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Feckin Eejit!
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Discussion Starter · #1 ·
I've been talking to Ammadien about doing a subbie out of al to run this Duc Monster seat unit I picked up but ran into questions of structural integrity of bent tube and what grade of alu would be safe.
I weigh 200lbs (hoefully less soon :/ ), I'd like to be able to travel a bit on the bike as well and would like to have the option of either having a passenger or putting on some saddle bags in the future.

This is the rough design I'd like to follow


I'd prefer to mimic the frames bend and over all shape as much as possible for aesthetic reasons. The frame is one of the reasons I fell in love with these Triumphs.

I want to use Aluminum for the weight savings, but I don't want to end up sitting on my rear wheel... The existing sub is aluminum, square and round tube, heres some photo's of that





I'll probably source another stock subby to chop the tank and seat mount brackets from if building a sub like this ends up a reality.

The bike is heavy, the lump is heavy, I want to keep weight down as much as possible, especially this high up on the bike, but am open to using steel if it's the only realistic option.

Also, hows this for getting my bends uniform ?
http://www.instructables.com/id/How-to-bend-tubing/
The guy doesn't mention filling the tube with sand... but I would plan on that too.
 

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Feckin Eejit!
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Discussion Starter · #3 ·
His design is part of the inspiration for this actually. If I can use substantially less material to offset the weight increase to a large degree, I would consider steel.

I'm a fish outta water on this subject. Primary concern is safety really. Secondary is weight. But they're close to tied in importance.
 

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moron with a hammer
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Unbraced bends like to continue bending. Doesn't matter what they're made of.

Also, from a purely aesthetic standpoint, there are no sweeping curves on the frame. It's straight, bend, straight, bend, straight...

I'd be really tempted to keep the top tubes straight for most of their length, and then have them kink at the very end to meet in an up-and-over hoop. Have the lower tube do the straight-bend-straight thing. If you can't work it out so that all the bends are against tube, box them somehow. Do that, and you can use surprisingly thin-wall steel tube, and it won't end up being any heavier than aluminum. You also save yourself the grief of nesting large-radius bends.

Interested to see where this goes...
 

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Feckin Eejit!
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Discussion Starter · #6 ·
Also, from a purely aesthetic standpoint, there are no sweeping curves on the frame. It's straight, bend, straight, bend, straight...
I'm not quite sure how my brain just saw one continuous curve to the frame. But it sure did.
Christ am I nutso sometimes :nuts:

My sketch also had the seat way the fuck up there. This version is probably too low... Actual seat height would be dictated by subframe


This looks to be way less bend... but the little bump to mimic the frame looks like it's trying too hard.
 

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Feckin Eejit!
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Discussion Starter · #7 ·
Moving the seat down like that changes how much of the sub is visible. Maybe bends are just a bad idea.

I don't think I've ever had an aluminum rod (as opposed to tube) to compare weight.


If that angled support bit was a rod and the grey bits in the seat area were tube, would it have greatly increased structural integrity as well as weight savings over steel? Would steel tube be lighter than alloy rod at that point?

Would it more prudent to put the primary weight bearing bit directly under my ass? Or would it be best to have it as far back and figure out a truss or gusset or something to tie in the bit under my ass with primary load bearing bit?
 

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I shouldn't speak to material strength Aluminum vs Steel cause I dunno.

But I noticed the use of rod in your sketch. While rod is stronger than tube of same diameter, it is way heavy. Ya gain strength vs weight faster with larger dia tube. Even going thinner wall will be stronger and lighter with a step up in dia vs a thicker wall.

Here's part of the puzzle quoted from 4 wheeler network

Strength is derived from two dimensions for a tube of a given material: outer diameter and wall thickness. Thicker wall translates to greater resistance to denting or other impact damage. When it comes to bending, the strength comes from the distance of the O.D. from the center axis. A larger diameter tube with even very thin wall can be more rigid than a tiny O.D. with very thick wall.

For a solid rod, the excess material in the center (as opposed to the hollow tube) adds progressively lesser strength as it approaches the center axis. Hence, solid is often only slightly stronger in deflection than decent hollow tube....despite adding much greater weight.









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moron with a hammer
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I like the last drawing best so far.

Rather than running a thin rod, I'd run a hollow tube, say 3/4", with maybe a .090" wall, and rather than welding it solid, I'd use rod ends to attach it to frame and upper subframe. That'll keep the thin look, and essentially turn the upper subframe into a lever, keeping the support tube in compression. You can run relatively thin, small-diameter tube then, as there will be no bending forces on it. All the bending will be on the subframe, which you have to make stiff in every design you've shown so far.

I'd probably move the mounting point up a little, unless you intend to carry a passenger. Try to think of what's going to put the most direct load on the support rod.

Learn to love steel. You would not believe how thin it can be and still support a ton of weight.
 

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Most visually appealing design




Would it more prudent to put the primary weight bearing bit directly under my ass? Or would it be best to have it as far back and figure out a truss or gusset or something to tie in the bit under my ass with primary load bearing bit?
best design for two up. it mimics the stocker. and the weight bearing behind the driver (under the passenger) will be best for two up, then you won't have a cantilever.
 

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QUESTIONABLE ENGINEERING
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His design is part of the inspiration for this actually. If I can use substantially less material to offset the weight increase to a large degree, I would consider steel.

I'm a fish outta water on this subject. Primary concern is safety really. Secondary is weight. But they're close to tied in importance.
6061 aluminum is very comparable to mild steel in most aspects, while usually providing a much higher strength-to-weight ratio. I say "usually" because there are many kinds of "strength" and mild steel greatly surpasses 6061 in tensile strength and yield resistance.

In the case of the subframe, steel is a safer option because there is a lot of tensile force being put on the top mount point on the frame. All of your weight +passenger weight + force of both riders hitting bumps in the road work toward pulling those two rails from the frame.

Unfortunately for Speed Triples, this argument is quite moot because you will be bolting the subframe to the welded aluminum tabs on the frame. Those tabs have been known to crack off from the frame even when people bolt on a "top rack" to the back of the stock subframe and put more than a pair of gym shoes on it. Happened to a couple people on Triumph RAT. Those frame lugs just weren't designed that well, so that will be your weak point no matter how well you design the subframe :/
 

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Feckin Eejit!
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Discussion Starter · #15 ·
Unfortunately for Speed Triples, this argument is quite moot because you will be bolting the subframe to the welded aluminum tabs on the frame. Those tabs have been known to crack off from the frame even when people bolt on a "top rack" to the back of the stock subframe and put more than a pair of gym shoes on it. Happened to a couple people on Triumph RAT. Those frame lugs just weren't designed that well, so that will be your weak point no matter how well you design the subframe :/
I had no idea that the frame lugs were a weak spot on these. Rep and thanks for the heads up.

Looks like the plan'll be to eventually convert to monoposto and keep it a toy rather than trying to figure out some magical solution for touring and good looks... Bummer... Looks like I'll have to pick up something for long range at some point.
 

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Feckin Eejit!
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Discussion Starter · #18 ·
Thanks for the awesome photo's. I saw your build on t.rat. Really dig the look.
Are the ends of the tube where they mount to the frame bungs a piece of hardware that you weld into the tube? What are they called so I can google?

Any idea what that sub weighs? Going to try to take the oe unit off and get it on a scale this winter. I really don't want to add any weight to this bike as our triples are heavy lumps.
 

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heres the chromoly subframe i made for my buell.



 
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Thanks for the awesome photo's. I saw your build on t.rat. Really dig the look.
Are the ends of the tube where they mount to the frame bungs a piece of hardware that you weld into the tube? What are they called so I can google?

Any idea what that sub weighs? Going to try to take the oe unit off and get it on a scale this winter. I really don't want to add any weight to this bike as our triples are heavy lumps.
Thanks! You won't find them on google. I have a friend who is a machinist. I took him a piece of the tubing I was using for the frame, had him basically replicate the ends on the stock subframe and had him machine the end to fit inside the tubing and then I welded it together. Not sure what it weighs, I know its heavier but its no race bike so I am not really concerned with any additional weight. I honestly dont think I will notice a performance difference with an extra 10 lbs or so on the street.
 
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