MOTOR MOUNT BUILD FOR CUT HALF VW

[Murray Randall]

I'd like to share my experience building the motor mount for my Casler cut half VW built up with magneto, alternator and a starter. Please make any criticisms, suggestions and observations.  Engine weight is 80 lb, mag 5 lb, carbs (2) 2 lb total, starter 6 lb. The starter does not have to be used but the rest is commited. The mount was a pretty serious build despite almost copying Sam's and Joe's mounts. The luxury motor items I requested had me bending all four elements of the basic ring dog bones for clearance. For the ring vertical sides .625 X .049, the ring horizontal top and bottom .625 X .035, the four corner fuse to ring arms .625 X .049, the two diagonal fuse to ring triangulators .5 X .035. Along the way I was using a finite element stress analysis program. The loads I input were 100 lb vertical, 100 lb thrust, 50 lb side to side and 67 ft lb torque. With these loads and the tube above sizes the stresses were all under 10,000 psi. The loads input were not intended to represent service, but the safety factor should be satisfactory. The max stresses were at the forward end of the four corner fuse to ring arms. Using those tube sizes kept the stresses pretty uniform. Nothing is loafing out there, all carrying their share. The weight of mount is 2.4 lb. So all up I hope to see 100 lb with prop fire wall forward and all engine goodies. The pics show also that I favor swing out motor mounts, primarily to simplify mag service while keeping the motor as close to the firewall as I can. Attached are pics of the mount, engine with ring in place before arms, and a plot of showing the finite element model.

The mount pucks were made first copying Sam and Joe, bolted securely to a 1/2 aluminum jig plate, ring elements bent, ring element fit and welded. The ring was then bolted securely to the motor, arms fit and tack welded, removed from motor and finish welded. Everything was TIG welded. tacks and finish welds. I'm old fashioned and would have liked to gas it but the distorsion was tough enough as it was.

The stress analysis I do cause I can and it does give me a bit more confidence. I do not see it as anything like necessary! The input loads were just thumb sucks and motor mounts are really vibration problems more than static load problems. So frankly the analysis maybe just window dressing. The cut half motor sure hands you a tough motor mount job. Think that issue over carefully when you face that decision. The resulting mount sure is the most robust mount I have made but the cut half guys have all said that you need every bit of it. Merry Christmas!

[Murray Randall]

I forgot to say that I actually welded the diagonal triangulating tubes from the opposite corners than the analysis. I was meaning to change the analysis model  but have not done it, yet anyhow.  The change doesn't get me too excited.    Murray Randall

[Steve]

Looks strong and the right materials - just say'n, provide anchor points for safety cables...

We want to keep you in harassing range...

Steve

[Dan_]

Murray,
Great workmanship there. 

I don't see the reason for the thick wall and large O.D tubing.  Or the ring. 

I could see the ring possibly if you needed to weld something to it...  (I know the dynafocal mounts use a ring, but I think that is because you usually have to weld a couple of the motor mounts tubes to them.)

Here is a pic for a Cessna mount.  It is holding up an engine and prop that weighs 4 times the 1/2 vw and potentially has twice the power...  



I would even delete the 2 tubes at the top of the Cessna one that would go to the center of the firewall.  The back-plate on the engine is all the spreader you need for your mount tubing... and you still lack a diagonal like the Cessna one has on the bottom.

Just my casual observations, I'm no engineer or aircraft designer.

I would be interested in hearing what the computer program says about one like the Cessna.  Keep up the good work and thanks for sharing.

[Sam Buchanan]

Murray,
Great workmanship there.  

I don't see the reason for the thick wall and large O.D tubing.  Or the ring.  

I could see the ring possibly if you needed to weld something to it...  (I know the dynafocal mounts use a ring, but I think that is because you usually have to weld a couple of the motor mounts tubes to them.)

Here is a pic for a Cessna mount.  It is holding up an engine and prop that weighs 4 times the 1/2 vw and potentially has twice the power...  



I would even delete the 2 tubes at the top of the Cessna one that would go to the center of the firewall.  The back-plate on the engine is all the spreader you need for your mount tubing... and you still lack a diagonal like the Cessna one has on the bottom.

Just my casual observations, I'm no engineer or aircraft designer.

I would be interested in hearing what the computer program says about one like the Cessna.  Keep up the good work and thanks for sharing.

Yep, the cut-case mount is a bit challenging, one reason why Leonard advocates the full case.

Here is my version:



Joe Spencer successfully used 1/2" tubing with his 37hp engine, but the 45 promptly destroyed the small diameter tubing. My mount is 0.035" x 5/8" and I had to repair a cracked tube at twenty hours. I added a couple of gussets and have had no more problems. The Lycoming mount uses 1/2" tubing but that engine is much smoother running than the 1/2 VW. The 45hp VW really pounds the mounts.

I readily admit my mount is designed with TLAR engineering (That Looks About Right), but so far so good. If I was doing it over again and was committed to the cut-case, I would probably just extend the nose of the fuse with longer/modified longerons and skip building a mount. Or use a full-case engine..... 

[Dan_]

Here's my TLAR from Sams photo...  It's bad I know, but all I can do in "paint"...  If possible I'd use the spools like Sam did at the firewall on the engine back plate as well, Ala Cessna.

[Sam Buchanan]

Here's my TLAR from Sams photo...  It's bad I know, but all I can do in "paint"...  If possible I'd use the spools like Sam did at the firewall on the engine back plate as well, Ala Cessna.

My TLAR analysis of the above design creates concern about the mount points at the upper left, upper right, and lower right positions where there is no three axis triangulation. The 45 has a very pronounced rocking couple around the crankshaft axis and this would result in those unsupported mount arms flexing. It seems to me they could fail due to the flexing.

The reason for the "ring" in my design is to prevent the mount points from moving independently of each other so the tubes in the mount aren't subjected to bending forces. But I don't drive a train so may not know what I'm talking about...

[Murray Randall]

You bet these cut 1/2 mounts look wicked strong, over built to me.  Attached is a pic of the last mount I did for my O-235 Cub.  Does it look maybe a bit stronger than the Cessna but sure is similar and was an easy build.  It is more like what we're all used to and I did no analysis.  The Eagle mount I made weighs 2.4 lbs so how much could be saved? I used the .049 cause I was sure that I would have trouble welding the .035 to the the heave wall donuts. When I get big thickness variation and narrow angle cluster intersections my welds deteriorate. The arms kept me from getting my helmet in to see the weld clearly. It was all hard for me.

[Sam Buchanan]

I really like the way you "pinned" the mount to station 1, that has advantages over the spools I used. Even though the mag isn't hard to remove from my engine (I left enough room between it and the firewall, and I don't have all the starter stuff) being able to swing the mount or remove it by merely pulling some bolts firewall-forward is a nice touch.

[Murray Randall]

The swing out motor mount idea comes from Super Cubs. You still have to support the motor when swinging then but they stay in position when one side is released. If you run your carb cables, oil temp/pressure and wiring near one hinge side you can leave all that attached to the motor. When you want to move the motor you don't have to get one wrench (vise grip) on the nut way up under the boot cowl and one wrench on the bolt, its all done outside and only for two bolts not four. Its no big deal but it helps a little bit, is no harder to make, no cost, no weight penalty.       Murray

[Dan_]

The reason for the "ring" in my design is to prevent the mount points from moving independently of each other so the tubes in the mount aren't subjected to bending forces. But I don't drive a train so may not know what I'm talking about...

Not on the Cessna.  The engine back-plate seems to me to be doing that job...  Perhaps it depends on how the rubber there is used.

We are talking about relatively short tubes and it is not an area you want under built...

The ones who drive the trains rarely talk about it... (seems to me).  One thing is for certain, force naturally travels in straight lines.  Anytime you make it travel through a bend or offset you provide it a spot to attack.

Properly executed dynafocal applications have the welds so close to the cups for the rubber, I don't see why the ring is even there...  The tubes usually can be welded to the cups where the ring is welded...  Again, how the rubber is used is probably the deciding factor...  

No ring:



Scott Casler's own CX 4...


Ring:

[Sam Buchanan]

Here's my TLAR from Sams photo...  It's bad I know, but all I can do in "paint"...  If possible I'd use the spools like Sam did at the firewall on the engine back plate as well, Ala Cessna.

I knew there was something about this image that didn't look right. This design wouldn't work for an engine with the single carb because the diagonals would foul the intake manifolds. Might work with dual carbs, however. My design has everything carefully located so there is no interference with stuff, and it is still a tight fit.

[weasel]

As an alternative Cassler offers a bed mount system.  We have the LE set up that way. you can see it on this Vid.

[Murray Randall]

I'm not getting anything on Scott's price list on the bed mount. But looks like it could be a bit heavy and might not be an easy build. Any idea of the weight?  Thanks  Murray

[Dan_]

Murray,

Here is a good example, it is on "Bodacious"...  Just move the carb back about 2 inches.