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7Fan7

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Posted Reply with quote  #1 

Hi everyone, I'm currently doing a project at the university on Lotus-7 style kit cars here in the UK. Essentially, we have a carbon-fibre tub that replaces the middle part of the chassis where the passenger sits. So the front and the back remain with the original steel frame chassis, except the passenger tub.
I was just wondering though, if anyone had any idea why carbon fibre passenger tubs are not being used in seven replicas? Because from several articles across the web, it seems to be very beneficial, providing extra stiffness of at least 20% and a weight-saving of at least 40kg, which I believe is quite substantial. I understand cost would be a possible cause of the reluctance to adopt carbon fibre (as part of the chassis) but besides that, are there any other reasons?

Below is a similar project carried out, as you can see the middle bit (passenger tub) is made out of carbon fibre composite material and is bolted to the steel frame. 
[WP_20160316_11_03_35_Pro] 

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TOC_Admin

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Posted Reply with quote  #2 
It is an interesting question, and one that probably comes down to cost and complexity - most kit car manufacturers are "old school" engineers and steel chassis with various panelling methods are a well proven formula with well understood design principles and costs.

The closest we've probably come is to have carbon fibre bodywork panels replacing fibreglass panels in some areas.

There are kit cars that have had fibreglass monocoque's, for example the GTM Libra (picture below) which would work equally as well in carbon fibre, but I can't say it's anything I've come across on a 7 style car... and for those who like to read more into things than written, this doesn't mean there aren't any, just that personally, there aren't any I can recall.


[gtm-libra-monocoque] 
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7Fan7

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Posted Reply with quote  #3 

Quote:
Originally Posted by TOC_Admin
It is an interesting question, and one that probably comes down to cost and complexity - most kit car manufacturers are "old school" engineers and steel chassis with various panelling methods are a well proven formula with well understood design principles and costs.

The closest we've probably come is to have carbon fibre bodywork panels replacing fibreglass panels in some areas.

There are kit cars that have had fibreglass monocoque's, for example the GTM Libra (picture below) which would work equally as well in carbon fibre, but I can't say it's anything I've come across on a 7 style car... and for those who like to read more into things than written, this doesn't mean there aren't any, just that personally, there aren't any I can recall.


 


Thanks for that information. Yeah, I would have thought most kit car manufacturers are old school indeed but when you come to think of it, the industry (in particular lotus 7 style kit cars) have existed for more than 20years but there has been little or no innovation made on these cars. Besides, of course, the occasional upgrades of bodywork panels, seats, etc, but the chassis essentially has remained the same.
Interesting though that the GTM Libra opted for fibreglass instead. Certainly a huge benefit on the weight-saving side, but in terms of structural stiffness, I'm in doubt. But as you suggested, cost and complexity are major determinants in the adoption of carbon fibre. 

Thanks again for the information!

Cheers!
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warboys

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Posted Reply with quote  #4 
I suspect that the volumes that the cars are made in is also a factor, plus the need for an autoclave of sufficient size. The increased cost of the materials vs the benefits of the materials doesn't make it viable. Not sure what level of tests and approvals would be required for this to be viable too. I think carbon panels are far more prevalent now and will continue to be as the costs come down.
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Gazza

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Posted Reply with quote  #5 
Hi 7 Fan 7,

Looking at the car that the project team has put together looks very impressive and well laid out as regards to the engine bay area.

My eyes are drawn to the cockpit tub area which appear to have been made from what appears to be a carbon fibre / honeycomb sandwich material to give the thickness. 

My question is how have the joints been bonded / joined?

Matt has already raised the point that in order to provide a mechanically strong / secure joint I would think that there might be a need to lay the carbon fibre matting into a mold which will then need to be cured in a autoclave.

Also yes I have to agree with your comment on the GTM that it is heavy but it was also very strong as all of the suspension and engine parts were fixed / supported by the tub, and can personally vouch for the weight as I have built one with my son many years ago using a Honda Vetec engine.

One other issue that carbon fibre has is the potential to be the new asbestos with all the nano fibres that are generated when cutting, filing, and then breathed in settling in the lungs where they could then become a issue in years to come.

Have you had the car out on the road or track yet and how does the carbon fibre react to being twisted and shaken around.

The other questions is what are the weight saving v cost for a more traditional build?

Gazza 
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7Fan7

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Posted Reply with quote  #6 

Quote:
Originally Posted by Gazza
Hi 7 Fan 7,

Looking at the car that the project team has put together looks very impressive and well laid out as regards to the engine bay area.

My eyes are drawn to the cockpit tub area which appear to have been made from what appears to be a carbon fibre / honeycomb sandwich material to give the thickness. 

My question is how have the joints been bonded / joined?

Matt has already raised the point that in order to provide a mechanically strong / secure joint I would think that there might be a need to lay the carbon fibre matting into a mold which will then need to be cured in a autoclave.

Also yes I have to agree with your comment on the GTM that it is heavy but it was also very strong as all of the suspension and engine parts were fixed / supported by the tub, and can personally vouch for the weight as I have built one with my son many years ago using a Honda Vetec engine.

One other issue that carbon fibre has is the potential to be the new asbestos with all the nano fibres that are generated when cutting, filing, and then breathed in settling in the lungs where they could then become a issue in years to come.

Have you had the car out on the road or track yet and how does the carbon fibre react to being twisted and shaken around.

The other questions is what are the weight saving v cost for a more traditional build?

Gazza 


Hi Gazza,

I apologise for the awfully late reply. Thanks for the compliment but the kit car depicted in the picture is actually a prototype (similar to the one I'm working on) developed by Axon Automotive and installed on Westfield.
I'm not quite sure which joints do you mean but I'll answer from both the perspective of the Steel to CF joint and CF beam to beam joints.

For the steel frame chassis to Carbon fibre tub joints, these are mostly by nuts and bolts. I understand that there would be concern over the strength of these joints as they are potential sources of failure. However, a test that has been carried out reveals that they are sufficiently (in fact exceedingly sufficient) able to withstand any forces, with a relatively large safety factor applied during the FEA analysis.

To address the carbon fibre beam to beam joints (which essentially make up the Tub), they are bonded together, with sort of a "cap" connecting the beams using strong adhesive. In fact the Finite Element Analysis results, using a safety factor of 3, have proved that the strength of the joints between each CF beams are stronger than any other part of the car. A similar study conducted by Cranfield University (I think) assessed the torsional resistance of the entire chassis, inclusive of the carbon fibre tub, and it did performed well above the target (the weakest point being the torsion in the steel frame, and a bit at the nuts and bolt joints). In any case, the failure of the structure, upon crash, would first damage the steel part, while leaving the carbon fibre tub more or less intact, which of course is dependent on the force applied. But essentially, the CF part is where you and I would sit when driving, so having a safer build would definitely make us feel better haha.

Lastly, I'm currently conducting a very quick survey about this CF thb, and I have included some benefits and details of the CF tub in the following link below. So I would greatly appreciate if you could take the time to answer it; your answers would be entirely anonymous. 

https://goo.gl/forms/kyjYgLPVGPcp3OaE3

Thanks a lot for your questions, I'd be glad to answer if you have any more!

Cheers

 

 
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7Fan7

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Posted Reply with quote  #7 

Quote:
Originally Posted by warboys
I suspect that the volumes that the cars are made in is also a factor, plus the need for an autoclave of sufficient size. The increased cost of the materials vs the benefits of the materials doesn't make it viable. Not sure what level of tests and approvals would be required for this to be viable too. I think carbon panels are far more prevalent now and will continue to be as the costs come down.


Hi warboys,

Thanks for the reply and the information that you have highlighted in your post. 

Regarding the level of tests and approvals required, I believe should you use the car for racing, only then you are required to comply with MSA and FIA regulations but from MOT, having a carbon fibre tub (as part of the chassis) is not a problem; there's been some research into that conducted by Axon Automotive in conjunction with Westfield if I'm not mistaken. I agree that carbon panels are definitely more prevalent now, but those are more of a visual appeal objects rather than structural improvements. Considering the fact that the Lotus 7/Caterham 7 chassis hasn't changed much over the past 20 years, it is perhaps time for some innovations haha. 
Anyway, I'm currently doing a very quick survey, which will take less than 5minutes, I'd really appreciate if you could take the time to fill it in. It would really help us understand owner's point of view regarding the tub. Here is the link

https://goo.gl/forms/kyjYgLPVGPcp3OaE3

Cheers!
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DJT

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Posted Reply with quote  #8 
The Westfield FW400 had a full carbon chassis/monocoque, and while it was very light (400kgs with K-Series), it was too expensive to be a commercial success.  http://westfield-world.com/mod_fw400.html
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7Fan7

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Posted Reply with quote  #9 
Quote:
Originally Posted by DJT
The Westfield FW400 had a full carbon chassis/monocoque, and while it was very light (400kgs with K-Series), it was too expensive to be a commercial success.  http://westfield-world.com/mod_fw400.html

Hi DJT!

Thanks a lot for the insight that you gave. I'd appreciate if you could fill up this survey, which will take less than 2minutes to complete, which basically assesses your perception of the product. Your responses will be entirely anonymous. You can find the survey on the link below:

https://goo.gl/forms/9VRAdBDIoFmU524J2

Cheers,
7Fan7
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