[francypants]

I wanted to cut out this flat portion of the subframe outlined in red below to make a little more floor space for a custom tray I plan on fabricating (so I can delete the side panels and larger battery/box).

Anyone think this would compromise the subframe at all?

I also plan on welding a loop on the end, which I feel should add overall rigidity, so I think it should be okay.

 

[NorthernThrux]

You'll be fine. That's not a stressed member at all.

 

[rodburner]

Do it..

 

[francypants]

Haha. Very nice. Thanks guys!

 

[norton74]

You'll be fine. That's not a stressed member at all.

How do you know?

I don't know if that piece is stressed or not; but, I wouldn't start cutting frame parts unless I knew for sure that there would be no structural compromise with this mod.

 

[rodburner]

I'll save my smartazz comment until Northern has a chance to reply,thank you...

 

[NorthernThrux]

How do you know?

I don't know if that piece is stressed or not; but, I wouldn't start cutting frame parts unless I knew for sure that there would be no structural compromise with this mod.

The glib answer is 12 years of university including a Ph.D. in Engineering Physics and 30 years of employment where FEM is one our tools of the trade.

But you don't need any of that to see the frame is not stressed there. Just follow the loads. In fact the way Rodburner did it is even more rigid than the rear plate that Triumph uses. All that plate does is provide a place to bolt a fender and the seat to. The torsional rigidity is provided by the corrugated plate right behind the battery box.

NT

 

[norton74]

The glib answer is 12 years of university including a Ph.D. in Engineering Physics and 30 years of employment where FEM is one our tools of the trade.

But you don't need any of that to see the frame is not stressed there. Just follow the loads. In fact the way Rodburner did it is even more rigid than the rear plate that Triumph uses. All that plate does is provide a place to bolt a fender and the seat to. The torsional rigidity is provided by the corrugated plate right behind the battery box.

NT

Again how do you know? Your assessment of load paths by cursory visual examination may or may not be sufficient.

It is my experience that brackets are rarely used to serve only as an attachment point. It is much more efficient to design plates such as this to serve more than one function like increasing frame stiffness and providing a point of attachment of other parts.

If you have run an appropriate spectrum of load cases through a proper finite element model with and without the plate, then you would know. If you have FEM as a tool, you should have used it, or at least performed some closed form calculations.

I know that I wouldn't be knackering up my frame without a lot of due diligence.

However, it's your bike and your life. You do what you will.

 

[francypants]

Spirit of the Seventies actually cut out the entire middle section from the subframe, only keeping a flat brace at the end for their Scrambler build. I don't need need that much clearance, but that along with what NT and rodburner have said make me pretty confident that I should be more than fine. Thanks for everyone's input!

 

[NorthernThrux]

Again how do you know? Your assessment of load paths by cursory visual examination may or may not be sufficient.

It is my experience that brackets are rarely used to serve only as an attachment point. It is much more efficient to design plates such as this to serve more than one function like increasing frame stiffness and providing a point of attachment of other parts.

If you have run an appropriate spectrum of load cases through a proper finite element model with and without the plate, then you would know. If you have FEM as a tool, you should have used it, or at least performed some closed form calculations.

I know that I wouldn't be knackering up my frame without a lot of due diligence.

However, it's your bike and your life. You do what you will.

Yeah, sorry. I thought it was implicit in my post that I did some FEM modelling. But it wasn't satisfactory, as I don't know the type of steel used in the frame, so it was hard to introduce the material properties.

What I ended up doing was thermoelastic stress measurements of the frame. I have a friend who runs a company that does stress testing for General Dynamics in our town. So we used his high res differential thermocamera setups to do some measurements. I'm afraid I can't post the pictures of the frame after an hour of riding on the street or track. Although the technique isn't classified, his company's capabilities are. It's actually a relatively simple technique as you might imagine, but it shows you the hotspots for stress.

The rear panel (which is what I was referring to - not the red highlighted middle panel behind the battery in the OP) has virtually no stress, even at the welds. The red highlighted piece by the battery does have some, but the hot spots are at the brace that is just behind it. It's the stressed member in this setup. The front and back flat extensions provide some anti shear properties (left side of frame moving relative to right), but most of that is in the rear part of the panel as the brace that is right at the connection of the left and right frame members to the spine also provides torsional and shear control.

It's not a very sophisticated design. It is actually massively overdesigned. I bet you could make just as stiff a frame at 1/3 the weight with an alloy and some clever bracing. Of course alloys are way more expensive than this steel.

 

[TB-T100]

I bet you could make just as stiff a frame at 1/3 the weight with an alloy and some clever bracing.

That is one aftermarket piece that I would really like to see! Surely there must be a few of us willing to pay up for loosing a few punds from the rear subframe.

 

[GDCobra]

That is one aftermarket piece that I would really like to see! Surely there must be a few of us willing to pay up for loosing a few punds from the rear subframe.

It mayIt may be relatively easy to create a lighter frame but not to do so and keep it looking as it does now, there is a good reason that modern bikes look different to 'classics', if you follow a design evolution which has weight and cost as the main criteria I suspect an aluminium perimeter frame with large thin-walled sections and mono-shock rear end. I know Ducati have made good use of their trellis frames but these are particularly suited to a V (or L) twin and to make it light weight probably increases costs for materials and production. I’d like a lighter Thruxton but if Triumph had to charge 50% (or greater) more for it I suspect I’d go in another direction.

 

[NorthernThrux]

Streetmaster makes a complete chrome moly frame for our motors. Just $3200, based on the Mule motorcycles design. Supposed to be really light.
Rmember the motor on our bikes is 200 lbs, so the rest of the chassis is about 260 lbs for a stock bike.
http://www.streetmaster.net/frame-kit.html

 

[GDCobra]

Any idea what that $3200 will net you?

You'd think that having gone to all the trouble to create the thing they'd not only quote a weight but the saving over the original kit

 

[TB-T100]

Yes, the complete frame has been discussed alot. Trouble with it (apart from price) is that most everything else needs to be custom. What I would like to see is a lightweight replacement rear subframe. Maybe even with integrated batterybox. And that would work with all other stock (or aftermarket parts designed for stock frame).

 

[bucksfizz]

Of course alloys are way more expensive than this steel.

Um, aren't all steels alloys?

 

[TB-T100]

https://www.youtube.com/watch?v=pACePi441ds&feature=youtube_gdata_player

 

[NorthernThrux]

Um, aren't all steels alloys?

Yes indeed. LIght alloys was implied, but I guess I should have been explicit.

 

[NorthernThrux]

The easiest pickings for weight reduction in the Bonnie frame isn't the rear subframe. It's the two lower cradle pieces. They are heavy and all they do is stop the motor from rocking. You could shave 15 lbs with properly designed light alloy tubes in their place easily.

The spine piece that connects the headstock to the rear frame is also very heavy. But no easy way to replace that. One could drill out some of the material, but that would require quite an engineering analysis to do it safely.

The rear hoop is hollow and not that heavy. The cross brace originally discussed in the first post is also quite thin. You can shave about 7 lbs cutting off the upper part of the passenger peg triangles as I did. That still leaves you the lower part for a mounting point for the exhause and in fact foot pegs. As I mentioned the frame is overdesigned. Any sub-250 lb passenger won't even be able to deflect the lower triangle piece even if they stand on the foot pegs.

Bottom line. The cradle pieces are any easy replacement part to produce and sell. It's what the alternate oil cooler people use and it could actually be made to optimize this usage as well. The rest of the frame isn't so easy to modify without cutting, and that means we'll never see aftermarket parts for it for liability reasons.

 

[norton74]

Northern,

Glad to see that you did some real analysis and experiment to satisfy yourself that your changes are not making your bike unsafe.

A more sophisticated approach is to use structural optimization. Using topology optimization and an allowable envelope would provide the ideal topology of the frame. This would be followed up with size and shape optimization with deflection and stress constraints to develop the most efficient design. I am pretty sure that this approach is not routinely done by the majority of motorcycle manufacurers. The usual is trial and error as a frame design evolves.

Too bad though, there is much potential for lighter and more efficient motorcycle structures with more advanced CAE.