Can anbody give me a good answer?

[XKRacer]

I am having a discussion (we shall call it that for now) on another forum regarding power to weight ratio.

I say 2 identical cars both with the same gearing and power say 500BHP but have a different weight, one has been stripped out, and lets say one car weighs 1500kilo and one car weighs 2000kilo

Given a long enough straight road the lighter car will have a greater top end than the heavier car due to it's power to weight ratio.

The lighter car will have 333BHP per ton.

The heavier car will have 250BHP per ton


Am I wrong?

[VIVA GT]

I would have thought that the reduced weight would improve acceleration (disregarding any traction issues) but think that overall top speed wouldn't be affected as this is more of a power to aerodynamics equation, and not weight.
(Obviously, improved acceleration would allow you to reach this top speed earlier, or for you to achieve a higher top speed on a shorter straight).

[davec]

The lighter car would be faster but not for the reasons given. Terminal velocity is when the cars power becomes equal to the drag (as you go faster aerodynamic drag increases and equals the force of the engine)

BUT on this race the heavier car would have more rolling resistance and therefore when added to the aero load ( the same for both ) you get a higher backward force.

you could of course say that the heavier car would be sitting lower on it suspension and would therefore have les drag under the car but i believe that this would be going a tad to far.

Hope this Helps,

David

[Dave Brand]

It all depends on gearing. Given identical gearing, if both cars can rev right up to the rev limiter in top gear, then the lighter car will be no faster; if that's not the case, then the lighter car may be marginally faster, but I suspect the difference would be minimal, as the only effect the lighter weight would have at steady speeds would be to slightly reduce rolling resistance because the tyres are carrying less weight.

[XKRacer]

I am getting the feeling I am going to be prooved wrong here

But assuming the power does not outreach the gearing, I would of thought the lighter car would have more top speed


Isn't this prooved when a fast car that is aerodynamic then fits a spoiler to create down force reduces it's top speed due to drag etc?

[phoenix]

Given that the gearing, frontal area and CD factor allow for a top speed of, say, 150mph plus, the top speeds of both cars will be more or less the same - within 2mph I would say.

The higher the top speed attained, the lesser the difference will be between the two cars, as a higher percentage of the engine power is 'lost' to aero drag. Say the top speed attained by the lighter car is 180 mph, then the power lost to drag will be just under 400bhp! The heavier car would achieve 178.5mph with only (ONLY!) 389bhp lost to drag, but the same total of 500bhp 'consumed'.

Of course the car with the better power/weight ratio will accelerate more quickly (subject to traction grip being sufficient) and therefore achieve 'terminal velocity' in a quicker time/shorter distance.

[JohnD]

Originally Posted by XKRacer

I am getting the feeling I am going to be prooved wrong here

But assuming the power does not outreach the gearing, I would of thought the lighter car would have more top speed


Isn't this prooved when a fast car that is aerodynamic then fits a spoiler to create down force reduces it's top speed due to drag etc?

XK,
I think that proves the point against you. If on adding the spoiler, the same weight as the spoiler was taken out of the car, would it still have the same top speed? No. It weighs the same but has more drag >> less top speed.

Top speed depends on power, not weight (excepting the suspension height, rolling resistance etc.)
John

[GORDON STREETER]

Unfortunately its not a sliding scale. The top speed of a F1 car with 800 + hp is something like 230mph. Whereas a purpose built car with the same power to weight would probably be able to do at least 100 mph more.
It is all down to drag at the end of the day .

[phoenix]

Originally Posted by GORDON STREETER

Unfortunately its not a sliding scale. The top speed of a F1 car with 800 + hp is something like 230mph. Whereas a purpose built car with the same power to weight would probably be able to do at least 100 mph more.
It is all down to drag at the end of the day .

The BAR Honda managed 246.908mph (average of two runs) at Bonneville in 2006 - 4 years ago yesterday in fact. It is still the F1 land speed record holder.

They had a vertical fin in place of the rear wing and a lower downforce front wing than used in racing, but of course there would still be a lot of drag from the open wheel configuration.

[GORDON STREETER]

Yes but it had to be modded to do it . Adrian Newey (who I have raced against) told me that an F1 car in race set up can only pull about 230 mph.
The point I was trying to make was that power vs speed isn't linear as you need a damned sight more the faster you go. I'm not going to google around pretending I know the facts regards power vs mph , But of course you know that

[fourWheelDrift]

Originally Posted by GORDON STREETER

Yes but it had to be modded to do it . Adrian Newey (who I have raced against) told me that an F1 car in race set up can only pull about 230 mph.
The point I was trying to make was that power vs speed isn't linear as you need a damned sight more the faster you go. I'm not going to google around pretending I know the facts regards power vs mph , But of course you know that

Drag is proportional to the square of the speed so power is proportional to the cube of the speed so all other things being equal (and they never are) to go twice as fast needs eight times the power.

I'm so sad I didn't have to google it :-(

[phoenix]

Originally Posted by GORDON STREETER

Yes but it had to be modded to do it . Adrian Newey (who I have raced against) told me that an F1 car in race set up can only pull about 230 mph.
The point I was trying to make was that power vs speed isn't linear as you need a damned sight more the faster you go. I'm not going to google around pretending I know the facts regards power vs mph , But of course you know that

I wasn't arguing against you, Gordon - just agreeing you were in the ballpark!

[Goran Malmberg]

I imagine that the problem is about with power vs. weight of a rolling vehicle, and therefore what difference that makes on a potential top speed. We must therefore reduce away all extraneous factors so that we only have to do with the issue of factors involved. Air resistance is the major factor, then we have rolling resistance. We can think of two identical cars in terms of air resistance and it is tricky with how weight affects the rolling resistance in terms of a practical experiment. In any case, since rolling resistance is a minor factor so it will not differ much at top speed with a weight difference of 25%. But the lighter car will go a little bit faster.
(As fare as I understandable)
Goran

[XKRacer]

Thanks for everyones help on this........And I do understand the theory perfectly and agree....but.....I know from my experience driving minis....if you have your foot on the floor you will do 90mph all day long.....If you have the same mini with 4 up you will be lucky to get 75-80mph all day long so how does the original theory hold up in that circumstance?

[MGDavid]

Originally Posted by XKRacer

Thanks for everyones help on this........And I do understand the theory perfectly and agree....but.....I know from my experience driving minis....if you have your foot on the floor you will do 90mph all day long.....If you have the same mini with 4 up you will be lucky to get 75-80mph all day long so how does the original theory hold up in that circumstance?

could be aero? - Mini with 1 runs level, mini full runs nose up, tail down

[phoenix]

Originally Posted by XKRacer

Thanks for everyones help on this........And I do understand the theory perfectly and agree....but.....I know from my experience driving minis....if you have your foot on the floor you will do 90mph all day long.....If you have the same mini with 4 up you will be lucky to get 75-80mph all day long so how does the original theory hold up in that circumstance?

There is a huge difference between a mini at 90mph and a 500BHP car at 180mph.

For the cars mentioned in the OP

The aero drag at 90mph is a quarter of the drag at 180mph
At a steady 180mph, 80% of the power is consumed by drag and 20% by moving 'the load'
At a steady 90 mph 50% of the power is lost to aero and 50% to the load
At a steady 75 mph 34% of the power is lost to aero and 66% to the load

So, with a smaller engine the same applies. With four up in a mini at 75 mph about 66% of the power is used to overcome the load, leaving only 34% to overcome aero drag - hence the lower top speed (except downhill where more power is available to overcome the aero as less, thanks to gravity, is being used to carry the load)

[JamesH]

Originally Posted by XKRacer

Thanks for everyones help on this........And I do understand the theory perfectly and agree....but.....I know from my experience driving minis....if you have your foot on the floor you will do 90mph all day long.....If you have the same mini with 4 up you will be lucky to get 75-80mph all day long so how does the original theory hold up in that circumstance?

Could it be that you need power to accelerate, and you need power to counteract the aero. Once you are at a certain speed you only need to overcome aero - Newton First Law (?) (ignoring rolling resistance). But to get to that speed you need to accelerate, and it comes to a point where to accelerate the load in the car requires more power than you have left after taking off the power required to overcome the aero. The more load, the more power is required to accelerate it (F = ma), so a more highly loaded car cannot reach such a high speed as a lightly loaded car. But if the car started at the high speed, it could maintain it, because it does not need to accelerate. That would explain why when going downhill you can reach a higher maximum speed AND maintain it when back on the flat (you will slowly reduce speed due to increased rolling resistance).

Or something.

[Dave Brand]

Originally Posted by JamesH

The more load, the more power is required to accelerate it (F = ma), so a more highly loaded car cannot reach such a high speed as a lightly loaded car.

Close, but not exactly right! Everything else being equal, for a given accelerative force, higher mass just means slower acceleration.

[JamesH]

Originally Posted by Dave Brand

Close, but not exactly right! Everything else being equal, for a given accelerative force, higher mass just means slower acceleration.

But that would imply you can ALWAYS accelerate, but we all know you cannot do that.

My point was that all else isn't equal - as the aero resistance (and rolling resistance) has increased to a point where you don't have enough power to accelerate at all. Since your mass is higher is a heavier car, you actually need more force to accelerate it, that force cannot be provided by the power as all the power is going towards overcoming rolling and wind resistance. Remove some weight, and your power requirements to accelerate become lower, get below the air and RR threshhold, so you can accelerate slightly.

[JohnD]

Rolling resistance - heavier load the tyres are more compressed.
You could experiment with higher inflation pressures!
I suspect that you would need very hard tyres indeed to ensure that the contact patch was the same size under load.

John

[Dave Brand]

Originally Posted by JohnD

Rolling resistance - heavier load the tyres are more compressed.

It's also a function of tyre design - a heavier car on low rolling resistance tyres may have less overall rolling resistance than a lighter car on soft, sticky tyres!