There's been a lot of discussion about cutting weight out of a 968 recently, some of it focused on the hatch glass, so I thought I'd calculate the weight of the glass hatch compared to that of a lexan one. My hatch glass measures about 42" x 60" (or 106.7 cm x 152.4 cm), and is 5.5 mm, or 0.55 cm thick. This gives a volume of 8943 cm^3. The density of automotive window glass is about 2.5g/cm^3, which gives a weight of 22,359 g, or 49.3 pounds (453.6 grams in a pound). This is much heavier than I had thought - I re-measured the glass to make sure I wasn't drastically off (I wasn't).



Lexan hatches are available in thicknesses of 1/8" and 3/16". The density of lexan (polycarbonate) is 1.32 g/cm^3. Using 3/16" (which is 0.47 cm thick, or only slightly thinner than our glass hatches), the volume of a 3/16" thick lexan hatch is 7642 cm^3, giving a weight of 10,088 g, or 22.2 lb. So, at least in theory, the weight of a lexan hatch is about 27 lb less than a glass one. Please check my math.



This is a lot greater weight savings that I had thought, enough to make me question my results, especially when Lindsey Racing lists the shipping weight of their 3/16" lexan hatch at 20 lb. Does this match anyone's results? Anybody remove their hatch recently (e.g. to re-seal it) and weigh it? 49 lb just seems too heavy to me, but that's what my calculations give.

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are you sure about those measurements? that seems pretty big. after all, it's not rectangular either. did you factor that in?



the OEM hatch complete weighs 59.4lbs (dug that one up)



i have a hard time believing the glass is that thick. that's .217". i've never seen glass that thick in a car. it should only be 4.8mm or so, like the rest of the glass (roughly 3/16"). that would reduce your figures by about 13% or 7lbs. i thought it was in the upper 30s, but i'll buy 42#.



3/16 glass weighs 2.45 lbs/sq ft. with your dimensions, that makes the glass again 42lbs or so.



3/16" lexan weighs 1.071 lbs/sq ft. making it weigh 18.74lbs



that makes the savings, assuming dimensions, of a bit over 23 lbs.

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I measured the glass thickness around the edge with calipers (smashed hatch) and it is 6mm thick. Your 27lb calculation is very close to my estimate I gave in the lightweight battery thread (12kg IIRC).

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pretty sure that glass is laminated. that would make the center of it plastic, and a lot lighter. if it is indeed 6mm thick, then it is almost certain, which would make the center layer about .030 of that thickness, and therefore again reduce the square footage weight figure.



even so, we know the entire assembly weighs 59.4lbs



all we need to know now is the real weight of the frame, or the real weight of the assembly with lexan.



i should have that in the near future, as there is a car with one on it coming back here soon.



whether it is 23 or 27 though, it's really moot, as that is not enough weight to mess with for any car to be used on the street. that was confirmed to me when i saw this car after having had it on there for a few years. it was very hazy, and like all acrylics, it was very yellowed due to sun exposure. yes, losing weight can make a difference. as long as you don't plan to leave the car in the sun, and are ok with regularly polishing it, it may work out for you. i've gone that route myself over the years. it was always on a dedicated track car though, with peel-away films on it (we learned to add that one the hard way the first track session, when it came back all pitted and scratched from track debris)

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A little off-topic because it's not about the weight of the hatch, but if that glass ever breaks the only replacement choice will likely be the lexan alternative, unless you're lucky enough to find a used 968 hatch at some salvage yeard..the latter being a very low probability, IMO .

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21.5 pounds for the hatch w/lexan and another 10.5 pound for the spoiler, stop lamp and hockey sticks. Bringing the total weight when I did mine to 32 pounds. Don't know how much the rear wiper weighs if you still have that. I'm back to glass. The lexan vibrated in the middle with the car running,and was the wrong tint compared to the rest of the glass. http://www.968forums...__1#entry150045 The hatch glass is solid safety glass not a laminate. DS968- You can use a propane torch to heat up and remove the glass from a 944 frame, Paint stripper to remove the old primer, re-primer the glass and set it into your old frame. The old primer is the problem with adhesion and the reason it separates. I did it and now have a spare. Here's a couple shots of the glass(upside down) before and near the end. I was going to publish the procedure but never got around to it. I agree with Flash. Lexan is not well suited for the street. But if it's your only choice, it will have to do.

       

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safety glass? really? that's surpising, given the size and location. interesting.



59.4 less 32 is again 27lbs, though based on your weight, i suspect your lexan is only 1/8" thick, because at that weight (21.5) it would mean the hatch assembly itself only weighed 2.76lbs. with 1/8" it would make the hatch assembly 8.69lbs, which seems more likely.

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The 944 glass is the same ?! Wow, had no idea - I had two 944s for over 18 + years and always thought the rear hatch was a couple of inches smaller all around than the 968 one ( and not just the spoiler, lol ) , but just goes to show you that looks can be deceiving .

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could be, at least on the 951 and probably the S2 - the only difference in part number is the first 3 digits (968 vs 951)

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I think my estimate of 60" wide for the hatch glass was a little large - it averages probably closer to 55". The 42" for-and-aft measurement is pretty accurate, though. I measured the thicknes through the hole where the wiper goes, and it definitely looked like solid glass, and I'm confident in my 5.5 mm measurement. This would bring the theoretical weight of the glass to about 45 lb, and that of the lexan to about 20 lb, which is closer to Lindsey's shipping weight estimate. Any way you slice it, the weight savings is pretty substantial for a piece that high up on the car - quite a bit more than I had expected.



But from the inputs from multiple people, I agree 100% that this isn't something you'd want on a street car. It's also superfluous for a noncompetitive DE car. I'd only do it if the demand, and therefore the value, of 944/968 glass hatch windows suddenly shot up, covering the cost of converting to lexan.

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while it may seem like a lot, you have to remember that it is spread across the back of the car, mitigating it's effect in polar moment. i would guess that only about half that weight will really have an effect in body roll. the other half will be very minimal in effect.



also, there is a certain amount of stiffening the glass does that lexan will not, as is indicated by the vibration issues noted above. not sure how much, or if it is a real issue, but it would be there to some degree.

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That makes sense, but I have a follow-up theoretical question (hopefully still relatively on topic, because it involves the weight of the hatch window). I agree that the impact on the polar moment of inertia of a lightened hatch window will be diluted by the fact that the weight savings is spread over a relatively large area. But doesn't the fact that taking 20+ pounds off such a high point of the car, by virtue of the effect of lowering the center of gravity, allow you to theoretically lower the car a bit more? The reason I ask is that my understanding is that it's the vertical separation of the car's center of gravity and its roll center that drives a significant portion of the car's tendency to roll in corners, and that the closer these two are in the vertical plane, the flatter the car will corner. You've spoken a lot about the fact that lowering a car too much is bad, I presume largely because it increases the separation between the C of G and the roll center. Wouldn't lowering the C of G, which is actually fairly easy to do in a 968, mitigate this effect, and allow you to lower the car more than would otherwise advisable?



And yes, I'll start a new thread on the whole topic of how low you should go, but I'm curious as to your thoughts of the impact of taking weight off a high point of the car like the hatch window on cornering attitude. Thanks.

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yes, it has that effect. i suppose it might calculate to a small change of effective lowering, given that 20lbs of a 2900lb car is .6% of the weight.



yes, you can change the roll center of the car. there are control arms that can do this, and coupled with lowering the steering rack, you can get there in the front. kind of hard in the rear though.



regarding removal of weight back there, i would have to see the car on a chassis dyno to see what's going on in the rear to be sure, but i suspect it twists quite a bit back there, which is why cross bracing is added in roll cages. that's why i see so many cars without cages that are sprung too stiff jumping all over the place. there is a point where the chassis acts a bit like a spring. it's the nature of a unibody car. it's designed to transfer loads across the car. when you get too much spring, it can't do that anymore.



so, yes, you can get some benefit of the weight loss, but i question whether or not that is actually a good thing after factoring in everything else.



this is exactly why they put the boat anchor in the cabs. it was needed to slow down the twisting of the chassis.

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Oh. When you said 'boat anchor' I thought you were referring to the spare tire. You must be referring to the big 'ballast weight' in the RR fender well?

I've wondered what the heck that thing was for. Any potential benefit in removing it? How much does it weigh?

Thanks

--Michael

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[quote name='flash' timestamp='1404261900' post='159427']

i would have to see the car on a chassis dyno to see what's going on in the rear to be sure, but i suspect it twists quite a bit back there, which is why cross bracing is added in roll cages. that's why i see so many cars without cages that are sprung too stiff jumping all over the place. there is a point where the chassis acts a bit like a spring. it's the nature of a unibody car. it's designed to transfer loads across the car. when you get too much spring, it can't do that anymore.

[/quote]



Now this is off topic, but I have to profoundly agree with that statement. As I mentioned to you on the phone once, after installing my new suspension, on the drive to the shop to do the weld-in roll bar/chassis brace install, the car drove horribly - you could literally feel the chassis flexing all over the place. But once I got the brace in there, it was a completely different car - now, despite the firm springs, driving it on the street doesn't bother me at all. I feel like I could drive it a thousand miles without any problem. Interesting how this stuff all interacts.

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mlb - benefits - livelier suspension. less weight, therefore quicker car.



detriments - new corner balance needed. can be a bit twitchier. the rear will be less settled.



cloud - yup - that's why i try to avoid blanket recommendations on setup. i need to know everything about the car, from its use, to the driver style, to what else has been done.

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It still amazes me how much of a performance difference you can actually feel with vs. without a passenger in your car. With just a 150 lbs person riding along ( so less than 5% of the car's weight + driver ) the car "feels" as of its at least 50 % slower, not 5% slower. Ugh, that sucks. No wonder so many are obsessed with every ounce of potential weight savings .

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lol - i think your buttometer needs serious calibration. when i have a passenger, or a full tank, versus not, the difference is actually very close to the percentage of weight. 0-60 is a bit less than 2 tenths of a second. 1/4 is a bit more than a half second. it's not the end of the world. the car does feel a bit heavier in the corners, but again, not the end of the world.



that being said, i have a couple of changes i am working on to reduce some weight (other than my fat ass). i've added quite a bit to the car over the last couple of years, improving creature comforts and such. i am now working on pulling some of that back a bit with lighter materials.



i'm not going nuts over this though. it just doesn't make that big of a difference, and i have more than enough power to make up for it.

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[quote name='flash' timestamp='1405741718' post='160282']lol - i think your buttometer needs serious calibration. when i have a passenger, or a full tank, versus not, the difference is actually very close to the percentage of weight. 0-60 is a bit less than 2 tenths of a second. 1/4 is a bit more than a half second. it's not the end of the world. the car does feel a bit heavier in the corners, but again, not the end of the world.[/quote]



Maybe my buttometer does indeed need "sensitivity" training and admittedly it's always been poorly calibrated since for example I could not feel much of a difference in the way the car rides with the strut brace vs without, or even after a completely different suspension was installed vs the stock set up, except when the shocks are set at almost the higher degree of stiffness or soft ends of the scale, anyway, back to the added weight seat of the pants feel. I would tend to question the performance numbers differences you put up in spite of the fact that logically they make perfect sense, and that's how physics should work , but I swear every time I have someone in the car and do that, it feels like someone just attached a 747 to my car's tow hitch ( if I had one, lol ) . There is no way that pronounced of a difference slows down the car by only 5% - I'll have to test this in real life with the Dynalicious and see for myself .

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yup - give it a go. you'll be amazed. our inner ear is actually quite adept at sensing the slightest motion change. that's why it "feels" so different.

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