Volvorsport
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| posted on 6/12/09 at 02:21 PM |
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the darrian , is not solely a race car chassis - its a rally car , and needs to be road registered , in fact its operating enviroment is much harder
than using it on the road .
if youve designed for stiffness the strength will be there .
obviously feeding a suspension load into a flat panel is not a good idea , since it has no stiffness .
the murataya , is a grp monocoque spawned from the minari - do you suppose that might be engineered correctly ? and not fly into a million pieces
.
im not being negative here .
GRP/polyester , correctly used + designed will be superior to steel in strength/weight ratio and most of the successful designs started of being built
in sheds .
[Edited on 6/12/09 by Volvorsport]
www.dbsmotorsport.co.uk
getting dirty under a bus
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boggle
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| posted on 6/12/09 at 02:31 PM |
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i used to cut pannels for the ballistic sector.....most stuf was grp and kevlar, its extreemly strong and fairly light....i remember a company who
made a composite go-kart chassis.....
so how about titanium tube as a chassis???
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David Jenkins
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| posted on 6/12/09 at 02:55 PM |
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I must admit to being amused by people who are prepared to spend a fortune to save a few Kg on what will, in all probability, be a road or track-day
car. The return on investment is ridiculously low.
I once followed a long-winded and occasionally heated topic on a different forum regarding weight saving on a road-going car - the builder was
spending ludicrously high prices to get the weight down on his car, against the advice of many on that forum. However, I had met that person at a
show - and he must have been 18 stone, if not more! The biggest way to reduce weight on the track would have been for him to shed 6 stone... THEN he
could have started to worry about reducing the weight of his car.
The steel space-frame has lasted so long because it works - it's not too heavy, it's strong, and it has quite a bit of built-in redundancy
(i.e. a few things can fail before the whole structure fails). It is also easy to maintain and/or modify. I'm not sure that enough people have
the skills to make a composite chassis with those qualities.
[Edited on 6/12/09 by David Jenkins]
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boggle
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| posted on 6/12/09 at 03:00 PM |
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i allways thought that if a car became too light it started to become unstable??
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PLEASE NOTE: This user is a trader who has not signed up for the LocostBuilders registration scheme. If this post is advertising a commercial product or service, please report it by clicking here.
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Theshed
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| posted on 6/12/09 at 03:23 PM |
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That's not a rant!
Kenny you are quite right to point out the fact that most of these suggested variations on the space frame theme are vanity projects i.e. because I
want to. On the other hand that's not so very different to most other hobbies.
There is a great tradition of shed built composite cars over here some good many not so good. In the main the worst ones probably never get near a
racetrack or road.
What we need now is a threat entitled "Well if you really want to do that here are a few tips......."
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aitch
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| posted on 6/12/09 at 07:43 PM |
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software genuine free / shareware
is there a good genuinely freeware or shareware cad
id like to get started with some form of 3d design but cant afford to buy at the moment
i know ill be getting ou of date software and not the best but anything wil be better then nothing
im using vista home premium
aitch
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richmars
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| posted on 6/12/09 at 09:51 PM |
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Not free but you can get Pro E (at least in the US) for personal use for $250.
Wait a bit and I'm sure you'll soon get people showing you how to steal other CAD packages from the internet.
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aitch
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| posted on 6/12/09 at 09:58 PM |
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quote:
Originally posted by richmars
Not free but you can get Pro E (at least in the US) for personal use for $250.
Wait a bit and I'm sure you'll soon get people showing you how to steal other CAD packages from the internet.
i have "borrowed" software from time to time but always bought it if i liked it, for now im just looking for share or freeware and will
buy when i can
aitch
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kennyrayandersen
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| posted on 6/12/09 at 10:43 PM |
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quote:
Originally posted by aitch
quote:
Originally posted by richmars
Not free but you can get Pro E (at least in the US) for personal use for $250.
Wait a bit and I'm sure you'll soon get people showing you how to steal other CAD packages from the internet.
i have "borrowed" software from time to time but always bought it if i liked it, for now im just looking for share or freeware and will
buy when i can
aitch
try here to start:
http://www.freebyte.com/cad/cad.htm#2D3DCADSystems
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kennyrayandersen
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| posted on 6/12/09 at 10:48 PM |
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quote:
Originally posted by Volvorsport
the darrian , is not solely a race car chassis - its a rally car , and needs to be road registered , in fact its operating enviroment is much harder
than using it on the road .
if youve designed for stiffness the strength will be there .
obviously feeding a suspension load into a flat panel is not a good idea , since it has no stiffness .
the murataya , is a grp monocoque spawned from the minari - do you suppose that might be engineered correctly ? and not fly into a million pieces
.
im not being negative here .
GRP/polyester , correctly used + designed will be superior to steel in strength/weight ratio and most of the successful designs started of being built
in sheds .
[Edited on 6/12/09 by Volvorsport]
Again, just in case you are referring to something I said, it most certainly can be done, there is no question about that. It's just not cheap,
easy, or the best way to go for a personal home-brew car when you compare to a steel frame (IMO). If you got deep pockets, then nothing but time and
money are preventing virtually anyone from eventually getting something to work. Knowledge will help you get there faster.
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kennyrayandersen
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| posted on 6/12/09 at 11:01 PM |
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quote:
Originally posted by boggle
i used to cut pannels for the ballistic sector.....most stuf was grp and kevlar, its extreemly strong and fairly light....i remember a company who
made a composite go-kart chassis.....
so how about titanium tube as a chassis???
Titanium is a great material -- one of the few with which using a really good process you can get 100% of the parent material strength with a weld or
even a casting. However, pound for pound it pretty expensive. Could it be done -- most certainly, and it would require a lot less expertise than a
composite chassis. I'm not sure whether the composite (graphite epoxy) or titanium chassis would be more expensive. I'm thinking the
cost somewhere around both nuts and both kidneys.
It comes mostly in round tube and due to cost you'd really like to optimize the tubes before you built. Again, this is a case where
understanding structural analysis would come in mighty handy, or you could be slapping some pretty expensive parts onto your chassis unnecessarily.
You would probably be looking at commercially pure Ti rather than the 6Al4V, which is stronger, but even MORE expensive!
Ti has about 1/2 the stiffness of steel, abut more than 50% more stiffness than Al
numbers in Msi
Al = 10.4
Ti = 16
Steel = 29
As can be seen steel is pretty stiff. Fortunately the stiffness of the chassis is more dependent on the space-frame aspect rather than the individual
members. Still, it's something that should be considered. I still get the feeling that people are looking a steel as the poor step child
rather than the firstborn son. You can do better than steel, sure, but you will get spanked for it (some folks like rope burns -- know what I mean
wink wink).
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Benonymous
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| posted on 7/12/09 at 12:26 AM |
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Agreed Kenny, steel is very hard to beat as a chassis material. I read somewhere that a welded joint can be left basically tacked (20%) and be 60% as
strong as a full length weld so even if your welding skills aren't exceptional a reasonable full length weld is still very strong. The other
aspect of composite vs space frame is the fact that a large amount of the chassis needs to be re-thought. A simple exchange of one material for
another is not going to work. You still need to mount your engine and suspension along with a million other details, all of which need to be planned
and designed.
Also, a suggestion for a basic CAD package to get started with would be Google Sketchup.
http://sketchup.google.com/
You will not be able to export a DXF file or anything like it to feed into a CNC machine or similar but it will allow you to get to grips with 3D
modelling. It's free and legal.
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kennyrayandersen
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| posted on 7/12/09 at 02:15 AM |
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quote:
Originally posted by Benonymous
Agreed Kenny, steel is very hard to beat as a chassis material. I read somewhere that a welded joint can be left basically tacked (20%) and be 60% as
strong as a full length weld so even if your welding skills aren't exceptional a reasonable full length weld is still very strong. The other
aspect of composite vs space frame is the fact that a large amount of the chassis needs to be re-thought. A simple exchange of one material for
another is not going to work. You still need to mount your engine and suspension along with a million other details, all of which need to be planned
and designed.
Also, a suggestion for a basic CAD package to get started with would be Google Sketchup.
http://sketchup.google.com/
You will not be able to export a DXF file or anything like it to feed into a CNC machine or similar but it will allow you to get to grips with 3D
modeling. It's free and legal.
You bring up an excellent point. If all you are trying to do is make black aluminum (quasi isotropic layup (equal number of plies in all directions
(45/0/0/45)s, for a typical 8 ply layup (note the little s means symmetric about the s)), then it’s really a lot of money for not much (the density of
Al is .101 Lb/cu. in. whereas graphite is about .056 Lb/cu. in. (though the home builder probably won’t get quite that light on the composite)) (Note
that the cost has quadrupled or quintupled (etc.) and the time to complete has gone up by the same factor). Composites really are only useful for
parts that are subjected to a maximum of two directional loading (like a shear panel). If they are loaded in all three directions (like a lug or
fitting) it gets horribly difficult [since you introduce weak failure modes such as inter-laminar tension which gives only the strength of the epoxy
which is only a fraction of the strength that exists in the fiber direction].
The only real way to make the composite pay off is to make the chassis completely from the beginning with the composite in mind. As I stated earlier,
this would likely be some type of composite tub with a metal frame front and aft. The aft frame would tie the rear suspension together and also tie
the suspension to the tub. The front frame would do the same for the engine and front suspension. Of course with a mid-engine car the rear frame
would double to hold the engine and rear suspension together, and that sub assembly could be then bolted to the tub. The tub could likely be extended
to gather in the non-engine end suspension with some thoughtful design.
You could probably more cost/time-effectively save weight by just making the monocoque tub out of aluminum. You’d save most of the weight for a
fraction of the cost. With aluminum you would want to keep the stresses below the fatigue limit for the material. The question might be asked – is
it a lowcost anymore? I guess it’s all relative. To me the concept of Locost is bang for the buck. Even the Ultima uses a steel frame… think about
it.
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Benonymous
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| posted on 7/12/09 at 09:00 AM |
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quote:
Even the Ultima uses a steel frame… think about it.
So does the Norma M20!!
PHWOAAAR!
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aitch
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| posted on 7/12/09 at 02:10 PM |
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still reading and researching
Ok here it is
i read and understand everyones posts and have little option other than to concurr that production of a composite chassis is neither the easy nor the
cheap route, and defonately not the quickest form of construcion, however, i dont believe that it has to be massively expensive either, GRP in place
of carbon for most of the structure will save on cost immensly, all the same forms open to car builders are also available to the homebuilt aircraft
industry, steel space frames, wooden construction, rivetted aluminium and composite. without doubt steel is cheapest and quickest.
There really is an element of """ this is how i want to do it """
and for me this is still composite construction, i have looked at various ways of introducing "HARD POINTS" into the structure for
mounting suspension, engine, gear box, diff etc.. and feel the easiest way is to include steel sbframes withing the GRP composite structure so that
loads are introduced over a larger area, with hard points included in the honeycomb to allow forces to be distributed between inner an outer skins
Looking at other posts it would seem most peoples "low costs" build costs run upwards of £3k and up to £5k-£6k
i am looking at a build being completed in 2-3 years from now with the design phase being at least a year possibly 2 so have pleanty of time to source
reasonably priced engine diff and donor compoments for the steering and suspension, all of which will be needed before the design can be finalised,
My Aim therefore is a BEC mid engined, chain drive, using modified bike wiring loom, GRP composite chassis with steel subframes (included within the
composite structure) and build cost of £5000 rolling chassis (driveable) with possibly another £1000 for the body work
i have most of the tools required so there will be little expense required there and plan to out source as little as possible, if at all.
aitch
[Edited on 7/12/09 by aitch]
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aitch
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| posted on 7/12/09 at 08:27 PM |
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and then
as im looking around trying to decide on a few other issues, mechanics and suspension im having second thoughts
for sure the engine / diff / suspension arrangement lends itself to the open arcecture that i can only get with a space frame, maybe composite tub
with spaceframes frnt and rear ????? only problem here is i could end up with the worst of both worlds, complexity in design and construction and
weight of steel
mmmmmmmmmmmmmmmmmmmm
aitch
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kennyrayandersen
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| posted on 7/12/09 at 10:35 PM |
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quote:
Originally posted by aitch
as im looking around trying to decide on a few other issues, mechanics and suspension im having second thoughts
for sure the engine / diff / suspension arrangement lends itself to the open arcecture that i can only get with a space frame, maybe composite tub
with spaceframes frnt and rear ????? only problem here is i could end up with the worst of both worlds, complexity in design and construction and
weight of steel
mmmmmmmmmmmmmmmmmmmm
aitch
Actually, you’ll find that a lot of folks do it that way. It’s not impossible to do it all composite as I previously stated, just really difficult.
Using a space frame (hollow steel tubes) tied to the corners of your tub I think is the right way to go. If you search on pictures of monocoque, tub
etc. you should get some good ideas. Do keep in mind that the stiffness of glass reinforced fiber is nearly 4 times less that graphite, but it is a
lot cheaper (that’s why you see it used for bodies, but not so much for chassis). I know you are ‘set’ on doing it this way, so after you look at
some examples of ‘working’ cars, make some sketches and post them; I, and I’m sure others, would be happy to give you some constructive comments.
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aitch
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| posted on 7/12/09 at 11:10 PM |
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quote:
Originally posted by kennyrayandersen
quote:
Originally posted by aitch
as im looking around trying to decide on a few other issues, mechanics and suspension im having second thoughts
for sure the engine / diff / suspension arrangement lends itself to the open arcecture that i can only get with a space frame, maybe composite tub
with spaceframes frnt and rear ????? only problem here is i could end up with the worst of both worlds, complexity in design and construction and
weight of steel
mmmmmmmmmmmmmmmmmmmm
aitch
Actually, you’ll find that a lot of folks do it that way. It’s not impossible to do it all composite as I previously stated, just really difficult.
Using a space frame (hollow steel tubes) tied to the corners of your tub I think is the right way to go. If you search on pictures of monocoque, tub
etc. you should get some good ideas. Do keep in mind that the stiffness of glass reinforced fiber is nearly 4 times less that graphite, but it is a
lot cheaper (that’s why you see it used for bodies, but not so much for chassis). I know you are ‘set’ on doing it this way, so after you look at
some examples of ‘working’ cars, make some sketches and post them; I, and I’m sure others, would be happy to give you some constructive comments.
ill probably change my mind 20 times while searching for parts, ultimately a GRP chassis although froma design point of view im thinking its a bit
ambitious for a first build, although still remains my aim
aitch
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Benonymous
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| posted on 8/12/09 at 02:05 AM |
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There's no doubt you will change your mind aitch! This forum is a great place for kicking ideas around and we all do it  I know
I'm getting a bit single track here but I'm in love with the Norma M20 How's this for a chassis?
Triangulation in extremis! The structure is not just for torsional stiffness but also for crash safety. In addition to the multitude of tubing, the
makers add shear panels to the whole structure which ends up making it look look a monocoque tub!
Just a question Kenny. If one was to construct a chassis from square steel with the intention of fitting a full set of shear panels, would you use
lighter steel from the beginning?
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kennyrayandersen
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| posted on 8/12/09 at 08:55 AM |
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quote:
Originally posted by Benonymous
There's no doubt you will change your mind aitch! This forum is a great place for kicking ideas around and we all do it I know
I'm getting a bit single track here but I'm in love with the Norma M20 How's this for a chassis?
Triangulation in extremis! The structure is not just for torsional stiffness but also for crash safety. In addition to the multitude of tubing, the
makers add shear panels to the whole structure which ends up making it look look a monocoque tub!
Just a question Kenny. If one was to construct a chassis from square steel with the intention of fitting a full set of shear panels, would you use
lighter steel from the beginning?
The mind changing for me happens when the cash has to clear edge of the wallet opening. Nothing quite defines reality like that. All the B.S., cheap
talk, boasting, bravado, and Tom-foolery are stripped and you get to decide is this REALLY what I want to spend all that hard-earned cash on?
That’s a heck of a chassis there! I’d be afraid it would be too stiff in a crash, but without knowing all of the gages tube length and what not there
is no way to tell. It’s certainly a bit more complex that most se7enisk chassis. I’m working on a helicopter right now, and though I’m not a crash
specialist, it’s been interesting working with those guys to come up with details that actually cause the structure to fail in a predictable and
sequential way.
With regard to the tube sizing: generally if you are building a tub, it amounts to a box inside another box. Since you have two walls on either side
of you and you have the skin on top and bottom of that section, it would seem that you could certainly reduce the axial members since the strength
comes from that bending section. The I of the upper section has gone from one 1 inch rectangular section in the typical steel frame to a whole upper
skin in the case of a tub-type monocoque -- the skin providing a lot more moment of inertia, since all of that area is at the extreme fiber of the
section (which decreases the stress).
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twybrow
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| posted on 8/12/09 at 11:12 AM |
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aitch - your iea of a composite tub with fromnt and rear frames sounds much more realistic. Just think about the following:
- How are you attaching composite and steel together?
- How does the load from one transfer into the other?
How are you going to jig/align the structures?
If I were you, I would start making lots of composite parts, using various materials. Get a feel for what works and what doesn't. do plenty of
readings, looking on u-tube etc and build up some knowledge of the materials and processes need to use them.
If you are going to use CSM (chopped strand mat) and polyester, this is a very forgiving material to use, but very difficult to master - start
practicing now! I don't think Build models to gauge stiffness/torsionsal rigidity of different designs, and cut up/test some of the composite
pieces you make, to understand the effect of your skill on the performance.
I think you will get there, and I'd be happy to help where I can.
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aitch
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| posted on 8/12/09 at 02:10 PM |
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quote:
Originally posted by twybrow
aitch - your iea of a composite tub with fromnt and rear frames sounds much more realistic. Just think about the following:
- How are you attaching composite and steel together?
- How does the load from one transfer into the other?
How are you going to jig/align the structures?
If I were you, I would start making lots of composite parts, using various materials. Get a feel for what works and what doesn't. do plenty of
readings, looking on u-tube etc and build up some knowledge of the materials and processes need to use them.
If you are going to use CSM (chopped strand mat) and polyester, this is a very forgiving material to use, but very difficult to master - start
practicing now! I don't think Build models to gauge stiffness/torsionsal rigidity of different designs, and cut up/test some of the composite
pieces you make, to understand the effect of your skill on the performance.
I think you will get there, and I'd be happy to help where I can.
i would use glass cloth and epoxy resins, slightly more expensive but will transfer stresses in a nore prediatale way with far less material and
therefore weight.
the tecniques are all very well doccumented, take a look at this wing spar which demonstrates just how strong a seemingly thin walled and skinned box
section can be, these spars are rated at over 9g loading http://www.cozybuilders.org/chapters/chap14.html
heres a link to the firewall
http://www.cozybuilders.org/chapters/chap15.html
and to the fusalage construction
http://www.cozybuilders.org/chapters/chap06_1.html
in order to be able to bolt on in this case the engine mount the firewall or bulhead usses plywood instead of a foam core, the forces as you can
imagine in attaching a 6 cylinder rotax enfine are going to be huge and similar meathods should transfer well to a car tub
my concerns are that to interconnect steel subframes to composite tun with GRP the weight will probably end up in excess of that using one meathod of
construction or the other.
i will be looking into all three meathods, all composite, all steel and combination of the two, as i plan to add a body it may be that i go for steel
and gain experience biulding a composite body shell (less critical but wil gain me experience) before a second all composite build
aitch
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Benonymous
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| posted on 8/12/09 at 11:28 PM |
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Good point on the home built planes aitch. I'd forgotten how many of them are composite monocoque designs. So the problems of load paths and
transitioning from tub to space frame have been solved many times over on these types of machines. As Kenny has pointed out, we can all educate
ourselves and find solutions the problems we encounter with chassis engineering.
[Edited on 8/12/09 by Benonymous]
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kennyrayandersen
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| posted on 10/12/09 at 06:52 AM |
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You should be careful when looking at the aircraft website because there is more going on there than a casual reading indicates. The loads from the
engine are not, in fact, dumped into the bulkhead [exactly]. There was a mention of hard points, no? What a hard point is is a point on something
that is designed to carry or react a point load (like the engine support frame). Generally, the loads of an aircraft are [eventually] carried by the
outer skins (fuselage bending, shear, and torsion loads). The loads from the engine must find their way to the outer skins since this is where the
load wants to be (stiffest load path). So, if the engine frame attaches to the bulkhead, you would expect to see a tension fitting on the backside of
the bulkhead that carries the load from the engine mounting frame through the bulkhead (hardpoint) and into the skin – this is how a ‘hard point’
functions.
The point being, is that in all of this you still don’t want to load a flat plate (or the aft wall of you tub) in a direction normal to the web (it
just doesn’t work that way – there is no out-of-plane stiffness to a web (shear panel)). Instead, what you will be doing is tying the aft frame that
supports the engine/suspension to the outer corners of the tub where the load then can be carried in the outer skins of your tube (and also along the
axial tubes or angles that make up the corner detail of your tub. You will do this with some type of fitting.
It’s kind of all about having a place for all of the load to go. And by place, I mean a direct path.
[Edited on 10/12/09 by kennyrayandersen]
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Benonymous
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| posted on 11/12/09 at 05:54 AM |
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So is this the way it's done Kenny?
The bulkhead is blue and made of steel tube. It bolts into hard points in the tub where the red cylinders (bolts) are. This way the load is fed into
the outer skin of the tub and not into the rear plate of the tub. If they were bolted into the rear plate I agree, the load on the rear plate would
be trying to pull it out of the back of the tub.
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