Rear engine vs. mid engine ? Cayman vs. 911.
 

I came across this article.
10 Ways the 2014 Porsche Cayman Is Better Than the 911

What is this polar moment they are discussing?
What is this commenter talking about?

Any truth to this comment about sliding in a mid engined car, and supposedly not being able to recover from it, since there is to weight imbalance?
Can someone review why rear engine is ideal for braking?

They'll get no argument from me.
Out there on the edge, the mid engine does its best work.
The rear engine does give a better warning of dropping off the cliff, but the edge is farther away with the mid motor.
My old 914 would gobble up any species of 911 (GT2,GT3, what ever). at autocross.

Nothing drives like a mid engine car. F1 cars do not have the same layout as a 914/Cayman by mistake

Braking and acceleration in rear engine are good because of weight transfer and traction of weight on drive wheels. As far as polar moment. Imagine an iron bar with the weight evenly distributed. Now image a pipe, with weight at the ends, that weighs the same as the bar and is the same length. Which do you think would be easier to make spin?

I always think of a 15 pound bar that's 3' long vs a 15 pound bowling ball.
Grab the bar in the middle and see how fast you can rotate it back and forth. Now do the same with the bowling ball.

polar moment of inertia (as I understand it) refers to the distribution of the mass in an object. For example, in an old corvette- the weight is in the front from the engine. In an old 911, the weight is in the back. These are high polar examples. A mid engine car has the majority of it's mass in the center, so it is not a high polar object.

I think it was a 1980's panorama article that described this and used a thrown dart as an example- with the front of the dart (the weight) having the most mass(and inertia), and the feathers in the back- so the mass continues to fly forward as the feathers provide drag- keeping the dart flying straight.

A 911 is like a dart thrown backwards, which gives it it's unique handling characteristics.

Having the engine in the middle theoretically keeps the moment of inertia equally distributed between front and rear- giving the car a potential advantage in neutral handling.

Thats my story and Im sticking to it.

http://forums.pelicanparts.com/off-topic-discussions/737448-polar-moment-inertia.html

This is a good thread with pictures and explanations.

Yes.

911 people (I have had 3) will tell you that layout is better. But the engineers know mid engined layouts are better. That is why the most expensive race cars in the world have mid engines. The 917 was mid engined.

I also had a Boxster. That was easy to corner with. In the 911 you had to have a lot of experience to really play with it.

911s are great at getting around a race course and tons of fun, but I believe that the ultimate way to get from corner to corner and through the corners is a mid-rear car. With a mid-rear, you've got the low polar moment from the weight being in the middle, but you've also got a slight rearward weight bias that gives better traction.

It's all about Physics.

the only reason the 911 has a rear engine is to fit the rear seats.

its no coincidence that all porsche hyper cars have been mid engined, and all racers besides the 911 series have been mid enigned.

that all being said, in the right hands, rear engined does seem as capable as mid engined. i think its trickier to drive them well though.

Bingo. It's about balance between a low polar moment and having enough weight on the rear wheels to aid traction for acceleration. The big Can Am cars of the early 70s had rear weight bias with could be adjusted because the engine was between the wheelbase.
Has there ever been a race car with mass that can be moved while under way?

Think of a low polar moment of inertia as the resistance to changing direction, because mid-engined cars tend to be very responsive to inputs. To me that's the biggest different between a Boxster/Cayman and a 911, the mid-engined twins are instantly responsive.

I've had the opportunity to drive my buddy's Cayman R. Pretty hard to want a modern 911 after driving it, excluding a GT3 or something.

All I can say is I've taken corners on the street at stupid speeds in my Cayman with little more than the tail stepping out a few inches or the fronts scrubbing a little. I can't say the same for my old 997.

There's a fair amount of BS in that article, and a definite bias from the author, so digest it with a little salt. Or quit reading that website, which is more or less worthless anyway.

There was a test done a while back (can't remember who did it) that set out to compare a 911 and a Cayman, without a power difference being the deciding factor. In the end, the 911 proved to be the faster car around a track. That wasn't a test of what's theoretically possible, just a real-world result based on how the cars were delivered from Porsche, which is what matters for most people, since most buyers don't change their cars much. Each car had advantages and disadvantages compared to the other, but the 911 proved to be the better tool around an entire lap.

There are big differences between the two cars ( I happen to find the Boxster the most fun to toss around) but don't assume the mid-engined cars have the most mass right in the center. They don't: it's still towards the rear, just not as far rearward as a 911. Yes, they might turn in a little more crisply, but I don't know of anyone that *****es that a 911 won't turn into a corner well. They spin a little more easily, so you need to be a little quicker to catch one. Given the electronics they have now, nobody should get that out of shape, anyway.

A 914 is a different beast altogether. Lots of fun, but tricky at the limit. Their autocross prowess has as much to do with their huge weight advantage over just about any car, as it does where the motor is placed.

The backwards arrow analogy wasn't related to the polar moment of inertia of a 911, but to the fact that its center of mass was behind its center of aerodynamic pressure, which is what makes it so unpleasant in cross winds. That's another issue, altogether...

If you look at F1 cars, or other racing cars, you can usually find that they do not have a 50:50 weight distribution. That's not an optimum distribution for a rear wheel drive car. It's not all that unusual to see a bias that approaches 70% on the rear, depending on the intended use for the car.

JR

I didn't even bother with the article, my comment was in general comparing a Boxster/Cayman with a 911. If you are looking for Porsche buying advice from Edmunds, you're already too far gone for help. Calling one of them better, unless you care about lap times, is totally subjective. They each have their own unique charm, but it is a much different driving experience. Personally I find the Boxster to be the most pure fun of any modern Porsche, it has a lightweight and tossable feel that even the Cayman can't match. It is also the slowest around a track, back to that whole not caring about numbers thing.....

Mercedes in DTM in the 90's used it, slabs of metal on a moveable sled

Retrospective>> Awd Tourenwagen Meisterschaft - Speedhunters

Also in F1 but with mercury moveable ballast system, it never got as far as asking the FIA for permission as internally within the team it was deemed to be just too much of a health risk

It's interesting to note that F1 has evolved towards a fairly conservative f/r weight distribution of around 46/54, give or take a percent or so. Note that you have more freedom to play with the distribution if you don't care to be at the minimum wieght limit, since the rules specify a minimum weight for each axle, but that's like asking a supermodel to eat pizza before a major shoot. Ain't going to happen....

reducing weight is always more important that the distribution of that weight.

Maybe. Read up on Porsche's bergspyders from 1968, or so.

JR

Having moved from a 914 to a 911, I will share this (and imagine others have observed the same).

A 914 will let go, often with little notice when it appears at its limits. It can require constant attention to keep it balanced and not spin (and spin and spin).

The 911 always lets you know the weight it back there. The key is learning how to use its weight to an advantage (ie, like Vic Elford quoted above says - use the weight o help the car turn for you. Anticipate the slide and control it). You lift, you rotate, you plant, you steer a little bit.

I had hoped my Boxster would be like the 914.. it wasn't. It understeered and lifting never rotated the car, just slowed it down.

Now, was that safe? sure, and in an era of frivolous lawsuits, it keeps Porsche in business.

Are mid-engined best? overall, yes but most really good mid-engine race cars are actually 46-54 or so on the front/rear for traction (handling) reasons.

low polar moment just means the car can be rotated w/ less force than a car w/ higher polar moment because the weight is more toward the center of rotation.

A commonly seen example of changing polar moment is an ice skater spinning, when the arms are pulled in you can see the rate of rotation increase noticeably, when the arms are extended the rate of rotation slows noticeably. This is because the total energy or rotation is conserved during the transition from one geometry to another.

In a car polar moment is more or less fixed, it does decrease a bit due to gas usage

Cayman has lower polar moment than 911.

Sometimes it's better to have a higher polar moment, on acceleration and deceleration are 2 prime examples.

Another point is that the inputs don't have to be so finely controlled w/ a higher polar moment, A 911s tail will always let you clearly know what it wants to do and responds to drivers input well, you want to rotate more? OK, let off the gas, the car will rotate more. You want to stop rotation? OK, gas it it, the rear plants and the car squirts ahead. You do have to steer a bit while all this is going on. In a Cayman you will want to be more subtle and not have to counter steer as much.

Someone mentioned that you can lower the polar moment of a 911 easily, this is true, a 911 w/ f/g or c/f bumpers as opposed to the stock rugged bashers will have a lower polar moment and will rotate more easily, If you have a chance to back to back drive the difference is amazing.

After getting used to the Cayman it is a completely different drive than my old SC. The 911 can rotate on a dime, sometimes whem you don't want it to :) but it can shoot out of corners like no tommorrow. The Cayman feels like it's on rails around a corner with no real feel of a rear break away, but a re-correct after the corner that takes a second before you can apply the power properly. Both understear but I managed to tune most of it out of the 911, I haven't had the chance to do it on the Cayman yet.

Both are completely fun cars to drive, just very different.

911, weight in the rear. The front of a 911, because there's not much mass/weight, is very easy to get to change direction. IE, you turn the steering wheel a little and the front end goes where you point it like it's telepathic. The rear end because it has all of the weight, wants to keep going in a straight line and takes a lot more force to get it to turn (hence the bigger rear tires). On a spirited drive, you can get that to work for you. A 911 can be a formidable car in the right hands.

A mid engined car (in this case, mid-rear), with the weight mostly in the middle (but as stated, still usually with a rearward bias, is still going to be pretty good at turning because of the low front weight, and since the weight is more centered, the rear is also a little happier to rotate for you. Because of that, a mid engined car is usually better at changing directions than cars that aren't mid-engined. But, a car that's happy to change directions (ie, spin faster) can also require very quick reflexes to catch when it does spin.

A balanced car that's forgiving and easier to drive is a car that's balanced, but has a high polar moment, think 944 with the motor in the front and the transmission and differential in the back, same as the newer corvettes. I could swear that there's an article out there where Derek Bell says that the 944 is better than the 911 because of it's balance and since the weight is divided between each end not as quick or likely to spin. I think the gist is that the 944 is very easy and forgiving to drive because it's got a high polar moment.