Carbon fiber bicycle hub and spoke assy mold

General Composites Discussions Composites Talk
#1

Hello,
I an new to this forum. I work with students as a mentor. We are working on single passenger urban commuter vehicle design. I would like to ask for some review and suggestions on the way we would propose to make a carbon fiber/hybrid bicycle wheel assembly. I have attached a *.docx file with pictures of the mold concept.

I need some specific suggestions on how to create the tapered rectangular rubber bladders that are used in the spoke to apply pressure to the CF prepreg. Thanks for looking.
kenkad

#2

Hi, Kenkad. I’m new here, too.

About the bladder, I´ve just solved a similar problem by using casting silicone.
I´m not using air to get pressure. I´ve done mechanical force, which is deformating the silicone and so expanding it against the mould´s walls.

My part has one open side, and I´m doing pressure on that side.
But in a long bladder, you could do a cilyndrical hollow under rest and filling it with a slightly bigger cilynder in order to expand the bladder…

I used the part´s mould to make the bladder. Reduce the laminate´s thickness and cast the silicone (nice results with shore 13).
Thickness reduction can be accurately made by use of specific thickness wax sheets.

I´ve read a lot about avoiding direct contact between silicone and epoxy, but I haven´t had any problem with the silicone if completely cured. In any case, you could always isulate silicone by wrapping it or similar.

By the way, the silicone is a perfect reléase material and I don´t need to use any reléase agent at all (well, obviously you have to coat the mould later, for the epoxy…)

Hope it helps.

#3

Here is a technique I thought might be worth a try.

Find a small cell foam that can be easily hot wired, like is used in home built aircraft. Hot wire the shell of the insert bladder, and then re-install the ends to essentially make a hollow foam box. then attach a short cylinder to the narrow tapered end (maybe 1 inch long. Now take latex (from hobby store), like is used in making thick balloons, and put two coats (drying between coats) or more on the foam. Break off the short cylinder and punch hole into foam box. Short cylinder created a hollow nipple for the air pressure fitting. Now you have a balloon around the foam box.

Alternative is to use what is sometimes referred to as a ‘rocket balloon’, made in china. May have to use one inside another for strength.

I believe that the foam box approach is the right approach except I do not like having to make latex balloons. The foam box is needed to keep the CF from wrinkling. Maybe using the rocket type of balloon inside the foam box would work and expand the foam box appropriately.

I guess we will just have to try various techniques.
kenkad

#4

Blackstone Tek make carbon wheels for motorbikes and a magazine toured their factory and took some pictures of their wheel building process. Their process may help you work out your approach.
Here is the link to the article and photo gallery - http://www.sportrider.com/features/146_1106_blackstone_tek_bst_factory/

#5

kyle,
Thanks for the link. I was trying to find a way to not make the hub/spoke assembly in two halves and then join them. That is straight forward, but, requires higher quality molds because you have to go back and machine each half before joining (constant refixturing issue). Now you can better understand why I want to use the hub integral to the assembly and why I plan to use off-the-shelf rims. With my approach, I cannot use silicone inserts because I have no way to get them out.

When I have a bit of free time, I am going to try the hot wired tapered rectangular (hollow with some thin wall) foam insert and then put a ‘rocket’ type balloon inside to expand it (just a one spoke mold test). Then I can pull the balloon out and melt out the foam with some type of solvent that will not affect the CF.

I was just wondering if anyone else had tried this before.
kenkad

#6

Why not take a sheet of polystyrene foam as a disc centre and layup 2 sheets of carbon on the outside? The whole wheel would act as one big foam sandwich and mold and process is far easier.

#7

Just to be clear I’m suggesting to leave foam in place

#8

Since months ago I want to make a carbon disc whell. I was thinking to make with foam inside, because this make to hold together the 2 sides of Carbon. I will use an aluminium rim and a cnc machined hub.

#9

glsssboy & fantoni,
Sorry, I have been away from the thread on other issues.

You are both probably right. I did not consider doing two discs and then laminating them together. I will still do the hub layup like I said. Making the two discs can be done with an infusion process quite accurately. I would need to find a way that is workable to mate the discs to the hub and rim.

I would put some ribs into the inside of the discs. That should be easy enough. Extended flanges on the hub would work there to bond the discs to the hub. I would still be using the hub mold I had planned before with very slight modifications. Just need some clever way to attach the Aluminum rims later and be able to deal with the spoke protrusions to the inside at the same time. Still have to have a recess for the air valve anyway. I think that this requires separately molded inserts to connect the very accurately fixtured rim, hub and two disc halves to ensure everything is aligned. This extra part is very important because it would easily allow for minor variations in rim size. Rim roundness would be handled by the fixturing.

Ribs inside the discs can be done by foam inserts. So I would need two molds, one for the disc halves and one (using a rim as part of the mold) to make the rim to disc mating parts.

Thanks for the suggestions. I will try to redo some 3D CAD drawings over the holidays.
kenkad

#10

I made dragster front wheel similar in design. I made the wheel in one go with no post bonding. Each side was layed up apart with one side having a lap. I used a tube bag in each spoke, they were over twice as long as they needed to be, as I turned half of it inside out and fed it back into itself. This meant they could slide in as the pressure came on. The bags were joined to the outside bag and the inner half had the end sealed. Before closing the mould I put the tube bags into the half with the laps, and covered them with a dusting of talc to help them slide.

The biggest problem I had was getting a useable bead.

#11

Hi kenkad,

Leave a foam core in place and let the sheer strength of the foam do the work…no need for any ribs.

I’d be very interested in any drawings you come up with

#12

It will be after New Years before I can get it done. I have a project I am mentoring for a graduate student (not CF work) in electronics and I have got a great deal to get done for her. When you are old, sometimes thinking is harder.
kenkad

#13

I hope everyone had a happy holiday. I had a bit of time and just had to get my thoughts on paper and 3D CAD with regard to using a disc spoke design instead of the individual spoke design. I have attached a revised document on the procedure as I had promised to do. I want to again thank those responsible for the disc spoke suggestion. It is far easier than the individual spoke approach.
kenkad

#14

Hope everyone had a happy holiday. I have revised my original document and have attached the new version here. I want to again say thanks for the disc spoke suggestion. That approach is shown in the attachment and it is much easier to do than the individual spoke approach.
kenkad

#15

Check out these DT Swiss EXC 1550 wheels with an internal rib - very cool and Im sure quite difficult to manufacture
http://is.pinkbike.com/p4pb4637752/p4pb4637752.jpg

#16

A solid disk wheel would be hell in wind. My old Harley fat boy had a solid rim and wind was a bit tricky on that. I could imagine a solid wheel on a bike

#17

This wheel is not for a regular bike type of application. It will be used for single passenger urban commuting vehicle designs, three wheeled designs. Side winds are not an issue due to the stance of the vehicle and its low center of gravity.

I added the method of attaching the rim in the final assembly process but have not modified the document on this forum. Also, I added using 2 inch woven tape around the outside edge to provide a more finished/decorative final appearance. The mold is scored (small V groove) to provide a tape edge boundary during the tape lay in initial process.
kenkad

#18

Hi kenkad,

How did you end up attaching the rim? Also, can you explain about the v groove? I did not understand.

#19

glassboy,
The small V groove was just to provide a boundary for the layin of the woven tape. Sorry, I do not have a JPG of that.

Attaching the rim cannot be done by simply tapping into the composite. I choose to use a scheme that integrates very short composite tubes (two or more) into each of the spoke protrusions. Then, during rim assembly, I would place a metal rod that is tapped for the particular spoke hole. Typically, this metal rod’s M5 tapped hole would be off center end-to-end wise (spoke holes are offset) and on center diameter wise. This has to be determined in advance when you have selected the rim you are going to use and know the exact location of the spoke holes, both offset and circumferentially. The composite tube takes the compression load OK in my opinion.

The composite tubes are located by something like a nylon pin that is slightly smaller than the inner diameter of the composite tube. I would suggest a tube with an OD of 0.400 and ID of less than 0.325. This allows the use of M5 rim attachment screws.

The tape weave is segmented as I had stated. The four short segments are placed in the area of the spoke protrusions first (you will have to punch holes in the weave for the nylon pins) and then the four longer segments are placed second. You can figure all the rest out. I have attached some updated JPGs.
kenkad