Life's a drag

[Propellor]

Read an article recently, about aero drag and motorbikes. Saying how unaerodynamic a motorbike was compared to a car. That a good indicator of the drag is how much power is consumed to propel the vehicle to a certain speed, given that aero drag is by far the most deciding factor at high speed.

I go along the power consumption thing, but I have difficulty with what represents drag.

Reason I say this is because it seems that a modern car will need double the horsepower to achieve the same speed as a bike. Looking around the 130 mph mark.

So if consumed horsepower is the indicator of drag, a bike Seems to have less of it.

Clearly there's more to overall drag than just a slippery shape.

Ok, when it comes to saving the planet, or at least the oil reserves, how efficiently the respective vehicle musters up its necessary horsepower is a different thing again.

[CrazyFrog]

I think you would have to compare a motorcycle and car of the same weight for the comparison of the effect of drag to be meaningful between the two. 

[Propellor]

I think you would have to compare a motorcycle and car of the same weight for the comparison of the effect of drag to be meaningful between the two. 

I'm not sure weight has that much to do with it. Ok, more weight will probably add slightly to rolling resistance due to increased friction in the bearings and maybe tyre contact. More weight generally adds to the bulk of an object, which may affect drag, depending where and how the bulk is placed.  But, by the same token you could quite conceivably take a motorcycle and add weight to it but also make it have less drag. Quite easily, in my eyes.

Weight affects the rate of acceleration in a big way, yes. But once the vehicle stabilises out at a given speed it isn't accelerating any more, but the drag still counts.

Acceleration rate sometimes limits top speed because you simply run out of road!

[CrazyFrog]

Weight affects the rate of acceleration in a big way, yes. But once the vehicle stabilises out at a given speed it isn't accelerating any more, but the drag still counts.

Yeahbut, if a 747 is cruising at 500mph and three of its four engines cut out, it won't be able to maintain that speed in level flight.

It always takes more power (hp) to accelerate to a given speed and maintain that speed for a heavy vehicle than a light one, all other variables being equal.

[Propellor]

Weight affects the rate of acceleration in a big way, yes. But once the vehicle stabilises out at a given speed it isn't accelerating any more, but the drag still counts.

Yeahbut, if a 747 is cruising at 500mph and three of its four engines cut out, it won't be able to maintain that speed in level flight.

It always takes more power (hp) to accelerate to a given speed and maintain that speed for a heavy vehicle than a light one, all other variables being equal.

First sentence doesn't change a thing I said. Second sentence doesn't take into account time and wrongly (IMO and with respect) places emphasis on weight when considering top speed.

Let me put an example forward of what I said earlier.

Take a bike. Say... A gpz500.  Take all the bodywork off and all steel structure supporting it. Run the bike on as long a road as it takes to reach top speed.

Put the bodywork and supporting structure back on, thus ADDING WEIGHT. Run again under exact same conditions but still allowing as long as it takes to reach top speed.

Which version will reach the higher speed?

[CrazyFrog]

Which version will reach the higher speed?

If you added the exact same amount of weight back in by filling the frame tubes with lead, then it would be slower. If you add the weight back in by adding the bodywork back on, it may be faster, depending on whether the design of the bodywork reduces drag from the air by enough of a margin to offset the additional weight.

At least this is my understanding, but it's a long, long time since I did physics at school......

[Propellor]

Which version will reach the higher speed?

If you added the exact same amount of weight back in by filling the frame tubes with lead, then it would be slower. If you add the weight back in by adding the bodywork back on, it may be faster, depending on whether the design of the bodywork reduces drag from the air by enough of a margin to offset the additional weight.

At least this is my understanding, but it's a long, long time since I did physics at school......

Filling the frame tubes is a brilliant thought experiment. It doesn't change anything with regard to the shape. But I disagree with your conclusion. It would lower the top speed very slightly due to very slightly increasing rolling resistance (not to be confused with inertia!) IMO. But only for that reason. It doesn't change the drag at all so I would expect it to reach near enough the same speed. I would expect adding back the fairings etc to make a noticeable change upwards in top speed because the drag is lowered even though the weight is increased.

Regarding rates of acceleration, that's a different thing.

There's a very simple graph in kevin Cameron's sportbike handbook which has available horsepower plotted against mph with the curve being the combination of rolling and aero resistance. The bike will keep accelerating until the power available is no longer able to overcome those two combined factors. We're not taking time into account here, only ultimate top speed. Nowhere on his graph does it mention weight.

[CrazyFrog]

There's a very simple graph in kevin Cameron's sportbike handbook which has available horsepower plotted against mph with the curve being the combination of rolling and aero resistance. The bike will keep accelerating until the power available is no longer able to overcome those two combined factors. We're not taking time into account here, only ultimate top speed. Nowhere on his graph does it mention weight.

It probably doesn't take weight into account as it's effect, given the similarity in construction of modern sports bikes and the available power, is insignificant. However, ride a CG125 at top speed an then repeat the experiment with somebody slightly smaller (to reduce the effect of drag as much as possible) but equally heavy sat snugly behind you and I'll guarantee you wont reach the same speed.

If weight had no effect then surely the makers of Moto GP bikes wouldn't spend millions on trick carbon and magnesium components trying to reduce it?

[Propellor]

Crazy frog and meself have fired a few posts between us, but can I just add, ladies n gentlemen, this is not an argument! At least not from this end and I don't detect any of such from crazy frog. 

[CrazyFrog]

Indeed not, just an interesting discussion, and I suspect we are both right in parts.

We just need someone to come along who has more knowledge of physics (not difficult in my case!) to set the record straight!

[Propellor]

A) It probably doesn't take weight into account as it's effect, given the similarity in construction of modern sports bikes and the available power, is insignificant.

B) However, ride a CG125 at top speed an then repeat the experiment with somebody slightly smaller (to reduce the effect of drag as much as possible) but equally heavy sat snugly behind you and I'll guarantee you wont reach the same speed.

C) If weight had no effect then surely the makers of Moto GP bikes wouldn't spend millions on trick carbon and magnesium components trying to reduce it?

A) possibly. That's certainly true. There's little to choose between em at any time. But, reading the chapter in context, I think it is just demonstrating what limits top speed. Available horsepower versus the combined effects of rolling and aero drag. The book considers just the principles which lead to performance. These principles just happen to lead to the sportsbikes we now see.

B) however the pillion affects rolling or aero drag will determine the top speed. The weight, in proportion to the power of the cg125 will lower the acceleration rate considerably. But if you have a road long enough and conditions don't change, then the theory, as I interpret it, says it should reach a very similar top speed. Chances are, in the real world, you'll just run out of road or the conditions will change enough to render the experiment unscientific.

C) has no effect on what? It affects acceleration no question. It affects dynamics. It affects lots of things which determine the chances of winning a race. Also, racetracks have straights of a very finite length. Very short, relatively speaking. So a heavier bike will need a longer straight to reach the same speed. That won't win it the race though.

[Propellor]

Indeed not, just an interesting discussion, and I suspect we are both right in parts.

We just need someone to come along who has more knowledge of physics (not difficult in my case!) to set the record straight!

Good man.

Yes! Or someone who goes to the salt flats. Their objective, I predict, would be to lower rolling resistance, lower aero resistance and increase power. If adding weight tipped the balance of that equation in the right direction they would add it.

[Steve H]

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.

[themoudie]

Having visited The  Denny ship model experiment tank at Dumbarton at the weekend (The Scottish Maritime Museum) can I throw a large 'spanner' and suggest that the formulae to calculate ship resistance and powering might be similar to the 'slippery' ponderings you are having?

William Froude's laws maybe applicable.  I quote "Froude considered that the total resistance (Rt) of a model or ship could be subdivided into two parts with little interaction, and each part being subject to different scaling laws i.e. resistance due to skin friction (Rf) and resistance due to wave making known as residual resistance (Rr), such that Rt = Rf + Rr. (J Craig Osborne, 25th, January, 2007)".

There are then Froude's empirical formula for frictional resistance Rf = f A V 1.825 and further proportional relationships, with speed being the square roots of two vessels lengths and the resistance of the two vessels being the cubes of similar dimensions. These are all summarised from the pamphlet purchased at the museum.

Can you treat air the same as a liquid? Because, I perceive them to be similar in their movement when an object passes through them. The mathmatics appear simple, but I do not know if they are applicable?

Maybe reference to something in this 'Google' search will do the job; "wind tunnel simulator".

All of it way beyond my CSE Maths of 45+ years ago! 

Good luck with your deliberations and hypothesis.

My regards, Bill

[Propellor]

Nice one bill.

Is that the same froude as in the water dynamometers I wonder? 

I think you are right to associate the two mediums, but only to a point. Gas is compressible for one thing.

The thing with acceleration and reaching a theoretical top speed is that as you near the top speed figure the rate of acceleration tends to slow down. So when you're dealing with high weight and little power the last little bit of acceleration takes (seemingly) forever, so in the real world it seems that you never actually make it. The theory says, given the right amount of time and perfectly stable ambient conditions, you will make it.

One thing I do know is that crazy frog is a gentleman and I respect his opinion.

Cheers

Andrew.