Power to weight ratio vs weight distribution vs aerodynamics
Discussion
Max_Torque said:
D = (Cd x ((? x V^2) / 2) x A ) + (d-rolling x V) + d-static
Ttyre = (Pengine - Plosses) / V
Ftyre = MIN ( Fnormal x ?(tyre) || Ttyre / Rtyre )
Fvehicle = Ftyre - D
Avehicle = Mvehicle / Fvehicle
Simples ;-)
(in fact, it is simple; Power to Weight matters when you are going slowly and Power to Drag matters when you are going fast! )
Most of that is pretty simple, GCSE physics covers it.Ttyre = (Pengine - Plosses) / V
Ftyre = MIN ( Fnormal x ?(tyre) || Ttyre / Rtyre )
Fvehicle = Ftyre - D
Avehicle = Mvehicle / Fvehicle
Simples ;-)
(in fact, it is simple; Power to Weight matters when you are going slowly and Power to Drag matters when you are going fast! )
Edited by Max_Torque on Saturday 13th January 14:39
If the figures are really meaningful - the reason the TVR is "slower" to 60 (if we must) - or, not a lot faster, would be down to how much power you can actually put down on the road. Light weight is great, but not proportionally great, because traction also decreases. On a less than perfectly grippy road TC comes in even on my 4WD "just" 420bhp 911 so with just the light rear wheels driven and barely or no driver aids getting even 300bhp to translate into forward motion rather than smoke and noise is a challenge. I don't know about the comparable static weight distribution but the Corvette has what is deemed to be a front-mid engine, which puts more weight on the rear wheels.
PhillipM said:
Even then it means very little - it really is just marketing b*****t - what good is 50:50 weight balance if you've achieved it by hanging very heavy objects over the wheelbase at either end of the car, compared to something like a Lotus with 60:40 weight balance and all that mass concentrated in the centre of the car as close as possible to the yaw axis?
50:50 weight balance is far from ideal in almost anything (how many race cars do you see chasing absolute 50:50 weight balance over minimising MOI? I'd guess almost none) - it's just marketing stuff that the press gobbled up.
Everything in the centre of the car, with 4wd, then 50:50 on top, and you're going somewhere. But in isolation it doesn't really matter.
I think jagnet's response to the same post makes more sense than yours. Because we are not talking about race cars here, so there is very little point in worrying about how many race cars chase 50/50 weight balance .50:50 weight balance is far from ideal in almost anything (how many race cars do you see chasing absolute 50:50 weight balance over minimising MOI? I'd guess almost none) - it's just marketing stuff that the press gobbled up.
Everything in the centre of the car, with 4wd, then 50:50 on top, and you're going somewhere. But in isolation it doesn't really matter.
Edited by PhillipM on Sunday 14th January 14:30
samoht said:
I interpret 'as well as possible' to be about objective capability.
First priority, as small as possible. Anything bigger than two people and a suitcase is excess bloat and makes a sports car worse at being a sports car.
Second, having got it small, a carbon-fibre monocoque / panels, magnesium wheels etc to make it as light as possible.
Engine - turbo for power density. Either a four-pot or a rotary (because both are small in service of the above, and make enough power if turboed).
Suspension - double wishbones.
This yields a few possible paths:
1) 2.0 turbo four-pot, transversely mounted behind the cabin. Should be able to get about 400hp and good driveability with variable cam lift and one of today's variable geometry turbos. Basically like an extreme version of the new Alpine, maybe get it down to a tonne with the carbon chassis. Would be v quick, although I'm not sure to what extent the transverse layout gives a higher c.g.
2) Front-mid engined with a rotary turbo. Again, 400hp should be very reasonable with v.g. turbo, one tonne with a carbon chassis (the RX-7 is all steel and 1275kg). Should have a lower c.g. than (1), but a less rearward weight bias so slightly less traction.
3) Mid-engined with a longitudinal flat-four like the new 718 Cayman, but smaller and lighter. Take power off both ends of the engine and have another gearbox at the front for 4wd (like the Ferrari FF does). Would be heavier, but the 4wd capability would make it quicker in the wet because you could get the power down.
In all cases, approx 400hp/tonne would make it really very quick, and it would probably top out around 180mph at a guess. Being small, light and fairly well balanced (somewhere between 50/50 and 40/60 f/r distribution) should make it handle, too.
I'm not sure I agree with as lights as possible. Or even as small as possible. Light yes. As light as possible probably not. Too light is not really the best. I think it's about finding a balance. As many have said here and I agree, too light may become counterproductive. If you could strap some sort of high tech engine the size of a coke can, which makes 1000 bhp ,to a car that is the size of a kid's pedal car just large enough to fit and adult, would you really want to?First priority, as small as possible. Anything bigger than two people and a suitcase is excess bloat and makes a sports car worse at being a sports car.
Second, having got it small, a carbon-fibre monocoque / panels, magnesium wheels etc to make it as light as possible.
Engine - turbo for power density. Either a four-pot or a rotary (because both are small in service of the above, and make enough power if turboed).
Suspension - double wishbones.
This yields a few possible paths:
1) 2.0 turbo four-pot, transversely mounted behind the cabin. Should be able to get about 400hp and good driveability with variable cam lift and one of today's variable geometry turbos. Basically like an extreme version of the new Alpine, maybe get it down to a tonne with the carbon chassis. Would be v quick, although I'm not sure to what extent the transverse layout gives a higher c.g.
2) Front-mid engined with a rotary turbo. Again, 400hp should be very reasonable with v.g. turbo, one tonne with a carbon chassis (the RX-7 is all steel and 1275kg). Should have a lower c.g. than (1), but a less rearward weight bias so slightly less traction.
3) Mid-engined with a longitudinal flat-four like the new 718 Cayman, but smaller and lighter. Take power off both ends of the engine and have another gearbox at the front for 4wd (like the Ferrari FF does). Would be heavier, but the 4wd capability would make it quicker in the wet because you could get the power down.
In all cases, approx 400hp/tonne would make it really very quick, and it would probably top out around 180mph at a guess. Being small, light and fairly well balanced (somewhere between 50/50 and 40/60 f/r distribution) should make it handle, too.
This again reminds me of the Corvette vs Alfa 4C challenge on Top Gear. For those who haven't seen it, Clarkson bet was that the 4C would lap faster than the Corvette because it was lighter and so would gain time around the curves, while the Corvette would have to slow down too much for the curves, etc, etc, etc you know, the usual cliche bias against U.S. cars. But Hammond won in the Corvette. Even if the 4C could go around the curves faster, in the end was about balance.
Now I know the 4C doesn't have your suggested 400hp/tonne. It has 270. But the Corvette only has 300 too. But the point is balance.
If you want a car to drive just around tight curves all the time and nothing else, then yes, the 4C. Just as if you want a car only to drag race, then a Veyron. But we normally need a balance of the two. So I think as light as possible may be as bad as too heavy. Balance is the key IMO.
Notanotherturbo said:
I used to use the rolling road of a TVR specialist and he said their factory figures were massively inflated - as much as 20%. You smell a rat if you looked at their old stats , always a perfect rounded up number - 300 - 350 -400. 1/4 terminal speed is the best figure to judge the faster car as it takes traction out of the equation
That's interesting to know. Since I heard the Corvette can actually be the other way around, with some engines making actually making more power than advertised, this combination could well explain the whole thing. 996TT02 said:
If the figures are really meaningful - the reason the TVR is "slower" to 60 (if we must) - or, not a lot faster, would be down to how much power you can actually put down on the road. Light weight is great, but not proportionally great, because traction also decreases. On a less than perfectly grippy road TC comes in even on my 4WD "just" 420bhp 911 so with just the light rear wheels driven and barely or no driver aids getting even 300bhp to translate into forward motion rather than smoke and noise is a challenge. I don't know about the comparable static weight distribution but the Corvette has what is deemed to be a front-mid engine, which puts more weight on the rear wheels.
I agree. Too light is not the answer.It gets more interesting when you compare wildly different cars
Subaru Impreza 300bhp V MX5 Turbo 225bhp
The Subaru boys do better than they should in initial acceleration due to the 4WD but some would lose around a track and wonder why when they had a 75bhp advantage - because their drivetrain with 3 diffs was using approx 75bhp whilst the MX5 was only losing approx 25bhp.
Then the 150+kg weight difference sealed their fate - IF the MX5 had a good enough driver - the Impreza is an easy car to drive fast - particularly on bumpy roads.
Subaru Impreza 300bhp V MX5 Turbo 225bhp
The Subaru boys do better than they should in initial acceleration due to the 4WD but some would lose around a track and wonder why when they had a 75bhp advantage - because their drivetrain with 3 diffs was using approx 75bhp whilst the MX5 was only losing approx 25bhp.
Then the 150+kg weight difference sealed their fate - IF the MX5 had a good enough driver - the Impreza is an easy car to drive fast - particularly on bumpy roads.
Certainly power/mass ratio (sorry folks, I refuse to cater to the terminology of the unwashed masses 
), when it comes to top speed it's irrelevant. Mostly. If we fix power and aerodynamic factors, then the mass affects 0-vmax time. However, if you're on a hill, the more massive car is going to be slower going up and faster coming down 
. I think everything else has been physicsed to death.
), when it comes to top speed it's irrelevant. Mostly. If we fix power and aerodynamic factors, then the mass affects 0-vmax time. However, if you're on a hill, the more massive car is going to be slower going up and faster coming down . I think everything else has been physicsed to death.SPKR said:
I agree. Too light is not the answer.
I am not sure you can be too light . OK , I am using circuit metrics , but no hypercar would see which way a bike engined Jedi(other bike engined single seaters are available) went , in wet or dry . With 20% of the power ....Just like the Veyron couldn't even keep up with a 230bhp R500 at Dunsfold coppice said:
OK , I am using circuit metrics...
And therein lies your problem.It's very easy indeed to be too light, for a road car: without aerodynamic downforce, you're up against the basic physics of sprung:unsprung weight ratio. On our crappy, poorly maintained road surfaces, very lightweight cars are often very seriously limited in pace because they're skipping around all over the place, with very inconsistent levels of grip.
Edited by Equus on Tuesday 16th January 17:45
Equus said:
coppice said:
OK , I am using circuit metrics...
And therein lies your problem.It's very easy indeed to be too light, for a road car: without aerodynamic downforce, you're up against the basic physics of sprung:unsprung wight ratio. On our crappy, poorly maintained road surfaces, very lightweight cars are often very seriously limited in pace because they're skipping around all over the place, with very inconsistent levels of grip.
996TT02 said:
If the figures are really meaningful - the reason the TVR is "slower" to 60 (if we must) - or, not a lot faster, would be down to how much power you can actually put down on the road. Light weight is great, but not proportionally great, because traction also decreases. On a less than perfectly grippy road TC comes in even on my 4WD "just" 420bhp 911 so with just the light rear wheels driven and barely or no driver aids getting even 300bhp to translate into forward motion rather than smoke and noise is a challenge. I don't know about the comparable static weight distribution but the Corvette has what is deemed to be a front-mid engine, which puts more weight on the rear wheels.
This.....my comparison being experience of Mkiv Supra's doing the quarter mile, due to the modding scene you good control test where you essentially have the same car with hugely different power to weight ratio's.Non turbos about 157bhp/ton always got beat by any twin turbo'd car as traction wasn't an issue (in the dry), the turbo's though ranged from stock 220bhp/ton ish to still twin turbo but modded and about 280bhp/ton and then highly tuned 400+bhp/ton cars.
Now the 280 range often beat the 400's down the quarter mile, traction being a major issue for the high power cars when on road legal rubber. Higher terminal speeds but hard to launch hard and fast.
Auto's in the 280bhp region also nearly always beat the manual cars due to the auto smoothing the shifts and there only being 2 shifts compared to more on the manual and it all being left to the human (fuelled with adrenalin!) on the manuals.
GroundEffect said:
Fix your damping/unsprung mass then
Of course, but you can only go so far. There's a practical limit to how much you can reduce your unsprung mass, and no matter how good your dampers are, they'll be transmitting the inertia of that unsprung mass hitting a bump to the sprung mass within roughly two cycles of the suspension frequency.Equus said:
coppice said:
OK , I am using circuit metrics...
And therein lies your problem.It's very easy indeed to be too light, for a road car: without aerodynamic downforce, you're up against the basic physics of sprung:unsprung weight ratio. On our crappy, poorly maintained road surfaces, very lightweight cars are often very seriously limited in pace because they're skipping around all over the place, with very inconsistent levels of grip.
Edited by Equus on Tuesday 16th January 17:45
- the flyweight Elan not only rode far, far better and gripped even more so compared to heavyweight TRs and MGs etc
- Gp B rally cars were light , powerful and terrific over rough stuff - and the svelte little 205T16 and Delta S4s destroyed the heavyweight Quattroes
-apart from Grandad putting a sack of cement in his Triumph Herald's boot to tame its wayward rear end , nobody has gone quicker by adding weight, rather than lightness , surely ?
Ultimately, it all comes down to resistance and applied force.
As long as the applied force (engine power) is more than the resistance (drag, internal engine/transmission resistance, rolling resistance, etc.), the car will accelerate.
The faster the car goes, the bigger the resistance and at some point, all the forces balance. This is when it reaches it's top speed.
Weight distribution won't affect the resistance much, since what friction it increases on the back tyres, it decreases on the front tyres, so they cancel out.
As long as the applied force (engine power) is more than the resistance (drag, internal engine/transmission resistance, rolling resistance, etc.), the car will accelerate.
The faster the car goes, the bigger the resistance and at some point, all the forces balance. This is when it reaches it's top speed.
Weight distribution won't affect the resistance much, since what friction it increases on the back tyres, it decreases on the front tyres, so they cancel out.
coppice said:
the flyweight Elan not only rode far, far better and gripped even more so compared to heavyweight TRs and MGs etc
It did indeed (I've owned four Elans, over the years), but having said that, it was hardly a major achievement to outperform TR's and MG's!The Elan was a fantastic little thing, but it was very softly sprung and under-damped by modern standards, and also lacked outright grip. Part of the reason it could get away with such supple suspension was that it had very low unsprung mass (4.5" x 13" rims), which would be unachievable today because we demand wider tyres and bigger brakes.
It also wasn't that light in absolute terms.
I'm not talking about Elise S1/Elan weights, here - they can certainly be made to work on the road, with suitable springs and dampers - but really lightweight cars (sub-550kg): you were quoting the Jedi, which weighs in at about 320kg (I've driven Jedis too, and used to own a Megapin).
The lightest road car I've owned weighed around 427 kilos (with circa 190bhp). I found that its pace was frequently tempered by grip on the public roads - despite some very trick components to minimise unsprung weight (wheels that weighed just 3.4kg each, for example) and high quality dampers from Penske, properly valved to match the car.
Nanook said:
Call it mass if it makes you happy, it's measured in kg (usually), but as soon as you start accelerating it about, it becomes a weight.
No, weight is the force due to gravity acting on the mass. I think you're getting it mixed up with inertia (which, in linear mechanics, is directly linked to mass).Edited by Solocle on Wednesday 17th January 22:35
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