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Learning how to build Lotus Seven replicas...together!
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PostPosted: Thu May 12, 2011 11:24 am 
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I know theres a thread about it somewhere but can't seem to find it. I'm interested in using the stock brace like this http://thumbs4.ebaystatic.com/m/mW0Ora3 ... lQ/140.jpg (I know its a tiny picture) and cut it down, then affix it to the differential and the firewall for more rigidity.

Some people do this and some don't. I'm sure it has advantages, but are there any major disadvantages if I don't? Also if you have a link to a thread about this then that would be very helpful! Thanks!


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PostPosted: Thu May 12, 2011 12:36 pm 
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Look at how the Miata uses the PPF. The diff is solid mounted to the engine but the engine/diff assembly are rubber mounted to the chassis. You could rubber mount the chassis end and it would be fine. The long arm reduces the load at the mount however using the PPF in this fashion seems like the heavy way to do it.

You do not want to rigid mount one end to the chassis and the other to the diff unless you replace the bushings and solid mount the diff. If everything isn't similar stiffness, the stiffer mount will carry a disproportional amount of load and will be more prone to failure.

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PostPosted: Thu May 12, 2011 1:04 pm 
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Ahh okay makes sense, thanks for the response! Now as far is not using the PPF, is it a horrible idea if I don't?


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PostPosted: Thu May 12, 2011 10:36 pm 
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Nope not a bad idea either way. I was in a similar scenario - I opted to make a custom nose mount that incorporated a rubber bushing.

Just make sure the mount is strong enough - if the rear axle is seeing 600 ft-lbs+ of torque, that mount will need to react some of that same 600 ft-lbs plus some extra strength to handle shock loads.

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PostPosted: Thu May 12, 2011 10:48 pm 
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Once again, very true. I almost don't want to make a mount just because I know it will end up taking a day or more the way I work (lol), but I also don't want make a decision I regret and can't change later.

I'm going to be running a stock 1.6 with turbo, and only looking for 230ish at the wheels. Hmmm, any links to places this was discussed before?


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PostPosted: Sun May 15, 2011 11:01 am 
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I'll be running the same configuration..........see my build log for my solution.

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PostPosted: Sun May 15, 2011 12:35 pm 
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Hey Jaf, love your build! Very well detailed, good pictures, and good work. We carry a lot of similarities in our builds too, with the Miata parts and turbo's and things. Also, I REALLY like your trans mount.

Anyway, my Diff is rubber mounted, which would make your design pretty useless to me considering most agree that a rubber and solid mounted driveline part is a bad idea. Maybe I could use the same kind of setup but bolt through some sort of bushing instead of weld it...


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PostPosted: Sun May 15, 2011 8:47 pm 
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My opinion is to use the Miata PPF. The weight is negligible and the fact that you can tie the transmission to the differential is a big positive. This way you can also keep a pretty good alignment and pinion angle. Of course, others have different opinions but I went with the PPF, shortening it 11". It does take up some additional room in the passenger side but you can work around that. There is a reason Mazda added these.
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PostPosted: Sun May 15, 2011 9:16 pm 
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Racepedals wrote:
There is a reason Mazda added these.


I totally agree! Thats why I get pretty uneasy about not using it. I really like the way you shortened it, I'm just trying to stick to a minimalistic, simple approach. Still looking for someone to say DON'T! or DO IT! :mrgreen:


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PostPosted: Mon May 16, 2011 8:32 am 
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Mazda used a PPF on the Miata and didn't use one on the RX-7. What's with that? There are millions of cars out there that are IRS RWD and have no PPF. There is no "right" answer. Nobody can tell you one way will work and the other will fail. If you are looking for a single answer, I can say with absolute certainty "DON'T! or DO IT!". :lol: The differential can be mounted in either fashion, depending on your wants and needs. I can't remember anybody on this group ever talking about differentials that fell on the ground, or flipped around. As already stated, remember that all wheel torque is resisted by the mounting of the diff in some form, so build the structure accordingly. it really is not that difficult to implement either way. You will have tougher decisions to make and engineering challenges along the way.

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PostPosted: Fri May 20, 2011 10:14 am 
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Everything about the Miatas seems to be designed to keep things light. I have a feeling that this beam, not found on many cars, may have been installed to resist a lot of the the loads developed by the drive train without having to beef up and stiffen the rest of the chassis along the way from the engine to the wheels. The shock and vibrations of resisting all these forces is also isolated somewhat from the rest of the chassis (and the peoples) by mounting the engine, transmission, drive shaft and differential all on flexible rubber joints.
So using the PPF and plenty o'rubber mounts may result in less shock, but lets face it, this is not going to be a quiet smooth ride, and we have a fairly light platform already. You have been building your own PPF beam with your chassis right beside your transmission and differential. So I'd say you should be able to come up with suitable mounting brackets for the diff and add any sensible bracing/gusseting to mate it to the frame and let your frame do the work. The rear mount of the transmission would not want to be rigid, but you would want to pay attention to your mount strength and limiting movement from forces exerted there.

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PostPosted: Sun May 22, 2011 8:43 am 
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The PPF was used to tie the Transmission to the diff in a solid fashion while (as mentioned above) allowing both to be soft mounted to the chassis.

This was done to eliminate/isolate vibration in the chassis (and more importantly to the designers, the driver).

That's it.


Do you need it in a seven?

No.


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PostPosted: Sun May 22, 2011 4:24 pm 
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Tralfaz wrote:
This was done to eliminate/isolate vibration in the chassis (and more importantly to the designers, the driver).


Thats one of the thing I have noticed as I keep researching the topic. We all know the Japanese wanted the Miata to be the "perfect" little sports car, so they did some (sometimes odd) things specifically to make it a great experience for the driver.

Although I'm slightly concerned about being comfertable in my 7, I'm more concerned with going fast :D

Thanks for all the posts, guys, this is a good discussion. At this point I'm very probably leaning towards a "No PPF" decision.


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PostPosted: Sun May 22, 2011 5:21 pm 
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Ds1 wrote:
At this point I'm very probably leaning towards a "No PPF" decision.
Remarkably enough, this discussion has me leaning the other way. I think I'll use a shortened PPF in my next build...but then again it'll be wider than book by a couple/few inches so passenger encroachment won't be as great an issue as it is with a book frame.

Racepedals, do you have any more photos of where your PPF is rejoined? And maybe a big pic that'll show where the 11" section was removed?

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PostPosted: Sun May 22, 2011 6:47 pm 
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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.

Up till this point I was of the mind that a PPF was an unnecessary encumbrance. I've taken more than few PPFs off of Miatae and RX7s and they can be a bugger to get off after 50k miles.

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