80 replies to this topic

[cbstdscott]

I can not stand it any longer. As much as I love my 1st gen, the chassis flex is more than I can stand. I know that fender braces will not solve all of my problems (like help me lose 20 lbs. and re-grow my hair) but it will go a long way toward making the front end of the CRX much more fun to live with.

I have been studying the Kakabox Build and I have a good idea of how I will design my braces. I expect to tie into all four of the bolts holding the doors to the chassis, run the forward "arm" at least as far forward and tie it in at the end and somewhere in the center as well. The critical element is the 45 degree triangulation to give the arm its rigidity. And I am probably going to add a piece to strengthen the connection between the down tube and forward arm in the corner.

The Kakabox Build uses steel box tubing. I am leaning toward using aluminum, largely because the chop saw I have access to does not have a carbine blade . But if I brace it correctly and use the right grade of tubing, the aluminum should be OK.

Installing fender braces means removing the old fenders which are being held together only by packing tape. I suppose this is the right time to get a set of decent new fenders. I could just lay some fiberglass on the inside of the OEM-style fenders for extra strength.... but how cool would it be if I could source some CF fenders?

But before I start the Fender Brace project.... I am working up my courage to drop the rear axle and install a set of new swing arm bushings. I will get that done during Spring Break (yeah, I am taking a Spring Break this year).

Maybe I will have the car all done by the time Doug has his Mid-State affair in Bakersfield.

Scott

[87rex]

You go Scott!!!!!! I'll be watching to make my own copies!!!!!

[kakabox]

I have been studying the Kakabox Build and I have a good idea of how I will design my braces. I expect to tie into all four of the bolts holding the doors to the chassis...

You might want to reconsider using all four of the door hinge bolts. If you use all four bolts, you will have to hold the door in place while you install the brace. Leaving the top door hinge bolt to hold the door on, and not part of the brace, allows you to install and remove the brace w/o the door falling off.

The Kakabox Build uses steel box tubing. I am leaning toward using aluminum, largely because the chop saw I have access to does not have a carbine blade .

But if I brace it correctly and use the right grade of tubing, the aluminum should be OK.

You don't need a carbide blade to cut mild steel tubes. FWIW, I use a commonly available (Home Depot) 10" Norton abrasive metal cut-off blade in my Delta compound miter chop saw.

Mild steel provide more than adequate stiffness & strength if the brace is designed properly. It's also more forgiving in fatigue to being welded (properly) than aluminum. I know you're very concerned about weight, however, the Kakabox mild steel braces only weigh 3.5 lbs. each.

Also, you'll want to pay close attention to the brace connections to the chassis. Make sure you have enough quality shear connections for good load transfer. The effectiveness of the brace is really only as good as it's connections to the chassis. If you can't effectively get the chassis loads into the brace, then it can't do it's job!

One more thing, watch for possible interference btwn the fender and the brace. Before final welding/install, install the brace and make sure that the fender fits over it w/o the brace touching the fender's inner surface. Also check the inner fender liner for interference.

Cheers!

[cbstdscott]

You might want to reconsider using all four of the door hinge bolts. If you use all four bolts, you will have to hold the door in place while you install the brace. Leaving the top door hinge bolt to hold the door on, and not part of the brace, allows you to install and remove the brace w/o the door falling off.

Ah ha! The voice of experience. Thanks for the tip.

And yes, I figured the real value of the brace will be in the way it is tied to the chassis before the firewall.

I am still fighting with myself on steel vs. aluminum. I have access to some exotic stuff (titanium fender braces?) I will have to do some engineering studies...

Scott

[strudel]

You might want to reconsider using all four of the door hinge bolts. If you use all four bolts, you will have to hold the door in place while you install the brace. Leaving the top door hinge bolt to hold the door on, and not part of the brace, allows you to install and remove the brace w/o the door falling off.

Point of interest. When I took a few doors off they didn't just fall off. If the door is closed you should be able to take all four bolts out IF the door has not been off before. Reason: years of grime I suppose. If it has been off then I would suggest using some sort of caulking just around the edges and it should have enough stability to remove the four bolts and still hold the hinges in place. This is only a suggestion not a tried and true methodology. JS

[kakabox]

Point of interest. When I took a few doors off they didn't just fall off. If the door is closed you should be able to take all four bolts out IF the door has not been off before. Reason: years of grime I suppose. If it has been off then I would suggest using some sort of caulking just around the edges and it should have enough stability to remove the four bolts and still hold the hinges in place. This is only a suggestion not a tried and true methodology. JS

Maybe, maybe not. Not worth the risk imo.

To make these braces, there is a fair amt of mock up involved...installing, measuring, removing, installing, tacking, removing, installing...etc., etc. In doing so, you'll be screwing the hinge bolts in and out, over and over again. If at any point the hinges slip, the door will pivot down from the door latch and come to rest on the door sill. Not the end of the world, but it can take a bit of fiddling to re-hang the door so it shuts the same as before.

Ask me how I know.

Never one to discourage "engineering studies"...imo mild steel is more than adequate for this application. Without a Finite Element Analysis of the car's monocoque to find the internal loads/deflections, or a chassis bending/torsion test w/strain gages applied in this area, it's really only a guess (albeit educated) at what kind of stiffening these braces will provide. In other words, since you don't know the loads/deflections, it's not worth the cost and fabbing hassles to use "exotic" materials. Just keep it simple and use good engineering judgement.

If you knew what the loads/deflections were in this area, then you could optimize the design using the exotic materials...just my opinion, once again!

Cheers!

[strudel]

J.

I must agree with you totally. I suppose one could do all the mock up work using only two bolts and get it set up for the final install. Then add an extra tab top and bottom for the final fit. Or just have the tabs tacked and then have it marked so that a two hole tab can be welded on for the final go. Am I way off base here? JS

[Jaminak]

Why not make the brace out of a 'sheet' of aluminum like other Civics? I would imagine it would be much lighter. I know it wouldn't have the lateral rigidity, but this shouldn't need that. It would seem to be much easier to make too. And no welding.

Here's a decent pic I found:

[kakabox]

I suppose one could do all the mock up work using only two bolts and get it set up for the final install. Then add an extra tab top and bottom for the final fit. Or just have the tabs tacked and then have it marked so that a two hole tab can be welded on for the final go. Am I way off base here? JS

Jerry, I don't understand...sorry ...but, there's absolutely more than one way to go about this, and probably a smarter way than I used!

If you wanted to remove all four hinge bolts for the brace brackets, I'd prop up the door using shims on the rocker panel, under the door, then carefully close the door, and remember not to open it when the hinge bolts are removed!

In theory, having two bolts at the top hinge connection instead of one would provide a slightly better clamp up footprint and twice the ultimate shear capability, but, I doubt you would ever notice the difference...you'd need to strain gage the area to find out for sure.

FWIW, I think adding the two additional shear connections at the middle of the down tube is more important as it helps to provide an additional shear load path and stabilizes and stiffens the down tube.

[kakabox]

Why not make the brace out of a 'sheet' of aluminum like other Civics? I would imagine it would be much lighter. I know it wouldn't have the lateral rigidity, but this shouldn't need that. It would seem to be much easier to make too. And no welding.

Here's a decent pic I found:

Yes! There you go! Absolutely...assuming the load is all in plane shear w/no lateral bending.

As long as when installed there is no load path eccentricity, that should work great. Flat sheet/plate can be very stiff/strong in shear.

However, as you said, no lateral load capability, but ultra light.

Comments edited out...see my 3/8/08 post below.

[strudel]

I guess I wasn't very helpful at all! Goes to show what I don't know.

[rpr]

If you have access to a welder already, just weld the brace in, no?

I know I have a better pic than this, but I can't find it:

[kakabox]

Why not make the brace out of a 'sheet' of aluminum like other Civics? I would imagine it would be much lighter. I know it wouldn't have the lateral rigidity, but this shouldn't need that. It would seem to be much easier to make too.

On further thinking, I agree w/you that in this application, the brace (the Kakabrace and the "flat plate" one above) really isn't effective for out of plane bending loads.

The reason is that the bolted connections on the chassis are only really effective in shear. Sure they'll react some tension load from any deflection induced bending loads, but, they shouldn't really be counted on to do so. They're only welded 'nutserts' in relatively thin gage metal. Appling a tension/compression load on them is not a very stiff load path...it's putting a punch load on a membrane. On thin gage material, that's not a stiff load path. However, for shear loads the bolted connections are very strong and a stiff load path. That's why I use them. I don't count on those welded nutserts taking any appreciable tension loads (there's some small kick loads due to the eccentricity of the cg of the brace's axial load members and the bolted reaction points). So, even though there might be some out of plane bending (lateral stiffness) capability, the bolted connections are not really effective in reacting that load. That means the brace is only really effective for in plane shear.

Bottom Line: given enough good shear connections to the chassis, I think these 'sheet' metal braces can be effective for local stiffening. FWIW, I've never seen these braces, but I'd guess they are at least 3/16" thick or thicker...I'd make them at least 1/4" thick if from aluminum.

Cheers!

[kakabox]

If you have access to a welder already, just weld the brace in, no?

I know I have a better pic than this, but I can't find it:

Depends on of well you reinforce the sheet metal that the brace is welded to.

Just like the termination points of a roll cage, the tubes should be welded to a hard point w/enough footprint to distribute the punch load. Typically that involves welding a thicker gage doubler that has enough "footprint" to distribute the punch load into the surrounding sheet metal structure w/o flexing the local area.

If you just weld a tube to the thin sheet metal, you may run the risk of the joint cracking out around the weld over time due to the flexing of the sheet metal at the weld. But, it really depends on the loads and deflections at that part of the chassis.

Just my thoughts.

[cbstdscott]

A plasma cutter and a perfect set of dimentions would be the ticket to making something like a flat plane fender brace. I can only wish that one of our CNC people could whip up something like that.

The discussion about loading on this brace is interesting. I wish I was more hip to the engineer's lingo...

My main concern is the lack of rigidity in the "front clip." I would guess that torsional load (twist in the for/aft plane) would be best handled by a strut brace that is tied securely to the firewall. I am just begining to scheme a way to tie my existing front strut brace to the firewall.

The fender braces that I am dreaming of would control the front clip from flexing in an arc pivoting from the top of firewall. These would have no effect on torsional load.

I have been studying Tom's braces... They are somewhat smaller and more simple, but possible as effective as the Kakabox braces. Perhaps I will use a hybrid approach to my design.

Hey Marcus! Are you following this discussion? There is a pent up demand for this product!!!

Scott