared. If we were exploring the rate of change of air resistance, it would be cubed.
Click to expand...
also
Rohan said:
C'mon guys, calculating hp is basic junior school stuff.
Have we all forgotten that much.
Click to expand...
 
We are just going round and round in circles here.

There is also quite a lot of baiting going on there too....
If 90% of those extraneous posts from everyone were deleted, it would make good clean reading.
The entire thread was finished really in the first page of posts - and we if we had left Jims "rise exponentially" unexplored...
But thrashing out these issues may have raised some awareness of some of this stuff.

I still have a couple of pages of charts and calcs, that I did in regard to my old Dommie that I had as a schoolboy.
It was well past its best by the time I got it, but I couldn't have afforded a Commando then.
Speed/gear change points, a stab at a torque curve, gear ratios - basic stuff.
Nortons claimed 29.5 hp* for that all iron 500cc twin and 90+ mph top speed, but owners had long suggested that was rather fanciful, you'd need a long steep hill to get anything like 90 mph out of one.
That also works out to quite a lot of rpms for an engine/cam that wasn't real revvy...
Nortons also claimed good brakes with those skinny little 7" brakes, and the road tests always said that "the braking wasn't up to Nortons standards", so this was seemingly a game the testers had to play then too.
We diverge from C'dos, slightly, this is ancestor history. !

*If they'd claimed 30 hp it would have had to do 100 mph.
Which the Marketing Dept probably wanted, but Engineering weren't going to play along,
so it ended up as 29.5 hp.
?? Don't know this, at all, but its not too hard to see the game in action.
What did Triumph claim, the T100 moniker suggests it would do 100 mph. Did they ?
 
we? you have the floor, make/state your case & what is the main point?

you finish here with questions

Rohan said:
We are just going round and round in circles here.

There is also quite a lot of baiting going on there too....
If 90% of those extraneous posts from everyone were deleted, it would make good clean reading.
The entire thread was finished really in the first page of posts - and we if we had left Jims "rise exponentially" unexplored...
But thrashing out these issues may have raised some awareness of some of this stuff.

I still have a couple of pages of charts and calcs, that I did in regard to my old Dommie that I had as a schoolboy.
It was well past its best by the time I got it, but I couldn't have afforded a Commando then.
Speed/gear change points, a stab at a torque curve, gear ratios - basic stuff.
Nortons claimed 29.5 hp* for that all iron 500cc twin and 90+ mph top speed, but owners had long suggested that was rather fanciful, you'd need a long steep hill to get anything like 90 mph out of one.
That also works out to quite a lot of rpms for an engine/cam that wasn't real revvy...
Nortons also claimed good brakes with those skinny little 7" brakes, and the road tests always said that "the braking wasn't up to Nortons standards", so this was seemingly a game the testers had to play then too.
We diverge from C'dos, slightly, this is ancestor history. !

*If they'd claimed 30 hp it would have had to do 100 mph.
Which the Marketing Dept probably wanted, but Engineering weren't going to play along,
so it ended up as 29.5 hp.
?? Don't know this, at all, but its not too hard to see the game in action.
What did Triumph claim, the T100 moniker suggests it would do 100 mph. Did they ?
Click to expand...
 
Aren't we allowed to ask questions here anymore ?
That 100 mph theme, again.
Been a big part in the brit motoring scene, for quite a large part of the past century even.

Most discussions on forums move forwards.
Lately here we seem to be looking in the rear view mirror, a lot ?
The question is, are folks gaining any knowledge or insights ?
Thats is why we are here ?
 
And just yesterday, someone posted about the 1st ever 100 mph lap by a Goldie at the IoM.
That really is a milestone.

http://www.iomtt.com/~/media/Files/2016 ... 2024.8.pdf

Apologies for the non-Commando diversion, but this sort of 100 mph stuff really has been the yardstick/icing on the cake so to speak, for motorcycling and motoring, for many many years.
Once-upon-a-time the motorcycling public could only dream about such things...
 
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