Restomod Day 18 - Building


The suspension work continues! The rear design with short arms plastically deformed after the first flex, so I ended up changing to a more complicated design with longer arms. The new design should also allow for equal length and thickness arms on the front and rear designs.

Front Design

Front Flex Front Unloaded

A closer look

Front Flex Front

Rear Design

Based on the front design, but with different packaging for the rear dimensions of the chassis.

Rear Flex 3/4

Rear Flex Top

Rear Flex Back

The rear suspension design has to deal with quite a lot of travel given the length of the arms as designed.

Rear Flex Back Compressed Spacing

Each bushing is ~2mm, so it's between 3 and 4 mm of suspension travel

Rear Flex Back Compressed Spacers

A different view of the spacing

Rear Flex Back Exposed Spacing

A closer look at the right side

Rear Flex Back Focused Spacing

This leads to a huge amount of flex in the rear suspension, which led to plastic deformation when I compressed it fully.

Rear Flex Full Compression

Even when unloaded the suspension is still deformed after one compression cycle

Rear Flex Unloaded

You can see the results of the deformation near the mounting screw

Rear Flex Post Compression Focused

Here you can see the stress deformations. Of note, the fractures tend to travel along the printed fill lines and seem to originate from where the screw mounts the suspension.

Rear suspension with stress fractures

A closer look at the left side

Rear suspension with stress fractures macro

Checking to see how it looks

The new suspension has a high effective spring rate, so the body rides relatively high.

Classic Mustang on chassis with new suspension

New Rear Design

The new design lengthens the arms to reduce how much the plastic needs to flex for a given length. This "dog-bone" packaging does make the arms much weaker; however, this approaches a similar spring rate as the original XMods suspension design.

New dog-bone rear suspension 3/4

When printed the gap is almost not there. It's just enough to be able to gently snap apart after taking the part off the print bed.

Dog-bone rear suspension top

With no load the suspension sits slightly flexed

Dog-bone rear suspension back

Stress-Testing the New Rear Design

Under compression. Note significantly reduced curvature compared with the original rear design.

Dog-bone rear suspension fully compressed

Here's a closer look at the new design after it'd been forced through its range of motion a couple of times. The fractures seem to spread from the end of the split between the mount and the arms. There's a similar pattern here of the stress travelling parallel to the fill direction.

Dog-bone suspension with stress fractures

Infill direction

Dog-bone suspension printed infill pattern

Close up of the stress pattern in the new design

Dog-bone suspension with stress fractures macro

Slicing Up the New Design

Looking at the slicing for the new design, it's easier to see the fill direction with the default settings.

Slicer view of stressed layer

The stresses seem to concentrate along the red fill

Top down slicer view

I was originally concerned that printing the version number into the part would cause it to break more easily; however, the tension stresses are primarily on the other side of the part. It appears that the design resists compressive forces well enough that there isn't visible deformation on the top.

Slicer view of exposed top layer

To adjust the design and see if I can get better stress patterns (and less permanent deformation) I decided to try increasing the shells to align the fill along the direction where it would be most stressed. In the XMods design, the lower A-arms can handle a lot of the longitudinal and lateral loading from the road, so the upper A-arm should mostly be stressed vertically (in an ideal world for this design). Even in a non-ideal world, the forces should primarily act to bend the suspension instead of shearing it, so the aligned print shouldn't have too much downside.

Slicer view of new infill pattern