Wheel Hop

[spd4me]

For the past 3 years I've struggled with excessive wheel shimmy, shake, bounce, whatever. Have had the wheels balanced several times without any relief. A friend started working at new dealership and I took front wheels Compomotive 15" 3 piece with B.F.Goodrich 245R/60 15" for him to look at. Their Hunter GSP 9700 balance machine showed a "Road Force" of 47 pounds which means as the wheel spins, where it is not round, if exerts 47 pounds of force down on road. He also checked the rim which was .038" out of round (up and down as rim turns) and .023" side to side. He put wheels on machine 3 times just to make sure it was on right. Before I check with rim manufacturer, has anyone else had this problem? One solution is to shave tires, if you can find anybody that does that any more.
Bob

[bstandley1]

Bob,

I'd recommend a search on this site for wheel balance or road force variation; there are many informative posts. Based on my personal experiences, your tires have way too much road force variation. The tire that repeatedly surfaces as the one with consistantly lowest variance is the Yokohama Avid S/T .... Hope this helps. Below is an e mail response I received from the U.S Compomotive wheel distributor.

Bob

Bob Standley

Thank you for your interest in Compomotive Wheels and at your business in the past.

I have attached a file on tire balancing. Hopefully that will give you some pointers on supervising the balancing of your wheels. It is possible that your existing wheels have been tweaked and that there are no signs of damage. However, before we get to the expense of shipping wheels back and forth, you might want to try the balancing one more time. If you are satisfied that the balancing was done properly and you still have a problem, you will need to dismount the tires before shipping the wheels to us. Once the tires are dismounted, put the wheels on the balancing machine by themselves. That will help determine if it is a wheel problem or tire problem. Depending on how much you use your Cobra, the tires may well be your problem. Also make sure that everything in the front suspension and it's alignment is okay. We're not trying to blame something else, it's just that it takes a pretty substantial bang to knock an HB out of round. The majority of times that we work with customers on wobbles the fault is not in the wheel. When we ship wheels they have less then .020” run out. That is undetectable on a balancing machine. Please let me know how you make out.

Thank you again for your interest. If there’s anything else you need, please do not hesitate to get back to Alan or I.

Norris D. Boyd

Motorsport Specialties, Inc.

Importer and Distributor for Compomotive Wheels in the United States

For mail use: mailto:NDBoyd@usacomp.com

Our web site is at: http://www.usacomp.com/

[JeepSnake]

Hey Bob,

I think the general consensus is that the BFG tires are typically the culprit. I've got them, and my Dad does, too - they seem to constantly be getting out of balance. They seem to do okay in the skinnier sizes (the 225/75R15's on my Chevelle are okay), but the wider ones just don't balance all that well, and don't seem to hold a balance job for long.

Maurice's advice to me shortly after Sandy and I got our car was this:

1) Balance the tires & wheels with weights on BOTH sides of the rim. You can't really balance with weights just on the inside. (This advice has been repeated to me by tire men on all manner of tires ranging from hi-po street tires to big offroad 4x4 tires).

2) Have your alignment man set up the specs so that the BFG tires wear out in approximately 200 miles so you can justify a new set of Yokohamas to the wife! :)

Hope this helps - I'm off to the basement to work on brake bias adjustments! :(

[spd4me]

bstandley
Is that an inquiry you made to Compomotive before my post or after? My spin out is .034". I hate to spend money on new tires (mine have very little wear) and it still be the rims. But maybe I should try the tires first. I can get installed to spin rims when tires are off to further check rims.

Jeepsnake - yeah the wives are always wanting equal financial rights.

[bstandley1]

Bob,

I contacted the Compomotive folks over a year ago in my effort to get rid of a chronic tire vibration problem. I ended up replacing my Dunlop's at 3400 miles! Even after replacing the tires, aligning the car and having wheel weights affixed to both sides of the wheels, I still had vibration. A local tire dealer did the Hunter 9700 Road Force trick and determined that one of my front tires had 32 pounds of Road Force ... too much! tire dealer replaced the offending tire ... problem solved!!!! You could purchase a set of Yokohama Avid S/Ts. (you can buy them at Discount Tire for $55 - $60 a piece). Hope this helps.

Bob

[Naumoff]

Is this hop just when you drive down the road or when you do a burn out?

[spd4me]

It starts at about 60 mph and gets real bad at 80 mph.

[eliminator]

My guess is it's not the rim's.

It's a tire and alignment issue, I know the BFG tires are an issue and I bet the front caster is off.

[will butterworth]

car to tireman Maurice told me to go to, wheel balanced on car, both front tires lopsided but good tread , starts hopping at the normal road speed I drive at most 60 to 80mph, most annoying, will spring for pair of yoko,s . Will they be compatible with rear tires,can,t rem. what they are, but what came on car.?Will-alabama Lopsided tires don,t bal. very well at all, so I go down road with arms jerking side to side, just tell folks it is my arthritis.

[Jerry Cowing]

Just to continue this thread a little further for troubleshooting and general education , here's a little primer on vibration. First, it is necessary to understand that all solid bodies act as "spring/mass/damper" entities; that is, they have "natural frequencies" at which they will vibrate. To illustrate, imagine a 2X4 clamped horizontally by its end to a bench. Deflect the board downward a few inches and let it go; it will vibrate up and down at a constant frequency with gradually diminishing amplitude until it comes to a rest. The frequency at which it vibrates is its "natural frequency." The amount of movement at the end of the board is described as the "amplitude" of the vibration and is a function of the displacement used to deflect the board. The number of oscillations which the board goes through before coming to rest is an indication of the "natural damping" characteristics of the board. If the board is made of a dense wood which exhibits heavy damping, it will come to rest after just a few oscillations. If, however, the board is lightly damped, it will bounce up and down for quite a few oscillations before stopping.

If, next, we make a series of small, equal force inputs at regular intervals to the board, it will begin to oscillate at the frequency equivalent to the input force (termed the "forcing function"). If the wood is reasonably damped, it is able to dissipate the energy of the forcing function input at the same rate as the energy is input and will cause the oscillations to achieve and stabilize at a given amplitude, dependent on the amplitude of the forcing input. (Some materials, however, have very light damping, and it is possible for the amplitude to actually increase for a given sinusoidal input at the body's natural frequency; this will cause the amplitude of the body to keep increasing until structural failure.) All solid bodies, including your tires, exhibit these vibratory characteristics in both linear and rotational motion. In the case of your tires, the mass is essentially the weight of the wheel/tire/brake disk assembly, with lighter masses leading to higher natural frequencies. The spring constant of the assembly is a function of the tire pressure and suspension spring stiffness, with higher spring constant (stiffer springs or tires) also leading to higher natural frequency. The damping constant is primarily a function of the tire sidewall construction and the shock absorber damping factor; heavier damping tends to dissipate the vibration quicker but damping that is too heavy causes the vibratory loads to be transmitted back into the supporting structure (the car frame) rather than being dissipated as heat.

Many folks have a mistaken impression that if the wheel is out of balance, the amplitude of the vibration will get progressively worse as wheel rotational speed (the "sinusoidal frequency or period") is increased. I've heard tire shop gurus tell unsuspecting customers: "I spun it up to 120 mph and it balanced perfectly." The problem is that the vibrational amplitude does NOT increase linearly with an increase in frequency. The amplitude rather will increase as the tire/wheel approaches its natural frequency, then the amplitude will begin to decrease at a logarithmic rate as the rotational frequency increases further. Translated into English, that means that the amplitude of the vibration is zero at rest, increases to a maximum amplitude at its natural frequency (rpm), and then falls off rapidly as the frequency of rotation (rpm) increases. The amplitude of the vibration depends on several characteristics of the wheel/tire assembly; namely, the mass, the tire pressure (which affects the tire's "spring constant"), and the tire structure (which affects the tire's "natural damping"). Unfortunately, for 15 inch radial ply tires, the natural frequency most commonly falls at an wheel rotational frequency which equates to roughly 60 to 65 mph. This is why many folks seem to complain that their steering wheels start to shimmy at 60 mph and the shimmy seems to disappear at about 70. The vibratory amplitude decreases logararithmically with increasing rpm at a rate approaching 10 dB per 10 mph of speed increase, so that the vibratory amplitude at 80 may only be 1% of the amplitude at 60. That low level of vibration is not apparent to the average driver and explains why the guru was so successful at balancing the tire at 120 mph without really solving the initial problem of vibration at 60. The moral here is that the tire guru should be instructed to balance the tire for the speed at which the vibration is the worst (which for most cars usually happens around 65 mph) rather than at some elevated speed. If you find that your tires bounce the worst at 75, then this tire rpm is the natural frequency of your tires/wheels and the place where the vibation feedback will be greatest, so it makes sense to balance them at this speed. Hope this makes sense and helps.

[Jerry Cowing]

Forgot to emphasize one of my points above besides balancing your tires at the speed where you get the most vibrations.

The more tire pressure you add to the tire, the more vibration you will feel because of the damping effect I spoke of above. You have to be careful not to let too much air out of the tires as it can cause your tires to overheat and fail. Since our cars only weigh about 2500 lbs and most tires are designed for cars much heavier, we can safely run much less air in our tires than the manufactuers recommend and still be safe. Also remember that tire pressure should ALWAYS be measured when the tires are cold. If you have to drive 5 blocks to get to an air compressor, you'll need to let the car sit for about an hour for the tires to cool enough to get a good pressure reading. Running BFGs at 24 lbs or less cold pressure will reduce vibration by increasing damping. Above 24 lbs will increase vibration because the side walls become harder and transfer the vibration to you through the body. I run 24 lbs or less for everyday driving and 20-22 lbs (HOT)when racing on a road course.

Note: for 4 wheel driving off road, you want higher tire pressures for a stiffer sidewall and better steering control. It's a wive's tale about letting tire pressure out when driving in the mud -- that's a bad idea. High tire pressures also protect your tires off road against sidewall punctures.

[Master]

Jerry, a very through explanation of what goes on with the tires at various speeds. ;) To the average guy 20psi cold works well even with BFG rocks.:D

[Slither]

Nice write up Jer... sounds like a technical background showin' through there;).

[Jerry Cowing]

Master,

I agree 20 lbs is usually plenty when measured cold as the tire pressure will increase about 2-4 lbs when they heat up after driving. I never go above 24 lbs and usually run less than 24 lbs.

After racing awhile, the tire pressure will increase quite a bit so I usually have to let some air out after a few laps to get back down to 20-22 lbs.

The best way to determine correct tire pressure of course is to use a temperature guage. After getting your tires hot, measure the temperature on the outside, middle and inside of your tires and then you can determine how much air to add or subtract and also if your alignment needs some tweeking.

[spd4me]

I'm glade there's someone with a PHD in tire balancing. This forum is great. It all makes sense now. I did have the tires balanced twice and the second time seems to be a lot smoother.
Bob

[spd4me]

I'm glade there's somebody on this forum with a PHD in tire balancing. I did have the tires balanced twice and the second time is a lot smoother.