Alloy Wheel Design & AnalysisUsing Pro Mechanica to resolve a major issue preventing a series of

wheel designs from entering production

@MartinDirker

A pretty wheel……• Did not meet durability requirements• Unable to be sold

• The design was flawed in some way• I had to find a way to fix it quickly and cheaply

What did we see on test?

• Crack initiation points

Analysis (Pro Mechanica) model

• High stress zone correlated well with crack initiation points

Very good correlation

• Both on front and rear faces

What to do?

• Many changes were made to improve the load path between the hub and the rim.

• This load path was very restrictive and I worked to improve it by adding material where it would have maximum benefit.

• I added material under the cap, altered the metal saving pocket on the rear and rotated the bolt holes

Fill in material under the cap• And rotated bolt holes

Altered metal saving pocket shape• Changed from rectangle to v-shape• Rotated bolt holes also shown

Tapered sides show v-shape metal saver

Straight sides show rectangle metal saver

Improved load path shown

• Thicker material where beneficial

Straighter load path

• Increased stiffness at hub

Lower stress for same load

• No cracks expected

Stiffness at hub area illustrated

• Less displacement for same load

A long time ago (the 90’s) in a land far away there was a company making alloy wheels. They made many wheels and sold many wheels, but they made a few more than they sold. Why? If a wheel does not pass durability testing it is not certified for use.

One design family was a particular problem and I was hired to solve this one and many others and prevent similar problems from happening again. The company had invested a lot of money in low pressure dies which were unusable.

I had a look at the wheel and built an analytical model to simulate the test and to see whether my design changes were useful in reducing stress. Pro Mechanica by ptc.com, if you’re interested.

The wheel was quite attractive but the load path needed from the hub to the rim was very constricted in the area under the cap. you see, when load is transferred, it likes an easy path. If we restrict that path, we get high stresses and potential for a crack to start.

I looked to 'fill in' the volume under the cap, especially from the bolt holes to the spokes and that made a big difference to the stress.

I then also looked at the load path down the spoke and improved it by adding some material near the hub area but less towards the rim. For particular fatigue tests, bending moments are greatest at the hub end of the spoke and reduce to zero at the rim end.

The final tweak was to rotate the bolt holes so that no holes were directly in line with a spoke. for a 4 bolt pattern, this meant a 15 degree rotation. This helped reduce the stress around the bolt holes as the load path was improved by not having to go around the bolt on both sides. The load tended to go on the side where more material was provided by the rotation. We should recognize one of our roles in design is to provide adequate load paths for the components and systems we design in order to have safe, reliable products.

This series of modifications were very successful and similar changes were made to the rest of the wheel family, consisting of different diameters. All those idle moulds were put to use to make wheels (and money)

If you'd like to know more about alloy wheel design, say so in the comments and we can start talking.