Power to weight ratio vs weight distribution vs aerodynamics

Power to weight ratio vs weight distribution vs aerodynamics

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spyder dryver

1,329 posts

215 months

Saturday 13th January 2018
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Huff said:
Example: I've a 420Kg BEC with c.170hp, which on paper is comparable with either example above.
And so, up to 80-90, and if I can get the power down /a good launch, it could beat either of the above to 60, maybe 90.. ish; certainly I've seen better times.

But above that forget it: even with tiny mass, tiny frontal area and even a very small Cd*A number - I simply don't have the excess power to swamp aero drag in the same way. At 100 all 3 cars will require 55-70hp to overcome their cumulative (aero+frictonal) drag: at which point I have perhaps 80-90bhp in hand to accelerate me+car (500kg); the TVR an optimistic amount more, let's just say 'some': and the Corvette perhaps c.400 (but 3x my all-up mass thus, at ~100mph, perhaps 1.6x my effective P:W ratio).

It's simple maths, really: if you want high speed, high-HP romps away*. 'Static' P:W, and the car's weight balance have very little to do with it. My 2p.
Being the owner of a 430kg BEC (a Phoenix with Tim's old battle damaged bodywork on it) I have to agree with the above.
But put your BEC side by side with the Corvette at Cadwell and he wouldn't see which way you went!

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
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jagnet said:
Perfect weight distribution? For straight line acceleration in a RWD car you want the mass as much to the rear as possible to gain the most traction from the driven wheels, in which case the original Beetle has better weight distribution than the Corvette when it comes to chasing 0-60 times.

The idea that a 50:50 weight distribution for a car is somehow "automotive perfection" was just a marketing tag line dreamed up by BMW, but it stuck and became "fact".
When it comes to balanced weight distribution I will have to disagree with it being just marketing. For straight line it makes sense that having more weight over the driven wheels is better than well balanced for a 2WD car. But for handling and going around a track, good balance is fundamental and 50/50 helps to make weight transfer easier to manage in my opinion.Balance is a good thing for handling in general.

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
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Bill said:
Grip plays a big part in 0-60 times, so weight can be an advantage. The Sag driver is probably having to feather the throttle in the first three gears. biggrin Come to the first corner and he's braking later, cornering faster and accelerating harder out the other side.
Do you mean the TVR would lap faster than the Corvette? I'm not so sure. I was surprised watching a 911 vs C6 test because I thought the Corvette would get the 911 in the drag race and the 911 would lap faster. But actually the 911 got the Corvette in the drag race because it was a dual clutch paddle and the Corvette a stick. Then the Corvette lapped faster than the 911, which I would have never thought. Also reminds me of the 4C vs Corvette challenge on Top Gear. The Corvette no longer fits the usual mantra of U.S. cars being only good for straight lines. For the last couple generations it's hanging with the best.

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
quotequote all
All great points.

Sure the claims might not stand up. But some of the data are from magazine tests such as Motor Trend. Not from the manufacturer, if that makes any difference.

I see the results with the Z06 were close. But I would have thought the Sagaris would at least smash the base Corvette with those specs.

Since apparently the aerodynamics are taken care of with the Sagaris, if you are to believe reviews and articles of the time, I think the most reasonable explanation is, as some have said, the Sagaris is not making 406 bhp. Upon further reading I already found sources listing 380 bhp rather than 406 bhp for the Sagaris. Gearing could also be the culprit here, sure.

But I didn't mean to turn this into a Corvette vs Sagaris topic. I wanted to discuss the issue at hand in general.

All the other points are also well made. Powerband, grip, differences in technique, drivers, road surface and all. I guess this is the difficulty of making a road sports car. If you are building something dedicated, such as a dragster, all you have to worry is that.

So with the interest of pushing the conversation away from Corvette vs Sagaris, if you had to build a sports car which did as well as possible on everything, what would you give priority to? From power to weight, weight distribution, power, aerodynamics, gearing etc? I know it's basically impossible to be good at it all. So let's bias it towards handling as I think that is more useful on the road than something like top speed. You can never really use it.

But let's still keep the car fast. An Elise handles like a dream but it's not what I would consider truly fast.

So what's your recipe?


spyder dryver

1,329 posts

215 months

Saturday 13th January 2018
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Do you mean "top speed" fast or "lap time" fast. The current vogue seems to be lap time at "The Ring".

A lightweight BEC with less than 200bhp can lap a circuit in a time the Corvette driver could only dream of.
In a straight line the Corvette would catch the BEC and then walk away from it.

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
quotequote all
High top speed would be nice but not at the cost of handling and acceleration. But also not something that will handle great but can barely reach 320kmh-200mph. A compromise would be great. But biased towards handling rather than top speed.

Equus

16,766 posts

100 months

Saturday 13th January 2018
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SPKR said:
But also not something that will handle great but can barely reach 320kmh-200mph.
God forbid that one would have to suffer something that could barely reach 200mph as one's daily driver.

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
quotequote all
Equus said:
God forbid that one would have to suffer something that could barely reach 200mph as one's daily driver.
Wait, I didn't say daily driver. I said road car.

He asked what I consider fast enough and I told him. Basically it doesn't need to be Bugatti or Koenigsegg fast. 200mph today is not really that fast. It's basically mid range for supercars.

Equus

16,766 posts

100 months

Saturday 13th January 2018
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SPKR said:
200mph today is not really that fast.
OoooKkkkk.

Two questions:

1) When (assuming you've actually passed a driving test) did you last drive anywhere close to 200mph on the road?

2) When do the schools go back where you are?

spyder dryver

1,329 posts

215 months

Saturday 13th January 2018
quotequote all
SPKR said:
200mph today is not really that fast.
You are Andy Green OBE AICMFP

grumpy52

5,565 posts

165 months

Saturday 13th January 2018
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One of the factors in acceleration is the gearbox .
Ratios and speed of change can ŕeally have an unusual effect on times .
Most testers don't lift while doing gear changes .
Having the engine drop lots of revs between ratios is not ideal for good acceleration. Or good lap progress .

SPKR

Original Poster:

226 posts

75 months

Saturday 13th January 2018
quotequote all
grumpy52 said:
One of the factors in acceleration is the gearbox .
Ratios and speed of change can ?eally have an unusual effect on times .
Most testers don't lift while doing gear changes .
Having the engine drop lots of revs between ratios is not ideal for good acceleration. Or good lap progress .
Yeah. Random data is difficult to compare. Much easier if it's all set by the same driver, same day and same conditions. But I needed the data to illustrate the point. Although the point was to start a discussion about the roles played by the different attributes and how they interact and balance with each other. Rather than a discussion only about that specific comparison. Although we have had some very good points being made about the comparison nonetheless.

Edited by SPKR on Saturday 13th January 23:44

jagnet

4,094 posts

201 months

Sunday 14th January 2018
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SPKR said:
When it comes to balanced weight distribution I will have to disagree with it being just marketing. For straight line it makes sense that having more weight over the driven wheels is better than well balanced for a 2WD car. But for handling and going around a track, good balance is fundamental and 50/50 helps to make weight transfer easier to manage in my opinion.Balance is a good thing for handling in general.
Without doubt good balance is important, but balance imho has more to do with suspension tuning than weight distribution. With bad suspension tuning even a car with front biased weight distribution can be made to handle like a first gen Corvair.

For a RWD car more rear bias (maybe 40:60 f/r) is preferable - better traction accelerating, better braking, better turn in, better acceleration out of corners. With 50:50 as soon as there's any weight transfer there'll be under utilised axles whether the weight transfer be fore or aft.

The difficulty is, for a front engined rear wheel drive road car getting any sort of rear biased weight distribution is nigh on impossible without severely compromising passenger space, so make it 50:50 and then get the marketing department to claim that's perfection. It's better than 60:40 f/r for sure, but it's not "perfect".

coppice

8,561 posts

143 months

Sunday 14th January 2018
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Trouble is , that far too many people think that huge power always overcomes lardy weight . It doesn't - it may do for headline 0-60s and top speeds but look at the times for any speed hillclimb and you will see that a little bike engined single seater(no weight , tiny frontal area etc ) will annihilate any road car. Or look at lap times of a Formula Ford 1600 at Cadwell - 100bhp-ish , skinny tyres, 40 year old design no aero grip but 450kilo . In practice this means near 911 GT3 pace .

'More power means you are faster down the straights but less weight means you are faster everywhere'- Colin Chapman of course

SPKR

Original Poster:

226 posts

75 months

Sunday 14th January 2018
quotequote all
jagnet said:
Without doubt good balance is important, but balance imho has more to do with suspension tuning than weight distribution. With bad suspension tuning even a car with front biased weight distribution can be made to handle like a first gen Corvair.

For a RWD car more rear bias (maybe 40:60 f/r) is preferable - better traction accelerating, better braking, better turn in, better acceleration out of corners. With 50:50 as soon as there's any weight transfer there'll be under utilised axles whether the weight transfer be fore or aft.

The difficulty is, for a front engined rear wheel drive road car getting any sort of rear biased weight distribution is nigh on impossible without severely compromising passenger space, so make it 50:50 and then get the marketing department to claim that's perfection. It's better than 60:40 f/r for sure, but it's not "perfect".
Fair enough and I see where you are coming from now.

And I agree with what you are saying. Although I'm a bit on the fence about better turn in. Could you elaborate on the technical points which make you say that?

I totally ignored the suspension side because I would like to think it is already sorted. I'm starting from the point where it will be already properly set up for the type of car. But absolutely, suspension is very important too.

But I see where you are coming from. The word "perfect" was probably not the best choice to describe it. I should probably have used something like "optimal weight distribution for a front engine car" or "as good as it gets weight distribution for a front engine car". I guess that would be more fair. Because around 50/50 is as good as it normally gets for a front engine car. Do you know of any RWD front mid engine road sports car with a rear weight bias such as 40/60 f/r? It would be interesting to know if there is something out there. But I'm thinking that would be difficult to achieve for a road car.



mwstewart

7,553 posts

187 months

Sunday 14th January 2018
quotequote all
SPKR said:
Fair enough and I see where you are coming from now.

And I agree with what you are saying. Although I'm a bit on the fence about better turn in. Could you elaborate on the technical points which make you say that?

I totally ignored the suspension side because I would like to think it is already sorted. I'm starting from the point where it will be already properly set up for the type of car. But absolutely, suspension is very important too.

But I see where you are coming from. The word "perfect" was probably not the best choice to describe it. I should probably have used something like "optimal weight distribution for a front engine car" or "as good as it gets weight distribution for a front engine car". I guess that would be more fair. Because around 50/50 is as good as it normally gets for a front engine car. Do you know of any RWD front mid engine road sports car with a rear weight bias such as 40/60 f/r? It would be interesting to know if there is something out there. But I'm thinking that would be difficult to achieve for a road car.
I seem to recall the perfect RWD road car weight distribution is something like 53 pc to the rear.

In fairness to BMW I suspect 50:50 is a pretty good setup for a road car not used in absolute extremis, so there is a 'perfect' element to it within the context of intended average use.

anonymous-user

53 months

Sunday 14th January 2018
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mwstewart said:
SPKR said:
Fair enough and I see where you are coming from now.

And I agree with what you are saying. Although I'm a bit on the fence about better turn in. Could you elaborate on the technical points which make you say that?

I totally ignored the suspension side because I would like to think it is already sorted. I'm starting from the point where it will be already properly set up for the type of car. But absolutely, suspension is very important too.

But I see where you are coming from. The word "perfect" was probably not the best choice to describe it. I should probably have used something like "optimal weight distribution for a front engine car" or "as good as it gets weight distribution for a front engine car". I guess that would be more fair. Because around 50/50 is as good as it normally gets for a front engine car. Do you know of any RWD front mid engine road sports car with a rear weight bias such as 40/60 f/r? It would be interesting to know if there is something out there. But I'm thinking that would be difficult to achieve for a road car.
I seem to recall the perfect RWD road car weight distribution is something like 53 pc to the rear.

In fairness to BMW I suspect 50:50 is a pretty good setup for a road car not used in absolute extremis, so there is a 'perfect' element to it within the context of intended average use.
I agree! If you're having to bias away from a neutral static balance for traction, for a road car, you're giving up dynamic performance for ultimate lap time, which is stupid (for a road car). Of course these days, the power outputs of even the super saloon cars is now so high they must be 4wd anyway, meaning they can be back to 50:50 and suffer no traction issues.....

PhillipM

6,515 posts

188 months

Sunday 14th January 2018
quotequote all
SPKR said:
When it comes to balanced weight distribution I will have to disagree with it being just marketing. For straight line it makes sense that having more weight over the driven wheels is better than well balanced for a 2WD car. But for handling and going around a track, good balance is fundamental and 50/50 helps to make weight transfer easier to manage in my opinion.Balance is a good thing for handling in general.
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.

Edited by PhillipM on Sunday 14th January 14:30

samoht

5,631 posts

145 months

Sunday 14th January 2018
quotequote all
SPKR said:
if you had to build a sports car which did as well as possible on everything, what would you give priority to? From power to weight, weight distribution, power, aerodynamics, gearing etc? I know it's basically impossible to be good at it all. So let's bias it towards handling as I think that is more useful on the road than something like top speed. You can never really use it.

But let's still keep the car fast. An Elise handles like a dream but it's not what I would consider truly fast.

So what's your recipe?
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.

Notanotherturbo

494 posts

206 months

Sunday 14th January 2018
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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