attaching FG rear bumper

[RWebb]

when the body shop re-attached the R bumper on my 1973.5 911, the idiot riveted it thru the tail light housings -- so one would have to drill out the rivets to remove the bumper or the tail lights!

obviously, I'm looking for another way

[1] I know some have cut apart the stock steel bumper ends (early cars) and used those steel strut pieces. But [a] it ruins them, and [b] it seems heavy.

[2] Then, there are the fiberglass support struts that come with most R. bumpers. They are shaped like the steel ones, but I hear they are way too flimsy to get a good fit -- my car is a street rod so fit is important.

[3] Some have carved the support struts out of Al billet. Also seems pretty heavy & would be very time consuming.




[4] I was thinking of fabbing something out of Al tubing - the square tubing can be had from Home Despot.

[5] But I also got the idea of using the FG struts, as in [2] above, but reinforcing them with some of the Al tubing.


Thoughts? Ideas? Pics of your install?

Other ways to approach the problem - while maintaining the twin constraints of light wt. and good fit?

Thx!

[jpnovak]

Randy,

I was only able to achieve a good fit using a similar method to your description. The fiberglass bumpers that I have installed are not perfectly level and not stiff enough to only mount using the stock attachment point at the front edge and rear bracket.

Instead of using rivets that need to be drilled I installed blind nut rivet inserts. I have the same holding power but the bumper can be easily removed with a few screws. I used low profile screw heads in the light housing socket.

I have some pictures that I can post later tonight.

Jamie

[911pcars]

subscribing. I have some ideas, but nothing validated.

Sherwood

[afterburn 549]

As I remember...bent some Al 3/16 ? put a 90deg bend in it overlaid it into the back of the F.G bumper and bolted it to something on the car . I did not use the car crash stuff at all.
With the rear fender lips bolted to it and the little fender tube supports (like stock ) all became quite stiff.

[barney911rs]

For a "street rod" I would not think a little weight to mount that would matter much. just my $.02

[RWebb]

i don't care about ounces, but if i can save a pound, i will....

Sherwood - your ideas are always welcome... even musings.

[911pcars]

Originally Posted by RWebb

i don't care about ounces, but if i can save a pound, i will....

Sherwood - your ideas are always welcome... even musings.

Well, this is untried yet. The trial-fit on my FG rear bumper is a disaster. Not only are the ends too wide by 1.5" on either end, but the mounting brackets are at some stray angle (not parallel with the chassis mounting surface). Notwithstanding some necessary surgery to narrow the overall width, here goes:

In lieu of these crooked FG brackets, which I'll cut off, I was thinking of drilling a series of matching holes along the lip between the wheel well and tail light openings. The FG bumper would be sandwiched between the lip and some shaped, flat al. plate to back up the FG and the 6-7 fasteners in this area. You could get fancier by using Riv-nuts on the metal lip to reduce the hassle factor with a nut and bolt. Either method would require modifying the weatherstripping normally residing in this gap to accept the threaded fasteners.

Was also thinking of then reattaching the cut-off brackets, then reglassing them in as another attachment point.

Mind you, this is all theoretical, just a plan.

I'm all ears for other ideas too.

Sherwood

[Zeke]

I mount bumpers by attaching loose brackets to the car and fitting the bumper up to its best position (prefit before, of course) with lots of bondo where things meet up. After the bondo hardens, I remove the entire ass'y and glass it all in solid.
Every car and every bumper is different. It's like my day job as a carpenter; no 2 doors alike anywhere.

[RWebb]

thanks - milt - what are your brackets made of?

Early911S bbs posts say your procedure works best, but that the FG brackets (struts) are too flimsy.

BTW - the ends of my bumper were cut off and repositioned and fit just fine there -- it is the middle third [near the inside of the taillights] that needs to be attached

[RWebb]

re #1 above, here are pics of the stock steel brackets:

the top piece of curved sheet metal is the stock bumper itself - the next lowest piece of sheet metal is the strut (bracket), which is spot welded to the bumper - those spot welds would have to be drilled out, destroying the bumper
- anybody know how much these bumpers sell for?





below is a view from below the car, showing the bottom of the strut
the two slots on the RH side are where the strut attaches to the body
you can see the curvature of the stamped steel to give it strength




in the view below, we would be standing outside the L fender and looking thru it; the curvature of the strut is evident
- note that the triangulated part in the R background does not really hold up the bumper - instead, it provides support and a mounting surface for the vertical bumperettes




a view as if we were standing on the center line of the car
- the strut is pretty heavy duty
- it also looks like it would be a lot of work to cut the main strut loose from the tringulated part for the bumperettes




finally, the top of the strut
- note the metal forming a complete triangle for the bumperettes and the curve in the main strut
- seems like you might not need that bend...



does any body know how much the steel strut (bracket) weighs by itself?

[RWebb]

I estimated the wt. of the steel bumper support brackets [one layer only - w/o the "triangulated" part] by using the sheet metal density info in:
http://forums.pelicanparts.com/porsche-911-technical-forum/436083-thin-metal-rs-models.html#post4245754

The metal appears to be 1 mm thick. Dividing it into sections and measuring those gave me an area of 98 inches2 for each one = 1.36 ft2 for both.

At 1.61 lbs/ft2 for 1 mm thick sheet steel, that gives a total wt. for both of 2.2 lbs.

[Zeke]

The brackets I used were either flat or angle steel with an angle folded over to meet the inside surface of the bumper and give a modicum of protection. Not exactly a weight saving device. When I both get better at AL welding and have the proper equipment, I would do all of this with 1/8th" AL sheet or maybe some different thicknesses welded together in a box, channel and flat configuration. I would like to see all that run a FG bumper to have some hidden lateral protection from one mounting side to the other and maybe extending a bit towards the corners.

[RWebb]

sq. tube Al weighs 344 g for 4 ft. length of 3/4" tube (1/16" thick) - this is just the stuff they had at Home Depot -- maybe something smaller or stronger for its wt. could be had....

if I reinforced a length of the FG strut with say 1 ft. of this, it would add 86 g (0.19 lb.) + the wt. of the glue &/or rivets, whatever + wt of FG strut itself + bolts to hold it to the chassis (which are used with the steel struts anyway)

hmmmm.....

[felix_browne]

I always just figured that the whole 14lbs of frp bumper could be supported by the normal body attachments through the body (3 on each side through the edge of the qtr. panels as well as the curved bottom edge support bars). Also, at least on midyear and carrera tail light housings, the light housings have a large hole on the bottom edge nearest the license plate panel that you can put a bolt with a big washer through to sinch up a large gap in fit. That's the way i have always done it but if you want some frp repros of the support tubes pictured in rweb's post, I have a number of sets not being used on shelves in my garage ready to be glassed onto the back of your repro bumper. just let me know if you would like them.

[felix_browne]

If you want pics of how i do it on my cars i'm taking the bumper and muffler off in like a week or so and i can get some specific shots if there is something you definitely want to see.

[RWebb]

thanks - yes, i'd love to see pics

my '73 tail light housings don't seem to have that large hole

also, i can't figure out how to access the area on the inward side of them as it is up above the muffler

[David]

My FG 964 bumper is attached with about 15 bolts through the top front of the bumper into the existing sheet metal below the reflector and along the bottom of the rear quarter panels. The center bolts are accessed by removing the center reflector; I use small allen bolts so I can hold them with a little allen wrench through the square openings under the reflector. I also use the stock support rods at the lower front corners of the bumper.

[RWebb]

sounds like a bumper car -- mine is an early car, so no way to do that

[Elombard]

Felix yes please show a pic of those supports.

[DanS911]

Timely post for me too (amazing how that happens, isn't it?) I will be putting front and rear bumpers on my '76 as soon as we get the motor done this week.

All info greatly appreciated. Felix pix would be nice.