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Balancing wheel tension with the TM-1 Spoke Tension Meter
Level of Difficulty: Intermediate to advanced
This article will discuss the use of the TM-1 Spoke Tension Meter to properly balance wheel spoke tension. See also related articles:
The TM-1 can be used to accurately and reliably measure the tension of each spoke in a wheel. The TM-1 will also measure the average tension of all the spokes in a wheel, and the relative tension between all the spokes in the wheel. It works on nearly any bicycle spoke, no matter what the diameter, material, or shape. The TM-1 Tension Meter is a precision measuring instrument and should be used and stored with care. To avoid damage, it is recommended that the TM-1 be stored in it's original packaging or suspended from a bench hook. It should always be kept clean and dry. As new spokes are developed, check the TM-1 Tension Meter page for updated reading conversion tables.
Wheels that are strong, reliable and long-lasting have spokes that are properly tensioned. Tension is the amount of force pulling on a wheel's spokes. Spokes that have low tension will continue to loosen as the bike is ridden, resulting in shortened spoke life and a wheel that requires continuous re-truing. As the wheel rotates while in use, the spokes that are on the bottom next to the ground actually lose tension momentarily. This loosening each revolution is a "stress cycle", and low tension wheels actually see a greater stress cycle than wheel with relatively greater tension. A high stress cycle fatigues the metal and leads to spoke breakage.
Spokes that have too much tension can result in deforming and/or cracks near the nipple holes of the rim, as seen in the image below. Notice crack at red arrow.
Too much tension can also lead to failure of the hub flange. The wrench flats of the spoke nipple can become deformed and rounded by forcing the nipple to turn while the spoke is at too high tension.
In addition to achieving proper spoke tension, it is also important for all the spokes in the wheel to have approximately the same relative tension. Relatively great differences in tension between each of the spokes will result in a wheel that is not laterally stable and that will come out of true more easily and more frequently.
The recommended tension for spokes in bicycle wheels can be as low as 80 Kilograms force (Kfg) and as high as 230 Kilograms force. As a rule of thumb, it is best to set tension as high as the weakest link in the system will allow, which for a bicycle wheel is usually the rim. Therefore, to obtain a spoke tension recommendation for a specific wheel, it is best to contact the rim manufacturer.
Rim true, or run-out, is dependent upon spoke tension and on the original manufacturing tolerances of the rim hoop before the wheel is even built. Good quality rims may vary in round less than 1mm before being built.
Measuring Tension of a Spoke
1. Measure the diameter of the spoke using the included spoke diameter gauge. The smallest slot the spoke fits into determines the diameter. A measuring caliper can also be used to measure the spoke diameter. The diameter at the middle section of spoke will determine the appropriate spoke-type column on the Conversion Chart.
4. Using the conversion table, find the column corresponding to the material and diameter of the spoke being measured. Follow the column down to the row corresponding to the spoke's deflection reading (as determined in step 3). The number at this intersection is the actual tension of the spoke in Kilograms force (Kgf).
The TM-1's conversion table converts the tool's deflection reading into Kilograms force. Other units of force sometimes used are Newtons and pounds force. One Kilogram force is approximately equal to 10 Newtons or 2.2 pounds force. As an example, a spoke tension is 105 Kgf. This spoke would be equal to approximately 1050 Newton or 210 pounds force.
The conversion table is based on the diameter and size of the spokes. The table does not refer to "gauge" sizes, commonly used in the bicycle industry. Gauge systems are arbitrary assignments of numbers to the relative size of the wire or material. Gauge sizes are often used for sheet metal, needles, shotgun size, and wire. There are several systems used depending upon the specific industry. For example, in the electrical industries, wire gauge is determined by a formula based on resistance per unit length. In the wire weave industry and the bicycle industry, the gauge system is Washburn & Moen Company, where the numbers are arbitrary and have no particular meaning other than relative size. The gauge system can be confusing because the size of the wire diameter decreases as the gauge numbers get larger. Relatively smaller gauge numbers mean a relatively larger wire.
Gauge systems often result in confusion for manufactures and users. Two well known spoke manufacturers give different millimeter diameters for what they both call "16 gauge" spokes. Because of this, it is best to simply use the measured diameter sizing of the spoke. The TM-1 conversion chart has tension columns for different spoke diameter, measured in the middle of the spoke where the TM-1 engages.
There are four bladed or "aerodynamic" steel spokes. The end of the spokes are not concerned in the tension reading, only the middle. Measure both the thickness and width of a blade (major and minor measuresements). When taking a TM-1 reading measurement on a bladed use care to hold to tool square to the blade profile. If the tool is held crooked it will not give an accurate reading
Measuring Average Spoke Tension
The average tension is the sum of the individual spoke tension measurements, divided by the number of spokes measured. Each wheel has two averages, one for the left side spokes and one for the right side spokes. If the spoke hole flanges of the hub are centered between the locknuts of the hub, it is possible for the left and right sides to have nearly equal average tension. However, if the spoke hole flanges are not centered between the locknuts of the hub, the tension between the left and right flanges will vary. Generally, on most rear wheels, the gear side (right side) will have greater tension then the left. On front wheels with disc brake mounts, the disc side will have more tension.
To determine average tension:
1. Take deflection readings of all the spokes on the right side of the wheel.
Record these numbers.
Tension Conversion Calculator
The conversion tables are now available as an Excel® spreadsheet. This spreadsheet will quickly convert the reading into tension, and also produce a graphic illustration showing the spoke tension balance. See Tension Conversion Calculator for more information.
Using the TM-1 Tension Meter Conversion Table
The Conversion Table may be downloaded as an Adobe® PDF file here. This table will be updated as new spokes enter the market. Additionally, if a computer is available, download the TM-1 Tension Conversion Calculator, a Microsoft® Excel spreadsheet for speeding tension conversion.
The TM-1 is calibrated for 16 different types of spokes. Round steel spokes are calibrated regardless of brand. Some of the bladed on odd sizes are proprietary designs, and these are noted below. The blade sizing is based on measurement, not on manufacturers nominal names:
Below is a simplied Conversion Table showing only three spokes. The deflection readings that are read off the tool are the left most column. The corresponding kilograms force is listed below each spoke diameter.
Notice that there may be some blanks for extreme low and high readings. If a 2mm spoke reads 16 or less from the tool, the tension is below 51 Kilograms. This spoke would be quite loose. However, a 3.2mm x 1mm bladed steel spoke is 89 kgf when the tool reads 14.
The table does not include separate columns for butted spokes. In testing spokes at the Park Tool Company, it was determined there was no significant difference between straight diameter spokes and butted spokes. A butted spoke is basically considered a shorter straight gauge spoke.
The pointer of the tool will point at a number on the scale. For more accurate tension results, the tool can be read to finer precision. If the point repeats just off a number, the reading can be quartered. For example, the image below shows an example of a 24, a 24.5, a 25 and a 25.25. The 25.25 show the pointer just off the 25 mark.
Tension Balancing and Relative Spoke Tension
Relative tension is the tension of a spoke in comparison to the tension of one or more other spokes. A wheel with spokes that are within plus or minus 20% of the wheel's average spoke tension is generally considered to have acceptable relative tension. As explained above, the spokes on one side of a wheel may be tensioned differently than the spokes on the opposite side. Therefore, it is important to only compare the tension of a spoke relative to spokes on the same side of the wheel. To determine relative tension:
1. Determine the average tension of the spokes on the right side of the wheel.
Rim manufacturers have set tension recommendations from as low as 80 Kilograms Force to as high as 230 Kilograms force. Generally, the heavier and strong the rim, the more tension it can handle. A light rim may weigh from 280 grams to 350 grams. A heavy rim may be said to weigh 450 grams or more. Additionally, rim eyelets may help distribute the load on the rim wall. A lack of eyelets on a light rim may imply less spoke tension is required. Always consult the rim manufacture for the most up to date specifications. Note that these manufactuers give specification for the wheel without tire, or without inflated tire. Tire pressure will have the effect of lowering the tension of the wheel. Generally, do not try to account for this drop by adding more tension than recommended by the manufacturer.
Below are some specifications:
The TM-1 Tension Meter is calibrated at the Park Tool manufacturing facility. Generally we do not recommend readjustment of tool. If the tool is worn or damaged and appears to be inaccurate, return to Park Tool for recalibration. Park Tool will recalibrate and return the tool for a reasonable charge.
It is recommended for professional mechanics and service departments use a non-riding wheel to help in tool calibration. Remove the axle from a wheel so that it is not rideable. Mark a spoke as a reference spoke and measure this one spoke. Write the date of the measurement directly on the rim. This same spoke can be used to double check the original calibration. If the tool is reading different than this "reference spoke", the spring tension can be changed. If the tool is reading low, take the tool off the spoke and turn it upside down. Remove the spring from the fixed moving stud, and then thread the adjusting screw inward toward its stud. This will decrease tension on the TM-1 spring, resulting in a higher deflection reading. If the TM-1 is reading high, increase tension at the adjusting screw.
The Park Tool Company would like to thank Colin S. Howat of the KTL Lab, Lawrence, Kansas, for his assitance and help in developing the TM-1 and the conversion table.