While one doesn't want to drag this discussion down to some sordid level, when it comes to small cars the fact that the front seats in a two (three?) door slide forward does leave a lot more room for the sort of activities often carried out down quiet country lanes at night.
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One door for each passenger and a third one for the chest of drawers, washing machine, self-assembly grandfather clock or similar.
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interestingly when the mk2 escort was at its peak in rallying times it was always the 3 door shell that the rally cars were based on ,but the 4 door shell was stronger ,due to the torsional rigidity built in by way of the "b" pillars
michael caine voice ............not a lot of people know that
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We had a 3 door Escort Estate at work as a run-about in approx 1980.
What about the new Mini Clubman - arguably that has 5 doors, 1 on one side, 2 on the other and 2 at the back!
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That is interesting bb. Of course they wanted to promote the sporting looking model which in low spec may even have been cheaper than the 4-door.
In rallying either they were depending on their drivers or some overall engine advantage, or they must have had to stuff an extra-expensive roll cage in the thing to keep it stiff. Both probably.
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"self-assembly grandfather clock"
I'd like one of those - do they exist?
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Sorry RR, I don't follow . Surely dumping old fridges , washing machines and mattresses is probably easier using a 5door than a 3door , or have I missed something?
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One or two of them certainly looked like old fridges (begars can't be choosers), but I was not actually referring to white goods.
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"In my opinion 3 door cars don't look as 'nice' as the 5 door version" I think the reverse although with some obvious exceptions like the 3-door Sierra!
Wasn't the Sierra Cosworth built with 3 doors simply to use up the stock of surplus early 3 door bodyshells, or is that an urban myth??
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I think 2 (or 3) door rallycars are done to save weight - any increase in strength comes from the roll cage as said.
I generally prefer the look of 2 big doors rather than 4 little ones, depends if you need them or not. Just be happy we have the choice in the market, why would anyone think that less choice is a good thing as per the original post?
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The torsional stiffness of structures is significantly diminished by openings - I would always expect a three door shell to be stiffer in torsion than an equivalent 5 door shell.
This is because the strength of one straight member makes little difference to the torsional response of a structure.
To get a simplified picture of what is going on, imagine twisting a shaft - say a long prop-shaft. If, conceptually you were to open out and unfurl the metal tube, initially you would have a long rectangle. Under a twisting load, the rectangle would become a parallelogram. The material deformsin shear. This is similar to what is going on when a car twists - small squares drawn on the surface of the stressed panels would become parallelograms. Material near the centre of the structure - say the transmission tunnel** does virtually nothing in torsion, while materila at the extremities, the roof and the sills bears a great deal of the torsional loading.
** The transmission tunnel has a great role to play in stiffening the floor of the structure, and stiffening the bulkhead to reduce scuttle shake, but it does little in torsion.
Now, that's all about stiffness - strength is another question, best adressed in a rally car by a roll cage.
Number_Cruncher
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My view is that 5 door hot hatches were bought out to satisfy a need in the market (family people etc) three doors were the purist route.
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Er, NC, roll cages wouldn't have extensions to the front and rear suspension towers if they were nothing to do with stiffness...
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having welded many mk2 escorts up i can assure you NC the torsional rigidity in the mk2 was better on the 5 door,to be honest im sure you have done welding in your time? and to say the drive tunnel doesnt help torsion is to me playing with words ,
sometimes real world look ins are better than text books especially if you are on your back replacing inner floor sections
over to you :-)
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>>better than text books...
Hmm - tiresome and irritating sniping - without the theory any cars on the road today would be truly carp! - it's the tool that's allowed us to progress from Victorian engineering into modern weight efficient structures.
How many times have you actually measured the torsional stiffness of a chassis? How many times have you designed a structure for torsional loading, and done the calcs to demonstrate it will work before the metal is cut?
BB - I've done my fair share of welding, spannering and electrical fault finding, I've also done a bit of study, and theory. I've designed structures, had them built, and seen them pass (and fail!) during subsequent testing. **Both** the practical and the theory have a contribution to make. Using the theory and text books you deride so frequently can prevent people wasting time money and effort putting metal where it won't make any difference. Even the computer aided design techniques which are currently used to design and stress car bodies are useless without the designer/analyst having some grasp of the theory.
The tunnel does virtually nothing for torsional rigidity - in fact, the floorpan would contribute more torsional stiffness to the body if it were completely flat. As a tunnel, it provides only slight resistance to the left hand floorpan moving forward and the RH moving backwards under torsion. The tunnel does contribute to bending stiffness and to avoiding scuttle shake.
Yes, the roll cage can augment the torsional stiffness already there, but, considered from the point of view of torsional stiffness per unit added weight, it's a very poor way to stiffen a monocoque chassis - the panels working in shear are many times more structurally efficient in torsion. In resisting a local intrusion during an accident, thin panels are useless, and the heavy tubes of a roll-cage are quite useful.
Number_Cruncher
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Granted NC, the roll cage is really there to prevent the shell deforming and crushing the driver in the event of a roll or impact. But my impression is that competition engineering is pretty rule-of-thumb. And I definitely remember reading (I've never done it) and seeing that roll cages are often welded to the body at certain points, with triangulated load-bearing extensions to the front and/or rear suspension turrets.
If that isn't using the cage to stiffen a floppy monocoque I'll, er, do something unspecified and not too nasty.
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Agreed Lud, it is being used to augment the stiffness of the shell, while also providing occupant protection.
In clubman circles, yes, competition engineering is rule of thumb (that's why there are so many hopeless entries!), but, once you move up the ladder a bit, then there is some performance analysis done in the design of structures like roll cages.
Once you get to the level of designing a car body, then typically finite element modelling is done, where the shell structure may be broken down into hundreds of thousands of small patches, and the stress and deformation of each element can be calculated under known loading and constraint conditions. For a given loading type - in this case torsional stiffness - one might then rank the elements in order of their strain energy per unit volume. The elements at the top of this list are the parts of the structure which are contributing most to the structural rigidity, while those languishing at the bottom of the table are along for the ride! However, as hinted above, if you change the mode of loading, or the constraint condition, elements which previously were inactive may become crucial.
The most modern software can use this ranking by strain energy to refine the design, and re-run the analysis in an iterative manner, to produce an optimum structure. For example, the wing stiffeners of A380 were optimised using this method.
Number_Cruncher
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Computer aided design has made modern car bodies stiffer, yes. But some are still a bit floppy!
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Some interesting ideas!
I did some Finite Element analysis of car structures in my university days. Although number of opening does reduce rigidity of a structure but that's not the end of story. Further rigidity to crash can be added differently like side impact beam etc.
A proper designed car [with 5 doors + sunroofs] etc. can be made more impact resisting than a 3 door car with inferior design. It all depends on design and material used. Nowadays the trend is to make the front and rear of car less rigid so that accident impact is absorbed to protect passengers.
If we go by just theory, then convertibles should be banned from safety point of view :) If accidentally [or intentionally] someone throws something from a high building and it lands over your head on an open top car - just imagine what could happen!
Regarding transmission tunnel and rigidity. Yes I agree with what NC said, just a simple experiment - fold a flat paper like /\/\/\/\ and it can carry some load but a flat paper won't. However, it isn't the reason why we have transmission tunnels! They are just to carry drive shaft only.
Modern car design is extremely complex and the calculations are simply mind boggling!
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>>A proper designed car [with 5 doors + sunroofs] etc. can be made more impact resisting than a 3 door car with inferior design.
True, but let's assume a 3 door & 5 door are equally well designed, the 5 door will be heavier to get the same strength as the 3 door.
Gareth
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Just discovered another possible reason for 3 door cars. They are, apparently, narrower, on some cars, and will hence be easier to slot through gaps.
Width of current Corsa - from the Parker's website
3 door 1713 mm -- 5 door 1737 mm
I say apparently, because Parker's may be wrong. With the Clio & 207, the width given was the same for 3 and 5 door models.
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Just discovered another possible reason for 3 door cars. They are apparently narrower on some
>>Width of current Corsa - from the Parker's website 3 door 1713 mm -- 5 door 1737 mm I say apparently because Parker's may be wrong. With the Clio & 207 the width given was the same for 3 and 5 door models.
Frankly, I cannot see any reason why that should be so. In all examples that I have looked at, the 3 and 5 door models are the same width, as measured at the door mirror's extremities.
I'll check elsewhere but would imagine that the info. in Parker's guide is erroneous. I have read many such errors in car magazines and guides!!
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I rather like most 3-door designs for their cleanliness of line and I don't really see why a bigger door should be harder to get out of...
I agree 3-door cars are, by their very nature, aesthetically cleaner and less fussy. But longer doors can be a problem if, for instance, you find yourself parked in a Sainsbury's car park with a Chelsea tractor on either side...
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