Reinforcing base of S4 Screen

[Bruce Crowthorne]

The base of my windscreen frame is cracked where it joins the wing.

I assume that this is a fairly common issue.
How have other people re-inforced their cars?

I am thinking about putting in a reinforcing rod in the "A" post and running it through the upper part of the wing and joining it on to the existing reinforcing structure inside the wing.
Does that sound like a good idea, or has anyone got a better mod?

Thanks
Bruce

[bill308]

My friend Tim used carbon fiber to repair his S3.

Bill

[Elanintheforest]

The most common cause of this is people using the pillar as a steady or something to grab onto when getting in and out of the car. If the fibreglass has cracked right through then you're going to have to repair it properly on top and underneath. Otherwise I would just treat it like a deep gel coat crack. I can't see any amount of re-inforcing with tubes, carbon fibre or metal struts doing anything to protect against the gorilla, it will just transfer the stress somewhere else! Same goes for the areas around the door handles, boot hinges, filler cap etc.
Mark

[nebogipfel]

Bruce, I could never get a definitive answer on this one because I'm pretty sure I can remember seeing a S3 shell with reinforcing rod in the corners of the screen pillars. It may have been added at some point in the cars life of course.

When I restored my S4 shell I added 1/4" mild steel rod into the corners and glassed it in. I actually ground a recess for it first. I took mine from about half way up the pillars and well round the corner into the area under the wing.

I don't think there is anything to be gained by joining it to the existing framework providing it is well glassed in.

You do have to take care to get it as neat and close as possible.

I think it adds considerable strength in that area but do agree you shouldn't let people drag themselves out of the car by the screen pillars!

[Bruce Crowthorne]

Thanks John. Sounds like my intention may be right.
I was considering griding a slot in the upright, drilling through the A post and glassing the rod onto the wing in a similar fashion to the existing strengthening structure.
I'll try and get some stainless rod, just in case.

Thinking about the Carbon fibre idea, I have a piece of woven carbon fibre here I may use that to back fill and strengthen the slot I cut.

Bruce

[nebogipfel]

Bruce,

I used bright mild steel because it was readily available and easy to shape. I don't think rust is an issue because it is fully glassed in but I guess stainless would eliminate the possibility.

I suppose carbon fibre might add a bit of strength. A downside might be the difficulty of making it conform to the shape in a confined area ..........perhaps it would work on the outside of the pillar and into the top of the wing?

I certainly added some extra layers of glass to both the inside and outside of my pillars but there is a limit to how thick you can make it.

It is a weak area on the Elan shell and I remember one car in particular where the pillar had actually detached completely

[M100]

Adding carbon fibre to the layup in that area will IMHO do next to bugger all. Unless the reverse side of the existing laminate is properly abraded and completely dewaxed any additional material will easily delaminate. The reverse side is the best side to beef though, but glass fibre is sufficient for the job if all the existing cracks are ground out and the layup is done properly.

Laying carbon up on the outside and running the layup into the wing area is just asking for fabric print through and gelcoat/paint cracks as the unbalanced lamination moves about as it warms and cools in service. In addition wetting out carbon cloth with polyester resins can sometimes be very difficult and the alternative of using epoxy is not recommended either as the coefficient of expansion of epoxy layups is different to polyester.

Adding steel will distribute the loads to somewhere else which might be a good thing (but smells slightly of TVR) To realistically make the existing screen pillar stiffer you need to increase the thickness of the layup (with a couple of layers of glass cloth/csm), or radically enlarge the cross section (maybe by modifying the inner trim and foam filling the enlarged core)

[nebogipfel]

The problem is you cannot radically increase the thickness of the pillars without encountering problems with the fit of everything but you can make them a bit beefier without re-designing the car.

I agree that in theory adding steel will redistribute the load to some extent but the pillar is much more likely to crack at the corner than halfway up its length.

The glass laminate should be properly ground out and re-laminated from both sides and when done properly will be as strong or preferably stronger than original.

Mark, there is no reason why a woven texture should show through the final finish if the repair is properly repaired and prepared prior to painting.

Can't comment on mixing carbon in with glass but I would suspect that if it is possible to properly wet it out it would give no trouble. That said I would also suspect that it would offer nothing over a well made glass laminate especially in such a small area and like woven glass would be a bugger to wet out and form properly into the corners

Bruce, I didn't, but I can't see a reason why you couldn't extend the steel rod nearly to the top of the screen if you wanted but it would be a bit tight.

All I can say is that it worked for me

[bill308]

Bruce,

It's about 10 years since Tim had his windshield frame repaired with carbon fiber. the repaired area still looks perfect.

I agree that one has to be carful with the finished product. It is not designed to be used as a hand hold to get in and out of the car.

As to the repair, I did not see the repair layup until it was finished, but if it was me, I would follow good practice and grind out both the front and back side in the affected area, plus some, to a tapered shape sufficient to ensure perhaps 3 layers of cloth can be accomodated accros the crack area and still arrive at the correct finished geometry. As in any structural shape, highest loads will be at the surface, so internal layers will provide progressively less benefit. I don't know how difficult it will be to wet out the repair layers of carbon fibre, but we're talking about relatively small areas here, so it seems with care, a proper job could be done. Use of epoxy, like perhaps West Systems epoxy, used for boad building, might be advantageous due to it's superior mechanical properties relative to polyester resin. Orientation of the fibers, primarilly in the length wise direction, will also add additional strength. I sometimes do this with coarse fiberglass cloth where I strip out some individual yarn, slightly separate the individual strands, and orient them in the desired direction, in this case axially or lenght wise.

Use of a steel rod for the core, is also a good idea. For this application, I would also prefer to use stainless, especially if polyester resin were used to bond the cloth or matt overlay. Polyester resin is not impervious to moisture, epoxy is, if the overlaying thickness is at least 5-10 mills.

Different coefficients of expansion due to temperature are something to keep in mind. A steel core probably should be uncoupled from the overlaying layup by not going out of your way to ensure perfect bonding with the steel. If one looks closely at how steel rods are glassed into the Elan body, it appears some axial movement is allowed as the joint is not completly filled, just lightly wrapped. Polyester resin will likely expand and contract more with temperature change than will a glass/restin matrix, especially if the lenght is significant. Carbon fibre expands and contracts an almost negligible amount due to temperature change, that's why they used it on the Hubble space telescope framework. Still, over a limited length of say 4-5 of the repair the relative movement of the material is small and the underlying glass matrix should accommodate the movement due to creep at the repair interface.

A proper repair should be at least as strong as a new frame and be visually undetectable.

Bill

[Bruce Crowthorne]

Thanks Guys, sounds like a combination of a rod (stainless if i can get it) and grinding back the area and reinforcing it with resin and cloth is the consensus.
I'll skip the carbon fibre idea.
I have some fine glass fibre woven roving which a can use and apply several layers in a relatively small thickness and align the fibres to take the maximum stress.

Not a lot I can do to avoid gorillas using the screen to pull themselves out of the car.... however I use an Elise as a daily driver and if a gorilla swings on their A post often enough the windscreen end up cracking!!! So I am used to that one.

Thanks
Bruce

[Bruce Crowthorne]

I managed to fix it at the weekend. I ground back the glass fibre at the joint between the wing and the frame and discovered that the frame was originally made as a separate part from the shell.

Mine has a steel reinforcing rod in there already, so that solved that problem.

On mine it appears that when it was built someone didn't make the joint particularly strong. There was a gap of about 5mm between the frame and the wing. They had pushed in some chopped strand mat and just coated it in resin!

I cut out the crud and bonded in some woven mat to fill the gap.
I ground back the glass on the outer side and put on three layers of fine woven mat.
On the inside I ground it back and put in a further three layers of the fine mat.
In each case I laid the weave in the direction of maximum stress.

Since I had uncovered a part of the metal rod, I unpicked some fibres from a coarse woven mat and back filled the slot with the fibres and resin, I made sure the fibres were laid in line with the stesses.

Thanks for all the opinions and advice
Regards
Bruce

[nebogipfel]

discovered that the frame was originally made as a separate part from the shell.

Bruce, I think it's more likely that the gap was a previous break which has been repaired. I'm pretty sure the screen frame is layed up as part of the shell.

Glad to hear all went well. Sounds like you've made a top job

[bill308]

Good job Bruce. Glad it came out well.

Here's a trick I learned when extra strength is required in a repair.

The strength of the fiberglass/resin matrix is really all in the glass fibers. It is sometimes possible to lay up a repair or new part in such a way that the ratio of glass to resin can be improved. To do this, one can lay up extra layers of cloth and resin and squeeze a good portion of the resin out, leaving a higher relative glass content. Squeezing is typically done with board or plate, lined on both sides of the repair with wax paper, and clamped with C-clamps, screws, bolts, Vice-Grips, weight, or whatever you have available. This works particularly well with flat panels where you have access to both sides. Just be careful not to squeeze too much resin out or you can end up with a somewhat dry matrix. Still, you can end up with twice as much glass content for a given volume, which translates into additional strength and stiffness.

Bill

[M100]

Bill, that's similar in some ways to the method used by Lotus on their VARI moulded cars, just use the minimum amount of resin that you can get away with.

It was used on the Elite/Eclat/Excel/Esprit/Elan M100 and in a modified form on the Elise S2 (the S1 was hand layup)

Basically it was done more to save a boat load of cash as resin is expensive and glass is cheap. Broken panels sometimes look like chalk dust though

The only Lotus cars I think I've seen a published fibre fraction for was the Type 14 Elite. Somewhere in Dennis Ortenburgers book is the price per pound of glass and resin.

I've a few scrap Elise and M100 panels around, one day I'll weigh a sample disc, burn it and see how much resin Lotus used.

[ianf]

I find a steel reinforcing rod keeps the screen pillars safe. I keep it under my seat and use it to rap the knuckles of anyone who tries to use the pillar as a lever to get out of the car. This becomes a more useful accessory as one's friends grow older!

Ian