The chassis con
Discussion
Yes, understand what Kitcarman is saying. From what I've seen in the past, a typical ladder chassis has a torsional stiffness of about 1000 - 1500 lbft / degree. "Spaceframes" vary enourmously depending on the type of construction. I think a typical roadgoing TVR / GD chassis would be 2000 - 3000lbft / degree but I'd be interested to hear some of Chris' predicted figures from his Finite Element models. "Spaceframes" with pillars and a roof structure can abviously be made a lot stiffer (like about 5000 lb ft / degree) and I was told once (but can't remember accurately) that TVR's racing V12 monster had about 18,000. (obviously with a big diagonal cage member across the windscreen aperture). 18,000 also sticks in my head because I seem to remember that was the quoted stiffness for the old Austin 1800 "Landcrab" monocoque. I think modern monocoques are a lot stiffer again but haven't seen any figures - I'm guessing at about 50 -60,000 lbft / degree for something like a big Merc or Beamer??? Clearly with differences like that it suggests (as you point out) that there's a lot more to good handling than a torsionally stiff chassis! Someone who does this sort of thing for a living told me that you need a chassis about ten times stiffer than the difference between the front and rear suspension roll stiffnesses to make it handle reasonably well and about 100 times stiffer before you start making something reasonably rattle and scuttle-shake free. Again, I'd be interested in any figures that anyone has obtained to help build up a picture of what's out there.
kits-r-us said:
Mark B, TVR's of the Chimaera generation are space frame. I've been to the factory a couple of times and it's remarkable how similar to kit cars the construction is. Just make sure Danielle is taking you around the factory when you visit (if your ever decide to), let's just say she's a bit
Having looked at Chimeara's and Griffiths they are all backbone style chassis. They may be constructed in a triangulation manner, but it is a long and strong back bone that the front and rear suspension hangs off, this is a back bone chassis AFAIK. (Although i am prepared to proved wrong)
I have a picture of a Chimeara chassis clearly showing it to be a back bone, not a space frame chassis.
This is very similar to a Gardner Douglas Cobra AFAIK and very dissimilar to a Pilgrim or such like.
I would call the thing in the picture below a backbone chassis. I was led to believe this was the norm for TVR(was the case on my old 350i anyway but I don't know about the new ones). A spaceframe I always thought was a triangulated frame which more or less goes round the extremities of the vehicle to get maximum torsional strength in all directions from minimum weight. The backbone needs to be much heavier to get the same stiffness.
Avocet said:
I think modern monocoques are a lot stiffer again but haven't seen any figures - I'm guessing at about 50 -60,000 lbft / degree for something like a big Merc or Beamer???
I dont think that they are even close...remember the Mac F1 was at about 25000ftlbs/degree and most others are well below that.
I believe that the Ultima is at about 3500 ftlbs/deg (but I will happily stand corrected) and everything else these days fit between these two.
Andy
gdr said:
I would call the thing in the picture below a backbone chassis. I was led to believe this was the norm for TVR(was the case on my old 350i anyway but I don't know about the new ones). A spaceframe I always thought was a triangulated frame which more or less goes round the extremities of the vehicle to get maximum torsional strength in all directions from minimum weight. The backbone needs to be much heavier to get the same stiffness.
Thank you GDR, my sentiments exactly. One thing with backbone chassis are though, they can also be used to develop the vehicles handling as the torsional stiffness is quite 'tuneable'.......
...but if the TVR has a "backbone" chassis what would you call the old Lotus chassis that were essentially the same basic shape (big stiff bit down the middle) but made of pressed rather than tubular steel? I've heard these referred to as "backbone" chassis but never TVRs (well, until the other day, that is)!
Avocet said:
...but if the TVR has a "backbone" chassis what would you call the old Lotus chassis that were essentially the same basic shape (big stiff bit down the middle) but made of pressed rather than tubular steel? I've heard these referred to as "backbone" chassis but never TVRs (well, until the other day, that is)!
Technically speaking, I think you will find that the Lotus has a “backbone” chassis, whereas the TVR has a “space frame backbone” chassis. Den
kitcarman said:
...but if the TVR has a "backbone" chassis what would you call the old Lotus chassis that were essentially the same basic shape (big stiff bit down the middle) but made of pressed rather than tubular steel? I've heard these referred to as "backbone" chassis but never TVRs (well, until the other day, that is)!
Den
Technically speaking a space frame back bone chassis - What???????
It's a back bone chassis, it just happens to be made from tubular steel in a triangulated manner. It's a back bone chassis, not a space frame!!!!!
kitcarman said:
Mark,
Upon the basis of your argument, ordinary production cars have space frame chassis. Alternatively, an Ultima has a monocoque chassis.
I think you’ll find that my descriptions are correct
Den
Kitcarman, I am not about to get into an argument with you, modern cars do not have a space frame they have a monocoque as I am quite sure you know. This is due to them being constructed of pressed or hydroformed steel and welded together with sheet steel to get the rigidity, not in anyway shape or form a space frame....... I am afraid I have not seen an Ultima chassis so I can't comment...
There is no such thing as a space frame back bone chassis, I think you would have been better describing it as a tubular backbone chassis, but by no means is it a space frame, that is a totally different concept. The manner in which a back bone chassis is constructed is irrelevent, it is a backbone chassis, ie gets its strength and rigidity from a central 'back bone' which 'picture frames' are bolted on front and rear for the suspension points........
Just like an E Type is a monocoque construction, although it has a framework on the front that holds the engine, bonnet and suspension onto.
A Stylus has a semi monocoque chassis, ie partly tubular and partly sheet steel.
A Striker/Caterham/Westfield/Locost have space frame chassis.
A GD Cobra has a back bone chassis where as Pilgrims AFAIK are semi monocoque
The fact is the space frame is a chassis type that effectively goes around the occupants and powertrain and the suspension hangs off and does not use large amounts of sheeting for it's structural integrity..
Sorry for this to go on and on, but the original comments about Kitcar chassis being a con wound me up. I also think it is important in a technical (ish) forum for peoples understanding to be put right, just like I am happy to be corrected if I make a wrong statement (which happens regularly) ....
Sorry kitcarman, but don't
me with a wrong description.... Mark B :rantmodeoff:
For what it is worth, I was informed that the spaceframe chassis of the Hawk HF3000 (Stratos Replica) was tested to have a torsional rigidity of 6000 lb/ft. And that was without the ally floorpan or rear bulkhead fitted, nor the central tub. When rivetted and bonded in place, these would add to the overall strengthness in some degree.
Sounds pretty impressive to me, at least.
Sounds pretty impressive to me, at least.
Take a look at http://autozine.kyul.net/technical_school/chassis/tech_chassis.htm
It specificaly states under "Backbone Chassis"
"TVR's chassis is adapted from this design - instead of a rigid backbone, it uses a lattice backbone made of tubular space frames. That's lighter and stronger (mainly because the transmission tunnel is wider and higher). "
So Den's descriptions sounds 100% correct from this.
It specificaly states under "Backbone Chassis"
"TVR's chassis is adapted from this design - instead of a rigid backbone, it uses a lattice backbone made of tubular space frames. That's lighter and stronger (mainly because the transmission tunnel is wider and higher). "
So Den's descriptions sounds 100% correct from this.
Ferg said:
Mark,
Our 'other car' is a Multipla.....(pause for ridicule from people who've never owned or driven one).......and it's described as being a spaceframe! How's that work??
'If you can't think of a good thing to say about a car (Fiat ????), don't say anything.'
'nuff said about the Multipla!
Ferg said:
Mark,
Our 'other car' is a Multipla.....(pause for ridicule from people who've never owned or driven one).......and it's described as being a spaceframe! How's that work??
I have heard that they are actuall very good MPV's, once you get over the rest of the driving population either recoiling in horror or laughing hystericaly
Sorry not to post earlier! I've been very busy lately.
The "con" I reffered to is the usual advertisement blurb along the lines of our spaceframe is very stiff and light etc etc. Most kit car spaceframes would actually be beaten by a properly designed ladder frame on stiffness and weight.
Here are some results as requested. My results are not based on actual chassis drawings but on diagrams and pictures from magazines and the internet.
These figures only take into account the basic welded steel structure of tubes and panels and are subject to the usual differences between builds and any inaccuracies due to the simple analysis used.
Alloy panels and other bolted, bonded or riveted on bodywork will increase the stiffness.
The Caterham 7
Very stiff for its weight and very well made. I have attempted to include the effect of the alloy panels for this one. I'm basing my analysis on pictures that are a few years old so things may have moved on. The stiffness is 2800 ftlbs per degree and the weight is about 180 lbs. My attempt at estimating the panels contribution may be off a bit so this is a bit approximate.
Ultima spyder and coupe
I've not seen it but everyone who has reckons it's well made. The stiffness is 2475 and 3267 ftlbs per degree of twist respectively. The weights are 265 lbs and 300 lbs respectively. I think that these weights are an over estimate as the mass calculation routine on my finite element analysis sometimes does overshoot but given other peoples results for stiffness I think my stiffness results are not too far off. If Ted Marlow has an accurate value I'd be very interested to know it! Take a look at the Ultimav12.ca website.
Porsche 550 replica
The stiffness is 1136 ftlbs per degree of twist and the weight is 172lbs. More complex four tube replica chassis probably approach 2000 ftlbs and use 3 inch diameter 1/8 inch wall tubes.
Lotus 23 replica
The stiffness is 1449 ftlbs per degree of twist and the weight is 100 lbs.
This assumes round 1 inch dia 16 gauge tubes
Cobra ladder frame using 100 x 50 x 3 mm rails, substantial footwell and dashboard structure and panelled in transmission tunnel
The stiffness is 4785 ftlbs per degree of twist and the weight is 381 lbs
Cobra ladder frame as above but using 80 x 40 x 3 mm rails
The stiffness is 2865 ftlbs per degree of twist and the weight is 319 lbs.
Simple X braced chassis using 100 x 50 x 2 mm rails with no other reinforcing structures for cars with narrow, Seven or Duce hot rod style, bodywork (the thin tube walls are to avoid an excessively heavy chassis on a seven type car)I included this in my original post.
The stiffness is 1400 ftlbs per degree of twist and the weight is 133 lbs.
Lancia Stratos replica. Not far off the figure quoted in an earlier post!
The stiffness is 6579 ftlbs per degree of twist and the weight is 300 lbs.
Claimed stiffness value when tested by STATUS was “over 6000”
Book claim for Morris Minor
The stiffness is 4000 ftlbs per degree of twist.
Book claim for original Lotus Elan backbone chassis with bodyshell mounted
The stiffness is 4300 ftlbs per degree of twist.
Magazine article claim for Lotus Elise
The stiffness is 7350 ftlbs per degree of twist.
Magazine article claim for BMW sports saloon
The stiffness is 13000 ftlbs per degree of twist.
Due to money, space and time it is going to be a while before I get started on anything in my garage. I am thinking of a mid engined lowcost but also hanker after a Porsche 550 or RSK with updated detailing (no beetle headlights!).
I'm glad this has kept you all interested!
The "con" I reffered to is the usual advertisement blurb along the lines of our spaceframe is very stiff and light etc etc. Most kit car spaceframes would actually be beaten by a properly designed ladder frame on stiffness and weight.
Here are some results as requested. My results are not based on actual chassis drawings but on diagrams and pictures from magazines and the internet.
These figures only take into account the basic welded steel structure of tubes and panels and are subject to the usual differences between builds and any inaccuracies due to the simple analysis used.
Alloy panels and other bolted, bonded or riveted on bodywork will increase the stiffness.
The Caterham 7
Very stiff for its weight and very well made. I have attempted to include the effect of the alloy panels for this one. I'm basing my analysis on pictures that are a few years old so things may have moved on. The stiffness is 2800 ftlbs per degree and the weight is about 180 lbs. My attempt at estimating the panels contribution may be off a bit so this is a bit approximate.
Ultima spyder and coupe
I've not seen it but everyone who has reckons it's well made. The stiffness is 2475 and 3267 ftlbs per degree of twist respectively. The weights are 265 lbs and 300 lbs respectively. I think that these weights are an over estimate as the mass calculation routine on my finite element analysis sometimes does overshoot but given other peoples results for stiffness I think my stiffness results are not too far off. If Ted Marlow has an accurate value I'd be very interested to know it! Take a look at the Ultimav12.ca website.
Porsche 550 replica
The stiffness is 1136 ftlbs per degree of twist and the weight is 172lbs. More complex four tube replica chassis probably approach 2000 ftlbs and use 3 inch diameter 1/8 inch wall tubes.
Lotus 23 replica
The stiffness is 1449 ftlbs per degree of twist and the weight is 100 lbs.
This assumes round 1 inch dia 16 gauge tubes
Cobra ladder frame using 100 x 50 x 3 mm rails, substantial footwell and dashboard structure and panelled in transmission tunnel
The stiffness is 4785 ftlbs per degree of twist and the weight is 381 lbs
Cobra ladder frame as above but using 80 x 40 x 3 mm rails
The stiffness is 2865 ftlbs per degree of twist and the weight is 319 lbs.
Simple X braced chassis using 100 x 50 x 2 mm rails with no other reinforcing structures for cars with narrow, Seven or Duce hot rod style, bodywork (the thin tube walls are to avoid an excessively heavy chassis on a seven type car)I included this in my original post.
The stiffness is 1400 ftlbs per degree of twist and the weight is 133 lbs.
Lancia Stratos replica. Not far off the figure quoted in an earlier post!
The stiffness is 6579 ftlbs per degree of twist and the weight is 300 lbs.
Claimed stiffness value when tested by STATUS was “over 6000”
Book claim for Morris Minor
The stiffness is 4000 ftlbs per degree of twist.
Book claim for original Lotus Elan backbone chassis with bodyshell mounted
The stiffness is 4300 ftlbs per degree of twist.
Magazine article claim for Lotus Elise
The stiffness is 7350 ftlbs per degree of twist.
Magazine article claim for BMW sports saloon
The stiffness is 13000 ftlbs per degree of twist.
Due to money, space and time it is going to be a while before I get started on anything in my garage. I am thinking of a mid engined lowcost but also hanker after a Porsche 550 or RSK with updated detailing (no beetle headlights!).
I'm glad this has kept you all interested!
Mr2Mike said:
Take a look at http://autozine.kyul.net/technical_school/chassis/tech_chassis.htm
It specificaly states under "Backbone Chassis"
"TVR's chassis is adapted from this design - instead of a rigid backbone, it uses a lattice backbone made of tubular space frames. That's lighter and stronger (mainly because the transmission tunnel is wider and higher). "
So Den's descriptions sounds 100% correct from this.
But the first photo in the "spaceframe" section of the same article is of a Tuscan chassis. So I guess even the author of this article is confused! Am I right in thinking that Mark B is saying that the crucial element of a spaceframe is that it has to have tubular side members?
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