Slimy38
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| posted on 13/4/12 at 07:39 AM |
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Impact of 'adding lightness'
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.
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matt_gsxr
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| posted on 13/4/12 at 07:48 AM |
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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.
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cliftyhanger
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| posted on 13/4/12 at 07:55 AM |
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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.
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scudderfish
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| posted on 13/4/12 at 08:02 AM |
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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.
Reducing driver mass by 10kg is often a lot cheaper than reducing car mass by 10kg. Less pies not more carbon fibre.
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Daddylonglegs
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| posted on 13/4/12 at 08:13 AM |
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Eat carbon fibre, double the benefits 
It looks like the Midget is winning at the moment......
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loggyboy
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| posted on 13/4/12 at 08:16 AM |
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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)
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kjouk
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| posted on 13/4/12 at 08:51 AM |
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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.
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Peteff
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| posted on 13/4/12 at 10:09 AM |
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Bit of a rant.
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.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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loggyboy
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| posted on 13/4/12 at 10:12 AM |
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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 beleive its a phrase pinned to Colion Chapman, whose designs did add lightness, for example, making the gearbox a stressed component of a chassis,
meaning you removed chassis weight, and didnt replace it as you would have had a gearbox of similar weight anyway. So its not ALL cases where you have
replace something. And also if you look as the weight of the car as a 'whole', then by switching a component with a lighter one is adding
lightness to the whole weight.
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CNHSS1
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| posted on 13/4/12 at 10:18 AM |
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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...
"Racing is life, everything else, before or after, is just waiting"---Steve McQueen
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bobinspain
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| posted on 13/4/12 at 10:44 AM |
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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.
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Slimy38
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| posted on 13/4/12 at 10:48 AM |
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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 have to admit I do agree with the sentiment, hence the quotes....
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RK
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| posted on 13/4/12 at 11:51 AM |
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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.
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FASTdan
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| posted on 13/4/12 at 01:42 PM |
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I always assumed 'adding lightness' was intended as being gramatically incorrect in an ironic sort of a way.....?
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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alistairolsen
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| posted on 14/4/12 at 11:50 AM |
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From what I know of the man it would have been VERY tongue in cheek!
My Build Thread
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zilspeed
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| posted on 15/4/12 at 10:28 AM |
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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)
Sorry, not buying this at all.
Your point regarding weight transfer (completely valid and I agree with 100%) is entirely different to a static weight difference between two
otherwise identical cars.
Let's take it to its logical conclusion.
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 ?
The whole concept of light cars not being able to put their power down ignores half of the argument.
Weight that isn't there doesn't need to be accelerated, braked and turned into corners.
Light cars may, in a static situation, have less weight pushing down on the contact patches of the driving wheels.
But they are far less difficult to move and the fact that they have that lessened grip is offset by the car being much more able to be moved with less
difficulty than a lardy thing that actually needs all that grip to move it.
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loony
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| posted on 15/4/12 at 11:24 AM |
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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  (with CarB wheelspin at 1th and
2nd gear)
Maybe it's not professional tool, but you can change lots of parameters.
Link to CARTEST site: http://www.cartestsoftware.com/cartest4.5/
Lucas
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MakeEverything
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| posted on 15/4/12 at 04:10 PM |
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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 (with CarB wheelspin at 1th and
2nd gear)
Maybe it's not professional tool, but you can change lots of parameters.
Link to CARTEST site: http://www.cartestsoftware.com/cartest4.5/
Great bit of software - i just bought it!!
Not sure about wheelies in mine though.....
 
Wheelie!
ETA: I had the length over all incorrect by 1000mm! Thatll teach me for guess-timating it!
This simulator is excellent. Itll be interesting to see how close the simulation results are to the real thing.
[Edited on 15-4-12 by MakeEverything]
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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