Noodle
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| posted on 17/3/04 at 08:05 PM |
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Chassis Design/Materials
Whilst rummaging around in the garage looking for a burglar 'deterrent' (i.e. hospitalisation device), I got to considering some 25mm sq
16swg RHS, the material that we're all familiar with through Uncle Rons book. Whilst going the the motions of an imaginary violent
'self-defence' assault, it struck me that the stuff's pretty heavy. So that got me thinking - "There must be a better
way".
We create space frames then hang redundant sheets all over them. Could we not use less materials, different materials, different methods that would
achieve the same level of stiffness but with less weight at a reasonable cost in our home workshops?
There are the obvious carbon fibre/aramid materials but are very expensive (well, they used to be). Anyone experimented with aluminium honeycomb? Or
for the locost idiom end-grain balsa on aluminium sheet? Thinner tubes that are bonded to avoid welding heat distortion?
I sketched out a small chassis a while ago using honeycomb but it dawned on me that I didn't know how to do a decent join with perpendicular
faces (and that the design was crap  )
To save you typing, I know (roughly) about the FW400
And just think, with the weight you've saved, you could drink more and still be faster.
Cheers,
Neil.
Your sort make me sick
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Staple balls
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| posted on 17/3/04 at 08:16 PM |
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i think
this has been gone over quite a lot before, honeycomb etc was deemed too expencive.
but then, if you wanna do it... do it
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JoelP
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| posted on 17/3/04 at 08:33 PM |
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i was wondering if fewer big tubes or many small tubes would be better. i decided it was probably dependent on the situation. Also, liberal use of 22g
welded sheet.
Cymtriks will undoubtedly be able to cast some light on this thread anyway!
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Noodle
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| posted on 17/3/04 at 09:22 PM |
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quote:
Originally posted by Staple balls
honeycomb etc was deemed too expencive.
 Even end-grain balsa with ali sheet?
What about semi-monocoque construction?
Your sort make me sick
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Staple balls
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| posted on 17/3/04 at 09:28 PM |
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not too sure.
do a search for honeycomb and maybe composite, should throw up a bit
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Mark Allanson
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| posted on 17/3/04 at 11:32 PM |
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Someone on the site used to do a 'magic box' front end, with all the brackets in the right place. This could be coupled to a GRP twin
skinned monocoque, filled with foam with a similar box at the rear for sierra IRS.
There are a lot of boat designer/builders in my neck of the woods, they use all kinds of techniques to add strength without adding weight, paper rope,
cadboard formers etc to create GRP tubes within the structure. I am sure a GRP triangulation within the structure would add as much strength as ERW.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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dozracing
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| posted on 17/3/04 at 11:47 PM |
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You can buy for around £250 per sheet 8x4 aluminium honeycomb sandwich panels (see technical resin bonders website and hexcel).
These you can route out one skin and fold the other and bond and do overlap joints etc, to form and nice bath tub type chassis. You then need to use
bulkheads and machined inserts to take the loads of the things that you bolt to it. All possible, doesn't cost the earth (but far more than
steel tube!!) and will be awesomely light and stiff.
The hexcel website tells you all the techniques for fabricating stuff with these panels.
Kind regards,
Darren
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britishtrident
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| posted on 18/3/04 at 08:21 AM |
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The valid reason for going monocoque isn't to save weight but to increase stiffness.
A lot of people in the racing car world have tried honeycomb, only to decide it wasn't iworth the extra hassle except for making a main bulkhead
or perhaps a floor from it.
The Locost shape isn't ideal for a monocoque but a stiff structure can be made as per the Qantum (and now even Robin Hood !!!) chassis. Another
good monocque material is GRP -- Davrian made good use of it and it.
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stutz
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| posted on 18/3/04 at 08:01 PM |
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What about using chromoly? Check out a few homebuilt aircraft sites, they all use chromoly tubing to build their spaceframes. Just checked it comes
in all the sizes required by 'the book'.
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robinbastd
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| posted on 18/3/04 at 09:56 PM |
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I've read several fibreglass/composite books and scoured the net looking for details on the construction of fibreglass/foam sandwich monocoques
and can't find enough detail. (thickness of foam core,what weight cloth,how many layers,etc) Any ideas???
IIRC it was Merlin that made the "magic box"
Ian
Only a dead fish swims with the tide.
http://smuttygifts.com/
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Mark Allanson
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| posted on 18/3/04 at 10:16 PM |
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I used to drive seariders in the Navy, we used to drop them 20' into a force 8 sea, with the 85BHP outboard running dry at 8000rpm, smash into
heavy seas at 40 knots plus, it never seemed to flex at all, but I did once tear the inflatable bit off the bow and we nearly sank whilst showing off
to my parents at Hayle Bar.
I think we should find one, jigsaw it in half, count the laminations and measure the foam thickness, or just fit wheels to the damn thing!
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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Alan B
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| posted on 18/3/04 at 10:19 PM |
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quote:
Originally posted by stutz
What about using chromoly? Check out a few homebuilt aircraft sites, they all use chromoly tubing to build their spaceframes. Just checked it comes
in all the sizes required by 'the book'.
Many reasons not to.
Very expensive
Needs more care/skill/equipment etc. to weld
It is only stronger...NOT stiffer...so by taking advantage of the strength wall thicknesses can be reduced, but at the expense of stiffness...IOW it
needs a lot of redesign to take any advantage of the material
I'm sure you will find the aircraft frames are well designed structurally...no bending loads...pure tension and compression
A lot of this applies to monocoque of course...it's got to be redesigned to take advantage of the materials.....
Many of us on here know a lot about the theory of monocoques...BUT very few with any real-life experience..
Bottom line..IMO...unless you are experienced with non-book construction methods....don't do do it...
Up to you of course... just purely IMO
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britishtrident
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| posted on 19/3/04 at 08:50 AM |
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quote:
Originally posted by robinbastd
I've read several fibreglass/composite books and scoured the net looking for details on the construction of fibreglass/foam sandwich monocoques
and can't find enough detail. (thickness of foam core,what weight cloth,how many layers,etc) Any ideas???
Ian
With foam cored composite structure the mechanical properties foam core dosen't really come in to bending and tensile calculations its only
purpose is to hold the upper and lower skins apart, like the central web on an "I" beam, what maters is the distance the core holds the
skins apart.
Like wise the mechanical properties of the plastic (usually polester or epoxy) matrix material that embeds, bonds and supports the fibres plays
almost no part in the over all strength and mechanical properties of the material.
As for the mechanical properties of a grp laminate it depends very much o the orientation of the glass fibres in the layers and the order in which the
layers are built up, get it wrong and you get sheer stresses between the layers of fibre, which can cause de-laminatio or twisting under load.
The most successful GRP monocoque has been the Davrian, it used mainly CSM (chopped strand mat) but with layers of woven material in critical areas.
The thickness varying between about 1.5 and 4 mm on race cars and somewhat thicker on rally and road cars say 2 to 7 mm. The main structure
consisted of two side pods of fairly thin lay-up joined by floor and roof, as the floor had to take various local loads it was of a thicker lay-up
with a additional local thickness around the suspension mounts. The transverse bulkheads were the most important part of the structure --- marine ply
was used on some versions.
******* The loads from suspension, engine mounts seat belts what have you were all put into the structure in shear*******
The roll cage was bonded into the structure but the structure was designed to be stiff enough with out it, or the roof.
This structure gave a car that was heavier than the equivalent racing spaceframe (made from 1.2mm tube) but a lot stiffer.
[Edited on 19/3/04 by britishtrident]
[Edited on 19/3/04 by britishtrident]
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imull
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| posted on 22/3/04 at 11:19 PM |
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Hello, first post on here. I have been thining of building a composite Locost chassis once out of uni so am interested in this thread. Carbon Fibre
(CFRP) is well out of my budget so GRP is the choice.
It is far more resiliant and repairable tha CFRP but weight vs strength is obviously down.
There are better GRP cars out there. I am building a Clan Crusader that has much much better tosional rigidity and general strength than the Davrian
and is made in a different way. The rear cross member mounts all the pick-ups and engine mounts and fits into a moulded recess in the floor pan which
bolts through some reinforcing into the chassis basically
One shell I have was rolled at over 100mph and has shown very little in the way of delamination around the seems...
The Clan was designed and built fast road/competition use by former Lotus engineers and is generally considered to be the best and most effective of
all Imp derivatives. The chassis is a full monocoque consisting of plywood and GRP with some plates bonded in for mounting the cross member and front
wsh bone pick up points. Bonded into the chassis itself was a GRP rollcage that managed support a skip and several tons of water in order for the MSA
to pass it for competition use on the Manx.
If you have the budget, you would be daft not to go down the honeycomb route and to glass around it.
You want the minimum number of seams and sharp angles as possible otherwise torsionary rigidity will suffer and fatigue will accelerate due to
increased stress concentrations.
As far as copying the marine industries processes, that will be big bucks as they enerally use a mix of Kevlar, Carbon and Nomex board
[Edited on 22/3/04 by imull]
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ProjectLMP
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| posted on 23/3/04 at 04:56 PM |
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Older rivetted/bonded honeycomb construction tubs are notorious for 'loosening up'. Most of these chassis are in a constant state of
repair. Not ideal for a street car. To avoid this you need to use some pretty exotic bonding methods such as those uses by Lotus. However, you
can't do this at home because you need an oven to bake the chassis in.
Home of the Astronomicalcost Mid engined LMP project
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britishtrident
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| posted on 23/3/04 at 06:31 PM |
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quote:
Originally posted by imull
snip
out of uni so am interested in this thread. Carbon Fibre (CFRP) is well out of my budget so GRP is the choice.
Yes that is correct
It is far more resiliant and repairable tha CFRP but weight vs strength is obviously down.
Strengthi is not my carbon fibres are use the are STIFFER the glass or kevlar.
There are better GRP cars out there. I am building a Clan Crusader that has much much better tosional rigidity and general strength than the Davrian
Pull the other one ! first you should take a look at THE "works" Clan the ex-Johnnie Blades, ex Kenny Allen works modsport car that has
been owned trundled by Stan Share for about 25 years for most of which it has been 10,000 fatigue cracks driving in close formation. It had be raced
hard by Blades and with famous successfully by Kenney Allen (mainly due to Allen sorting of the handling and brakes and the ultra special 1140
Carter engine which was without doubt the best Imp engine I have ever driven) but at the end of Allen's steward ship the shell was about raced
out.
This works car was a very different annimal from the road shell it was substantially strengthened (where as proper modsport Davrian such as the Mk8
shell made for Ian Forrest were thinner than the road cars) , a few people have tried to build relicas using road Clan shells and failed.
"and is made in a different way. The rear cross member mounts all the pick-ups and engine mounts and fits into a moulded recess in the floor pan
which bolts through some reinforcing into the chassis basically"
Actually the floor in front of the (Imp) crossmember was a major failure point on early Washington built Clans, I have seen road going clans split
across the full width of the floor in this area.
One shell I have was rolled at over 100mph and has shown very little in the way of delamination around the seems...
The Clan was designed and built fast road/competition use by former Lotus engineers and is generally considered to be the best and most effective of
all Imp derivatives. The chassis is a full monocoque consisting of plywood and GRP with some plates bonded in for mounting the cross member and front
wsh bone pick up points. Bonded into the chassis itself was a GRP rollcage that managed support a skip and several tons of water in order for the MSA
to pass it for competition use on the Manx.
The strength of the "paper" roll cage in the Clan is not in doubt. ****But significantly the the Blades-Allen-Share car has a steel roll
cage.
If you have the budget, you would be daft not to go down the honeycomb route and to glass around it.
You want the minimum number of seams and sharp angles as possible otherwise torsionary rigidity will suffer and fatigue will accelerate due to
increased stress concentrations.
As far as copying the marine industries processes, that will be big bucks as they enerally use a mix of Kevlar, Carbon and Nomex board
[Edited on 22/3/04 by imull]
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imull
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| posted on 24/3/04 at 07:54 PM |
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quote:
Originally posted by britishtrident
snip
out of uni so am interested in this thread. Carbon Fibre (CFRP) is well out of my budget so GRP is the choice.
Yes that is correct
It is far more resiliant and repairable tha CFRP but weight vs strength is obviously down.
Strengthi is not my carbon fibres are use the are STIFFER the glass or kevlar.
My mistake, wrong word choice
quote:
There are better GRP cars out there. I am building a Clan Crusader that has much much better tosional rigidity and general strength than the Davrian
Pull the other one ! first you should take a look at THE "works" Clan the ex-Johnnie Blades, ex Kenny Allen works modsport car that has
been owned trundled by Stan Share for about 25 years for most of which it has been 10,000 fatigue cracks driving in close formation. It had be raced
hard by Blades and with famous successfully by Kenney Allen (mainly due to Allen sorting of the handling and brakes and the ultra special 1140
Carter engine which was without doubt the best Imp engine I have ever driven) but at the end of Allen's steward ship the shell was about raced
out.
This works car was a very different annimal from the road shell it was substantially strengthened (where as proper modsport Davrian such as the Mk8
shell made for Ian Forrest were thinner than the road cars) , a few people have tried to build relicas using road Clan shells and failed.
Have talked to people who own both (each very highly tuned) and in both cases they say the Clan is the better car. Another Davrian owner that I know
was apparently told by Tim Duffee that the Clan was a more efficient design.
quote:
"and is made in a different way. The rear cross member mounts all the pick-ups and engine mounts and fits into a moulded recess in the floor pan
which bolts through some reinforcing into the chassis basically"
Actually the floor in front of the (Imp) crossmember was a major failure point on early Washington built Clans, I have seen road going clans split
across the full width of the floor in this area.
commonly used reinforcement in this area has hugely increased strength. One shell as stated was rolled at top speed (somehow) and other spent several
seasons forest rallying with sign of wear and tear there...
quote:
The strength of the "paper" roll cage in the Clan is not in doubt. ****But significantly the the Blades-Allen-Share car has a steel roll
cage.
Probably because the MSA demands them now. Also, the paper cage may be very strong but you would have to be stupid to try and compete without a stell
one too, even if not required by the blur book!
Mine will have fully triangulated multi point cages reinforcing suspension etc.
Should also have mentioned that both my Shells are Luff shells that as standard are hugely stronger than the standard ones.
You will have to forgive my ignorance, but I have never even heard of Jonnie Blades (roundy roundy stuff isnt really my thing). From what I have been
told the RR and RC Clans were very different cars. As mine has been built for forest rallying it is more substantial than the original road cars.
HOWEVER, it is also still a wee bit lighter than the road cars published weights! It is also by no means the lightest of the Rally Clans.
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Noodle
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| posted on 24/3/04 at 08:00 PM |
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Alec Issigonis made some sort of home-brew lightweight special in the 50's that used a laminated balsa (I think) for increased torsional
rigidity. I don't think any special high-pressure ovens were constructed to cure it etc.
Surely, if we can bodge home made TIG welders, de-rusting equipment etc, someone must have some super-cheap ideas?
Cheers,
Neil
Your sort make me sick
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mranlet
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| posted on 25/3/04 at 07:51 AM |
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A technique used in Japan in production-class race cars is to mix up 2-part foam and inject it (with a syringe) into the voids in the unit-body.
Although this technique requires bracing of the frame to prevent flexing, the results boost rigidity up to 75% from what I've heard.
2-part foam isn't the lightest thing so it won't help that much for straight line performance, but the increased rigidity will improve
handling more than it would be penalized by increased weight.
At the same time, you may be able to use much thinner steel tube to save weight...
Just a thought.
As for Carbon Fiber Monocoque, unless you have the funds for pre-preg cloth, the molds to form it, a room-size curing chamber (with both atmospheric
pressure and temperature control) and the means to fuse it to the aluminum honeycomb, it probably isn't a practical possibility.
-MR
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britishtrident
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| posted on 25/3/04 at 10:54 AM |
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quote:
Originally posted by Noodle
Alec Issigonis made some sort of home-brew lightweight special in the 50's that used a laminated balsa (I think) for increased torsional
rigidity. I don't think any special high-pressure ovens were constructed to cure it etc.
Surely, if we can bodge home made TIG welders, de-rusting equipment etc, someone must have some super-cheap ideas?
Cheers,
Neil
Even older than that early 1930s when Issigonnis was working at Humber and Alvis -- it was mainly plywood -- not sure if it was balsa ply but it
followed the type of techniques used by De Haviland aircraft.
The suspension double wishbone and was sprung by inboard rubber bands.
Of course in the late 1950s/early60s Marcos use Marine ply chassis up to 1968 or so -- Jackie Stewart raced one at Charterhall only to have
turned to matchwood after a start line shunt.
I was tempted to try a bonded in marine ply rear cockpit bulkhead in my Locost as a bit of finite element work on the book chassis revealed just how
much the rear bulhead flexes, but that has went by the board as I have decided to use Westie style seatbelts with the reels in the centre inspired
by this url http://www.gibbs111.fsnet.co.uk/centrall.htm
[Edited on 25/3/04 by britishtrident]
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cymtriks
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| posted on 28/3/04 at 09:38 PM |
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If it's stiffness for minimum weight you want then one tube is best. This is the idea behind backbone chassis as on the Lotus Elan (original)
and Esprit. However this kind of chassis requires a much more substantial body shell as this needs to be virtually self supporting away from the
central chassis and will also need to support bits that would normally be chassis mounted like the steering column and the door hinges.
The Lowcost chassis can be stiffened up a lot for no extra weight. My suggested mods take the stiffness from around 1200 ftlbs per degree to around
2700, reduce the weight and use less tubes.
If you used my mods but in 18g tube and 20g sheet then you'd have a light weight chassis that would still be 50% stiffer than the book design.
The only down side is that you'd need to be carefull about how loads were fed into the chassis. For example tubes G1/G2 may need to be in 1.5
inch deep tube to resist the engine weight or extra triangulation to the engine mounts may be needed.
The standard 16g tube is probably over engineered but is cheep, available, proven and can be made into a stiff structure by an amateur welder.
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DaveFJ
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| posted on 28/3/04 at 11:38 PM |
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Anyone looked at RHE's latest offering ?
Dave
"In Support of Help the Heroes" - Always
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britishtrident
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| posted on 29/3/04 at 08:05 PM |
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quote:
Originally posted by cymtriks
If it's stiffness for minimum weight you want then one tube is best. This is the idea behind backbone chassis as on the Lotus Elan (original)
and Esprit. However this kind of chassis requires a much more substantial body shell as this needs to be virtually self supporting away from the
central chassis
snip snip snip
The Elan was really a monocoque design to which a backbone subframe was added to solve problems durring development,.
To meet the strict approval regs in Australian states where truly ott torsional tests are mandatory Oz buliders have evolved mods mainly to the
chassis sides, tunnel and rear bulkhead that more than treble the torsional stiffness.
At one time I considered using a folded steel central tunnel with the dual goals of more stiffness and more internal space.
[Edited on 29/3/04 by britishtrident]
[Edited on 30/3/04 by britishtrident]
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