Front suspension turrets in aluminium, in an all-steel unibody? 1

[kingnero]

Just saw the above image on LinkedIn.
As a welding engineer, I'm wondering:
1) is it correct, that the fromt suspension turrets are in aluminium, (cast/deep drawn?)
and if so,
2) how are they connected to the steel frame (conventional welding isn't an option, that leaves deformation joining, fastening using bolts or rivets, adhesives, ...) ?

I'd say these are fairly high (cyclic) loaded parts, so aluminium seems not logical, but on the other hand I have zero experience in car frame, unibody or chassis design, so there's that if this is a stupid question...

 

[BrianPetersen]

In the one that I know about, those are castings, and they are attached to the adjacent parts by a combination of adhesives, rivets, and folding the edges of adjacent steel panels around the edges of the aluminium part to encapsulate the adhesive and then spot welding the assembly in place. Tesla has something like this in production although that's not the one I've looked at.

 

[kingnero]

Thanks for confirming this exists at least. It still strikes me as an odd materials choice for that particular place/application.

 

[BrianE22]

Are there a lot of attachment points? If so then maybe the aluminum is there just to have some material near the areas of attachment.

 

[BrianPetersen]

The strut tower and the area just forward of it tends to be a complicated structure welded together from several smaller pieces. Doing it as a casting lets a bunch of that be built as one piece and yet incorporate even more attachment points for odds and ends like battery, windscreen washer bottle, coolant reservoir, electrical distribution block, etc. You'll notice that the rail structure underneath it - which is part of the frontal crash structure - is still steel. The curved structure above it, also steel, is for low-offset frontal impact.

 

[GregLocock]

Apparently that's a 2015 Volvo XC90. One solution is Atlas Copco BG Rivets.

Cheers

Greg Locock


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[SwinnyGG]

FSW solutions for bonding aluminum and steel panels do exist, but I think 2015 might have been before that technology was really volume production ready.

 

[TugboatEng]

Honda pioneered aluminum frames in their off-road motorcycles in the 1990's for this very reason. They used castings in the complex areas of the frame such as the head tube and sub plate. They were able to produce the entire frame with some 7 welds if I remember correctly.

 

[kingnero]

Thanks for your insights. The use of a casting seems logical (for the versatility of attachment points), but I am still unsure if aluminium is the best choice of material for this (for fatigue reasons). But, I am sure those who decide in this know better then me...

 

[SwinnyGG]

Well... aluminum has been in use in automotive structures for a looooong time at this point. How many fatigue failures of automotive structural components have you seen or heard about?

 

[kingnero]

I'd say the use of aluminium alloys in body in white is minimal so far, so that statement isn't really a good representation of the situation...

 

[SwinnyGG]

Aluminum BIW components have been in use for more than 20 years.. this isn't a recent development

 

[GregLocock]

Ahem. Panhard Dyna X. 1948-54. 47000+ built. Also of course every LandRover.


Cheers

Greg Locock


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[BrianPetersen]

Lots and lots of cars and trucks have aluminium control arms in the suspension.

Aluminium complete unibody: Tesla Model S, Model X, some high-end Audi models, the original Honda Insight, the original Audi A2 minicar.

I own three motorcycles that have frames and swing-arms made of aluminium, generally welded together from a combination of pressings and castings. The steering neck, and the rear suspension attachment points, are rather high-stress areas. This has been commonplace since the 1980s.

As far as this particular Volvo unibody goes, the loads on an upper strut mount of a MacPherson strut suspension are reasonably predictable. The front crash-bar is designed to absorb energy by deformation once; the stresses imposed upon that part during normal use are negligible.

 

[CWB1]

If you really want to question the industry's judgement, consider the common use of glue as a fastening method.

 

[BrianPetersen]

... a.k.a. "structural adhesives". Not exactly Elmer's school glue.

If you drive a car that has been built in the last 3 or 4 decades, the windscreen and rear window are glued in. Before that, they were just held in place with a funky-cross-section rubber gasket.

 

[SwinnyGG]

Not to mention that the 'glue' we're talking about in this case makes a joint that is stronger than a weld on day 1 and stays that way through the entire life of the assembly...

 

[kingnero]

Anyone cares to guess what part of all BIWs in general, consists of aluminium?
For the thousands of Panhards there are millions of other cars without aluminium, so I am not really convinced (yet) that "no fatigue failure reports" really means something.
And yes, I am well well aware of structural bonding, have more than once done the comparison between welded and bonded joints and adhesives certainly have their particular advantages.

 

[SwinnyGG]

aluminium seems not logical, but on the other hand I have zero experience in car frame, unibody or chassis design

For someone who said this in their original thread, you seem awfully determined to disagree with a bunch of people who know these types of assemblies intimately

 

[gruntguru]

Suffice to say that fatigue analysis is sufficiently understood for car makers to build chassis in most materials. Lotus Elise appeared in 1996 with full aluminium, adhesive-bonded chassis. Elise production continued for 25 years (Lotus still makes other models using the same technique) and there are plenty of 25 y.o. Elises still going strong.

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