Chassis bracing / gusseting

So, despite a lot of good ideas, nobody has yet to solve the chatter problems on the FZ / R7 / Dunlop combo.

Clearly, the frame on this thing is pretty spindly. The one team that seems to have made the most progress is N2, they use R1 forks (larger diameter) which leads one to believe the added stiffness is what's helping.

For us that means yet another set of forks and triples. Not willing to go that far yet.

Westby is supposedly working on some kind of frame brace for the front of the bike, but I haven't seen anything yet.

Chassis gusseting and bracing is not allowed in the MA rulebook, but it IS allowed for club racing in Superbike classes.

I honestly wouldn't know where to start with potential bracing. Offering more support around the steering head seems the likely first choice. You can see how the chassis would probably benefit greatly from more support between the front and rear, but since the main goal is to improve front end feel I'm thinking starting at the front would be better.

Wondering who on here has experience with this and if it's worth a shot- frames are not crazy expensive and we're willing to experiment a bit.

First thought:

 

What does it take to do a real stress analysis? I would think the right type of nerd would have those tools? Just a guess.

 

yeah like GMD or similar.

 

Not Familiar with stress analysis. Eric Grey had the GMD franchise down here, he passed away. Really just throwing shit at the wall at this point. But everyone needs a project......

 

I have a lot of info on this... let me look around.

I braced an old 70's yamaha triple cradle frame. Pretty happy with the results.
Ran some FEA on solidworks with the frame braces too.

 

Oh wait... you are working on a "modern" bike with the engine as a stressed member.
maybe I won't be of much help.

 

Is the engine a stressed member of the chassis on the R7? Just looking at the above pictures, it looks like the swingarm bolts to the frame, not the engine.

 

If it isn't... that would be the worst frame in existence.
I think the engine forms PART of the structure, but doesn't completely replace it like in a ducati.

 

looking at the frame, the top frame rails look like where the bike would flex.

as well as twisting above the swingarm area.

a couple of the old BMW airheads that raced had a triangular plate from the engine/trans to the top tubes in addition to the bolt on braces from near the steering head to the swingarm mounts of the bike.

i think i would do some kind of crossed brace in that space tying in the front structure to just above the swingarm mounting area, but there will likely be stuff in the way. the front gussets are a good idea as well. I'd triangulate it myself with braces but either way would be good.
I would think about bracing that front space under and behind the steering tube as well.

Here's some ideas... a lot of this example is overkill, imo. If he had fea, he could evaluate the contribution of each of those add on's and see which were worth keeping.
https://oldskoolsuzuki.info/archives/315

maybe something like this to start...


chop the rear subframe, make it a shorter and stiffer structure

 

You're potentially stepping into a deep pool. I recommend you do some reading first rather than just welding shit on. Although with a steel frame, there's not much risk with welding shit on other than wasting time/money. Google search for "Motorcycle Handling and Chassis Design the art and science .pdf" It's a text by Tony Foale. The actual copies are expensive collector things, but the .pdf should be easy to find. It has a bunch of math, but he explains things well to a non-math type as well.

Just looking at the R7 frame there are a couple points that stand out to me, but in general it should be useful to mentally compare it to a modern twin spar frame.

The headstock on that type of design lacks lateral support at the top. So if you imagined pushing the front wheel straight back towards the engine, all would be good. But with any left/right in addition to back, the top of the headstock could easily flex left/right. The tall cross section of a twin spar provides rigidity here, same with the trellis frame. That's also why that section of tubing behind the headstock is so much bigger than the other tubes, trying to keep some rigidity.

The swingarm pivot is basically backwards. The swingarm should be inside the frame (side to side) since that will overall be more rigid. I would guess that's why the R7 has those bolt on aluminum braces.

I'd say look to a 916/999 frame as inspiration. My gut answer would be to roughly mirror the bottom headstock tubes for the top, and add a single triangulating diagonal. Basically the front part would look like the beginning of a trellis frame. Then add the R7 bolt on braces and see what it feels like.

 

what type of metal to use would be my question .. 4140?

 

This is one of the dozens of runs I did planning my frame mods. Solid works FEA let's you apply any type and force of twist or bend in the frame and calculates displacement of the structure. Even animates it for you.

Once you do this, you kinda get an eye for knowing what works and what doesn't.

A trellis frame is of course a very good design... problem is most cradle chassis do not lend themselves to trellis conversions. You would really just be building your frame from the head tube to swimgarm mounts practically from scratch.

I started doing a r75 but got bored and stopped...

 

Interesting. @JCW the areas you drew are all occupied by the engine, cylinder head, or airbox, so yes, the engine is a "stressed member". The swingarm pivot area is very narrow, and that is the area that is reinforced with plates on the R7. The flexing of the chassis during exit was helped by this.

@Busdriver02 as you said, the steering head doesn't look very well supported laterally. My thought is that with the addition of the R6 front end and high grip tires, it's the flexing around the steering head that is getting us into trouble. I have been reading alot of Kevin Cameron, and I understand that stiffer is not always better, however as I said above some people are having better results with larger diameter forks which leads me to believe increasing stiffness in that area might help. Or it may be transferring MORE load into the steering head and flexing it even more. I don't know. It seems that the combo of the new tire (more grip, higher sidewall), the R6 fork, and the frame as it sits creates a harmonic with chatter that we can't get rid of. Ride height windows of +/- 20 mm have been tried (by other riders) with no luck.

My original idea was some kind of bolt on bracing tube between the steering head and the downtube that goes to the engine mount because 1. we could try it without wrecking a frame, and 2. we could take it off for MA races where bracing is not (yet) legal - but I can't see how that would change the twisting moment from a leaned over motorcycle.

Putting the frame in solid works doesn't look as complex as I originally thought, although the shapes are not all round, i.e. the frame is not strictly made of tubing as you can see on the top "spine".

 

This is where I would look. I bought the pdf version and it is very informative on where and how to triangulate and the pitfalls to avoid.

 

Just read your original post/thread about the stripped hole in the engine case...
Wow. the frame is probably practically doing nothing in that midsection and all the twisting is going through the engine case.
I'd focus on some way to brace that first...
Too stiff doesn't exist for frames like this (and old steel cradle frames), imo.

it's not gonna be easy as you said there stuff in the way... but I think you could get something in there.
Something crossed in an X (like in my first frame pic for my xs750) would be IDEAL, but rarely easy.
maybe two side rails, then additional bracing across to attach them to each other and to the top of the frame...
I like messing with these frames, it's fun.



I'm telling you the difference some good quality bracing made in my xs750 was easily noticeable and you didn't have to be riding 9/10+. something as simple as going over a hard bump felt different.

 

This is easier than changing forks?

 

If you're actually dealing with a harmonic (flexing frames and fork tubes are both undamped springs after all), then you might not have to change the frame's effective spring rate much to fix your problem. Might also explain the R1 forks, just moving one of the components' natural freq is enough.

 

Here is the fix

 

I actually have a huge list of things to try. This would be last on the list, of course. I'm hoping to have an organized test day coming up.

I was hoping to design some kind of bolt on brace to try, but that's not looking practical.

Another one of my ideas is to reduce the triple clamp offset and rake out the front a little more, then add some rear ride height. This is a simple change, just need the additional inserts for the triple. This would shift some weight forward without disturbing the geometry of the front end (theoretically). Going to mess up the swingarm angle though.

 

I have zero experience with R7s, but here's what comes off the top of my brain:

If you're trying to eliminate chatter specifically, we need to understand what this is. Chatter is an overlap of the harmonic wave of the fork/wheel/tire and frame where the amplitude gets increased at a certain frequency. So what we're ultimately after is just changing the natural frequency of either the wheel/fork or the frame so these natural frequencies wouldn't line up at this point anymore. In layman's terms, we want one of those things to be stiffer or softer if it can't be damped.

Is there any way of using a stiffer fork, or creating a fork brace to increase the natural frequency of the fork? Conversely, you could do the same to the frame, but it'd probably be much more difficult. Regardless, the theoretical part is rocket science, but the practical part isn't. You're just triangulating the headstock to increase stiffness and seeing what results you get.

Using the photo you posted, it seems like the upper part of the frame has very little lateral support because the frame uses a 'backbone' style beam that goes over the top. There's very little triangulation there. Look at this frame for example, make yours look more like this





It seems the hardest part of mimicking this would be making the frame wider and supporting the triangulating braces. It'll end up looking goofy in the end, but it'd definitely work. All that being said, it'd be much easier to start with some larger diameter fork tubes or something. Remember stiffness goes up by the 4th power when talking about tube structures, so even a minor difference in tube diameter will make a large difference in stiffness.