I have a feeling this is going to have been discussed many times, but here goes. With everything else being the same on a car, if you halve the
weight, what happens to the 0-60? My O level physics suggests force = mass x acceleration, so halving the mass and keeping the force (engine power)
the same should double the acceleration?
I suspect there will be a certain amount of diminishing returns as other things come into play (tyre grip being an obvious one). And I was
considering this from halving the weight of a 2 ton tin top rather than trying to scrape every ounce from a 700 kilo seven.
If we define the unit of lightness as per tonne (i.e. 1/mass in tonnes).
Then acceleration is linear in lightness.
This is all from a theoretical physics perspective. So completely true for a rocket in outer space, I doubt it will be quite as simple in your Mondeo
likely it will just bugger it up.
But the idea is correct. Do not forget to add in the mass of the driver though. That is difficult to change, though not impossible. Likewise
aerodynamics and friction which are "constants" (though air resistance increases by a factor of 4 every time you double speed, friction
possibly similar)
On light weight cars I reckon the greater impact is agility. After all nobody really cars how fast to 60 a car is, not really exciting stuff unless
you are into drag racing.
quote:
Originally posted by cliftyhanger
But the idea is correct. Do not forget to add in the mass of the driver though. That is difficult to change, though not impossible. Likewise aerodynamics and friction which are "constants" (though air resistance increases by a factor of 4 every time you double speed, friction possibly similar)
On light weight cars I reckon the greater impact is agility. After all nobody really cars how fast to 60 a car is, not really exciting stuff unless you are into drag racing.
Eat carbon fibre, double the benefits
The main effect of adding lightness when it comes to accelerating is reducing grip, just look at the difference in qtr mile time between a 1tonne RWD card and a 1tonne FWD car with identical power, would be as much as a second in it, which would probly equate to .5 to .75 of sec 0-60 as thats where all the difference is gained or lost. All because the weight its transfered to the driven wheels on a RWD. (touring car starts are a great example of this, just watch the BMWs shoot off compared to the rest of the FWD field)
The returns would be fairly linear for a heavy tin-top assuming good tyres and not too much power. A quick calc suggests an reduction in time of about
1.75x, so 0-60 of 4.5s vs 7.8s for 225bhp.
The tradeoff is more slip vs better acceleration, in this case traction is limited ~0-10mph for the heavier car and ~0-20mph for the lighter one. The
more power the less benefit you get from lower weight.
In the extreme cases (low weight/high power) you can be traction limited to beyond 60mph, still makes sense to go higher bhp/ton though as you can use
bhp to overcome aero drag >75mph and lower weight to get around the corners quicker and carry more speed into the straights.
Adding lightness is a stupid expression which really annoys me (well a bit anyway) If you stick something on it adds weight, if you remove something it subtracts weight and if you then replace it with something lighter you are adding weight again but not as much. You can't add lightness you can replace heavier components.
quote:
Originally posted by Peteff
Adding lightness is a stupid expression which really annoys me (well a bit anyway) If you stick something on it adds weight, if you remove something it subtracts weight and if you then replace it with something lighter you are adding weight again but not as much. You can't add lightness you can replace heavier components.
i think Chapmans full famous quote was 'simplify and add lightness'.
whilst possibly not grammatically correct, its a mantra all race cars should be built by.
Chapmans obsession with no part on the car only doing one job (such as gearbox/engine stressed member) is why he revolutionised single seaters, albeit
sometimes went too far and they were somewhat fragile...
I think the formula for total drag is 1/2 x rho x v^2 x s x cd x L.
(It was something like that whan I was a lad anyway). The important thing is that our cars have a very poor cd (coefficient of drag) and the v^2 in
the equation means that total drag (on the performance we experience) is dependent on, and increases as a function of the square of the speed.
As was pointed out by an earlier poster, the Total Drag at 40 mph is a function of 40^2= 1600, whilst TD at 80mph is a function of 80^2= 6,400.
It explains why squeezing an extra few mph out of 'Supercars' in the 200mph range takes vastly more horses and 'bleeding edge'
aerodynamics.
quote:
Originally posted by Peteff
Adding lightness is a stupid expression which really annoys me (well a bit anyway) If you stick something on it adds weight, if you remove something it subtracts weight and if you then replace it with something lighter you are adding weight again but not as much. You can't add lightness you can replace heavier components.
You are taking his words far too seriously. He was merely commenting on the impossibility of the task of getting speed under the well established rules of physics.
I always assumed 'adding lightness' was intended as being gramatically incorrect in an ironic sort of a way.....?
From what I know of the man it would have been VERY tongue in cheek!
quote:
Originally posted by loggyboy
The main effect of adding lightness when it comes to accelerating is reducing grip, just look at the difference in qtr mile time between a 1tonne RWD card and a 1tonne FWD car with identical power, would be as much as a second in it, which would probly equate to .5 to .75 of sec 0-60 as thats where all the difference is gained or lost. All because the weight its transfered to the driven wheels on a RWD. (touring car starts are a great example of this, just watch the BMWs shoot off compared to the rest of the FWD field)
quote:
Originally posted by zilspeed
Car A has 180bhp, front engine, rear wheel drive. It weighs 1200kg.
Car B has 180bhp, front engine, rear wheel drive. It weighs 450kg.
Both car have a front rear weights split of 60/40
Which car will accelerate harder from a standing start ?
(with CarB wheelspin at 1th and 2nd
gear) quote:
Originally posted by loony
quote:
Originally posted by zilspeed
Car A has 180bhp, front engine, rear wheel drive. It weighs 1200kg.
Car B has 180bhp, front engine, rear wheel drive. It weighs 450kg.
Both car have a front rear weights split of 60/40
Which car will accelerate harder from a standing start ?
Using old "CARTEST" - with Caterham Super7 at 180bhp I got 0-60 in 7,2s for Car A and 3.9s for Car B
Maybe it's not professional tool, but you can change lots of parameters.
Link to CARTEST site: http://www.cartestsoftware.com/cartest4.5/
