Best light, rigid metal or alloy?

[Wzed]

I picked up a Monster seat that Ducati made for models destined to serve in the Italian police.  Basically, it’s like a standard seat cut in half, with no provision for a passenger seat.  Why did I do this?  Because it’s 2lbs lighter and I’m a weight weenie (bike is currently about 330lbs wet).  To get it to work, I need to run a metal spine that attaches to the seat and then back to the stock seat latch location (I can’t move the latch, as the motor’s in the way). I plan to also have a hunp come off this spine to provide some structure to keep the seat cowl from rattling (Imagine a “D” shape lying on its flat side, about an inch in width), as I’ll be bonding the cowl to the seat and spine to get rid of the cowl mounting hardware and frame tabs (more weight!).  The question is, what should I make the spine/cowl support out of?  Titanium was my first thought, but I think it’s a little flexible?  Also, it’s my experience that Ti isn’t that much lighter than aluminum when used for small applications.  So what should I use?  If the part ends up weighing more than a pound, it’s not worth the hassle.

[koko64]

330lbs

A pic would really help.

[Wzed]

330lbs

A pic would really help.

When it’s “finished,” I will.  It’s not painted, so everything is raw carbon and gel coats.  How did I get it that low?  BSTs, ever piece of titanium PTO Parts made for the S-R bikes, a completely new harness built by revival cycles, carbon tank by ETI, and lots of little tricks here and there.  Basically, the only thing that is still stock is the frame and parts of the motor.  There’s still a lot of weight I can wring out of the motor internals, but I’m not ready to do that yet.  Oh, and a Ti frame, but it’s a street bike and I think I might have some difficulty titling it/insuring it.  If I was going full tilt, I think I could get it into the 300 teens or even high single digits.

[stopintime]

"Wet" as in wet with a ~full tank?

Normally, I'd think it takes MUCH more than wheels, battery and a carbon tank to get to 330 wet. Not even a TPO account would be enough...

Anyway - maybe you could remove half the seat components of a stock seat - leaving just a little plastic back to the lock. Attach a carbon seat cowl with silicone, glue or whatever.

[Wzed]

"Wet" as in wet with a ~full tank?

Normally, I'd think it takes MUCH more than wheels, battery and a carbon tank to get to 330 wet. Not even a TPO account would be enough...

Anyway - maybe you could remove half the seat components of a stock seat - leaving just a little plastic back to the lock. Attach a carbon seat cowl with silicone, glue or whatever.

All fluids plus one gallon of gas.  Wheels, rotors, breaks, Ti everything (including a Ti shock spring), lightened flywheel, lightened stator, battery, harness - everything.  Like I said, the only stock part is the frame, swingarm, and engine.

As for gutting the stock seat, the plastic is a lot of weight.  You may be right that after building the structure I’d get the same result as gutting the seat, but I have police seat, so that’s what I’m going to use - it’s the more elegant solution.

Soooo...anyone known which material would make a better part?

[stopintime]

Carbon fibre 

If it's not going to be visible, you can make it from carbon 'wrapped' foam.

[koko64]

Yeah, Kopf's right, it's gotta be carbon or the weight could creep up with other materials.

[S21FOLGORE]

Also, it’s my experience that Ti isn’t that much lighter than aluminum when used for small applications.  So what should I use?

Soooo...anyone known which material would make a better part?

So, I assume you know that
Titanium is NOT lighter than Aluminum.

Right?
As a raw material, titanium weighs more than aluminum.
(Titanium is 60% heavier than aluminum. But 45% lighter than steel (and almost as strong as steel.)

The strength is the key word.
You can make aluminum motorcycle frame but you can't make aluminum ply bar.


You know how they could reduce the weight of Mitsubishi A6M2 (commonly known as Zero fighter)?



Look at all those holes.





And, Pop Hideo Yoshimura ...


... did this to Wes Cooley / Mike baldwin 's GS1000 ...

Cam chain sprocket


Rear sprocket



(Sorry for low quality pics. They were taken from low resolution video file.)

...  back in 1978, Suzuka 8hours. There was no titanium parts on these "Super bikes".







If, weight saving is your #1 priority, everything else is secondary (cost, aesthetics), you can get titanium bar drilled (skeleteonized) in Pop Youshimura / Mitsubishi A6M2 fashion.

You can also get ideas from skeletonized knives and such ...

KA-BAR skeleton combat



Neil Blackwood's Grifter (S30V)


Bark River knives full tang construction (handle scale removed)


Becker BK-9

[Speeddog]

Low density and low stiffness go hand in hand for most materials.

Density is lb/in^3.

Stiffness is lb/in^2, actually Young's Modulus.
Which is proportional to density.

So, if you keep the general part geometry the same, making the part a bit thicker as you substitute lower density material.....
....you end up with the same finished part stiffness and weight, irrespective of the material.
No free lunch.

Aluminum, titanium and steel all have proportional density and stiffness.
You can build an airplane fuselage with aluminum skin because it's 3x as thick as an equal weight steel skin and won't buckle or cripple locally.

So it's using the material's properties correctly for each application.

In your case, mounting the cowl and the seat independently to your existing rather handily located frame tubes is the lightest approach.

[Wzed]

All great replies!  I always assumed Ti was lighter than Al because that's what all the go-fast bits were made of, and in the bicycle world, Ti frames were usually lighter than Al (as well as motorbike frames - why does NCR make Ti frames but not Al ones?  Bimota made Al frames).  Now I'm thinking maybe a piece of carbon might be the way to go - I can form it any way I like, and it will be plenty strong and stiff for the application.  Really, the only function of the part is to keep the attached to the frame - it won't actually support any weight, as the seat sits on the frame just like a normal, full-length seat. 

[S21FOLGORE]

For the bolts and nuts,

Aluminum bolts are light (roughly 1/3 of the weight of the steel bolt in the same size) but the strength is also 1/3 of the steel.
(See what Speeddog said just above your post?
1/3 of the weight of the steel because of 1/3 of density. Because of 1/3 of density, (if the same geometry) 1/3 of the stiffness.)
So, you can only use them for, things like side cover, front sprocket cover, chain guard, etc.
Those anodized in pretty color aluminum bolts were pretty popular back in the 90s (mostly came from StreetFighter magazine and Performance Bike's reader's special...)

Titanium bolts are usually made out of 6AL-4V, commonly known as "Grade 5 Ti", and they are almost as strong as 10.9 grade steel bolts.
So, as long as you buy the bolts from trustworthy vendor, they are safe to use. (eBay and Amazon DON'T qualify as trustworthy vendors.)
And they stay shiny.

They are nice for bling. To make enough difference in weight saving , you'll have to use bunch of them.

As for the frames ...

companies such as NCR have some restrictions about the frame design (aesthetics), I believe.

For example, this (NCR Ti frame) looks "proper" Ducati frame, and no one would argue about aesthetics.



However ...

Spondon Aluminum beam frame for Ducati



some people would reject this kind of frame just because of the "looks". (They can't accept the frame that would make Ducati looking like FZR.)




BTW, since aluminum has only 1/3 of the strength of the steel, you have to figure out the shape that can add the structural strength.

The good example would be...

1978 Suzuki GS1000 frame (steel double cradle)


1985 Suzuki GSX-R 750 frame (aluminum square tube(box section) double cradle)


1996 Suzuki GSX-R 750 frame (aluminum beam type frame)

[Wzed]

For the bolts and nuts,

Aluminum bolts are light (roughly 1/3 of the weight of the steel bolt in the same size) but the strength is also 1/3 of the steel.
(See what Speeddog said just above your post?
1/3 of the weight of the steel because of 1/3 of density. Because of 1/3 of density, (if the same geometry) 1/3 of the stiffness.)
So, you can only use them for, things like side cover, front sprocket cover, chain guard, etc.
Those anodized in pretty color aluminum bolts were pretty popular back in the 90s (mostly came from StreetFighter magazine and Performance Bike's reader's special...)

Titanium bolts are usually made out of 6AL-4V, commonly known as "Grade 5 Ti", and they are almost as strong as 10.9 grade steel bolts.
So, as long as you buy the bolts from trustworthy vendor, they are safe to use. (eBay and Amazon DON'T qualify as trustworthy vendors.)
And they stay shiny.

They are nice for bling. To make enough difference in weight saving , you'll have to use bunch of them.

As for the frames ...

companies such as NCR has some restrictions about the frame design (aesthetics), I believe.

For example, this (NCR Ti frame) looks "proper" Ducati frame, and no one would argue about aesthetics.



However ...

Spondon Aluminum beam frame for Ducati



some people would reject this kind of frame just because of the "looks". (They can't accept the frame that would make Ducati looking like FZR.)




BTW, since aluminum had only 1/3 of the strength of the steel, you have to figure out the shape that can add the structural strength.

The good example would be...

1978 Suzuki GS1000 frame (steel double cradle)


1985 Suzuki GSX-R 750 frame (aluminum square tube(box section) double cradle)


1996 Suzuki GSX-R 750 frame (aluminum beam type frame)

Got it - very good to know!

[Wzed]

Just want to reiterate my appreciation again - lots of great info in this thread!  Thanks to all the posters who took thet time to post such detailed, informative replies.