Forged vs. CNC

I’m working on a full-suspension mountain-bike frame design and am trying to figure out how process should be used when creating the rocker link. Not the chainstay or seatstay, but the small little arm the connects everything to the shock.

What are the positives and negatives associated with forging versus CNC. Should I specify titanium, 7000 or 6000-series aluminum, magnesium? What about compression molded carbon-fiber (like the Look 585 bottom-bracket?)

Thoughts? Sugggestions?

Most of them I have seen are forged Al. Even on the high-end carbon frames. I don’t know if carbon could take the stress and Ti is just about impossible to forge and would be silly expensive to machine and get a minimal performance increase over the Al. Machined Al may not also have the strength. I don’t know much about Mg to comment.

Whats the price point for your final product? Volume?

For a low run, higher cost CNC would probably be better and easier to source.

For a high run, lower cost forging will most likely have a higher tooling cost but lower part cost.

I would not want to use CF - as a linkage for a few reasons. Part cost would be super high, your manufacturing would need to be extremely consistent because CF will fail catostrophically and if there are any flaws in the part you’ll wind up with someone getting a shock up their back side and suing you for a faulty frame.

Titanium will just be insanely expensive.

6061 Aluminum is probably your best bet and is most commonly used for bike components. It’s affordable, machines easily, is strong and will fail more gracefully than any of the other choices.

I know Trek uses magnesium but only on their upper end bikes.
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Even then it’s a forged in two pieces and welded together (I think) and the lower end model get Aluminum. Ti would be crazy and maybe too flexible. Carbon would fail too easily. If Trek or someone else hasn’t figured it out yet, then it probably isn’t feasible yet. Trek makes everything out of carbon these days.

The strength of the forged vs CNC’ed part would be an issue but I’m wondering if just doing a CNC and then heat treating would be strong enough. Not sure off hand.

One of the big benefits of CF is that it is an anisotropic material (stronger in one direction than others). While it makes it really nice for parts where the stress is by and large in one direction, you get less of a benefit if you’re stresses are more 3-dimensional.

Don’t invent if you don’t have to. The reason so many of the arm.'s you see are forged is because forged parts can be stronger than CNC (with proper stress relief)

Titanium is tough (but not impossible) to justify for anything that doesn’t fly.

My understanding is that while magnesium has better mechanical performance in the short-term, aluminum will last longer. I agree with 6061… readily available and strong.

We’ve seen some really funky effects of magnesium corrosion, especially if it’s on a mountain bike where you could be getting a lot of exposure to salt water.

If it’s treated correctly it’s less of an issue, but it likes to grow a bit of a fuzz after a while. :open_mouth:

Aluminum gives you the most options for post processing as well (polishing, brushing, anodizing, etc)

Thanks SO much for all your help, guys. My client wants to utilize compression-molded carbon but I’m going to suggest we simply stick with CNC’d aluminum.

Someone mentioned heat treat after CNC, forget this, you only do that after welding. For a first design 100% go for CNC Aluminium. 7075-T6 is ideal but you can’t weld it at all so make sure thats only for the rockers/links. Anything like the swing arms will be easiest in 6061 then post heat treat and post final machining size of bearing seats.

Will it be strong enough? Someone should be doing the calcs or copy the sizes from a Knolly or something. Someone said Titainium it too flexible - strange comment, look at the properties of the material, it can be stiff if you want. Alum gives geometric stiffening due to low density so its normally the best/easiest for full suspension. I image the carbon route may be an option once the design is 100% dialed - similar to upper links on a V-10 or Nomad.

Guess I was thinking in terms of tubing. Ti give you a pretty plush ride in my opinion. But there is less fatigue over time vs. steel.

True as a general comment on frame “feel”, and agreed on the effect.

This comes from Titanium not being prone to fatigue, hence the frame can be designed to flex, where Aluminium HAS to have enough material to be stiff or it will fatigue and fail. A suspension frame should be stiff because the movement comes from the spring/damper, so you lose that possible advantage of Titanium.

That it does. We’re certainly on the same page here.

probably there’s enough information given for you to act.

But, specifically, your question - forging a part into near net shape does two main things, work hardens the piece especially at the surface, and aligns the metallic crystalline grain structure imparting extreme strength. Often forgings are abrasive blast finished to remove the work hardened surface; for example all aircraft landing gear structure are cast-forged-abrasive blast finish-final machined.

Machined parts from ingot or billet have the as cast metallic crystalline grain structure, essentially random and not as strong as above.

You may investigate powder metallurgy, better than machined, not quite as forged. Involves tooling.

Simple… CNC the prototype then with your engineering FEA tools (Design Engine can help) shrink size as you design the production forged part. If you manage the grain direction and strike the aluminum with a ‘consistent’ grain direction each time you can estimate the stress and strain better… therefore loosing even more mass.

Is this a DH monster or a cross country grinder? You have to consider usage to get the best answer. However, I worked at a machine shop and we did lots of custom one-off aluminum parts for trophy trucks. One simple rule we lived by: Don’t over engineer it.

Maybe two rules - don’t under engineer it either :slight_smile:

I like to under engineer so I can test till failure. That way I know how light to make the part. I know where the part will fail so therefore I leave open possibles for making the part stronger where. In the case of the first plastic derailleur the part seamed under designed until the engineers added glass fibers to the material and managed where they gated the part. They got their structural integrity in the direction they wanted by undersigning the part.

SRAM engineers are bad asses and not everyone can do the things they do.

Yes they are. SRAM Red is one of the most beautifully constructed component groups I’ve ever seen. Both mechanically and aesthetically.