Casting Part II (Post # 52)

I'm back to making rubber parts again.  This time the challenge is the four bump stops (rubber travel limiters to keep The Alfatross' front suspension from travelling down too far) in the front suspension, two on each side mounted to the upper A-arms.  

Here's one of the bump stops in place in the front suspension
before removal.  Dessicated, cracked, and compressed, the
rubber parts were in need of replacement.

After removing the bump stops it was easy to see that they weren't up to the task of keeping the car from topping out anymore.  Maybe there is a secret stash of bump stops for 1955 Alfa Romeo 1900s somewhere, but I doubt it.  I didn't even bother to ask around.  Besides, I liked the idea of refurbishing the originals and continuing to use them.  

The steel and aluminum parts cleaned up easily, and the rubber tips had already peeled off two of them.  Evidently, supporting the chassis on jack stands for years may have been good for the tires, but it let the suspension compress the bump stops until they lost their original shape. 

All of the rubber tips are in such bad condition that it is impossible to tell
what the original profile was like, so I'm just guessing.  The old rubber peeled
off the steel shaft easily.  

I knew I could replace the rubber tips of the bump stops using a two-part RTV rubber compound called Flexane, but there were a couple of obstacles.  I didn't have an un-deformed original from which to get the proper shape, and I didn't know exactly how hard to make the rubber. I decided to just reproduce the existing shape without trying to make it longer or more pointed.  Flexane can be combined with a third additive called "Flex-add" to make the product more flexible and "rubbery," but the only way to achieve the right hardness is by trial and error.

The first set of experiments was devoted to getting the right proportions of the three Flexane components using one-ounce condiment cups for testing.  I allowed several days for each cup to cure before subjecting them to toughness and hardness tests--including smashing them with a heavy hammer!  I finally came up with a combination that I thought would work.  

The only way to achieve the right hardness was by trial and error.  As you can see from the numerous test cups, there was a lot of error!   

Next I tried making molds in clay in which to form the rubber tips, but couldn't get a symmetrical shape. Because I had four of these to make I thought it might be cost-effective to make an aluminum mold.  Only problem was that I don't have a lathe or machining equipment . . . but I know somebody who does!   

Harold Williams is a fellow Car Table Guy (more about the Car Table in a later post).  He is also an avid builder of radio controlled vehicles (http://www.youtube.com/watch?v=crHlnNX_HF0&feature=youtu.be), which means that he has the means, motive, and opportunity to machine all kinds of small parts out of aluminum. The mold I was looking for was just a simple cup, really, and I know he would have preferred something more challenging. But he did it anyway.

Making the mold in Harold Williams' shop.  All he asked in return was dibs on the first ride
in The Alfatross when it's finished!

Making the cast was easy.  I put a separator film inside the mold, mixed the Flexane formula, filled the mold, put the steel shaft in contact with the surface of the Flexane, and clamped it in place making sure it was vertical and stable. 

The finished mold and the first bump stop product.

Original bump stop (left) with the first example
made  with the new mold.  

Now all I have to do is make 3 more . . . and go on to the next rubber casting project.  This one was easy, given the shape of the part.  But there are some real challenges ahead.  It's a good thing Harold likes challenges . . . .