vibration at certain speeds

[rh71]

I have mentioned this as a response to another post but that was many many months ago and I was hoping people have new ideas or even solved their problem...

What would cause a vibration in the seats (not steering wheel) when accelerating or even coasting down between 55-70mph? Anything higher or lower and it's smooth again. I have had them balance 2 times (there are weights on the rims) and did a wheel alignment (to make sure) as well... problem remains, on different roads so I know it's not just a stretch of highway, etc. This does not occur at any other vehicle speed. Number of passengers doesn't matter either. I can actually see the seat vibrate too.

From the old thread it looks like I'm not alone on this issue. I have replica wheels which were spun to ensure they were not out-of-round on Toyo Proxes S/T. They are at 36psi (would different tire pressures cause this at certain speeds only?)... I ask because this used to happen at a different range (70-75mph only). The stock 19s never had this problem.

**EDIT 11/4: see resolution below.

[motordavid]

Strictly fwiw, the ol'Conventional Wisdom suggested that feeling it in the seats
was due to rear wheel situ.
Another spin for the rears may show one or more out of round, with a slight bent rim, etc.
GL,mD

[Barcius]

Rh71,

I suggest you mount your stock 19s at the back and check if the problem goes away. From what you describe, it looks like it is a problem at the back and not the front since your steering wheel does not vibrate. It should not take you long to swap them back and forth.

Let me know what you find out.

[Denalio]

You could have them road forced balanced.

[poleposition]

Originally Posted by Barcius

Rh71,

I suggest you mount your stock 19s at the back and check if the problem goes away. From what you describe, it looks like it is a problem at the back and not the front since your steering wheel does not vibrate. It should not take you long to swap them back and forth.

Let me know what you find out.

+1

[rh71]

The 19s are already sold. I will have to get them road-force balanced at the dealer then. I asked to have this done last time but they wanted me to at least get the wheel alignment first because that was probably never done at my independent installer. The stealer's alignment & balancing aren't cheap at over $150 each service - and I wasn't willing to spend over $300 just for a "maybe it may fix it" so that's why I'm posting again here. I suppose it irritates me enough to want to get this done properly this time. I want to have them take care of this since they are supposed to be held to higher standards than an indy shop... am I wrong?

[rh71]

what do you guys think of this person's response at another board? Seems he's saying this is somewhat expected.

At times like this when I have a problem getting tires to balance and not knowing for sure which one(s) are doing it, I just drop the air pressure a little in them one at a time until the vibration goes away.

Reason? A tire at normal inflation will bounce a little more vigorously at higher pressure and less so at lower pressure.

Now - this is for diagnostic purposes only as I don't run at lower pressure to just compensate for a bad balance. I then break the tire completely free of the rim and clock it into a different position and re-balance it again.

If you just re-balance the tire you can chase your tail for hours adding 1/4 oz here and taking 1/2 oz there and it just never ends. I never spin a tire more than two times to balance it - why? It makes no sense to move fly speck for hours when it won't make much difference except to a 200+ MPH Indy car.

Quarter ounces at 7.5 inches from epicenter at less than 100 MPH can't be felt nor will it affect the tire in the long run.

There are (kinda) rules of thumb for tire-size and road-speed imbalance factors:

13 inch tires seem to be bouncers at about 42-49 MPH
14 inch tires bounce at about 50-56 MPH
15 inch tires seem to act up around 55-62 MPH.


There appears to be a corollary to the size/speed relationship when they are out of balance. It's just a hint, not a solid rule.

I would - for good reasons - check for loose wheel bearings, bad tie rod ends or slop in any suspension/steering parts - very carefully too.

[motordavid]

I still vote for my orig. hypothesis of a rear wheel(s) way off balance, out of round and/or a bend in one...
Edit: PS...in the time it takes to fiddle with tire pressures, (done when tire is "cold", no less), one can
be down at a quality tire joint and have it diagnosed.

[rh71]

^ I'm just a bit wary of taking it to a regular tire shop... if they (Mavis discount tires) treated my mint-condition Isuzu like a POS (dented the door) I don't want to think about what they do with nicer cars (including joy rides). Do you guys bring it to Joe Schmoe tire shops? What's really considered a quality tire shop? I trusted that independent shop that specializes in rims & tires and they couldn't get it right to begin with. I wasn't about to bring it back to them a 3rd time.

I guess what I'm asking is... should I pay the dealer their asking price as opposed to doing it somewhere less reputable but cheaper for a supposedly simple job like this?

[motordavid]

Long Island: big place. There has to be a decent tire specialist joint in your area. Maybe some fellow poster from your zip will rec'd one...no, I doubt many of us go to cheapazz tire joints.

Try GOOG, Yahoo Yellow Pages, et al.
Easy stuff, imo.