Any Stress/Design Engineers and Fibreglass pro's?

[skid]

Hi Guys,

I have a little project on the go for my car and need some help.

I want to fit a hard top to my Cobra though all manufacturers who have done this have problems with air pressures at high speed. Dax had their rear screen pop out!

I'm looking for advice from Stress/Design Engineers, I have many pictures from all profiles of an existing roof and details of fixing/locating methods.

Does any one know of a good fibreglass specialist in the Thames Valley area? I'm possibly looking to modify an existing roof, or make a mould and lay up a new one.

All help would be much appreciated.

I'm also posting this in general enquiries.

Many thanks.

Skid

[chris_n]

Presume you've seen what Dave Brookes from DB replicas did to fit a Hawk hard top to a Dax ...

thread on cobraclub with details

End result was impressive to say the least ...



Might be worth e-mailing him through his website www.v8cobra.com ?


>> Edited by chris_n on Thursday 12th February 13:48

[skid]

Cheers Chris

Actually I'm a customer of Daves and I'm going to use one of his roofs, but he doesn't guarantee it over 50mph and his fiberglass specialist is pissing him around a bit. Apparently Dave has passed on the project to this guy, but it's still sat in the fibreglass workshop complete with Dax body and screen (at Dave's expense) but with all progress in it.

I wanted a roof from day 1 but getting pissed off myself as need it for Le Mans. It's not Daves fault.

I only have a few months and need professional help bastardising one of Daves roofs with additional strength and supports, plus aerodynamic/stress advice re the screen etc.

All help much appreciated as time ticking by far too quickly.

Skid

[Purple AK]

Hi Skid.
I think Daves top was a modified Hawk? Although he may not want to entertain you it may be worth trying to talk to Gerry, His customer with the yellow LeMans races it! So one assumes it is up to the stresses involved.
Cheers Chris (Resident Muppet!)

[chris_n]

skid,

Please keep us updated on anything you find - I will follow this one with interest as I have in the past made enquiries of Dave myself for my Dax

I have considered buying a Hawk hardtop myself and trying to replicate what Dave has done, less the clever bootlid, as with all due respect to him that seems to be the hard bit of what he's done and I wouldn't be that bothered about keeping the boot useable witht the hard top on. Trouble was that Hawk want about £700 for a Le Mans style hardtop and at that price I wouldn't want to ruin it!

My immediate thoughts on the rear screen problem (as a bodger rather than any kind of expert!) were that relieving pressure on the rear screen is always going to be more effective than resisting it, so to that end came up with the following thoughts:

holes or vents let into the rear screen would make a significant difference though could obviously have an adverse effect on weather-tightness.

Alternatively it ought to be possible to find someone who could put louvres in the flat perspex sheet that formed the rear screen.

Lastly, you could make an ally frame for the rear screen then mount the top on hinges and rig up the bottom with little pop-out catches like you got in side rear windows of older cars. Opening these up would make the bottom edge of the screen stand away from them hard top, and relieve any pressure.

HTH.

Chris

[grahambell]

Might be worth you having a word with Andy Burrows at Gardner Douglas. Not sure how well their hardtop would fit a Dax body, or what speed it's OK up to - though knowing Andy it won't be low! Good things about the DG harptop are that it doesn't fit across the boot lid and it has small gullwing doors incorporating the side glass. See www.gdcars.com

[Avocet]

It would look daft but you might be able to fit a roof spoiler to destroy the airflow over the screen and thus reduce lift on it. That said, there are plenty of fast cars whose rear screens stay in place. What were you going to make the screen out of? I think that as long as the flange has a big enough area and you use the correct bonding agent and primer, you should be fine. Call in at an "Autoglass" type depot and see if they'll tell you what they use. If it's a "Perspex" screen, I think "Sikaflex" would probably work well. Use their high-strength 295UV (ultraviolet resistant) adhesive or have a look on their website.

[skid]

Hi Guys,

Thanks for all your comments.

Had a chat/email with Dave on Friday and things have progressed with the Glassfibre specialist. However he's developing it with a hatch to allow those with rear mounted petrol fillers access to them.

I'm not happy about having a petrol tank venting inside the canopy, or a hatch which I'm sure will be a weak point in the structure.

I am thinking about venting the rear perspex with 3 circular holes so pressure should equalise at speed. Unfortunately I don't understand enough about aerodynamics to estimate if holes higher or lower in the screen would be best. As for weather tightness you can buy translucscent rubber 'bungs' which I will use to plug the holes in bad weather. It should be a problem plugging these holes as don't expect to be travelling at speed in bad weather.

I'm looking to get the top without the hatch and would rather spend a few extra quid on relocating a recessed aston style filler onto the rear wing so wont need the hatch anyway and wont be worrying about the vapour venting in the cabin.

I will keep you all informed as I'm sure many will be interested. I will keep with the Dave Brookes design (derived from Hawk cars but with MUCH modifications) as it's the more streamlined 'LeMans' style which the GD isn't.

Cheers again.

Skid