I have to design the drive axles for a sports car.

I'll be getting the details next week but for now it seems I have to do the lot. From calculating the stresses and simulation to selecting the materials. I don't have to do the actual machining but that's about it.

Has any of you had a similar project? Any help would be appreciated.

Carbon fiber, 30mm in diameter, probably good for at least 5000Nm (rough guess).

Either that or wood. Pine ought to do the trick. Balsa if you want to go really lightweight.

Disclaimer: I majored in EE and never took strength of materials. Take this post with a huge pile of salt.

I'm sure about a 500mm cast iron shaft will be strong enough to handle the torque, might not be good for your wheel bearings though.

I think I'll go with the CF. This is supposed to be a sports car so it must be light.

How is the elasticity of CF?

Maybe I'll just use some steel tube. Or maybe titanium!

Maybe not titanium because it's supposed to be a limited edition sports car. Only a few hundered produced but still affordable. For the rich enthusiast atleast.

Again, I don't have any background in this kind of stuff besides what I see in racing, but AFAIK, CF doesn't really have much elasticity. It's solid right up to its failure point, then it shatters.

Quote from March Hare :

I think I'll go with the CF. This is supposed to be a sports car so it must be light.

How is the elasticity of CF?

Maybe I'll just use some steel tube. Or maybe titanium!

Maybe not titanium because it's supposed to be a limited edition sports car. Only a few hundered produced but still affordable. For the rich enthusiast atleast.

I'd have thought steel tubes would still be the best option for a low volume production car. In fact I don't think I've ever seen any other material used on a racing car, F1 probably use more advanced materials but if you're worried about the cost of titanium then I think you can consider composites out of the question.

Carbon composites can be elastic. Just see how far you can bend a carbon rod before it breaks - 180° or so (depending on the rod). Look at CF fishing rods... etc.

But it depends on the weave, the epoxy resins, fatigue life, strength in other areas.

For a low volume car were you are even SLIGHTLY worried about costs, and don't know where to start, then 100% stick to steel (and possibly steel alloys perhaps). You'll regret everything else, either with cost, time, component life or durability.

That's what I thought. Steel tube is the only solution that makes sense. Have get some info about alloys though.

The weight reduction that is gained by using tube instead of rod is huge! It can be 50%!

Quote from March Hare :

The weight reduction that is gained by using tube instead of rod is huge! It can be 50%!

Sorry, but...duh!

I reckon you can probably remove ~70-80% material to make the tubing? Obviously dependant on your wall thickness..which again will be dependant on the what your torque requirements etc are. Anyway, I bet you could get some good stuff off-the-shelf anyway...but I guess that defeats the design brief!

What course is this for anyway? Engineering degree?

Sounds interesting anyway. Should be able to learn a lot of basics by researching all these possible materials.

Quote from March Hare :

How is the elasticity of CF?

It all depends on the way you layer it, the types of weaves you'll use and if you plan on adding any "veins" to it and of course the resin you'll use. It can range from pretty damn elastic to selectively stiff in one axis to totally stiff all depending on your construction procedure - don't ask me to quote numbers, I only know of this from a hobbyist's hands-on approach. Also tubes apart from being obviously lighter can be much stiffer than a solid axle as well.

If it's the same material, then a tube is never stiffer than a solid bar (in bending or in torsion). However, the stiffness:weight ratio improves with bar. But nevertheless, you're still reducing the second moment of area when you remove materal, and that's impossible to get back without increasing the diameter, or changing the material.

Odd, I was under the impression that deflection of hollow shafts/axles was less than solid ones - perhaps that's not the sum of what stiffness is about though?

I thought that solid axles were used where strength is a concern (bearing weight for example) and hollow were deflection is a concern (transmitting torque being one I suppose).

On a similar tangent, hollow would also have the possible advantages of applying a splined design to the section which could further reinforce it without increasing weight dramatically.

Even with axial splines (or radial for that matter) you've still reduced the SMoA from that of solid bar, so it deflect more for a given torque. But it will weigh less, and that could be key.

e.g. Anti-roll bars. Often hollow on racing cars because although the bar is quite as stiff as a solid bar, it might be 40 - 60% lighter, which is a good thing. Road cars have solid bars because weight isn't an issue and cost is.

Under torsion the strength of the shaft increases ^3 versus the diameter. So if the diameter doubles, the "strength" goes 8x.

This means that hollow shaft can be stronger versus the weight.

Also most of the load is handled by the "outer edge" of the shaft.

Personally I wouldn't touch carbon fiber. Unlss you know that stuff you should avoid it. It handles differently than steel under various conditions.

By drive axle, what do you actually need to design, march hare?

Aha - so just up the diameter of the hollow one and you're a winner.

After several breakages of various items: carbon fibre is only as good as that one damned air bubble that's trapped in it - if you can vacuum bag it properly and be sure you get no bubbles between plies you're on the right course. A single bubble can mean near-instant failure when stressed.

How detailed must the design be?
CF is a difficult material and it'll be hard to come up with ISO-compliant life-expectancy calculations that actually mean something when you have no testing facility to back up your assumptions.
Steel is a lot easier to handle calculation-wise and its behaviour is well understood. It is easy to design shapes for steel since it's easy to judge what production-processes can be used.

Really, the first thing I'd do is check out the axle of a real sports car at a local car cemetery/scrapyard. That'll prevent you from setting your first steps in the wrong direction and shorten the brainstorming-phase significantly.

Vain

Or just get this torrent file: Stepney.F2007.REPACK.[McL4r3n].torrent

Quote from Hyperactive :

Under torsion the strength of the shaft increases ^3 versus the diameter. So if the diameter doubles, the "strength" goes 8x.

This means that hollow shaft can be stronger versus the weight.

Also most of the load is handled by the "outer edge" of the shaft.

Personally I wouldn't touch carbon fiber. Unlss you know that stuff you should avoid it. It handles differently than steel under various conditions.

By drive axle, what do you actually need to design, march hare?

Yes. Absolutely true. Take a solid shaft and make it twice as thick, it'll be 8x as stiff as the original bar. Take a tube and make it twice as thick (OD, with the same wall thickness and it'll be ~6x as stiff as the original tube.

But take a solid shaft and drill out the middle, and it'll be less stiff. If you THEN increase the diameter it might regain the original stiffness (and even weigh less than the solid, thinner bar did), but often there isn't enough room to do this.

Hmm, does a kart count as sportscar?

Thank you all for your input.

Ian, it's for automotive engineering. I'll get some cradits from it. I'm gonna be an engineer... well sometime in the future anyway. 30 and studying...

Quote from Hyperactive :

By drive axle, what do you actually need to design, march hare?

I'll get the details next week but as far as I know now the whole shebang.

From diff to hub. Good thing I'm on a driveline cource right now.

The two important things are:

1. weight
   
2. weight

Well cost is there somewhere but...

Quote from xaotik :

Or just get this torrent file: Stepney.F2007.REPACK.[McL4r3n].torrent

:ices_rofl

Quote from xaotik :

Or just get this torrent file: Stepney.F2007.REPACK.[McL4r3n].torrent

Hahahahahahaha

Quote from March Hare :

The two important things are:

1. weight
   
2. weight

Well cost is there somewhere but...

I would still stick to steel

Unless you're one of these pretend engineers that think steel is heavy.

Compared to titanium it is a bit heavy.

But as the cost of machining titanium is monstrous and the stuff itself costs an arm and a leg...

Besides steel sounds cool. Anyone remember Remington Steele?