Road Force Balance and what it means to vehicle dynamics

I have a 2004 Monte Carlo SS with the FE3 (I believe) suspension package. I am chasing a bounce in the car that can be felt at low speed but goes mostly away at 60 + MPH. This is not a consistent bounce, it is worse at lower speeds. I cannot remember how long this has been going on. The car has 86K miles and I keep this car maintained. The wheel bearings were replaced at 60K miles. I replaced the axles last week because there was movement in the shaft I could visibly see. I replaced the lower engine mounts and the did a common modification of flipping the radiator support mounts to tighten up the motor in the cradle. The dog bone mounts seem very tight. The tires are new because the old ones were worn past my comfort level. They have around 3000 miles on them. They were installed using the GSP9700 balancer. The pass criteria for this shop was 26lbs of force measurement on the machine. One tire was 14 lbs, the other 3 measured very near the limit all over 20 lbs. The factory struts are still in place and very stiff.

Finally my question, can this 20 + lbs of force cause a bounce in my seat at speeds as low as 20 MPH due to the stiff suspension on this SS package car?

Each car is different when it comes to how much road force variation tolerance there is… The 26 lbs spec the shop has provided you is just an all around maximum average… Some vehicles can handle significantly more, others can only handle force variations in the upper single digits. In my opinion, 26 lbs is too much for some pickup trucks to handle, let alone a car.

Another issue to keep in mind is that you have three tires in the 20 plus force variation range… Each tire vibration produces a sine wave (Graphical view of the vibration over time) There are times where the peaks of each sign wave match up with each other… its like putting batteries together in series… the amplitudes stack up… When this happens the vibrations can become severe and when out of phase, feel not as strong.

Excessive Road force variation will result in a rhythmic vibration that is directly proportional to the rotation speed of the tires. If we think about it, if one tire has a one hard spot or high spot, the tire will create a vibration one time for each rotation of the wheel… if there are 2 hards spots that for each tire rotation, there will be two vibration pulses. When you say your seat bounces… I am assuming it is a continuous shaking/vibration?

Typically runout issues will generate slower speed vibration like you have indicated… This would include bent rims, bent axles, improper wheel/hub centering. etc. Again there are exceptions, each car can respond a bit differently. Yes excessive road force variation due to just the tires can also create such vibration.

The GSP9700 balancer can be an art form to use properly… Yes it shows each wheel/tire assembly out of balance level as well as force variation, it can even show you whether the rim itself is any good (Bent, out of round) Setup is critical when using these machines, there are proper wheel to machine adaptors that are required for proper wheel centering. Tires may need to be match mounted to the rims to acquire the best road force variation numbers. All of these requires a trained operator… Today tire balancing has become an art and science, not a job just anyone can do anymore. The GSP9700 also will provide the road force variation print outs for each wheel tire assembly tested. (Did you get them) Were the tires force matched?

I have a few more questions for you…

One: how long has the vibration been there, i know you have your not sure exactly how long it has been. Was there any change in the vibration characteristics with the new tires (Better/worse)?

Two: Is the vibration worse first thing in the morning when the vehicle/tires are cold? Does the vibration diminish as you drive?

Three: Can you feel the vibration in the steering wheel or is it just in the vehicle body/seats? Has performing a wheel/tire rotation changed the vibration characteristics?

I agree with jgree. The problem is not the road force value which you would feel at higher speeds. At low speeds you are looking for something else. Have the techs measure your bare rims for run out. The GSP9700 has a procedure to do that. If they don’t know how, it’s time they learned.

But, honestly, I think the problem is not tires and wheels. With the values you’ve quoted, you should be feeling some vibration in the 50 to 70 mph range. Since you aren’t reporting that, I’m thinking the problem at low speeds is something mechanical - driveline, trans - something like that.

Check for liquid in a tire.

Let me preface this by saying the issue is there intermittently. It is felt at higher speeds but most notable at low speeds. If you have ever driven through a parking lot of Walmart at the pedestrian crossing and noticed the paint stripes bouncing your car that is about the amount of bounce I can see at 20. This cars suspension is stiff. It is completely factory and so are the wheels on this car.

  1. How long has the vibration been there, I am not positive but I have noticed it ever since this set of tires have been on the car or since around December last year. I had not noticed the vibrations before the tires were exchanged but the old tires were very noisy and cupped from a missed tire rotation early in the tire life. I am not 100 % positive that it started at the tire change.

  2. I have not noticed a diminish during driving. They are about the same cold and hot. Maybe even a little worse after being driven. I noticed yesterday that after about 4 miles, they were pretty bouncy at 20 MPH in stop in go traffic.

  3. Rotating tires did nothing to the feel. Sometimes I feel it in the steering wheel but mostly in the seat. It is not a small amount of bounce when it is at its worst. It can ride like glass on some of our newer streets one day and bounce the next on the same stretch of road.

I watched this tire shop owner install the tires on my car. I watched the tire balance machine as he used the tools to measure the rims, with tires on, and then had to break down the tires and rotate them on the rims. He worked one tire 3 tires before the machine claimed it was under the 26 lbs limit. The rims appears straight and true on the tire machine.

The car spent 3 weeks with the only transmission shop I trust. They replaced the axles, checked the output extension for twist and warp. They checked the differential for signs of uneven wear. They are the ones that changed the motor mounts. I can feel a difference in the car that the mounts needed to be changed because of other dynamics that are different. The oscillatory period between high and lo points is also the same between OD and D when driving at higher speeds. I am just not sure what to chase now. The period between bounces is so slow that tires is the only thing that comes to mind now.

“I watched this tire shop owner install the tires on my car. I watched the tire balance machine as he used the tools to measure the rims, with tires on, and then had to break down the tires and rotate them on the rims. He worked one tire 3 tires before the machine claimed it was under the 26 lbs limit. The rims appears straight and true on the tire machine.”

This may seem obviously simple, but I’d like to share my story on this. I had a similar experience with trying to get a tire balanced. After balancing and mounting, the out-of-balance condition would re-occur… back on the balancer… This happened a couple of times, each time the weights would be placed in a different location. Soon discovered that the cause was dut to the procedure for deflating the tire to reposition it. When the mechanic used the valve stem extractor, the inside nub remained in the tire… free to float around. By the time he discovered this, he had three nubs in there. This is typically not a problem when changing tires because the nub doesn’t remain behind.

Would you hear it rolling around inside when you pull the tire off and shake it? That will be an easy check.

He did that after try Number 3. That is how he discovered it.

I took the car back to the same tire shop 1000 miles after the tires were installed. He checked them again on the machine with the same results as the first check. No change in tire weights… I found that slightly odd but just went with it since it was just 1000 miles of driving.

This may seem a bit obvious, but are you using the inflation pressure on the vehicle tire placard? It’s usually located on the driver’s door post, but sometimes in the glove box.

If you were using the max written on the sidewall - like 44 psi - the tire would feel “bouncy” at slow speeds and would seem to smooth out at speed.

The car claims 30 PSI for the tire pressure on the information sticker and I have tried 32, 30 and 28 with almost no difference that I can tell. Now, is my tire gauge correct may be a different matter.

I have a 95 Crown Vic that we bought in August for my son to learn to drive. The Vic sat still for 4 years with a bad motor. We replaced the engine and drive the car regularly. The flat spots on that car bounce very similar to the Monte Carlo. That is another pointer to the tires. I just replaced the tires on the VIC this weekend and it all went away.

If a rotating, perfectly balanced tire produces over 10 pounds of any kind of variation, I would call that tire defective…Tire manufacturers strive to build tires with no belt overlaps or ply variations which will produce tires that will not roll smoothly and create suspension and or steering inputs…

If one of the tires was hard to balance I’d take the car back where they installed the tires and have them check them to see if one of them is out of round. I remember my dad putting a new set of tires on a car one time years ago and there was a vibration when driving, he took the car back and they found an out of round tire.

At 20 mph, the tires are hardly doing anything - they are more like solid wheels - and therefore, road force isn’t a factor. If you are feeling a boucniness at that speed then it is something other than the tires. I wonder if the low speed circuit in the shocks has failed. That would make the shock stiff for slow inputs, but normal for the usual and high speed stuff.

This was the first set of tires I have ever had installed and balanced using this GSP9700 machine. I didn’t know to ask for printouts on the results for each tire. I will in the future.

FordMan1959, the tire that was hardest to balance did end up getting replaced. It was 35 lbs of force at its best point. I just pointed out that the shop owner went through the process of trying to use the tool to check the run-out of the rim and re-installed the tire multiple times on the rim. He appeared to know what he was doing. These are still suspect in my mind.

CarpiRacer, I go back to the analogy of driving over the painted stripes on the ground at Walmart. You can almost not feel them when walking over then but driving over then that thickness is enough difference that makes my car bounce from just that deviation. If the Road Force was the measure of run-out that deviation would be a similar issue.

I have been wondering how struts fail and the characteristics of a failed strut. I knew most get softer and become unable to dampen the spring changes. I was curious if the strut could become too stiff. The ones on the rear feel as though they may be locking up. It is very hard to get motion from them applying the Heimlich to the interior of the trunk…

I was also wrong in my first post above. This car has the FE4 suspension package according to the RPO code label in the trunk not the FE3. That is probably an even more tight suspension package from what I can gather from the RPO codes listings found on the web…

Gentlemen, thanks for all the comments on this. I do appreciate the advice and ideas.

Shocks (struts) can fail both hard and soft. There are also different circuits for different shock travel speeds. When driving slowly, you use the low shock speed circuit and when driving faster, you use the high shock speed circuit. Either circuit can fail and make the failure only apparent under that one situation.

I’m inclined to think this problem is related to a shock, strut, bent wheel rim or hub, or possibly even a dragging brake caliper.

I went back to talk to GM on this. The shop foreman was out to lunch but later the service adviser said he was pretty certain that GM sets the limit for a road force balanced tire at 22 lbs. I have 2 over that limit I believe. I will talk to the shop foreman later today. The Adviser also said the FE4 was stiffer meaning less tolerant to tire vibrations. This could be the answer I was looking for. Now how to get the tire shop and the manufacturer to work on this 25 lb limit issue.


No, that is NOT the answer. Your problem is at low speeds. Balance and force variation cause high speed vibrations. The tires are NOT the source of your problem. Continuing to pursue road force is going to be a waste of time. Spend your time and money looking elsewhere - and shocks is a likely place to start.

Even if the tire appears to have excessive runout?