loopingz wrote:
Hi there,
I love your project mke. I have seen you want to build cf wheels on a budget.
As a pro (aerospace composite engineer) and hobbyist board builder I would like to stress some points here.
Prepare yourself to waste some wheels at least you have to budget it.
What can go wrong.
By conception matting on the screws will be hard. Be monolotic around there. Ply orientation and quantity is vital at least a minimum thickness is vital.
The shape on the 308 is not very carbon fiber friendly. Koenigsegg put some tubic branch and everything is rounded.
By tooling. Vacuum pump go for industrial straight. My 3 free fridge pump + relays and microcontrollers is around 500$ and yet not that good (3 pump is for redundancy here). Oven you will need one and you will need to cook high temp 200C minimum 400C would be better. Mine is "almost free" but reach only 70C. You need to control ramp up and ramp down temperature. Failure to do so = trash.
By building. You can have mass reaction (better to constantly mix resin and never more than half inch high in your bucket), avoid it at all cost. You will have bubble visible outside and hidden inside. How much can you tolerate?. Warping, that is a wheel you need to avoid it a lot. Milling afterward should be limited to a minimum! You can have vacuum failure, oven failure, epoxy too humid... Prepreg can remove some issues. Molding will be key. Infusion is interesting too but it might be trickier for a start. If you want to use some filling material you may want use herex/airex, c70-75 or stronger. Avoid honeycomb too much added pain...
You can have delamination too and a lot more possible issue.
It is doable still with your energy I am sure you can manage. But take your budget and multiply by a factor 2, 3 or 4. There is so much to learn!
A lot of good points here.
Looking at the koenigsegg design and listening to the video I think they used tubular spokes mostly because they had a process to make them and he seems pretty proud they they are hollow.....but I don't see any structural benefit of the shape? They may have varied the wall thickness to make it more optimal though.
I am taking some liberties with the 308 wheel design to make it a bit better for composite. Specifically where the spokes meet the hub and rim have been altered significantly....and I'm still not loving what I've done honestly. I still need to play with that a bit.
A piece the really bothers me is the ribs...aka stress risers. Where my head is right now is I don't see the car with black wheels, I just don't so they will be painted....I was thinking maybe something a little bit translucent but it will probably just be silver paint. If that is true and I'm not trying to get a show quality surface layup then my thought is to make the ribs from fiberglass so they are way less stiff and under way less stress...making them effectively just decorations. With this approach I can have decent radii on the corners of the carbon spokes to reduce stress and make layup easier but still achieve a 308 appearing final wheel.
Thee are currently sections of the design that are hollow. Laying up and molding hollow is hard.....so the plan is to use foam in the "hollow" space. The foam will not serve any function purpose once the wheel is cured so the strength of the foam or potential delamination is not a concern....ideally I would have a hole and dissolve the foam out but it adds so little weight my thought is just ignore it.
In a similar thought as the foam, I'm also pretty certain there will be excess epoxy is certain regions....unneeded weight for sure but I don't think it causes any actual structural concerns? so I'm not sure just how hard to work at minimizing this issue?
My plan is to make a fiberglass wheel or 2 or as many as it takes to get a working layup process sorted out. Then test layup a couple of the more challenging sections in CF.....then attempt a full CF wheel. I'm pretty sure a well designed 8x18 front wheel should be about 11lbs....mine will be a few lbs over that kind of on purpose. I mentioned post machining and allowing for this is part of the extra weight, design thickness/layout would be completely out of the way of any machining....so the hub say will have 2 or 4 extra layers slapped on like a spacer, it serves no structural purpose so mill it flat and to size paint it and call it done. Same on the bead surface....the tire can't go anywhere but it would be nice if it ran true so a few extra layers to know I'm not cutting anything structural and it should be good I'd think.
Right now I'm leaning toward a room temp epoxy.....it saves a ton of money both material and equipment. My fear is working time. So small batches as you say (I've already make the big batch mistake once) but I'm thinking of going 1 or maybe 2 steps further. I want to see how thick the stuff gets in the fridge as starting with 40-50F resin should turn 2hour pot life into 3-4hour. If that path isn't enough then working in a cold room might be the go.....a few 2x4s, plastic sheet and a window AC unit should give me a 60F work space which should turn the 2 hours time to 4 hours, then flip on the space heaters and make it an 80F space for the cure time.
If I can't make good looking sample parts on this path it will be time to re-think the effort as the prepreg path would be a lot more investment and doesn't leave me with the same flexibility for future projects.....I've been planning a composite rear decklid for example and the autoclave for something like that would be an insane investment.