Aluminium monocoupes and kit cars
Discussion
I have been thinking about kit cars again and was wondering why to the best of my knowledge there are no build it your self aluminium monocouple design's out there? I know that robin hood had a go with the light weight witch does not seam to have gone well. Having said that in the US a number of aeroplanes are built from in kit form using this method so it can be done and I assume with a significant degree of safety that and access to cheap laser cutting, the idea of a flat packed monocoupe kit seam to make sense at leasest to me.
Blake
PS I have no idea of the correct spelling of monocope
Blake
PS I have no idea of the correct spelling of monocope
A couple of reasons are that you don't see traditional folded-and-rivetted aluminium monocoques (<<< correct spelling) are:
- they're very time consuming to build; lots and lots of holes to drill and rivet, so they're are much more expensive and labour intensive for a manufacturer to produce (and it would be difficult to offer a kit of pre-folded panels to allow the builder to do the time consuming bit, due to issues of jigging accuracy).
- They lose a very substantial part of their stiffness in regular use, due to the rivetted joins 'relaxing'. A typical 1970's aluminium F1 car would typically lose something like half of its stiffness over the course of a single season's race use, so you can imagine the impact (pardon the pun) of 60,000 miles over bumps, kerbs and potholes.
Edited by Sam_68 on Friday 5th November 23:41
Sam_68 said:
Interesting stuff.
I guess I had imagined the panels being shipped pre cut with all the holes cut by laser then the builder would just fold the panels then rivet the lot together, this still leaves you with the problem of folding accurately mayby that could also be pre folded as well. As for the relaxing I take that is why the structures are now bonded? so that the rivets just hold the structure while it is gluing then the bond is in the glue?
While the example you gave of the 79 F1 car is interesting I presume that this means that the lotus was of a simaler level of stifnes to the tubula constructed cars that pre dated it?
A friend of mine is building a small mid engined car with a galvanised steel monocoque. It is laser cut, folded and holes laser cut to a 0.01mm tolerence or something stupid. It is then glued and steel riveted together. All self jigging. It was all CAD designed with a tubular engine frame bolted to the back.
It's super cheap if you were to buy 10 at a time just takes a couple of days to stick together. It's also very light and rigid.
It proves it could be done. He is doing it because he can with no plans to do anything really apart from design and build himself a car.
I would imagine manufacturers stick with what they know and know they can sell. The current market isn't really one that you would want to pitch something new and radical. Or is it?
It's super cheap if you were to buy 10 at a time just takes a couple of days to stick together. It's also very light and rigid.
It proves it could be done. He is doing it because he can with no plans to do anything really apart from design and build himself a car.
I would imagine manufacturers stick with what they know and know they can sell. The current market isn't really one that you would want to pitch something new and radical. Or is it?
Myself and two collegaues freinds are designing a car for Road/RGB championship and Aluminium monocoque is the way were going. Considering that we have all worked in fairly high level motorsport this is not a decision based on hopes and dreams.
Remember F1 went: Steel Space Frame - Aluminium Monocoque - Carbon Fibre Tub.
We can't afford a Carbon Tub so Ally monocoque is the next best thing. As mentioned the peices can be laser cut from a DXF CAD file, its self jigging (if designed right) and doesn't suffer heat distortion like steel. Also the monocoque becomes its own bodywork so no s
tty GRP panels required.
I don't know why somebody is going on about riveting, rivets should only be a supplementary joining method after proper Aluminium bonding. Its not even hard to bond either, Permabond and Loctite sell the stuff off the shelf for a reasonable price.
Remember F1 went: Steel Space Frame - Aluminium Monocoque - Carbon Fibre Tub.
We can't afford a Carbon Tub so Ally monocoque is the next best thing. As mentioned the peices can be laser cut from a DXF CAD file, its self jigging (if designed right) and doesn't suffer heat distortion like steel. Also the monocoque becomes its own bodywork so no s
tty GRP panels required.I don't know why somebody is going on about riveting, rivets should only be a supplementary joining method after proper Aluminium bonding. Its not even hard to bond either, Permabond and Loctite sell the stuff off the shelf for a reasonable price.
I believe its cheaper to produce an Ally Monocoque tub than a space frame dependant on complexity of the spaceframe. But I also think the up front design costs may be a little more than a spaceframe based on discussions I have had with a manufacturer. But I have never been to convinced on that as I have had to get a spaceframe into CAD for laser cutting and it takes a while. It may be the stress analysis aspect that takes more work on Ally.
The Tig welded Ally stuff done by US based kit cars does seem quite stiff but weighs about the same as a spaceframed car when finished.( ie GT40)
Wooden Monocoque thats the answer.........
The Tig welded Ally stuff done by US based kit cars does seem quite stiff but weighs about the same as a spaceframed car when finished.( ie GT40)
Wooden Monocoque thats the answer.........
ceebmoj said:
rdodger does your friend have a web sight or are there any pictures / info on line? would youer friend be intrested in posting some information up?
I guess you are also right that it is about doing what you know.
I'm not sure. He does post on here so he may pop up and post some teasers for you. I guess you are also right that it is about doing what you know.
ceebmoj said:
As for the relaxing I take that is why the structures are now bonded? so that the rivets just hold the structure while it is gluing then the bond is in the glue?
Yes, but don't confuse the extruded aluminium structures that the likes of Lotus and Aston Martin use (even though they refer to them as 'monocoques') with traditional ali monocoques made from thin sheet and/or honeycomb. The Elise functions more like a space frame, with bonded (and rivetted) joints only at a limited number of 'node points'. The thing you've got to remember about the epoxy adhesives used to bond ali monocoques (be they the traditional type or the Elise type) is that they quite flexible, relatively speaking. If you look at the adhesive on an Elise chassis, it looks and feels almost like a very stiff 'Sikaflex' type of silicone adhesive. This is fine where it results in a small amount of flex at a small number of 'node points' on the Elise's pseudo-space frame, but where you have the cumulative effect of a small amount of flex in lots and lots of long seams of a folded/bonded/rivetted traditional ali monocoque it adds up to an appreciable amount of 'give' to the structure as a whole.
For this reason (and the reason I'm going on about it, Dr DD), is that the adhesive element contributes only a limited amout to the overall stiffness: even when also bonded, an ali monocoque will still lose a large amount of stiffness in long-term use as a result of the rivets losing their effectiveness.
Of course unless the adhesive fails (which it shouldn't), it will stop the degradation of the structural stiffness at a certain point, whereas a purely rivetted monocoque will just get baggier and baggier until it becomes intolerable and/or falls to pieces, but even with bonding the stiffness of a traditional ali monocoque will degrade to the point where it is significantly worse than a (much cheaper and easier to fabricate) steel spaceframe.
ceebmoj said:
While the example you gave of the 79 F1 car is interesting I presume that this means that the lotus was of a similar level of stiffness to the tubular constructed cars that pre dated it?
No, even the original 'bathtub' monocoque of the Lotus 25 was a major step up on the spaceframe F1 cars that predated the monocoque era, and the ground effect cars like the Lotus 78 were a good bit stiffer still.Many people don't appreciate just how flexible the old spaceframe cars were. A good 60's F1 spaceframe wouldn't be any stiffer than a modern Caterham. You won't find Caterham quoting figures for their spaceframes because they're actually pretty dismal in absolute terms: you'll be doing very well to see much more than 2000lb.ft per degree from a Caterham (or other 'Seven' type) spaceframe, whereas IIRC Lotus claims about 7,700 lb.ft per degree for the Elise and even a limp and wibbly 'production' convertible like an MX5 will be twice as stiff as the Caterham (albeit with a lot more weight).
Now I've said before on this forum that I think chassis stiffness to be an idol much worshipped and little understood, and you don't need mega stiffness for a very lightweight, relatively compliantly-sprung car. But the problem with a traditional ali monocoque is that not only will any serious everyday use will see it 'relax' to a stiffness no better (and possibly a lot worse) than a spaceframe, but at the same time it will be more expensive, more complicated to build and more difficult to repair.
For race use, where weight is everything and you can throw the tub away at the end of a season's racing, it made sense (until it was superseded by the composite monocoque), but for a road car it has several disadvantages with no major advantages by way of compensation.
Edited by Sam_68 on Friday 5th November 18:20
Sam, interested in your comments about adhesives in the assembly of an ali monocoque. Firstly I know very little about this subject so quite happy to listen to educated comment! A significant point about Sika is that it is a PU sealant not silicone - there is a massive difference in adhesion & flexibility, I wouldn't dream of using a silicone sealant for anything I actually wanted to stay in one piece! but I will quite happily bond cycle wings to stays with Sika & know they'll stay attached.
I would have thought if you were looking at long seams such as down the length of the side of a monocoque chassis, if the items were properly keyed & cleaned then surely just a thin coat of Sika between the faces b4 rivetting up would result in a very strong & long lasting joint. Further if this proved too flexible, then I seem to recall we've been sticking airliner monocoques together with epoxy adhesives (not dissimlar to Araldite!) for many years & not only do they seem to stay in one piece (thankfully) but retain air pressure without significant leaks too, so presumably their seams must have pretty good integrity?
I assume when they were making things like the Lotus 49 there were no adhesives used (from what you've said about loss of stiffness over a season) I wonder why not as we were making alloy bodied aircraft decades b4 this - intriguing, maybe Colin Chapman missed a trick???
Any informed comment would be welcome.
I would have thought if you were looking at long seams such as down the length of the side of a monocoque chassis, if the items were properly keyed & cleaned then surely just a thin coat of Sika between the faces b4 rivetting up would result in a very strong & long lasting joint. Further if this proved too flexible, then I seem to recall we've been sticking airliner monocoques together with epoxy adhesives (not dissimlar to Araldite!) for many years & not only do they seem to stay in one piece (thankfully) but retain air pressure without significant leaks too, so presumably their seams must have pretty good integrity?
I assume when they were making things like the Lotus 49 there were no adhesives used (from what you've said about loss of stiffness over a season) I wonder why not as we were making alloy bodied aircraft decades b4 this - intriguing, maybe Colin Chapman missed a trick???
Any informed comment would be welcome.
Sikaflex is PU not silicone.
Stick your panels on your Caterham with it and your torisional rigidity improves immensely. (a racers trick)
Mickrick, no not mine. Mines all top secret until its registered. It's not a GT40 either. Re: Ally monocoques go take a look at RCR at http://www.race-car-replicas.com
Nice kits, unusual business terms if buying in the UK. Like give us lots of money, no guarantees you'll get your kit (i.e ESCROW). Given what happended with Roaring Forties and RCR car was too much of a risk for me.
Stick your panels on your Caterham with it and your torisional rigidity improves immensely. (a racers trick)
Mickrick, no not mine. Mines all top secret until its registered. It's not a GT40 either. Re: Ally monocoques go take a look at RCR at http://www.race-car-replicas.com
Nice kits, unusual business terms if buying in the UK. Like give us lots of money, no guarantees you'll get your kit (i.e ESCROW). Given what happended with Roaring Forties and RCR car was too much of a risk for me.
Sam_68 said:
No, even the original 'bathtub' monocoque of the Lotus 25 was a major step up on the spaceframe F1 cars that predated the monocoque era, and the ground effect cars like the Lotus 78 were a good bit stiffer still.
As you have said at other points that this type of construction is not all that ridged is it just the case that the ealer era space frame cars where just particularly wibelly? or that space frame construction has moved a long way forward?Thanks for all of the informative posts I was looking at picks of the RCR Superlite Coupe on a tread on heay the other day and it looks like a nice piece of kit. I know that the javan is a hunny cone construction but the frame at both ends seams to make nice use of later cut possibly self jigging construction http://www.javansportscars.com/javan.html. I guess as always I just have a lot to learn however the idea of using CNC laser cutting seams to bring a number of benefits with it.
singlecoil said:
How about using steel, and seam or stich welding it (or even spot welding it)? Steel is heavier than aluminium, but it is also stronger, so thinner stuff can be used.
:cough: GTM :cough:It works, but it's not outstandingly light, and it rusts.
or, indeed...
:cough: Robin Hood :cough:
Stainless steel, so it doesn't rust, but it's heavier and more difficult to work.
Both prove that its certainly possible, but both also suggest that there's no overwhelming technical advantage to be gained?
Russ Bost said:
A significant point about Sika is that it is a PU sealant not silicone - there is a massive difference in adhesion & flexibility
And, of course, neither is used to bond monocoques - that's usually an epoxy-type adhesive. But I was trying to make the basic point that they are all more flexible than the materials they bond together. If you have a lot of joints that are more flexible than the materials they bond together, the cumulative effect is that the overall stiffness will tend to be reduced toward that of the bonding adhesive.Russ Bost said:
I wonder why not as we were making alloy bodied aircraft decades b4 this - intriguing, maybe Colin Chapman missed a trick???
My understanding (and I will admit to having only a passing interest in aircraft structures) is that use of structural adhesives on aircraft is fairly recent and that the stressed skin aircraft that significantly pre-date the Lotus monocoques used structural rivets (and where pressurised, their air-tightness was relative; they have compressors, of course, to top-up the leaks).You may have noticed that aircraft wings flex quite a bit.
You'll also notice that it is possible to build perfectly viable aircraft out of sticks and canvas, cross-braced with bits of tension wire. They're stiff enough to function as aircraft, but I wouldn't recommend the same techniques for building high-performance cars.
The stiffness:weight requirements are biased rather more toward the weight side of the equation for aircraft than they are for cars...
Also remember that even Lotus were nervous about using structural adhesives as the primary means of assembling the Elise chassis, and resorted to belt-and-braces rivets for comfort.
Lack of the ultimate in rigidity in itself isn't necessarily a bad thing, mind you. I've argued many times on these forums that the obsession with chassis stiffness can be taken way too far. A chassis needs to be stiff enough to do its job whilst remaining as light as possible to maximise performance. The original Lotus Seven (with much more compliant tyres and springs that we'd use these days) handled well enough to become a legend with about 700-800 lb.ft/degree. My Sylva probably has around 1100-1200 lb.ft/degree, yet handles just as well as my carbon fibre Westfield or my previous Lotus Elises each with perhaps with 5 time the stiffness.
The point is not whether it's possible to build a traditional alloy monocoque that's stiff enough to remain functional after extended use: it is.
The point is that to do so is much more expensive, complex and time-consuming than to build a simple steel spaceframe, and after any significant amount of road use, any advantage in terms of stiffness:weight is likely to be degraded to the extent that it simply doesn't make sense any more, in technical terms.
ceebmoj said:
...is it just the case that the earlier era space frame cars were just particularly wibelly? or that space frame construction has moved a long way forward?
No, spaceframe construction hasn't moved forward by a huge amount; triangulation is triangulation and some of the early Chapman and Costin spaceframes were just as 'pure' as anything produced since. It's just that other technology - and the demands it places upon the chassis - have changed, so the optimum stiffness:weight compromise expected of the chassis has changed too.The early chassis were designed to be as stiff as they needed to be to do the job, whilst being as light as possible. But they had narrower tyres that generated smaller cornering and braking forces, more compliant springs, so the instantaneous forces being transmitted from the road were lower, and (by and large) less powerful engines, so they didn't need to be as stiff as they do now.
Remember that the Lotus 25 that offered such a big leap forward in chassis stiffness over its spaceframe F1 predecessors was powered by a 1.5 litre engine with a power output (195bhp) that would be considered unexceptional in a modern 'Seven'.
The corollary, of course, is that while a good modern spaceframe is a lot stiffer, it's heavier too: Caterham couldn't aspire to building a car-engined Seven as light as the original (full bodied) Lotus Eleven in their wildest dreams, despite all the magnesium split-rim wheels and carbon-fibre mudguards that money can buy.
To add some numbers to my earlier comments, now that I'm home from the office and can look up the necessary references:
Martin Ogilvie (who helped design the Lotus 78 and, coincidentally, also designed my carbon monocoque Westfield) reckoned that the Lotus 78 has a chassis stiffness of about 3,000lb.ft/degree when the tub was new, and that this was degraded to about 1,500 lb.ft/degree after a season's racing. So to put it in perspective, a fresh tub started out being rather less stiff than a Mazda MX5, and after a season's racing ended up (to quote Martin) 'about the same as a Caterham Seven without the side panels on'.
...it didn't stop the 78 dominating the Formula 1 championship and revolutionising the design of single seat racing cars, mind you.
Edited by Sam_68 on Saturday 6th November 00:19
CAV and their GT40 uses a stainles steel monocoque
GTD I believe too...
RCR indeed a nice aluminium mono
and Tornado now makes a Carbon GT40 mono...cool
http://www.tornadosportscars.com/GT40/Monocoquecha...
by the way, Wiesmann also makes Alu monocoque for their MF4 and MF5 cars, but those are expensive.
GTD I believe too...
RCR indeed a nice aluminium mono
and Tornado now makes a Carbon GT40 mono...cool
http://www.tornadosportscars.com/GT40/Monocoquecha...
by the way, Wiesmann also makes Alu monocoque for their MF4 and MF5 cars, but those are expensive.
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