Hi all,

I'm planning to make a structural part - a top box support rack for my motorbike (bolts via 2 bolts on each side of the rear subframe). It needs to be able to carry around 15kg to be safe. The metal bracket it's replacing was made from 5mm thick iron as far as I can tell and snapped. I plan to make a carbon replacement around 10mm thick if that's feesible (final part should weigh around 2kg I think)

I've made a quick prototype in cardboard, coated in some household epoxy glue to check the shape and fitting, with a plan to use 0.7mm aluminium sheet to make the pattern\mould.

I'm almost at the stage of ordering all the bits I need from easy composites to make the part as a project over the next week and a half. Though I have a few questions so I get the right bits

1)how thick can one wet-lay carbon - is 10mm too thick - will the heat from curing cause problems?

2)Can I post cure wet lay carbon of this thickness in a standard home oven(without vacuum) by small incremental increases in temperature and cooling to improve composite strength?

3) Which epoxy would give me the best mechanical properties?

4) Is wet-lay significantly weaker than pre-preg\unsuitable for this purpose ?I'd avoided prepreg due to cost of the kit, and as I hope to use quite a lot of unidirectional carbon tape to reinforce the struts that attach to the motorbikes rear subframe (where most of the force on the carbon should be exerted in one direction. If pre-preg and vacuum is needed and going to result in a considerably stronger part then I'm happy to take that route.


Many thanks in advance,

Tom

10mm is pretty thick, but I think you would get away with it if you use a low exotherm resin like an infusion resin. However for structural parts, I would at the very least be vacuum bagging it. Pre-preg or infusion would give the best results obviously, but if this is the only part you ever make then it's probably not worth the investment. 

You mention the existing one snapped, where and can you determine for what reason? Fatigue, Overloading, Abuse??

If the "iron" Rack snapped, then it was probably severely overloaded, a lot of the brackets for top-boxes are nowhere near safe for loads over 10 - 15 kg..  and they are usually made from cast aluminium or steel tubing.

A 10 mm thick layup of CF from "standard 200GSM weave will be massively overengineered, especially if properly consolidated by vacuum or mould pressure.

Rethink your design to reduce thickness in all the areas that does not need it, add steel reinforcements at mounting points...  Not a trivial task compared to buying a new rack

Posting a picture of the broken rack, along with images of your ideas will make it much easier to offer advice!

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Hi,

     I am interested to know what strength  reduction you would suggest between:-

1. Pre-preg

2.Vacuum bag

3. Hand lay up

  This would apply to flat panels, Same sizes and fibre type.