mschueler03

I've had my C4 about one month now. Purchased it with about 49,500 miles, and have just crossed 50,000. The rear tires were put on in 2019, and have about 3,500 miles on them. The front tires were new about six months ago. I have noticed with the targa top out, I am getting serious shaking/vibration at approximately 20 mph, 30 mph, 50 mph, and 60 mph. This only lasts a couple mph's, and then disappears. I have read about "cowl shake" quite a bit as well. I am having the balance on the tires checked this weekend. Any thoughts if it's just "cowl shake," or could tire balancing and/or flat spotting on the rear tires be the possible culprit. FYI, the issue is far less noticeable with the targa top in place. Thank you for any and all input.

Cheers

Nowhere Man

keep the roof on

Andy Seynaeve

What brand of tires? I had bad tire vibrations with my Nitto tires. It was a cold tire thump that sometimes took hours to go away. Got rid of the shake getting Continental Extremes.

hawk454

More like you are noticing the flaw in the chassis without the A-pillar to B-pillar structure. There are many threads about the sometimes dangerous chassis behavior without the targa. The design engineer noted that the original intention was supposed to be T-top but marketing thought no top was better. Only convertibles were stiffened with an x-brace underneath the chassis to control chassis movement. Your best solution would be the targa brace or targa truss (or even AFSB) bar often sold in batch runs.

https://www.motortrend.com/how-to/02...t-bar-install/

https://www.corvetteforum.com/forums...rga-brace.html

PS It is always possible you have old tires with flat spots. Check date codes on tires to be safe. Especially if you still have original C4 rims. My tires were ten years old with some cracking starting to show.

Cheers,

FAUEE

That sounds more like a tire or alignment issue to me.

Nittos do flat spot pretty bad, so if it's been sittingg for a week or so, then you go run out and it's doing it right off the bat it could be that.

vader86

My Nittos flat spot almost immediately, so thats a big part of it.

The targa being out is probably the rest of it. The car just needs the top left in for best rigidity.

mschueler03

Nitto on front and rear. Could be the culprit. I will start there. Thank you.

Paul Workman

with what is said. However, if you intend to go "topless' regularly, one solution to frame flex w/ the top off is a set of the frame stiffening flex bars, ICO Vette2Vette. Reports are they "make all the difference" when riding with the targa off.

Joe K

Not under the same circumstances as you but I found the front shocks not being compressed properly at the top mount bushings caused a horrible shake at track speeds. That with the top on. Just something you may want to look at and eliminate from being a source.

HTH,

Joe K

GREGGPENN

One easy way to see if it's your tires is raise the pressure. See how it affects any shake, rattle, and roll! Ok...just shake. If most of it disappears, it's more an issue of your tires. I got a set of Sumi's 10 yrs ago where 1 was essentially "out-of-speck". It sucked but I had to pay shipping to get it replaced. I found it via road force balancing. That's different (and costs more) than spin balancing. (yeah...both are spun). You'll find some who store/park their cars with inflated tires to avoid GETTING flat spots too.

Seems I read over inflation and driving on HOT pavement can relax a flat spot?

There was also a thread about frame welds...that they might get weaker, were designed for 20yr life span, etc... Frame bracing and/or V2V bars can't hurt. I put a "camber" brace across the front of mine. It helps but doesn't eliminate shake from low pressure. Cowl shake is simply my cue to check tire pressure! LOL

That said, I've never had shake at speeds that low. 20mph? 30? Really? That sounds like BAD ROADS! BTW...if your shaking is in the same section of town every day...consider roads as the problem.