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Adding my 2 cents: an aluminum monocoque Caterham would no longer be seen as a «seven», a symbol that this company has fought very hard to keep for itself. I am assuming that no one here would suggest that aluminum tubes would be suitable. Serious studies or modeling have been made on the subject and I believe that to achieve a comparable torsion (or bending) resistance an alu chassis may perhaps end up heavier than steel. A final point about Sevens: if the market aimed at by a manufacturer is ordinary thrill seeking road users a case can be made and has that the 1200-1400 lbs is as light as it should get. With engines in excess of 240 bhp road driveability (according to some professionals testing the car in ordinary road conditions) shaving more weight or adding more ponies might be too much of a good thing. (Just a thought)

You wouldn't see this as a 'Seven', then?:


(Australian PRB aluminium monocoque)


If you use plain sheet aluminium, perhaps. Not if you use honeycomb sandwich panels.



I've owned four 'Sevens', and none of them weighed as much as 1,200lbs. The lightest was about 940lbs, wet, and when I was running it, had a 175bhp engine. It's now had an engine upgrade and is running 214bhp. It was fiercely quick, but perfectly benign and usable on the road. It's currently for sale again, if anyone has $40K burning a hole in their pocket.

I'd actually say that about 1,250lbs should be the upper limit for any well-designed and fabricated Seven. Any more than that and the designer/manufacturer hasn't paid enough attention to detail.

The subject was «Caterham» sevens.


Then if the goal is indeed 1250 pounds, the steel tube bird cage is perfectly adequate. Why bother building anything else if road use is the market?

All this being said, I have nothing against monocoque cars, when justified

One of my Sevens was a <<Caterham>>.

That didn't weigh 1200lbs, either.

Caterham has built cars that don't look like traditional Sevens (the 21, and they very nearly entered into production with a mid-engined coupe that's a version of what has now become the Alpine), so they're obviously not too averse to straying from the S3 formula.


The goal is less than 1250lbs. Preferably considerably less than. That's the point at which I'd think 'You know what? This is getting a bit lardy... I don't think I'll bother."

Then there's the fact that spaceframes are typically as limp as boiled spaghetti (especially when you build them down to that sort of weight, in a 'Seven'-style chassis). My lightest 'Seven' - the monocoque one - was also the stiffest, by a factor of at least two.

I guess one could argue the monocoque vs space frame ( an interesting subject) until the cows come home.
I find it difficult to second guess the motivations of a manufacturer such as Caterham or Donkervoort. The fact that both have not abandoned the bird cage chassis technology is a sign ...to me ( humble garage builder) that welded steel tubes in «one off» (or small production sports cars and light aviation) are probably the most effective approach to chassis building for road car application.

For what it's worth, had they gone ahead with the partnership with Alpine (and it is rumoured to have been Renault who pulled the plug on Caterham, not the other way around), the car that Caterham would have built was an aluminium monocoque.

Would you have any photos of those?

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Here's a diagram and a photo of a a chassis that used it in practice:



It was discussed in detail on one of the threads I linked back on page 2 of this one, before the Great Photobucket Debacle so I can't be bothered explaining again how it works, beyond saying that in addition to the basic stiffness imparted by the transverse torsion boxes, they also ensure that the side trellises do they job they're intended to do, without distortion. Structurally, it's really very clever, but it does dictate a fixed seating arrangement.

Back purging the insides of tubes with Argon is good practice even for the welding of the mild steel tubes we use. That's a reason we use mild steel and the wall thicknesses we do. Argon is a lot heavier than plain air and will sit in a well. So I think titanium is weldable with good standard practices. I see professional welders using diffusers under their work and other tricks to do good shielding. Frames are a worst case situation because of their size and shape.



Thanks Cheapracer...



If you do a good job with the driveshaft tunnel it can be a strong backbone. There is at least a small issue with the Locost chassis because of how the backbone connects into the front of the frame, the FEA we did show it yielding with a reasonable load.

The tunnel though always bothered me a bit, I resented the investment in material there when it seemed it could be used other places which would have an additional benefit of also providing more protection for occupants.

For Caterham the Seven shape is pretty specific and I think there was some legal finding there to differentiate Westfield for example. I found when doing the FEA that just raising the upper frame rail a few inches could increase the stiffness %25. A frame tube %25 larger is twice as stiff and will also contribute to overall stiffness and safety. One of our organizations specifies 1 3/8" x .083" tube for roll cages on light cars so that seemed a reasonable choice. Using that spec for tubes that corresponded with a roll cage produced a chassis with something like 7k-8k lbs./deg stiffness.

As Cheapracer also mentioned having a truss under the scuttle, connecting that to engine bay braces was a really big win for stiffness. It turns out this can also connect to a removable bar over the passenger seat and that little bar increases stiffness by another %50 for just 3 pounds of weight.

I don't think my frame is unique in this way, these approaches would apply to any frames of this style. Using the driveshaft tunnel material to invest in a larger diameter upper rail and raising the upper rail can mostly be hidden under the bodywork, except for the part under the driver's elbow. Improving bracing under scuttle and engine bay are pretty much hidden.

_________________
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I am not aware of this. For what it's worth too.... Your point reminds me of something else:
the original idea to build monocoque vehicles hinged on the fact that the structure of the roof provides a great deal of structural stiffness for less weight than a ladder or space frame chassis. To my knowledge the Alpine has always been a coupé . Monocoque would be an obvious choice for this type of vehicle.

@Sam_68
Thank you for posting that information.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Well, here's the Alpine chassis we're talking about, if anyone is interested:



It will certainly be stiffer as a coupe, but it's not a million miles different to the aluminium chassis used on cars like the Lotus Elise and Morgan Aero, both of which were designed from the outset as open cars, of course:

What you are suggesting may be right. And if you are I will be glad to learn. Up until today my understanding was that in the monocoque construction the ideal shape is the egg shell. Take the roof out and you have to start introducing more bracing , channels, etc. What follows is not from me, I pumped it out of carthrottle.com.


A coupe body with a roof is naturally strong. Any forces being applied through the dynamics of the car are dissipated throughout every corner of the car’s body, with the roof holding it all together in the centre of the vehicle. Take away the roof and the forces can suddenly only be channelled through the door sills, placing much larger amounts of stress through the floor of the car. So to stop the chassis over-flexing and placing abnormal strains on the other structural components of the car, additional beams and panels have to be implemented within the design to counteract any flex.

Frame rails helping to stiffen the chassis of an MX-5
Frame rails helping to stiffen the chassis of an MX-5
These traditionally come in the shape of cross bars, reinforced sills and specific structural panels placed in areas of high stress. Additional frame rails running down the chassis alongside the sills meanwhile are a common aftermarket structural modification, using mild steel to give the car a much stronger backbone from axle to axle.

Cross bars or X-bars (effectively additional cross members) on the other hand take any load from any side of the car and dissipate it to wherever else the bar is connected to. Full-length cross bars can be used depending on the severity of the flex at hand.

Unfortunately, all this extra steel contributes a chunk towards a car’s overall mass, making the majority of convertibles heavier than their coupe variants. Even though the roof is absent, the accumulation of mass from the convertible roof, additional beams and bars manage to surpass the weight of a fixed simple roof, thus effecting the acceleration and handling of the car at hand.

For example, the BMW E46 M3 convertible sits at a staggering 180kg (or one Alex Kersten during peak bulking season) heavier than the hardtop, thanks to an electric convertible roof and all the additional underbody support needed. This made for a 0-60mph time of 5.5 seconds, 0.4sec slower than the hardtop.

I wouldn't dispute any of what you've said.

Certainly a stressed roof will add stiffness to a monocoque ...but it will also add stiffness to a spaceframe or ladder frame.

Certainly, large door openings are a problem to an open monocoque ...but they are also a problem to an open spaceframe (and while ladder frames are not much affected by the addition of doors, that's because they're so terrible to start with that it's difficult to make them any worse )

The definition of a monocoque also becomes pretty blurred at times. Are the chassis I pictured above monocoques, or merely deep-section perimeter tube ladder frames, or platform chassis? Their manufacturers refer to them as monocoques, but to my mind their external surfaces need to form the exposed external surfaces of the car for that definition to be true (which might mean that the Elise chassis is a Monocoque when used on the 340R, but not when it's used on the Elise... go figure). I think most of us would agree that they're not triangulated spaceframes, though, and they're definitely aluminium, which make them relevant to this discussion.

@Sam_68
Thanks for posting those photos.

I've seen the Lotus chassis. However, I've not seen the Alpine chassis before. What is really "seductive" about both is the sort of right-angled design and construction of them. If you contrast those two with the GT40 chassis (of course, it was designed to be a coupe) they both look so much simpler. On the face of it, they seem simpler to design and construct than the tube chassis I've already done for my Locost build. That's why I say they're "seductive."

The GT40 chassis seems to be more complex and aeroplane-like than the Lotus or Alpine. I wouldn't want to make one in my garage. The other two look do-able somehow. I do know the Lotus has a combination of fastening mechanisms used such as bolts and adhesives. I don't know if anything on it was welded. That would require a much higher level of skill and expensive TIG equipment.

How was the Alpine chassis assembled, or do you know?

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886