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Posted by: rbutler914 Jul 27 2023, 09:43 PM

I’ve been doing a bit of freeway driving lately and I have a weird steering wheel shimmy, noticeable between 60-80 mph. The strange thing is it dissipates after about 5 seconds, the returns about 5 seconds later and continues like that. There is no accompanying wheel vibration or shaking, just the steering wheel. Any thoughts would be appreciated.

73, 1.7, all stock, slightly lowered by previous owner.

Posted by: 914Sixer Jul 28 2023, 06:03 AM

Your front steering geometry is probably incorrect from lowering. Need to check to see if steering rack spacers are installed. Tie rods must be level with rack and struts.

Posted by: Shivers Jul 28 2023, 06:22 AM

If it was my money I’d throw it at having my tires computer balanced at a reliable shop. I hate to say it but tire shops don’t always hire quality techs. I had a front caliper stick and it vibrated above 50. But you’d smell brakes burning if it was that.

Posted by: r_towle Jul 28 2023, 06:42 AM

Like any old car, tires need to be balanced or replaced
Front end needs to be aligned.

Check date codes on your tires.
Tires, especially as the get older, get flat spots when parking for too long.

I’m a firm believer, based upon a bad experience, to replace tires at the 6-8 year life span.

Posted by: VegasRacer Jul 28 2023, 09:08 AM

Replace your tires and have them Road Force Ballanced. I bet the shimmy problem goes away.

Posted by: 930cabman Jul 28 2023, 10:25 AM

Depending how much she is lowered, I might put my $5. on the geometry

Posted by: emerygt350 Jul 28 2023, 05:51 PM

It could so easily be any of those. Start with the cheapest thing, balance and alignment. At least that will be something nice to have regardless if it fixes the issue.

Posted by: tomeric914 Jul 29 2023, 01:56 PM

Does it have hubcentric wheels and rotors?

I did all of the suggestions above, road force balance, alignment, tighten wheel bearings, etc. Nothing worked until I created my own hubcentric grease caps (since my front rotors were not hubcentric). Since then, completely smooth and vibration free.

I recently created hubcentric rings for the rear wheels to ensure no vibration, but that's another thread...

Posted by: Porschef Jul 29 2023, 02:13 PM

Eliminate loose wheel bearings first, free, and easy...

Posted by: fiacra Jul 29 2023, 06:15 PM

Lots of good suggestions and I suspect the answer is in there somewhere. It's really a process of running down the checklist and arriving at a diagnosis by elimination and discovery. I would only add that I would start by swapping the front and back tires and seeing if that eliminates the problem. If it doesn't, on to the next thing. If it does, you have a bad tire or wheel. Won't cost you a thing (except time) and you've quickly eliminated two potential causes for your problem.

Posted by: NARP74 Jul 29 2023, 07:48 PM

Doesn't sound exactly like bump steer. But still worth checking out the rack alignment after some of the other items.

Posted by: Geezer914 Jul 30 2023, 05:04 AM

Don't waste money on an alignment until you check the ball joints, tie rods and wheel bearings.

Posted by: 914Toy Jul 30 2023, 12:41 PM

Start with having the wheels balanced.

Posted by: technicalninja Jul 30 2023, 12:45 PM

Wheel balance issues ALWAYS show up somewhere between 55 and 80.

At one small range of speed (5mph 55-60 or 65-70) it vibrates.

Above or below that range it's smooth.

I don't know WHY it's always above 50 but it is...

Wheel balance issues.

Have that crap happen earlier and you can still feel it a little bit in all ranges.

Bent wheels.

Jack car up and spin each wheel on one of the front hubs.

Look at rim run out and tire run out. A bubble in the tire, even a small one, can wreak havoc in the NVH department.

Let us know what you find.

Posted by: rbutler914 Jul 30 2023, 02:00 PM

Wow, thx for all the great suggestions. I’ve already tried balancing the fronts. Swapping with the rears will happen next. Btw Fuchs 2 liter wheels with 205/50 tires. I have a suspicion that front end geometry and/or worn components might be the issue, that’ll be next. The fact that it comes and goes and constant speed and road surface in puzzling though. I will definitely report back.

Posted by: Superhawk996 Jul 30 2023, 02:15 PM

Mini white paper of the day:

The reason is related to the tire diameter and the resonant frequency of the steering column / steering system.

The tire vibration in question is called Radial 1st Harmonic (R1H). It is a force disturbance that occurs once per rotation of the wheel and tire assembly. R1H is different than tire balance - R1H is related to the tire itself. A tire can be perfectly balanced and still have high R1H forces. Road force balancing tries to compensate out both tire balance force and this R1H force. Road force balancing is superior to old fashioned dynamic balance.

Not having hub centric wheels is another source of R1H that can come into play independent of what’s occurring in the wheel and tire assembly itself.

There are other harmonics such as Radial 2nd Harmonic and Radial 3rd that can cause problems too but there are less prevalent.

R1H frequency is directly proportional to vehicle speed. When the frequency
Of the disturbance coming from the tire aligns with the resonant frequency of the steering column / steering system you get shimmy.

Steering system resonant frequency is usually around 20-30 Hz. If you do the math to work out tire rotational speed vs frequency you’ll find that 50-60 mph is the speed where the tire is rotating at 20-30 hz. When the tire frequency aligns to the steering system resonant frequency - you’ll have the worst shimmy.

Posted by: Superhawk996 Jul 30 2023, 02:24 PM

The reason it comes and goes is because of phasing between the right and left wheels.

The R1H component for right and left tire are not usually occurring at exactly the same place (in degrees of wheel rotation).

The right and left tires will have slightly different diameters due to tire pressure and tread wear. This means they rotate close the same amount between the sides but not not exactly the same.

Because of the slight difference in rotational speed between them, they will periodically align where both R1H forces occur at the same time - doubling the forces acting on the steering system. This is when the shimmy will be at its worst.

Then due to the rotational speed difference, R1H forces will eventually go out of phase between right and left. The shimmy will dissipate for a bit. This pattern repeats over and over.

Sometimes you can immediately break the shimmy if you are able to turn slightly. As you turn the right and left tires have to rotate at different speeds through the turn. Forcing that speed differential can move the R1H forces out of phase more quickly than it would naturally occur when travelling in a straight line.

Posted by: 76-914 Jul 30 2023, 07:21 PM

Interesting info. So why doesn't this happen to all cars or new cars for instance. BTW, metal propellers have to deal with harmonics that overlap at a certain rpm.

Posted by: Superhawk996 Jul 30 2023, 08:49 PM

It does occur on new cars. I’ve spent many working hours looking at warranty data from unhappy customers.

The chances of it occurring on new cars is greatly minimised by the OEM controlling tire uniformity to tolerances that are less than what you can typically buy retail.

In fact, what often happens is that the OEM is paying the tire supplier to sort tires to a tire uniformity specification. Tires that don’t make the OEM spec are sold into the aftermarket by the tire manufacturer.

There is also another technique called match mounting.

Match mounting requires that the spot on the tire that has the high R1H force is mounted on the rim where the lowest radial runout spot is. This helps minimise the force disturbance that R1H causes.

The problem is match mounting is much more effective with steel wheels that are never perfectly round. Aluminum wheels on the other hand are machined round and have much less radial runout - therefore less effective at obtaining a benefit from match mounting.

Everything is a compromise between the OEMs willingness to pay for tire sorting and the ability of the tire supplier to produce enough tires without generating excess scrap or more high spec tires than the aftermarket can absorb.

Posted by: technicalninja Jul 31 2023, 07:09 AM

Old school tires used to have a red or yellow dot which was supposed to be mounted dot next to the valve stem for just this purpose.

I've always wondered how the manufacture figured out where the heavy spot was without actually mounting the tire on a wheel and spinning it.

Thanks for the explanation SuperHawk.

Nicely done Sir...

I don't need an explanation for the question above.

I don't care that much anymore but your posts made me research "road force balancing".

The only problem with that process is that it REQUIRES a serious tech that will break bead and remount a tire for balance purposes.

I'm guessing it will be 1 out of 10 tire monkeys who are willing/able to do this.

I also don't like to have the monkeys dismount/remount a tire multiple times.
Each time the tire is stretched/ twisted a bit more and I believe a tire has a limited number of dismount/remount cycles before it gets damaged...
I do understand break bead, rotate, and re-bead is not the same as a full dismount but I don't trust the 18-year-old kids that most tire shops employ.
I wish they'd just let ME use their equipment.
They won't.
I've asked!

Rick

Posted by: Superhawk996 Jul 31 2023, 07:18 AM

Gearhead by birth . . . Automotive engineer by trade for nearly 30 years. Let me know next time you want another mini white paper

Posted by: Superhawk996 Jul 31 2023, 07:35 AM

Sorry can’t help myself For anyone else that might be curious.

Two piece split rim. All done within an automated machine on the tire production line.

Posted by: technicalninja Jul 31 2023, 07:43 AM

So, they do actually mount it...

Makes sense now.

I got a set of Continental DWS for my M3 a while back.

Noticed NO weights on wheels.

Pissed off BEAST!

Take them back.

The Manager advised he toured the Conti plant a while back and they showed him their segmented molds that allow super precise injection of rubber from many points instead of just a few. Makes for tires that are silly close out of the box.

I'd balanced the wheels without tires and the balanced wheels plus the stupid close tires made for near perfect balance.

1 wheel had 1/4 ounce of weight on it. Just one!!!

Posted by: rbutler914 Jul 31 2023, 02:49 PM

Thoroughly enjoyed the “white paper, thx so much for taking the time to share. Looking forward to solving this mystery but gonna have to wait a while, since I’m getting ready to head to Sturgis.

Posted by: 76-914 Jul 31 2023, 04:57 PM

@http://www.914world.com/bbs2/index.php?showuser=27135 I just ran across this video last week.It is Kumho Tire Mfg and you will see the split rim machine that checks the tire balance inn this episode of "How a tire is made". https://www.youtube.com/watch?v=GK4IDPpjheA

https://www.youtube.com/watch?v=GK4IDPpjheA

Posted by: Mikey914 Aug 3 2023, 10:00 AM

If you balanced the tires, my money is on the above.
I made the rubber to replace on the tie rod ends because of this exact issue.
It’s pretty easy takes a press. Or you can cheat and use a bench vice.
$22 a set for rubber and new metal insert, or we have completed sets for exchange.

https://914rubber.com/rebuilt-eye-bolt-set-1

This and the ball joints are most likely your problem at this point.

Posted by: Biggles Aug 3 2023, 03:24 PM

If all the obvious things have been checked like people have already suggested, check the caliper pistons arent binding and pulsing on the disc. I replaced wheel bearings, checked wheel balance etc etc and this was eventually the problem.