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Carbonfibre Push Rods - Good Idea?

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JasonB15/06/2014 10:15:00
17800 forum posts
1947 photos
1 articles

I've been quietly building a small 4-stroke aero engine recently and for a bit of a contrast between all the Aluminium thought that some carbon fibre for the pushrods would look good. My only worry is that the ends of the rods may be too abrasive and wear away the tappets etc, what do the collective minds think on this?

Tappets are silversteel and the other end of the rod fits into the socket of an Allen screw to provide adjustment, engine is not likely to see much use after I have run it to satisfy myself that it works.

Thanks, J.

Clive Hartland15/06/2014 10:17:55
2550 forum posts
40 photos

The lighter the moving parts are the better, perhaps make CF rods but have metal ends? Would CF be lighter than hollow steel/Alu tube?


Neil Wyatt15/06/2014 11:02:30
17686 forum posts
697 photos
77 articles

I would cap the ends, in common with glassfibre, carbon fibre rods can fray at the ends.


Ian P15/06/2014 11:20:34
2352 forum posts
98 photos

C/F is not very good as bearing material so you definitely need to have end terminals of something compatible with the tappets etc. Carbon fire eats carbide tooling!

What sort of length and diameter are we talking about? also are the ends concave/convex spherical?

The lightest would be relatively large diameter but thin wall, composite tube with end fittings that are bonded into the tube ID.

Ian P

Steve Withnell15/06/2014 12:06:26
796 forum posts
215 photos

I did a bit of work on a waterjet cutter for carbon fibre a long while ago - when it was "new" and I have a vague memory of the materials guy saying it was great for bending forces (wing skins, fishing rods) but weak in compression? Lost in my cluttered brain.


fizzy15/06/2014 12:37:25
1705 forum posts
115 photos

I can only tell you of my experience with them in full size race engines - they are rather prone to splintering and snapping!

Neil Lickfold15/06/2014 12:49:51
590 forum posts
102 photos

It all depends on the modulas of the carbon being used and it's layup, and resin. Most prepreg resins have quite low melt/ soften points, so you will need to look for a resin that can handle temps higher than you think the part will be operating at. That being said, also the glue used to bond the end caps will also have to be capable of withstanding those temps as well. Carbon has quite a high compressive strength relative to it's tensile strength. Kevlar only has tensile strength. Quite often carbon shafts will have a thin skin layer of glass on the inside and outside to help keep the carbon together as a structure and to offer some hoop strength. Most of the carbon will be inline with the rod with a glass cloth providing the hoop and encapsulation.

Approx density of carbon , resin, glass is about 1.6 to 1.8 grams per cc , depending on mainly on the resin to fibre ratio. Ideally you would like resin to be the least amount as possible , that is about 28% to 34 % depending on the diameter of the filiments being used.


JasonB15/06/2014 13:19:51
17800 forum posts
1947 photos
1 articles

Thanks for all the replies

Size of rods is 1.5mm dia x 65mm long

Adding steel ends from say 2.5mm may just fit into the M2.5 cap screws so could be an option.

There is not a lot of clearance so tubular rods are out though I do have these in Ali on another engine and they are quite a light option.

Not sure what the actual composition is as they are just some rods I have from an R/C Heli

Looks like it may not be worth teh bother so could just go for blackened metal rods which would give the same look as weight saving was not really an issue.


PS Pic of the rockers finished yesterday


Bob Brown 115/06/2014 18:19:23
1008 forum posts
127 photos

Have you considered titanium?

ronan walsh15/06/2014 18:31:03
541 forum posts
32 photos

I looked into pushrods a lot as i am into classic british cafe racers of the 50's and 60's. There are a lot of parts suppliers catering for the classic racers. There have been pushrods made of all sorts of materials, carbon fibre, aluminium alloys of various types, titanium alloys, but for the best combination of strength and lightness most settle for thin wall high carbon steel tube with end caps fitted.

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