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Discussion Starter #1
I was wondering what more experienced personnes think about the old backbone frames Tony Foale built in the 70’s





They seem to be of extremely simple construction, lightweight, and from several simulations with autodesk’s frame analysis software seem to handle forces with minimal distortion.

Tl;dr
Want to have one made for a t500 engine, good idea or not?
 

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Here are my thoughts from a lifetime of being a motorcycle road race fanboi:

The single spine frame design you posted above worked really well for light weight two strokes. Foale used that design for large race bikes as well as road bikes, but it quickly evolved into the wishbone style frames we know today, where as he was still building those single spine frames for competitive racers until 1978 for TZ250's and 350s.

The biggest advantage is the direct connection between the swingarm pivot and the steering head. The only concern I have is that unless you pay special attention to the geometry and how the engine is used as a stressed member, you could end up with some frame twisting where the front wheel and rear wheel are moving out of alignment during cornering. If built properly and to his original specs it shouldn't be an issue, but it is something to consider. The heavier the bike and the more powerful it is the more likely you are to see those twisting forces - hence why the big bikes moved to wishbone frames and the smaller ones used the single spine for longer.

A standard T500 is about 408 lbs dry. With the right equipment I could see you having a road bike in the low 300lb range wet (like about 325lbs) which would make the thing pretty spirited.
 

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Discussion Starter #5
cbx honda:

View attachment 87225


z1 900 frame:

View attachment 87233

View attachment 87241

compared to the t500 frame, see how much wider the swingarm pivot supports are and how they attach further up the spine backbone?
I see it, it’s because the SA pivot is so far below the backbone. In order to deal with the force on the pivot having a larger inertial moment it required bracing. The advantage there is that the shock lies. Parallel to the backbone so the forces transferred are almost perfectly in line with the spine so there is little strain from that source.

Interestingly, it was suggested here


That the backbone frame may actually have more torsional stiffness than a triangulated wishbone style frame of similar weight.
In terms of lateral stiffness however it is severely lacking by comparison.

I was planning on using a 2.5” by 0.95” 4130 steel tube for the backbone. The image of the 500 frame i posted seems to indicate that he used used 2” diameter tubing, but to be certain in added and extra .5” to it.
This page seems to indicate that he would have likely used thinner walls at 2mm than the 2.4mm i would use.


I figured i could build it and see how it feels, and add a small tube on each side from the SA pivot up to the neck with minimal weight penalty. This would maximize both lateral and torsional strain.
 

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Discussion Starter #6
Unfortunately i won’t be able to have the pivot inline with the spine, it’s about 8” lower.
This also makes designing the rear suspension in a way that gives a progressive rate geometry a little tougher as it limits the angles that it can be mounted at without mounting the shock under the spine. I’ve got a hunch about this one, but i’ll see how it mocks up in Inventor.
 

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.095" is pretty thick for a large diameter tube. Do the analysis with thinner gauge and see how much it changes the results.

Can you rotate the T500 motor slightly forwards (nose down a little) to get a straighter run in the top tube?
 

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Discussion Starter #10
I know, i know cyorg.
But i was hopeful that i could use a non linkage setup and still obtain a progressive rate. Far less complex, and a lot lighter.
It’s easier to think harder now and build less later.
I can do it with the overhead shock, but it means that the angle between the shock and frame increases as it compresses which i was hoping to avoid. If i can get the SA pivot to line up a bit closer it will be okay to mount on the top.
 

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Discussion Starter #11
.095" is pretty thick for a large diameter tube. Do the analysis with thinner gauge and see how much it changes the results.

Can you rotate the T500 motor slightly forwards (nose down a little) to get a straighter run in the top tube?
I’m worried about the tubes buckling if i go too thin, plus the total weight of the frame and sub frame should be between 15-18lbs

I can get the tube a lot closer after a second check. It will raise the neck a little, but the forks have some spare length and if i set the rake angle a little steeper (2-3 degrees) it should be okay.
 

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the tube frame idea is surprisingly good, moreso than you'd think. the thing that makes them really sweet is the ability to flex along the whole tube if built right, a lot of the challenges of designing frames is making them flexible enough not to overstress certain points, the more energy you can get the whole frame to deal with, the better, and with the big tube frame, you can also very easily "tune" the amount of flex. I played with tube frames in solidworks, as well as a walt siegl frame I own, I have ridden the siegl frame on the track and loved the handling, I could make the tube frame act the same without as much stress in "hotspots" as walt's frame had.
 

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I’m worried about the tubes buckling if i go too thin, plus the total weight of the frame and sub frame should be between 15-18lbs

I can get the tube a lot closer after a second check. It will raise the neck a little, but the forks have some spare length and if i set the rake angle a little steeper (2-3 degrees) it should be okay.
For thin walled tubing, you get way more strength increasing the diameter of the tube rather than the wall thickness.

Area moment of inertia (the resistance to bending of a hollow tube) is approximately pi x R^3 x T. R is OD and T is thickness. So stiffness is porportional to T, but porportional to the cube of radius.

Torsional resistance or polar moment of inertia is similar DC-11V5.png
 

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Discussion Starter #14 (Edited)
For thin walled tubing, you get way more strength increasing the diameter of the tube rather than the wall thickness.

Area moment of inertia (the resistance to bending of a hollow tube) is approximately pi x R^3 x T. R is OD and T is thickness. So stiffness is porportional to T, but porportional to the cube of radius.

Torsional resistance or polar moment of inertia is similar View attachment 87313
that's true
but i'm not talking about the frame strength/stiffness here, i'm talking about the tube resisting kinking. The larger the diameter for a given thickness, the more liable it is to kink. As the diameter increases, the curvature decreases, so you get less support from the arc and it behaves more and more like a flat piece. This is only for localized stress though.

so i redid the sketch of the frame to include a beam for the forks and a few for the swingarm so that i could place the appropriate loads and constraints
3 tests on the frame with no engine support.

enough force on the lateral test to bottom out the planned spring rate.
and substantially more on the other 2
1400 ftlbs of torque twisting the neck with the SA pivot pinned
and 900lbf pushing the neck across with the pivot pinned.
if the frame ever sees these magnitudes, the frame holding is the least of my worries.
i am content with the results

Edit: the images do not show the actual displacement of the tubes, they are adjusted to for easier viewing.
 

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Flexi frames :rolleyes: you like that do you. I never really enjoyed that wallowing through the corners, wiggling on hitting bumps and vague handling, rubber hinge in the middle of the bike feeling myself, I buy bikes with the stiffest frames possible.
 

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Discussion Starter #16
Nope, not a fan of flexi frames.
Luckily this version was lighter than expected so there is wiggle room to add additional stiffening.
 

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OK, so what are your results?

Tubes have to bend before kinking. The area moment of inertia tests that very property. A beam's ability to resist deflection for a given cross sectional area.

that's true
but i'm not talking about the frame strength/stiffness here, i'm talking about the tube resisting kinking. The larger the diameter for a given thickness, the more liable it is to kink. As the diameter increases, the curvature decreases, so you get less support from the arc and it behaves more and more like a flat piece. This is only for localized stress though.

so i redid the sketch of the frame to include a beam for the forks and a few for the swingarm so that i could place the appropriate loads and constraints
3 tests on the frame with no engine support.

enough force on the lateral test to bottom out the planned spring rate.
and substantially more on the other 2
1400 ftlbs of torque twisting the neck with the SA pivot pinned
and 900lbf pushing the neck across with the pivot pinned.
if the frame ever sees these magnitudes, the frame holding is the least of my worries.
i am content with the results

Edit: the images do not show the actual displacement of the tubes, they are adjusted to for easier viewing.
 

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Discussion Starter #19
The results indicated that the forces that would cause significant distortion of the frame would more likely result in a crash long before that point of distortion is reached.
 

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......ok

Don't go under...
50mm=1.96"
1.2mm=0.047"

May not need to go over
75mm=2.95"
2.0mm=0.078"

The thicknesses sound about right... I almost always use 16g tubing which is 065. occasionaly 18g tubing for subframes or bracing which is 049.

084 is 14g, 095 is 13g. Like Teazer said those typically are not in the realm of sport chassis building.


Harley's maybe...

but whatever, nobody comes here to listen anyway...

The results indicated that the forces that would cause significant distortion of the frame would more likely result in a crash long before that point of distortion is reached.
I meant comparing a thinner walled tubing at one larger size diameter.
 
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