Rear Bumper Weight Reduction
I’m not happy with driving a 2060 lb. electric Midget. I had high hopes of a 1700 lb. car when I was considering going electric. Starting with a newer rubber bumper Midget didn’t help, and adding 475lbs. of batteries sealed the deal on being overweight. The rubber bumpers are known to be heavy and seemed like a good place to start when reducing the weight of the car. My front bumper weighed 50lbs. and my rear bumper weighed 35lbs. I have seen the many conversions of rubber bumpers to the earlier classic bumpers, but I didn’t want to deal with all of the body work involved with that approach to weight reduction. I also saw that someone had removed the steel from the rubber bumpers to lighten them. I was able to do this myself, and I’ve documented the process so others can try this if they want to.
We’ll start with the rear bumper. Remove it by finding the four nuts in the trunk that hold it onto the car. It will practically fall off once those nuts are removed. Once off, you’ll be able to see the backside of the steel box beam that is inside the rubber bumper.
To remove the beam you’ll have to drill out the pop rivets that hold the rubber part to the steel.
There are two keeper straps that are on top of the rubber part. One is on the bottom side of the bumper and is easily seen, the other is on the top side and hidden by an extra cosmetic flap of rubber.
After you have drilled out the rivets, you’ll have to pry the keeper straps off, and then separate the rubber part from the steel beam. If you’ve picked the correct drill size for removing the rivets, then the keeper straps will fall off. The cosmetic flap of rubber makes it hard to get the drill onto the rivets below the flap. Be careful not to accidentally cut through the cosmetic flap when attacking the rivets on the top of the bumper. Use screwdrivers to work the rubber off the steel beam.
When it’s apart you’ll be able to appreciate the size of the steel beam used in the bumper. You can also see that the rubber part has molded in internal ribs that make it stiff enough to be able to remove the steel and still have it hold its shape.
Now that it is apart, we can remove as much of the steel beam as possible. I decided to keep the mounting features of the beam because they had studs in them. Making a lighter wooden part with studs, and getting the alignment correct seemed to be too much work for the few extra pounds I would save. The remaining rivets in the mounting features would allow me to get them back into the rubber part with perfect alignment. I used a metal cutting blade for a skill saw to do the cutting of the steel beam. It worked very well and made the cutting easy. Use hearing protection and safety glasses when doing this.
Here are the mounting features back in the rubber bumper. Their original rivets and holes locate them, so I know they are in the correct place.
The rivets alone hold these parts into the rubber very well. I decided to glue these back together using Gorilla glue. This is the original gorilla glue, which is a single part urethane adhesive. Single part urethanes cure by absorbing moisture from the air. The directions tell you to wet down the surface you are gluing to. This glue will foam up as it cures and can expand to the point of pushing things apart. The fit was tight enough that I didn’t devise a way of clamping the rubber to the steel parts while the glue cured. It worked fine for me, but someone else might consider clamping this while it cures. To fill the open bumper so it doesn’t collect dirt, I used some expanded polypropylene foam (EPP) I had. This is water proof, tough and light. I also glued it into the bumper with Gorilla glue.
Here is the finished product. I painted the foam black so it wouldn’t be noticed. The total weight reduction for this work was 17.5 lbs. The original bumper weighed 35 lbs, so I cut the weight in half. If someone wants more weight reduction, then fashioning wooden mounting features will cut the weight down more. I ended up doing this for the front bumper because the mounting features for it were so heavy that I would have only reduced the front bumper’s weight by 15 lbs. Creating from wood, and aligning the front mounting features were much easier to do for the front bumper.