flyrotary@lancaironline.net Mailing List Archive

The RX-8 flywheel/flexplate is designed to have the starter gear enter from the front as related to the engine. The RX-7 has the starter entering from the rear as related to the engine. As a result the RX-8 flywheel will not work with the RX-7 starter. That is my opinion as I remember the layout. I am willing to stand corrected by more knowledgeable folks.

Bill B

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy
Sent: Friday, July 15, 2011 2:41 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Flex Plate Failure - Improvements

I'm still in analysis mode on this myself.

So far the possibilities are:

1. The flex plate may have been warped when installed ( I bought several that were) I think the standard should be close to zero runout or wobble on the flexplate in view of this failure.

2. The flexplate could have had cracks when installed (I also bought several that had cracks and they were not visible unless you looked closely)

3. The dang things (RX-7 flexplates) are not strong enough for boosted 13Bs or 20Bs.

I do NOT think the problem is fore & aft travel of the input shaft. The RD-1x drives have had thrust bearings from day one. There is no thrust load at all on the damper, flexplate or crankshaft imposed by the drive.   This was not the case with the Ross drive I used at first.

I have over 1000 hours on the same flexplate on my NA 13B. I have only 55 hours on the 20B.   But I'll be checking both. I did inspect both flexplates VERY carefully before installing.

If I found that a change were necessary, I would consider two possibilities.

1. Use a modified aluminum racing flywheel used with an auto counterweight. (Or possibly a lightweight steel one if there is such a thing)

2. Use an RX-8 flexplate with an adapter plate made of aluminum or steel sheet to go between the flexplate and damper. The RX-8 version looks stronger than the RX-7. I briefly studied this when good RX-7 flexplates were getting hard to find. I did not develop a formal procedure for doing this but it looked like it would be pretty easy. Anybody with spare time out there want to take a closer look? Hint: The damper bolt pattern would be offset 45 degrees from the RX-8 torque converter bolt pattern.

Tracy Crook,   RWS

On Fri, Jul 15, 2011 at 1:47 PM, Bobby J. Hughes <bhughes@qnsi.net> wrote:

Bill summed up what I’ve been thinking the last few days. I’m not changing anything until Tracy makes a recommendation. It could be fore and aft movement on the shaft, flex plate out of round, worn rubber isolators or a combination of all these items. We may need a scheduled maintenance plan to replace items like the rubber isolators or other items that wear and loosen tolerances.

I will however be inspecting for this problem before most flights.

Bobby Hughes

(playing with fiberglass again)

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Bill Bradburry
Sent: Friday, July 15, 2011 10:03 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Ok Here are the important flywheel measurements

David,

I can’t help but think that you may be fixing something that is not broken. I realize that the flexplate came apart on your plane, but that doesn’t mean that the flexplate is the cause of the problem. That flexplate holds up quite well in the car with a heavy torque converter attached to it. Think of the forces when a car under heavy acceleration changes gear! As long as the forces are along the plane of the plate it is very strong. Not so much in the forward/rearward directions. In fact that would cause metal fatigue and cause it to break just like yours did. I think that what ever is causing the endplay is what you should be looking at. I suspect that if you check out the transmission on the car you will find that the endplay of the input shaft is controlled by thrust bearings or some such device. Lynn may know the answer to this??

That msg from Al Wick really resonates with me. Tracy had to install a thrust bearing to prevent the drive shaft from hitting the bottom of the pilot bearing and causing an eccentric shaft failure on the engine. Maybe you now need to add a cage similar to what Al was discussing that will trap the shaft end play completely. That, to me, makes more sense than adding a bigger flywheel. If you strengthen the weak spot, you will then discover where the next weakest spot is because that will be the next failure!

This is a real opportunity for you and the rest of us with the same drive. Please study this a little more before you begin the fix!

Bill B

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of David Leonard
Sent: Friday, July 15, 2011 12:42 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Ok Here are the important flywheel measurements

I finally got to the hangar and measured out my S5 turbo flywheel and the remains of my S5 flexplate. The results are very encouraging, even as good as the possibly could be to make the swap, but I only had ten min at the hangar before I had to leave to pick up my daughter from daycare, so they are one try and slightly sloppy... I make no guarantees. The height measurements of the flex plate were particularly suspect because, well, mine is all warped and broken.

Ring Gear Diameter - Perfect fit. As we suspected, same size, same tooth spacing.

Ring Gear Fore-Aft Placement - As measured from the underside of the big nut. (for all further reference, aft means aft in the CAR). The flex plate puts the aft side of the ring gear 1.290 inches from the front of the big nut. The flywheel puts it 0.913. So the flex plate puts the ring gear (appx) 0.377 in. further aft. This is actually fortuitous because it just so happens that the ring gear thickness is .365 ... So all we have to do to fix the discrepancy is weld and extra ring gears onto to the flywheel ring gear and there will be teeth that the starter can reach:(bottom of the page.. the cost $80)
http://mazdatrix.com/flywheel.htm
It may also be possible to fix the discrepancy by changing the starter... will look into that.

Damper Plate Placement - On the flywheel, the mounting surface is proud (or aft) of the ring gear. On the flex plate, the mounting surface is recessed (or forward) from the ring gear. And it all pretty much works out. That position on the flywheel will vary somewhat depending on which flywheel you have and how much wear you have on the clutch slip surface. But on my (I assume well-worn) flywheel the placement was 0.965 while the flex plate was 0.950. Pretty damn close and given measurement error it might me meant to be right on. Or, any difference could be made up by trimming down the spline on the damper plate or removing or adding spacers between the two.

So in my mind, a flywheel is the way to go. Stout, easy to get, pick your weight, only mods seem to be drilling the mounting holes and welding on a second gear ring.

--
David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net