Road force balance and straight trak

Discussion in 'SRT Hellcat Wheels and Tires' started by Tire God 2.0, Jun 4, 2016.

  1. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

    Apr 24, 2016
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    Challenger SRT Hellcat
    Hunters road force balancers have a loaded roller that applies about 1300 lbs of force to the tire to simulate it on the road. Radial force variation is the variation of radial forces acting on the tire/wheel assembly which can cause vibrations that feel like an imbalance. Every action has an equal and opposite reaction, and everything below the suspension is unsprung weight which translates to what you feel when you drive. Radial (radius) is the force applied from the top of the tire to the center point of the wheels center bore. And this force variation changes in each section of the tire due to the tires construction. Keep in mind no tire is perfectly round, nor perfectly built. The forces that cause issues stem from the sidewall stiffness not being the same around the tire. The sidewall splices (where the sidewall ply overlaps, can sometimes be seen as bulge or valley) are generally the stiff spots. As you drive you hit these spots at a high rate depending on the rpm of the tire, and it causes a vibration. Wheels are not perfectly round either. Every wheel has a low spot. The match mounting of tires on wheels can drastically change the way the tire rolls and also change how it rides. So the loaded roller applies force and measures the tire to find the stiff spots (every spot that creates a vibration is called a harmonic). After it has finished measuring the tire, you measure the runout of the wheel. Once this is done, it tells you to mark the tire at the stiffest spot (first order radial harmonic), and mark the wheel at the lowest spot. Then you turn the tire on the rim so that the marks match, which puts the stiffest spot of the tire at the lowest spot of the wheel which makes the tire/wheel assembly the most uniform. This can best be thought of as there being coil springs in the sidewalls of the tires, but one spring is much stiffer than the rest. And when the stiff spot hits the ground, it is absorbed by the low spot of the wheel.

    Straight trak

    Terms: Tire conicity, Tire eccentricity, Ply steer, radial pull, lateral force, tangential force.

    Tire conicity, or the tendency of a tire to be shaped like a cone. When you roll a cone on a flat surface it will roll around the small end of the cone. This same principle applies to tires and if bad enough can cause the vehicle to pull. All tires have conicity, as no tire is perfectly round as stated above. Normally it's such a small amount the tiny force is absorbed by the suspension and is not enough to make the vehicle pull. In some cases where it causes a pull, it is known as a radial pull. The conicity stems from the following: different sidewall geometries on the same tire, the tread may not be moulded properly(mould defect) or off center belt.
    Ply steer has the same effect only usually a much more severe condition where if you let go if the wheel you would be in the next lane within 10 seconds or less. It creates a crab walk effect, this stems from the belt ply angles shifting during the curing process where the rubber is vulcanized and if the plys shift not only can it cause a pull, but also belt edge separation and tire failure. The outer most ply has the dominant effect, which is why they pull towards the outside of the tire. In rare cases there will be a radial push, where it pulls towards the inside sidewall, ply steer can also cause push but it's rare.
    Tire eccentricity, or how far from round a circle is. Most of the time, the first harmonic is also the high spot in the tire. The load roller also measures tire runout or "eccentricity".
    Tangential force does not contribute to any of these but none the less good to know. The Tangential Axis is parallel to the road, in the
    direction of travel. This is the axis where the
    driving force is applied to the tire.

    Lateral force. The straight trak feature measures the lateral forces in the tires that are caused by the above, it displays them on the screen in the configuration they are on the vehicle after they are all measured. It will give the option to arrange the tires on the vehicle to create the least amount of pull, which means if there are two tires pulling, it will put them on the same axle so that the pull from each tire cancels out one another(they pull against each other). This works for minor pulls, or at least until the tires are rotated. But in most cases it will allow the user to determine which tire or tires have the pull and can be sent back to the manufacturer under warranty which is the ideal method.

    In some cases, the conicity can actually be improved by road force balancing, because match mounting creates a more uniform assembly. If you measure them and see the pull, you should road force the tires then remeasure the lateral force to see if it's fixed.

    Other notes: Yellow and red colored dots on the sidewalls.
    This is an old method of match mounting that in some cases is good enough, but should only be used if the shop does not have a quickmatch or road force balancer. The yellow dot is the lightest spot in the tire, and should be matched with the valve stem due to it being the heavy spot in the wheel. This will make the assembly call for less wheel weights to balance. The red dot is the high spot of the tire, very few wheels have a marking for the low spot but if it does the red dot should be matched to it. This is not as effective as a loaded runout measurement because the tire changes drastically under load (on the car). The quickmatch balancer measures loaded runout or "eccentricity" and the rim runout and provides a better match mount than using the colored dots, due to it being a loaded runout measurement rather than an unloaded like the red dot. But this still does not account for sidewall geometries and stiffnesses, which is why the road force is the ideal method above all else.

    Honorable mention: michelin builds their tires with three sidewall splices exactly 33.33% around the sidewalls. So even if they are stiffer than normal, it won't cause a radial force variation. Very rarely does a michelin tire cause an issue with any of the above. They hand check every tire before it leaves, and they use what's called an ASTEC machine to measure every tire which is a multi million dollar machine that supercedes the road force balancer. This is why regular people prefer michelin tires, and they don't even know it. They just know it's the smoothest tire they have ever driven on.

    Lastly, in some cases radial force variation can't be fixed due to a number of factors. Bent wheels, or the tire having the second harmonic almost just as stiff as the first. Ideally you never want the second harmonic to be half of the first or more. If this is the case they can be sent back to the manufacturer. In very rare cases there are some tires that just don't match well to some wheels, the balancer has a match maker feature that assigns a match code to each tire and wheel. If assembly #1 is tire-12 wheel-3 and assembly #2 is tire-5 wheel-10, it will instruct the user to swap the tires on the rims and then proceed to road force balance. The match code numbers are arbitrary but are useful in some cases.
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