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Why Mid-engine
4

Why Mid-engine

Why Mid-engine

(OP)
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?


RE: Why Mid-engine

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.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why Mid-engine

Perhaps the current obsession with 0-60 mph, 0-100 kph times? AWD is of course a better solution but for a RWD supercar, mid-engine provides a better launch.

je suis charlie

RE: Why Mid-engine

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.

RE: Why Mid-engine

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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RE: Why Mid-engine

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

RE: Why Mid-engine

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.

RE: Why Mid-engine

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.

Quote (dgallup)

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.

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.

Quote (2020 Porsche Taycan Turbo S Track Ride)

Were Team Lotus genius Colin Chapman alive today, the thought of the 5,100-pound, four-door 2020 Porsche Taycan lapping a racetrack might spark his own spontaneous combustion. How would the Formula 1 master of "add lightness" reconcile racetrack handling with an EV's elephantine weight?

As we enter the twisting part of the Fontana course's infield, Long doesn't hesitate to slam over the curbs … tha-whump! We bounce hard; the Taycan is tough. Hanging on through the esses, the weight is obvious; direction changes are sharp, but not the scalpel cuts of a mid-engine car with its concentrated drivetrain.
Squashed low in the car, the battery and its heft are helpful, because weight transfer diminishes overall cornering grip due to tire grip's nonlinearity with load. But because the weight is spread out like a pancake, it increases the polar inertia (even more so in skateboard-platform EVs). That's bad through these infield esses, where repeated swiveling around its axis requires extra energy and eats away tire grip that's better used for lateral acceleration.


Norm

RE: Why Mid-engine

(OP)
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

RE: Why Mid-engine

Buggar, this why people crash when they try to swerve to avoid hitting a deer. Most cars and drivers are poor at this maneuver.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

RE: Why Mid-engine

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why Mid-engine

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 www.km77.com 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: https://www.youtube.com/watch?v=0uR6bfMeefM

High and softly-sprung SUVs/CUVs have trouble with this: https://www.youtube.com/watch?v=iA-IiKjyEf8&t=...

Tesla Model 3 does very well ... the low positioning of the battery helps: https://www.youtube.com/watch?v=XRCWBFSQsp4

Front wheel drive can perform well: https://www.youtube.com/watch?v=d_v9sy32Z5I

And I know you want to see failures ... https://www.youtube.com/watch?v=MJOgmIuqYrM

RE: Why Mid-engine

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why Mid-engine

Quote (dgallup)

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.
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.)

Did someone say mid-engine layout increases the polar moment of inertia? I think the reverse is true.

je suis charlie

RE: Why Mid-engine

(OP)
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.

RE: Why Mid-engine

"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.

RE: Why Mid-engine

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.

RE: Why Mid-engine

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.

smile

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.

RE: Why Mid-engine

Quote (enginesrus)

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.
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.


Quote:

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.
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

RE: Why Mid-engine

(OP)
Going back to the significance of a mid-engine vs front engine for a lower vehicular polar moment of inertia:

MOI of 4 wheel/tire assy’s x 150-lb ea. x 5’^2 = 15,000-lb ft^2

Front engine MOI from 2-ft offset = 800-lb x 2’^2 = 3,200-lb ft^2 - Insignificant in comparison, therefore engine placement does not affect the MOI that much.

Bob

RE: Why Mid-engine

What axis are you calculating this MoI around??

RE: Why Mid-engine

Buggar - wouldn't it be better to sum the inertias of the wheels, powertrain, and everything else lumped as a rectangular mass about the CG . . . and find the difference between the totals?


Norm

RE: Why Mid-engine

(OP)
Norm, yes it would but I thought I'd try a ruff 'n dirty first.
The MOI was considered midway between wheelbase and track.
Bob

RE: Why Mid-engine

When you steer with the front wheels, at least for an instant until weight transfer and suspension movement and tire compliance start having extremely complex confounding effects, the car yaws around the (un-steered) rear axle. (The front goes sideways and the rear stays put relative to a frame of reference travelling with the car's initial motion.)

RE: Why Mid-engine

Quote (Bob)

Norm, yes it would but I thought I'd try a ruff 'n dirty first.
The MOI was considered midway between wheelbase and track.
Bob
It's not particularly difficult as a spreadsheet. Though I do think you would need to include a manual goal-seek step to capture the effect of the CG moving as a result of moving a couple of rather heavy items by significant distances.

I'm getting numbers within Greg's range for the total but numbers for the engine plus transmission more consistent with yours.


What Brian and Greg have noted above reads just like a free body diagram of a beam that suddenly goes out of equilibrium due to the appearance of a new force at one of the support locations. I'm not surprised that this is not addressed directly in any of the enthusiast magazines or even the various message boards. Easier to use the dumbbell analogy (no, I'm not going to say it).


Norm

RE: Why Mid-engine

One other small thing. The Corvette has always (since C2, and I'm not sure about C1) been "mid-engine". It has been front-mid up until now, and now it is changing to rear-mid. And the transmission has been at the back since C5. So your starting point for a MoI calculated around the rear axle, or around the CG, was favourable to begin with. The vast majority of the drivetrain has always (since C2) been between the wheels.

The big difference now, I predict, will be what the driver feels. Up to now, the driver has been very close to the rear wheels, and what happened out front was way out front. Now, the driver's head will be close to the middle of the car.

I don't have experience with what that will feel like ... only the opposite. My front wheel drive Fiat Ducato van is as opposite to a Corvette as you can possibly be. Drivetrain hanging out front outside the wheelbase, driver sitting almost on top of the front wheels, what happens out back I have no idea, but being front wheel drive, what's out back is just following along anyhow. When maneuvering at low speed, you certainly get the impression of moving sideways as you steer. It's slow. It understeers. It's not nimble. It's tall. But it hauls my roadracing motorcycle around, and it's my portable garage, and that's the point.

RE: Why Mid-engine

Quote (Greg)

Quick back of envelope calc suggests FE to ME is worth 1200 kg m2, which is significant, typical PMOI is 3000-5000 kg m2
Hmmm . . . summing MOIs about the rear axle line makes that 1200 number look much better (I got 1500-ish overall for the model I'm using), without going too far outside that 3000 - 5000 (5700-ish, ± 750-ish). Both of which would of course depend on specifics.

Looking at this from the rear axle line shows a far more significant effect going FE to ME than looking at it from the CG.


Norm

RE: Why Mid-engine

What matters to the vehicle dynamics of a car or truck is referred to as k' (Kay prime). It is [ the ratio of the Z (yaw) inertia to the vehicle mass -1.0 ]. A value of zero gives you a bar bell. This factor is the player which determines what tire and suspension properties are needed to deliver prescribed Steering Gain, yaw overshoot (yaw damping) and lateral acceleration response time. THESE factors are usually determined by 'competitive assessment' (i.e. what do drivers want or prefer in a designated 'handling emphasized' market slice. A synthesis done by determining the tire and suspension properties necessary to achieve these goals is easily done. What pops out of it is a k' value that is needed to obtain a set of tire properties (lateral force and aligning moment stiffness) needed to meet such goals. Sometimes these goals are unobtainable because there are no such tires which can deliver the specifications.

For example, an equivalent BMW 5 Series on it's 'best' handling/feeling tires (Continentals) can NOT be made from a Cadillac CTS-V because the V8 engine plus it's transmission location can not deliver a k' of -0.1 Instead, it's k' of 0.2 penalizes the needed tire stiffnesses to produce the balanced car. Yes, you can utilize different front and rear tire sizes, but now we are into marketing, part numbers, wheel rims, tire rotation, aftermarket tire availibility, warranty and a few other things some car buyers, dealers (and some LAWYERS) will NOT tolerate.

Here's a simple example:

Lets say you drool over a Kleptomatic Level-III handling car that has nimble handling, great road feel, a wapping max lat and costs less than 70,000 denariuses (denarii ???).

You measure a few and here are it's metrics:

Steering Gain (g/100 deg SWA at 100 kph): 1.80
lateral acceleration response time (sec.) 0.28
Yaw Velocity Peak to Steady Ratio 1.05
[Same as a zeta of 0.69 ]

for k'= 0.2:
Front Cornering Compliance == 4.54 deg/g
Rear Cornering Compliance == 1.83 deg/g
[Understeer == 2.71 ] deg/g
Overall Steering Ratio == 11.80 deg/deg
This much understeer kills your max lat.
The low steer ratio makes for way too high of a steering gain at 250 kph.

Next is k' = 0.0
Front Cornering Compliance == 3.87
Rear Cornering Compliance == 2.01
[Understeer == 1.86 ]
Overall Steering Ratio == 14.40
This is do-able, but where will all that front compliance come from? Roll steer, soggy bushings, high effort steering feel, high caster ???

Then there is k' = -0.02
Front Cornering Compliance == 3.51
Rear Cornering Compliance == 2.21
[Understeer == 1.30 ]
Overall Steering Ratio == 16.82
That 17:1 ratio means its a dog in the parking lot (low speed gain).
The low understeer might make some lawyers nervous in light of the replacement baloney skins some clueless tire dealer will try out.
These low cornering compliances will require some stiff tires, possibly half tread, produce poor isolation from road bimps, be VERY expensive, and result in a gigantic steering gain at max speed. And, the tires can't make up the whole enchilada. You are gunna need some extra compliance(s) (roll steer) this is the Porch Glider solution. Not worth a crap at max lat so Happy Tails to you, Roy Rogers. How about the Lond Rova approach ? Cheewing gum mounted steering gear ?

So, what's in YOUR wallet ?

RE: Why Mid-engine

BTW: Summing up point masses * distance^2 to get total inertia can yield a 40 - 50% error because you also need to add in the inertia(s) of the 'point' masses. ( Remember your parallel axis theorem ? ) For a dressed engine + transmission, this ain't something to ignore. Cross-products, too. This is the same issue that confronts the designer who is all wound up about where to put the 'roll axis'. The sprung mass has it's own moment of inertia and transferring it to the roll axix has it's own set of requirements and specifications based on resulting roll and yaw rate frequencies. (Remember that one of them is speed dependent and one is not. How did Shakespeare 'splain it ??? : "Never the twain shall meet".

RE: Why Mid-engine

OK, for the folks watching at home- so if there's no technical basis for switching the 'vette from front (front-mid) to mid (rear-mid), then we conclude it is purely marketing-driven, perhaps to attract younger buyers who are cross-shopping (or will only consider) the much pricier import competition? Seems an odd strategy, but in an environment where the experts conclude branding it a 'Mustang' is the best strategy to sell a full electric crossover, I'm clearly out of my depth . . .

RE: Why Mid-engine

@drwebb - the mid-engine Corvette is hardly an overnight happening. Rumors of near-future introduction of a mid-engine Corvette have been bandied about for at least 50 years. Closer to 60 if we're counting Duntov's 'CERV II' effort.


Norm

RE: Why Mid-engine

Quote (Ciba)

BTW: Summing up point masses * distance^2 to get total inertia can yield a 40 - 50% error because you also need to add in the inertia(s) of the 'point' masses. ( Remember your parallel axis theorem ? ) For a dressed engine + transmission, this ain't something to ignore.
Understood.

But I think for these sorts of back-of-napkin efforts you can safely ignore the 'own inertias' of the masses that you're moving . . . as long as it's only the difference in MOI that you're looking for (in this discussion, those 1200-ish numbers). Of course, you'd have to include them if as part of the FE to ME changeover significant changes in the 'own inertias' of the things that were moved occurred.


Norm

RE: Why Mid-engine

Part of the reasoning may have been boredom. There are few things more expensive than a bored engineer. If they had to crank out yet another FR car they knew they'd be compelled to duct tape a few new bits and pieces onto that tired old structure. By going to MR they got to redesign lots of things and abandon some sacred cows, and have a completely new car to develop.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why Mid-engine

Quote:

OK, for the folks watching at home- so if there's no technical basis for switching the 'vette from front (front-mid) to mid (rear-mid)

Its the classic argument - its technically superior, but most owners are neither skillful enough nor in a position to use its potential so yes, its marketing driven as most luxury products are.

RE: Why Mid-engine

Exactly. What does a manufacturer do to promote their particular supercar? How many buyers track test before they buy? They all want to own the fastest thing around a track but have to rely on some scribe's opinion - or the lap time at the Nurburgring with a hotshot behind the wheel.

That - and the 0-60 (0-100k) time as I said earlier.

je suis charlie

RE: Why Mid-engine

It's not about satisfying buyers who go golfing with their image cars or grocery shopping or Dream Cruising down Woodward Ave.

When the company decides it wants to win at Daytona, Lemans, fastest lap at the 'Ring, whatever, in a World Market, and can predict the sales and price break, a platform is initiated. Cadillac is another GM division who wanted some racing DNA whether it sells or not on Monday. It strengthens the backbone of the whole division. When a person who has been on a racing team goes back to the bread and butter vehicles, the product only improves, faster timing, new tools, styles, quality, fitment, testing machines, analysis techniques, even the way they walk.

Management (Reuss) knows this.

RE: Why Mid-engine

I worked on a couple of start from scratch supercar programs, neither got even to prototype stage. They weren't especially interesting to work on once the crazier ideas got thrifted out, although I would have liked to have worked on the V12 with a central power take off, (essentially two V6s nose to nose). The crankshaft dynamics of that would have been interesting.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why Mid-engine

Greg: Was Reuss there when you were doing this ? BTW: I remember him being a coop student in the N&V Lab At Milford P.G. Never hinted who his dad was or required, requested, or hinted at any special consideration. Sharp Dude. Good Listener...

RE: Why Mid-engine

Mark did a stint at Holden (in Oz) to get the G8 over here in the U.S. .

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