Fabricating Custom Knuckles with plate steel (Page 1/7)
zkhennings JAN 20, 05:19 PM
I want to make some custom knuckles for my build,

Benefits:

I could use whatever brakes I want

Any hub I want

And any suspension points I want (custom suspension)

ccfiero and coppertop have made some sweet knuckles with plate steel and a brake

I scoured the internet looking for others who have made knuckles from a plate steel and welded construction, I could only find some 4x4 knuckles made this way.

I believe the Slalom front suspension made by HELD has drop spindles made from plate steel and tubing.

I am just looking for advice or tips on constructing knuckles in this manner and am open to other ways of doing it as well.

Obviously one goal is too keep the knuckles lightweight, but at the same time I would not want knuckles to fail on me. I am planning on using FEA with the cad file I produce to determine whether it will be strong enough to withstand track abuse

I have substantial CAM and CNC milling experience as well so the possibility of milled aluminum knuckles came to mind, though I would most likely need to mill on a 5 axis machine and I have never done that before. There would also be higher material costs and I have seen that machined knuckles are not as strong as cast knuckles, though the source was referring to a specific application so I do not know if this is a general rule of thumb or just one instance.

Let me know your input, I am in the beginning stages of design so I am open to anything.
DKcustoms JAN 20, 07:39 PM
I will be following this thread.

Should be interesting regardless of where it goes, can't wait to see the outcome(s).
ericjon262 JAN 20, 08:03 PM
Machined vs cast depends greatly on material cast, and casting method used. I would imagine just about anything machined from a chunk of billet aluminum would be stronger than a cast aluminum counterpart.

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Will JAN 20, 09:20 PM
Lots of OEM's cast knuckles in aluminum... you can compare older steel spindles to new aluminum knuckles to get a feel for where the casting has to be thicker to take the same loads.

I would suggest starting with 1/2" or thicker plate steel water jet cut into a shape that will accept the hub of your choice (mine would be C6 Corvette ZR1) and the brake calipers.
Then drill holes to bolt on ball joint bosses. If you have access to CNC equipment, you could make your own bolt on ball joint bosses. I think most or all of the universal ball joint bosses sold by race car fabrication supply houses are intended to be welded on.
There are also bolt on or weld on steering arms available from the street rod industry.

If you have the capability to move the bosses an steering arm around, then you can play with camber, caster, kingpin inclination and ackerman MUCH more easily than if you welded up one set of knuckles at a time.
KurtAKX JAN 20, 10:22 PM

quote
Originally posted by zkhennings:
Obviously one goal is too keep the knuckles lightweight, but at the same time I would not want knuckles to fail on me. I am planning on using FEA with the cad file I produce to determine whether it will be strong enough to withstand track abuse



I assume you have access to a decent FEA package through an institution of higher learning. If you spin this right, you can probably get class credit for it. I think back when I did an FEA project for class credit it was a driveshaft yoke, but that's going on 10 years ago now.

The real challenge with the analysis, as I see it, is getting some idea for what the design loads at the knuckles are. Any load values you put in will produce a pretty Blue-green-yellow-red topography, but it's really meaningless if it doesn't correlate to what's happening on the car.

Then again, that might get you some credit in another class, if you set up one of your current front knuckles with some strain gauges and calibrated them and then took some data on a skidpad, slamming sideways into a curb, and so on.

Perhaps you can ballpark something like....
Load in Z: MASS_vehicle x %weight_front x 3.5G x safety_factor (to simulate hitting a bumpstop in suspension compression, with one wheel carrying all frontend load)
Load in Y: MASS_vehicle x %weight_front x 2x max_expected_cornering_G x safety_factor (to simulate peak instantaneous lateral load on the front)
Load in X: MASS_vehicle x 1.2G x safety_factor (to simulate worst-case load on knuckle during max-effort braking with decent tires)

^Really just talking out my ass here, spitballing, I'm a powertrain engineer, so I don't look at the numbers involved at the wheel ends much, if at all.

Of course, you'd want the knuckle to be able to handle all the forces simultaneously for situations that happen at the track like running off-track in a corner, where you're some combination of piling on the brakes and/or turning at max ability to avoid going wide, and also crashing over the rumble curbs on the outside of the corner.

You also have to consider the fatigue properties of your choice of material, since it has to be able to endure some lower levels of these cyclic loads tens-to-hundreds of thousands of times in the part's life.
zkhennings JAN 21, 09:54 AM
Perfect, thanks for all the responses. I looked up some rapid waterjet companies, has anyone used a company for waterjetting that they would recommend?

Kurt, thanks for the input on loading, unfortunately I graduated recently and my FEA access is through my work now, so no class credit for me.

Ultimately do you think plate steel knuckles would end up heavier than a machined aluminum knuckle of the same strength? Because I'm definitely trying to reduce unsprung weight by being able to use lighter calipers and don't want to totally negate that with heavy knuckles. I assume a steel construction would be heavier but roughly by what percentage?

[This message has been edited by zkhennings (edited 01-21-2015).]

dobey JAN 21, 10:20 AM

quote
Originally posted by zkhennings:
Ultimately do you think plate steel knuckles would end up heavier than a machined aluminum knuckle of the same strength? Because I definitely trying to reduce unsprung weight by being able to use lighter calipers and don't want to totally negate that with heavy knuckles. I assume a steel construction would be heavier but roughly by what percentage?



30-40% heavier is generally what you're going to get with steel over aluminum.

I've been looking around for the possibility of using some knuckles from other cars on the Fiero, as well, but haven't found anything that will let me use the hubs I want. As a result, I've also been pondering with the idea of designing and machining some custom billet aluminum knuckles for the Fiero. The plan to design them is to design them in a way to accept a few different bolt-on hub types. When I get my mini mill set up, and converted to CNC, I should be able to make some easily enough, once I get the design done. As a first design, I'd probably make them to fit the stock 84-87 suspension, since I have an 87 GT, and am not necessarily trying to redesign the suspension yet. I just want the brake dimensions and design, and wheel sensors, off the Matrix/Vibe, to start with.

[This message has been edited by dobey (edited 01-21-2015).]

zkhennings JAN 21, 10:35 AM
I have been looking into building a CNC and might do so in the near future, this would be a fun first project to run on the mill. Almost all of the CNCs I have seen for home use have no coolant setup, have you seen any that do? Seems to make endmills last a lot longer than just using compressed air.

I will most likely make knuckles in the near future with plate steel and I will make them customizable as Will was suggesting. Then I will know the ideal geometry and if I am unhappy with the unsprung weight I can machine them from aluminum.

[This message has been edited by zkhennings (edited 01-21-2015).]

2.5 JAN 21, 10:37 AM
Cool idea.
olejoedad JAN 21, 11:56 AM
Are these knuckles intended for street use?