Ring mount- why?

I am sure there will be others with a better explanation;

It's all about control of engine movement particularly at start & stop. The gearbox would seem  to be the main offender.

Bed mounts do not do a good job of controlling this movement - I know I have one in my Sonex. This is due to the effect of leverage (principle of moments). With all the support/control at the bottom of the engine,  there is less control of the movement compared with a Ring Mount that has wider support mounts, usually at 4 points, around the whole periphery of the engine (less leverage).

The Ring Mount also conferse the potential for softer mounting rubbers, that may dampen vibration better.

I would also speculate that a Ring Mount offers easier adjustment (than a Bed) for engine/prop thrust line, by shimming the firewall attach points.

The down side (always one) is the Ring Mount is likly to cost more to produce (make your own instead) and may be heavier.

One day I may fit one.

HI all

Just to be clear, Rotax has never made a bed mount.  Mounting points are provided however the better option is a ring mount.  To explain we have to understand what kind of vibrations, movements, are coming from the engine.  It is important to remember we are dealing with a 4 cylinder boxer engine.  What that means is piston 1 and 2 move in and out at the same time.  They are not on the same stroke but the mass movement is the same.  The 3 and 4 do the same at 180 degrees from the 1 and 2.  

Piston movement in the Rotax design creates movement of pressure internally, so 1 and 2 pump to an extent toward 3 and 4 in a dance between the cylinders.  This creates an interesting pulse internally.  To hold the case from the mounts under 1 and 2, and then under 3 and 4, as in the bed mount design, we will see internal vibrations that are very hard to damp out.  The ring mount solves most of this in that it holds the engine from the ignition side and this fore and aft movement, or pulse, is much easier to damp.  The long bed mount arms under the engine from the firewall forward vibrate  lot and a lot of these designs crack mount tubes.  (not just on Zenair)  After years of working with OEMs to switch to the ring mounts we have reduced most of the issues with cracking and failing mounts.  The desire to build the kit for less was the prime motivator for OEMs to use bed mounts but in my opinion it is a mistake.  

As an example, especially on the 912 ULS with its almost 11 to 1 compression bump inside, the engines were originally sold without the large starter and without clutch systems in the gearbox.  This was never and issue in the 80 HP, it had only a 9 to 1 compression.  With the high compression engines it was soon found that bed mount systems....Zenair, Rans, just to name a few, had horrific vibration issues with bed mounts and the high compression.  The aircraft were fine when the gearbox was shimmed to max but shortly after some wear internally the engine would vibrate so bad that fuel would spew from the carb vent tubes and shake badly.  This was noted as the pilot would be coming down to just below peak torque on throttle down.  Descents were scarry for some.  The solution, since it was too late for the kits to be changed and the OEM did not want to redesign, was to install the overload clutch.  (circa 2003 large starter and overload clutches were added) This absorbed the inputs internally to allow them to use the ULS with the existing bed mounts.  

Today bed mounts are rare (thank heavens) and almost all OEM have the Rotax ring mount or a clone ring mount to fit special airframes. (RV12 is a Vans ring mount) 

This is what i know, some may disagree but feel free to try run one ULS without a clutch on a Zenair with bed mount.  ( I don't recommend it) 

Cheers

RW - What of start up/shut down movement control???

Maybe because a ring mount is better.

All engines will exhibit a startup pulse, even ones without a gearbox.  (direct drive type) This is normal in that the pistons push the crank hard on the power stroke and shortly after this another piston tries to slow it down with compression on a different cylinder.  The damping of these tortional moments must be absorbed into the mount between the airframe and engine.  Once running there is very little movement when you get above initial start.  On a geared engine like a Rotax the damping for the most part for torsional movement is done by the dog and ramp internally, that is the rattle you hear on both start and stop.  Higher frequency vibrations however are more complex.  The overload clutch absorbs those moments better in that the excitement at a frequency on the up or down RPM passage internally can be damped by micro slippage inside the clutch.  

If you have an opportunity to get into the Rotax training this is covered mostly in the heavy maintenance class.  The service class shows how to check the friction torque as long as you have a clutch witih 30 degree free-play.  The only way to judge otherwise is to listen, and feel, the gearbox at idle.  If the start up rattle does not go away at your idle speed it suggests it is undeershimmed.  This then creates excessive wear inside the gearbox.  You can normally test this by start, idle...if it rattles raise your idle with the throttle.  If it becomes smooth that tells you your idle is too low, or the gearbox damping is too low.  In that case time to remove it and have it re shimmed.

Cheers