Structural question for mountain bike frame

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#1

I have been really lusting to make my own mountain bike frame. I am essentially giving up street riding my motorcycle for family reasons and want to take to the trails instead. I also want to incorporate an electric drive system into the frame for kicks.

I envision making a system that is an electric mid drive ( powered by a motor located located near the crank where you pedal and driven by a belt or chain for boost).

My idea for frame design is similar to the size and geometry for standard downhill bikes. I would like to create a large hollow triangular frame section from the head steering tube to the seat to the bottom bracket. I would like to mount the motor inside the hollow frame (essentially a monocoque, but it would need one side of the frame open to be able to mount it all in there).

Triangular frame
One solid side and one cut away side for access to mount the motor and electronics.
Frame width at least 3" left to right

sketch.jpg
sketch.jpg1024×576 56.9 KB

My question is how much of a compromise would it be to have the inner portion of the triangle of the frame cut away on one side?
What about adding some features for rigidity to the non cut away(solid side)?

please excuse my rough sketch.

#2

You may want to check out the Vivax Assist. It’s a motor that fits down the seat tube so will save the headache of needing to cut the frame.

#3

It’s a pretty elegant design but I’m looking to make something totally custom and a bit more substantial than the power of that one.

#4

Use 2 of them - One in the seat tube and one in the down tube. Haha.

#5

anyone have any insight?

Would building to about 3mm thick be strong enough?

#6

i would say 3mm would be overkill for the majority of the frame, it will weigh a ton

most places will only need to be a millimeter or less , around the neck, seat pole and crank you can beef it up

#7

Thanks for the reply!

Ya at 3mm thick it would be thicker than needs to be in all areas I suppose, comparing to the 6mm aluminum equivalent (in terms of strength)… Gives me confidence that it would last though.

What do you think about the geometry? Any doubts about the one side being solid and the other having a large cut out?

#8

personally i dont like that, i would like symmetry on either side of the frame , or the frame to be a space frame type construction, and then a cover that screws on either side

#9

Because i have insight of more than 30 carbon fiber frames i can tell you these:
For a mountain bike -

  1. The top tube thickens is about 1.2 mm with 3 layers of 330 gr uni

  2. The down tube is about 2.5 mm at the down section and 1.5 -2 mm at
    up section and it consists of 7 layers of uni (330 -240 ) and around 4-5 at top.

  3. The bottom bracket has 8 - 12 layers of 300 gr uni and actually it needs to be beefy for the bottom bracket inserts etc.

  4. The seat post has about 5 layers of 300 gr Uni

  5. Where there is a hole or a possibility of abrasion plain 200 gr is used ( at the bottom brackets , at cable management points , at fixing holes etc)

ALL OF THE ABOVE are very general and every case is by far different to the other. For this reason i would discourage from doing this . You will have spend a lot of time to a result that is bad . But this is your thing. Especially in a trail bike you have a lot of fixing spots at the frame and a lot of things that can co bad. Even if you end up with a good bike in terms of carbon fiber knowledge and execution you will probably have a bike that does not ride well because of geometry issues. With what material you plan to make it ? prepreg ? Hand lay up?
The only way to have a acceptable outcome for me is prepreg , but then what are the materials of the mold will be ? Aluminum ? At the end of the day maybe you will end up with an unfinished project to which you will have spend far more time from how it looks. This is what happened to me … :frowning: hope you will do better

#10

Thanks for the insight. I get what you are saying about the complexities of the project…

My idea initially was to use removable silicone forms internally and have thermal expansion of the silicone compact prepreg. The challenge would be the tooling since the silicone can exert such tremendous force that the mold (if not made of alloy) would be destroyed.

Second thought was to use OOA prepreg with bladder molds and get compaction that way, or third possibly even infusion.

My thoughts were to use multiaxial fabrics.

#11

The symmetry wouldn’t really matter to me, I would cover it with a similar panel anyways so all that would be seen are some screw holes.

#12

I"d go with the bladder over the silicone. The silicone is better, in my opinon, but might be a pain in this sort of a shape. The bladder might be easier in the long run, but harder to make the initial bladders. As for tooling, it’d be the same for either more or less. We make parts using silicone like you mention, with aluminum molds, and then cook them in a heated press. The parts come out great, especially if you really polish the mold.

My coworker broke his mountain bike recently, has it sitting on his desk in a bunch of pieces. He cut it up to look at the inside. I’ts a pretty thick piece. I can measure it?

As for material… Bikes are usually unidirectional largely. I’d imagine it’s because you can control ply orientation much better with uni. Thus you can optimize for particular stresses at different points in the structure. While uni might be less damage tolerant, if you have enough layers to make the laminate isotropic, you can minimize this weakness. Also, many bike manufacturers prefer to use higher modulus carbon to create as rigid a frame as possible.

sounds like a cool project. The molds might cost a little bit to make but if you make them well, they will give you some great parts.

Have you done the cad and any simulation to get a better idea of what you need to do? Sounds like you’re just thinking about right now?

#13

Sammy, if it’s not much trouble I definitely would be curious what the thickness of the broken bike would be. Thanks!

Silicone would be great I know. I would do it with silicon except it is SO difficult to first-dial in the thermal expansion, and second-to determine the proper compression/pressure you get with temperature as it expands… Using calibrated sheets/wax one could very easily account for the laminate thickness needed but the difficult part would be accounting for the variable expansion due to the silicon in X/Y/Z with a part that is not planar… This is mainly why I rule out the silicone option. …Unless you have any suggestions? Or know any resources for it. The frame would be about 5-6"wide ~27"long and ~15-17"tall. that would mean that the expansion of the silicone would be at least 4X greater in the length vs the width of the bike. So accounting for that with calibrated sheet/wax would be tough since I’m not sure how to estimate the PSI exerted with expansion of the silicone… http://www.iccm-central.org/Proceedings/ICCM18proceedings/data/2.%20Oral%20Presentation/Aug26(Friday)/F03%20Simulation-Based%20Design%20and%20Optimization/F03-6-IF1192.pdf This explains a little bit of the thermal expansion vs pressure but it seems it is so highly dependent on geometry and calibration that I don’t think I could get close enough to make it work and would very likely destroy the tools…

Bladder would be good because I can place the bladder and pressureize but also place the whole thing into a vac bag and get an extra 14psi out of the whole setup without necessarily needing a mold that is any stronger…

High mod. would be great for minimizing thickness and weight at the same time but it also makes the frame a bit more brittle compared to the lower mod. carbon. It would take crashes a bit better at the expense of a little weight perhaps no?

I have done no CAD or anythign of that sort. I’m initially trying to visualize the process and the challenges and see if I can get past these with the budget and materials I have available. I definitely don’t have access to aluminum molds or a press. I’m trying to determine if pouring a solid mold (casting resins and aluminum powder or carbon powder-which itself might have less CTE) or standard layup would yield the greatest strength.

#14

Ok, i’ll take a few measurements tomorrow… i did it the other day, but don’t remember what the measurements were.

i agree with you about the silicone… hard to keep uniform pressure. The bladder will be much more consistent in this way.

Yes high mod would break more easily, though unidirectional is generally much less damage tolerant. From what I’ve seen most bikes are mostly unidirectional, except for the outer ply. I would imagine it varies from company to company and applications. The uni is also more efficient from a production stand point, though takes a little longer to layup. It also gives you more control in ply orientation. I read an article a while ago about bike companies doing unsymmetrical type laminates witha +/- 30deg orientations. Again I"m sure it varies from company to company.

I hear you on working on the cheap… Ive never tried making a mold to take these kinds of pressures using a castable material. It could work though, the thickenss you might need could make it expensive. Would suck to make it, then have it crack apart. I wonder how much is gained, in strength/durability, by having these high pressures? Compared to vacuum? To control the expansion in one direction would be difficult I would think… maybe if there was open space in one direction it would control it? To do it right would probably be difficult and not cheap.

i’ll take some measurements of the frame pieces tomorrow, if I’m awake enough to remember in the morning! :smiley:

#15

As a huge mountain bike nerd and with some experience in composites I thought I’d throw in my two cents:

-Mountain bike geometry and suspension kinematics are extremely complex and the details and compromises are interrelated. For geometry, it’s probably best to simply copy a typical modern enduro bike like a Specialized Enduro 27.5 in the appropriate size. For Kinematics, you can get by with a simple single pivot as long as you’re not mounting the shock in a way that creates a regressive shock rate and as long as the pivot is near the top of the chainring to prevent excess pedal kickback or squat. See http://linkagedesign.blogspot.com.es/ if you really want to go down the rabbit hole.

-I don’t think epoxy/aluminum powder molds are compatible with expanding silicone mandrels unless you really know what you are doing or have giant presses to work with. Definitely not for something as large as a bike frame. I built an obscenely thick (3" each side) epoxy/aluminum pellet mold for a silicone plug and still broke it on the first cycle. If i had a large heated press to hold the mold halves together it probably would have worked but i tried to through-bolt the two sides together. It was weeks of work to build that mold and the silicone insert, hundreds of dollars of materials, for nothing. Either way, not something likely to be able to pull off for a 1mx1m frame.

-If I were going to do a carbon frame, I would either: build a jig of 80/20 extrusion and do a tube-tube construction where you miter and bond carbon tubes then wrap the joints. Or, mill a mold on a CNC router from tooling foam and build the frame in left/right halves with overlap joints and then bond the two halves together. If you think about it enough you can probably come up with some ways to get a strong overlap joint. Both of these ways only require a vacuum bagging setup for the actual composite work.

Without access to affordable CNC equipment time I would say the only reasonable way to do it is the tube-to-tube bonding and wrapping method.

I’ve repaired a handful of carbon frames and the numbers Creator threw out are pretty accurate.

Don’t want to discourage your project at all but I’ve run all the same thought experiments and also have had a lot of (failed and successful) composites experience.