![]() The drawing below shows the out of balance weight denoted by the orange out of balance, and the gray correction weight that is mounted slightly inwards and at a slightly imperfect phase angle. The net result is a waveform that tends towards zero but does not quite cancel out. Additionally, it is extremely unlikely the counter weight will be positioned at the perfect angle. Realistically, a counter weight will be of a slightly heavier and positioned at a radius slightly inwards of the wheel rim. In a purely theoretical situation, an exactly matching counterweight will cancel the sine wave produced by the out of balance but this is rarely achievable in a practical situation. It is recommended that this video is watched to explain this in greater depth. This is due to maximal acceleration always being perpendicular to the out of balance. In perfect conditions, the wheel orbit lags the out of balance by 90 degrees. The phase of the out of balance and the corresponding movement of the wheel are of importance. This is intended as a rough guide but should prove to be accurate in most scenarios. The Y axis is the balance limit weight based on equation (5). The bicycle velocity and hence the rotational speed of the wheel increasesĪ graph below is shown for differing weights of wheel assemblies against speed. ![]() The wheelset becomes lighter (typically a front wheel will require a tighter tolerance than a rear wheel).Some key points to note, the Permissible imbalance becomes smaller in the following conditions : In most situations, weight is applied at a slightly shorter radial distance and hence a greater weight must be applied. The calculated balance weight is based on a radial distance of 311mm. Thus a static or single plane balance can be used to correct imbalance. The diameter being 700mm+ vs an axial length of approximately 130mm and in reality, the portion carrying the weight at that diameter is only around 30mm thick. The diameter is much larger than it’s axial length. Variations of it can be traced back to pre 1900’s.Ī bicycle wheel is by definition a slender object. This ISO standard is used for everything from Jet Engines to Hard disk drives and is widely accepted as a definitive standard. Rotor balancing is covered in an ISO technical standard 21940 Section 11 which documents vibration and mechanical rotor balancing grades. Imbalance causes a number of symptoms that often overlap other issuesĪcceptable levels of imbalance (Balance Tolerance) Poorly manufactured wheels will often have a significant phase angle difference (see more below). Once built up, this light spot will usually become the heavy spot due to the weight of the tire valve. Typically, a wheel will have it’s light spot (when it is not assembled) at the valve hole because there is physically less material there. Excess resin build up (carbon wheels are frequently made in quadrants).The primary causes of imbalance are due to physical properties such as valves but a large element is due to imperfect manufacturing techniques. Static (Single Plane) Imbalance Causes of Imbalance It should be noted that almost every industrial machine that rotates has some basic balancing carried out on it. This is depicted below in the diagram with the orange rectangles representing counterweights. Hence balancing must occur on two planes, the first is near the outer edge of the wheel and the second is towards the inner edge of the wheel. A car wheel doesn’t have a particularly large diameter in comparison to it’s axial length so it is not considered to be a slender body (thin). The most common type that people will have come across and the most obvious will be dynamic balancing, this occurs when car wheels are balanced. There are two fundamental modes of balancing. This post will look at the engineering behind the balancing of bicycle wheels. A lot of professionals balance their wheels and this has filtered down to the amateur ranks wishing to eek out the maximum amount of performance. Bicycle wheel balancing is a topic that is often discussed but has not ever been dissected in detail.
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