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> Torsional rigidity of the 914, Differences roof on / roof off / steel roof?
Beebo Kanelle
post Jan 5 2019, 08:17 PM
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Does anybody anywhere know where to find the torsional rigidity of the 914 with the roof off vs. roof on vs. steel roof (916) ?

Thanks in advance.
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Chi-town
post Jan 5 2019, 09:05 PM
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I've never seen an FEA of the whole car but I would be interested in knowing also.
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mepstein
post Jan 5 2019, 10:15 PM
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No difference with the regular top. It's not bolted on. The steel top helps but the A pillars move easily so nothing like a full roll cage.
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Dave_Darling
post Jan 5 2019, 10:25 PM
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I am told that there actually is a difference with the stock top on. Even though not bolted, it is attached and helps a little bit. Anecdotal evidence: A nationally-known autocrosser lost a half second on a particular course at one point and then got it back later in the day by noticing that one of the top latches had become unlatched and latching it again.

Sadly, I have no numbers to back that up.

--DD
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jmitro
post Jan 5 2019, 10:37 PM
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that sounds like placebo effect to me.

I have no firsthand or secondhand knowledge, but I would suspect as mepstein said it makes no difference whatsoever with the targa top on or off. Too much slack in the rubber weatherstripping
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914forme
post Jan 5 2019, 11:11 PM
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The front A Pilars are the same as the 944, pretty much the same setup. Same gauge, same structure, etc.... Difference could be in the base as it attached to the cowl section.

Some of the GTs have bolted down tops, and they added reinforcement to the lower A pillar not the whole thing. these did make a difference.

916 steel roofs made a difference, just like any coupe does.

Nothing will be as good as a proper cage.

In Pano reprinted up fixin' they did do a test of frenquency variations on the 914 chassis. interesting read, I might have to look it up again.
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Tdskip
post Jan 6 2019, 08:41 AM
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Not what you asked, but bracing the internal core structure of the car the longs and rear firewall is the place to do it rather than the top unless you want to loose the roof and weld the car closed.

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Chi-town
post Jan 6 2019, 09:09 AM
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A bolt down metal top would make a big difference as it completes the "box" just like in a FRC Corvette or Supra Targa.

Bracing the longs is a band-aid for not having a solid roof structure. The longs flex because there is no top to the "box".

An A pillar to roll hoop brace setup would probably do more and weigh less. The trick would be making a fiberglass top with a metal structure and a way to actually bolt it on.

Of course a full cage takes care of the issue all together but is not feasible for street cars.
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mgphoto
post Jan 6 2019, 09:20 AM
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The factory 914/6GT’s had steel reinforced roofs, steel rods front to rear and an x pattern left to right.
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Mark Henry
post Jan 6 2019, 09:47 AM
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I have two 914's here one is rust free ( (IMG:style_emoticons/default/rolleyes.gif) ) unmolested, the other repaired, GT kit and inner long stiffener kit.
Putting my hand flat across the hoop/top part line, pushing the car around, the OG car you can feel the part line wiggle. The car with the inner long kit the part line has no wiggle whatsoever..
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ConeDodger
post Jan 6 2019, 01:21 PM
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QUOTE(jmitro @ Jan 6 2019, 01:37 AM) *

that sounds like placebo effect to me.

I have no firsthand or secondhand knowledge, but I would suspect as mepstein said it makes no difference whatsoever with the targa top on or off. Too much slack in the rubber weatherstripping


I doubt placebo effect was in play. National level autocrossers are remarkably consistent. They can often predict improvement run to run. Mostly because unlike novice drivers, they know where they lost time. When they lose time, it’s usually spectacular. Ie: they spin...

I have driven my 914 without a top and without a windshield. That window frame wags like a golden retriever when dad comes home! (IMG:style_emoticons/default/laugh.gif)
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VegasRacer
post Jan 6 2019, 02:18 PM
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Elvira has a cage plus some additional reinforcement.
On the track, with the top off . . . the top corner of the windshield frame will flex almost an inch vs. the corner of the cage. (IMG:style_emoticons/default/yikes.gif)
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GeorgeRud
post Jan 6 2019, 02:23 PM
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I notice quite a difference between my original -6 with the original chassis and my conversion -6 that has 914Ltd’s stiffening kit installed. Years ago, Bruce Anderson and Garretson Enterprises did actual measurements of body flex.
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Dave_Darling
post Jan 6 2019, 03:49 PM
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I believe that the guy whose car they tested was actually the same person who told me the anecdote about the latch....

--DD
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Curbandgutter
post Jan 6 2019, 05:13 PM
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Hello, I did run some numbers on the torsional rigidity of the 914. Here's the link http://www.914world.com/bbs2/index.php?sho...p;#entry2396780

See POST#10

Bottom line is that the stock 914 chassis is very flexible, 3,326’# per degree from my testing. Although I didn't say it at the time, I ran the numbers with and without the doors and found no real difference. From my "door" tests, it's safe to assume that the roof will not be any different. There is too much flex in the hinges, and latches before the roof and doors begin to provide support.

Based on testing, I can say that, other than a full roll cage, that the next best thing is to stiffen the longs. Then, if you wanted more, a welded steel roof like the factory. There is a reason why the factory went with a welded steel roof in the 916.

I'm almost to the point where I can test again with the full roll cage in place. Based on modeling, I'm expecting a 10x stiffer chassis now.

In testing, I found that one of the longs had a stress crack in it. I don't know if that was just mine, or if most of the 914's out there are running around with cracked longs.
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rgalla9146
post Jan 6 2019, 05:14 PM
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This is a great topic.
I want a 914 coupe.....with AC
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Chi-town
post Jan 6 2019, 05:18 PM
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I think I'm going to move forward with a bolt on roof frame (IMG:style_emoticons/default/idea.gif)
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73-914
post Jan 6 2019, 07:41 PM
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QUOTE(rgalla9146 @ Jan 6 2019, 06:14 PM) *

This is a great topic.
I want a 914 coupe.....with AC

It's called a Cayman
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Beebo Kanelle
post Jan 6 2019, 07:49 PM
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It appears that the 914 has a stiffness of 3,326 '#/Degree

Thanks, that gets me in the ballpark.
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Curbandgutter
post Jan 6 2019, 08:21 PM
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QUOTE(Beebo Kanelle @ Jan 6 2019, 05:49 PM) *

It appears that the 914 has a stiffness of 3,326 '#/Degree

Thanks, that gets me in the ballpark.


Yes, sorry I should have stated that instead of forcing you to go look for it in the post. I updated my response to show the number. (IMG:style_emoticons/default/smile.gif)
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