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Learning how to build Lotus Seven replicas...together!
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 Post subject: Re: Monocoque - why not?
PostPosted: March 17, 2017, 7:19 pm 
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Why not..

This is all my opinion and it should be treated as such.. 

...the reason why it just doesnt make sense in that it our projects (7 type home built cars) just wont see the true benefit of what an aluminum semi monocoque offers, extreme stiffness beyond of what a seven requires.

Dig into the history of racing dating back the late 70s. Tubs and underbody aero were introduced by lotus. To keep the car suspended properly after the additional several tons of effective weight were added to the car via downforce, spring rates had to be massive. The prosche 956 used a variable diameter and variable rate tiatinum spring to help conteract the force, as well as offering a wheel rate near equal to spring rate.

Essentially chassis improvements were born as a result of higher suspension loading. A chassis not stiff enough would deflect prior to the suspension.. largely making it impossible to tune..

Again my opinion. As our cars arent nearly as highly sprung as an aero car, the need for additional stiffness just isnt there justifying the need for alternative/advance chassis construction. Dont get me wrong, theres always a benefit of adding stiffness to a point.

As far as the engineering goes. I just dont see it as big as a hurdle in our purposes. The reality is that we most likely would overbuild it and incurr a weight penalty, where as a experienced p eng would build it lighter to the same strength.
 
Keep in my mind that no airplane I have flown or worked was an engineering home run. They are still subject to the tests of daily abuse. You can apply FEA models all day long but there is no substitute for validating your design through a trial or test. Theres a good book I read about the test flying of the dehavilland otter and twin otter. Lets just say airplanes were crashed and people died as a result. Specifically calculating the flutter on the tail at high speed. It was eventually improved and put into service but remains the limit for the planes, and most aircrafts performance.

This trend continues today as new materials are brought into use on modern aircraft. Just ask airbus how there aluminum wing rib replacement program is costing for the A380. Bassically it was engineered to be light and flex properly, but the ribs cracked early into the service life and had to be replaced by a directive.

Anothet example is the A350 wing tip flutter... and easier fix, it was dialed out with stability augmentation.. stuff that didnt show up in design.

Im rambling here, but hopefully this illustrates that you can design all day long with best practices but you still be in " your own no- mans land" untill you put it through enough stress that the weak spots finally show. Unfortuneatly public road are the testing ground for self designed cars, and your life and others depend some safety factor, the good news is that we tend to overbuild imho.

The porsche 956 and 962 are great examples of how a design evolved. Originally the tubs were sheet metal. Over time as weakness showed up, the front bulkhead was replaced by a milled one and the sheet stock in the tub was replace with aluminum honeycomb.

At best a home designed semi mono tub constructed from inferior materials with poor methods is going to yeild either an underbuilt structure somewhere between a maintenace pig to abrupt catastrophic failure to an overbuilt overweight expensive structure that defeats the purpose all together..

I was watching a seminar on additive manufacturing recently, not because it has anything to do with my job, but just because I am a dork. The engineer summed it up well.

"Anyone can build a bridge, but only an engineer can build a bridge that will barely stand".

I think we covered why welding is not ideal on aluminum. Bonding of aluminum and its alloys is out of  the locost league and too risky, im sure all of the aerospace/aviation people would agree.

That leaves mechanical fastening.. a whole other topic, and one I believe in if your making parts fro aluminum.

Lets keep this rolling.
Andrew

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Last edited by airframefixer on March 18, 2017, 7:32 am, edited 1 time in total.

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 Post subject: Re: Monocoque - why not?
PostPosted: March 17, 2017, 10:05 pm 
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I'll keep it going.............Why not.............it's your build.

Early on, in my "design phase" I went deep down the monocoque rabbit hole only to pop out and see the light of day and it was steel space frame and composite skins. The space frame design is tried and true, easy to modify/repair, and forced me to mature my welding skill set. Composites skins are definitely lighter than aluminum but so is my wallet, and the time required to fabricate it is orders of magnitudes longer than any off-the-shelf sheet material. Why'd I chose them........it's my build, and I love working with composites. I got the experience to play with all kinds of different materials, and my composite fabrication skills have grown as a result.

Why didn't I do monocoque..........two words, Robin Hood. I have a bunch of pictures of the Robin Hood design/build process if interested. While doing the research I also read the horror stories of the modifications needed to make these cars road worthy, pass inspection requirements, and/or just to keep them moving down the road without falling apart. There are better monocoque designs (a bunch already listed in the thread) but for my budget, build time-line, skill set, and ultimate goals, I didn't really feel like I could fabricate a car that would be lighter/faster/safer than my above design path.

If you have the time, the money, and the drive to build a monocoque chassis..............go for it. There are quite a few experienced builders on here that will offer help. There are also people that will tell you horror stories so I hope you have thick skin. Ultimately it's your time, money, effort and eventually your butt, your passengers butt, and the rest of the road going public that will be at risk. So do you research, sharpen your pencil, ask lots of questions, and start your build log.

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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 4:56 am 
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rx7locost wrote:
A monocoque Seven was attempted by a company called "Robin Hood" Do some research. In a few minutes I found that they changed the design to include a steel frame inside the aluminum shell.

The Robin Hood was an abortion by a company that was notorious for its poor design and construction quality. Their monocoque was in stainless steel, incidentally, not aluminium. Their spaceframes were lousy, too, so I wouldn't rule out a construction technique just because Robin Hood did it badly. ;)

By contrast, the Quantum Xtreme is a well-respected and robust stainless steel monocoque that has been available for a number of years here in the UK.

Image

In Australia, there was the very well respected (but costly) PRB aluminium monocoque:

Image

If you want to use sandwich panels (though I wouldn't touch the SIPs that Graham mentioned), there have been examples like the Westfield WiSPER, Westfield FW400 and Strathcarron SC5a/Javan R1, though the actual tub design on the latter was very poor:

Image

Image

Image

Monocoques in general have been discussed at some length on these threads:

https://www.locostusa.com/forums/viewtopic.php?f=39&t=15283

https://www.locostusa.com/forums/viewtopic.php?f=39&t=17162


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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 9:34 am 
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Posts: 78
Sam_68 wrote:
rx7locost wrote:
A monocoque Seven was attempted by a company called "Robin Hood" Do some research. In a few minutes I found that they changed the design to include a steel frame inside the aluminum shell.

The Robin Hood was an abortion by a company that was notorious for its poor design and construction quality. Their monocoque was in stainless steel, incidentally, not aluminium. Their spaceframes were lousy, too, so I wouldn't rule out a construction technique just because Robin Hood did it badly. ;)

By contrast, the Quantum Xtreme is a well-respected and robust stainless steel monocoque that has been available for a number of years here in the UK.

Image

In Australia, there was the very well respected (but costly) PRB aluminium monocoque:

Image

If you want to use sandwich panels (though I wouldn't touch the SIPs that Graham mentioned), there have been examples like the Westfield WiSPER, Westfield FW400 and Strathcarron SC5a/Javan R1, though the actual tub design on the latter was very poor:

Strathcarron looks like just a box made by core panel.............


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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 9:53 am 
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Posts: 78
airframefixer wrote:
Why not..

This is all my opinion and it should be treated as such.. 

...the reason why it just doesnt make sense in that it our projects (7 type home built cars) just wont see the true benefit of what an aluminum semi monocoque offers, extreme stiffness beyond of what a seven requires.

Dig into the history of racing dating back the late 70s. Tubs and underbody aero were introduced by lotus. To keep the car suspended properly after the additional several tons of effective weight were added to the car via downforce, spring rates had to be massive. The prosche 956 used a variable diameter and variable rate tiatinum spring to help conteract the force, as well as offering a wheel rate near equal to spring rate.

Essentially chassis improvements were born as a result of higher suspension loading. A chassis not stiff enough would deflect prior to the suspension.. largely making it impossible to tune..

Again my opinion. As our cars arent nearly as highly sprung as an aero car, the need for additional stiffness just isnt there justifying the need for alternative/advance chassis construction. Dont get me wrong, theres always a benefit of adding stiffness to a point.

As far as the engineering goes. I just dont see it as big as a hurdle in our purposes. The reality is that we most likely would overbuild it and incurr a weight penalty, where as a experienced p eng would build it lighter to the same strength.
 
Keep in my mind that no airplane I have flown or worked was an engineering home run. They are still subject to the tests of daily abuse. You can apply FEA models all day long but there is no substitute for validating your design through a trial or test. Theres a good book I read about the test flying of the dehavilland otter and twin otter. Lets just say airplanes were crashed and people died as a result. Specifically calculating the flutter on the tail at high speed. It was eventually improved and put into service but remains the limit for the planes, and most aircrafts performance.

This trend continues today as new materials are brought into use on modern aircraft. Just ask airbus how there aluminum wing rib replacement program is costing for the A380. Bassically it was engineered to be light and flex properly, but the ribs cracked early into the service life and had to be replaced by a directive.

Anothet example is the A350 wing tip flutter... and easier fix, it was dialed out with stability augmentation.. stuff that didnt show up in design.

Im rambling here, but hopefully this illustrates that you can design all day long with best practices but you still be in " your own no- mans land" untill you put it through enough stress that the weak spots finally show. Unfortuneatly public road are the testing ground for self designed cars, and your life and others depend some safety factor, the good news is that we tend to overbuild imho.

The porsche 956 and 962 are great examples of how a design evolved. Originally the tubs were sheet metal. Over time as weakness showed up, the front bulkhead was replaced by a milled one and the sheet stock in the tub was replace with aluminum honeycomb.

At best a home designed semi mono tub constructed from inferior materials with poor methods is going to yeild either an underbuilt structure somewhere between a maintenace pig to abrupt catastrophic failure to an overbuilt overweight expensive structure that defeats the purpose all together..

I was watching a seminar on additive manufacturing recently, not because it has anything to do with my job, but just because I am a dork. The engineer summed it up well.

"Anyone can build a bridge, but only an engineer can build a bridge that will barely stand".

I think we covered why welding is not ideal on aluminum. Bonding of aluminum and its alloys is out of  the locost league and too risky, im sure all of the aerospace/aviation people would agree.

That leaves mechanical fastening.. a whole other topic, and one I believe in if your making parts fro aluminum.

Lets keep this rolling.
Andrew


agree

even I still haven't build my own kit car, I had doing research about monocoque for few years, and try to design it---include 7 style & Mid engine, also elan backbone chassis

what I feel is, if u have no calculation, u always feels the chassis is not strong enough, after u think it is ok, it is too heavy, too much supporting component, but if u see other chassis designed by engineer, u think: OMG it is to simple, looks like very week, is it really work?

maybe it will be easier after we have a fully open source monocoque chassis design just like 7


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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 12:32 pm 
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KinFung wrote:
Strathcarron looks like just a box made by core panel.............

Yup, that's exactly what it is (aluminium skinned, aluminium honeycomb core 'Cellite' panels, to be accurate).

They basically made no attempt whatsoever to stiffen the opne top of the 'box' by any sort of perimeter reinforcement - they just rely on the inherent stiffness of the material itself.

It's a lousy design, but it did still work well enough to be functional.


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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 12:37 pm 
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KinFung wrote:
airframefixer wrote:
..."Anyone can build a bridge, but only an engineer can build a bridge that will barely stand"....


...also elan backbone chassis...

Now, if we're talking about bridges that barely stand, the Elan backbone chassis is a fine example.

If you've ever handled one, you'll know that the metal of the main backbone is thin enough that it will 'oil can' and deflect under thumb pressure, yet it's stiff enough to make the Elan structure overall considerably stiffer than the spaceframes used by most 'Locost' and 'Seven' style kit cars.

It is marginal and prone to cracking in several areas, though: particularly around the diff. mounts and at the upper front chassis fork.


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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 2:01 pm 
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airframefixer wrote:
..."Anyone can build a bridge, but only an engineer can build a bridge that will barely stand"....


Or for that matter, ever get into an original real-Lotus Seven? The thing breaks every dogma of this gathering about what must be for a car to be built. Is this where we mention the wood construction of the Morgan? But both worked when kept into their intended uses.

I think that many of the things we discard would work fairly well in the restrictions of the use intended (HP, speed, offroad versys track versus fun car, etc). Having said that, crashworthiness is another issue entirely.

Would I want to be in that vintage Seven in a crash? Oh hell no. Nor would I want to put more HP into the thing, or stickier tires (outside of the design envelope) nor would I take it off road, at least intentionally.

So backing up -- in making said monocoque car, what would be the intent of the thing? Track car with 700 hp and 700 ft-pounds of torque on a LSx motor? Mileage winner (Jack's cars) with a 38 hp tractor motor? Tell me the design envelope, and how much risk you're willing to take here.

Heck, for all I know, we can do balsa wood-encapsulated cardboard?

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 Post subject: Re: Monocoque - why not?
PostPosted: March 18, 2017, 9:10 pm 
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Sam_68 wrote:
If you want to use sandwich panels (though I wouldn't touch the SIPs that Graham mentioned), there have been examples like the Westfield WiSPER, Westfield FW400 and Strathcarron SC5a/Javan R1, though the actual tub design on the latter was very poor:

Image

is this fiber glass panel?


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 Post subject: Re: Monocoque - why not?
PostPosted: March 19, 2017, 3:26 am 
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KinFung wrote:
is this fiber glass panel?

Yes.

It is 'Cellite' panel, which is available with fibreglass, aluminium or carbon fibre skins. That car was designed and built by the ex-Team Lotus Formula 1 Chief Designer, Martin Ogilvie, with whom I have had the opportunity to work on another similar project.

The Westfield FW400 (the picture immediately below it in my post above) was another one of his designs, similar in many ways to the WiSPER but done in carbon fibre. I owned the FW400 in the pic for several years.


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 Post subject: Re: Monocoque - why not?
PostPosted: March 19, 2017, 9:55 am 
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Sam_68 wrote:
KinFung wrote:
is this fiber glass panel?

Yes.

It is 'Cellite' panel, which is available with fibreglass, aluminium or carbon fibre skins. That car was designed and built by the ex-Team Lotus Formula 1 Chief Designer, Martin Ogilvie, with whom I have had the opportunity to work on another similar project.

The Westfield FW400 (the picture immediately below it in my post above) was another one of his designs, similar in many ways to the WiSPER but done in carbon fibre. I owned the FW400 in the pic for several years.

what the meaning of 'Cellite' panel is? I can't translate it


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 Post subject: Re: Monocoque - why not?
PostPosted: March 19, 2017, 10:12 am 
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KinFung wrote:
what the meaning of 'Cellite' panel is? I can't translate it


Cellite is just the manufacturer's brand name for their product.

See data sheets here: http://www.trbls.com/wp-content/uploads/2013/03/Cellite-220-620-Panels-March2013.pdf


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 Post subject: Re: Monocoque - why not?
PostPosted: April 11, 2017, 3:00 pm 
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airframefixer wrote:
...the reason why it just doesnt make sense in that it our projects (7 type home built cars) just wont see the true benefit of what an aluminum semi monocoque offers, extreme stiffness beyond of what a seven requires.

...Dont get me wrong, theres always a benefit of adding stiffness to a point.


I imagine that reaching that point is rare, i.e. only with 3D structure at the sides of the cockpit.

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 Post subject: Re: Monocoque - why not?
PostPosted: April 12, 2017, 4:42 am 
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NoahKatz wrote:
airframefixer wrote:
...the reason why it just doesnt make sense in that it our projects (7 type home built cars) just wont see the true benefit of what an aluminum semi monocoque offers, extreme stiffness beyond of what a seven requires.

...Dont get me wrong, theres always a benefit of adding stiffness to a point.

I imagine that reaching that point is rare, i.e. only with 3D structure at the sides of the cockpit.

"How much stiffness do you need" is a whole different debate in itself?

The usual rule of thumb is that 10 times the roll stiffness of the springs is plenty, and beyond that you're well into the realms of diminishing returns. But the fashion in UK hillclimb and sprint cars at the moment is for all the roll stiffness to be at the front end - which is approximating toward a tadpole trike arrangement, where chassis torsional stiffness is not nearly so important.

As another thread not far from this one demonstrates, it's not absolute stiffness that's important, anyway - it's stiffness:weight ratio. No point in achieving high stiffness by simply bodging on an ever increasing number of tubes.

Stiffness:weight ratio is where monocoques come into their own, of course: despite being a 'traditional' Seven form with no 3D structure alongside the cockpit, the Westfiedl FW400 was 2-3 times the stiffness of an equivalent panelled spaceframe and about 100lbs lighter. I've got designs that do use 3D structure that are dramatically better still - in both directions.


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 Post subject: Re: Monocoque - why not?
PostPosted: April 13, 2017, 7:37 pm 
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Sam_68 wrote:
"How much stiffness do you need" is a whole different debate in itself?


To say the least, though I don't recollect seeing it here.

Interesting rule of thumb, I hadn't seen that one before.

Makes sense as far as chassis compliance affecting desired roll stiffness distribution, but there may be other considerations i.e. squeaks & rattles.

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