Is it common for a windshield to break when driving a 914 after installing the roll cage?

I just drove the car around the neighborhood after tuning the Webers. I now have a diagonal crack running from the upper front corner of the passenger side window down to the lower center of the window.

I rebuilt this 1970 914 after pulling it out of the desert two years ago. It has non adjustable KYB shocks in the rear and Bilstein struts in the front. I installed a stock front and rear swaybar. Both are non adjustable. I have the front end lowered as far as it can go. The rear is still near stock height. I will drop it when I get coil overs.

I also put in a 6 point cage kit from Tangerine. My welder followed the installation instructions to the letter. Very happy with the roll cage kit.

The car does ride really rough and hard. There is zero body roll. I did notice deflection between the upper frame of the windshield and the top front bar of the roll cage when I would go over rough sections of road.

Is it that after stiffening up the frame by adding the roll cage and stiffening up the suspension that the 1970 windshield finally broke after I hit a few bumps in the road?

Is this common? Is this why most racers just cut the front windshield off?

I don't think this is really common but it also does not surprise me. I broke a windshield last year when I put a wheel off-track and it bottomed really hard. My cage is welded-in at 10 points and the suspension is pretty much all bearings and heim joints (no rubber). The combination of a very stiff suspension and having everything tied together with the cage must transmit the shock all thru the car. With a stock car there is more chassis flex and rubber to absorb such things.

Glass does "age"... but very slowly. Combine that with a stiff suspension and possibly old butyl rubber holding the glass in, and the weak link becomes the glass itself.

What spring rate are you running? Is anything binding? Perhaps from lowering the front too much, you're actually bottoming the front a-arms on the frame?

Have you replaced the windshield before?

I put in Chris's cage too and added to it. I also welded a tab on each side of the windsheild frame (near the top) to the cage. I used butyl to install windshield. No problems with cracked glass after many autocross and timetrials.

No, this is the original windshield. I am running stock torsion bars in the front and don't know the spring rates in the rear. They came with the car. This happened after I lowered the front end. Could be bottoming out.

I was planning on adding the tabs too. I am an aerospace structural engineer. The cage is good enough to pass the tech inspectors, but there is no lateral stiffness in the front of the cage. There should be a bar under the dash going from the left front pillar to the right front pillar. Also a diagonal is needed from the center of that bar down to the point where the passenger side front pillar attaches to the frame. I also need to add struts between the front pillars through the firewall to the top of the shock towers.

I am debating as to whether to just cut the windshield off or not. This car had rust int he hell hole so it will never be a concourse car.

IMHO, cutting the windshield off should be left as a final solution to a very specific goal of making the car competitive in certain classes. Otherwise, it's really going to make it hard to be able to sell to someone else in the future (it will always need to be stored inside).

From the way you're describing ZERO roll/no complacency in the front end, I'm betting on it's bottoming out. Jack it up and look for rub marks.

Sorry to tell you, but YES.

I had NEW glass on my gray AX car which had just a welded in bar with a forward bar to the pass long. Jacking the car up by the front donut cracked my new windshield.

I replaced it and only jacked the car by the rear donut from that point on. The whole car raised nicely from the rear.

I broke a 40 year old windshield the first time on track when I got a little "air time" after cutting a corner a little too tight.

Replaced it with lexan my self

Did you make a pattern first or how did you get that fit so nice?

I made a template from hard board and routed it:

No it is very common for this to happen. When you Track a 914 with Rubber it was never intended for, the Lateral G's exceed the car's Torsional Limits. Putting a Cage in can put different forces within the car. Especially if the cage was done in correctly.

The Factory knew there were problems with windshields cracking & Windshield A-Pillars flexing. Here is the Factory Fix on a Factory GT

Click to view attachment

What thickness lexan did you use and did yo have to do any heat forming? thx, Bill

different racing clubs have different thickness requirements.

my memory is that 1/4 often works...
but verify that

Here's why the windshield broke.....the original glue had dry rotten and the windshield came loose. Again this is a California car I pulled out of the desert. Every piece of rubber on the car had dry rotted and has been removed. I stripped the chassis down to bare metal...This car is strictly for vintage racing and will never be returned to street condition.

In previous test drives, I heard something rattling. I could not tell what it was. Now I know. It was the loose windshield banging against the frame...it finally broke.

On close inspection, the metal trim around the windshield had finally come loose and I could see gap between the windshield and the frame(see photos).

Word to the wise: Have your OEM glass windshield removed and professionally reinstalled if you plan on racing the car. Without safety tabs, this windshield could have popped out on the track. That would have REALLY sucked.....

I will look to make a lexan windshield like Stu Gray.

Thank you everyone for your kind words of wisdom and help with this problem. Thanks 914World....

That's why I made my comment about the butyl rubber drying out. By now, all of these cars need the original glass reinstalled, regardless of whether they're racing or not.

yep...this one will sneak up and bite you. I would have never know had the chrome trim not come loose.

1/4" and no heat forming required. The flat sheet will conform.

Stu

How much for the flat sheet lexan?

Thanks


Eddie

Bills Racing ...

The front glass on my teener had a big diagonal crack. IIRC it was after the cage.

I suppose it can be expected ...

Big 3.2 six
Big torsion bars, 225lb springs, big front sway bars,
Big 235/255/17 R tires
Big Wilwood Superlite II brakes.

Something has to give ...

Mine has been replaced with cheap glass, foam tape, universal rubber trim and racing clips.

A common occurrence back in the day with the 914GT.....even with the factory mods. In one word :FLEX. Pissed off some Porsche racers who bought them.

When you say foam tape, is that double sides adhesive?

I assume letting the window float int he frame is somehow the answer here. The addition of the racing clips then keep it from popping out.

Thanks everyone for the discussion. I think we have touched on something here everyone on the site needs to consider....914 windshield can break unexpectedly. Best thing to do if you plan on racing the car is to have the windshield removed and reinstalled before this happens....or go to a polycarbonate(Lexan) substitute.

http://www.plasticareinc.com/Merchant2/mer...Code=poly_sheet

Polycarbonate Clr 1/4" (.220) for $130 @ 30in X 60in
Self-Aligning Rivet-nut inserts: 25 for $30 Ordered with tool
Self-Aligning Rivet Nut, 10-24 Internal Thread, .020"-.100" Material Thickness
http://www.mcmaster.com/#catalog/121/3308/=vyf3d2

And neoprene material: ¼” thick ¾” wide self adhesive one side