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 Post subject: Re: Monocoque - why not?
PostPosted: April 17, 2017, 6:25 pm 
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Omaha Vette Graveyard wrote:
They are just started to get into double-butted tubing last year, for example.


I thought the primary benefit was increasing the wall thickness in the weld area to make welding easier? I have a hard time seeing the benefit in a car chassis. You can get 0.028" wall thickness in 4130 and while its easily welded with a TIG, its too fragile for a car - I've literally seen a person bend a frame member just from their own weight.

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 Post subject: Re: Monocoque - why not?
PostPosted: April 17, 2017, 8:09 pm 
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Maserati Tipo 61.

Why in the world is Caterham still using steel? Shouldn't a brilliant engineering company that is seeking the best possible optimization for their frames be using exclusively carbon fiber monocoque?

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 Post subject: Re: Monocoque - why not?
PostPosted: April 17, 2017, 10:52 pm 
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Sorry if my last post sounded snarky.

I'm not trying to insult any person or any company, but engage in a discussion that leads to a better understanding of the benefits/drawbacks of different building materials and approaches. None if it is really relevant to anything I'm building or plan to build, one way or the other. I really have no dog in this fight, so to speak. I'm only trying to argue the other side in this particular thread, to help the discussion develop in a helpful way. In that regard, I do think I made some headway, though there does still seem to be a confusion of weather our comparison is monocoque to space frame or steel to carbon/aluminum.

No, I do not believe that Caterham has found the ultimate solution for a steel space framed car in the shape of a Seven (let alone a carbon or aluminum space frame) or that they were ever really seeking to do that. There are lots of reasons, from the fact that their primary goal is profit, to tradition, to customer base, to marketing, to lots of other stuff. I understand benchmarking Caterham (probably the best thing to benchmark), but that is different from benchmarking the steel space frame approach.

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 Post subject: Re: Monocoque - why not?
PostPosted: April 18, 2017, 10:31 am 
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Omaha Vette Graveyard wrote:
... Why in the world is Caterham still using steel?...

Because they're running a business. Building the best of something doesn't mean anything if the target audience can't afford it and goes elsewhere.

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 Post subject: Re: Monocoque - why not?
PostPosted: April 18, 2017, 12:17 pm 
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KB58 wrote:
Omaha Vette Graveyard wrote:
... Why in the world is Caterham still using steel?...

Because they're running a business. Building the best of something doesn't mean anything if the target audience can't afford it and goes elsewhere.



Yes, and other reasons.

I think the steel space frame is also part of the Seven tradition, which is important for marketing, and it makes Caterham cars a little different from most other modern autos. I think it's interesting that Reynolds approached Caterham, not the other way, and that it's being called 'bicycle tubing'. It tells us a bit about Caterham and who they think their customers are.

Also, I think the folks at Caterham, after many years of development, probably understand that the frame is only one component of what needs to be a very well-tuned machine, and the characteristics of the frame (stiffness) have a large effect on how the car drives, performs, feels. A significantly stiffer frame would need a full re-development of chassis tuning, and the end result might (or might not) be faster than the original, but it would certainly change the driving feel and not necessarily for the better.

Making a frame lighter (and therefore the car lighter) generally just makes it faster and makes it feel better. Making the frame stiffer is not always an improvement. Race cars of types I have the most personal experience with are not made as nearly stiff as they could be, but are purposefully left more flexible. How much more flexible is a matter of team tuning philosophy and driver preference.

If you've ever ridden a really good steel-framed bike back-to-back with a really good carbon fiber bike you notice that the carbon bike is stiffer but 'dead' feeling and not necessarily better to ride. Titanium bikes have their own character. Aluminum is similar in stiffness to carbon, but to me it feels better.

I think my two cents are long since spent here, so I'll bow out of this thread.

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 Post subject: Re: Monocoque - why not?
PostPosted: April 18, 2017, 8:04 pm 
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a.moore wrote:
Omaha Vette Graveyard wrote:
They are just started to get into double-butted tubing last year, for example.


I thought the primary benefit was increasing the wall thickness in the weld area to make welding easier? ...

The actual benefit to Caterham and Reynolds was the ability to part the UK Government from a substantial chunk of money, via a research funding bid. ;)

...or perhaps that's just me being cynical!


If you review my past posts, you'll find that I'm actually quite critical of Caterham in some respects. I certainly wouldn't call them 'a brilliant engineering company': they are very conservative in some respects. Some of their suspension 'upgrade' solutions, in particular, are dreadful bodges as a result of them not being willing to deviate too far from their traditional, inherited design.

Never the less, despite (or perhaps because of) their conservatism, their basic spaceframe is the best optimised 'Seven' type chassis that I know of. I know of stiffer 'Seven' spaceframes, but they're much heavier, and I know of lighter 'Seven' spaceframes, but they're much less stiff. If anyone can come up with an example that offers this specific type of spaceframe with a better stiffness:weight than the Caterham, then I'm genuinely interested: I'm using the Caterham figures to benchmark against my own design, so if there are reliable figures for a better alternative, I want to know about them!

It's a shame OVG has decided to bow out of the thread without answering my specific questions: I'm also genuinely interested to know which other cars have used butted tubing before Caterham; and if there's a way of reliably welding complex titanium structures without a full argon tent, using backyard levels of technology, then it would be very valuable knowledge.


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 Post subject: Re: Monocoque - why not?
PostPosted: April 19, 2017, 2:56 am 
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KB58 wrote:
Omaha Vette Graveyard wrote:
... Why in the world is Caterham still using steel?...

Because they're running a business. Building the best of something doesn't mean anything if the target audience can't afford it and goes elsewhere.

Exactly so; as Westfield found out with the FW400 (although it was only the best for a limited value of 'best'... the ones built were badly finished and cobbled-together prototypes, and were really too extreme for regular road use by most people).


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 Post subject: Re: Monocoque - why not?
PostPosted: April 19, 2017, 8:55 am 
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Omaha Vette Graveyard wrote:
Why in the world is Caterham still using steel? Shouldn't a brilliant engineering company that is seeking the best possible optimization for their frames be using exclusively carbon fiber monocoque?


They could, but it wouldn't be a classic 7. Which is what their customers mostly want.

Lotus learned that lesson hard with the Mk4, and Caterham has been slapped down by the market when they've moved too far from the baseline. I quite liked the Caterham 21, but they sold, what, a dozen a year for a few years? before Caterham pulled the plug.


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 Post subject: Re: Monocoque - why not?
PostPosted: October 17, 2017, 9:57 am 
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Been away for awhile; just adding some info and a few points for future readers:

Bike builders (high-end custom builders, not just the big companies) have pretty much sorted out welding titanium tubing without huge purge tents. Purging the insides of the tubes, a really good custom trailing shield on the torch, plus setting the particular joint partially in a diffuser purge box can leave a weld that easily stays silver colored, or nearly so, when it's done by an experienced builder.

The Maserati Tipo 61 was supposed to have used butted steel tubing.

Donkervoort makes carbon fiber everything, but they use a steel frame. I think it's still brazed. I haven't seen any tests, but it looks like a lot of the stiffness they would get from that car would come from the way in which the attach things to that steel frame, especially the tunnel.

If you want any accuracy for comparison, you cannot bench-mark a frame (or frame plus panels) for stiffness, you must benchmark the fully assembled car. You might be surprised how much stiffness can be gained when things like seats and glass are added. I forget which model, but there was a BMW sedan that was something like 15 percent less stiff with the fold-down rear seat option.

Incidentally, the Seven seems like it would be an ideal candidate for structural seats. The right aluminum seats attached appropriately at the back, on the bottom and perhaps on the sides in a few places could make a worthwhile contribution to chassis stiffness, if one were looking for more of that. But, I guess that's the rub; I'm not sure anyone is actually looking for more stiffness from a Seven.

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 Post subject: Re: Monocoque - why not?
PostPosted: October 17, 2017, 2:22 pm 
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Omaha Vette Graveyard wrote:
Incidentally, the Seven seems like it would be an ideal candidate for structural seats. The right aluminum seats attached appropriately at the back, on the bottom and perhaps on the sides in a few places could make a worthwhile contribution to chassis stiffness, if one were looking for more of that. But, I guess that's the rub; I'm not sure anyone is actually looking for more stiffness from a Seven.


The more I think on things the more I think you are right.

Being able to place the very heavy body weight (in my case) and keep it where you want it plus the additional rigidity, not to mention the expense of seats, makes a very good case for adjustable pedals & steering wheel.

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 Post subject: Re: Monocoque - why not?
PostPosted: October 17, 2017, 3:14 pm 
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Omaha Vette Graveyard wrote:
If you want any accuracy for comparison, you cannot bench-mark a frame (or frame plus panels) for stiffness, you must benchmark the fully assembled car. You might be surprised how much stiffness can be gained when things like seats and glass are added. I forget which model, but there was a BMW sedan that was something like 15 percent less stiff with the fold-down rear seat option.
Non-structural items do NOT need to be on the car to achieve an accurate comparison or benchmark. Sure, a windshield adhesive-bonded into a non-triangulated structural frame becomes a structural item to at least some degree. But a traditional Lotus/Locost windshield/frame is not. Seats also are entirely non-structural. Either the seat has a stand alone frame that is not meaningfully utilized by the vehicle structure, or they are little more than a cushion (sometimes with a hard backing) that is non-structurally fastened to the main vehicle structure. Making the former tie into the structure would generally be uncomfortable, impractical, and not cost effective. Making the latter structural would be redundant, impractical, and not cost effective.

The reason the fold down rear seat option dropped the stiffness in the BMW is not because the seats themselves were structural in any meaningful way, but because without the fold down seats they could place a simple structural brace across the gaping hole in the chassis structure that was only put there so that said pass through convenience option could be offered in the first place.



Omaha Vette Graveyard wrote:
Incidentally, the Seven seems like it would be an ideal candidate for structural seats.
The real 'rub' is that the original 7's (and 'book' Locosts) already did/do this with their 'bench' style seat cushion usage.

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 Post subject: Re: Monocoque - why not?
PostPosted: October 19, 2017, 8:59 am 
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Omaha Vette Graveyard wrote:
The Maserati Tipo 61 was supposed to have used butted steel tubing.


I've seen this said before, but never backed up with any evidence. Do you have a source?

Given the small diameter of the chassis tubes used on the Birdcage, it seems unlikely: if they'd known what they were doing to that degree (the Birdcage chassis is usually held up as an example of how not to design a spaceframe), then they'd have been using fewer and larger diameter tubes that might then have justified butted walls. Butting a small-diameter tube would just be plain nuts.

Maserati manufactured bicycles, and certainly some of those products had butted tubing (in the 1970's); I'm wondering if this is where the myth originates.

Omaha Vette Graveyard wrote:
Incidentally, the Seven seems like it would be an ideal candidate for structural seats.

The Costin 'perfect spaceframe', as discussed previously on this forum, uses this concept in a very clever manner. But as you are perhaps suggesting, if you have the slightest interest in creating a stiff chassis, you would start with basic arrangement of the 'Seven', anyway.


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 Post subject: Re: Monocoque - why not?
PostPosted: October 19, 2017, 10:24 am 
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Actually, modern unibody cars rely on the glass for a substantial portion of their rigidity.

How much difference the components added to the chassis change the actual stiffness numbers is going to depend on a lot of things, but one cannot simply discount it. Even on a Seven-type chassis, any components that are bolted, riveted, or bonded to the chassis (even rubber mounted) will change the stiffness numbers. Perhaps not very much in most cases, but in some cases it can make a substantial difference.

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