Mid-engine, Rear- engines superior for aero?

[aerohead]

Originally Posted by equation112

I feel I have to disagree with much in this post Matt... I'll break it down into smaller chunks.



first off, while you are partially correct that in a street vehicle (indeed even race vehicles to a large extent) over and understeer are determined by weight distro and wheel alignment, you are overlooking an equally important (perhaps even more important) aspect of the equation: suspension setup - in particular the relative stiffness of the sway bars front to back.

Second, 99% of new cars are front engine/FWD (just call them FF and be done... FR would be front engine/rwd, MR is mid/rwd etc.) not because of steering characteristics, but because it is easier to design (and cheaper to build) them that way, and also allows the manufacturer to maximize a given vehicle's utility so it can appeal to as many potential buyers as possible. let's face it: MR configurations have very low useable interior volume when compared to FR's (hence why that layout is generally reserved for high-performance vehicles), and FR's are inferior to FF's although not by as large a margin.

I also have to take issue to your characterization of understeer as being preferable to oversteer. there is a saying you might be familiar with:

oversteer makes passengers go 'holy $h!T!!!' while understeer makes drivers go 'holy $h!T!!!'.

A small caveat here... understeer *is* preferable to snap oversteer, but a smooth and gentle oversteer is *far* preferable to any understeer, regardless of the drivetrain layout of the vehicle. a properly setup suspension will always tend towards oversteer. as an example, all of my scariest moments in cars have been during understeer events, where my car threatened to slam into a wall or launch itself into the air towards the outside of a curve (many ramps around here have large curves with no railing on the outside radius). every time I have experienced oversteer it was such a non-event that my heart didn't even bother to beat faster.



there are a number of variables there to consider... what year is the MR2? (the 1991 and 1992 SW20 models were prone to snap oversteer because of their suspension setup, which was nerfed by toyota for the 1993 and later models). also, oversteer should be seen by an autocrosser as something to be used and not fought - when the back breaks loose, drift it. that or your friend had something setup in a non-optimal way - it could have been as simple as using the wrong tires for that surface that day. certainly there are other avenues of investigation that I am not mentioning. if the MR2 in question suffered from understeer, your friend would have had a hard time being competitive - and might not have been able to stay on the course.



RR cars with engines any larger than the original VW beetle and of course the 'thing' have severe handling problems - a serious tendency towards snap understeer in particular.

I understand that oversteer/understeer characteristics for passenger cars are directly related to manufacture's recommended tire inflation pressures.

[aerohead]

Originally Posted by Peter7307

The grille is only one part of the whole picture but yes superior aero is more easily achievable with the engine mid or rear mounted. Then again you could say the same for 2 doors versus4 doors versus wagons.

Citroen managed very respectable Cd numbers from their cars which were mainly 4 doors and front engined and front drive as well.
Check the CX , GS models for the 4 doors and the SM model for the 2 door.

Pete.

Might want to check out Professor Morelli's CNR "banana-car of the 1970s,developed in conjunction with Pininfarina's wind tunnel.

[aerohead]

Originally Posted by equation112

ah yes... understeer is scary. thing is I just dont see why many people buy into the idea that understeer is more forgiving, when the very definition of understeer is the complete and total loss of steering effectiveness. this is doubly bad in a FF type car, where understeer results in both a loss of steering AND propulsion AND over half the braking power simultaneously. suprisingly enough however the solution to both over and understeer is pretty simple: MORE power (ie NOT more brakes, because braking in a curve is likely what caused the problem in the first place. drifting a car during oversteer really isnt hard at all, because you still have the ability to control steering, the majority of the braking and power... matter of fact a car in oversteer is actually nowhere near it's true handling limits, while a car in understeer is far beyond its' limits. what bites people about it is the fact that it this is all somewhat counter-intuitive.

I forgot all about the 911's... yeah their handling is not all that impressive, though the new AWD models have rectified the deficiencies to a great degree.

Low speed understeer is considered an attribute by carmakers for the 99th-percentile motorist.If your driving in oversteer conditions,your driving beyond the weather or posted speed.

[Frank Lee]

Originally Posted by aerohead

I went back into my library to make sure I was using proper terminology when describing terminal understeer for rear engined,rear drive cars.I believe I chose the correct word.

Nope. Rear engine cars definitely have oversteering tendencies. Think about what "under" and "over" steer really means.

[equation112]

Originally Posted by aerohead

I understand that oversteer/understeer characteristics for passenger cars are directly related to manufacture's recommended tire inflation pressures.

tire pressure is only part of the equation, and not so much as your sources would have you believe. suspension setup is easily as important at all speeds - I believe it is even more so, so long as the tires are in good condition and inflated to within a certain range of psi of each other. for instance, my tc from the factory is recommended to have tires at about 34f/32r. when I increased the pressure recently (my current tires are rated for 51psi), I set it for 48f/45r, in order to maintain the pressure ratio from front to rear within an acceptable relative range.

Low speed understeer is considered an attribute by carmakers for the 99th-percentile motorist.If your driving in oversteer conditions,your driving beyond the weather or posted speed.

oversteer can be induced in a number of ways at any speed, but I fail to see how understeer could ever be considered a desirable attribute under any circumstance or at any speed. remember, when a car understeers it is effectively losing all steering control (like I stated previously), as opposed to an oversteer. personally, I am utterly terrified when the front end breaks loose, because at that point the car is going to go wherever the hell it wants, and I have effectively no say in the matter.

remember class... understeer=bad

[Kohote]

I agree that understeer is bad in terms of maximum performance and speed around a racetrack. But it takes a much more skilled driver to drive a loose car at the limit than it does to drive a tight car at the limit.
This is why pretty much every single road car sold today with the exception of some sports cars and exotics has a tendency to understeer. Sure there are ways to induce oversteer but the engineers design the handling characteristics so that the majority of drivers will only experience understeer. When a driver experiences understeer their natural reaction is to lift or start braking- most of the time the driver then regains control and is safely on their way. When an average/untrained driver is surprised to find their car exhibiting oversteer it is much less likely they will regain control.

[aerohead]

Originally Posted by Frank Lee

Nope. Rear engine cars definitely have oversteering tendencies. Think about what "under" and "over" steer really means.

Hi Frank! I understand where you're coming from,as the Porsche 356,911,912,and Karman Ghias I've driven all had oversteer when at the limits of adhesion.--------------------------------- The thing about what Korff was warning about,which directly effects any of us streamlining,is that since the C.G.of rear-engined and even mid-engined cars can be behind the centerline of the wheelbase,should the C.P. lie ahead of C.G., in a strong wind gust(as Otto might experience if he crosses into Germany from Belgium and goes full speed on the Autobahn) from the side,the car will act like an arrow flying feathers-first,and attempt to switch ends.In such a scenario,with the front coming around to meet the back,would constitute "understeer",of which it might be impossible to recover from,as the front has exceeded the limit of adhesion,and no amount of steering correction or throttle modulation may be enough to recover,with powered rear wheels pushing the car further off course.Remember,I'm not talking about rounding curves at speed,just barreling straight down a superhighway in very strong crosswinds,something that has already claimed lives.

[aerohead]

Originally Posted by equation112

tire pressure is only part of the equation, and not so much as your sources would have you believe. suspension setup is easily as important at all speeds - I believe it is even more so, so long as the tires are in good condition and inflated to within a certain range of psi of each other. for instance, my tc from the factory is recommended to have tires at about 34f/32r. when I increased the pressure recently (my current tires are rated for 51psi), I set it for 48f/45r, in order to maintain the pressure ratio from front to rear within an acceptable relative range.
(my comment in response)
equation,let me play Devil's advocate for a minute here.I don't want anybody's blood on my hands,so I'm gonna be kinda anal on this issue.------------------ Whether I or anyone else likes it,carmakers design low speed understeer into their cars.And yes,understeer resists turning.Thing is,at the limit of adhesion(and I'm talking about typical commuting around town), in a panic situation,if the front of the car breaks loose(and it's designed that they do) all the driver need do is apply throttle and steer in the intended direction.The front of the car will drag the back of the car around with it.With ALL the other parameters mentioned to describe and predict how a car will handle,the last thing they do,is select tire inflation pressures which guarantee that the front will break loose first(understeer).
(original quote)
oversteer can be induced in a number of ways at any speed, but I fail to see how understeer could ever be considered a desirable attribute under any circumstance or at any speed. remember, when a car understeers it is effectively losing all steering control (like I stated previously), as opposed to an oversteer. personally, I am utterly terrified when the front end breaks loose, because at that point the car is going to go wherever the hell it wants, and I have effectively no say in the matter.

remember class... understeer=bad

(my response) Understeer is considered desirable by the carmaker,for low speed driving.Oversteer is desired at high speed for the reasons you mention,however is terrifying to anyone not trained in how to deal with it(nearly everyone in the U.S.).----------------------------- My whole point has to do with high speed directional stability which is directly related to C.G. If an ecomodder streamlines their car,such that the C.P. of their car is positioned ahead of the C.G. of their car,in certain situations,they will die.

[Unheard]

I spend most of my driveing life on tiny little country roads my car can either understeer or overseer depending on the way it is driven. In the lupo i have to break before a corner and power through it if i break into a corner i will get massive undersear the end up in a hedge its very very scary and once it starts to happen there is hardly anything that will correct it apart from a tree or curb. Overseer is much more predictable if i slow or brake before a corner i can then just power into it, if i start to feel the overstear i know letting off the power will correct it. You can also feather the amount of steer you put into the wheel. So driving into the corner you will almost twitch the steering wheel putting in a bit of steer then backing of then a bit more than backing off. this is ideal on a front wheel drive car you get use to the balance of the car. Its like how you drive a car up on 2 wheels you feel the balance and add the steer in to correct the car from falling back down onto to wheels or in this case spinning out.

On a front engine rear wheel drive car understeer is not as scary as you have the ability to blip the power to get the back end to swing out to correct the lack of steer.

IMO under or over is not better it just depends on the car and the ability of the car and driver to correct it and the driving conditions.

Personally like oversteer for the tight bend british roads i drive on. On long gradual turns oversteer is not as good as g-force is not with you to give the same feed back to calculate how much steer to put in so you tend to overcompensate and it gets very tiring.

YouTube - Top Gear - Oversteer and Understeer

YouTube - What's behind "top gear explain understeer and oversteer"

[nwbabybronco]

Understeer is not complete loss of steering. Understeer is when the turning radius of the car is greater than it should be with a given steering wheel angle. The front tires are losing lateral grip in the turn, but there is a spectrum. At its worst, understeer sends the car straight toward the outside of the corner, as described by Equation112.

Oversteer is when the turning radius is tighter than the steering wheel input. The rear is sliding. Somewhat counterintuitively, oversteer spins the car to the inside of a corner.

In general, the drive wheels will determine whether the front or rear slides. FWD understeers, RWD oversteers. This can be complicated by engine placement, which definitely affects turning.

My FR ranger understeers going into corners, but with throttle will severely oversteer because of its poor longitudinal weight balance. My Golf would understeer terribly, which was a tire and possibly suspension issue. Only three times, at my worst behavior, did I ever get it to oversteer. My Passat, with better tires, has a very smooth and controllable understeer. That is the turning radius can be comfortably adjusted with throttle, which made poor weather simple to drive in.

So, I guess my point is absolutes are dangerous to use and all generalizations are false. Back to your regularly scheduled thread.