Miata Roll Bars and Cowl Shake: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Oct. 5, 2008 5:18 p.m. Woody Dork

I have a pair of 96 Miatas. One is a street car and I use the other one almost exclusively for HPDE type events. I had originally planned to go Spec Miata racing with it, but instead I have only used it for track days.

I installed a Hard Dog bolt-in roll cage, which completely eliminated all traces of cowl shake.

I'm thinking about getting rid of the track car and setting up the the other one for HPDE use. Since I don't really want a full cage in a car that will see occasional street use, I'd prefer to go with a four point roll bar instead. After driving my caged car, the cowl shake in the other one is really noticeable. I suspect that a roll bar will make it a little better, although, obviously, I don't expect it to be as stiff as a full cage.

Can someone who has a Miata with a roll bar offer some insight into it's effect on cowl shake? My car has all the usual factory chassis braces already and a Racing Beat sway bar brace. I've heard good things about the butterfly braces, but I don't see myself adding one of those.
Oct. 5, 2008 5:56 p.m. Nitroracer Dork

I have a nearly stock 91' running a four point hard dog roll bar and the cowl shake in my car is still very noticeable. My car is lacking any of the chassis braces the later cars have but I think the butterfly brace or some form of subframe connector would be the best for reducing the problem.
Oct. 5, 2008 7:40 p.m. mad_machine SuperDork

yea.. having had several non-miata roadsters, the 4 point bar is shoring up the wrong end to eleminate cowl shake
Oct. 6, 2008 6:47 a.m. wreckerboy SuperDork

I have both a stock '90 and a '90 built for SM. The race car has had both a bolt in four point bar from Hard Dog and currently has a custom welded in cage.

Both cars when stock had a fair degree of cowl shake. Adding the four point bar eliminated it to a fair degree. Adding a cage took it out of the equation entirely. Adding a shock tower brace to the street car did nothing for eliminating cowl shake, nor should it have.
Oct. 6, 2008 8:10 a.m. mad_machine SuperDork

something I am going to do to my fiat.. adding an under cowl bar. Just a 4 point loop that bolts to the front sills, loops under the dash to bolt to the firewall next to the heaterbox and with a couple of tabs to reinforce where the windsheild bolts on.

It does not need to be a large a tubing diameter as the autopower bar the car currently has, but anything has to be a big help.
Oct. 6, 2008 10:49 a.m. Keith SuperDork

You could also try seam welding. There's a big seam around the door sill on the Miata that's easily accessible - remove the sill covers and pull the rubber seam cover off, and you'll see it. I'd probably pull the seats and pull the carpet back, but you could weld this up with only an hour or two of work.

http://targamiata.com/diary.php?UID=4

The frame rail reinforcements from Flyin' Miata (disclaimer, I work there) will also improve the car's rigidity nicely. I measured a fairly significant improvement in torsional rigidity when they were being developed.
Oct. 6, 2008 12:22 p.m. Kramer New Reader

I have a '96 with the Hard Dog dual diagonal roll bar. I still get some cowl shake, but only at the 68 mph range (common to Miata's). I have my tires road-force balanced, which helps greatly, and if the cowl shake happens, I just increase my speed. I had a careful balance job done on my R-compounds, but no road force balance.

The roll bar did stiffen the chassis greatly, though. A few weeks ago, I had the car on jackstands in my garage. I could only get a good load on three of the stands, unless I went under something that was sprung.
Oct. 6, 2008 12:52 p.m. confuZion3 HalfDork

What is cowl shake? I don't think I've ever noticed this. I'll keep an eye (or buttcheek?) out for it though now that I'm curious.

What about seam-welding the windshield frame? I heard somewhere that doing this makes that part substantially stronger.
Oct. 6, 2008 1:16 p.m. Keith SuperDork

Cowl shake is basically a shudder in the dashboard - it's not as prevalent in the Miata as in old Brit roadsters, but it's a good indication of chassis torsional rigidity.

The 63-ish mph shake comes from out-of-round tires, in my experience. You're exciting the chassis at its resonant frequency. Solutions are to change the frequency (which is why bracing sometimes works) or to remove the vibration that's exciting the chassis.

The seam in the door opening continues around the windshield. Definitely worthwhile, but it gets more difficult to weld without spattering the glass or setting things on fire.
Oct. 6, 2008 1:32 p.m. thatsnowinnebago Reader

Keith wrote:

... You're exciting the chassis at its resonant frequency. Solutions are to change the frequency (which is why bracing sometimes works) or to remove the vibration that's exciting the chassis.

Miatae are just so easily "excited"
Oct. 6, 2008 2:41 p.m. npartist None

~65mph is a first order excitation by the rotating items (brakes and tires mainly). Its based on overall tire size so smaller tires (ie miata) will be at a little lower speeds.

Basically if your tire is unbalanced, it means at ~65mph that heavy spot will "hit" at just the right time to magnify the unbalance and thus induce more shaking in the car.

Sorry for the tangent.
Oct. 6, 2008 3:47 p.m. RussellH New Reader

I had a hard-dog single diagnol rollbar and the cowl shake was still there. For me the biggest difference was when I bolted-in the hardtop with fixed mounting plates. It wasn't gone but it reduced it a significant amount.