Vibration from under driver seat

[ryanblair12]

Recently my b7 s4 manual has started to vibrate in my seat. It’s like a shuddering from something under drive seat that is most noticeable around 50mph. It seems to go away at 60mph. I suppose there is minor steering wheel shake as well.
I have replaced control arm bushings with alignment afterwards and recently had wheels balanced rotated. Power steering just flushed as well, Other than that, i haven’t done much to service suspension or steering in the last couple years and car is at 120k miles.
Also I’m not sure if it’s related but the car does make a humming sound (best way to describe it is like blowing into lid of an empty glass bottle of that makes sense?) at lower speeds like 40mph or less. Bottom line the car seems to shake a bit more than usual at medium speeds. Does not seem to make a difference whether in gear or neutral.
Any suggestions where to start guys? Shocks/struts are not leaking. Wheel bearing? Maybe tie rod? Let me know what y’all think...

[ryanblair12]

Also I should say!!!! When tires were balanced the tech said it had a bit of inside shoulder wear.. might have something to do with it?

[idotavant]

Recently my b7 s4 manual has started to vibrate in my seat. It’s like a shuddering from something under drive seat that is most noticeable around 50mph. It seems to go away at 60mph. I suppose there is minor steering wheel shake as well.
I have replaced control arm bushings with alignment afterwards and recently had wheels balanced rotated. Power steering just flushed as well, Other than that, i haven’t done much to service suspension or steering in the last couple years and car is at 120k miles.
Also I’m not sure if it’s related but the car does make a humming sound (best way to describe it is like blowing into lid of an empty glass bottle of that makes sense?) at lower speeds like 40mph or less. Bottom line the car seems to shake a bit more than usual at medium speeds. Does not seem to make a difference whether in gear or neutral.
Any suggestions where to start guys? Shocks/struts are not leaking. Wheel bearing? Maybe tie rod? Let me know what y’all think...

Vibrations are difficult to diagnose without thorough and knowledgeable inspection

If the tires were balanced, we’ll rule that out for now, but it could still be a bent wheel or improper balance

Consistency is a good indication as some vibrations are more constant, while others are a little intermittent

The humming is probably unrelated. Could be a wheel bearing. You may need it to get worse to figure it out

A good mechanic/technician might put it on a rack and go through all the drive shaft/cv joints. A very small amount of play can cause a vibration you describe, without making any noise

Cv axles can often cause an intermittent vibration that can become greater or lesser. The center driveshaft to the rear can also oscillate in its worn rubber carrier bearing mount. This can create a vibration too, but I am less familiar with that

You could check the axles yourself if you can safely get the car up high enough. Yank on each shaft looking for radial (vertical) play. Look for torn cv boots as well. The center drive should droop in its carrier normally. Inspect the rubber carrier for tears. Please be safe under there


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[Rilla]

Was it a road force balance?

[VinnysS4]

Check to make sure the down pipes have plenty clearance around the crossmember.


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[Nitro182]

My downpipes were knocking , this was only at slow speed through town or sat at the lights/a junction. Was a strange one as the knocking increased via rpm , at first i thought it was something serious with the engine.

Id check the prop shaft center support bearing

[Zolli]

My opinions, for a Super-Legit(TM) internet diagnosis:

Humming sound is probably a wheel bearing, because we know for sure my 8-year-old isn’t in the car. So there’s no other source of slightly annoying humming.

Steering Wheel shake I think is due to a wheel out of balance. If you recently had them balanced, my guess is one of the weights fell off. Could be a tie rod end, too. Could also be an air / nitrogen mix from filling the tires, though that would only be noticeable at high speeds.

Related to Vinny’s idea, I had my cats on my downpipes replaced, so the repair weld was a tad off - enough to have the pipes slightly contact not the cross member at the transmission but the flat crossing plate thing that is under the shifter box. It was just enough to transmit some vibration through to my butt. I remedied that by getting a silicone soldering mat (resists temps up to 900*C, cutting it to an appropriate size and shape, and jamming it between the pipe and the crossing thing. Butt soothed, ever since.

[Rilla]

Could also be an air / nitrogen mix from filling the tires, though that would only be noticeable at high speeds.

What?? You need to explain what you are talking about.

[Zolli]

What?? You need to explain what you are talking about.

Sure.

Some places in the States will fill tires with nitrogen gas because it’s lighter than regular air. I guess the idea is to reduce the rotating mass. I’m sure we’re talking about mere grams, but whatever, that’s what some retailers do in an effort to provide “value-added” service. Costco is a big company that does it, as well as Big O Tires, Firestone, and Discount Tire I believe. All of these places offer to “top up” tires for free. But gas stations and typical shops just use regular air.

Whenever a tire is filled with nitrogen, it’s supposed to get a green cap on the stem. Not all places do this, but most do. Regular air gets black caps. If a person replaces their tire stem caps with a sweet aftermarket or vanity cap, then someone else might not have that indicator of which gas the tire is filled with. This can lead to accidental mixing, i.e., nitrogen filled but topped off with regular air, or regular air-filled but topped off with nitrogen.

So if a person has a mix of nitrogen and regular air in their tires, what you have is a certain amount of weight - due to the different weights of the different gases (or basically the non-nitrogen parts of regular air) - that wants to settle to bottom of the inside of the tire. Again, it’s mere grams, so it’s not much and really not noticeable at low speeds, say 30mph or in city driving. It just kind of sloshes around at those speeds.

But at higher speeds, like 50+mph or highway driving, centripetal force causes the greater mass of the regular air to stay pressed against the same spot on the inside of the tire, like swinging a bucket of water real fast around your head: the water stays in the bucket because of this force and you don’t get wet. This phenomenon then causes the heavier regular air to act as an unbalanced weight applied to the wheel and causes vibration at high speeds. I don’t know how much or how little of a gas mix would be necessary to cause the vibration or at which speeds depending on the mix ratio.

Anyway that’s what I was referring to.

[ottocycle]

Sure.

Some places in the States will fill tires with nitrogen gas because it’s lighter than regular air. I guess the idea is to reduce the rotating mass. I’m sure we’re talking about mere grams, but whatever, that’s what some retailers do in an effort to provide “value-added” service. Costco is a big company that does it, as well as Big O Tires, Firestone, and Discount Tire I believe. All of these places offer to “top up” tires for free. But gas stations and typical shops just use regular air.

Whenever a tire is filled with nitrogen, it’s supposed to get a green cap on the stem. Not all places do this, but most do. Regular air gets black caps. If a person replaces their tire stem caps with a sweet aftermarket or vanity cap, then someone else might not have that indicator of which gas the tire is filled with. This can lead to accidental mixing, i.e., nitrogen filled but topped off with regular air, or regular air-filled but topped off with nitrogen.

So if a person has a mix of nitrogen and regular air in their tires, what you have is a certain amount of weight - due to the different weights of the different gases (or basically the non-nitrogen parts of regular air) - that wants to settle to bottom of the inside of the tire. Again, it’s mere grams, so it’s not much and really not noticeable at low speeds, say 30mph or in city driving. It just kind of sloshes around at those speeds.

But at higher speeds, like 50+mph or highway driving, centripetal force causes the greater mass of the regular air to stay pressed against the same spot on the inside of the tire, like swinging a bucket of water real fast around your head: the water stays in the bucket because of this force and you don’t get wet. This phenomenon then causes the heavier regular air to act as an unbalanced weight applied to the wheel and causes vibration at high speeds. I don’t know how much or how little of a gas mix would be necessary to cause the vibration or at which speeds depending on the mix ratio.

Anyway that’s what I was referring to.

You are aware that our atmosphere is made up of 75-80% nitrogen? What is inside the wheel holding these molecules in place so that they can be unbalanced?

[Zolli]

You are aware that our atmosphere is made up of 75-80% nitrogen? What is inside the wheel holding these molecules in place so that they can be unbalanced?

Centripetal force. Outward pointing forces due to angular momentum. To be clear, this concept wasn’t my invention or idea. But it is a thing, I suppose.

[ottocycle]

Sure.

So if a person has a mix of nitrogen and regular air in their tires, what you have is a certain amount of weight - due to the different weights of the different gases (or basically the non-nitrogen parts of regular air) - that wants to settle to bottom of the inside of the tire. Again, it’s mere grams, so it’s not much and really not noticeable at low speeds, say 30mph or in city driving. It just kind of sloshes around at those speeds.

Do you have any idea of the weights you are talking about? you are talking 80 thousands of a gram per tire. Tire balance machines are only accurate to 1 gram. Also the centripetal forces are equal in a radial direction from the axis. So each oxygen molecule will be subject to same forces and will be evenly distributed along the circumference of the tire. There is no way for the oxygen molecules to bunch up and create more than a gram of difference that would be enough to throw off the balance of tire. Your assessment lacks real world knowledge. Go home and make yourself a daiquiri or any other ice drink of your choosing and let me know when it gets out of balance. I'll bring the rum of your choice.

[Zolli]

Do you have any idea of the weights you are talking about? you are talking 80 thousands of a gram per tire. Tire balance machines are only accurate to 1 gram. Also the centripetal forces are equal in a radial direction from the axis. So each oxygen molecule will be subject to same forces and will be evenly distributed along the circumference of the tire. There is no way for the oxygen molecules to bunch up and create more than a gram of difference that would be enough to throw off the balance of tire. Your assessment lacks real world knowledge. Go home and make yourself a daiquiri or any other ice drink of your choosing and let me know when it gets out of balance. I'll bring the rum of your choice.

Shoot, man. Welcome to the internet. There’s a ton of stuff I don’t know, and I’ll bet it’s the same for you. You need to ease up. Even I don’t think that’s a significant factor (I mentioned it was mere grams) and I thought I was clear enough about that. Guess not.

I like Zacapa. I know you were doing your best flame job, but I’m always down for drinks, even with someone being unnecessarily rude.

[Zolli]

I should add: thanks for the correction, Otto

[idotavant]

I think nitrogen is used bc it doesn’t have any water vapor in it and because it doesn’t escape as easily


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[Rilla]

I think nitrogen is used bc it doesn’t have any water vapor in it and because it doesn’t escape as easily


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This is true but good shop compressors have moisture removal. Also, the n2 purity is only going to be in the low 90s. Add to that the fact that shops add the n2 to a tire full of air.

[Rilla]

Centripetal force. Outward pointing forces due to angular momentum. To be clear, this concept wasn’t my invention or idea. But it is a thing, I suppose.

Centrifugal is the one you’re thinking of. But the gases are evenly distributed within the tire… so similar in weight plus they are warm and getting stirred. Think: carbon dioxide is a heavy gas but it doesn’t settle into the ground right?
Also for fun, Air is 28.97 grams per mole and N2 is 28.01 g/mol. A tire might hold a coupe moles of gas when pressurized, so a tire filled with n2 could theoretically be a gram or two less.


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[Zolli]

Centrifugal is the one you’re thinking of. But the gases are evenly distributed within the tire… so similar in weight plus they are warm and getting stirred. Think: carbon dioxide is a heavy gas but it doesn’t settle into the ground right?
Also for fun, Air is 28.97 grams per mole and N2 is 28.01 g/mol. A tire might hold a coupe moles of gas when pressurized, so a tire filled with n2 could theoretically be a gram or two less.


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Awesome. Yes you are right.

[idotavant]

This is true but good shop compressors have moisture removal. Also, the n2 purity is only going to be in the low 90s. Add to that the fact that shops add the n2 to a tire full of air.

Yeah, I don’t think the benefits are worth the trouble, I was just giving the actual reasoning shops use to upsell it. (Not because it’s lighter)


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