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Discussion Starter · #1 ·
Thinking of doing (having someone make one or help them make it) a custom minimalist subframe/tail for my 2011 Speed Triple. It will bolt onto the stock subframe tabs on the rear of the frame.

What tubing (chromoly, stainless)? Would like to stay away from aluminum just for ease of welding. Can chromoly and stainless be formed, as in a curve bent into it?)

What's the smallest diameter I can get away with? This will be a solo seat, the only thing it'll have to hold up is me and the CDI and relays. Battery on this bike is under the front of the gas tank.


Was thinking of something similar to these ideas. Basically I want as little of the tubing or subframe to be visible as possible.





Those this Joe Kopp RS Racecraft bike uses CNC'd plates, it's sort of the look I'm going for.
 

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sickboy
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Personally I would stay with 7/8 round tube at a minimum and use at least a 14 gauge but a 12 gauge would be best. Chromoly and stainless will both bend exactly like steel and weld very similar too.
Also, you can use chromoly if you want but thats so little tube it really will make no difference than just using regular steel tube. Stainless too will work fine if you want but again no point unless you want to leave it raw for that stainless look.
 

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Discussion Starter · #3 ·
I'm looking for the lightest stuff I can use. I only weight 170lbs, not 220 like most of you fatties.
So if I can get away with smaller diameter, albeit with a little thicker gauge I would go that route as well, most for the fact that the smaller tubing looks better.

With that red Sachs bike, what diameter would you estimate they used?
Whatever I use will get painted or powder coated either way.

Update: just measured the stock aluminum subframe tubing. It is 1" aluminum. But of course it's made to hold the weight of 350 lbs, plus the weight of all the crap it carries without passengers. What diameter under 1' in chromoly or stainless could I get away with given that?
 

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sickboy
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Honestly depends on your design. Figure out exactly how you want it and post up. you can get away with anywhere from 16 gauge to 10 gauge
 

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lɐʇuǝɯᴉɹǝdx&#4
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I think I'd be comfortable going as low as 3/4" 14g for a one up, maybe even 5/8" if it was good steel (1020 or Chromoly, maybe some grades of stainless) and care was taken in design and construction. That's what the Sach's bike looks like to me. If going that small I'd also look at using carbon fiber tubing instead of metal tubing, both for the pimp factor and because its stronger & stiffer without to much more cost when looking at moderate lengths of small diameter material.

Update: just measured the stock aluminum subframe tubing. It is 1" aluminum. But of course it's made to hold the weight of 350 lbs, plus the weight of all the crap it carries without passengers. What diameter under 1' in chromoly or stainless could I get away with given that?
From what I figure, 3/4" steel tube can be as stiff as 1" aluminum, assuming you can bump up wall thickness by 50%. This is based on rules of thumb that steel is twice as heavy by volume, but also twice as stiff, and thin tube stiffness goes up with the cube of diameter. So you'd give up a bit over half the stiffness by using smaller OD tube, but get it back by using a stiffer material (steel) and more of it. Of course, the downside is the new design weighs over twice as much. Its also probably overkill, as aluminum structures need to be (much) stiffer than steel to avoid fatigue failure; steel can live with more flex. Which leads back the long way to the skimpy tubing I felt OK with.
 

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A lot of this is purely subjective talk.

You need to read a book on material strength. Find out what each material will flex with a given load at a given length. Then do your math.

Properly done, 1/2" 16g stainless wouldn't flex for shit with a 400 pound load on it.

Our old superbike sub frames had minimal 1" square tubing with 1/2" cross bracing. 1/8th wall.


Go get yourself 2 feet of each type of tube you're considering. Secure it dead horizontal on end and hang weight off it until you have a measurable deflection. Divide by 2 to get tensile load.

Then figure out at what point your 170lbs would create more force against. Either landing a jump at full weight or what have you. Remember, most common welding filler material has a tensile load in the 10s of thousands of pounds.
 
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:Stupid:

Hybrid's post is good advice, some knowlage of material strengths along with good design could allow you to go smaller or thinner walled on your subframe

ive gone as small as 1/2" for the lower rails with 3/4" the top because of design & have also made self suporting seat units from sheet steel or carbon

The other thing that comes to mind is while you may only be a lightweight like myself, if/when you sell the bike on could the next fat arse owner break your subframe & injure himself or someone else IE over engineer if in doubt & think of the end user
 

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I've been looking into this exact same stuff because I will be building my first completely custom
Subframe. I decided to use 1in square tubing for the top of the subframe and 3/4in round tubing for the bottom rails and gussets. I was going to go all 1in square for the whole subframe but then remembered my frame is going to be steel tribe so the square subframe wouldn't look right. But seeing as the top rail is going to be hidden by the tail fairing I chose to use it for the top rail because it's stronger and use the round tube on the bottom to match the frame. It's not going to Be a 2 up bike so I'm sure the 3/4 round would work fine but I'd rather be safe then sorry
 
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