LocostUSA.com

I think I have to contact my Board Administrator to post a spreadsheet

Testing the attachment function here...no real message. Anyway, should be an attached file. Just click reply, then scroll down to the "Upload attachment" window and follow the instructions.

Let's see if this works...

_________________
Stan
Dauntless Racing
You can have it fast, cheap and reliable -- choose any two!

Let's try a spreadsheet...ah yes, the xls extension is not allowed.

_________________
Stan
Dauntless Racing
You can have it fast, cheap and reliable -- choose any two!

You should be able to to zip it and then it can be uploaded.

Walt

Why would you want the torsional stiffness of a modern hatchback if you car has only one third the mass? I also think there is space for rim bracing and also space to raise the frame rail. It might start to look different than the Super 7.

On my frame I arranged for strong bracing under the scuttle. The addition of a bolt in tube from the center of the dash across the passenger seat to the upper rail raises the frame stiffness by about %50, for about 3 lbs. cost. I don't think passengers can tell the difference so it seems like a reasonable compromise.

If you want to drive in competition it helps to have a roll cage and once you start to have that size tubing on the car it seems to make sense to use it and then stiffness is possible. In general I think these frames respond well to larger diameter tubing.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Well, there are two schools of thought on that one. One is that you can't possibly have too much stiffness (subject to weigh limitations).

The other, which I subscribe to, is as I stated above:

"Good chassis design (I would argue) is about stiffness:weight ratio, not absolute stiffness. It's therefore about placing the material you use so that it works hardest for its living, and using only as much of it as you need to deliver adequate levels of stiffness, whilst minimising structural weight."

The basic rule of thumb for race cars, which I've seen quoted by several well-known designers, seems to be that you need a torsional stiffness of at least 10 times the (road spring and ARB) roll stiffness.

But even on a very lightweight road car, I'd be setting myself a target of 2,500 lb.ft/deg minimum, whereas many Seven chassis struggle to reach half that.



Yes, that's a very valid point. If you want to build a Locost that looks like a traditional 'Seven', I think you just have to accept that it's a lousy design - both in terms of structural stiffness and aerodynamics - but that the limitations the design imposes are just part of the character of the vehicle.

Here on this thread, with its topic of carbon fibre construction, though, I'm making the assumption that many people will be interested in more progressive design ideas? Indeed, based on my experience with the design of the chassis I pictured earlier, I'd go so far as to say that the traditional 'Seven' form is heavily sub-optimal, if not outright inappropriate, for a composite monocoque chassis.

Have you looked at the KITCARANALYSIS_V2.doc files? Without adding upper cockpit changes, the mods suggested make a book-looking chassis and get you the following:

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

For torsional stiffness, some people recommend the 10% rule. Somewhere on this site I did a write-up on calculating chassis stiffness vs. wheel spring rate per this rule.

That rule is recognizing that we're dealing with springs in series so as long as the chassis is 10x the stiffness of your suspension you've reached the point where it isn't providing any additional performance or tuning benefit. (1/Kequiv) = (1/K1) + (1/K2) + (1/K3) +.....+ (1/Kn) where K is your spring rate - if one is particularly soft you can see that it ends up dominating the overall stiffness number but if one is really stiff it becomes almost zero in the grand scheme of things.

Its always been a good enough close approximation from my experience. We had FSAE cars that were below 10x and it took a major suspension change to see any effect in the handling whereas the stiffer cars would react from a smaller change to spring or bar rates.

Speaking of high sills, these were two of the chassises - 90% of the stiffness increase came from added the four tubes near the driver's shoulders. The only downside to this method versus making the sills wider is that you don't do nearly as much to strengthen the cockpit from side impacts. The tube typically used in these cars doesn't do jack when faced with a pole-like object - if you used rollcage sized tubing like Car-9 or even rectangular section in some areas you could probably achieve a similar benefit while increasing safety.

_________________
-Andrew
Build Log
Youtube

Yes, I'm aware of them. Their originator regularly used to preach that a simple ladder frame was a far better solution than a spaceframe. He's gone quiet on that subject in recent years - I think he probably got sick of us taking the rip out of him!

Not that I'm saying the results aren't valid (there is insufficient information to be sure ether way) or that it isn't possible to achieve those sorts of torsional stiffnesses with a 'Seven' spaceframe - the Caterham chassis gives very similar figures and is slightly lighter than the 'Cymtriks' chassis - just that a great many designs don't.

... and the figures being quoted by Cymtriks/achieved by Caterham represent about the practical limit of what can be achieved by a traditional 'Seven' spaceframe chassis, whilst being at the lower end of what would be considered reasonable target figures, given the sort of spring rates people are typically using these days.

Since this thread started out discussing the possibilities of a carbon spaceframe, I thought it might be the appropriate place to draw attention to Westfield's latest project.

Story here: http://www.pistonheads.com/news/ph-britishcars/westfield-working-on-all-new-car/32952

If the links to the websites of Westfield's partners give the game away, then it'll be a carbon spaceframe using resin infused braided tubes, with a rotary engine producing (120bhp) from a weight of 62lbs.

My Car9 frame is similar to a Seven chassis, but has a higher upper rail. The FEA says 12,000 ft-lbs./degree with the extra 3 lb. cockpit tube and 8000 without. It's about 175 lbs. and includes the tubes of a competition roll cage. I don't think the Seven is poorly designed by any means. It is just a great deal lighter and the ratio of weight to stiffness favors a heavier car. In a Seven you could not remove many tubes before it was no longer a car even. You could remove a lot from my car, but it would lose stiffness pretty rapidly. You can see that because that one cockpit bar gives a %50 increase in stiffness.

That Westfield carbon frame is interesting and quite light. I would guess it's pretty stiff because of the large section in those tubes.

It's a valid point that larger section and fewer tubes are also a stiff approach. I don't think the person advocating a ladder frame was wrong on this point and I would say the Lotus Elan and Europa with their large backbone are yet another take on this. The whole thing comes down to what diameter tubes are used.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

When you're talking about spaceframes vs. ladder frames, vs. backbone chassis, it comes down to second moment of area, all other things being equal.... and anything approximating to a ladder frame in the accepted sense of the term has a lousy second moment.

I read the first few pages of posts in this thread and then skipped to the end, so I might have missed a similar comment. If so I apologize for the redundancy. But if I were going to build a carbon fiber/epoxy frame I'd design a monocoque chassis, much like Chapman did with the Lotus Elite. A truss or space frame is intended to get the maximum strength vs. weight compromise out of a wooden or metal structure. Composites give you much more freedom to develop an optimum design.

Or possibly a compromise. When I was first learning about FRP, one of the tricks I learned was to split the paper tube from a roll of wrapping paper and place that strategically in your lay-up to develop an integral stiffening member. The paper only held the cloth in place until the resin set up, and did not add significantly to the weight of the structure.

You could overcome the weakness the Elite experienced by incorporating metal (not aluminum) attachment fittings to distribute structural loads.

But I think that in the end, once you've designed for all the loads you aren't going to save much weight over the steel. And if you design for similar impact resistance you'll actually be significantly heavier.

For example, high end carbon fiber racing bicycle frames range from about two and a half to three pounds. The lightest steel bicycle frames are about a pound heavier, with production steel frames such as my Soma ES coming in around 4 lb for a 54 cm frame. Bicycle Quarterly magazine tested a carbon fiber bicycle that was designed for touring a year or so ago and found that after the carbon fiber frame was designed with attachments for fenders and racks there was virtually no weight advantage.

_________________
Jerry Henneman

Jack of many trades
Master of unfinished projects
build blog/log at https://jerryslocost.wordpress.com

Yeah, there has been a lot of discussion in the middle!

High-end steel bicycle frames use technology that is not routinely used in car spaceframes, and when you do, the costs approach or exceed those for a composite monocoque.

As it happens, Caterham, here in the UK, is currently involved in a project to use some of that frame tech (Reynolds tubing, with 'butted' tubes - thicker at their ends, to allow welding, and thinner in the middle to save weight), with the objective of taking 10% off the weight of their standard chassis.

It would be a very light 'Seven' spaceframe that would get below 125lbs with a reasonable degree of stiffness, whereas the relatively crude and heavy Westfield FW400 carbon monocoque was 20+lbs lighter than that, and much stiffer, with more to come on both stiffness and weight.

I have designs for a 2-seater, 'budget' carbon tub for a lightweight road car (albeit not a 'Seven' type) that are nudging below 60lbs, whilst remaining much stiffer than an equivalent spaceframe.