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Shimmy Under Braking Conditions

4K views 33 replies 20 participants last post by  Rob@WretchedMS 
#1 ·
I have a severe shimmy when applying the brakes. I have slotted and drilled Baer brakes. I don't see them warping but I would like to check. I have a dial caliper but since the needle would drop into every slot what would be the best way to check. Could it even be crappy tie rod bushings? The bushings themselves are intact and look fine. Input would be awesome.
 
#3 ·
Rotors
 
#6 ·
Material build up causing the pad to get knocked back and forth.

Fix, Turn the rotor and get better pads.
 
#7 · (Edited)
Is this a new problem or have you had it a while?

Did you put the rotors on or did someone else?

Were the hubs cleaned before installing the new rotors?

How many miles on this setup?

Are the pad material deposits uniform across the face of the rotor or do you have banding/streaking?

Are the rotor a uniform color or is there color banding/streaking or spots?

What kind of use do your brakes see (DD, AutoX, drag strip, etc.)?

Are your wheel lug nuts torqued in increments following 30-60-90 ft-lbs in a repeating star pattern?

Bad tie rod ends would probably create wobble all the time, not just under braking. If they look good and the dust jackets look fine and you see no grease then they're fine.
 
#8 ·
I've had a slight shimmy for prob 6k, but it has gotten extreme pretty recently, someone else put them on, no idea if the hubs were clean, 36k about 15k on the rotors, pretty even wear, every other day driver, torqued properly just put new rubber on; that's what she said haha. I'll just have to take the rotors off and take a look at the hubs, rotors etc.
 
#11 ·
Steering wheel shake under braking; GuTOo 2005!

Greetings Guys & Gals; I'm new to forums. I came across this a few weeks ago and it seems to explain my problem. My shake was @ 60+ cruse, then I had them road force balanced twice. Now It only shakes during serious braking and the smoother the surface the worse it seems. If I put OE wheels (48mm BS?) back on, the car is as smooth as glass. The shake is with after market wheels (42mm BS) even though they are lighter!

First, let me THANK the person* that initially put this PROBLEM into words. I don’t know who you are, but Thank you. I changed some of the wording, trying to give this explanation better definition. Hopefully I succeeded! Robert Holden Jr

"All systems have natural frequencies in multiple modes. The suspension is excited by all vibrations, and the system reacts to them. It’s obvious when GM-Holden made this suspension that beyond the first mode the other mode frequencies are low enough to make significant motion in the system. The first mode is the vertical motion acting as suspension. The second and third modes would be front to back and side to side. The Problem is that the system is too sensitive in these directions. As an engineer* I'd increase the stiffness in the front to back and side to side directions. The radius rod would be removed all together and be replaced by a more rigid lower A arm. This of course would be an engineering challenge on a modern GTO. MacPhersons suspensions are known for transferring road feel directly to the driver, and many auto companies use this design for that reason.
Natural Frequencies are always confusing to those who don't understand them, when the system is excited it ALWAYS resorts to is natural frequency. It just so happens if the tire-wheel vibrates at that same frequency, then resonance begins to occur. The issue is the input vibration from the road & or tire-wheel combo is too close to the natural frequency of one or more of the modes of the suspension.
Many owners have removed the input frequency by having well balanced tires and non-worn suspension parts. This stops exciting the system, stopping resonance.
Changing the Strut and spring won’t change the other two axis’s by very much at all. The Radius rod and lower control arm are suspect. You want to move the stiffness up by several magnitudes to stop low frequency vibrations. I'd think the bushings, rod, and arm need to change. The materials and geometry from GM are not performing to our expectations."
 
#13 ·
Rotors need replacing.
 
#14 ·
steering wheel shake

Greetings Guys; Now that I’ve figured out how to get back to this spot on the forum. If rotors are my problem (.001”-.003”driver & .001”-.002”pass side run out) why would the shake disappear when I put the OE 17” wheels and tires back on?? And I’ve never seen any other car over sensitive (key word when owning a GuTOo) to .003” of run out. If in fact it is a frequency problem it will be tough to fix. I read at least 3 different forums looking for guys that have had or have this curse and have the guys been successful fixing it long tirm? I’ve been told that I’m anal (analytical) you ready, this is my ritual of torque’in lug nuts. Star pattern of course 1st cycle 46 lb ft; 2nd cycle 76 lb ft; 3rd cycle 91 lb ft and finishing up in a circular pattern to 96 lb ft. That’s my way of checking that I didn’t miss any in star pattern. I know I’m nuts! Anyway you might say that it is the aftermarket wheels (17” x 7.5” x 43mm BS?) Elbrus and tires from Tire Rack. I should mention 31,000 little ole man miles on the GuTOo, brakes (all my cars of the last 30 years) usually last me 60,000 to 80,000 miles. I live in the country and don’t charge red lights. That Elbrus set has been balanced 5 times and tires replaced once. We chased the shake from 58-72 mph cruise to smooth cruise, I’m real happy but not for long. And now with 2000 miles I’ve got this steering wheel shake, but only when I get on the brakes firmly. I’ve just Had it with the Tire Racks Chinese wheels. I like the look of the Chinese wheels, their lighter, clean up easer, but the shake or the wheels gotta go. Please note Holden center caps, I drew (ultimate GTO website yr 05 page 29 to see drawing) up some adaptors and had the local shop make them out of black nylon. And Thank you Rob@WretchedMS for the rotor suggestion. Please everybody all suggestions welcome. I’ll try two things this spring more air, I usually run 33 psi front. Stand watching the center cap while wife or buddy hammers the brakes from 5mph. And if the center cap moves back, chime in guys! I’ll try posting this in a couple of places in hopes that I’m successful placing it in one place. PS or BS: my 19th cousins back 500 years ago give or take a 100 were sent down under because they were undesirable, I'm from the desirable side of the family. Thanks guys Robert Holden Jr.
 
#17 ·
front end frequency

Greetings Rob & guys; I'm not sure what I'm asking – looking for either! The suspension is OE except for the rear springs (Kings). What if I replaced – up graded (brand?) the radius rod bushings? I've read that some guys have more shake after bushings! Just in another guy says .003” is too much run out on the rotors. OK, but if they are the problem, why does it disappear when I re mount the OE 17”? Thats why I don't think its rotors. I sense that you (Rob) have had about 500 times more experience with these cars than I have. So let’s explore the possibility of a frequency problem, humor me. Does anyone make a radius rod that is of a heavy gage? Or a better – thicker lower control arm? I’m thinking that could be a way of changing the frequency of the front end. Many thanks Bob Holden Jr.
 
#20 ·
Greetings Rob & guys; I'm not sure what I'm asking – looking for either! The suspension is OE except for the rear springs (Kings). What if I replaced – up graded (brand?) the radius rod bushings? I've read that some guys have more shake after bushings! Just in another guy says .003” is too much run out on the rotors. OK, but if they are the problem, why does it disappear when I re mount the OE 17”? Thats why I don't think its rotors. I sense that you (Rob) have had about 500 times more experience with these cars than I have. So let’s explore the possibility of a frequency problem, humor me. Does anyone make a radius rod that is of a heavy gage? Or a better – thicker lower control arm? I’m thinking that could be a way of changing the frequency of the front end. Many thanks Bob Holden Jr.
Cliffs: It's your Chinese wheels, possibly including tires now. And Rob has at least 100 times more experience than you give him credit for. Don't go messing with any more stuff until you understand this. You say the 17" stockers are fine. There's a hint.
 
#18 ·
Why would you entertain replacing the radius rod instead of the radius rod bushings? Talk about throwing the baby out with the bathwater. Again, replace the bushings and turn the rotors.
 
#21 ·
front end frequency

Greetings Guys; Mother nature took the wind out of my sails and she forgot to bring in spring!
Just a little sarcastic humor, I only gave Rob 500 times more experience than myself and Wang feels that I shorted Rob credit by 100 times. So I would like to make this right. So Rob I now grant you 5000 times more experience that me, hopefully that will be enough.
I've given my question more time, I love these forums. Yes of course new bushings, does it need to be said. I'm long winded anyway, and some of you mite say that's an understatement. I've found at least 6 different styles of radius rod bushings maybe more? So please chime in and recommend a bushing set that would be suitable for a late middle aged (67 year old) hot rodder. lets put the past (1969) in perspective, a ram air IV off the show room floor would turn a 14 second flat. Even a stock 2004 would blow the 69 away, so take note guys we are living the golden era of hot roding NOW.
Back to radius rods, I see that at least one or more after market rods are out their. My continuing education will be to measure the diameter of a stock rod and look for a thicker one. Any of you guys have this info. Thanks for your help guys, R Holden Jr.
 
#22 ·
You could start by calling Rob @ Wretched Motorsports again. Competitive lines of bushings are sold by the sponsors Duckman @ Kollar Racing, and BMR Suspension. There may be other sponsors as well, not meaning to exclude anyone.

Congrats on driving so fast for so long and living to tell about it!!! :thumbs:
 
#23 ·
the shake

Greetings Guys; Thanks for the thoughts Wang. And I did call Rob, also picked his brain, Thank you Rob. Even with his (Robs) help I'm still undecided what path I will take. I have 4 additional scratched 17" OE wheels now that I will be refinishing and removing that damn corner that is so hard to clean. Since I have NO shaking with my 17" OE set that still have 2 of the OE tires mounted on them. I'm considering putting my sticky tires on these refinished wheels and selling the Elbrus wheels with the center cap adapters (fits Pontiac - Monaro caps). The other choice will be to continue running the Elbrus wheels and try fixing shake with new RR bushings, turning and checking rotor balance. Tomorrow the weather mite be good enough to take the car out and do the stand on the brakes @ 5-10 MPH test watching the hub center. So all you guys that have replaced the RR bushings please tell me the brand you like and why, I would like to hear the pros and cons of the various brands please. Thank you guys, Rob Holden Jr.
 
#24 ·
Good evening; The roads are clean enough to put some miles on now. So I did the 3 to 5 MPH stop test watching the center of the front hub and it moves back over an inch! Looks like my 31,000 OE RR bushings need replacing. One of the many guys that helped called my shaking steering wheel a shudder. What ever ya call it I hope new bushings will fix it. Thanks guys Bob Jr.
 
#29 ·
Rotors don't warp. They get uneven pad deposits and wear.

I have the steering wheels shakes when braking on my 05 gto, It happens at around between 40-60mph something like that, anyways maybe the rotors are warped.

I bought the new rotors and pads from ebay about 6 months ago and they are normal blank oem equivalent parts. When I first put them on the steering wheel shakes wernt their it was smooth braking but after about 2 weeks the shakes came on. I thought I bedded them and was easy on not braking to hard for the first few days since the rotors and pads where new.

If i have the rotors resurfaced does that weaken the structure of the rotors?
No, you can resurface them just fine. In the future, buy better pads. Bad pads are the main cause of brake shimmy. I ran EBC Yellow pads for 25,000 miles without a hint of brake shimmy. Slapped on some Advance Auto Parts ceramic pads and all of a sudden I've got nasty brake shimmy and a nice squeal to go with it.

It's all about the pads.
 
#28 ·
I have the steering wheels shakes when braking on my 05 gto, It happens at around between 40-60mph something like that, anyways maybe the rotors are warped.

I bought the new rotors and pads from ebay about 6 months ago and they are normal blank oem equivalent parts. When I first put them on the steering wheel shakes wernt their it was smooth braking but after about 2 weeks the shakes came on. I thought I bedded them and was easy on not braking to hard for the first few days since the rotors and pads where new.

If i have the rotors resurfaced does that weaken the structure of the rotors?
 
#31 ·
Since my issue is pads I got a quote from Julian at Harrop Performance Products for pads on the VT-VZ performance brake kit.

Front DB439 $ 130 AUD
Rear DB440 $100 AUD
Plus Freight

These pads are Bendix ultimates and only available through Harrop.

It may be time to convert to something else.
 
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