I’m thinking of building a composite Mtb frame, probably a hardtail to start with a foam or balsa core.

I’m a bit concerned about the main areas of stress (head tube interface/BB interface etc)and wondered what kind of loadings/design philosophy goes into the design of commercially produced frames.

I’ve got a feeling that a lot of frames are made ‘off the cuff’ and then tested to make sure they are strong/stiff enough without much actual design going on.

I don’t really have the time/funds to build a load of frames and test to destruction, so I’m probably going to model using finite elements, wire frame first of all, then shell.

I found this, which has a lot of good info:

http://sheldonbrown.com/rinard/fea.htm

Anyone else have anything they can share?

Cheers

CEN test loads and directions would be a good point to start at, and/or calculate the force a rider would exert on the bike for a set number of situations and factor in the safety factors.

EG 18 stone rider, dropping 8ft to flat etc.

Have a load of FEA stuff from my Dissertation which was a 5″ travel FS,slack AM frame (10 years ago :wink:) but haven’t been able to find the ZIp disc/cart it’s on 😕

Thanks Loco, good idea to look into the CEN testing stuff.

Now safety factors… what would be sensible for a composite bike?..

I remember my composites module at uni, once you’d factored in material SF, loading SF, workmanship SF, Layup SF, Void ratio SF, etc etc you ended up with something like 17! (Civil structures)

I reckon that deflections rather than failure will be the governing factors on a composite bike…

I seem to remember using a factor of 10 as a starting point and then working from there.
Saying that it was a while ago and in Aluminium, my mate deigned a full carbon beam frame for a 1000cc motorbike to the point it could have been produced so he’d be the man to ask, however he’s emigrated to NZ so abit difficult to pick his brains!

Brant maybe able to give some basic stats.

I’ve got a feeling that a lot of frames are made ‘off the cuff’ and then tested to make sure they are strong/stiff enough without much actual design going on.

What makes you think that?

How about reverse engineering a metal frame? Measure the tubes and calculate the failure loads then work from there with the composite model.

I’ve got a feeling that a lot of frames are made ‘off the cuff’ and then tested to make sure they are strong/stiff enough without much actual design going on.

What makes you think that?

Not sure… because it may be cheaper to do it that way? and once you have somthing that passes the tests there’s no question about its suitablity etc.

Good idea irelanst, it would require cutting up a frame to get the wall thickness though..

it would require cutting up a frame to get the wall thickness though..

Good point, although you can get the specifications for Reynolds tubing easily enough here which could be a good place to start?

Mikeyd, I can mail you some steel frame dims with related CEN info if of use. Tubes dims, approx geo etc of known or easy passes. Theoretical from models or actual, but actuals are often test to CEN + a percentage of cycles, not always to destruction so not sure how effective that is for reverse eng. Got the same for Al but tube shapes vary more so more complex to model.

Good point, although you can get the specifications for Reynolds tubing easily enough here which could be a good place to start?

Great, thanks

I can mail you some steel frame dims with related CEN info if of use. Tubes dims, approx geo etc of known or easy passes. Theoretical from models or actual, but actuals are often test to CEN + a percentage of cycles, not always to destruction so not sure how effective that is for reverse eng. Got the same for Al but tube shapes vary more so more complex to model.

That would be really useful jamesco, thanks: mike dot davies at aecom dot com.

I think you guys are right, get a feel of what a simple steel mtb can handle and use as a starting point/check to my calcs/design.

Ages ago, I did some interesting stuff with strain gauge and accelerometer data logging on real bikes/riding and verifying this against FE models for an MTB company – don’t have any of the data any more, but it shows it’s out there.

I know Trek use Abaqus, and the other Dassault Systems products (probably tie it with Catia for developing ply-books for the composite frames directly from the FE results) – they’re always (it seems!) turning up the Simulia news letter.

Not sure… because it may be cheaper to do it that way? and once you have somthing that passes the tests there’s no question about its suitablity etc.

Sure, crack on. How hard can it be. I mean, Specialized and co just dream up a couple of ideas on an envelope and then just make it and tick a box and sell it.

What could possibly go wrong?

Judging by the amount of cracked frames that appear on here I’d say that very little actual design goes into them.

Hello Mikeyd,

This will certainly be an interesting project for you. It will be fun and a little challenging. It will involve a thinking side but will also test your practical skills.

I would really ask yourself how much you want to do this. If its purely as a hobby, great carry on. However, if your aim is to obtain a bike that you will be happy with and use on a regular basis, then I would think very carefully.

I would be interested in knowing what facilities you have to do this and also how much money you want to spend. At this point I could advise you the best way to do it. lets not forget that design is closely related to manufacture. With composites you are always designing with the intended manufacturing process in mind.

bristolbiker – Member
Ages ago, I did some interesting stuff with strain gauge and accelerometer data logging on real bikes/riding

I’d love to try that. Been temped to strain gauge up my frame and go for a ride. Unfortunately my datalogger set up is a bit bulky for a ride, doable but a bit bulky.

mikeyd,
Did what you’re planning a few years ago, started with a hard tail, then a full suspension. Very much suck it and see in terms of how thick to make everything, I think it’s well over engineered but I didn’t want a failure.
Photos here:
carbon bikes

Hi Chief,

I understand the concerns you’re talking about, having made a canoe and a couple of surfboards I can confirm that they are not perfect, and in no way have I saved any money!

However, I’m happiest when I’m making something, and a bike seems like a good next project.

I would be suprised if I got any change out of £500 for a first attempt.

Facilities wise, garage/workshop setup, good selection of tools to make jigs etc.

Initial thoughts for manufacture are to ‘sculpt’ the shape out of medium density foam (like the type you shape surfboards from), fit hardware (BB shells, headset tube, dropouts etc).

Then either layup with CF/GF, wet out with epoxy then wrap with perforated tape to get rid of excess resin.

Or

Vacuum bag in sections, but I’m not sure about that, I’ve only every vac bagged on a plate/mould

Richmars,

I have seen some really crappy efforts at making carbon bikes and I have to admit that when I clicked on your link I was expecting the same.

Pleasantly surprised! well done, its a good job. I am interested to hear how it rides? I guess you used stock tubes?

Cheers

Richmars,

Nice work, pretty much exactly how I was seeing mine come together in my head!

How much do you reckon you spent?

some good general info here if you havnt seen it:

chief9000,
Thanks, I was just trying to do something that worked, not the best looking. The first frame (the road one) used stock tubes and lugs, I ‘just’ glued it together. The 2nd and third were all mine, The tubes were made from a foam core (turned on a homemade lathe!)so not very round. The full sus. didn’t really use tubes, but based on foam cores or moulds made from MDF.
I used the hard tail for a while, but didn’t use the full sus much once I knew it worked.

mikeyd,
Spend, not sure, carbon from ebay was about £100, but still some left, epoxy is £30 or so, then mostly my time and consumables, like brushes for glue. At the time I had access to a mill and lathe, so the metal hard points I made myself (even the suspension pivots), again using Al alloy from ebay (they said it was a good grade, no way of checking!).
(Brought the bottom bracket shell from a frame builder supplier, about £10).