Stressed Motor...

[Flynt]

So I am hoping to learn something here...  I don't understand how the FJ went from "stressed engine" to "unstressed engine", and thus bigger frame, with the '91 MY. 

I have a fair understanding of Vector Statics and have looked at all the mounts in some detail.  Not sure anything changed when the rubber was added other than an isolation element that takes out part of the vibration while the engine is still bolted directly to the frame through a link just like the earlier bikes...  structurally equivalent I think and the frame is still benefitting from the cross engine through bolt (I think this is the key part of frame integrity with both designs).

I'd like to know why the frame was increased in size...  but my contention is the best mod platform is the '91-93 due to the inherently stiffer frame.

Now that's a popcorn moment!   

[Pat Conlon]

So I am hoping to learn something here...  I don't understand how the FJ went from "stressed engine" to "unstressed engine", and thus bigger frame, with the '91......

Frank, I don't know where that came from....did you read it somewhere? Having bad dreams at night?
I have always thought of the FJ engine as unstressed, meaning that the front end forces are transferred to the steering stem and then to the neck and then thru the frame rails....as are the swing arm forces, transferred to the frame via the pivot bolt. IOW The front and back forces are absorbed by the frame.
When I think of a stressed design, the engine cases transfer the loads with no frame connection between the front or back load path except thru the engine case.

I would like to learn more....

[FJmonkey]

Frank, I don't know where that came from....did you read it somewhere? Having bad dreams at night?

Found this page: http://fjclub.co.uk/#/buying-a-fj/4546920434

-Aftermarket rubber mounts for the earlier generations (do these exist? in the first three generations the engine was a 'stressed member', not so in the last gen; does that matter?)



How does this sound?

From the FJ Hunting thread...

[Flynt]

From the FJ Hunting thread...

What he said...  I think I had heard this before as well, but likely referencing the same source.  Doesn't make sense to me, so thought I'd start a discussion amongst the best and brightest FJers on the planet.

Frank

[rktmanfj]

I'd like to know why the frame was increased in size...  but my contention is the best mod platform is the '91-93 due to the inherently stiffer frame.

Now that's a popcorn moment!   

Frank, not saying that you are wrong at all in this case, but (for instance, when Honda was developing the CBR900RR) testing revealed that there is a point where there is too much stiffness in a m/c frame, which is actually detrimental to handling.  They found the same thing about wheels when developing the Comstars.

One of the m/c tests from back in the day detailed the reasons for the later model FJ frame, but I don't have it handy right now. 

     

[fintip]

I am pretty sure I read somewhere in trying to understand this myself (I cant seem to find a good resource talking over frame dynamics, if anyone has one, Id love to study up) that the rubber stops the transmission of vibration into the frame, but that increases stress on the mounts, or something to that effect. Thats as close to an explanation as I got. I also remember reading about the.... gpz900 from 86 I believe? that was the first bike to just use the engine as a part of the frame.

Then theres the comment on fjclub elsewhere, if I remember right, that handling of Japanese bikes sucked before the FJ because of the frames. Why a perimeter frame helps handling (aside from factors like weight distribution, flex-stiff, and rake), I am not sure, and again has me back to that first parenthetical comment.