11 replies to this topic

[Canuck]

Is anyone intimately familiar with carbon fibre monocoque construction? I'd like to explore a couple of project ideas and I'm not sure where to start. I'm looking for some technical resources as I've been unable to find what I need.

[bettonracing]

Wow.... now that's a generic question fer ya...

Try yahoo or google. Seriously. Images and word search. Then go search in some tech forums (this one included) [www.eng-tips.com, www.fsae.com>>forums] and u'll have enough information to ask a question specific enough to merit an educated response, assuming u won't find the answer in your search. (Note: I responded so I'm not claiming to be one of these highly educated responders).

No "thank u" reply required. Save the front page for the "... 9th order vibration @ 23,000rpm..." and "...depepnds on the order of differential equation used..." posts if u can. We need to drain these brains while they're still alive...

Kurt

[dc21]

New tubs cost about £15,000 and require a lot of work. It is not something you can do in your garage. In fact, you'd struggle to do an aluminium monocoque in your garage. May I ask what the project is? I may be able to put you in touch with someone who can help.

Dave

[Canuck]

Just trying to gather some background on the feasibility of doing a limited-run of tubs with a hotrod project. Rather yet another 'traditional' 'glass body on a tube-frame, we'd like to examine monocoque construction. If I understand it correctly (a fairly big if), monocoque design would allow us to reduce the weight of the chassis, thus reducing the amount of power required. That it's expensive and cutting-edge for street-vehicles goes without saying.

I have a very cursory understanding of how the tub is laid up but it goes without saying I don't have an autoclave in my back yard, nor even the remotest understanding of the engineering and design process required.

In so much as we'll have to have the tubs made outside our own facilities, it would still be beneficial to find someone on this side of the Atlantic to deal with for sheer cost reasons alone.

[Greg Locock]

Firstly, for a street machine CF is waste of money, by and large. S glass is a lot easier to handle and almost as stiff once it is laid up.

I have designed and helped build a fully composite body, for a solar car. Designing the hardpoints is critical - something the aerospace people still have yet to get to grips with.

The health problems associated with the resin systems are non trivial. Where I live we have a lot of surfboard and windsurfer makers. We now have a lot of 50 year old surf shop proprietors with chronic skin and chest problems. For this and other reasons I prefer polyester resins.

[Canuck]

There's enough talent and experience around me that if we just wanted to replicate an existing body in carbon or what-have-you, we could do a nice job of it. Engineering a monocoque that looks similar to the original but has the all-important hardpoints was the very first red flag that cropped up.

I don't claim to understand composites, but if the GPa of carbon is 500+ and S-glass is ~90, how is S-glass as-stiff?

I can't argue that from a strictly technical standpoint, CF may be a waste in street vehicles. The same could be said of 400hp sedans (in so far as our legal road speeds here are somewhat low), 180hp motorcycles and Z-rated tires.

This project began when I stumbled into a rather nice old farm truck, looking for a good home. I took some pictures and tweaked them until I had a shape and stance that I liked. The amount of modifications necessary to achieve the finished product in steel are both arduous and almost overwhelming. However, even if I managed to get the body as wide as I'd like (30s trucks aren't exactly large inside), there's not enough height within the channelled cabin to stuff my 6', 220 self into it if I'm to keep my knees from blocking my view out the side windows. By utilizing a monocoque structure, I can eliminate any structural steel from below the cabin and regain 4 or 5 inches of interior height while retaining the outward channelled appearance. There's the transmission and driveshaft to pass through the cab, but that's addressed with the additional cab width. If we're going to go through the time and effort of stretching and widening the cab, we might as well make it comfortable enough to use. It would appear that will require a monocoque design, so making a handful makes more sense than making just the one.

There's a second project we'd like to take on, inspired by the Ariel Atom (unavailable here), again with an eye towards eliminating the tube frame while maintaining the structural integrity of it all.

[MclarenF1]

Originally posted by Canuck
Just trying to gather some background on the feasibility of doing a limited-run of tubs with a hotrod project. Rather yet another 'traditional' 'glass body on a tube-frame, we'd like to examine monocoque construction. If I understand it correctly (a fairly big if), monocoque design would allow us to reduce the weight of the chassis, thus reducing the amount of power required. That it's expensive and cutting-edge for street-vehicles goes without saying.beneficial to find someone on this side of the Atlantic to deal with for sheer cost reasons alone.

I have a very cursory understanding of how the tub is laid up but it goes without saying I don't have an autoclave in my back yard, nor even the remotest understanding of the engineering and design process required.

Weight reduction is definitely an advantage with a composite structure. BUT, only if properly designed. The exact procedure for determining the number of plies and their orientation are not something easily explained over a forum. So please forgive me if this seems overly simplified. Typically, I start as anyone should, with good free body diagrams. I then use with a "best guess" laminate to started. Then taking section cuts of the loft to determine IE's, QG and JG terms. Stiffness weighting is ESSENTIAL when working with composites, which is why I include the E and G terms. From there you can determine the running axial and shear loads. Once these have been determined, you need to utilize a composites analysis program or a lot of time, paper and patience. There are some commercially available programs out there, or you can take your time and develop your own (MS Excel works fine for this). I use the first ply Hoffman failure criteria when doing quick initial sizing rather then progressive ply failure. From this you will have an idea of whether your structure is strong enough. The catch is that for most automobiles the stiffness is critical, this adds another level of complexity to analysis.

Once the hand analysis is complete I will start on the FEM, comparing the hand analysis to the FEA results. Not sure if you have access to one, so I won't go into any more detail then this.

Also, you can’t just bolt items to CFRP without some sort of insulating barrier to prevent galvanic corrosion of the fasteners/brackets. Therefore all metal parts in contact with the carbon must be made from stainless or better yet, titanium.


Erik

[Greg Locock]

I got the strength vs weight data from an analysis we did of some actual parts - I think the carbon parts came out about 15% stiffer per unit weight. That's because the realistic details you need on a part (resin, hardpoints, core) weigh the same for both an S glass and a CF structure. You would see a bigger difference on a body rather than chassis parts, thinking about it.

[McGuire]

This subject really comes home to me because I am 6'3", 210 lbs and I am way into hot rods. Channeled cars have insufficient legroom and interior volume for me, and so do 1930's vintage truck cabs. (I don't really fit in a 300SL Gullwing either, sob.) When you combine these two negative features you have an extremely challenged package in my view, if you are after a vehicle that is truly comfortable and satisfying to drive.

I would not worry too much about power-to-weight ratio in a street rod because the first part of the equation is just too damn easy to adjust.

Most hot rods are already overpowered by normal automotive standards . That's the defintion of a hot rod, basically. if you are really intending a driveable car, I would go for elegance, simplicity, and robustness of construction and let the thing weigh whatever it is supposed to weigh.

By monocoque do you mean 1) a composite floor and chassis structure bonded into the original steel cab which becomes a stressed member, 2) bonded into a CF or 'glass replica as stressed member, or 3) a monolithic chassis/floor pan structure to which the cab, original or replica, is bolted?

As I see it, #1 is right out while #2 is really problemmatical. #3 is the only truly practical path, and there a steel chassis structure will do a better job for you than a composite one. If you can design a mid-frame structure to maximize leg room and interior volume, you can do it just as well and far more easily with steel. In my opinion.

[McGuire]

A crucial factor in a channeled car is where the rear transmission mount ends up relative to the seat and driver position. An X-member is out of the question if you want any legroom...if you are widening the cab perhaps a central backbone frame or member is the way to go.

Some old hot rod tricks for lowering the floor in a channeled car...first, suspended pedals are a must. Then you need to maximize whatever volume you have between the frame rails and reclaim it as legroom. One method I have used more than once to fabricate a highly-contoured floor pan & transmission tunnel from fiberglass...first using layers of chicken wire carefully make a precise form, cheating every clearance around the frame and driveline as much as you can. Then simply lay up fiberglass over the chicken wire. Finished on both sides it looks like a million bucks, it's strong, leak-and-squeak-proof, it will get you all the room you will ever get, and I can't imagine an easier fabrication method.

[Canuck]

Glassing the floor addresses the problem of getting tall bodies into short spaces, but it fails miserably in trying something new. Half the impetus is my desire to do something out of the ordinary.

Interesting note about the final weight of components. While watching the Discovery Channel's "How it's Made" series, they detailed a light aircraft build from a Canuckistani company. The entire shell was 'glass (though they neglected to be more specific) with CF reinforcement only where needed.

In the real world, can an s-glass monocoque be designed to work? As to whether I mean McGuire's 1, 2, or 3, I mean 2. A stand-alone tub that acts as a stressed-member to which the framework supporting the engine and fr/rr suspension is affixed. Whether said tub also contains the external appearance of the project is another matter entirely.

[McGuire]

With a 1930's pickup cab we don't really have a "tub"...more like the cardboard box a refrigerator comes in, but with large holes cut out on two sides. The size and location of the door openings severely compromises any stressed-skin or inner/outer boxed construction I can imagine. Perimeter rails or a central spine/backbone will be required to take the bending & torsional loads, I have to think...a frame by any other name.

For a monocoque hot rod, a roadster with fixed doors might be a more suitable candidate.