I have some intermittent shuttering happening on hwy. It's actually been there for a few years, but NTB was telling me it was the tires -- that when I bought new ones, it should be solved. BS!

So, I finally got new Sumi's on in April. It still has some vibration/shake. Yeah, C4's are a big-azz-tire-equipped cowl-hooded beast that often gets this type of complaint. But, a few weeks later, I got hacked-off and took it back for a re-check. Two wheels were out of balance.

After the rebalance, it's better -- but I still feel intermittent shutter/vibration at 70mph. Oddly, it's sometimes in the front...sometimes in the rear.

Are rear 315's difficult-to-impossible to balance? Are Sumis crap?

I call a few places and find a RoadForce balancer. Thinking about taking it by this week (and will owe about $130 for the service).

Later tonight, I'm thinking about the rear shocks....
I replaced the front ones after a saw evidence of leak/bounce a few years ago. The rears have always seemed fine. Even though I'm only up to 65k miles, they are 23 years old now! (Damn, is my car really THAT old? )

Obviously, new Bilsteins aren't cheep. But, I never replaced them before. Plus, I never considered that the heavier 315's on 11" wheels might need a stiffer shock to handle the extra weight? FWIW, I have no complaints when running 60mph or less.

So what do you think? And why?

 

I should also add that I never feel bouncing...and the shutter described above is closer to lateral shimy/shaking.

I wouldn't think shocks would do that.

Cruddy highways might...But then there's the bushing/sway bar issue. Should I consider that?

Or, and x-brace?

Or, just assume it's the tires?

 

I'm surprised you haven't answered your own question yet.
But to avoid a I'd really suggest you replace the rear shocks.

Because they reduce tire/suspension bounce.
Because the suspension requires a good tire,spring and shock absorber.
Anyone of the three is faulty, the other two cannot make up for the third.

Big tires and wheels and old shocks?

 

If I thought is was that easy, I wouldn't have asked. Plus, the question included recommendations for heavier rear wheels. Bilstein has multiple models.

Old shocks aren't necessarily bad either. Obviously, it wouldn't hurt to change them, but that might not be the issue. And, it would be nice is someone knows if/how to compensate for the heavier wheels. (e.g., what model...etc.)

Beyond that, since the symptom seems more like a "shimey", I'm leaning more toward a tire/wheel issue.

 

It's possible that the new tires are slightly out-of-round. That can happen even on more expensive tires. The Hunter GSP9700 Road Force balancer can find an out-of-round condition and it may be that the tires will have to be shaved. Internal cord separation is also a possibility. Carefully inspect the tire (both tread and sidewall) to see if there are any variations of the surface. Sometimes a visible indication of a separation won't be seen but just felt as the tire spins faster.

It's possible for the wheel to not be true; I don't think wheel weight by itself is a real issue as most aftermarket wheels are heavier than the OEM versions. Did you ever curb a wheel? How old are they? But those things will come out on that type of machine.

It's also possible that one of the u-joints are going out.

Start with the balance work and see if there is a problem with one of the tires or wheel.

 

 

I had my new Sumi 315s road force mounted and balanced for $12 each. I made sure the tech knew about the little circle printed on the tire lining up with the valve stem. They both balanced out at 1 or less, dunno exactly what that means but tech said "Wow" Potentially the Sumis will balance out good, it just takes the right tech and a little advice and a watchful eye. Wouldnt hurt to do it but shop that price around, its ridiculous.

Good luck with it.

 

Wow, mounting AND road force balancing for $12? Sounds like the cost of labor in FL must me lower than KS (which is also lower than CA btw.)

I called three places so far. I'm getting prices in the $25-$30 range PER tire. (That's unless they find normal balancing solves NTB's balance issue....in which case, it's $16/tire).

One place mentioned the ability to shave if necessary.

Keep in mind, road force balancing is different than spin balancing.

I think it was Caboboy who just posted a link for everyone in another thread about vibrations....
http://www.gsp9700.com/how/index.htm

 

have similar issues on the 67
Had a nice shake and what I thought was 100% driveline vibration starting at 63. Fooled me!
Turns out not one wheel was perfect or even close.

One had to be replaced altogether

Road Force machine will tell you how far out they are or if it can be balanced. If its out of spec it will say

Watch them when they do it otherwise youll get a run of the mill "ok" balance job. They tried balancing my bad wheel 2 different times said nothing about the wheel being bad kept adding weights.


Get them balanced inside AND out it will make a world of difference. Who cares how it looks if its smooth
Saved me from buying 3 wheels.
Shouldnt cost any more than reg balancing.

 

The explanation I got included the possibility of deflating and rotating the tire (potentially) several times. And, that a tech could spend up to 20 minutes per tire. IDK.

I also don't know if shaving is ever done.

I haven't checked the dot vs. valve orientation, but that may be a minor thing. Maybe that point in the tire is measured as the lightiest...and should/could offset the extra weight of the valve stem?

My issue is intermittent but fairly frequent. And, I feel it in the front and back...Sometimes both at the same time. Road surface definitely makes a difference too -- but it's still present (to a lesser degree) on perfectly smooth hwy.

I even wonder whether I mounted my driveshaft 180-degree off after the 383 install. But, the issue doesn't seem isolated to the rear. Seems more likely that its' NTB's balance job.

Also, since it's doing it on the front too...I have new Bilstein's up there. Kind of rules out the shock issue for the fronts.

Maybe it's the cam!

 

Ok maybe I was misinformed by the tire place about road force balancing sounds like it.

Sounds like I just got a spin balance.

May have to look for a place then...if it still does it.
Youll know if its the tire easy to tell. Bet its the wheel too many repops these days seem to be made "within a certain range" not spot on un less you pay big bux for nice wheels.

 

That link 5 posts up, has a locator.

I have (what's supposed to be GM factory wheels that are chrome plated...and the plating is still good.)

They came from a shop that takes (or used to take) GM vette wheels as cores. They do have the correct part number in the back...but I can say each of the 315's has between 8-10 ounces of weight on the inside of the rims.

Seems like the Sumis are crap, the wheels are crap, or they're mounted in the worst config (wheel to rim) possible.

We'll see!

 

I replaced both u-joints 5k miles ago (with the 383), but maybe it's eating them up? Not a bad thought!

As posted above, I should be running chrome GM stockers. They are older -- since GM wheels are getting in the 20-yr-old range now. But, they are exclusively fair-weather driven.

 

Ditto on ckg Ujoints theres a few there.

Not sure if wheel bearings would do that.

Experienced a vib at speed turned out to be a pinion bearing
Got a gear howl accompanying that also so unless you have that you may be ok. (lash moves when the bearing/race is scored)

 

Get the road force, I had the same issue w a sumi.

 

Think Im gonna get this done just to do it after reading this thread.

 

8-10 oz a wheel WOW!


I had a wheel induced shudder at speed.(it would balance out fine but new tires did not fix it. I'd go back to rebalance and it would be off. Finally fixed it bv going to new wheels (I was lucky and scored some factory C5 wheels bought from a guy that removed them from his C5 when it was new. )
I balanced my own tires. (They hunter equipment at last assignment, old coates equipment here ) I spun each wheel multiple times finding the tire mounting location that takes the least amount of wieght .... mounting balancing [checking amount of wieght] then moving tire 1/4 turn on the rim and repeating until finding location that takes the least amount of weight.

There is not 5 ounces on my entire C4 (all four wheels total) But I'm running Goodyear F1s not budget tires, my shocks have not reached the legal drinking age all bushings and suspension wear items have been replaced since I've owned the car. Cheap only gets you so far. I'd rather spend a little more for quality and end up with decent results than go cheap and live with disapointment.

 

That has been discussed in here for years and the only reason I never bought Sumitomos. They are the cheapest 315 tire you can buy and cheap means crap. They are also way narrower than 315.

Don't buy the cheap stuff, plenty of experienced threads on that subject.

 

The traditional triad of factors are:

Cost
Quality
Time

or

Cheap
Good
Fast








To put it simply, I can get old newspapers for free; however, I choose to buy Charmin. The results are probably the same but the experience is totally different.

 

Found out the wheels have 1/2oz weights, so cut what I said in half. And, that's only on the 11" rears. (Probably more necessary for the chromed factory wheels than the tires too)

It always easy to blame cheap.

After going to my buddy's Chevy shop, I found out the problem(s). In this case, the tires spin balance correctly, but Road Force did show 2 of the four can't be road forced balanced. This does result in some mild vibration (when not on smooth hiway).

This wasn't my only issue.

Because I have a cammed motor -- connected to a sidepipes -- connected to the frame, that's the other issue. The sidepipes are isolated [from the frame] with large rubber bushings but the cam (and my hot low rpm timing) can cause some "cam surge" that transmits thru the exhaust piping and therefore, the frame. When going 70+ on rough hwy -- in 6th gear, the combination can be annoying. I just wanted to make sure it wasn't problematic. (And, luckily, the problem rpm range is very narrow.)

I think the addition of the flex pipe (sections) in my exhaust might solve most of the problem.

If I ever convert to an HSR and start running 100mph, then I'll consider $1000 tires and dumping the sidepipes.