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Learning how to build Lotus Seven replicas...together!
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PostPosted: May 22, 2011, 7:53 pm 
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carguy123 wrote:
I'm with you Jack feeding the torsion into the frame seems like it would put stresses in other areas. By isolating the forces to the places nature (the engineers) intended seems to be a better choice....



Just to play Devil's Advocate.

These same engineers you mention didn't intend on a cut and shut PPF, or it installed in a different car, which wasn't designed for a PPF. Further the basis for the car you are building never had a PPF and seems to get along fine without it, hell it is considered one of the 10 best drivers cars etc...etc.....

Having said that, be thoughtful of the soft 'ear' mounts if you don't use the PPF....

Best,

T


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PostPosted: May 22, 2011, 8:05 pm 
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But will it get along better with it. The issue of chassis degradation has reared it's head several times on the forum and would isolating the forces help extend chassis life?

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PostPosted: May 22, 2011, 8:39 pm 
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The only thing I see wrong with the PPF is it's the wrong length. Other than that (and Racepedals' "cut and shut" looks like a clean and straightforward remedy for that issue), the PPF is a brilliant solution to the problems of handling pitch in the differential and providing support for the rear of the engine/transmission assembly...and all the mounting points are already built into the Miata diff and trans...and other than the cost of cutting the PPF and hiring someone competent to weld it back together again, it comes free with the donor car. No, a PPF is not necessary, and there are a number of ways to skin the diff mounting cat, but I don't think any of them are simpler, lighter, easier, or...locostier. Heck, you're going to have to shorten the drive shaft, might as well do the PPF too.

Racepedals, more photos please! Inquiring minds want to know.

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PostPosted: May 22, 2011, 11:19 pm 
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Not saying go one way or another but the Miata was also a unibody (aka monocoque) - light gauge sheet metal does not like loads perpendicular to the sheet especially loads that can spike to several hundred if not 1000+ pounds in an instant. The 0.065" thick tube wall diff mount used by the CMC kits that always failed is evidence of this. We're really comparing apples to oranges by feeling forced to incorporate a design element from a unibody into a tube frame chassis. Yes it could work but there are other factors that influenced Mazda's decision that the 7 does not have to deal with.

Now if you wanted to drop some chassis weight and didn't mind potentially sacrificing some torsional stiffness, going with a PPF and no structural tunnel (just a hoop and some sheet metal ala Locofinn's build or the original 7) would be a very elegant and very simple solution to a Miata build.... ;)

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PostPosted: May 23, 2011, 10:14 pm 
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Good conversation guys. We all know there are more ways to skin a cat. Thats what makes this forum so great. Just think where we all would be without the internet.

Anyway, back to the PPF. First, to respond to rx7locost, if my memory is correct, the 3rd generation RX7 does have a PPF. This is an amazing car and Mazda also did their homework on it. Even the windshield was thinner gauge than typically used on cars to cut down weight. We all know we are adapting components from various cars. If you can mount the engine, transmission, differential in a similar manner that the manufacturer used, chances are you will not run into major issues. I've read a few posts with different concepts to secure the Miata differential without using the PPF. Some work, some fail and had to be redesigned. In my case, I saw no reason to try something different. Plus, what better way to tie the drive train together without having to rig up some braces to prevent the differential from twisting. If I was using a different differential, then the mounts would need to be similar to what it came out of, in my opinion. The PPF functions like a torque tube which can be found on other cars....selling for mega$$$.

Once you connect the PPF to the rear differential and transmission, forces into the frame are basically eliminated. It's the mechanical advantage of the lever arm of the PPF that works so great. If I was not going to use the PPF, I would run a brace forward 1-2 ft and then secure it to the frame. Remember the equation Torgue=Force x distance.

Jack...sounds like you are on board with the PPF as you make some good observations as well. The only negative I know is the PPF will reduce the passenger width. MY build has a drivers width of ~17.25" and a passenger width of ~17" using a 44" wide build.

I cut 11" out of the PPF and did the tig welding myself. I also dye penetrant checked the welds for cracks.

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PostPosted: May 23, 2011, 10:37 pm 
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I can't weld aluminum and don't have any friends that weld aluminum... Man I wish we all were neighbors! :cheers:

I'm doing a 442e build, but I'm very averaged sized. I wanted a bigger frame so that IF someone else wanted to drive/buy my car they could without chopping down their shins like us Locosters chop up 1x1. So, using the chopped PPF wouldn't be too bad of deal.

Only problem is I DESTROYED the bolts when trying to get it off the rusty nose of the diff...


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PostPosted: May 24, 2011, 7:34 am 
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Ds1....bolts can be found on eBay. If you find someone that can weld aluminum, I would suggest you watch him and make sure he backgrinds the weld to insure full penetration. I actually made test coupons from the section I cut out and did a bend test to make sure I had obtained full strength of the joint.

Are you close to North Carolina? I would be glad to help you on your PPF.

Russ


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PostPosted: May 24, 2011, 10:39 am 
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Russ, thanks so much for the offer! Unfortunately I'm in upstate NY, so quite a ways away. I know a place that will weld aluminum for me but it will cost me something I'm sure. And you hit on exactly what I'm worried about - making sure thats a good weld!

Decisions decisions. The only reason I wouldn't use the PPF is for the sake of saving time, a little bit of cash, and keeping things simple. Its very easy to accidentally way over do your own car, and one of the goals of my build is to stay as simple as possible. BUT at the same time, I don't want to do a crappy job; I want to do things well and with integrity.


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