This post is purely out of curiousity, but has anyone designed/modelled a lightweight chassis?

I know we have had oversize chassis's both in terms of width, length and height, but wondered if anyone had done any analysis of the weight of various tube sections and designs in addition to the tortional/rigidty tests that some people on here have modelled...

i would be guessing perhaps a BEC builder might have??

Ned.

There is a book called "Chassis Engineering" by Frank Herbert (IIRC) in which the author discusses material selection, spaceframe versus monocoque, and methods for achieving maximum torsional stiffness (includes many pictures of wooden frame models and addresses rails-style frames, NASCAR rollcage frames, Transmission tunnel frames, etc). I'm sure Amazon.com has it.

The similarities between the frame they construct in the book and the Locost frame are subtle but important. Their frame is also meant for filling the body of a sportscar and accomodating a V-6 or V-8. Their frame is also made from aluminum and uses sheer panels in lieu of cross bracing in several places (front and rear firewalls, footwells, etc).

Check it out!

-MR

Martin at MK did an ali chassis for Ron at the beginning. It was finally taken over by a guy to complete. I have some pics somewhere.

It was beautifully made and welded. You could pick it up with one finger it was so light.

Martin still has some of the special order ali box section left!
Rescued attachment OTHER059.jpg

This one as it appeared at Autosport Show.
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thanks for the replies...

2 things...

firstly, i was of the impression an ally chassis was a bad thing due to fatigue of the metal etc

secondly, the blue car in the background is a vauxhall 16v engined lola, that i have driven at goodwood (136mph on limiter )

Ned.

MK on their website in their FAQ

Q. Can I get an aluminium chassis?
A. No. Aluminium chassis suffer from metal fatigue which leads to premature failure. The ones that were made were for demonstration puposes only

Why should metal fatigue be such a problem for a well designed chassis?

A Lynx helicopter for instance has a entirely aluminium construction and I would suggest far higher loadings combined with an enormous vibration problem, and yet they rarely begin to show signs of failure/cracking less than 10 years from manufacture.

But all helicopters get closely inspected at very regular intervals...

I doubt whether a Locost would be stripped and rebuilt after 500 hours of operation...

DJ

The lynx is a bit more complicated than a locost . EVERYTHING is structural, if we cracked one of the front screens, the aircraft was grounded until replaced, apparently it was a significant part of the structure.

I think the stress comes into play is the chassis is not rigid, the locost chassis is definately not that rigid - try tying a piece of string, so it is taught, from your roll bar to the front of the chassis and then jack it up.

The constant (even if small) flexing gives the aluminium a hard time and it ultimately fails

thanks guys, i though tthis was the case.

just wondered if anyone had tried anything with different size tubing or tried building a 'superlight' or equilavent something chassis....

Ned.

ps thought the BEC guys would be the ones to try it to get the power-weight ratios as good as poss..

fair points and yes we definately 'over service' our military aircraft - i would be interested to know the life expectancy of an ali chassis though....

I suppose if you honeycoombed all the appertures (except where you get in etc!) and welded ally sheet to both sides if the HC'd sections, the chassis might be stiff enough for ally?

You could build one and then do racer type testing. In other words. Build it. Then test it until it breaks. Fix it at that point and make it stronger. Test it again until it breaks.... Fix it at that point and make it stronger.

Etc Etc until it doesn't break anymore. Draw plans of what you have.... Now we have a lightweight aluminium one that doesn't break....

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Originally posted by VinceGledhill
You could build one and then do racer type testing. In other words. Build it. Then test it until it breaks. Fix it at that point and make it stronger. Test it again until it breaks.... Fix it at that point and make it stronger.

Etc Etc until it doesn't break anymore.

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No, the real truth is Martin just can't be bothered to make them due to the extreme hassle he had from some customers. It was during his earlier times when he had hassle from Ron etc. to contend with as well. Sort of put him off further developement.

The chassis is not easy to make in ali, it takes three times longer to make and a true finished cost for chassis alone would have been £2500.

The chassis as shown in pic was not fully completed and had further stiffening done to it later. I just happened to be there when the table was wheeled out.

Martin had sussed the design out OK and done other things but in the overall scheme of things the chassis life was limited. Especially if exposed to any salt water as in Sept roads when morning de-icing is done.

When I win the lottery (names on one somewhere!) it's either spend a fortune with MK as single commission build or go for a R500.

Actually went down to Caterham Midlands last week to ogle at the Porsche Silver R500 that's in just now. Just toooo damn pricy!

Found this detail of one of the welds. It's so beautiful a weld in ali.
The square tube was special order and he had to take about 500kg of the stuff.
I have other pics but no point posting them all.
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Ned. That's how they tested the cobras prior to le-mans. Don't believe me then take a look at Cobra Farari wars. On BBC4 quite a bit.

Ned

See one of the posts in pet peeves.

the chassis is pretty minimal already......



atb

steve

Some of the hillclimb boys use ali chassis but they are"lifed" and only expected to last a certain amount of time, no good on a road car, unless you plan to keep changing the chassis!

Cheers

Chris

[Edited on 6/8/03 by chrisg]

Some thoughts..

Aluminium is no lighter than steel for a properly designed spaceframe. Yes, it is one third of the weight but it also one third of the stiffness and is much more prone to suffer from fatigue. In order to restore the stiffness in a true spaceframe you will then need to tripple the tube weight which brings the weight of the chassis back to where you started.

Audi and others have made what they call spaceframe chassis. These mass produced cars are actually much closer to ladder frames in concept.

The Lotus Elise chassis is light weight, stiff and made of aluminium. It exploits the big tube principle of chassis design. Basically tube stiffness, for a given wall thickness, goes up as the cube of the tube size. So double the diameter and you have eight times the stiffness. Make tubes as big as those down the sides of the elise and you get high stiffness for very little weight. The elise chassis can be regarded as a very clever ladder frame.

For ladder frames big tubes with thin walls are better but for spaceframes it's usually the triangulation that counts. Theoretically it's all that counts. The book lowcost chassis is about equal to a decent ladder frame and there are much worse spaceframes on the market. Most kitcar chassis spaceframes are not properly triangulated. In most cases it really is that simple.

The easiest way to reduce the weight of the lowcost is to reduce the floor, tunnel and footwell thickness from the book 16g down to 20g (1.0mm) sheet. This has a very small effect on chassis stiffness but saves an estimated 30lbs / 14Kg.

In my pictures section I've posted a design for a high stiffness lowcost chassis. There is extra triangulation around the front suspension and tube R is replaced by two Y braces. To save weight the 3/4 inch tubes in the tunnel can be deleted. Leave the arch over tube B to help support the gear lever and handbrake mount though. Panel the tunnel in on the top sides and bottom to make it into a welded steel tube.

Those mods will bring the book chassis up from about 1200ftlbs per degree of twist to about 2700. Scroll backwards on this forum and you'll find my posts decribing this in more detail.

The double Y brace across the engine bay top is better than one long diagonal and is much better than the two short doagonals used on some seven type chassis.

I hope this is of interest!

I'm intrigued (purely academically) wether the use of tesioned wire cross braces could be used to replace some of the cross bracing and save weight ?

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Originally posted by protofj
I'm intrigued (purely academically) wether the use of tesioned wire cross braces could be used to replace some of the cross bracing and save weight ?

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cymtricks:

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In order to restore the stiffness in a true spaceframe you will then need to tripple the tube weight which brings the weight of the chassis back to where you started.

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As far as I know, aluminum can vary greatly...

T-16 will give you different results than high-grade aircraft Aluminum. The weight also varies greatly, people have claimed that Aluminum is 30% the weight of steel, which may be true of some forms, but the aluminum you would need for the rigidity (given that the design is minimally modified) would be more like 60% of the weight of steel. There are also composite aluminum materials, such as AMMC, which use are a synthesis of fiberous ceramics and aluminum. These materials exceed the torsional strength of an equal steel beam and are roughly 65% of the weight f steel. The only applicatoins that I know of for AMMS are in the engine-to-differential torque tube in late model Corvettes and in the housing of some turbojet aircraft engines.

The Chassis Engineering book to which I refered to earlier has a frame constructed of 2"x2" RHC aluminum with a 1/10" beam thickness. Their frame is approximated at 200-250lbs at a size that would fit within a late 80's Camaro and is designed to accomodate a V-8. The author points out that the use of aluminum will save you weight, but the durability is only practical for a race car that will only see one or two years of use and could be hazardous for a street-driven car.

The torsional stiffness of the frame in Chassis Engineering is worked out to be something along the lines of 8000lb/deg IIRC... The Locost frame as depicted in the book is only about 1200lb/deg. For further comparison, and a Mazda Miata is around 3800lb/deg, A Lotus Elise is 8000lb/deg, and a McLaren F1 can take 25,000lb/deg. (http://www.ultimav12.ca/chassis/ - a wonderful site that depicts frame improvements on an Ultima GT-R)

I've designed my frame with 2"x2"x.1" RHS steel (since it's cheap and readily available) and 18ga sheer panels. My frame also employs a quadrilateral transmission tunnel that is 12" tall, 18" wide at the base and 12" wide at the crest with sheer panels on the top 3 sides to serve as scatterguard and crossbracing underneath (which could be removed in the event of major transmission and driveshaft work). I tried to provide triangulation in every area that needed it and eliminated members that did not give substantial gain. I also joined some 2"x2" sections of RHS to make 2"x4" beams for high stress areas like the crosspieces at the front and rear suspension. This semi-monocoque frame should yeild a theoretical 6500-7000lb/deg.

Anyway, not to get too far off topic - I understand the point about wanting to see how lightweight frames effect performance, but it has pretty well been determined that an aluminum frame would be costly and difficult to adapt for a street vehicle.

A: The benefits of lightweight materials depend greatly on your application.

Is isn't about more metal, it's about where you put it. I hope this helps.

-MR

[Edited on 7/8/03 by mranlet]

not to detract from anyones ally discussions, though my original question should have been along the lines of lightweight chassis in steel!

as has been said in the peeves and rants section i wouldn't have the first clue or skills to weld anything other than mild steel!

I was just curious as I know (have seen chassis' at show's) that caterham use some small diameter tube (presumably in tension) on their superlight chassis and wondered if anyone had substituted this on a book or near book/modified chassis...

I guess the answer would be that you loose torsional rigidity unless adding more bracing/structural members as per cymtriks mods...

thanks,

Ned.

I would think the best way to reduce weight would be to use 3/4 round for diagonal bracing and substitute some of the less highly stressed members for 18g tubing instead of 16g.

LOL, we all kinda got a little absorbed into the idea of exotic materials, huh?

You could probably preserve the torsional rigidity if you essentially build one big trans. tunnel that would hook up to the front and rear suspensions.

One of my origional plans was pretty much one big piece that looked somewhat like a chicken bone with a groove cut out of it on the top side to hold the engine. The addition a thin bulkhead front and thinner one rear joined together by 1.5" RHS beams outboard and 3/4" tubes cross braced gave a very rigid frame considering its simplicity. The frame that I have now is really just a beefed up version of the same thing in order to accomodate the higher power levels that I have planned for it. Although, my current concept has really been developed into something more than the origional.

With a strong enough torque tube, you might be able to bold seats, suspension, steering, and fuel tank to it and have yourself a superlight - just don't get in an crash.

-MR

Well, for me at least-

Taking 30 pounds off the driver is the most cost effective way of getting a lighter car. Except for the cost of my new wardrobe.


My take on this is that it is easiest to get weight reductions by attacking the heavy stuff first. I saw one engine that a guy spent 3 months in evenings cleaning up the block of EVERY casting feature that wasn't required. Seems it pulled 15 pounds of. Smokey Yunick, on one of the stockblock chevrolet Indycar engines in the early '70's ground close to 30 pound off an engine block.

Do we really need big car (2500lb) rear axles for these things? Can we get away with a Spridget or (in the US) a Datsun 1200 axle? The banjo axle in my MGB weighs about 40 pounds less than the later tube type. So I guess I don't need to loose those 30 pounds- and it's unsprung to boot.

Same with the fronts- are we putting big brakes on for street cred or do we really NEED them on a 1200 pound car

The attraction for me in BEC is that you cut out 150-200 pounds off the CEC configurations.

Want to take some weight off the frame? Look at larger thinner section steel tubes. I think cymtriks underestimated here- I believe it's the 4th power of diameter change, not the 3rd, that affects tube stiffness as a function of cross section. This tubes will have the same load carrying ability in tension and compression as the smaller tubes, but may well be lighter (haven't done the calculations, so I'm not going to state this as fact.)

Everyone must (and should) follow their own path. Listen to others, and follow your own advice.

quote:

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Originally posted by mranlet
You could probably preserve the torsional rigidity if you essentially build one big trans. tunnel that would hook up to the front and rear suspensions.

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That's almost exactly what I'm talking about!!! Great picture Gecko!

With a little re-proportioning, that is essentially my frame plus a little extra metal for footwells, side rails, roof, etc. for protection...Oh, and I use a mundane RB26DETT in lieu of a turbine

How would you harness a turbine's power into shaft drive anyway? Do you just hook a shaft to the turbine's shaft?

Ned - There you go, frame simplicity at its finest!

-MR

Regarding the responses to my earlier post an aluminium part of the same size as a steel part will have about one third of the stiffness. For a spaceframe this means that you need three times the cross sectional area as its stiffness is mainly derived from tubes in tension or compression. So no weight saving for a true spaceframe of the same stiffness. Strength is different. A high quality material alloy part may be stronger than a low quality material steel part. It will still bend or twist three times as much though. Many components are overengineered anyway so changing to alloy often gives a worthwhile weight saving for no real world reduction in strength. If stiffness is an issue then for beams, as in ladder frame tubes, levers and bell cranks, the part can be made about 50% deeper in cross section which restores the stiffness and still lets you have a 50% weight saving.

As for tube sizes, for a given wall thickness, the relative bending and torsional stiffness is proportional to the cube of the size. The actual stiffness is the fourth power as pointed out above.

Must dash! We have to be somewhere else and the wife wants to be on the road.

If you look at say mountain bike frames, alu frames are significantly lighter than steel frames, last for years, and can be stiffer too, all at the same time. The stiffest frame Ive ever rode was a specialized M2 made from an Ally based alloy, (Duralcan I think), which was featherweight and extremely stiff.

The tubes on ally frames are generally of greater diameter than their steel counterparts, but of thin wall section. If you tried to build an Ali chassis with tubes the same small diameter as in the steel version you could be on to a loser, but if you up the diamter of the tubes, then you get the stiffness required to avoid fatigue and can use fairly thin guage to save weight...

I would actually like to know if anyone has much knowledge of using Cro-Moly as a chassis material and the ins and outs of construction methods using it.. ie welding / brazing and whether heat treatment to de-stress it is required??

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How would you harness a turbine's power into shaft drive anyway? Do you just hook a shaft to the turbine's shaft?
-MR

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