Why do wheels shake at specific speeds when a car’s alignment is off?

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Why is there a sweet spot (50-60mph) where a vehicle will shake when the alignment is off? How does the shaking start around 50/55 and get better after going over 60?

In: Engineering

4 Answers

Anonymous 0 Comments

It is caused by harmonics. The wheel actually shakes the whole time you’re driving, but there is a certain frequency of vibration that causes harmonic resonance, and the fact that a vibration shows up much worse at certain speeds is a good indicator that you have harmonics playing into the vibration mix. What happens is that as the tire rotates with either a misalignment or damage to the tire or wheel itself, it vibrates. As the speed increases, so does the frequency of the vibration. There is a certain point at which that frequency will cause sympathetic vibrations in other parts of the car, and that’s what you’re feeling when it really shakes at a certain speed, and you get out of that harmonic range when the vibration stops at a higher speed.

Anonymous 0 Comments

This happens when you hit a resonant frequency and the misalignment synchronizes with your suspension.

When you hit a bump, your suspension is going to bounce, and each bounce will take about the same amount of time, no matter your speed. A misalignment is going to cause a small bounce, the faster you go, the more often it happens. When these two times match up, they reinforce each other, and you get big suspension bounces. Then when you speed up a little more, they no longer match up, and the bouncing is not so bad.

You’ll often see the same sort of thing driving on washboard gravel or on a concrete highway with evenly spaced cracks between the blocks.

Anonymous 0 Comments

It usually has nothing to do with alignment, but balance (imbalance actually) of the wheels and tires or other spinning driveline components.

The “sweet spot” is when it hits a resonant frequency that makes it most noticable. Above or below that and it gets absorbed or dampened by other vibrations.

In the rare event you do feel a vibration caused by alignment would most likely be attributed to extreme toe-in. Camber and caster aren’t going to cause a vibration. In the case of toe, if it’s so far in that the tires are scrubbing along the pavement, that could potentially be perceived as a vibration in the steering wheel, but it’s not common, and it’s more likely to just wear the tires prematurely.

What you’re describing is very typical of first order driveline vibrations.

Anonymous 0 Comments

The mis-alignment is a minor part of the situation. ALL rotating objects have what is know as a “Critical Speed” where they will vibrate, (or try to, if sufficiently mounted to prevent the critical speed vibration from causing physical movement). Even huge power plant turbines have a critical speed and the proper method of dealing with that specific speed is to accelerate quickly through it. Bad alignment just allows the critical speed to become quite noticeable.