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I know some of y'all have shortened your PPFs by cutting out a section and rejoining the ends via heliarc, but can anybody advise me where to cut for a 9" length reduction? Or is there another way to achieve the same end? Like a bolt-on center section?

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Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

OK Jack-
I'll throw out the first comment and it will be controversial!

Why not eliminate the PPF. Many people have done just that and have a viable Locost. Here's a much older post that you even added comments.

http://www.locostusa.com/forums/viewtopic.php?f=39&t=1284

I built my first Locost without the PPF using the full Miata drivetrain. It's now on the third owner and is still doing fine.

Now let the comments fly!

Cheers,
Roy

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Build log http://www.locostusa.com/forums/viewtopic.php?t=16510

It'll be fine without if the high torque seen by the differential mounts can be kept in check. For a Miata NA engine, it makes 100 ft-lb. Step that up by the first gear ratio (3.136), then up again by the differential ratio (3.636). So on sticky tires, the differential is trying to spin backwards with a force of 100 x 3.136 x 3.636 = 1140 ft-lbs. This value gets much worse if the mounting points are close together, but regardless, it's a huge amount of force to be resisting, so the mounts have to be designed from the start to handle it.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Couldn't agree more. But your statement is true of any rear wheel drive car with IRS. Not many use a PPF to handle the torque.

Roy

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Build log http://www.locostusa.com/forums/viewtopic.php?t=16510

In this case the "why" is that the PPF solves some serious problems, like not having to build any other structures to deal with the differential trying to climb up its own ring gear, or the back of the transmission trying to fall out in the street. Another point is that a PPF comes free with every Miata, and I'm cheap.

I re-read that other thread you pointed me to, and it has some good info on PPF alternatives (including a couple comments of mine about ways that didn't work) but my preference for this project remains a shortened PPF.

You're right that every front engine rear drive manufacturer has dealt with with the problem of keeping the differential in place under acceleration (as aptly explained by KB58), and most of them did so without a PPF, but I think the PPF is quite elegant, and a lighter solution than most.

_________________
Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

There's a reason Mazda uses the PPF, as you stated the obvious of you need less structure to tame the diff, but have you noticed that with the cars of most manufacturers that don't use the PPF when they go to higher power or racing situations and larger tires, these guys are constantly chasing an elusive occassional miss shift that you never have with a Mazda.

Section the center. Keep in mind that the PPF also gives you massive head start on a driveshaft containment system

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

viewtopic.php?f=35&t=1715&hilit=short&start=15

Wkeeling took a foot out of the middle.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

If shortening the OEM aluminum PPF isn't elegant, I am reasonably sure that a similar functioning part could be made from a couple of trussed steel tubes fairly easily. I think I recall someone doing that some time back.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

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MiataV8, thanks for hunting that photo up for me -- I can count holes and see where a 9" snip will fit.

Carguy123 and rx7locost, good points. I'm now thinking (and I do my best thinking at dawn) of taking out the nine inches and using a bolt-on piece to join to join the ends together...that piece (a 12 gauge steel plate bent into a C and bolted to the flanges of the PPF?) could probably integrate drive shaft retention as well.

_________________
Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

I took 11" out of mine.

Racepedals, that's lovely. If I'd seen this earlier in my project, I might have gone for 11" too. I hadn't looked at cutting the PPF so close to the diff 'cause I wanted to have as long an unmodified lever arm as I could (in order to keep the weld as unstressed as I could), but I was probably being unnecessarily conservative because I'm such a crappy welder. Since I intend to take it to welder who knows what he*'s doing, that's not a big issue.


* Or she. This is the 21st Century, after all.

_________________
Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

Another option not mentioned is to narrow the ppf vertically as it approaches the trans then fit a bushing to the crossmember for a torque arm, similar to '80s GM F bodies. The long leverage arm greatly reduces the strength required of the frame at the attachment point and it can still support a close fitting, bolt-on-to-the-ppf safety loop, since the angle changes are limited to bush compliance at the arm and trans mount.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12