Topic: Engine as partial stress member

[rbmgf7]

I was curious if anyone here was brave enough to make their engine a partial stress member amongst the chassis? i've seen a few frames on satanicmechanics site and was wondering if anyone here has one. if so, more pics please 

[heffay]

"brave" might not be the right word.   

[markjenn]

I was curious if anyone here was brave enough to make their engine a partial stress member amongst the chassis?

I thought any solidly-mounted engine that is surrounded by the frame is a "partial stress member".

- Mark

[heffay]

in order to be stressed... the engine must be used as part of the frame.  if it is only cradled by the frame, it is not stressed.

i've never heard to it referred to as "partially stressed"... an engine is either a stressed member or it is not.

[markjenn]

in order to be stressed... the engine must be used as part of the frame.  if it is only cradled by the frame, it is not stressed.

Any engine solidly mounted to a frame that solidly attaches at points that brace the frame is most-decidedly stressed.  If you take the engine out of a CB750 the frame will definitely be less stiff.  And the engineers factored this into the design as they do with all solidly-mounted engines.

Some bikes totally rely on engine as the frame - some BMWs attach the steering head to front of the engine and the rear of the bike to the back of the engine.  Conversely, some engines are rubber-mounted and the engine does nothing.  I think there are lots of shades of grey in-between so I don't get the "they're stressed or not" statement.  I think the term "partially stressed" is designed to cover that grey area for lots of designs that distribute the load between the frame and the engine.

A CB750 doesn't have engine mounts on the cylinder heads though so it is definitely less stressed than many of the newer designs which do use the engine more extensively - the CB750 engine basically braces the two bottom frame rails, but the majority of the load is borne by the backbone.  I have no idea whether this meets the bar of "partially stressed" or not.

But forgetting the terminology discussion, it would help to know what the OP had in mind which I would assume is to use the engine more than it is now used.  Fashion an attachment point high on the engine?  Cut the backbone out and attach the steering head to the cyl head and the rear of the frame to the engine cases?

- Mark

[heffay]

it made sense up until you said take the motor out... now, why the heck would you need to ride around on a frame w/ wheels and no motor?

its kindof like saying the drywall in a house is a stressed member because it helps to add rigidity to the house... drywall should never be considered a stressed member or used as such.

[dusterdude]

an engine is used as a stressed member if the frame is not a perimeter frame.that is the engine is hanging from the frame mounts,ie just about any sport bike made in that last 15 years or so.

[rbmgf7]

if you placed a force between two engine mounts, then it does become a stressed member. however, you would only see this if you crash into something 

if you look at your frame, you could actually eliminate the frame rails between the two bottom mounts. this rail technically doesn't see any forces, it's all applied to the cases. now, only if you trust the case mounts to be sturdy and structural enough to handle it's own weight and applied forces from riding. but if you're anal about weight, you may save a pound 

i would say the engine was to be a stressed member if it had a mount on top like my KZ400. then it would help deflection by the moment caused by turning. if you look at the bike from the front, then it would be triangulated from the left/right and top mount points.

from my original post, i was sorta implying a partial stressed member like the motorcycles nowadays where there is very little frame structures. more "the engine is the frame and everything bolts to it".

just thought it would've been a cool topic 

[GammaFlat]

it made sense up until you said take the motor out... now, why the heck would you need to ride around on a frame w/ wheels and no motor?

its kindof like saying the drywall in a house is a stressed member because it helps to add rigidity to the house... drywall should never be considered a stressed member or used as such.

Frame up a wall and push on the wall before you put drywall on it.  Then put drywall on it and push on it.    There's quite a large difference. 

[heffay]

no sh!t sherlock...

i'm done arguing, i graduated MMI and we discussed engines as stressed members...
i also graduated w/ a design degree from CU and own a remodel biz, so i know what drywall is made for and what it is not.

[rbmgf7]

kinda like this bike from satanicmech.'s website

[fuzzybutt]

kinda like this bike from satanicmech.'s website

that pic gives me major wood

[cafe750]

it made sense up until you said take the motor out... now, why the heck would you need to ride around on a frame w/ wheels and no motor?

its kindof like saying the drywall in a house is a stressed member because it helps to add rigidity to the house... drywall should never be considered a stressed member or used as such.

Frame up a wall and push on the wall before you put drywall on it.  Then put drywall on it and push on it.    There's quite a large difference. 

Engineers actually use sheetrock as a structural part of a wall, when they specify a sheetrock or plywood shear wall in a house...but thats a completely different thread...

[martino1972]

how the hell did we go from building bikes to building houses so fast,normally that takes 2 pages,you guys are getting impatient..

i think,long story short... the cb engine is not designed to be used as a stress member of the frame structure,failure of engine cases may occure and while driving to work on a short raked bike,you will come home riding a chopper,with your tongue twisted up in the spokes...

[markjenn]

i think,long story short... the cb engine is not designed to be used as a stress member of the frame structure...

I don't know the degree to which the CB750 engine adds to the frames rigidity, but I can guarantee you that the frame is less stiff without the engine providing cross bracing, just like the wall is less stiff without the drywall.  By definition this means that the CB750 engine is carrying loads to some degree.  To what degree, I don't precisely know, but if you were to engineer rubber engine mounts for the CB750 such that the engine could no longer carry loads, you'd have to compensate by beefing up the frame.

There have been several motorcycle designs (BMW K100 comes immediately to mind) that were originally designed with solid engine mounts but were later revised to have rubber-mounted engines.  In every case, the frame had to be beefed up to compensate for the engine no longer having to bear loads.

- Mark

[martino1972]

yes,the engine functions as a cross member.....but you cant use it as a structual part of the frame,like lets say cylinderhead to engine case being the carrying load of the frame..

[malcolmgb]

not seen Rick Denoon's 400f mentioned http://www.denoonsp.com/serv011.htm is he still a member here?

[sohc4]

On any backbone backbone frame (like the one above) the engine acts as a stressed member. When Egli built a frame for a specific Kawa engine which had the engine mounted in rubbers, these rubber mounts on the engine had to be replaced with rigid parts to ensure stiffness of the frame.

Cheers,
Axl

[MarshallCS]

So what would happen exactly if you beefed up the mounts, and removed the under engine frame tubes between the front and rear mounts?

[strynboen]

cb 77 and cbx 1000 are thrue stressed engine used as part of the frame...they dont have tubes down in  front of the engine...the engine makes it aut for a part of the frame..

i have both bikes..they are close in konstruktion...a lot of details in build of the frame...old insparision

[MCRider]

Egli and MArtin

http://forums.sohc4.net/index.php?topic=67581.0

[lucky]

The CB750 engine was not designed to be a "stressed member" of the frame.
Matter of fact the CB750 frame is a double loop cradle frame made to not flex and it is easy to work on.

If you want a sport bike with the frame over the top, and the engine hanging down you need to start with an engine that is designed to take those stresses.

The aluminum frame motorcycle with the frame over the top is a nightmare to work on.
It blocks access and some access is gained by removing the gas tank.
Why make it hard on yourself. Use the stock honda frame and have easy access to working on everything on the bike.
Also changing to a mono shock is another bad idea. The frame is only 1/16 inch thick and why would you want to make it harder to work on. The twin shock set up makes it easy to remove one shock at a time if needed, while the other shock holds up the bike.  stresses are evenly distributed on the swing arm so there is no potential twisting on the swing arm.

Yamaha ruined their V-Max with this same idea.
Now the V-Max has a bulky aluminum frame that makes working on the bike difficult and of course the frame has to be bulkier.
If you are going to pay that kind of money ($20,000 plus) why not buy a Ducati and get a real steel tubular light weight frame that will last many years.

The aluminum frame is subject to cracking ,difficult to repair, and  can easily be damaged in a minor accident.

[MarshallCS]

The cbx1000 looks cool, really shows off the engine!

Sent from my Nexus 5 using Tapatalk

[strynboen]

yes cbx are a nice...there are 4 mounts in the head..who holds oilcooler and engine mounts..and 2 more back  down on the engine

klick on the lower photo to make it bigger...

[lucky]

The cbx1000 looks cool, really shows off the engine!

Sent from my Nexus 5 using Tapatalk

That CBX1000 was designed from the start to accomodate the stresses on the engine.