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PostPosted: November 28, 2006, 6:53 pm 
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chetcpo wrote:
It is set up for the Miata rear suspension unlike the book chassis which is made for a solid rear axle.


Meaning travel space? Does it still need the PPF?

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PostPosted: November 28, 2006, 9:19 pm 
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-DOUG- wrote:
chetcpo wrote:
It is set up for the Miata rear suspension unlike the book chassis which is made for a solid rear axle.


Meaning travel space? Does it still need the PPF?


Most of the rear structure is different since unlike a book locost which uses a 4 link and panhard rod setup to locate the solid rear axle, it is made to accept the Miata rear diff and a set of upper and lower A arms. It doesn't utilize the stock Miata PPF but it has other design features to help securely locate the Miata differential. Nothing special, and nothing that much different than the solutions many of us home builders have come up with. If it is specifics you want you may have to call them and find out how they do it, if they will tell you.

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PostPosted: December 24, 2006, 11:53 am 
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Not trying to argue, just stating my opinion for the sake of discusstion. For a uncompromising sportscar, I'm suprised that passenger comfort is held in such high regard.

If it were mine, the entire tunnel on the passengerside would be moved over as required. All the tubes in the tetrahedrons that terminated at the old vertices would need to be completely cut out and new ones fitted to the new vertices.

The ppf is outstanding. It is light, has no welds to fail, prevents angular changes in the driveline from engine loads, and best of all, it's free, sitting there in front of you. Without the ppf, the pinion angle needs to be dropped to compensate for loads compressing bushings as the pinion tries to climb the ring gear under load. If you go solid without bushings, noise and vibration will be transmitted into the chassis.

The long pinion rx7 will see less force at its mounts due to the longer arm but the driveshaft will be very short. The shorter the shaft is, the tougher it can be to get the ujoint angles you want. Ideally a driveshaft with ujoints should have a 1/2 degree angle at most to aid needle bearing lubrication and joint life. The steeper the angle, the lower the reliable rpm of the shaft. The trans output shaft should be parallel to the pinion except for bushing compression compensation. The trans output shaft should be higher than the pinion. Under load the pinion lift and reduce the ujoint angles.

Engine torque if the trans mount is bushed also affects the angles. Solid mounting a transmission while keeping rubber engine mounts can break the trans case. I have seen it happen on older domestics.

All this stuff is surmountable but it doesn't make any sense to do it. I can totally relate to the possiblity that a few of you guys don't feeling comfortable cutting up a chassis you bought or are concerned about modifying. I am not a total newbie. I have not spoken to McSorley in about 5 years while I have been caught up in other stuff. Just my humble opinion about something that is not being built for me.


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PostPosted: December 24, 2006, 1:27 pm 
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Hmm. Interesting points, worthy of further discussion. As you said, I understand we aren't agruing, I just wanted to try out some counterpoints and see how they fly.
Miatav8,MstrASE,A&P,F wrote:
Not trying to argue, just stating my opinion for the sake of discusstion. For a uncompromising sportscar, I'm suprised that passenger comfort is held in such high regard.

If it were mine, the entire tunnel on the passengerside would be moved over as required. All the tubes in the tetrahedrons that terminated at the old vertices would need to be completely cut out and new ones fitted to the new vertices.

The ppf is outstanding. It is light, has no welds to fail, prevents angular changes in the driveline from engine loads, and best of all, it's free, sitting there in front of you.


True perhaps, but the PPF isn't plug and play for issues other than the width of the tranny tunnel. It would have to be shortened over a foot (no easy way to do that) and on top of that you would need to cut through or omit much of the bracing (if not all) on the bottom of the tranny tunnel, sacraficing some chassis rigidity.

Miatav8,MstrASE,A&P,F wrote:
Without the ppf, the pinion angle needs to be dropped to compensate for loads compressing bushings as the pinion tries to climb the ring gear under load. If you go solid without bushings, noise and vibration will be transmitted into the chassis.


What you said up top regarding passenger comfort applies to this too IMO. In an uncompromising sports car who cares about some vibration. (If there's that much vibration something is probably wrong anyway right?) Lock the Miata diff carrier down solid and you don't need a PPF, which also allows you to mount the engine/tranny and diff carfier in whatever spatial releationship that you need. Using the PPF locks you into the same geometry used in the Miata, which IMO is less than ideal since you are not installing them in a Miata, but rather a 13" tall sled. You have to move the engine tranny up a bit to get sump clearance unless you have a 6" ride height.

Miatav8,MstrASE,A&P,F wrote:

The long pinion rx7 will see less force at its mounts due to the longer arm but the driveshaft will be very short. The shorter the shaft is, the tougher it can be to get the ujoint angles you want. Ideally a driveshaft with ujoints should have a 1/2 degree angle at most to aid needle bearing lubrication and joint life. The steeper the angle, the lower the reliable rpm of the shaft. The trans output shaft should be parallel to the pinion except for bushing compression compensation. The trans output shaft should be higher than the pinion. Under load the pinion lift and reduce the ujoint angles.

Engine torque if the trans mount is bushed also affects the angles. Solid mounting a transmission while keeping rubber engine mounts can break the trans case. I have seen it happen on older domestics.

All this stuff is surmountable but it doesn't make any sense to do it. I can totally relate to the possiblity that a few of you guys don't feeling comfortable cutting up a chassis you bought or are concerned about modifying. I am not a total newbie. I have not spoken to McSorley in about 5 years while I have been caught up in other stuff. Just my humble opinion about something that is not being built for me.


Well as you've probably guessed I have the engine and tranny mounts bushed and the diff carrier mounted solid. I didn't do it that way to save passenger space, but that turns out to be an added bonus. IMHO trying to adapt the PPF would require much more work and for minimal gain and perhaps at the cost of some chassis rigidity. Sure I will feel more vibration, but it's a purpose built autocrosser and track day car that just happens to be street legal, so who cares.

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PostPosted: December 24, 2006, 2:45 pm 
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All good points. I didn't realize the ppf had to be shortened. Raising the tunnel an inch or two in addition to widen it would be annoying if the chassis were final welded before fitting the driveline.

GRM's miata locost had nearly immediate failure with Ron C.'s original irs diff mounting. I was just curious if you guys knew about that.


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PostPosted: December 24, 2006, 3:01 pm 
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Yep, the CMC (no longer in business) people's revised solution (after the GRM failure) has failed people before too, but I think this is because they keep sticking bushings in there that allows the repetitive movement that eventually breaks something.

Once I get my car on the road I will give a full report on how bad the vibrations are from my solid mount.

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PostPosted: December 25, 2006, 12:24 am 
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Miatav8,MstrASE,A&P,F wrote:
GRM's miata locost had nearly immediate failure with Ron C.'s original irs diff mounting. I was just curious if you guys knew about that.


Yeah, I was there--it made it once around the building and onto the trailer, and that was it. The redesign (introduced at the Mitty) was good for two autocross laps. But it was <not> "...Ron C.'s original irs diff mounting," the diff mount was authentic original CMC engineering.

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PostPosted: December 25, 2006, 8:14 am 
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Thanks for the clarification. I've gone to the last two Mitty's, making sure to get my two short laps in a demo Elise. It is a shame CMC didn't have more knowledge. They were located about an hour from me. I would have been glad to haul the broken car away for you guys, free of charge! That would not have made a very good article though.


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PostPosted: December 25, 2006, 6:57 pm 
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Miatav8,MstrASE,A&P,F wrote:
I would have been glad to haul the broken car away for you guys, free of charge!


Steve's brother Chase drove it most of the way home, after a field fix that needed refixing every hour or so, despite easy throttle applications and mildest aceleration and highway speeds possible, before giving up and getting a trailer. That would have made a good article, though it wouldn't have been much of a CMC promotion. I've got to give those guys credit for confidence--I'm sure they thought it would work, despite a complete lack of reasoning to support that confidence. Faith-based car design rarely ends well.

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PostPosted: March 26, 2007, 12:42 pm 
Very late to the party here, I apologize. This was just brought to my attention.

CMC had little or no engineering talent from what I've seen. Feeding shock loads into the middle of control arms, a differential mount that appeared to be designed to fail as soon as possible - don't use them as an example for anything.

The PPF is in the Miata to make the drivetrain into one solid piece. Since the diff is firmly fixed in pitch (using aircraft terms to make Jack comfy) relative to the transmission, there's better power transfer. It is allowed to "roll" by the PPF. This allows the entire drivetrain to be isolated from the chassis of the car without transmitting too much vibration or compromising performance. It's a brilliant piece of kit and I don't know why all other cars aren't set up this way. The FD RX-7 and the RX-8 are.

However, things are different in our little locosts. NVH isn't really as much of a big deal. If we're going to put the weight into making a stiff bridge between the transmission and the differential, why not make that weight work for us by stiffening the whole chassis? That's the approach I took after my modified CMC mount (ie, I took out the bushing that caused immediate failure for the CMC cars) broke a year or so into the life of my car. The nose of the diff was bolted into a reinforced transmission tunnel. Westfield does the same thing, attaching the nose of the diff to brackets on the solid rear bulkhead as shown in that picture posted earlier. That portion of their frame is very similar to a Locost.

In my case, I also inserted solid diff mounts in the side bushings although Westfield did not. We'll see how their solution works, as there is evidence of much higher engineering abilities at Westfield than at CMC. My own car with the new mount has seen countless clutch drops on R-compound tires without so much as a loosened bolt and the Westfield has survived three track days so far but no standing start testing.

I don't know what Coveland does, I'd be interested to know. From what I've seen of pictures of their kit, it's a very close relative to mine.


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PostPosted: November 28, 2020, 6:41 am 
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I'm so late to the party, it must have ended ages ago, but I've found this thread really helpful. I'm building a replica 1936 BMW328 with a grp body made from a buck made from an original car. I've had to shorten the propshaft (called driveshaft in USA?) by 10 inches so have decided to do without the PPF.

Forgive me if I'm not giving credit to things already said, but a key purpose of the PPF is to allow the entire drivetrain to interact with the chassis only at the engine mounts and at the end of the diff "wings" An important feature is that the PPF is allowed to twist so that the forces are all distributed between these four points. This is why the wing mounting points are 2ft apart to distribute the high torque moment with reasonably low forces.

Taking out the PPF and mounting the gearbox housing and front of the diff rigidly to the chassis will mean very high forces at these points because the moment arms are so short! This puts a huge stress on the chassis at these mounting points where there is none if the PPF is used. So I plan to mount these points in bushes to allow some flex so most of the torque is taken up by the diff wings and engine mounting points as originally intended. I also plan to put bracing bars between the engine lifting lugs and the chassis - again, long lever arm implies lower forces. And I've seen pictures of some scarily weak plate arrangements for the diff wing mounting points making me wonder if everyone realises they are not just for supporting the weight of the diff!

I hope this helps for anyone thinking of losing the PPF. I've learnt a lot from this thread, and from this site. I'd overlooked the importance of careful alignment of the transmission and diff to protect the UJ bearings so I'll now be looking at ways to measure and adjust this.


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