nile13

Hoosier H6 and Hoosier A7 in 275/35-15 on 2,100 lb '2000 Mazda Miata with cool spherical shift ****. And the same car sitting for 6 months a year on old Azenis. Oh, also a pair of my own 993s in the last 13 years, including 8 winters as daily drivers and 5 winters of sitting on flat ground for 4+ month with Michelin PSS2, Toyo R1R, Dunlop Z1 StarSpec and a number of other tires. One with 140K miles and original LCA and the other with 100K miles and Walrod bushings for the last 20K. And, conversely, another Miata that sits for the other 6 months, previously on Michelin PSS and now on Bridgestone RE-71R. And girlfriend's Lancer sitting for 6 months a year on non-descript all season tires that simply do not have enough rubber content in them to actually deform under the pun weight of 2,600 or so pounds that this contraption weights.

I hope that is specific enough answer in this case. And, as a suggestion, please do look up the resonant frequency and it's effect on vibration amplitude as a function of vehicle's speed.

As a follow up - the OP's issue is the tires. If it's not tires, he should still start with tires and progress from there if vibration persists. LCA bushing are easy to check. But not the first thing.

NYC993

Dude, do you talk like this to everybody? Or did I hurt your feelings by questioning your "expert" opinion. Have a Lager from your hometown

So do they all have flat spots on tires all the time and you replace them? I don't get your point, but whatever.

Btw for the record and OP's benefit, I didn't say that worn tires is not where to start.I would start with them 3 years ago. It's just that 45-55mph range of vibration is very narrow and I'm not sure it would fully go away if unbalanced tire was causing it. But I fully accept that it could be. When I had **** on my tires, or tires were too worn where they became unbalanced, vibration would not go away at any speed, maybe it would get worse at some speeds than others. However, worn out LCA bushings often cause vibration right around 50-60mph, which I'm surprised nobody brought up.

nile13

No. But I do talk like this to people unfamiliar with the simple laws of physics and those who like to spend somebody else's money with impunity.

However, keep thinking that none of this applies to your Porsche. Because, of course, how could it.

NYC993

What's up with the accusations?

How is "those are symptoms of worn out LCA bushings. Have you done those?" equates to "those who like to spend somebody else's money with impunity" in a context of a thread where OP just had engine rebuilt.

Ok, if we have such simple laws of physics, maybe you can tell us exactly how big the flat spot on his tire that causes vibration at exactly between 45-55mph? Actually I don't care, have nice life. I'm out of this "discussion".

Churchill

Flatspotted tires will create vibration/thumping in relation to wheel speed, i.e. the problem gets worse the faster you go. That's why I said it may be front control arm bushings, which are notorious for creating a steering wheel shimmy around 50-60MPH. A flat spot will be bad all the time. They also don't really create steering wheel vibration, they create a thumping sounds and feel.

nile13

This is sad and no longer entertaining.

-nick

Physicist here, I think you fellas need an engineer though. Or just an interpreter You are both right, but you're talking about two different things.

Thing #1 An out of balance wheel will vibrate more as its rotation increases and isn't likely to show strong harmonic resonances.

Thing #2 An out of round (flat spotted tire) wheel will have a very strong resonate window.

My cars typically don't sit for long periods, but I have had summer tires flat spot from sitting in the past. Some tires are more susceptible than others. My old summer Dunlops from a few generations ago were notorious for flat spotting after only a few weeks. The deformed flat spot goes away after driving on them.

If the car didn't vibrate going into the shop. And it was there for a while with an engine out service. Then flat spotted tires are almost guaranteed to be the culprit.

OP- What are the front tires, how old are they, how long was the car in the shop, and how far have you driven since you picked it up? That info could help someone in the future who has the same symptoms and finds this thread.

myflat6

Do u know if your car was suspended on a lift for the duration of the service with the suspension extended? For what it's worth, I had a similar experience with my car after I had it up on jack stands for a few weeks while I was working on it. I had no issues with vibration/shaking before this. It ended up being caused by the front control arm bushings. I purchased walrods and sent my control arms to Andreas to have the bushings installed - no more shaking. My thought was that the old bushings finally gave out after being subjected to that twisting/torsion pressure for weeks. There are methods for checking the bushings by using a pry bar (search the site).

geolab

I bet that it is your rear that is shaking the steering wheel @ 50 mph
a motor out requires an alignment after, starting from the rear, and ending in the front.
moreover, what rear wheels do you have? any work done on rear wheels ?

stubble88

Maybe you did this already and I missed it but im saying first check the pressures.
I had the exact same shaking at around 50mph it was simply a 20psi front pressure on both tires. pumped it up to 36 and problem went away.
wheels and tires were sitting off the car for some time.

nile13

I am (was) an engineer.

This, as an engineer and from previous 30 years of experience, isn't what I've observed. One of the more interesting cases has been owning numerous AW11s (first generation Toyota MR2) which were well known in teh early 90s for a front wheel shimmy. We, collectively, spent countless hours chasing this and came up with countless theories. The culprit, however, turned out to be... overtightened lug nuts. The monkeys with air wrenches (this is before torque limiters were widely available to them) would put close to 100 lb/in of torque instead of spec 72 and the wheel would actually bend, The resulting vibration was pretty obviously resonant at around 65-70mph, exacerbated by a light nose of the mid-engined car. It would go away around 75mph almost completely.

So... very interested in why, scientifically, the bent wheel or unbalanced tire would not cause resonance.

Yep, that's the basis of my theory.

vincer77

Any mass-spring system excited by a cyclical force will experience resonance at a particular frequency. My experience is that out of balance wheels-tires will result in steering wheel shimmy at some speed window.

I would expect the same with an out of round tire.

The first thing to check is balance. IF THAT Is NOT The issue, Then Look At Control Arm bushings.

OverBoosted28

Wow, just as Geo was trying to get this thread back on track, with pertinent info. it goes right back to another topic. Yes! Go get an alignment!!! If you've assured tires/wheels are good/balanced/aired, cars aligned properly, but still have issues, then you start another search. GL

nile13

While I've known Geo for many years and have been a beneficiary of his excellent help and advice many, many times, I disagree in this case. The reason is that the _steering wheel_ shakes. Which, in my own experience, has always associated with front wheels/tires, not rear. As in, in 100% of the cases.

Rears wheels/tires could certainly cause vibration. But it would not manifest itself through the steering wheel.

squidge

Bought my car with ten year old tires, and it was infrequently driven by the previous owner. PPI found steering wheel shake and pointed at the tires. New tires did nothing. New front control arm bushings fixed it 100%. YMMV