911s: What are the advantages of a RR layout over FR or MR ?

Danny Tran

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I want to discuss the plain 911 (NOT the 4WD)

I heard the 997 is better balanced than previous 911s, but it keeps ne wondering if there are any advantages of having the engine behind the rear axle.

Anybody know the exact weight distribution of a 997S?
 

YF19pilot

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Re: 911s: What are the advantages of a RR layout over FR or

Re: 911s: What are the advantages of a RR layout over FR or

Danny Tran said:
I want to discuss the plain 911 (NOT the 4WD)

I heard the 997 is better balanced than previous 911s, but it keeps ne wondering if there are any advantages of having the engine behind the rear axle.

Anybody know the exact weight distribution of a 997S?
MR, and RR to a further extent, cause a car to become less "stable", thus making it more manueverable. It's a trade off between stability and manueverability.

More stability means that the car is more resistive to changes in its state. Less means that it's easier to disturb the car's state. This is of course affected by the placement of the engine and what tires are powered.

Of course, a porsche is more unstable, so it's easier to loose control of than a FF car.

Sumary: Engine in back makes car turn better, but harder to keep control over when making turns.
 

andyhui01

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^Don't MR and RR cars understeer badly if the corner speed is quite low (ie <80kph) because there is no weight at the front and the downforce isn't working at the slow speed?
 

Top Geek

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Well, I can tell you one advantage to a rear-engine layout, and that is grip off the line from all the weight over the rear wheels.
 

Viper007Bond

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Yeah, the only advantage really is more traction on those rear wheels. The disadvantages (easy to oversteer, somewhat cramped engine, lack of room for exhaust and stuff, etc.) outweigh it and that's probably why the Boxster and Cayman are mid-engined.

The 911 is just rear engined 'cause it's been like that forever and they're German. As Jeremy said, they'd rather engineer the hell of out of it to get power and stuff and use electronics and good suspension to make up for it rather than move the engine.
 

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andyhui01 said:
^Don't MR and RR cars understeer badly if the corner speed is quite low (ie <80kph) because there is no weight at the front and the downforce isn't working at the slow speed?
Not at all. It's all about shifting the weight to the front of the car to help the front end grip, and the intertia helping the vehicle turn into the corner. It is a different driving style to a FR car though - but it means you can brake later into the corner to help the front turn, and then get on the throttle sooner since the interia of the car is turning it around the corner much more so than with a FR car. I've had the MR2 oversteer (intentionally) doing less than 80...

Remember, there's a reason the fastest racing cars are MR :wink:

Oh, downforce isn't a factor for 99.999% of road cars in any case (and for those where it is a factor, it's not effective until waaaaay above 80kmh)
 

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epp_b said:
Well, I can tell you one advantage to a rear-engine layout, and that is grip off the line from all the weight over the rear wheels.
Either that, or you overdo it, and get HEAVY axle tramping ;-)

A 997 has about 40 of the weight over the front, so its not ALL on the back, they moved the engine a bit forward with every generation.
 

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There is only one main advantage for having your engine in the back.Good traction at the rear because the weight is on the rear axle.

While it is true that it makes the car unstable it is also its biggest disadvantage.A skillfull driver will take advantage of that on the track, but Joe Blow will just end up wrapped into a tree.Having an unstable car at the limit is not a desired design criteria.Its a flaw.You want your car to be neuntral handling wise, no understeer no oversteer just cornering grip like it is on rails.

It's difficult to achieve that with a FR layout it is even more so with MR or RR.Why the 997 is the benchmark in sports cars is only down to the fact that Porsche has been honing the same basic design for 40 years now.They are good engineers at Porsche no doubt about it but one would say that after 40 years they ought to have cracked it.And they do to a certain extent.

Finally, the list of disadvantage far outweighs the single advantage it provides.Engine packaging is difficult.Changing the oil on a 911 is almost an engine out job.There's way too much stuff in the way.Engine cooling is also difficult thats why they have changed to liquid cooling.And you have to make your car drive like an FR car which believes me is a big challenge.

Just note that the 997 is a very different animal when the PSM is turned off.
 

MXM

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You know how they say corner exit is most important part? Well, RR offers the best corner exit traction. Of course the weight shifting is the most noticeable too, which is a downside to unexperienced driver, and requires different approach to that of FR or even MR, but it's still effective.
 

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MXM said:
You know how they say corner exit is most important part? Well, RR offers the best corner exit traction. Of course the weight shifting is the most noticeable too, which is a downside to unexperienced driver, and requires different approach to that of FR or even MR, but it's still effective.
I remember Sir Jackie Stewart saying in the TG episode that the "entry of the corner is far more important than the exit of the corner".......
 
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VR6 said:
Wasn't it the other way around? thats how i seem to remmeber it.
I'm pretty sure it's the other way around, especially if there's a long straight after the corner then the exit is very important.
 

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alokharidas said:
MXM said:
You know how they say corner exit is most important part? Well, RR offers the best corner exit traction. Of course the weight shifting is the most noticeable too, which is a downside to unexperienced driver, and requires different approach to that of FR or even MR, but it's still effective.
I remember Sir Jackie Stewart saying in the TG episode that the "entry of the corner is far more important than the exit of the corner".......
switch that, it's exit > entry because your exit speed is greatly influencing on your next corner.
 

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Think of the center of mass for a car on the horizontal axis.

FF cars have the weight in front, meaning your axis for rotation is near the front of the car and the front wheels have to drag that weight sideways off the direction of travel. The light weight rear end simply follows the front. Result: understeer but very very small chance of a spin.

MR cars have the weight in the middle, equally (or nearly) distributing the weight between the front and rear wheels. The front wheels have to drag much less weight off the direction of travel and the greater distances gives the steering wheels the advantage of a lever with the fulcrum (rear wheels) on the other side of the mass. Result: better responsiveness, but a moderate chance of spinning the car with poor throttle controll.

RR setup, or cars with the engine behind the rear axle. Now the front end is very light and responsive, you have the maximum lever effect for moving the engine mass because the fulcrum (rear wheels) are just inside the mass. Result: it's very easy to get the front end moving in a different direction than the center of mass (engine) which has most the inertia and wants to keep going straight. This means that without very careful control it's easy to get the front end going on a different vector while the engine sails past the front wheels resulting in a spectacular spin.

This is very very simplistic and the placement of fuel tanks, batteries and other components along with the use of light weight materials can either balance or unbalance any basic setup.
 

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RR engine layout is also good for braking performance.
 

Blind_Io

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Svempa said:
RR engine layout is also good for braking performance.
Oh really?



*whistles smugly* 8)
 

Blind_Io

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I call with another three-headlight rear-engine! (although this one has an aluminum flat six aviation helicopter engine, not a V8)
 
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