F1freak

Quoting Feb 2004 Road&Track regarding the C6: "The front of the engine is on the axle centerline, for a front-mid-engine layout". Is there any advantage to a rear-mid-engine design(Porsche Boxster) over the C6 front-mid-engine layout as far as cornering capability is concerned? My guess is the rear-mid-engine design might have a small advantage at the track where you would push your car to the limits and beyond. Definitely something you shouldn't be doing on the street. I'm familiar with the concept of low polar moment of inertia as it applies to the rear-mid-engine design but not as it applies to the front-mid-engine design. Anyone?

[Modified by F1freak, 10:43 PM 5/1/2004]


[Modified by F1freak, 7:44 PM 5/2/2004]

SWCDuke

The primary advantage of the rear mid-engine layout is greater tractive effort due to the rear weight bias. Corvettes are definitely traction limited and at their power to weight ratio, a rear mid-engine layout would have an advantage on a track. This is why all purpose built race cars are rear mid-engine, however rear mid-engine cars can have tricky handling characteristics. Expert drivers can get the most out of them, but they are edgier, and can be a handful for a novice.

Also, rear mid-engine cars are tougher to package, and if generous cockpit and cargo accomodations are provided they tend to be big and heavy. Consider the Ford GT. It is bigger and heavier than a C6 and has ZERO cargo capacity.

I think the term "front mid-engine" was coined by Mazda to describe the RX-7, but all Corvettes since C2 have been this layout where the front of the engine was setback to the front axle centerline. Back in the seventies and eighties Porsche spend millions advertising the advantages of the front engine/rear transaxle high polar moment of inertia layout as the BEST architecture for a street sports car. They were right, but their customers kept demanding quirky 911s, so the Porsche family finally fired Peter Schutz and dumped the 924 and 928.

Apparently GM was listening too, or came to the same conclusion, because the C5/6 are a "front mid engine/rear transmission" architecture, and they are both very fast and amazingly forgiving, even with the active handling disabled.

Duke




[Modified by SWCDuke, 10:52 PM 5/1/2004]

F1freak

See that's just it SWCDuke, I've heard it both ways. I've heard and read that the rear-mid-engine design is "unforgiving" particularly in the hands of a novice. However I've also read that because of the lower polar moment of inertia, rear-mid-engine cars respond quicker to steering input and the same holds true for countering a steering action. In other words it is more responsive to steer and correct vs. any other engine layout/design. I've also read that when braking into a corner, weight transfers from the rear to the front, hence actually creating unbalance to a car which achieves 50/50 weight balance in static condition. In contrast, a 40/60 mid-rear-engined car,because of it's slight rear weight bias, may achieve a real dynamic balance(50/50) under braking. Quite frankly i don't know what to believe. Sometimes i get the impression that vehicle dynamics is still very much an inexact science.





[Modified by F1freak, 12:59 PM 5/2/2004]

SWCDuke

Mid engine cars have very different driving dynamics than front engine cars. It's tough to explain other than to say that they are very responsive and edgy. If they are set up right, they are very rewarding to a sensitive driver. If not, they are a handful and can be downright scary.

I have a '91 MR2. The first week I owned it, it almost got away from me twice. The problem was insufficient rear tire load capacity. Increasing the section width of the rear tires 20 mm increased total rear tire capacity by 400 pounds and brought the front/rear tire load capacity in to line with the 44/56 static weight distribution. I was also able to take nearly all the rear toe-in out which Toyota specified in a feeble attempt to mitigate oversteer, but dragging the rear tires down the road sideways tended to wear them out quicky.

With street high performance tires it still has a hint of oversteer. With DOT legal racing tires is is dead neutral and you'd think your brain was directly connected to the front wheels.

Duke




[Modified by SWCDuke, 6:01 PM 5/2/2004]

F1freak

Thanks for the insight SWCDuke. It was helpful as usual. Quite honestly, I'm sitting on pins and needles waiting to see what they do with the MR2 for 2005. The rumours abound. I've heard everything from a new generation MR2/Spyder to discontinuing them outright. If they were to redesign it as a 2400-2500 lbs., transversely mounted mid-engine V6 with 260+ horsepower, and it was under $35,000, it would probably be my next car. I know,I'm probably dreamin and I feel guilty talking about MR2's like that on this forum. I still love Vette's though. Thanks again Duke.





[Modified by F1freak, 6:14 PM 5/3/2004]

SWCDuke

The MR2 was an example of a great sports car marketed by idiots. In the early ninties I was part of a focus group that was given a preview of Toyota's new sports car, which turned out to be the Supra. Almost to a person we told them it was NOT a sports car. What is it they asked? A luxury GT car we replied. They were slack jawed. Off line I told the VP of Marketing that the Supra was not a viable alternative to the Corvette - a three liter turbocharged six is just not an alternative for a big torquey V-8. They thought it was and would win "conquest sales". They were clueless! But what do you expect for a bunch of Ford refugees.

They considered the MR2 to be a "secretaries car" - had no idea what they had - a terrific mid range sports car. It just needed a V-6 to replace the laggy turbocharged four. We told them to put the V-6 in it. They ignored us. Now the Supra and MR2 as I knew it are long gone. I don't fit in the MR-S. It doesn't have any luggage space, and I'm not a convertible fan.

I'll be keeping my '91 Deuce as long as I can.

That's one thing about GM. They KNOW what the Corvette is all about, and haven't screwed with the formula since Duntov defined its niche. Dave Hill and the current crew understand what Corvettes are all about, and just keep making them better. Dave Hill and Dave McLellan deserve a hearty three cheers for not screwing up the Corvette like Toyota screwed up the MR2!

Duke

Yibbutkeen

I used to own a 1977 ferrari 308gtb (sold it to help fund my ce z06)- that rear-mid engine layout was as perfect a balance as you could get. I never managed to lose it, even pushing it to the limit on a racetrack. Extremely easy to put the car exactly where you wanted it etc.

I'll have to get back to you on how the z06 compares - its in transit to me now, and it will be a while before I can push it.

Overall I can't imagine a better configuration than the mid-rear from a driving perspective. If you want to haul any kind of cargo beyond a set or two of golfclubs however...

LTC Z06

Duke I think you've seen this quote from my friend before, he's one of the head engineers at Porsche, "Porsche, a bad design engineered to perfection"

At auto-x speeds I was just as quick in his 996 (with GT-3 like suspension) as in my Z06, the only thing I did wrong in the 996??????? You guessed it, oversteer. The C5 with front-mid engine and near 50/50 weight distribution is an easy, forgiving car to drive fast. That being said, one can learn to drive anything fast.

Adam Bruce

To echo the posts above weight is one factor for sure, another is that motors mounted on the same axle as the drive wheels (i.e. Front Engien FWD or Rear Engine RWD) tend to have less drivetrain loss and thus put more power to the road.

FWIW the largest reason that manufacturers went to Front Engine FWD cars in the 80s is that using the same motor you get better mileage, more power to the ground (that is less drivetrain loss) and it is easier to package. the driveline all in the same area of the car.

SWCDuke

Theoetically, a transverse drivetrain should have lower driveline loss, because there is no hypoid gearset. This is somewhat offset by the fact that a transverse drivetrain does not have a "direct drive" gear like on a typical front engine rear drive layout. A typical front engine rear drive gearbox is about 99 percent efficient in direct drive and 95-96 percent in the indirect gears. On a transverse gearbox, torque is alway transmittend through a gearset, so all gears are about 95-96 percent efficient. This is somewhat mute because modern front engine/rear drive cars all have "overdrive" gearboxes and achieve good fuel economy by operating the engines at low revs at freeway cruise speed to minimize internal friction and pumping loss.

The anecdotal evidence I've seen of computing drivetrain percent loss by dividing RWHP by SAE net does not lead to a definitive conclusion. There are too many other variables that come into play. Many are small, but they add up.

The biggest advantage of FWD is packaging efficiency. You can package more people and cargo room into a given volume and mass, than an equivalently powered front engine/rear drive setup.

Duke

WhiteDiamond

Duke,

I have seen many articles lately about packaging effeciency between the front and rear axles. Apparently, there is a focus among some auto builders to get more of the weight centered in the area between the axles. My take on this is that front and rear over hand areas will now be heavily reduced or contain absolutely no major hardware/structure other than required bumper mechanisms and bracing. Understanding the front versus rear weight bias, how does concentrating the mass inbetween the axles help? It would seem that you would get a more precise control over the weight distribution under braking, acceleration and cornering since the polar movement would(should?) be reduced.

Todd

SWCDuke

For a given overall length, the longer the wheelbase, the more occupant room, so modern designs tend to go for as long a wheelbase as possible within the overall length, especislly on small sedans. Longer wheelbases also reduce pitching tendency, and the closer you are to the middle of the wheelbase, the better the perceived ride quality. I remember my dad talking about the great ride quality of his '49 Cadillac Fleetwood Sixty Special because of its 133" wheelbase. Now that was one honkin' car - wish he had kept it!

Placing the major masses near the CG reduces the yaw moment of inertia, which is why mid-engine cars are so responsive and have completely different driving dynamics, but they can be tricky for average drivers because they are so willing to rotate.

The Corvette has a relatively high polar moment of inertia, which makes it a bit less responsive than a mid-engine design, but it is inherently more stable and less likely to snap into oversteer than a mid-engine design. This "high polar moment of inertia" concept is what Porsche was trying to sell 25 years ago, but their customers just didn't get it. Such a design - front engine/rear transaxle - is the best design compromise for a sports/GT layout when you consider occupant comfort and luggage capacity, especially if the engine is set back so the front of the engine is at the front axle line. This is exactly the design archtecture of a modern Corvette! Placing the tranmission at the rear also allows for much more generous footwell width. Just compare a C4 to a C5!

You end up with a car that provides excellent comfort to two, generous cargo capacity, 50/50 weight distribution so it is both balanced and stable, and an excellent compromise of ride and handling for someone of less than Schumacher's driving prowess.

Duke





[Modified by SWCDuke, 10:27 PM 5/21/2004]

I Bin Therbefor

A some of one liners from some famous sources.

Dave McL in "Corvettes from the Inside" appears to be a Mid Rear man. He claims that GM ran some studies showing that the Mid Rear design would be 100 lbs lighter than the equivalent mid front.

Jim Hall of Chaparral claims you can't get too much weight in the rear of a mid rear if you can get the tires to carry it.

Several folks have re engined Fieros with V-8s. Everything from push rod small blocks to big blocks to North Stars. The small block swap is very close in weight to the cast iron V-6. Supposed to handle very nicely.

I've seen a couple of MR2s with V-6 engines. One a Toyota V-6 and the other a push rod GM. One of my daily drivers is a MR2, first gen with a supercharged 4. A great car for a commuter. Very sensitive to tire choice. Sufficient storage space for a commuter.

As for the 928, I remember when it was first shown and tested, the professional drivers who were quoted in the road tests, to a person, gave its handling great praise.

I've seen or heard of a lot of projects to put a small block into an RX-7 but I haven't seen one finished yet. Be interesting to see how that turned out.


[Modified by I Bin Therbefor, 9:38 AM 5/22/2004]

SWCDuke

Like Zora, Dave McClellen was a proponent of a mid-engine design. The basic reason why it never happened was more cost than technical. There was just no way a mid-engine design could be built without substantially raising the price point of the Corvette, and management did not want to "up-market" the car.

GM also realized that there were some handling and stability issues with mid-engine cars, and the Corvair experience was still pretty fresh in their minds through the seventies, and at that point it was all they could do to keep up with rapidly changing safety and emissions issues, so a new car was out of the question.

Once Dave got the go ahead to do C4 the mid-engine cost issue reappeared, so it was clear that the basic architecture would remain front engine/rear drive.

I think the idea of a mid-engine Corvette ended with McClellan's reign as Chief Engineer. I don't think Dave Hill ever gave a mid-engine design any serious thought. He knew the issues and knew it was a dead end.

Duke

LongTimer

This "up-market" issue is exactly why Cad wants a Cein type car for their V12 in develpment. If Caddy gets a middy chassis, I would expect to see some parts sharing for a Vette version with the Blue Devil 625HP Gen IV in it. Likely lighter than the Cad version.

Dylan Savage

SWCDuke - I had a supra before my corvette, and while it felt big and heavy (and it was), that thing was FAST. I road race mostly, and the supra had tons of grip, accelerated hard, and got the power down really well. Also the standard 4 piston brakes with 13" rotors in the front were a nice touch.

The MR2 with it's 1-piston brakes, 10" rotors, 185-60/14 tires, broken rear suspension (until 93), weaknut motor that breaks when pushed, and no standard LSD (again until 93) could hardly be considered a sports car. It is mid engined but the rear suspension geometry makes it about as oversteer-prone as a '72 911 Turbo. Heaven forbid you ever lift in a corner.

The supra would have been more or less perfect if it had been a 2-seater, lightened a bit (3000lbs?) with a single T61 turbo from the factory rather than the sequential twins. As delivered, I have to agree with you, it was more of a luxury GT than a sports car. The corvette fixes most of these deficiencies, aside from the brakes.

The MR2 is only more of a sports car than a supra or corvette in theory because of it's engine layout.. in practice a corvette or supra would lap an Mr2 within 20 minutes.

SWCDuke

The MR2 was the quintessential definition of a sports car - light, responsive, nimble, and economical - a true driver's car. Sports cars are not defined by 0-60 or quarter mile times or rear wheel power numbers.!

There was nothing wrong with the original MR2 suspension that higher load capacity rear tires could not fix. That's what I did to mine after a week of ownership and nearly two snap spins. I replaced the OE 205/60R-14s with 225/60R-14s, which increased total rear tire load capacity by 400 pounds and brought tire load capacity distribution into line with vehicle static weight distribution. I was also able to reduce the excess rear toe-in of 5 mm to 1.5 mm. It remains a mystery to me why/how Toyota could build the car so seriously "under tired" that is had obvious stability problems!

The 1993 suspension redesign merely took the "edge" off the car and made it understeer more and less responsive to a seasioned driver. I never raced a Supra, but there were many higher powered sports cars and sports sedans that could not equal my 1:12 lap times at the original Streets of Willow Springs course. The Porsche guys used to ask me how much boost I was running. I replied that it was just the 130 HP "tractor engine" out of a Camry, but that engine had a great, flat torque curve that was perfect for a short course.

The Supra was a very good car in it's own way. It did not fit the definition of "sports car" that drove MR2 owner's to purchase their cars, nor was it an alternative to the Corvette that Toyota thought it would be and win "conquest sales" from Corvette. In particular, MR2 owners scoffed at the vestigal back seat that was too small for real people, but added bulk and weight to the car. Real sports car have accomodations for two, only! I say the same thing to 911 owners.

The relative success or failure of a car is measured in the marketplace - actual sales relative to the manufacturer's projected/expected sales. By this definition the Supra was an abject failure (and so was theZR-1). Though initially quite successful, the MR2 also ultimately failed, and Toyota forced that failure by raising the price of the car 25 percent in four years - far in excess of inflation or value added. They left the low end sports car market to Mazda and the Miata went on to a long and happy success including to this day.

The C5 Corvette by this measure was a grand slam home run, and I excect the C6 will be the same. Both adhere to the "high end sports car" formula laid out by Duntov nearly 50 years ago. In addition to the expected light, nimble, and responsive requirements for a basic sports car, Corvette adds lots of power an creature comforts placing it in the "high end sport car" group that competes in performance, quality, and comfort with foreign "exotics" while being reasonably affordable both in terms or purchase price and operating costs that have been a hallmarks of Corvette's long successful run, which appears to have no end.

Duke


[Modified by SWCDuke, 7:18 PM 5/27/2004]

rbarta

SWCDuke - you have thoroughly explained the reasoning and history behind many cars and the front-mid vs rear-mid engine layout and it has been very informative reading.

I wanted to add that when Porsche put out the 928, the 911 was a rear engine layout, with the majority of the engine weight behind the rear axle, not mid-engine. You probably knew that. I don't know if that is still true today but I would guess part of what they did to lower it's propensity to spin was to move the engine forward a bit.

SWCDuke

Yes, I'm aware that the Porsche 911 and it's variants are rear engine. With it's high rear weight distribution and engine hanging out over the rear axle, the 911 is more yaw happy than a mid engine car. The early 911 instability was exascerbated by the semi trailing arm rear suspension, which added lots of toe and camber change with jounce and rebound.

The 911 was finally tamed - at least compared to the prior models- circa 1996 when a multilink rear suspension was adopted, but they are still tricky cars to drive.

I find then interesting in a quirky sort of way, but basically do not care for 911s or any other rear (as opposed to mid) engine car. I also remember some crazy rides and spins in Corvairs when I was in high school.

Duke

I Bin Therbefor

Check out the Mosler home page. That car has a lot of the engineering that Dave McL was mentioning in "Corvettes from the Inside". Best I can see, the only difficulty they're having is finding an appropriate gear box.