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Shaking from 40 to 60 miles an hour

Good morning, happy Friday to everyone.

My '87 S4 auto has a shake that develops at 40 mph and then goes away at 60 mph.

I have turbo twist front rims, but the tires our new and only have around 500 miles on them. The tires were professionally installed and balanced.

The motor mounts on the car are also new (Rogers's), and while needed, did not cure the 40-60 mph vibration. I put a new steering rack bushings at the same time as I did the motor mounts, they were also needed but appear not to have impacted the vibration at all.

I haven't been able to get the car on a steep enough he'll to have it accelerate it neutral from just below 40 mph to 60 mph to see if it has anything to do with being under power when the vibration happens.

The vibration feels like it's coming from the front end of the car.

I'm inclined to think that either I have a problem with a rim or a bad balance, but it seems weird that it would smooth out again once I get through that speed range.

Are there any weird vibration/harmonics that other people have dealt with at lower speeds that I should be aware of? Ideas?

Thank you in advance for the help
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-08-2018, 05:36 AM
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have you had it aligned? - and if you did was the shop aware of the fact you can not lift the car before aligning it? - i had a vibration at 75 mph - had it aligned at a local place that said they could do it - tires burned off in 2000 miles - took it to a specialty German car place - they redid the alignment and car drives straight with no vibrations - it did cost $300 for the alignment but what can you do?
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Old 06-08-2018, 07:04 AM
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I would look carefully for a knot or delamination on all the tires. It could be on a rear tire since it doesn't happen immediately from start. I had a set of Sumitomos that came with my GTS from So Cal. I added air during a cold weather event here in Texas. Drove the car after airing the tires and a rear tire delaminated on the inside sidewall almost all the way around. Turns out that that particular tire is not supposed to be used in temperatures below freezing. Not a problem in So Cal. Major issue on the plains of North Texas in the winter. Shocked me beyond belief that tires like that were even sold, but they were great tires in the summer.
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Last edited by stepson; 06-08-2018 at 07:12 AM..
Old 06-08-2018, 07:07 AM
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Good morning - thanks for the responses.

I have not had it aligned, tracks straight etc, so didn't occur to me. Probably worth doing, and thanks for the coaching on conditions to make sure it is done right.

I'll double check the tires John, would be disappointing to have a brand new set have a defect like that but stranger things have happen. Thanks for the idea.
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-08-2018, 08:06 AM
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Have you checked on you wheel bearings, axle, and tie rod ends too?
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Old 06-08-2018, 10:06 AM
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And here comes the slippery slope!

All good suggestions, thanks you.
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-08-2018, 02:08 PM
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Had the exact same situation a while back. Went back to the tire shop, and one of the wheel weights had come loose and fell off. After a re-balance, all was well.
Old 06-10-2018, 03:26 AM
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I love easy solutions.

Glad yours was a quick fix.

Thanks for the response and have a good weekend guys!
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-10-2018, 06:53 AM
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Any satisfactory outcome yet?
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John Curry (Drift King) 928OC member
Grand Prix White 1994 GTS AT (The GTS)
Cobalt Blue 1989 S4 AT (The Blue Car)
Indishrot 1984 Euro S AT (The Stepson)
Black Metallic 1984 Euro S 5 speed (The Schwartz)
Old 06-11-2018, 07:40 AM
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Hi John, haven’t had a chance to dig in yet. Been busy buying a 914.

:-)
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-11-2018, 12:08 PM
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Owning a 914 should occupy you for a bit. Sort of miss ours, but no AC was an issue.
Old 06-11-2018, 05:42 PM
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Quote:
Originally Posted by Danglerb View Post
Owning a 914 should occupy you for a bit. Sort of miss ours, but no AC was an issue.
Fun cars, but wildly different than our 928s. This one may be moved along pretty is still a priority.
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1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-11-2018, 06:50 PM
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Its probably the tire balancing, it could possibly be a defect in the tire. A rim could be out as well but they usually tell you when they try to balance them. If you feel it in the steering wheel it's a front tire, if the seats shake its a rear. Have the tires rebalanced. It could be something as simple as it threw a weight after the balance.
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Old 06-12-2018, 02:53 AM
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Hi Pete - thanks for the note.

I have a spare set of rim/tires for the front, I think I am going to swap them out and see if it is still there.

The shake isn't moving the steering wheel around, steering remains smooth (I think) despite the shake. Will get in the car today and pay extra attention to see if the steering wheel is trying to tell me something.
__________________
1967 912 (now w/ 50% few random holes in it)
911 w/ 3.2 (ugly but kinda fast)
1974 914 (3.2L swap underway)
1984 928s (with S4 engine and suspension)
1987 928S4
Old 06-12-2018, 04:34 AM
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