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Frame Bracing.

  • Thread starter Thread starter Ausben
  • Start date Start date
PJ.
A model would be useful, but it depends on what you are trying to get out of it. While understanding the internal stresses of the frame and understanding the relative benefits of adding braces is certain areas would be important, we really don't know the external loads as they vary with track conditions. So the primary benefits will be relative performance.
Generally models are developed with data to "verify" or calibrate the model. We are not likely to get that. Without that data it is hard to get absolute numbers.

If some one that is an ME here and wants to take it on as a project, I'm sure you would learn plenty and have fun at the same time. unfortunately that is not me :rolleyes:

Having said that, the diagram is useful as it makes it clearer, where the loading occurs and why the OEM frame is braced as it is. From a perspective of high performance frame bracing, I think keeping the torque diagram in mind is useful in incrementally changing the frame. Hopefully it makes clear why having those central "C" gussets is so important to resisting overall twist.

I would caution anyone that trying to start from scatch on bracing using a FEM would be the long way to go as we already have very good examples of the right bracing to add, we are now just trying to perform a relative merit assessment for street use.

Jim
 
posplayr said:
JED,
I added some freebody diagrams here; maybe this would help answer your question

http://www.thegsresources.com/_forum/showpost.php?p=1013240&postcount=4

Pos
Click to expand...
I think your loads are not quite right.

Diagram 1 you are missing the resultant force at the swingarm pivot bolt. That force would be pulling down and away from the frame at the pivot bolt.
The force acting at the fork wouldn't be vertical at that point. The force is at the wheel which is offset from the support so you would have an additional moment at that point in the frame. This moment could be shown as two counteracting forces one at the top of the stem and one at the bottom. The bottom of the stem is pulling in and the top of the stem is pushing out.

Diagram 2 is interesting but I dont agree that changing the axis of "A" would be beneficial. That brace is additionally resisting the moment developed from the wheels distance from the frame. I would imagine that that the load in the front would increase at higher speed cornering which would could torque the front stem. Having "A" braced at its angle would resist that.

Developing a static model wouldn't be hard but who actually knows the forces developed from riding in a turn? Looking at the frame like this really helps to explain why the monoshock is the modern rear suspension.
 
I think your loads are not quite right.

Diagram 1 you are missing the resultant force at the swingarm pivot bolt. That force would be pulling down and away from the frame at the pivot bolt.
The force acting at the fork wouldn't be vertical at that point. The force is at the wheel which is offset from the support so you would have an additional moment at that point in the frame. This moment could be shown as two counteracting forces one at the top of the stem and one at the bottom. The bottom of the stem is pulling in and the top of the stem is pushing out.

Diagram 2 is interesting but I don't agree that changing the axis of "A" would be beneficial. That brace is additionally resisting the moment developed from the wheels distance from the frame. I would imagine that that the load in the front would increase at higher speed cornering which would could torque the front stem. Having "A" braced at its angle would resist that.

Developing a static model wouldn't be hard but who actually knows the forces developed from riding in a turn? Looking at the frame like this really helps to explain why the monoshock is the modern rear suspension.
Click to expand...

I agree that the resultant force on the pivot is not right. After posting i realized that there was something askew. Given the assumptions , what are the pivot bolt forces?

Reading the rest of your comments, there is a common theme. You seem to think that the X-Z plane bending moment is significant if not more important that the torsion effects I described.

Given the relative motion of wheel axis (axles) during aggressive riding I don't see how this bending moment, within reason as being a major concern to handling. It is a concern for JED, if he cuts off is back triangle that is the primary supporting structure for the bike and rider weight but that is the exception. :rolleyes:
 
Last edited:
okay, what if i did this:

braced1.jpg


cut off the brown part

sorry, my mario-paint skills arent as good as yours, and my new PC doesnt have photoshop (yet;))
 
Last edited:
jed.only said:
okay, what if i did this:

braced1.jpg



sorry, my mario-paint skills arent as good as yours, and my new PC doesnt have photoshop (yet;))
Click to expand...

If you did that it would take you a LOT of time for very little benefit.
 
jed.only said:
okay, what if i did this:

braced1.jpg



sorry, my mario-paint skills arent as good as yours, and my new PC doesnt have photoshop (yet;))
Click to expand...
The most critical areas of the frame are the neck and the swingarm pivot. It's why all the new sportbikes, except Ducati, have spar frames that do away with the double cradle design and use spar frames that connect the steering head and pivot area in as straight of a line as possible and reinforce those areas. Even most of the touring bikes these days have spar frames. About all the areas you have in your picture might do is strengthen the top shock mount area, and not by much.
 
the reason im asking is im monoshocking the frame and i want to build a custom subframe that bolts on, like a modern bike.
 
Billy Ricks said:
The most critical areas of the frame are the neck and the swingarm pivot. It's why all the new sportbikes, except Ducati, have spar frames that do away with the double cradle design and use spar frames that connect the steering head and pivot area in as straight of a line as possible and reinforce those areas. Even most of the touring bikes these days have spar frames. About all the areas you have in your picture might do is strengthen the top shock mount area, and not by much.
Click to expand...


i left out the part about me cutting the brown part off.
 
jed.only said:
the reason im asking is im monoshocking the frame and i want to build a custom subframe that bolts on, like a modern bike.
Click to expand...

and then do this

the blue is my tabs and new subframe

braced1.jpg



then add whatever bracing i need.
 
JED,
The reason I emphasized that rear triangle is because that is probably one of the strongest parts of the frame.

Now it is build that way partially to carry the rider weight but mainly because the shock mount is at the upper rear part of the triangle.

Because a mono shock will not have that load U might be able to get away with it. U will be relying completely on the forward double cradle.

You need to look and see if you can find someone else that has cut off the rear triangle on a GS and see what they think. You are taking a week frame and dropping the strongest part and now trying to build it back up:-k.

If you could get an ME to help you , OK but given your lack of expertise in this area, I would look for other examples before getting the torch out. Maybe even get a GSXR.

The first gen is considered a pretty spindly frame as modern bikes go, but look at the mass of metal on the lower swing arm pivot in order to have a suspended tail.

This is one of the nicest "bug tails" I have seen,

http://www.gixxer.com/forums/showthread.php?t=200135

U are definably wanting to do OSS C and D will be accomplished by the mono shock mounts (look at PJ's bike). You are going to have to overcome not having the rear triangle though.

picture.php
 
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looks like the "cradle" is pretty much braced and the sub is not a "structural" part of the frame, as i suspected.
 
JED,
Of course when you mono shock the cantilevered tail section only has to support the rider weight. With the twin shock the weight is supported at the rear upper corner of the strength triangle.

The point I have been trying to make is that beyond the strength and support for the rear twin shocks, there is a certain portion GS frame rigidity is coming from this rear triangle as well.

You need to make sure there is plenty of cross bracing in the mono shock mount to increase the frame stiffness to account for this loss.

Pos
 
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