>>I would beg to suggest that ultimately (ie at the limit) most rear engined cars end up with the engines at the front, particularly when it is wet!
Polar moment of inertia?
someone like NC will be along shortly to explain it (hopefully?).
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What a lot of waffle and bull excrement, its at the front because over 100 years of design refinement have proved that it is the best place to put it.
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>>its at the front because over 100 years of design refinement have proved that it is the best place to put it.
It depends upon how you choose to define "best". I don't see many F1 cars with the engine at the front!
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Engines, especially nowadays, appear in various places and while in the front is the norm, plenty of cars have it in the back or are mid-engined, depending on the packaging needed.
You could in theory have a family sized car with the engine in the back using the same packaging as the Smart, its just it would look like a minibus if you extended the Smart's proportions to the length of a Focus.
Infact some people carriers have been mid-engined and microvans have often had the engine under the seats.
I would guess, it is all about the accepted asthetics of cars, whereas where practicality is more important, then engines appear in more diverse places.
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It depends upon how you choose to define "best". I don't see many F1 cars with the engine at the front!
I dont see many people driving to work or to the shops in F1 cars either.
Edited by Old Navy on 31/03/2009 at 19:08
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It's at the front because over 100 years of design refinement have proved that it is the best place ...
The engine is at the front because that is where the horse(s) always used to be - simple as that.
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...The engine is at the front because that is where the horse(s) always used to be. ...
Glad someone thought to change the position of the exhaust. :)
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POLAR MOMENT OF INERTIA
Definition: The resistance of an object to rotational acceleration. When the mass of an object is distributed far from its axis of rotation, the object is said to have a high polar moment of inertia. When the mass distribution is close to the axis of rotation, it has a low polar moment of inertia. A mid-engined car has most of its mass within its wheelbase, contributing to a low polar moment of inertia, which, in turn, improves cornering turn-in.
The question then is what is the centre of rotation? Is it the front wheels (or thereabouts?) when braking, or some movable point dependending on what the vehicle is actually doing at the time. In which case surely a front engined car would have the lower figure.
However if you can regard a car as a compressible body, how about the following extract!
Abstract The rotational dynamics of a body are governed by the values of its principle moments of inertia. These quantities are not directly observable, but they are related to the harmonic coefficients of the external gravity field and to the density distribution within the body, both of which can be inferred from appropriate observations. It is shown that, for the particular case of a spherical planet whose density varies as a power of the radial distance, the principal moment of inertia has an elegantly simple form. Application of this simplified case to the Jovian planets suggests that the density profiles outside the central core are approximately linear, with the apparent exception of Neptune.
Where are you NC
TGFG
p
Edited by pmh2 on 31/03/2009 at 18:34
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>>Where are you NC
Busy thinking about this!
Getting the info on how mass distribution works and affects car dynamics into a form where I can write about on this forum is going to take me an hour or two to work out. All I'll say for now is that it's not quite as simple as it might appear from the definition of mass moment of inertia!
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...'unreliable' Hillman Imps....
Lack of basic maintenance.
Aluminium engine - few people bothered to keep the antifeeze/corrosion inhibitor up to strength.
Cooling - there was a large cowling around the fan to duct air through the radiator. This cowling often split/fell off and owners didn't bother to repair or replace it.
When I was doing motorway breakdowns in the 1970s, we reckoned that if the sun came out, we'd be dragging in an overheated Imp or two and we were usually right.
Many had one or both of the above problems.
Perfectly good car if looked after correctly.
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Its no surprise (to me) that the front engined hatchback has become the "standard issue" car. Practical, safe handling, fast and powerful, and if required you can put all manner of bulky items in the back. Only serious IKEA addicts need roof bars.
As said above F1 cars dont have front engines, neither do combine harvesters, but hardly relevant.
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Its because horses find pulling easier than pushing and manufacturers are basically conservative.
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Practical safe handling fast and powerful and if required you can put all mannerof bulky items in the back.
Well you can have all the above in a rear engined car; sorry my mistake you can't get bulky items in the back.
Not sure how safe the handling of Peugeot 205 GTi' s was.
alfalfa
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its to save walking round the back to work on them
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>>but hardly relevant.
No, I disagree - it's absolutely central to what we are discussing, and is exactly the point I was making above. The vehicle package follows from the requirements of the specific vehicle type.
So, your example of a combine harvester is perfect - you want the cutters at the front, before any other part of the vehicle damages the crop - the driver needs a good view, and all the mechanical parts to make the vehicle move and to process the crop must fit behind there.
Putting the engine in the back of a modern car would make it less useful, less able to carry large loads, less apealling to the buyer, and so, it isn't done on any significant scale.
At the time when rear engined cars were more common, there weren't many hatchbacks, and so, for practicality, they were only competing with saloons, and the ease of use of either car wasn't much different.
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Putting the engine in the back of a modern car would make it less useful
>>At the time when rear engined cars were more common there weren't many
>>Hatchbacks and so for practicality they were only competing with saloons and the >>ease of use of either car wasn't much different.
As I said 100+ years of design refinement. I still dont see the relevance of specialist vhehicles designed for a specific use to the design of the "average car".
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I have to be honest that when I asked this question I never thought of the boot/hatchback/estate scenario. I don't fancy having the front windscreen as a liftable hatch to stretch a pair of ladders in!!
I can also accept that engines at the front help for maintenance / access etc but as more and more engines are becoming diy-proof, I wonder if, say 10 years down the line, whether they will be looking at putting engine under the floor and spreading the parts around where possible?
Or maybe by then we will all be driving large battery cars!!
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My understanding of the front or rear engine design question is that for a normal saloon / hatchback, having the engine at the front maximises the luggage space.
The front wheels do the steering and thus affect the width of any possible luggage space (just look at the small compartment in an original Beetle).
There may well be other factors, but I feel that it is simply as stated; the main reason is to maximise carrying capacity.
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The front wheels do the steering and thus affect the width of any possible luggage space (just look at the small compartment in an original Beetle).
For a 1930s design it did ok, but it's the front suspension design that limited luggage space. If you remember the 1302 / 1303 Beetles of the early '70s they had McPherson strut front ends and doubled the luggage space. The boxier front end of the VW 411 / 412 was much more generous.
Even the Renault 8 had a fairly big luggage space in the front, the Tatras luggage space was vast.
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>>whether they will be looking at putting engine under the floor and spreading the parts around where possible?<<
Isn't this what they have done with merc A Class. Glad I do not have personal experience of working on one (or paying!).
p
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Does straight line stability come in to it somewhere ? Having the weight at the front must surely make the vehicle more stable. try throwing a hammer backwards.
Also crosswind stability. With the centre of pressure behind the centre of gravity, as in front engine cars the tendency is to swing the nose into the wind. With a rear engined vehicle the nose would tend to swing away from the wind. Having driven a VW combi in a cross wind I can vouch for that.
My Toyota Estima with the engine under the front seats was far more controllable, but you needed to remove the seats to do any major work.
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Does straight line stability come in to it somewhere ? Having the weight at the front must surely make the vehicle more stable. try throwing a hammer backwards. Also crosswind stability. With the centre of pressure behind the centre of gravity as in front engine cars the tendency is to swing the nose into the wind. With a rear engined vehicle the nose would tend to swing away from the wind.
I'm not sure I buy that explanation. There was a test done back in the early '70s of a VW Beetle with a large fin stuck out behind it - imagine the back end of a Cessna so the tail was at least 5 feet further behind the rear bumper. This would certainly move the centre of pressure back in a crosswind, behind the centre of gravity.
They rigged up some wind machines from a film studio and did several drive pasts with measuring equipment over the car. The stabilizing fin didn't change the crosswind behaviour at all, but the wind passing over the car created a lot of lift, this made the swing axle suspension give more positive camber, less grip and therefore less stability.
Something like a camper van is pretty boxy and any design from the 60s is likely to be fairly awful, but for cars the crosswind stability is more to do with aerodynamics and suspension than weight distribution.
N_C, any thoughts?
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"overheated Imp"
A relative of mine got a nasty steam-burn that way from her Singer Chamois (a sort of Imp deluxe). She replaced it with an air-cooled NSU that was astonishingly roomy and pleasant to drive. I liked Imps, and built a Ginetta G15 that had the same engine*, but with a front radiator and some fore-aft tubing integral with the chassis. I still get pangs from that fibreglass smell...
*Adapted from a Coventry-climax design for fire pumps, so it had to be light, and therefore didn't mess up the balance too much. IIRC, the Imp only failed because development wasn't quite thorough, and let through things like the pneumatic throttle linkage. The bugs were ironed out soon enough, but the mud stuck. The engine made the Mini lump look neanderthal.
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*Adapted from a Coventry-climax design for fire pumps>>
As fitted to Green Goddesses, in a compartment on the nearside behind the cab. Used as a lift pump to supply duck pond water to the Goddess pump if no hydrant around. Both that pump and the Goddess were superb at shifting water, not fun to drive though, even if they did scatter the traffic.
Edited by Old Navy on 01/04/2009 at 18:11
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"As fitted to Green Goddesses"
You've reminded me of a unintentionally funny demonstration by a volunteer fire brigade, of fire-fighting at a race meeting for home-made hovercraft.
Part of the course was over water, and they set up their demo on a barge that they paddled out onto the lake. Plenty of water for the pumps, of course, but no forward planning when Newton's law of action and reaction kicked in, with the result that the barge took on a life of its own and effectively negated all their attempts to aim at the burning target.
It must have reassured the contestants no end!
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The engine made the Mini lump look neanderthal.
Yes, it did. Alec Issigonis must have got ulcers griping about it to his own management.
I passed my driving test in an Imp. Didn't like it much but it was driveable in that sort of satirically safe way required for the test.
Oddly, it belonged to a person who had been a younger boy than me at my last school. He had become a driving instructor.
Edited by Lud on 01/04/2009 at 18:49
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I got colleague to deliver a hang glider on the roof rack of a rear engined Skoda from Luton to the Lake district. He didn't talk to me for ages after. He reckoned the journey took twice as long as usual and he nearly dumped it a few .times. It certainly did nothing for stability.
A car club member uses Imps. If he does a lot of travelling over dry grass he reckons the cooling system sucks it up and blocks the radiator.
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"a lot of travelling over dry grass"
They have roads where you are, Tom..? :-)
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Sporting club member, trials, autotests, classic rallies etc.
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