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Tire flat spots versus alignment... how to tell?


Old 03-05-2010, 09:05 PM
Betsy W
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Default Tire flat spots versus alignment... how to tell?

What does it feel like to have tire flat spots?

Our '98 993 C2S has the 18" twists on it and Toyo Proxes T1Rs. I can't find the date stamp but the tire treads look pretty good and the rest of the car is well maintained. We bought it fairly recently; our first test drive we noticed what seemed like some fairly significant steering wheel shimmy at around 55 mph. The dealer rebalanced the front end (and yes, it was out of balance) but blames the ongoing shimmy on flat spots in the tires.

Since the rebalancing, the shimmy has diminished but not disappeared. It's been a snowy winter and we haven't driven the car very much, but my husband has been driving it every day this week. (To and from work, maybe an hour all told.) It's chilly outside (50 degrees max). As of today, after 5 days, the shimmy is still there.

I've accurately diagnosed alignment issues before and this sure feels similar. The car is smooth until about 50 mph, but then the the steering shimmies steadily (maybe an inch to an inch and a half of steering wheel rotation, back and forth), clearing up again around 65 mph. It seems to me that a flat spot should be detectable at a range of speeds, and that's not what we're experiencing. I've read a little about this online and the tire places say that flat spots should resolve within minutes of driving as the tires warm up. My gut tells me it's something in the alignment or the front end; the dealer says the wheels are balanced and the shimmy is due to flat spots and will disappear when the ambient temps increase and the car is driven more regularly.

So -- those of you who suffer from flat spots, what's it like? How long does it take to diminish? I trust my dealer but I'd appreciate some feedback from you guys.
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Old 03-05-2010, 09:33 PM
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Take it from someone who had flat spotted tires, sometimes the flat spots do not go away. Depends on tire condition and how long it was sitting. I had something very similar when I bought my 993. The front tires had lots of tread, but were beyond their six year life, hard as a rock with bad flat spots. Also had a noticable tire "drone" at various speeds. After puttying on new tires, it was like new...smooth and noticably quieter.
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Old 03-05-2010, 10:29 PM
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if there is a flat spot, usually it will never disappear. the shimmy should be present whatever reasonable speed you are running. if the shimmy only appears at a certain range of speed, say 55-65, it still might be a wheel balancing problem.
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Old 03-06-2010, 07:17 AM
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You can remove some types of flat spots by driving the car for about 30+ miles at different speeds with approx half the normal tyre pressure. This should allow the tyre to move and reform. Then go to 50 psi and run the car for several miles. Then back to normal pressure.

It might be high pressure first then low - seen the solution somehere but cant recall. (google tyre flat spots?)
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Old 03-06-2010, 08:17 AM
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There are two types of flat spots - those caused by loss of rubber due to a spin or panic stop ( ABS usually prevents those) .... and those due to deformation due to static (semi) deformation: both are evident at all speeds. The former is correctable by grinding ( generally not worth it unless one wants shaved track tires), and the latter often dissapates by driving.

Failed/broken belts within the tread package also do such things, although that is a more rare occurrence now.

A bent wheel can also cause a similar sensation - easy to check by jacking the front wheel(s), and hand spinning against a reference point .... like a coat hanger wire shaped as a pointer.

While in the air, do the 12-6 o'clock twist for wheel bearings and the 3-9 o'clock deal for tie rod ends.

Also, do the 'blind' feel test by closing the eyes and lightly pass your finger tips over the tread surface: you will be surprised how sensitive this 'human micrometer' test is. Often, this reveals wear patterns, sharp edges, and depressions not readily visible .... and points accurately to a particular suspension issue.

I noted a second 993 C2 listed in your stable ..... if so, I would be quick to swap front tires around and go for a joy ride ....
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Old 03-06-2010, 08:56 AM
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When I bought my 993, the tires would flat spot if it sat for a week or more w/o being driven. Mine wasn't as dramatic as what you describe, just a slight shimmy in the steering wheel mostly noticeable at around 40 - 50 MPH. It would go away after 20 mins or so of driving. I attributed it to very soft and old tires.

As Garth noted above, if you have another set of wheels you could try, give that a shot and it will help narrow it down for sure.

Good luck!
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Old 03-06-2010, 09:07 AM
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Were the tires road force balanced. If not that may be an issue as well. Weak shocks/struts will also lead to flat spotted tires. You should be able to feel the difference of tread height on the inside of the tire.
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Old 03-06-2010, 01:13 PM
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R1's are SOFT, I would make sure all the pressures are correct, even a few PSI from side to side will create a shimmy. Then I would over inflate a bit & drive them till they warm up, then cool'em off, & let our some PSI. If you are going to park for a while bump up the PSI. The heat from the engine when parked can soften the rubber & make them flat a bit. You are basically running a track tire.
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Old 03-06-2010, 01:40 PM
Steve Weiner-Rennsport Systems
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I would also closely inspect the front A-arm bushings and check deflection as they can cause a shimmy at that speed when worn.

Physically flat-spotted tires are pretty easy to spot when they are rotated. If the belts are deformed from sitting on the ground for a few months, one needs to drive the car and get the tires hot to see if they will get round again.

I'd find the date codes because tires over 4 years old may have become too hard to drive nicely again.
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Old 03-06-2010, 01:43 PM
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I had my wheels/tires balanced, then the symptoms did not go away, so I had them balanced AGAIN, and they seemed to be out, so needed more weights to be added...

just a thought,

good luck,

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Old 03-06-2010, 07:01 PM
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If the tires were out of balance to begin with and then re-balanced could some of this be related to cupping of the tires? This is where some of the tire has worn at a different rate than the rest of the tire leaving a high and low spots in the tread and then transforming into your shimmy.

I had a tire one time that had an actual chunk taken out of it, must have hit something sharp on the road surface at some time, this resulted in a shimmy and only became apparant after a visual inspection. After replacing both tires the car ran verrrrrry smooth like it was designed to run!
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