So, what’s up with new trend towards mid-engine performance cars? There’s mumbo jumbo about weight distribution. But no one has mentioned Polar Moment of Inertia. I’ve been watching films of race cars spinning out. Some recover quickly, some do not. Comments?
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Why Mid-engine 4
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GregLocock
Automotive
Fashion mostly. The C8 Corvette confirms your suspicions, when lazily driven around a circuit by an incompetent journo it was a bit quicker than a front engine Corvette, but by less than the difference in tires were worth. The lap times were about that of a stock Miata driven by someone who actually knows what they are doing. Mid engine cars are more difficult to drive fast. For instance the Lotus Carlton was easy to drive and have fun, compared with the Turbo Esprit.
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Greg Locock
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Cheers
Greg Locock
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BrianPetersen
Mechanical
Not exactly a "new" trend ... aside from perhaps the Corvette viewed in isolation.
Formula 1 and Indy cars and other true race cars have used that general layout for decades.
Supercars (Ferrari, Lamborghini, etc) have used the same layout as the Corvette is going towards, for decades.
In more affordable packages ... Porsche Boxster/Cayman have been around for some time, Toyota MR2 in the 1980s, Fiat X1/9 and Porsche 914 in the 1970s, Ford GT40 in the 1960s, certain Porsche models long before that. We probably shouldn't discuss the Pontiac Fiero ...
More weight on the rear drive wheels does allow for a harder launch from a standing start, it also allows harder acceleration out of corners on a racetrack. It may either remove the need for power steering or reduce the amount of assist (= interference with "feel"). Mid-engine cars have a reputation for snap oversteer, but snap oversteer can happen with any drivetrain layout, even with front-wheel-drive, if the suspension is wrong or the tires are wrong or the driver is wrong.
Formula 1 and Indy cars and other true race cars have used that general layout for decades.
Supercars (Ferrari, Lamborghini, etc) have used the same layout as the Corvette is going towards, for decades.
In more affordable packages ... Porsche Boxster/Cayman have been around for some time, Toyota MR2 in the 1980s, Fiat X1/9 and Porsche 914 in the 1970s, Ford GT40 in the 1960s, certain Porsche models long before that. We probably shouldn't discuss the Pontiac Fiero ...
More weight on the rear drive wheels does allow for a harder launch from a standing start, it also allows harder acceleration out of corners on a racetrack. It may either remove the need for power steering or reduce the amount of assist (= interference with "feel"). Mid-engine cars have a reputation for snap oversteer, but snap oversteer can happen with any drivetrain layout, even with front-wheel-drive, if the suspension is wrong or the tires are wrong or the driver is wrong.
You will see polar moment of inertia mentioned but only by the geeks as it's a harder concept to grasp and harder to quantify. It doesn't show up in simple performance numbers like 0-60 or skid pads. While I love to fantasize about all these exotic mid-engine sports cars, including the C8, the layout is just to impractical for me. I know they have multiple storage compartments in the C8 but they are all small and oddly shaped and being able to put one golf bag in a car does not carry any water with me. I need my car to be able to haul at least enough for a couple of weeks vacation. My front engine C6 Grand Sport has amazing performance (I've recorded 1.26 G on the street) and makes a great GT for vacation. I even brought home a 10' x 12' oriental rug a couple of weeks ago. Folded up it fit in the hatch perfectly. You will never do that with a C8.
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EdStainless
Materials
If you want handling and speed the first rule is light weight.
Where the engine is and which wheels are driven all come after that.
It is impossible to make a fast well handling car that is heavy.
In terms of handling dynamics it is more important to have good weight distribution.
If one end is heavy you have a situation where one end of the suspension has to be much stiffer, this gives odd dynamic effects in transitions.
DGallup, yes it is amazing that many common cars today significantly out perform high performance cars from decades ago. They are faster, stop and corner better, and are more efficient.
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P.E. Metallurgy
Where the engine is and which wheels are driven all come after that.
It is impossible to make a fast well handling car that is heavy.
In terms of handling dynamics it is more important to have good weight distribution.
If one end is heavy you have a situation where one end of the suspension has to be much stiffer, this gives odd dynamic effects in transitions.
DGallup, yes it is amazing that many common cars today significantly out perform high performance cars from decades ago. They are faster, stop and corner better, and are more efficient.
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P.E. Metallurgy
BrianPetersen
Mechanical
First gen Toyota MR2 remains one of my favourite fun cars that I've never driven an example of. Compact, light, manual everything. Too bad rust has claimed virtually all of them around here.
NormPeterson
Structural
More weight on the rear wheels also tends to improve hard/extreme braking, as the forward load transfer then works to equalize tire loadings rather than make them worse.
Agreed. PMOI hardly ever gets any mention addressed even in vague terms such as 'nimble', 'deliberate', and 'ponderous' or 'heavy'. So I was a bit surprised to read this in a Motor Trend newsletter this morning. Boldface mine.
Norm
dgallup said:
Agreed. PMOI hardly ever gets any mention addressed even in vague terms such as 'nimble', 'deliberate', and 'ponderous' or 'heavy'. So I was a bit surprised to read this in a Motor Trend newsletter this morning. Boldface mine.
2020 Porsche Taycan Turbo S Track Ride said:
Norm
- Thread starter
- #9
As an engineer, I always liked that zig zag maneuver between cones at high speed as a measure of transient response and the ability to recover control (vis a vis polar moment of inertia of vehicle) – I’ve seen several bad road crashes from vehicles ”failing” this maneuver.
I need to do the angular acceleration math to see if this is a practically measurable event – or am I reinventing the wheel?
Bob
I need to do the angular acceleration math to see if this is a practically measurable event – or am I reinventing the wheel?
Bob
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EdStainless
Materials
Buggar, this why people crash when they try to swerve to avoid hitting a deer. Most cars and drivers are poor at this maneuver.
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P.E. Metallurgy
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P.E. Metallurgy
GregLocock
Automotive
I do measure it, and it does result in a metric that is useful. The metric at a vehicle level is called yaw delay time, and is measured in a frequency response test, which basically involves wiggling the steering wheel at an ever increasing frequency and measuring the resulting yaw velocity. When this hits a 45 degree delay that's the number we use. What's happening is the PMOI is the inertia, and the steering compliance of each end acts as springs. It's a fourth order system that is dominated by the first order term.
But, it is comparatively easy to change the YDT by changing the understeer of the car (more understeer=quicker YDT).
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
But, it is comparatively easy to change the YDT by changing the understeer of the car (more understeer=quicker YDT).
Cheers
Greg Locock
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BrianPetersen
Mechanical
The manoeuvre of which you speak is unofficially called the "moose test"; it seems to be commonly referenced in Europe to the extent that there is an ISO standard covering how it is to be done. This is the test that the original Mercedes A-class failed by rolling over, and a number of other vehicles have had problems over the years including a certain generation of Jeep Grand Cherokee.
The Swedish publication Teknikens Värld has done this test for many years. A Spanish publication also does them and publishes the outcomes on their Youtube channel.
Mid-engine doesn't guarantee trouble. Here's an example ... although it's one with well-sorted suspension and stability-control:
High and softly-sprung SUVs/CUVs have trouble with this:
Tesla Model 3 does very well ... the low positioning of the battery helps:
Front wheel drive can perform well:
And I know you want to see failures ...
The Swedish publication Teknikens Värld has done this test for many years. A Spanish publication also does them and publishes the outcomes on their Youtube channel.
Mid-engine doesn't guarantee trouble. Here's an example ... although it's one with well-sorted suspension and stability-control:
High and softly-sprung SUVs/CUVs have trouble with this:
Tesla Model 3 does very well ... the low positioning of the battery helps:
Front wheel drive can perform well:
And I know you want to see failures ...
GregLocock
Automotive
Double lane change is a bugger to do analytically as there is no clear recipe for getting a high speed result for a given car. We do have a method that sort of works, but it is basically iterative messing about and can run for days.
The funny thing with Mercedes A is that they could have just fitted smaller front tires and the problem would have gone away.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
The funny thing with Mercedes A is that they could have just fitted smaller front tires and the problem would have gone away.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
To be fair, the lack of "frunk" space is not due to the mid-engine layout, rather the fact that mid engine sports cars tend to take the opportunity to put a steep slope on the front. (Another advantage of the mid-engine layout.)dgallup said:
Did someone say mid-engine layout increases the polar moment of inertia? I think the reverse is true.
je suis charlie
- Thread starter
- #15
I started to quantify the polar moment of inertia of a mid engine vs front engine car and see that a big issue here are the weight of the wheel/tire/suspension parts being hung so far from the c.g. that engine placement almost does not matter! But now I have to guess at some engine offsets and prove it.
gruntguru is correct IN mid engine lessoning MOI.
gruntguru is correct IN mid engine lessoning MOI.
BrianPetersen
Mechanical
"Polar moment of inertia" ... about what axis when there is tire side-slip involved - which there always will be, when there is cornering force involved, there is always an angle between the direction the tire is pointed and the direction of actual travel.
Study those videos and you'll see that rear end side-slip can develop into a swinging pendulum effect, regardless of engine placement. The initial turn (left, the way KM77 does it) gets the rear to have a slip angle to the right, and then the hard turn the opposite way (right) sends the back end swinging much more to the other direction. Nowadays ESP is called upon to catch this.
The sports cars (Cayman, or even the rear-engine 911), and the Tesla for that matter, have wide low-profile tires, which one would expect would keep that slip angle to a minimum. (Up until the point where they suddenly let go. Squishy high-profile tires give more warning to the driver before letting go)
Bouncy suspension with insufficient roll damping can start a pendulum effect in the roll direction, too, which surely doesn't help the tires maintain grip. In the vehicles that went up on two wheels, it's always after the second turn, not the initial one. The rebound from the springs adds momentum in the roll direction.
The sports cars (and the Tesla) have a low center of gravity and high roll stiffness and probably better dampers. High, squishy-soft-suspension SUVs ... not so much.
The Nissan Kicks and the Toyota Rav4 in the fail video are front-engine front-drive vehicles. The Rav4 may be all wheel drive, but the rear drivetrain in those is along for the ride unless the front wheels are spinning.
I don't doubt that the engine placement is a factor here, but it seems to be more tires, and suspension calibration, and center of gravity height, and (nowadays) ESP calibration.
I had a generation 1 Honda Civic once upon a time. It had wicked lift-throttle oversteer and was skittish on the brakes, and I spun it on dry pavement more than once. Rear suspension was MacPherson with inverted-A lower wishbones and a trailing link. I see now that this design is prone to going into toe-out in the rear when braking, and probably at other times as well. Today I drive a car that's almost the same size (Fiat 500) but which uses a twist-beam rear axle. That's still not a wonderful design, but at least it behaves.
Let's see a Ford E350 15-passenger van do that test. Something like that, has just about everything going against it.
Study those videos and you'll see that rear end side-slip can develop into a swinging pendulum effect, regardless of engine placement. The initial turn (left, the way KM77 does it) gets the rear to have a slip angle to the right, and then the hard turn the opposite way (right) sends the back end swinging much more to the other direction. Nowadays ESP is called upon to catch this.
The sports cars (Cayman, or even the rear-engine 911), and the Tesla for that matter, have wide low-profile tires, which one would expect would keep that slip angle to a minimum. (Up until the point where they suddenly let go. Squishy high-profile tires give more warning to the driver before letting go)
Bouncy suspension with insufficient roll damping can start a pendulum effect in the roll direction, too, which surely doesn't help the tires maintain grip. In the vehicles that went up on two wheels, it's always after the second turn, not the initial one. The rebound from the springs adds momentum in the roll direction.
The sports cars (and the Tesla) have a low center of gravity and high roll stiffness and probably better dampers. High, squishy-soft-suspension SUVs ... not so much.
The Nissan Kicks and the Toyota Rav4 in the fail video are front-engine front-drive vehicles. The Rav4 may be all wheel drive, but the rear drivetrain in those is along for the ride unless the front wheels are spinning.
I don't doubt that the engine placement is a factor here, but it seems to be more tires, and suspension calibration, and center of gravity height, and (nowadays) ESP calibration.
I had a generation 1 Honda Civic once upon a time. It had wicked lift-throttle oversteer and was skittish on the brakes, and I spun it on dry pavement more than once. Rear suspension was MacPherson with inverted-A lower wishbones and a trailing link. I see now that this design is prone to going into toe-out in the rear when braking, and probably at other times as well. Today I drive a car that's almost the same size (Fiat 500) but which uses a twist-beam rear axle. That's still not a wonderful design, but at least it behaves.
Let's see a Ford E350 15-passenger van do that test. Something like that, has just about everything going against it.
GregLocock
Automotive
Quick back of envelope calc suggests FE to ME is worth 1200 kg m2, which is significant, typical PMOI is 3000-5000 kg m2
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
enginesrus
Mechanical
It makes no sense, to manufacture a 700hp,or mid engine high performance vehicle for common street or road use. Especially for the US, as we have no autobahn with unlimited speed limits and such.
All such vehicles do is help fuel egos and dangerous situations for the average motorist on the roads. And if it is strictly a design thing to look a certain way, then why does a nice looking type vehicle have to be priced out of the average persons ability to purchase? They are not luxury vehicles, but some carry prices as high as or higher than.
All such vehicles do is help fuel egos and dangerous situations for the average motorist on the roads. And if it is strictly a design thing to look a certain way, then why does a nice looking type vehicle have to be priced out of the average persons ability to purchase? They are not luxury vehicles, but some carry prices as high as or higher than.
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BrianPetersen
Mechanical
Chrysler K-car wagon for you, then. Oh, I forgot, you don't like overhead-cam engines or timing belts. Chevy Cavalier, then. With the pushrod 4 cylinder and old skool 3 speed automatic with no newfangled electronics. Not that fancy DOHC engine in the newer ones that was actually capable of making the car move.
The Corvette is a halo car. It supports the brand. Some people buy them, not necessarily because of what it will do, but because of what it is capable of. And in order to do that, it has to be capable, in the right hands. By the way, a Corvette is half the money of a Ferrari.
The Corvette is a halo car. It supports the brand. Some people buy them, not necessarily because of what it will do, but because of what it is capable of. And in order to do that, it has to be capable, in the right hands. By the way, a Corvette is half the money of a Ferrari.
NormPeterson
Structural
Apparently you don't understand the appeal of driving something with dynamics closer to a race car on the street. While it's true that not everybody has sufficient discipline to drive such a car in appropriate fashion for street environments, that's not reason enough to make such cars unavailable to those who do have the requisite discipline. We aren't all like those in the various crashed-into-the-crowd-while-leaving-a-car-show videos and associated memes.enginesrus said:
For starters, low production numbers ==> higher unit costs. And when supply - or just perceived supply - is held down below anticipated demand, prices naturally tend to rise. I'm pretty sure that pricing philosophies for impractical purchases in general work differently than that for more pragmatic purchases (if you want it as bad as you think you do, you'll pay extra for that level of want).
Norm
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