Car Wheel
Tire - Shaping
The car wheels are relatively simple in terms of geometry, but the actual trick of achieving a convincing tire is in the textures. This wheel is by no means as convincing as you can get... but it does offer a reasonable result for some pretty reasonable effort.
Thanks should definitely go to thorn whose Cinema 4D Essentials videotapes introduced me to HyperNURBS, and gave me the idea for this wheel technique. (Okay, I admit, it's a pretty much a straight up copy, but I asked permission!!!) :-) Check out thorn's site here.
Using the side view of your car, determine what is a reasonable diameter for your tire to be. Draw a circle spline to represent this tire.
Then copy and paste this spline into a new file so we can model it externally and not worry about anything getting in our way.
Create a tube object. Above right is the default tube object before modification.
Modify the parameters of the tube primitive so that it is on the correct axis (+X in this case). Estimate the radii for now, but change the segmentation to 8 rotation segments, 3 height segments, and 3 cap segments.
Checking what we've got, it looks way too big, and also way too angular...
But that's okay, because this is a HyperNURBS tutorial! So...
...drop the object underneath a HyperNURBS parent, and it quickly becomes a smooth round cylinder again.
Now you can adjust the outer radius to fit the circle spline we brought in for reference, and the inner radius can be adjusted until it looks appropriate for a tire.
Also the height of the tube which represents the tire width can be adjusted to correspond with the overall tire shape you are looking for.
Now, with the tube object selected, use the Make Editable command under the Structure Menu.
This will enable us to work with the tire object on a polygon and point level... however, it means that the tube is now cut off from the basic parameter changes we can use with primitives.
I disabled HyperNURBS just to show you the mesh structure. It is very basic and simple, which is good for easy editing.
With HyperNURBS reactivated, the round tire shape comes back again, however you'll notice a couple problems.
One problem is that the curvature of the tire appears fairly faceted despite the rounding being done by the HyperNURBS. That's okay for now... it's simply a matter of our HyperNURBS subdivision settings. We want them really low right now to make things a little quicker in editing and rendering. Later we can turn the settings up.
The other problem is that sharp edge running around our tire. That's not at all natural looking, is it?
The reason that sharp edge exists despite being in a HyperNURBS object, is that the primitive actually had two parts... 'height' and 'caps'. When the tube was made editable, these separate parts were retained, even though it is all one single object. The edge and points where caps and side meet are duplicate, and therefore are not rounded or filleted with each other like you might expect.
To remedy this, select all polygons for the object, and use the Optimize command (with default settings is fine) to remove duplicate points.
What happens is that points in 3D space where there are two points coexisting are 'welded' together so the object is effectively continuous and not made up of height and cap segments.
The HyperNURBS object then has the chance to smooth these edges in a way that is more suitable for tire edges.
For aesthetics, I like a little bit of rounding to the surface of the tire. This step is not necessary, but is a good exercise in the kind of polygon manipulation that will be necessary soon!
Here, all polygons on the outside middle of the tire are selected.
Then they are scaled up using one of two options...
Option one is to constrain scaling to the Z and Y axes so that the polygons selected merely move outwards radially from the center of the tire.
Option two is to just scale uniformly in all three dimensions so that not only is there the radial outward movement, but also the polygons will get a little wider in the X direction. For the minute amount we are scaling, there is little implications to doing it one way or another now.
Looking at the shape we have now, it's clear that it could use a little bit of work on the inner radius to taper the tire wall inwards toward where the hubcap will be.
Also, I'd like to make the inner radius smaller, which is something I could have done back when we were playing with the primitive settings, but which I overlooked.
Select all polygons that make up the inner surface of the tire.
Using the scale tool constrained to only the Z and Y axes, scale the polygons 'down'.
When constrained to the Z and Y axes, the scaling down of the polygons will cause them to move inwards towards the center of the wheel, but the polygons will still retain their width in the X direction.
Now to create the inward taper of the tire wall, scale the same selection of polygons down on the X axis only. This will decrease the inner tire surface's width in the X direction, causing the slight slope we are looking for.
The reason we did not scale all axes at the same time is because we wanted the added control of non-uniform scaling in multiple axes. We could scale just a little bit in Z and Y directions, without affecting X, which could then be given it's appropriate adjustment independent of the other axes.
So the resultant shape is generally tire-like. It's not about to win an Oscar for best tire in a dramatic tutorial or anything, but it will do a passable job.
Once it gets a nice cast of supporting textures though, it may do far better than we expect.
We can now set up the tire to be ready for the texturing we will inflict upon it in the next few steps.
Here I have shown how we will set up selection sets for a couple parts of the tire, for use in our upcoming texturing stages. To the left is a selection set consisting of all polygons on one side of the tire. This selection set I have called TireWall (okay, no one said I was original!). To the right is a selection set consisting of all polygons around the outside surface of the tire, where the tread will go. For that reason, I have called that selection set, TireTread. ;-)
Now that we have a nice donut, we can get down to the REAL business of making a tire!