How to Build a custom fitting bumper by Gerry Holm
The standard bumper that comes with a car works with all bodies, but doesn't give a good fit for any one body. The standard bumper leaves gaps, sometimes fairly large gaps, between the body and the foam bumper. This gap can contribute to body breaks/cracks when the car hits something because the body shell is absorbing the impact before the bumper comes into play.
There are 2 general principles for improving this situation. The first is to stick with the standard bumper and apply foam weatherstripping to the body and build it up so that it fills the gap. The advantage of this method is that you can easily change bodies without having to change bumpers. The disadvantage is that weather stripping is not as dense as the foam bumper and is more easily compressed and might not provide much shock absorption as the method below.
The second method is to build a custom fitting foam bumper that will replace the standard bumper. The advantage of this method is that it provides the best shock absorption. The disadvantage is that it is specific to one body, so if you change bodies you have to change the bumper.
For this project I used the following:
Materials - A heavy piece of paper for the template and high density foam about 5/8" in thickness. I used a flip-flop which cost $2 (Thanks to Ward Vanderway for this tip). You can also use anti-fatigue mats that you find in hardware stores.
Tools - Utility knife (I used a breakaway blade model since it can cut all the way through the foam), drill, dremel with either spiral cutter or drum sander
Here we see the underside of the car we're working on. You can see this body has been repaired several times and has some foam weather stripping already.
The red line in the bottom right shows the size of the gap between the body and the foam bumper. No wonder this body is so beat up, there's no support in the corners.
On this particular HPI Toyota Trueno body, there's a molding bumper in the body shell that's impossible to fit with a custom foam bumper, so I'm using foam weather stripping to fill this.
The first step is to trace the outline of the back of the original bumper on the paper. The objective here is to get the shape of the bumper in the wheel wells. Also mark the locations of the holes.
We don't need to trace the outline of the front of the bumper. Even though that's what it looks like I'm doing in this picture.
Use a paper-punch, scissors or the utility knife to cut the holes for the post. It also makes the next steps easier if you cut the piece of paper down to a manageable size. But it needs to extend beyond the front and sides of the body, so don't cut it too small.
I've now mounted the template to the body for the next steps. With the Sprint 2, the adjustable body mounts make these next few steps time consuming. The Tamiya bumper is much easier to work on. But more about this in a second.
Now put the body back on. Trace the outline of the body on the template on the front and sides.
When you get done you should have something that looks like this. You will need to extend the lines from the standard bumper to meet the side points.
Cut out the template and re-install it on the car with the body attached. Now looking from the underside, check the template against the body. It's OK to have small gaps, but if the template is beyond the body it will need to be trimmed. Take small amounts off as you go. It's no fun making a big mistake and having to start over.
It usually takes me several attempts to get it where I want it to be. On this body, the lower skirt flares out which made my template too big when I traced it. You can mark the material that needs to be removed then take the body off and trim away.
I also label the template, so I remember which body it goes. You'll want to save this template for future use.
Now I trace the outline of my template on the foam material I'm going to use for a bumper. I'm trying to use a spot that might allow me to get another bumper out of this flip-flop, but most of the time there's not enough material left.
Using the utility knife, start cutting the outline of the bumper. Remember you want to cut on the inside of the line. I find it easiest to first take a shallow pass on the cut lines to make sure I get them straight up and down. You don't want to cut at an angle unless you're trying to match the profile of your body.
Make sure you're cutting on a surface that's safe for knifes. I'm using a mat cutting sheet. Just think how much trouble I would have gotten into if I nicked up the kitchen table!
It will probably take several passes to cut all the way through the foam. This utility knife's blade is fairly narrow, so it make following these curves pretty easy. Plus it extends to get all the way though the foam.
For the holes, I use a drill to drill out the mounting holes. On the Sprint 2, these holes are round, so it's fairly easy. On the TT-01 these are oblong and took a couple of different drill bits and some trimming with a spiral cutting bit on the dremel to get them shaped correctly. It's always better to go a little small since the foam will stretch.
Once you get the bumper cut out, install it back on the car and test it for fit. Even though I spent a lot of time making the template fit, it seems there's always some touch up work at this point to get it right. Here's where the spiral cutting bit on the dremel or the drum sander comes into play, since these make quick work of "sanding" away some of the material.
When you're done you should have a custom bumper (I haven't drilled the mounting holes yet). I'm not too worried about the hole from the flip-flop strap in the middle. It's too far away from the edge to have any effect.
It's also important to check the clearance of the tires to the back of the bumper. Move the steering from side to side to make sure the tires are not going to rub up against the back of your new bumper. You probably want at least 1/8" of clearance between the two. Also if you use different tires make sure you check all of them.
Here's my new bumper installed. It's not perfect, but it doesn't have to be, The gaps are fairly minimal and it' will do a much better job of protecting my front end. And I'll need it!