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Discussion Starter · #1 ·
I was thinking about how I could increase the stiffness on my next frame which will incorporate a single sided rear end and came upon wrapping the tubing with carbon fiber acting like gussets. I would imagine I could therefore run a thinner wall and save weight as well as increase stiffness if done properly.

Would this be viable and can anyone break down carbon fiber wrapping?

There are pictures of the latest solid models at this link on my website so you can see what I mean better.

http://www.nobsbikes.com/future_designs

Any opinions and ideas are greatly appreciated.

-Brian
 

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Um...

Generally, the best way to make the rear end stiff is to use a two-sided rear end.

I can't help you on the CF wrapping idea, but I'm pretty sure you'd be better off just making the bike have a swingarm with structural members on both sides of the wheel.

Cool website, btw. Good to see someone doing DH bikes for a change and actually thinking about wheel path and leverage ratios.

-Walt

sirknumskullgt said:
I was thinking about how I could increase the stiffness on my next frame which will incorporate a single sided rear end and came upon wrapping the tubing with carbon fiber acting like gussets. I would imagine I could therefore run a thinner wall and save weight as well as increase stiffness if done properly.

Would this be viable and can anyone break down carbon fiber wrapping?

There are pictures of the latest solid models at this link on my website so you can see what I mean better.

http://www.nobsbikes.com/future_designs

Any opinions and ideas are greatly appreciated.

-Brian
 

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Hi Brian,

Those are some pretty neat ideas. I like what your doing. IMHO if it has suspension like that it should also have 500cc bolted to it too but that is the old moto-crosser in me.

I know you want to do something g-whiz but I am with walt. It just makes sense to do a double sided swing arm. There have been a number of single sided swing arms in motorcycle road racing but generally those are to facilitate lightning fast wheel changes for 24 hour events. It required a piece that was heavier and bulkier to maintain stiffness and almost all current GP motorcycle are of a traditional nature. Same thing when MX went through the same phase in the 80's.

Generally wrapping metal tubes doesn't really work as expected. A couple of issues. You may get delaminations from the base material if it is not prepped correctly and the compaction of your composite is off. You will not increase stiffness for unit weight more than just oversizing the material (always maximize your diameters and take up the largest envelope possible) This is also a big reason that you see most DH bikes in Aluminum because it generally is a better material for these applications. That is coming from a die hard steel old fart but it is the case.

Read up on a guy name John Britten, if you don't know who he is already. He had a process in which he built a lattice type frame from steel (later from carbon) this located all his hard points (linkages, pivots etc) and then built a composite structure around this, enclosing the envelope and making super stiff structures. This guy and his team were downright mechanical geniuses.



hope that helps.

Dave Bohm
Bohemian Bicycles
 

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ya complicaaated little man!! (sorry, channelling the rum commercials)

forget carbon wrapping! it's for the birds!
very high tech birds maybe... but birds nonetheless!

you've got a tube up top, a tube down below?
groovy, then weld thinner gauge sheet between them to make a box section swingarm.
You get the end result of an internally reinforced extruded structure only custom built to your purpose (admittedly with lots of welds).
if you decide to get REALLY funky with the FEA programs you can weld in little reinforcing ribs (riblets?!) between the upper and lower tubes before you "plate them up" and add appropriate reinforcement exactly where you want it.
You can run a thicker rib between them like a funky I beam, but I suggest running thinner sheets on the outside edge of the tubes and ending up with a nice blended looking piece.

Strongly suggest you whip together several protos (unless you actually DO have access to the FEA funstuff programs, in which case, can I have some!?)
Because when you get the advantages of the reinforced structure, you quickly realize your target stiffness can be reached with thinner gauge tubing and you'll want to go back and start again. :D

OOH! and you can drill holes in the sideplates of the swinger and weld in little tube (tubelets!?) and get that old hot-rod style "drilled for lighter weight" look... those little tubes running on the same plane as the axle (side to side on the ...ermm.. sideplates?) could give you a seriously unique style touch. (sales point maybe?! :p )

trick stuff yo!
I like the caliper mount hiding in the swingarm with the rotor sandwiched, reminds me of jesse rooke's choppers...
So whacha using for a hub?
Axle that diameter's going to be tough to accomodate with conventional bits (impossible rearlly...
You could run a belt drive hub, since the belt drive pulley has to be MUCH larger than a regular cog to prevent the belt from binding, the freehub mechanism's diameter is irrelevant...
Also, IIRC trials guys were running some funky cranks with the freewheel mechanism on the crankarm/chainring instead of the rear hub, there's an option, have your rear hub as essentially a fixie hub with a belt drive pulley?
Frame-mounted gearbox might completely negate the entire need for any freewheeling on the rear wheel anyways.
You could always invert the caliper and attach the rotor's outside surface to the belt pulley, chopper styles, and then run the caliper inside of the rotor instead of outside...
Or just run a sprotor...

Pardon me, sleep deprivation from the 6 day old newborn and lotsa coffee, makes for interesting sidetracks.
 

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Discussion Starter · #6 ·
Thanks for all the input.

This Britten guys idea is what I was thinking. I was concerned about delamination as well from the steel but I figured that the epoxy that is used could be strong enough. I know this is by no means compeltely necessary, but it is to me. Ive wanted to make this happen for a long time and was working through some new ideas for maximizing stiffness with the 4130. I have been very happy with how it has worked in my previous frames but I am worried in this application that it is too flexy. Perhaps If I gusset it well enough and do all my FEA analysis then I can make something that will last a good amount of abuse. I like the idea of having a fast wheel change and the hub Im making for this will be a quick release style.

Im running through the analysis on it now and will be for the next couple weeks. Im not sure how I would incorporate the influence the carbon fiber would have on it because it all depends on the wrap i suppose. It would be cool to try nonetheless.

Im pulling out all the stops on this design. Id really like not to have to compromise on the ss rear end because its been done on motos before.
 

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ooh britten.. truly lustworthy.. IIRC he actually used his cylinder heads (custom cast doncha know) as the main mount points for the front and rear subframes (supporting the front suspension and bodywork and his butt respectively)

Nice to be able to use the engine as a stressed chassis member, but that part of things doesn't really apply to bikes... cause... ermm.. WE are the engine (and I don't even like rugburn, never mind being welded to a rock shox...)
 

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Discussion Starter · #8 ·
byknuts

Youre speakin my language entirely. Thats basically what my plan was to do. Ive gotten good at making gussets out of sheet 4130 and the plan was to lay across the tubing like you said and hog out holes in the middle where material is redundant for weight savings. Most likely little riblets like you said along it as well.

I do have Solidworks on my home computer and at college so I can do the FEA easily. Ive taken the courses on it as well. 4 months til graduation. This will be my ME senior Capstone project. Also have considered both the gearbox and using belt drive. Might be a hammershmidt modified for belts or a rohloff hub mounted fore in the front triangle with LSD. The pics of that are on my site I believe.

Plan is to use a 20 mm front hub and press out the bearings to press in a custom made tapered spline for a complementary 20mm splined axle. There will be a mini hub which holds the disc rotor and rear chainring that gets sandwiched and will also be custom made with spline or key way mounting to the axle. The rear end will effectively be fixed gear like on a motorcycle and the freewheel would be in the front which would assist shifting because it will always be running the drivetrain on the transmission end.
 

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sirknumskullgt said:
Thanks for all the input.

This Britten guys idea is what I was thinking. I was concerned about delamination as well from the steel but I figured that the epoxy that is used could be strong enough. I know this is by no means compeltely necessary, but it is to me. Ive wanted to make this happen for a long time and was working through some new ideas for maximizing stiffness with the 4130. I have been very happy with how it has worked in my previous frames but I am worried in this application that it is too flexy. Perhaps If I gusset it well enough and do all my FEA analysis then I can make something that will last a good amount of abuse. I like the idea of having a fast wheel change and the hub Im making for this will be a quick release style.

Im running through the analysis on it now and will be for the next couple weeks. Im not sure how I would incorporate the influence the carbon fiber would have on it because it all depends on the wrap i suppose. It would be cool to try nonetheless.

Im pulling out all the stops on this design. Id really like not to have to compromise on the ss rear end because its been done on motos before.
I'll apologize in advance, last fea-type program I had access to was pro-e, dunno what you're using, but you might be able to plug in different properties from the carbon manufacturer's info (re: "our sexy weave numero uno is 1400 KSI strength when single wrapped" etc) and tailor your fea's calcs?
Ask a couple suppliers of the carbon fabric you might choose to use and see if they can give you some numbers.
Yeah I know the epoxy and how properly you cured it makes a difference, but those are "manufacturer's defects" type of things that FEA programs can't account for anyways!
 

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hah! post-racing.... :D

ok, so why not try and get the axle made with shimano's centerlock on one side for the rotor interface (least there's rotors out there made to fit!)

My suggestion for a cheap and useful axle for the rear wheel assembly? car axle!
Ok, from a small car... :p
hey, they're made for single sided use already, some of them have fairly small diameter axle, and they're definitely strong enough.

Could also hacksaw the lower end of a lefty fork off... ahem.. and use their hub... :thumbsup:

This sounds like gangbusters... will keep an eye on it!
 

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Discussion Starter · #11 ·
I will most likely machine it out of aluminum so I can thread the end for an end cap that will mate with the outer bearing race. It will have to be stepped so that the inner bearing will becontacted on the race and located so everything will be nice and solid. I could try and graft a centerlock off of a hub and use that but most likely the 6 bolt pattern will suffice. I have a centerlock saint on the rear of my scissor jack proto and could take a look. I want to make everything custom though. Id rather not use someone elses stuff if Im going to go through the work of designing it. I like to see things from start to finish myself.

Speaking to the manufacturing problem, it is definitely a factor in this because I have never worked with carbon before. I have also never made a bike before my first fs DH bike and all my 1 off protos are holding up great to the abuse. I still feel like a crash test dummy sometimes when I consider its my own welding underneath me that im taking off the cliff.
 

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sounds fun as hell!
I'd volunteer to R+D (read:beat your stuff up) but the harshest riding I do is urban bashing, my offroading is relatively tame I think... :D

I gotta say, don't discount the ability to use something someone else has designed before as a PART of your overall design.
YOUR design is the suspension, how uniquely it's designed, how the rotor's held IN the swingarm, the axle, etc... how the rotor mounts up is a small part of things. ;)
But I DO certainly understand the whole "every last detail's gotta be MINE!" mentality!

Forge ahead... tons of fun awaits.
 

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Discussion Starter · #13 ·
I do a ton of urban riding in Boston. Its mostly what I do nowadays. Scissor jack has new holy rollers on it and ive been taking it off drops, doing stair gaps and trialsin it a bunch already, habits die hard. I had trials fs bike in mind when I designed it though. The super short chainstay is working nicely. STP geometry otherwise. I have taken the DVP prototype off a 20 foot drop and it didnt even bottom. I like big hucks as well.
 

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Cool desings, I like the one ALA Ducati rear arm...
I have several years experience on composites, I'm in the process to buil my own full CF. MT. bike. Thats why I'm on this forum.
answer to your question: yes you can add more strenght to your part with CF. There are several ways to doit. The one that I will use:
-Buy CF. sleeves. (like one continous sock)
-Place 3 or 4 CF. sleeves on the part.
-wet with a good expoxy
-add presure (bacuum baging) this will take the extra resin away, an will compact your layers together.
-theres is no way of delamination
-don't forget that CF reacts with metals (oxidation) so betwen the CF and the metal you should put a barrier, in this case a thin layer of fiber glass will work.
 

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Discussion Starter · #16 ·
Thanks so much for the nod Journeyc6. I knew it was a good idea and that way sounds like it would be the easiest to make it look clean and nice. The only problem I see is I wont be able to do shapes that are not of a uniform cross section. Now I could modify my design so that the rear end does not taper down to the rear axle so that this wont be a problem. I will definitely take it into consideration in any case.

Im sooo psyched to make this frame. Just keeps looking better every day. To the new year.
 

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sirknumskullgt said:
Thanks so much for the nod Journeyc6. I knew it was a good idea and that way sounds like it would be the easiest to make it look clean and nice. The only problem I see is I wont be able to do shapes that are not of a uniform cross section. Now I could modify my design so that the rear end does not taper down to the rear axle so that this wont be a problem. I will definitely take it into consideration in any case.

Im sooo psyched to make this frame. Just keeps looking better every day. To the new year.
Non-prismatic shapes are easy in composites. Although there is a link between cross section circumference and fiber angle, braided tubes can easily accommodate taper, and you can get decent properties if you pick the right nominal diameter/fiber angle. Using the steel as a form, you will need to be clever about your internal (most likely foam or a bladder) support to be able to use vacuum compaction.
 

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Discussion Starter · #18 ·
Hey I know I havent been on this one for a while but I thought those who were would like to see the progress on the single sided swingarm project.

http://www.nobsbikes.com/capstone_design_bike

I decided against the carbon wrapping once I did the stress and fatigue analysis because it is plenty strong as is. Will be finishing it up this week and next and should be riding it shortly thereafter. Ill be updating the pictures regularly.
 

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I'm all for this super creative design and it's cool to see a project come together, but a little constructive criticism: some practice welding might be in order before the next project.

PVD could probably give you some tips ;) . (actually this is true)
 

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rustola said:
I'm all for this super creative design and it's cool to see a project come together, but a little constructive criticism: some practice welding might be in order before the next project.

PVD could probably give you some tips ;) . (actually this is true)
Yes, it is a neat design. I know a lot of work went into that. I look forward to hearing your thoughts on how it rides.

Some thoughts. Rustola is right (ohh, that rhymes:thumbsup: ) Your welding really needs some work. It is one of those things that with some good instruction you could go from borderline acceptable to really good in a day or two.

From an engineering POV. I don't know about the double downtube. I did this once on a frame and it was nearly un-ridable. There is a lot less torsional rigidity with a double downtube such as your design, which brings me to my second thought. You will be an engineer soon and that frame is so complicated that it could never be produced at a reasonable cost. Try to simplify. I bet if you thought it out you could get the same performance and results in a much easier to manufacture frame. In fact some of the other designs on your website seem to be better in this respect.

So, tell us how it works. Learn to weld better and simplify, simplify. For what its worth:p

Dave Bohm
Bohemian Bicycles
 
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