Steering wheel shimmy |
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Steering wheel shimmy |
rbutler914 |
Jul 27 2023, 09:43 PM
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#1
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Newbie Group: Members Posts: 4 Joined: 5-June 14 From: Jackson, CA. Member No.: 17,439 Region Association: Northern California |
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. |
technicalninja |
Jul 30 2023, 12:45 PM
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#2
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Senior Member Group: Members Posts: 1,319 Joined: 31-January 23 From: Granbury Texas Member No.: 27,135 Region Association: Southwest Region |
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. |
Superhawk996 |
Jul 30 2023, 02:15 PM
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#3
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914 Guru Group: Members Posts: 5,891 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
I don't know WHY it's always above 50 but it is... 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. |
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