Black & Tan 332iT

[Bry5on]

Actual weights from the bathroom scale, without heat shields or ABS sensors, repeatable 3x:
E46 total 14lb  = 6.35 kg
E60 total 13lb  = 5.897 kg
E60 aluminum knuckle 5.4lb = 2.5kg

so in theory the superknuckle is .15kg heavier than the e60 setup and .3kg lighter than the e46 setup. So no appreciable weight benefit, but definitely a stiffness increase along with a much larger (and stiffer) wheel bearing that happens to bolt in.

I’ve also decided that I’m going to reduce the steering ratio on the next iteration to match the non-m 13.7:1 steering ratio (vs the M3 14.5:1 with the same rack) that I find preferable. After adjusting for wheelbase differences, here are some baseline comparisons from other cars:

911 ST converted to e46: 13.5:1
911 GT3 converted to e46: 12.8:1
E90 M3 converted to e46: 13.3:1
Wife's Macan converted to e46: 13.9:1

So 13.7:1 has some good company, I guess that’s why it feels so nice. The MK60 is easy to program with a write-in steering ratio, so I’ll have to knock that out along with this change as well to keep DSC in its happy place.

[Bry5on]

What about much higher, peaky upwards loads (e.g. hitting a pothole)? I would think the wheel would see more than 1G upward acceleration in cases like that.

Worst case scenario sounds like steady state cornering and then a sudden upward force at the wheel (e.g. taking a corner and riding the kerbs on track).

Yeah, I thought about this. I had a hard time coming up with a real number to use here because the tire is a nice spring, and so is the bushing. So the load it sees is maybe 2x the worst case? Worst case being braking and hitting a pothole I think. The part has a safety factor of just under 3 to a 1.6G load of half the car's weight (0 load on rear tires) for this scenario, meaning that it's got to take a 5G+ load wallop to risk permanent deformation. Any experts on modeling suspension impacts here?

[heinzboehmer]

I mathed out and ran four load cases:

What about much higher, peaky upwards loads (e.g. hitting a pothole)? I would think the wheel would see more than 1G upward acceleration in cases like that.

Worst case scenario sounds like steady state cornering and then a sudden upward force at the wheel (e.g. taking a corner and riding the kerbs on track).

[Bry5on]

Okay, on to some results. I mathed out and ran four load cases:
1) 1.6G braking
2) 1.6G cornering, outside wheel
3) ~.8G cornering, inside wheel (more load on inside at lower Gs)
4) 1G braking + 1G cornering, outside wheel

I thought long and hard about these load cases and determined these to be the enveloping ones. Would appreciate any thoughts of other load cases that I may have missed or should consider.

All load cases included a lot of extra tie rod force to compensate for the fact that you can also be turning the steering wheel. You can see that the Fusion FEA spit out some real localized high sresses at sharp edges due to constraints. This is generally expected and not cause for concern as the bulk of the material has very low stress. I opted to run the solver for a stiffness-driven part, as I figured that would give the best steering feedback, and it means that generally the part ended up pretty low-stress.

The part, with just barely enough clearance to install fasteners and get an extension through the top bolts:


The stresses on the part in various conditions ("Load Case 1" is 1.6G Braking):


The displacements:

[heinzboehmer]

It has an MK60 and 48 tooth wheel speed sensor just like the e46 - I get to keep ABS and traction control.

And bolt in vs press in wheel bearing.

[Bry5on]

Hi Bryson, sorry if I missed it but, besides the material weight benefit, why did you choose e60 design?

It has an MK60 and 48 tooth wheel speed sensor just like the e46 - I get to keep ABS and traction control.

[maupineda]

Hi Bryson, sorry if I missed it but, besides the material weight benefit, why did you choose e60 design?

[Bry5on]

I'm sadly out of town this weekend, but decided to check for clearance in the printer here at home. The knuckle is rather conveniently sized..


This model is 2kg (4.4lb) in aluminum and has a minimum safety factor of 4 under a 2G braking case and 1.6G cornering case.

[Bry5on]

Is this an optimal model generated by CG? The geometric complexity in areas like the tie rod mount look really difficult. Looks like you could keep 10% more material and simplify the shape, no? Cool nonetheless.

Yeah you’re right - can definitely make it simpler at very little weight penalty. I’ll run a few more simulations and computer generated designs then probably CAD a human-made version to compare stiffness and strength properties. I’ve been basically just adjusting the keep-outs and letting the model chug. There’s a new run waiting for me to get home and inspect. You can see how the BMW design sub-optimizes deflection for ease of manufacturing, they definitely left some performance on the table in those castings.

This computer generated knuckle may even be small enough to print in my home plastic 3D printer as a mockup, which would be pretty cool, and a handy validation for metal 3D printing.

Before I finalize this, I’ve still got to design a mount for a heat shield and a basic duct for brake cooling. I *think* I can make that from a single brake formed piece of aluminum at this point, but we’ll see.

[YoitsTmac]

Is this an optimal model generated by CG? The geometric complexity in areas like the tie rod mount look really difficult. Looks like you could keep 10% more material and simplify the shape, no? Cool nonetheless.

[Bry5on]

Okay a little practice and I'm getting better. Once you start adding all the keepout spaces for bolts, wrenches, ratchets, AND sensor and install direction, control arms, tie rods... you can see how BMW arrived at the e60 'bowl' shape design. I'm a few simulations in now and they're all coming in around 2-3kg with 4-7x safety factors in printed/machined aluminum and 5-15x safety factors in Ti. I used some pretty hefty loads, like 2G in braking and 1.6G cornering to build the model. I'm sure there's lots of mass optimization to be had here, but since I'm really trying to optimize for stiffness without invalidating strength, Ti probably won't be much of a benefit at all over Al, practically speaking. I'll keep you all posted.


Example aluminum solves

[Bry5on]

Okay, I couldn't help myself. I'm going to try the built in generative design tool in Fusion. Looks like the e60 hub will package within the e46 available space..

[Bry5on]

Made some progress modeling bank 2:


Interesting to note that as George Hill pointed out - the US bank 2 header crosses over to the *passenger* side of the car, where the euro bank 2 header stays on the *driver* side of the car. Odd!

Edit, some details:
Tube is 40mm OD, 1.5mm wall (37mm ID) and all bends in bank 2 are 40mm radius
Runner length is somewhere around 320mm when you include the length of the exhaust port
A post-step size of a 5/8" OD, .065" wall should work well with the stock pipes
I just purchased an exhaust textbook so that I don't royally screw this up.

[Bry5on]

Thanks to a fellow member here (thank you!), today I got a good scan of a front suspension knuckle for a future project. Figured I'd make it available here for anyone who wants to use it for design purposes: https://s.digital3dcloud.com/space/f...lang=en&loop=1

[Bry5on]

Progress. What's next.....