Guide: one way to make your own fibreglass orthotics

Discuss light weight issues concerning road bikes & parts.

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by zappafile123

I decided to spend my holiday away making some orthotics for fun. I figure my experience may be of interest to some weenies. By all means, I do not have any experience engineering or fabricating parts like this. Nevertheless, what I've done works for me. I have no doubt that there are better ways to do what I've done. If you have some tips for me, I'd love to hear them!

Why make your own orthotics:
  • Proper foot support will increase the sense of connectedness to the bike and improve your pedal stroke
  • If done properly, it will increase power output (assuming you had insufficient foot support to begin with)
  • It can address ailments associated with your feet. For example, I have super flexible feet that collapse flat underload along with some tibial torsion which has resulted in patella tendonitis due to my knee tracking inward through the pedal stroke
  • All up I spent $100AUD, Cobra9 orthotics cost $700 ($500 for the orthotic, ~$200 for the consultation fee)
  • You have real control over the product e.g. how stiff it is, its shape etc.
  • Its fun
Why get a professional to do it for you:
  • Points 1-3 in the above list apply
  • It goes without saying that a professional is in a better position to diagnose and treat various foot ailments
  • They will make a more refined product
  • You avoid the risks associated with working with dangerous substances
  • Cast
    • plaster of paris bandage wrap
    • someone to cast your feet
    • Vaseline
    • corner cement
    • gap filler/fine plaster
    • bees wax or any other wax/PVA based release agent
  • Orthotic
    • woven fibreglass sheet - 1 square meter
    • resin
    • hardening catalyst
    • measuring cup
    • disposable stirring implement
    • paper tape
    • high quality, strong scissors
    • cheap paint brushes
    • nitrile gloves
    • sandpaper: 80, 120, 180, 320 grit
    • mask
    • angle grinder with cutting disc
    • outdoor work area
Making the cast
Here is a video demonstrating how to make a cast of your feet with the plaster bandage:
How you make the cast has a huge impact upon the final product. Your foot can adopt different shapes and postures depending on how you load it. As such, you want to attempt to duplicate the posture your foot wants to take whilst cycling to make sure the orthotic is the right shape. I chose the above video because I wanted my foot to be unloaded (because it collapses flat under load which is precisely what I'm trying to avoid). If you look to Rocket7's youtube videos (, you see they have a totally different method which may be more appropriate to you
Once the cast has set and has been removed, generously line the inside with Vaseline. This will ensure the corner cement can easily come away.
When mixing the corner cement, mix it thoroughly and make sure the mixture is not too viscous so it fills all caps in the cast.
After you remove the moulds from the cast, you have to prepare the moulds for casting by sanding them into shape – flatten and square off the toes, then use gap filler to fill any holes. You also need to use gap filler to add some extra space around your foot (e.g. the bone on the side) to account for the fact that your foot will swell under load.
Once the mould is ready, apply a releasing agent so the fibreglass resin doesn’t bond to the mould. In this case I just used bees wax because that’s what we had on hand.
Making the orthotic
The next step is to prepare the fibreglass weave. I used sticky tape to make clean cuts, but this is inappropriate for small pieces (like I did). Its good for large sheets. I used 8 sheets per foot along with smaller sheets applied to the heel and arch interspersed between each layer. 8 sheets are overkill and create an orthotic that’s basically as stiff as the sole of a shoe. This will be inappropriate for most people. I’d suggest 4 or 5 sheets for an orthotic that’s still quite stiff but flexes a little under load. This is one of the real advantages of making your own orthotic – you can reinforce whatever you want and make it as stiff as you like. I probably could not have gotten something this stiff from a podiatrist.
I suggest you cut the large sheets so that there is plenty of room to play i.e. an area well in excess of what you need. The ones I made were just a bit too small (see photo below). If you make them smaller so that they’re only a little bigger then make sure you remove the tape before you start (or just don’t use tape at all)
Next you need to prepare your work station so all the things are within reach, so you can work quickly. Use a block you don’t mind spilling resin on. If possible, place the moulds on a raised platform so the sides of the mould aren’t touching any surfaces (I didn’t do this, but wish I had). This will make it easier to apply the sheets to the side of the mould.
Fibreglass resin is toxic, carcinogenic and produces noxious fumes. Make sure you avoid skin contact and do the job in a well-ventilated area. I’d say its best just to do it outside.
Once you mix the catalyst in the resin, you’ve only got a few minutes before the resin starts to set. Apply a layer of resin directly onto the mould. Lay the fibreglass sheet down. Use a stickling motion with the paint brush so you penetrate the resin into the fibreglass. Apply the reinforcing squares to the places you want them, then apply the next sheet and repeat. Do not use broad brush strokes to Apply the resin as this will pull and lift the sheet off the mould which will create voids in your laminate.
If you want deep heel cups, I’d recommend cutting the sheet at the centre of the heel, so you can wrap them around the heel more easily – otherwise forcing the sheet over the heel creates voids
Make sure you don’t wrap the sheets over the top of the mould – you need to be able to remove it after the fibreglass sets! Think of how your going to remove the mould once it sets.
Let the orthotic set overnight
Remove the mould gently so they can be used if necessary. As you can see below, some of the gap filler and all of the bees wax stuck to the laminate. Use water to remove the gap filler. I used a spoon to remove the bees wax – hot water would also do the trick. Use the scissors to cut away the excess fibre at the sides. Be careful not to cut too close to the orthotic as cutting does delaminate the adjacent fibres.
Once its prepared, use a sharpy to make out the desired shape of the orthotic and then use the angle grinder to cut away the excess material. Fibreglass dust is a dangerous substance. Make sure you wear a mask and do it in a place that makes the dust easy to dispose of.
The final step is to refine the shape of the orthotic with sandpaper. Again, be careful with the dust.

I'm considering doing the same thing with carbon fibre and a vacuum seal. The orthotics are hefty at 95g each. This weight is due to the huge amount of material I used and the excess resin you get with a cave-man approach to producing a laminate. I reckon you could easily cut 40g off each orthotic with superior methods.
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by SchallUndRauch

Nice job, thanks for sharing!

by Weenie

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