RobK

Hello all,

I have been working to reduce cowl shake on my convertible (details in sig) and in general make it more stable, even at some speed.

The car is 99% daily driver, drag racing not that often (only when I feel like getting kicked off the track--need a roll bar--Grrr), some auto-x (few times a year). One day, I'd like to get into real RR, but not RSN (roll bar discussion again).

I bought the car used 2 years ago--it was factory bone stock then, driven my someone's wife who "appeared" pretty sedate

So far I have added a STB (BMR CM 2pt), KBDD SFC (wish I bought bigger ones--only seemed to help rear stiffness), 1LE LCA's, & an adjustable BMR panhard.

Tires are new 245 BFG KDW's on all 4's. The previous tires, AVS Intermediate 255's road similar (same type of shake/nervousness), although the new ones seem to have better stability with transitions (width impact?)

The car has a new steering rag-joint (man, that helped!).

I'm going to buy some new Bilstein shocks (probably custom Strano valving) within the month because the current ones just stink--hopeing that will stablize it & improve the ride. A decent alignment (not factory) will be done when the shocks go in.

A pair of sway bars would probably be next -- still considering sizing options (lots to think about)

Looking beyond that, I was wondering if anyone thinks a 4pt STB (G2) would make a difference over a 2pt. I have heard the stories of cracked windshields, but I was hoping most of these were caused by 3pt (I could understand that).

I know a 6pt roll bar without swingouts would help a ton, but I'm not ready for a roll cage until I have done everything else I can to improve my car.

In general the car seems to shake a little (never seems settled at least through the steering wheel) going down even pretty level roads, except for the smoothest--maybe it's partly my roads, but my wife's SUV is rock solid. I can only guess the shocks are just shot, but it still feels like the body twists easily. Bounce test fails for the rear (they squeak too).

I've even had thoughts of having the KBDD's cut off & have something meaty put on, but not sure that would help.

Opinions? What else would help?

Thanks for any help.

drop top steve

I hear ya about the cowl shake, the first time I drove my 97 SS vert I pulled over and looked for a broken shock. I installed a BMR STB, it helped. I installed weld in BMR SFC's, that help more, lowered car 1.5 in, with jamco springs, KYB AGX adjustable shocks, BMR LCA (rubber/poly) and relocaters, adjustable PHB (poly) that helped some more. went to 274/40/17 bfg's and added a 21mm rear bar, already have a hollow 32 up front. At this point the car handled great and 80% of the cowl shake was gone. I have two friends with eibach/bilstien setups and I think my car rides and handles better, so do they. I added a Wolf bolt-in rollbar, five point, (I only use the side bar when I drag race) and that took care of the rest of the cowl shake, car feels rock stable now.

RobK

Steve,

Thanks for the data.

I am most interested in your rollbar comments. Are you saying that the bolt-in rollbar (assuming you have the non-removable cross-bar) with the swing out removed, the cowl shake greatly improved? I'm trying to understand where the flex is in these verts.

So far, for my car & my choice of parts, the STB has made the biggest difference. I barely noticed the KBDD SFC's.

Based upon your description, shocks are definitately next, followed shortly by sway bars.

After that, I'll start thinking about the roll bar again (tough as I have a 5 year old who rides in the back)

-Rob

drop top steve

With just four "points" bolted down you well see major improvment in cowl shake. The wolf unit rear bars bolt into the stock seatbelt retracter bolt holes, four bolts on each bar, you can keep the retracters in the car if you want. The two down tubes bolt thru the floor just behind the seats, again four bolts per bar. After I bolted these in I also added a few short welds that well be easy to grind out if I want to remove the bar. You can keep the back seat, and a five year old should have no problems getting in or out, not so for an adult. I left my seat out and filled in the few spots with out carpet. I think a lot of the flex we vert owners feel is front to rear, and the four point shortens the area that can flex.
I installed the bmr sfc, the oval tube, weld in, and saw a good improvment, I would have thought the DD's would have helped a lot also. I would say the rollbar did as much as the rest of the parts combined.

KLATOO

What do you guys think about removable/swing-out side bars vs. non-removable? Will the removable side bars provide extra chassis stiffness or not really?

I don't think I could convince myself to get the non-removable side bars but I would like to think that even the swing out ones will help.

LPEdave

I have the swing-outs that use a pin through the bar, and absolutely, they still stiffen the car up quite a bit. I can notice if I remove the two swing-out bars, the car flexes more than when they're in. Definitely get them if you're going to drive the car regularly.

Dave

KLATOO

Yeah thats definately good to hear. I don't think I could live with the fixed bars. If I could get the car to feel like my old hard top I would be really happy.

Now I just have to find somebody in northern CA with a Wolfe bar. Everyone says I won't hit my head but looking at pics I'm not so sure.

auto-Xer

I have a 4 point STB (G2). I think that windshield cracking stuff is an urban legend. Everyone has heard about it but know one actually knows of someone it has happened to. I could see how, but just havent heard of an actual account. I dont have any cowel shake, but I also have t-tops and g2 subframes as well. As far as the rollbar, just chuck the backseats entirely Do you really ever use them? Sounds like the Wolf rollbar might be a bit better than my autopower. I am a little unimpressed with them. They took about 2 months to make and deliver it (that might not be their fault though) but after they painted it, they wrapped it in plastic without waiting for the paint to dry! So I had these awful imprints all over my bar. Plus I wanted a CM/weld-in and they sent me a mildsteel bolt in (again, probably not their fault but the people I ordered it from). But the paint thing still makes me kinda mad... And the top bar is about 1" from the ceiling, so it blocks the rearview mirror quite a bit. Dont know why they didnt use more of the space in the car... Should have gotten a custom one made.

--Kevin

RobK

Steve / Dave / Kevin,

Thanks for all the great feedback.

Good to hear that the swing-outs provide benefit when installed--yet also provide the flexibility that they may not be there, and that just the 4-points do help stiffen the body.

I will be replacing my 2pt with a 4pt RSN--just ordered my Strano valvled Bilstein shocks, so that or sway bars will be next.

6-pt rollbar w/swingouts will probably come next year--I will just have to buy a simpson strap for my daughter

-Rob

SDA

Your KBDD have the potential to make the chassis as rigid as you want it to be,but they have to be modified in their installation. For a complete description of the process and some of the theory behind it : ls1.com in autocross+roadracing in SFC? Kind of long but includes pictures of the modification.

KLATOO

Hey, thats cool. I have been thinking about trying to weld the KBDDSFCs to the rocker panel for a good while now. I just need to find a welder in my area that is willing to work with me.

So I can't really tell. How many points does the sfc connect to the rocker panel? I just see one in the picture but the description said :

"I used the kb one inch square box rails to create a connection between front and rear sub frames with eight points of welding on the pinch welds of the rocker panels"

Can you help me understand how and where the SFCs are welded to the rocker panel?

If I can get this done my undercarriage will be pretty similar to the one pictured (I also have a BMR tunnel brace). His home made SFCs look way better than the KBDDS though.

SDA

The drivers side rail can be welded at four points to the pinch weld under the door.Using one inch square stock cut to fit the space closest to the front wheel well. You will need a c-clamp to close the space before welding as the factory pinch weld is irregular. The two center connections are about parrelel with the trans tunnel mount. The rear wheel well point is about two inches toward the center from the lca attachment point. On the pass. side it is the same except for the front wheel well.There you will need to make a two inch long piece, angle cut, to attach the pinch weld to the front sub frame across from the SFC attachment point. What all this does is reduce the leverage arm of the SFC's. In a 2-pt. that runs the whole span between front and rear sub frames, even a two or three inch box or tube is leveraged over the whole span, so it can flex a lot. A one inch box rail installed as described is never leveraged over more than a few inches, so very rigid laterally,minimum twist as well. It's easy to make the suspension too stiff. It's impossible to make the chassis too stiff.

KLATOO

Thanks for the info. I probably won't be able to do it in the same way since the SFCs are already installed and there is only 1/4" to 1/2" clearance from the SFC to the rocker panel. I will try to tie them in somehow though.