Life's a drag

[Propellor]

Read an article recently, about aero drag and motorbikes. Saying how unaerodynamic a motorbike was compared to a car.

Aerodynamic, as I understand it is a concept, rather than something qualitative. So saying something is aerodynamic just means that its been designed in a way that it lessens the disruption to the airflow over it and therefore reduces drag.
Motorbikes have shed loads of bits sticking out which can disrupt flow, amongst these are indicators, handlebars, footrests, mudguards, riders etc etc. As such the air flow is more likely to be disrupted and as such could be considered less aerodynamic. (Yes there are are exceptions, but we are talking generalities here).

A car, even if it is more aerodynamic (in design), will present a much larger surface area to the airflow and as such drag increased, hence the need for more power.

Weight has nothing to do with top speed, apart from as propeller mentioned, increasing drag on the tyres.

This is getting towards my OP point. A bike has a generally  un aerodynamic shape, but in its favour it is narrow and has less in contact with the ground. The losses it accrues by the air tumbling etc it seems to more than compensated for by having a narrow front and less in contact with the ground. The performance figures of a few examples seem to bear this out.

A cg125 was mentioned earlier. Its 11HP goes an awful long way compared to a car. Imagine 11HP in even a smallish car.

Turn things around. Take the power of an Audi Q7 and put it in a bike. The bike mashes it in the top speed stakes, no matter how much time the q7 is allowed to accelerate.

[themoudie]

Aye Andrew,

One and the same!

Link for edification: Froude_Hoffman

Am I not correct in thinking that if you add weight (Mass) and frontal area to an object then the two act in tandem to absorb kinetic energy and increase friction with the air? They are separate and individually calculable yet produce a combined effect on the objects ability to travel from A to B in a specific time. Once accelerated the mass has no effect and has a value of zero, whilst the friction with the air continues to rise. The reverse is true with deceleration, the kinetic energy stored in the mass continues to propel the object, whilst the friction with the air is reduced. Well, in my 1 + 1 =2 world it does, although I acknowledge the abilty to distinguish and separate the influence of the 2 forces and their respective effects requires more grey porridge neurons than I can muster!

Off to "sheet alley" to rest my neurons!

Arrivederci!

Bill

[Oldtimer]

Look at the difference in power to speed on static dynometer tests. You will go as fast as the engine will rev on the rolling road but take it out on the open road with weight and no aerodynamics and see the difference in top speed. Power falls off at the top end of the rev range so it would be impossible to achieve the top speed of the engine at max revs in the same gear as tested on the rolling road. Applying resistance on the rolling road will give you a brake horse power reading. All tests will be done at maximum throttle opening and revs. As a rule of thumb 40 BHP would get you over the ton.
This may not be to relevant to what your post is about but thought I would throw it in anyway.

[Propellor]

Look at the difference in power to speed on static dynometer tests. You will go as fast as the engine will rev on the rolling road but take it out on the open road with weight and no aerodynamics and see the difference in top speed. Power falls off at the top end of the rev range so it would be impossible to achieve the top speed of the engine at max revs in the same gear as tested on the rolling road. Applying resistance on the rolling road will give you a brake horse power reading. All tests will be done at maximum throttle opening and revs. As a rule of thumb 40 BHP would get you over the ton.
This may not be to relevant to what your post is about but thought I would throw it in anyway.

Good point. I think it's relevant. It proves a point. That maximum power revs will coincide with top speed. But the tricky bit is that you have to pre know what the theoretical top speed is, so that you can fit the exact gearing to coincide the two parameters. Oh eck. That is virtually impossible to know to the enth degree. On the road wind and gradients muddle up the science anyway.

[CrazyFrog]

Wow, this is starting to make my brain itch.

Great discussion though, conducted in a civilized manner and a credit to the forum.

I agree that once an object is accelerated to a given speed, it's mass is irrelevant. It only becomes relevant when trying to increase or decrease the speed again. However, in the real world rather than in the vacuum of deep space, a vehicle is effectively constantly having to accelerate to maintain a constant speed (due to frictional losses from air resistance and in it's  wheel bearings etc etc. slowing it down),otherwise you could switch the engine off once you have reached 70 on the motorway, and continue at that speed indefinitely! Does it not therefore follows that mass (weight) is relevant in both reaching and maintaining a given speed, and the amount of power required to reach and maintain that speed must vary with the mass of the object.

I now remember why I hated Physics at school! I can report though that regardless of the laws of physics, I have just de-baffled and re-jetted my SLR 650 and it now sounds fantastic and goes like stink....

[Propellor]

Wow, this is starting to make my brain itch.

Great discussion though, conducted in a civilized manner and a credit to the forum.

I agree that once an object is accelerated to a given speed, it's mass is irrelevant. It only becomes relevant when trying to increase or decrease the speed again. However, in the real world rather than in the vacuum of deep space, a vehicle is effectively constantly having to accelerate to maintain a constant speed (due to frictional losses from air resistance and in it's  wheel bearings etc etc. slowing it down),otherwise you could switch the engine off once you have reached 70 on the motorway, and continue at that speed indefinitely! Does it not therefore follows that mass (weight) is relevant in both reaching and maintaining a given speed, and the amount of power required to reach and maintain that speed must vary with the mass of the object.

I now remember why I hated Physics at school! I can report though that regardless of the laws of physics, I have just de-baffled and re-jetted my SLR 650 and it now sounds fantastic and goes like stink....

Very good point. My understanding of the physics is that if there's no change in the inertial status quo then you're not accelerating or decelerating,  imagine being in the,back of a huge truck playing ping pong. The truck is fully enclosed, travelling at perfectly constant speed on a perfectly flat road etc. You can play ping pong just the same as being stationary. That's because you are stationary, in an inertial sense. Relative to the thing you are stood in. The vehicle still needs constant but steady power to maintain the status quo against the effects of rolling and aero drag.

Dunno if that is lucid or not! On.my mobile so not easy.

[Moto63]

Enough of this mumbo, jumbo. Just get out there n ride it like ya stole it I say. LOL. 

[Andy M]

The very reason I gave up Aeronautics and took manufacturing. The physics is mind bending!

I always like the thermodynamic approach:

Energy is never made or destroyed,  only changes form.
Any process has multiple results and an efficiency ratio of less than 100 %

The bike converts chemical energy into movement, heat, noise, a bit of radiation etc. So does the car. The bikes proportions are less efficient. To add one Joule of potential to the bike you start with more petrol and the air you passed through gets slightly hotter as a percentage. Make the bike more aerodynamic ( it's a number without a unit, only used in the efficiency)  and you get cooler air and a faster bike for the same petrol. Add weight and you end up with either more potential energy or less speed cos Energy is mass times velocity squared according to both Einstein and Steve Davies.

Andy

[Propellor]

..........Add weight and you end up with either more potential energy or less speed cos Energy is mass times velocity squared according to both Einstein and Steve Davies.

Andy

I happen to pretty much agree with what you said prior to the bit I've quoted above. In fact the weight and potential energy thing I agree too, it's just that I think it may not be applied quite in context. I have no problem with threads drifting at all, I think it's good, but just to put my perception of the physics as follows.

Yes, more mass for a given speed equals more potential energy, but this potential energy was built up during the acceleration period and once a stable speed is reached it remains potential until such time as we start to brake, and then the energy is converted. This phenomenon I don't see changing the point previously under discussion. Namely that for a bike or car to achieve a certain max speed the factors are available power versus the combined effects of aero and rolling drag. With more weight you end up with less speed only if you allow the same time. Allow more time and eventually the same speed will be reached. Unless the extra weight has an adverse effect on either aero or rolling drag. As I've suggested, it may help!

Regarding einstein's equation, do you mean e=mc^2? If you do, C is the speed of light, is it not? A constant.

[Propellor]

............Add weight and you end up with either more potential energy or less speed cos Energy is mass times velocity squared according to both Einstein and Steve Davies.

Andy

How about another approach as a response to the above. My maths only goes so far, so I'm sticking my neck out!

To know velocity we need acceleration and time. To know acceleration we need mass and a force.

The torque produced at the rear wheel produces tractive effort. A linear force ultimately.  You'll need to know the mass then to get acceleration. Given a certain time you'll end up with a certain velocity. The higher the mass, the longer the time taken to reach the same velocity.

But that casually ignores the real world factors, as pointed out by crazy frog. That being that there are opposing forces all the while acting to tend towards preventing the production of required velocity. Namely: rolling resistance and aero drag. Both forms of friction, but still considered as opposing forces. I think!

Phew.

Don't know if that made any sense or even if it is right!

[Andy M]

Impulse: E=M V ^2

Total energy in the system is mass times its velocity squared. Snooker balls, playing marbles or Newtons cradle.

( The white ball of course runs slightly cooler, but that gets out of Newtons nice neat world into Einstein and Hawking and the mind bending stuff) 

Andy

[themoudie]

AAAAAhhhhhh! The bells! 

Regards, Bill

[Oldtimer]

A railway engine cruises for much of its journey out of gear and that is a heavy mass of built up kinetic energy pushing the whole thing along without power. Doesn't have a lot of resistance between rail and wheel.

[Propellor]

A railway engine cruises for much of its journey out of gear and that is a heavy mass of built up kinetic energy pushing the whole thing along without power. Doesn't have a lot of resistance between rail and wheel.

What you guys are on about here is surely inertia? I'm still not convinced that this applies to the scenario of a car or bike maxing out top speed. The inertia affects the rate of acceleration and deceleration. Which will affect top speed if sufficient time isn't allowed. But in that case the top speed will be false, because the vehicle would have still been accelerating as it was forced to come off the power.

The really heavy train runs on for ages, but then it would have taken  a very long time to reach the speed in the first place. Either that or a LOT of power. (Traction would dictate the amount)

Crazy frog suggested that to keep maintaining top speed against the two forms of friction, we are constantly accelerating. If he's right then what you are saying would apply. But I'm not sure he is right. Think of putting an accelerometer on there. Once top speed is reached, will it show a reading? I would suggest it would show zero. You would be in equilibrium between the force pushing you forward and the net force of aero and rolling drag pushing back against you. As soon as another force is added eg more wind or a gradient, then acceleration or deceleration begins again.

[Propellor]

Impulse: E=M V ^2

Total energy in the system is mass times its velocity squared. Snooker balls, playing marbles or Newtons cradle.

( The white ball of course runs slightly cooler, but that gets out of Newtons nice neat world into Einstein and Hawking and the mind bending stuff) 

Andy

I can see how a heavier vehicle travelling as fast as a lighter one would posses more energy. It would cause more destruction if impacted a brick wall. Or even how a lighter vehicle travelling at a much greater speed would cause the same damage as the heavy one travelling slower. So the equation tells us something in energy terms about a moving body. How do we apply that to the context of the op?