rear and mid-engines

[charlie_pank]

Having only ever owned a soft-top mk1 golf gti and my (2001 Caterham Academy) Kermit I can't speak from experience of lots of fancy cars. I'd like to know what the relative handling differences are between different configurations - eg. Kermit - engine in the front, drive at the back. Porsche 911 - engine at the back, drive at the back. Elise - mid-engine (I think) drive at the back.

 

How do these different configurations affect acceleration and cornering?

It is my understanding that in terms of grip in a straight line, the more weight you have over the driven wheels the better, and if those are the back ones, then even better still because the rear wheels take more weight during accelration. In terms of cornering you don't want the engine at the back because the inertia makes it swing out easily (but having less weight over the back wheels means they don't grip as well too).

 

Kermit the frog

[scooby dooby doo]

cornering wants a low polar moment of inertia so you can change direction with minimum force. this means all the mass goes in the middle (in a fore aft sense). hence mid engined. this is the dominant effect so F1 cars ended up being mid engined after experimenting with front and rear engined. the disadvantages are that cooling is more awkawrd (so long runs to the radiator at the front or sides)

 

acceleration means you want rear (or better still four) wheel drive and the mass towards the rear. but the handling is more important so you stick it in the middle still. you don't actually want the car to rock back too much or you'll unsettle it when you apply power.

 

braking is similar - you don't want the car rocking forward as you'll have to wait for that to finish before full braking is required.

 

the 996 is mid engined - the engine is between the axles. theres also a lot of other stuff you can move about to get the distribution right - eg the Ferrari 275 is a traditional front engined GT but the gearbox is at the rear.

 

the ring is a bit weird as it has a 3 mile straight. an R500 goes along here at about 145 mph, the 996 will do 185. that gains it a LOT of time...

 

 

HOOPY R706KGU what's a 'hood' 🤔 😬

[charlie_pank]

Thanks Hoopy. BTW I'm told by your new kid Rich that we work for the same company. (Are you "David M"?)

 

Kermit the frog

[sjwb]

996 engine between the axles 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔 🤔

 

Steve B

[Peter Carmichael]

The ring has a 2 kilometre straight.

[caterhamnut]

Had this 'where is the engine in a 996' debate whilst at Donnington at the weekend for the Touring Cars. Looking at a Porsche Cup 996, which has all the interior stripped out you can clearly see that the engine is behind the axle. You can look in the rear of the car at the bodyshell and see that the gearbox is between the wheels - the engine is behind this.

 

Front wheel drive cars are not so good off the line because as the power is applied and the car accelerates, the weight transfers to the back of the car (to the rear wheels) and the front lifts, thus reducing traction at the front (driven) wheels - the opposite affect of what you describe for rear wheel driven cars.

 

😬26,000 miles in 13 months!

angus@tinyworld.co.uk

[scooby dooby doo]

charlie_pank - I am David M, well thats what loutus notes calls me - it can't cope with two middle initials...

 

who are you? and who do you mean by rich?

 

Unlike the original 911 the engine in a 996 is far more mid engine than rear engined. I'm pretty certain its centre of mass is inside the wheel base. by this argument a caterham is mid engined as well I suppose.... hmm...

 

HOOPY R706KGU what's a 'hood' 🤔 😬

[caterhamnut]

Add to the list 'front/mid engined cars'. Some cars have the engine so far back in the engine bay (in front of the driver) that they are termed front/mid engined cars. Cannot for the life of me remember any examples of this, but I suspect that some bike engined cars have the bike engine so far back (because of its compact dimensions) that they could almost be termed this. As Hoopy says, it is all about getting the mass towards the centre of the car.

 

This weight balance is achieved by careful placement of components. For example, a Porsche 944 had the engine at the front, and a transaxle at the rear, which gave it a near perfect 50/50 weight distribution - as per Hoopy's Ferrari example.

The Mazda MX-5 has the battery mounted in the boot to help weight distribution.

 

F1 cars are built underweight so that they can use movable ballast to achieve the balance they require. The more underweight they can be (or the lighter the driver) the more freedom they have to move the ballast.

 

Front wheel drive cars usually have a transverse engine, which is almost above the axle line. Their transverse layout allows greater packaging flexibility to the designer than a longitudinal engine (as in BMW's) which usually requires either a longer bonnet, or they have a large transmission tunnel extending into the passenger space.

 

There are different priorities in designing a car (packaging, seating, manufacture, cost, safety, crumple zones etc) to those used when designing a sports car (ultimate performance, ulitmate handling etc)

 

😬26,000 miles in 13 months!

angus@tinyworld.co.uk

[caterhamnut]

As far as the centre of mass goes on a 996, it may well be inside the wheel base as it will be balanced by ancillary components, seats, fuel tank etc (and the driver I guess), but I am pretty sure the engine itself is behind the axle line.

 

😬26,000 miles in 13 months!

angus@tinyworld.co.uk

[Super_Rich_Bernie]

I used to have a Lancia Fulvia - which had its front to back mounted V4 engine entirely ahead of its front wheels, right at the front of the car - the engine was canted over to the left and the radiator on the right. This was quite a light and compact engine (alloy head, crankcase & sump) but still concentrated the mass just where you would think it shouldn't be. There can't have been much weight at all at the back of the car.

 

It was the best handling front drive car I have driven (certainly better than current Ford Puma or past Pug GTI) and fast too. It really came into its own on snow - Fulvias had an enviable rallying record.

 

In the past Lancia had gone to the trouble of a transaxle (Aurelia) to get 50:50 weight distribution.

 

Bristol are calling their new sportscar 'mid' engined - when in fact the engine is at the front - but behind the front axle line. They have been obsessed with 'correct' weight distribution since their earliest cars.

 

Jonathan

[charlie_pank]

So what we're saying is that considering the weight of the driver (and passenger where applicable), Kermit et al. are front-engined, mid-weight cars, thus having the best handling possible?

 

I find the handling amazing and am stunned by the roadholding around corners in Kermit, I am very, very cautious when it's wet. As I'm sure everyone is aware sevens do have a tendency to go sideways round corners if you're too heavy on the loud pedal and the road's shiny. Given the ideal weight distribution described in the last paragraph is the sideways inclination just down to the power to weight ratio of a seven?

 

Kermit the frog

[caterhamnut]

I think the cornering of a 7 is much more to do with its low centre of gravity (something F1 cars go to extraordinary lengths to lower). I don't know what the exact weight distribution of a 7 is, but the layout of the engine is what is often described as the 'classic sports car' format of front (mid!) engine/rear wheel drive. It does not necessarily give it the best handling possible.

 

A car can go into oversteer for different reasons - only one of which is too much power to the rear wheels, which breaks the traction and allows the car to swing out at the rear if you are cornering. I guess this can happen in a Morris Minor as well in the right circumstances, so can't really say if power to weight ratio has much to do with it. Obviously it is easier to spin the wheels and break traction in a more powerful car though. Older Porsche 911's were more prone to this as they had the added input of a pendulum weight at the rear of the car (Newer versions get around this with better weight distribution, and much more sophisticated suspension geometry).

 

I'm speaking in very basic terms here - there are people of this forum who can explain all this stuff in far more detail, talking about coefficients of grip etc etc, so forgive me if this is a little basic and jumps around a bit!

 

I would say that a front engined/rear wheel drive car gives a more progressive handling characteristic. A mid-engined car, whilst possible having higher cornering limits, may well be far more tricky or twitchy when it does finally reach the limit. Because the mass is in the centre (longitudinally) of the car once it goes, it goes, and a spin is harder to catch. You often hear this about Elise's for example, particularly the older version, where the handling was often described as tricky on the limit.

 

😬26,000 miles in 13 months!

angus@tinyworld.co.uk

 

Edited by - angus&tessa on 25 Sep 2002 18:21:13

[Roger King]

The best example of a front mid engined car is a Mallock.

[Joe 90]

This is so open ended, we could be here for ever ❗ I don't think my opinions are going to be that useful, but here are some of my experiences.

 

The 996 engine is behind the rear axle. The weight distribution of the 911 has varied over the years, but is usually between 35:65% and 40:60%. A lot of them are set up with a lot of understeer, but if you do lose the back it will spin very quickly. The joy of the 911 is that it has a lot of traction mixed with a lot of power. This means it accelerates well out of corners, and rewards smooth driving. The problem is that when a lot of power overcomes a lot of traction, your problems are a lot worse. Despite their common ancestry, I found the handling of the Carrera 4 very different to an old 2.2E.

 

I have also driven a mid engined GT40 replica for quite a few miles. That felt so glued to the road that I couldn't really tell you about oversteer, understeer or spin characteristics.

 

Some of the TVRs might qualify as front-mid engine, with their neat trick of running the exhausts forward so that the engine sits really far back. My TVR also had tremendous grip, but again I can't tell you about over/understeer because it was too damn unreliable

 

I think that if a car has sensible weight distribution, a low centre of gravity and proper suspension it will handle well regardless of where the engine is.

 

99,000 miles so far

[paul.davis]

Four corner weighting shows my seven to be 51% front 49% rear - not bad for an old Zetec!

 

Paul

 

Zeeeeeeetec . . . . . .

[Peter Carmichael]

The miracle of 50:50 weight distribution is only talked about by marketing men and not by engineers.

[caterhamnut]

True.

Why is the drive on 4-wheel drive cars often biased to the front wheels during normal driving, and only moved towards the rear when grip is required?

 

😬26,000 miles in 13 months!

angus@tinyworld.co.uk

[aph]

a 4*4 car usaully always has a 'torque' biased to the rear wheels but the front wheels always spin faster which means you get the stability of a front wheel drive with the ability to put down more power without the car understeering or trying to swap ends

[Joe 90]

I disagree. Cars that were designed originally as FWD retain a front torque bias, e.g. Golf synchro, while cars originally designed as RWD retain a rear torque bias e.g. Diablo VT, Porsche 911. Presumably the designers don't want to re-engineer the handling of their car.

 

99,000 miles so far

[Peter Carmichael]

Where is the weight?

 

How is the torque bias achieved?

[aph]

ahh i know what you mean now, what like a golf/audi syncro that uses a clutch that engages the rear drive when needed

[Morls]

Of the cars I have driven (with one notable exeption, thank you Mr Issigonis), the most FUN has always been in Front engined RWD, slightly tail happy things, from Capris, Sunbeams and 'proper' escorts throught Fiat Mirafioris, MGBs and Midgets to the 7.

 

I'm sure a better, or more serious, driver might appreciate the extra traction that 4WD or a mid engine offers.

 

I'm also sure a better driver could predict and control the pendulum effect of a Beetle (and it's derivatives ).

 

I imagine a non-enthusiast may be safer with an understeering FWD thing.

 

I suppose a dedicated engineer could redesign the whole car to cope with a less than obvious engine location and still make it work despite the theoretical problems.

 

But....if a Seven is for fun, not open competition, then I like the predictable and easily controlled loss of traction from the rear wheels. In fact... apart from the clams, it's my favorite bit.

 

Mark

[Lauben]

One question pops to mind, there is this talk about how an se7en will beat almost everything in the dry but will get a serious woppin in the wet? is it the lack of weight? as I understand traction and with my own experience as a crazed pizzadriver in the winter a well sorted light car will beat anything when the white slippery stuff hits the black not so slippery stuff and the same I would think should apply to wet roads, or do I suffer from some kind of missunderstanding that should have for a long time ago have permanently parked me, but because I didn't know about it........... or is it just the fact that know how to handle small fwd's and are ignorant of the finer aspects of rwd.

 

The Slippery Road and Welcoming Ditches Society of 1993