PDA

View Full Version : Miata rear bumper cutting; Cutting the rear bumper for airflow and aesthetics



Agent☣Orange
12-23-2011, 03:29 PM
DIY bumper cutting:

Materials: Dremmel with large cutoff wheels, metal spike, marker, level, goggles ( whole face protection or ski mask recommended).

To be specific, the bumper cover is the plastic part that will be cut. The actual bumper is the aluminum bar hidden by the cover.

I've done two different versions: short and long but the process is the same. Crawl beneath the rear of the car and study the area to be cut. You can see the actual aluminum bumper hidden by the bumper cover along with four plastic braces that attach the bumper cover to the aluminum bumper. The actual bumper is the reference part to use for the cutting line. Do not simply measure from the floor to the bumper because the car or floor may be uneven and so will your cut.

Decide how high up and how wide you want to cut the bumper. Ideally for me, it was to cut it all the way up to the bottom of the aluminum bumper. Any higher and the aluminum bumper would be exposed and may ruin the project. The width for this particular project was determined by the space between the center two of the four plastic bumper cover braces.

Application note: Putting the rear of the car on stands or ramps makes this project much easier. Your cutting lines will be based upon the aluminum bumper to the bumper cover, not the bumper cover to the ground.

Here is a view from below. There is the bumper and the four braces:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4382.jpg

Since I decided to cut the bumper cover as high up as possible without exposing the actual bumper, I use a metal spike and hold it against the bumper to push holes evenly along the bumper cover:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4383.jpg

http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4381.jpg

http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4384.jpg

For the long version (detailed later) I made about six punctures ensuring that that I got either end of the bumper. This is important because standing behind the car, I don't know where the aluminum bumper ends on each side. By spiking a hole at the each end, I now have a reference for my down cuts. For the short version, I used the two center bumper cover braces as my guide. I left them in place and my vertical cuts would be in between the two.

The horizontal cut line is easy; it's just a matter of connecting the dots (puncture holes). I used a plumb-bob to assist me with the vertical lines but most of it was a good eye and freehand. Using a marker, draw the lines for the final cuts. Don't worry about the ink as it can be cleaned up easily later.

This is the important part: check and re-check and then re-check your lines. Decide if you want rounded corners or straight. I chose rounded corners in the same diameter is my Dremmel sanding cylinder to make my finishing work easier. Also, rubber trim such as door edge guard works with soft, rounded curves but not with right angles.

Don your eye protection and preferably a face shield or ski mask because your face is going to be assaulted with molten plastic. Cut along your lines slowly and carefully. There will be globs of molten plastic that gathers in places so stop, let them cool and then break them off and continue cutting.

Application note: If you think you are unsteady with a cutting or sanding tool, mask off the entire area with thick paper to prevent scratches from a runaway device.

Do not worry if you deviate from a straight line! Cut inside your marked lines so that you can fine tune your cuts later with a sander. Even after that, you're going to cover the edge with trim so relax and keep cutting. It's going to look ugly for a while and you'll be tense.
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4390.jpg


Okay, now the bumper is cut, it's time to clean up the cut line. Using a sanding cylinder on medium speed, gently even up your cut ensuring that you get inside the edge as there are hidden plastic gobs that will prevent the final trim to go on evenly.
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4399.jpg

Appication note: Take it easy with the sander, removing small bits at a time. Sand a little and step back to check for eveness. You can easily remove material but you can never get it back.

So that's it for the short version:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/IMG_4453.jpg

http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/NBbumpercut-1.jpg



The long version:

The long version involves cutting the bumper cover almost from end to end and allows the greatest airflow. It does away with all four support brackets underneath leaving the bumper cover a little floppy. It can be made rigid again by applying 3M double sided tape or pads between the aluminum bumper cover and the actual bumper so this is not really an issue.

This particular Miata started with the short cut but later modified for the long cut. The same procedure from before applies here except this one is just longer. Just to reitterate one important tip though, make sure to puncture holes on either end of the aluminum bumper through the cover in order to guide you better.

It is up to you whether to round the ends or do a straight vertical cut. Since the Miata is is rounded everywhere, I think the long version should be rounded as well.

First I start on one side and freehanded a marker line that gave me the most appealing design. It took me several times with a chalk pencil to settle on the design would work best and I kept crawling back underneath the car to make sure my cut wouldn't interfere with anything.
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut9.jpg

Then I cut the piece out:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut11.jpg

and used it as a reference for the other side:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut5.jpg

similar to this but just flipping it over and taping it to the uncut part of the bumper to mark my line:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut7.jpg


Once the cutting was done, I cleaned up the edges as before:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut14.jpg

got out the roll of trim (good thing I got a roll!) and finished the job:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/2Extendingbumpercut22.jpg

and this project is done!:
http://i1194.photobucket.com/albums/aa375/YellowYata/Miata%20NB%20bumper%20cutting/BlackNBrearbumpercutwide.jpg

kung fu jesus
12-23-2011, 03:42 PM
this thread needs Jeremy to post up (ArtCenterMiata).

His original cutout on his NB is probably the cleanest, best executed job I have ever seen. It looked like it came that way from the factory.

Agent☣Orange
12-23-2011, 03:47 PM
Not a problem. Feel free to delete.

kung fu jesus
12-23-2011, 04:00 PM
no! it's great info! always good to see different ways to do it.

phillyb
12-23-2011, 04:04 PM
don't delete!
this is a good thread

Agent☣Orange
12-23-2011, 04:15 PM
Oops, didn't mean my response facetitiously. I'm just running around here like crazy trying to flesh out some new sections that don't have anything in them yet. I'm sure there's better material out there. I'm just getting the ball rolling. :cheerful:

revlimiter
12-23-2011, 04:15 PM
it's a VERY good thread! I'm impressed by the work here.

markcher
12-23-2011, 05:00 PM
Its a team game and by sharing of any ideas it can only go to strengthen the skills and knowledge of our members.

phillyb
12-23-2011, 05:01 PM
i think i'd like to do this eventually...

Oldgrayfrog
12-25-2011, 01:58 PM
If you even start to get serious about going to the track, this is probably on your list of to do's. Combine this with a similarly easy to understand post on making a diffuser (I really hope somebody does, I'm hopelessly uncrafty), and score some serious aerodynamic gains. Looking at the stock bumper from below makes me think of a giant cupped hand, grabbing at the under car air flow. Great post!

ShadowMX5
12-25-2011, 02:51 PM
Great write-up; I'll definitely be looking here again later on when I install my diffuser

MiataLife
12-25-2011, 04:36 PM
Subscribed! Turned out great!

Thumper13
12-25-2011, 10:31 PM
Me Likey !! Alex,I'll take a cruise down to visit and we can do the large cut on my rear bumper cover.

Agent☣Orange
12-25-2011, 10:45 PM
That's fine but just don't go telling people later that I played with your rear end or something ghey like that.

Thumper13
12-26-2011, 07:27 AM
Just remember,you're the one that started blabbing after our camping trip

kung fu jesus
12-26-2011, 03:25 PM
here's jeremy's bumper cutout:

http://i295.photobucket.com/albums/mm147/caboosage/mia2/DSCF1588.jpg

http://i295.photobucket.com/albums/mm147/caboosage/mia2/DSCF1606.jpg

Agent☣Orange
12-26-2011, 05:43 PM
Very nice curvature. End result is amazing!

bovaco
12-26-2011, 06:47 PM
Nice write up YellowYata, this has now worked it's way up to the top of my to do list.

Agent☣Orange
12-26-2011, 06:54 PM
Thank you much! The best part is that it's (almost) free. Thumper got confused and cut his front bumper instead but anyone else can easily do this mod.

ShadowMX5
12-26-2011, 09:16 PM
Oh God, that silver NB is surprise buttsecks.

kung fu jesus
12-26-2011, 09:29 PM
Jeremy is a transportation design student at Art Center in Pasadena (car design). He made good use of some fineline tape, tangents and the existing design to make my favorite customized OEM bumper to date. Unfortunately, the bumper was hit and a bit damaged. He had to trim more away to hide that damage and it lost a lot of that aesthetic pictured above.

Agent☣Orange
12-26-2011, 10:01 PM
That explains why it's so fantastic. I used an eyeball and crossed fingers.

phillyb
12-26-2011, 10:48 PM
that silver nb bumper looks awesome!

theothersawyer
12-26-2011, 11:15 PM
I have never liked the bumper cutouts, but damn! These pictures have totally changed my mind! Looking real good!

AntiochOG
12-27-2011, 01:31 AM
Seriously considering selling my not-yet-installed rear r lip and cutting my bumper. Then maybe someday a diffuser.

Stealth97
12-30-2011, 08:49 AM
This is something I have thought about doing for a looong time, but I am afraid of screwing it up, even though I have done more than my fair share of bodywork in the past.

Thumper13
12-30-2011, 09:12 AM
Thank you much! The best part is that it's (almost) free. Thumper got confused and cut his front bumper instead but anyone else can easily do this mod.


D'OH you mean it's not the front bumper :confused:

kung fu jesus
12-30-2011, 10:16 AM
This is something I have thought about doing for a looong time, but I am afraid of screwing it up, even though I have done more than my fair share of bodywork in the past.

That fineline tape make it easier. You can lay it out freehand and reposition it. Designers use it to lay out curves on larger profile drawings.

The tape usually comes in white and black. I am trying to find the actual name of it, I can't remember it. Most good-sized art supply stores should have it. You could probably use pin-stripe tape, too.

miata5620
12-30-2011, 01:10 PM
I like the lines that the silver one has with the cut out but at the same time when I start looking closer I hate seeing parts of the bumper support and other chassis parts that are now visible. Now that line with a nice diffuser out back would look good.

kung fu jesus
12-30-2011, 01:45 PM
it depends on how much you cut out. Jeremy painted everything behind it black, which makes sense.

I know some people have trimmed off the rear metal of the rear supports. Building a rear diffuser is tricky because of how the exhaust is routed/hung.

I am surprised no one has offered an exhaust/rear diffuser combo with a template for cutting the rear bumper. At least a rear bumper template you can print out on 8.5x11 sheets and tape together.

Ne0z
12-31-2011, 03:29 PM
I really think I want to do the stripping you have in the OP on mine. Might clean it up a bit...

http://i23.photobucket.com/albums/b384/Ne0z31/2011-10-09155904.jpg

http://i23.photobucket.com/albums/b384/Ne0z31/2011-10-09155811.jpg

HellaRareYellow
01-02-2012, 03:58 PM
Where did you find the rubber door trim? And what's is the technical name for it? Thanks in advance. :)

Agent☣Orange
01-02-2012, 04:21 PM
It's been so long, I don't remember where I bought it but if you search for U-shaped edge molding, there are tons of varieties out there. I got a bulk roll of 25ft at the time and it's textured with metal inside so you have to really force it on and then squeeze every inch with pliers. If you want just plain door edge guard, Aotozone sells a 20ft roll I think.

HellaRareYellow
01-02-2012, 05:18 PM
Sweet. I cut a whole in my engine bay and want to line it with that stuff. Thanks man!

Kolbotn
01-05-2012, 08:00 AM
Thanks for shareing Yata, will be doing this sometime during this winter:)

miatasaint
02-04-2012, 03:12 PM
Im really tempted to do this, but i dont trust myself at all. LOL

NAautoxer
02-04-2012, 04:43 PM
since i ditched my dual exhaust i've got a blank cutout on the left side. i was just going to fill with a reverse fog light, but i think i may end up doing this instead..

NAautoxer
02-04-2012, 07:24 PM
wife approved.. :D

fitftw
02-04-2012, 07:26 PM
The short version in the original post is fugly, but I REALLY love the long version. I cut my rear bumper on my NA similarly. Since I bought it with dual exhaust, my work was cut out for me and I just cut across.

Coupeloop
02-06-2012, 09:55 PM
yay for cut bumpers!
edit: after seeing how an NB is done... it looks way easier than cutting an NA bumper, since the plastic support also has to be cut (but of course that always depends on how much you want to cut it)
before:
http://i970.photobucket.com/albums/ae182/windchariot/Miata/IMAG1228Medium.jpg
after:
http://i970.photobucket.com/albums/ae182/windchariot/Miata/IMAG1229Medium.jpg
http://i970.photobucket.com/albums/ae182/windchariot/Miata/6780077935_6e71a5842e_z.jpg

NAautoxer
02-06-2012, 10:05 PM
and that answers my question.... i pulled my bumper tonight. god i love your car.. freak... in..... fabulous... so you do cut the support. excellent...

http://farm8.staticflickr.com/7022/6833481575_8be13b18ab.jpg (http://www.flickr.com/photos/80073081@N00/6833481575/)
DSC00153 (http://www.flickr.com/photos/80073081@N00/6833481575/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

http://farm8.staticflickr.com/7159/6833486283_dd68181f25.jpg (http://www.flickr.com/photos/80073081@N00/6833486283/)
DSC00154 (http://www.flickr.com/photos/80073081@N00/6833486283/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

Coupeloop
02-06-2012, 10:21 PM
thanks for the kind words.
oh and at the height that i cut mine, i had to cut the plastic brackets(black block thing) in half as well or else it will show.

NAautoxer
02-07-2012, 05:44 AM
yeah, like i said i'm just doing it because i've got the "i used to have a dual exhaust and cut my bumper for the 2nd tip"..

itzzspencer
02-07-2012, 09:01 AM
so thats how you cut it! thanks for the DIY! i want to do this but my my hands probably arent steady enough for this. :/

fitftw
02-08-2012, 02:42 PM
Here's mine...might be hard to see. Camera phone sucks and the car is black...needs to be touched up with a dremel sander.
http://a5.sphotos.ak.fbcdn.net/hphotos-ak-ash4/405399_2604938768404_1402947049_32073940_268001089 _n.jpg
http://sphotos.xx.fbcdn.net/hphotos-ash4/398658_2604940248441_1402947049_32073942_192255079 2_n.jpg

Coupeloop
02-08-2012, 03:07 PM
if you go to your typical autostore... I actually bought that door molding that goes long the edges of the door. They have it available in black, it really cleaned up the lines
you can kinda see it here...
http://i970.photobucket.com/albums/ae182/windchariot/Miata/6780077935_6e71a5842e_z.jpg

NAautoxer
02-08-2012, 09:24 PM
messed up exhaust cutouts BE GONE!!

http://farm8.staticflickr.com/7149/6844217353_326405cea4.jpg (http://www.flickr.com/photos/80073081@N00/6844217353/)
DSC00155 (http://www.flickr.com/photos/80073081@N00/6844217353/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

http://farm8.staticflickr.com/7021/6844221053_09293578da.jpg (http://www.flickr.com/photos/80073081@N00/6844221053/)
DSC00156 (http://www.flickr.com/photos/80073081@N00/6844221053/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

http://farm8.staticflickr.com/7033/6844229059_f037d1ccd8.jpg (http://www.flickr.com/photos/80073081@N00/6844229059/)
DSC00158 (http://www.flickr.com/photos/80073081@N00/6844229059/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

http://farm8.staticflickr.com/7006/6844243001_97d0b261a8.jpg (http://www.flickr.com/photos/80073081@N00/6844243001/)
DSC00161 (http://www.flickr.com/photos/80073081@N00/6844243001/) by fragile_this_side_up_retro_club (http://www.flickr.com/people/80073081@N00/), on Flickr

xjdesertfox
08-18-2012, 01:16 PM
http://i559.photobucket.com/albums/ss35/xjdesertfox/20120805_193424.jpg

pablomondragon
02-17-2016, 10:36 AM
yay for cut bumpers!
edit: after seeing how an NB is done... it looks way easier than cutting an NA bumper, since the plastic support also has to be cut (but of course that always depends on how much you want to cut it)
before:
http://i970.photobucket.com/albums/ae182/windchariot/Miata/IMAG1228Medium.jpg
after:
http://i970.photobucket.com/albums/ae182/windchariot/Miata/IMAG1229Medium.jpg
http://i970.photobucket.com/albums/ae182/windchariot/Miata/6780077935_6e71a5842e_z.jpg

What diffuser is that? Where did you get it? I'm planning on cutting the bumper on my NC and putting a diffuser in like that one

Mazdabp26
02-17-2016, 09:55 PM
What diffuser is that? Where did you get it? I'm planning on cutting the bumper on my NC and putting a diffuser in like that one
Amemiya Rsex7 diffuser my friend
Btw thanks for bumping i am about to cut my bumper this weekend

fwdtamiya
02-19-2016, 11:47 PM
Basically. It was actually a replica made by Shine Auto, IIRC.