Wheel pulsations while braking

[oldalbert]

I have been trying to analyze this problem. While braking down from 75 or so mph the suspension sometimes throbs rapidly and quite strongly - but not at all times. I have experimented with wheel balanciong and tire rotation, and I have come to think that it happens only when the tires are quite hot from hard highway driving on warm days.

So I had all four wheels rebalanced, and replaced on the same corners that they were on before the rebalancing. And filled with nitrogen gas. Today I drove all day, hard, without any pulsating. I think it may be because nitrogen does not increase pressure so much with temperature as does normal air. My theory so far is that the Goodyear tires come out of round when the tire pressure is high from heat.

I'm wondering whether anyone else has had related experience. W-rated Goodyears on 17 inch rims on 2001 STS.

By the way, the nitrogen does give a smoother ride on the highway.

[TDK]

Pulsation while braking is typically from brake disc's that are warped.

Have you had the disc's checked for warpage or other damage?

It might be time to get 'em turned true as part of a brake job. A good shop will let you know if they can be re-used.

Plenty of opinions at this site on what pads to use, "break-in procedure", etc.

Brakes have oppertunities to work differently due to heat, dust, etc - so the problem might be somewhat inconsistent or masked by other issues (such as wheel balance).

Some weird kind of front end linkage problem might be involved, but I really doubt it.

Have you tried less braking ?

Just kidding...

Good luck

[Scotty]

I have been trying to analyze this problem. While braking down from 75 or so mph the suspension sometimes throbs rapidly and quite strongly - but not at all times. I have experimented with wheel balanciong and tire rotation, and I have come to think that it happens only when the tires are quite hot from hard highway driving on warm days.

So I had all four wheels rebalanced, and replaced on the same corners that they were on before the rebalancing. And filled with nitrogen gas. Today I drove all day, hard, without any pulsating. I think it may be because nitrogen does not increase pressure so much with temperature as does normal air. My theory so far is that the Goodyear tires come out of round when the tire pressure is high from heat.

I'm wondering whether anyone else has had related experience. W-rated Goodyears on 17 inch rims on 2001 STS.

By the way, the nitrogen does give a smoother ride on the highway.

In my opinion this has nothing to do with you tires, especially since it occurs during high speed braking. I have the same problem. I have improved it quite a bit by RE-BEDDING them which basically cleans off the rotors and provides a cleaner surface. Do a search on bedding the brakes it has been discussed here quite a bit. If re-bedding the brakes does not help you may need a clean up cut done of your front rotors and then bed them. Read this article it explains why you are getting the pulsing, its due to a transfer of brake material to the rotors causing a grabbing and thickness deviation.., Mike

http://www.stoptech.com/whitepapers/warped_rotors_myth.htm

[JasonA]

So I had all four wheels rebalanced, and replaced on the same corners that they were on before the rebalancing. And filled with nitrogen gas. Today I drove all day, hard, without any pulsating. I think it may be because nitrogen does not increase pressure so much with temperature as does normal air.

My theory so far is that the Goodyear tires come out of round when the tire pressure is high from heat.

What, are you kidding? Nitrogen? I've never heard of such a thing. Where do you get nitrogen...and who had that idea? Did your tire shop recommend that? I've never heard of using nitrogen in tires before. Interesting!

It's my theory that the Goodyear tires come off any car I own/buy, and are then replaced with an equivalent size of much higher quality (which can usually be had for LESS than the price of the Goodyears).

[oldalbert]

Scotty and TDK:

It could be rotors as you suggest, but I don't so far think so.

The rotors do have some hard spots, but not seriously so. If rotors were the problem I think it would always be there, but is is not. It is too early to tell yet, but it does appear so far that the problem has been eliminated by the "softer" tires i.e., not so "pumped up" by temperature induced pressure increase. I have tire pressure sensers on my car, and the pressure hot stays pretty close to the pressure cold, when nitrogen is used. (Hence also, softer highway ride).

I noticed last summer that when the braking pulsations were so noticable (hot days, highway driving), also the wheels were pulsing a bit when I was not braking.

Which is what got me to thinking it was a going-out-of-round problem.

I have the impression from reading that this problem is worse with Goodyears, and relatively rare with Michelins. In fact is seems to be almost characteristic of Goodyears to be difficult to maintain balance. It is a new idea to consider. How about you trying the nitrogen too, and reporting on it?

[oldalbert]

Jadcock:

Search Goole "nitrogen tires pressure". There is lots of info there. Trucking firms have been using it for years.

[TDK]

Be dang carefull using nitrogen in an enclosed area!

It is essentially oderless and not readily detectable. It has the ability to displace air very efficently - in particular inside your lungs. A fellow worker and I were sampling pyrophoric catalyst (catches on fire is exposed to oxygen) insided a reactor that was blanketed (left with only N2 inside at very low pressure) by nitrogen (a common practice in chemical plants). He leaned over the open manway and fell over onto the grating! It only took one breath to knock him out! He recoverd but another guy I knew was not so lucky.

Years later at another facility, He was in an instrument shed and some nitrogen leaked from a tiny fitting while he was working on something else. They found him dead in the shed - after he missed lunch.

It can take your breath so easily and painlessly, that you don't even know what happened...

Yes, tire oxidation and inconsistent pressure changes is somewhat of a minor problem. Racers and truckers have used this inert gas for years. www.tirelast.com

I'd be dang carefull around it! More use in the general public can only result in some "problems". IMHO <_<

I still think you should have the roters/pads, etc checked. There could be one dragging a bit at times.

[Ranger]

I can recall tire stores offering nitrogen fills 30 yrs ago.

[bigfoo]

I get some 'grinding' and svery slight pulsation when hitting the brakes at low speed.. I can't figure out what it is and all the brakes look great and they stop just fine from high speed..... I think i need a new motor mount or something

[Ed Fenwick]

Aloha oldalbert,

I just replaced the front rotors and pads on my '92 Seville. The job was easy, took a couple of hours including cleaning the backs of the wheels. That took care of the pulsating problem. Try it you will like it!

Ed

[Bob D]

Everyone of my STS's "'92,'93 and 2000) all exhibit these symptoms about once a year or so..pulsing brake pedal. Sometimes it's the front rotors, sometimes it's the rear, but they ALL do it. I can't really tell you how many miles elapse between each episode, it just seems to happen to each one now and then. Turning the rotors cures it every time. The rotors have never been "warped", they just need cleaning up. I know it's not the pads, as I have varied the brand and materials (ceramics) over time.

Driving style maybe?..Hmmm...it does seem to happen to the ones my wife's been driving lately..

The '93 is due now, new pads too, along with a complete change over to Arnot passive shock/strut replacement..Starts tomorrow.

[DocFey]

I agree with the rotors. My 96 was doing a high speed stop pulse-my wife never felt it because she drives like your grandmother. I had the front rotors turned(10$ ea) then took off for Colorado and never had it again(didn't change pads). That was a year ago so I put on new rotors and brakes before going to the Colorado mountains again a couple of weeks ago. Bedded the pads and took off. Mountain driving can smoke your brakes but the $21 chinese rotors and $44 Wagner pads I installed worked great. From now on, I'll just put new rotors on for that price rather than turn the old ones. I put the pads and rotors on in an easy hour or so and had to dog it a little so my wife would get done mowing the lawn before I finished my task. I know some of you will howl about the cheap rotors but they work for our driving habits. If they work coming down off Trail Ridge Road from 12,000 feet, they are OK by me.

[oldalbert]

I think everyone is right - that the problem derives from the interaction of wheel unbalance and rotor hard spots.

When the wheel is "bouncing" due to unbalance, the brakes are able to slow the rotational speed of the wheel more while it is in light contact with the road, than when it is in heavy contact - so there is a rotational velocity variation as well as wheel unbalance motion. I suspect it is fore/aft wheel motion because the strut shock absorbers don't seem to damp it out. I think that the damping comes only from the mushiness of the contact between tire and road. The rotor frictional variances exacerbate the problem, and in the absence of significant damping, the amplitude becomes high. There is more rubber movement when the tire pressure is lower, and so more damping, which might be why the problem is much worse when the tires are hot.

The nitrogen definitely fixes it. Before nitrogen a great deal of vibration. After nitrogen, none. Also, since the tire pressure does not increase much with temperature, the ride is noticably smoother.

This is all theory of course, but borne out by experience.

I believe you that turning the rotors sends the problem away for a while, but the nitrogen is almost free, doesn't seep out so much where the tire meets the rim, gives a better ride, and just may be a permanent cure.

If anyone else is interested enough to try it, I would like to read their posts.

[RalPh]

Before you cut the rotors........

Do a search on "bedding brakes" here. Some very good information. Rotors don't really "warp", their surface becomes an uneven texture and proper re-bedding cures it.

Its easy to do with repetitive hard stops (you actually smell brakes, and see a little smoke from them).

Everytime the pulsations become annoying (twice a year) I simply go out and abuse the brakes for a while and it cures the problem

EDIT - Ooops, I tried the search I suggested above - musta been on the "Old" forum.

Anyway... hese's it described by our "Local Guru" - bbobynski

Originall posted by bbobynski

I doubt that your rotors are actually warping and it is

most certainly not from too much heat.  It is more likely a

variation in the surface finish of the rotors caused by rusting

of the rotors during intervals when the car is parked.  The rust

will clean off at the first brake apply but the sub-surface

pitting remains.  Over time this can/will get worse and worse and

since the rusting will not occur uniformly (the area where the

pad contacts the rotor during the parked situation can remain

clean or corrode worse depending on the conditions and moisture

when the car was parked)...  As the surface finish degrades over

time due to repeated rustings and cleanings by brake applies the

surface finish can get bad enough and non-uniform enough to cause

a roughness in the pedal and during brake applies.

I think this is your problem because even in cases where the

brakes are abused beyond belief during testing the rears seldom

if ever require any attention.  The fronts do 65-70 percent of

the work stopping the car so even if you beat the brakes the

rears rarely if ever care.

The solution may be as simple as using the brakes a little more

aggressively.  It doesn't hurt the brakes to use them.  The is a

wide band of "normal" or acceptable usage between "too gentle"

and "too hard" so it is unlikely you will hurt the brakes unless

you do a lot of stopping from 120 MPH ....

I have the same problem with my wif'e Corvette....she is not a

hard braker and the brakes need to be used hard occasionally to

clean them up and get any contamination off the pads.  The

brakes will get rough on her car from "lack of use" and I will

have to get them turned litely to clean the surface up.

It is very easy to check for a "warped" rotor by taking the tire

off, replacing several of the lug nuts to hold the rotor in place

and turn the wheel using a dial indicator and/or a fixed pointer

to dtermine runout.  I doubt you really have a warped rotor.  I

have seen MANY "rough" brakes fixed by lightly cleaning up the

rotors on a brake lathe even though there was no "runout" per se.

The rears are more susceptable to this because most of the

braking is done by the fronts so if you do only very gently stops

then the rears are hardly getting used at all and never clean

themselves up.

You can verify this by lightly applying the parking brake while

rolling down a side street with no traffic.  This applies only

the rear brakes and you can then fell the roughness in the rotors

and/or clean up just the rears.  When some of our development

vehicles are parked for a long period of time the rotors will get

rusty like this and I have had to make repeated stops from 35-45

using only the parking brake to cllean them up.  You might try

this to see if it helps.

Additionally, make it a point to occasionally do 10 moderately

hard stops from 45 or 50 to zero back-to-back.  Drive around for

10 minutes to cool the brakes and then  do 10 more stops from 45-

50 and let them cool again.  This beds in the brakes and bakes

any contaminants off the pads and rotors.  It is actually

beneficial to brake performance and brake life as it will prevent

any glaze from forming on the rotors and pads from the rosins in

the pad material.  If the pads and rotors "glaze" due to too easy

of a duty cycle then the temperatures actually can go higher than

normal due to the inefficient braking action on the glazed

surface.

Brake pads are cheap.  Use the brakes hard occasionally when the

oportunity presents itself.  I find that a car that is driven

every day and has a constant moderate brake duty cycle tends to

be much more stable and smmother over the long haul.  Parking the

car intermittantly for long periods (at the airport for several

days every week ) combined with very light brake applies is about

the worst situation for rough brakes aside from use on the

racetrack.

Next time, anyway, have the rotors turned instead of replacing

them as that only costs 15 to 20 dollars and you can turn a rotor

several times over it's life.  The dealer or garage will make

more selling you new rotors but they generally aren't required. 

Besides, if they do "warp" they will typically tend to warp much

less after they have been thermocycled for several thousand miles

and then trued by turning.  The heat stress relieves the rotor

and they will be much more stabile dimensionally  after being

stress relieved and trued.

Hope this helps....

[Scotty]

I think everyone is right - that the problem derives from the interaction of wheel unbalance and rotor hard spots.

When the wheel is "bouncing" due to unbalance, the brakes are able to slow the rotational speed of the wheel more while it is in light contact with the road, than when it is in heavy contact - so there is a rotational velocity variation as well as wheel unbalance motion. I suspect it is fore/aft wheel motion because the strut shock absorbers don't seem to damp it out. I think that the damping comes only from the mushiness of the contact between tire and road. The rotor frictional variances exacerbate the problem, and in the absence of significant damping, the amplitude becomes high. There is more rubber movement when the tire pressure is lower, and so more damping, which might be why the problem is much worse when the tires are hot.

The nitrogen definitely fixes it. Before nitrogen a great deal of vibration. After nitrogen, none. Also, since the tire pressure does not increase much with temperature, the ride is noticably smoother.

This is all theory of course, but borne out by experience.

I believe you that turning the rotors sends the problem away for a while, but the nitrogen is almost free, doesn't seep out so much where the tire meets the rim, gives a better ride, and just may be a permanent cure.

If anyone else is interested enough to try it, I would like to read their posts.

I respectfully disagree with your theory. Tires can have a multide of problems, some of which are out of balance side to side, out of balance up and down, out of round, wheel run out, to name a few..

You are talking about two different problems 1) an out of round or out of balance tire and 2) rotors that have material transferred to them causing braking roughness. MAYBE you have both problems but I tend to doubt it. However I can tell you that the ROTORS WITH BRAKE MATERIAL transferred to them DEFINATELY causes the high speed braking vibration SO BAD that its scary, but only on braking. IF you did in fact have a tire out of round or out of balance you would get the vibration whether you HIT the BRAKES or NOT....are you getting vibration without hitting the brakes? Look toward the rotors, did you read the article I posted for you?

What you are theorizing is not tire unbalance, but tire out of roundness, that is magnified by braking, that is doubtful, those out of round tires would pound regardless of whether or not you were braking. RE-bedding the brakes to clean them up may also help your problem as someone above noted, it helped me... Mike