How to Calculate V-Belt Tension

Updated April 17, 2017

V-belts, sometimes called fan belts, drive the pullyes that power components and accessories on an automobile engine, and in some cases generators and other small engines. V-belts differ from flat serpentine belts in that they have a V-shaped groove which allows them to seat deeply inside a pulley. The angle of the V offers more surface area for gripping purposes. V-belts can be found on pulleys that drive the automotive air conditioning compressor, alternator, smog pump, water pump and power steering units. Calculating the V-belt tension ensures that the belt will not slip, break or wear out component or accessory bearings.

Set the vehicle transmission selector in park or neutral. Apply the emergency brake. Raise the hood and remove the negative battery cable with a socket and wrench. Move to the front of the car and look down at each belt. There may be several that drive various accessories, such as the water pump, alternator and power steering. Use a dry rag to wipe each belt. Look for cracks, glazing or feathering of the fibres. Deformed belts should be replaced, rather than adjusted for tension.

Start with one belt at a time. Find the longest reach of the belt between pulleys. Locate the mid section of the belt and use your finger to push down on the outside flat spot of the belt, in toward the centre of the engine -- this movement represents the deflection rate. Refer to your owner's repair manual for the correct deflection rate of your belt, either for individual belts, or all of them as a whole. Hold a ruler perpendicular to the belt and push the belt with your finger again.

Read how far the belt moves across the ruler in inches. Your manual will state the correct number of inches of deflection. If the belt reads over specifications, you must loosen the bracket bolts to the accessory and put more tension on the belt, then re-tighten the bracket bolts. To loosen the belt, allow the accessory to move inward toward the middle of the engine, then re-tighten the bolts. Common deflection for standard V-belts can be 1/4 inch to 1/2 inch, for example. Measure all belts this way.

Use a tape measure to measure the span of a single belt from the point where the belt just leaves the pulley to attach to the next pulley. Make sure you pick the longest span. Write the number down in inches. Calculate by using 1/64 for every inch of span. For example, a 32-inch span times 1/64 equals 1/2-inch. The permissible deflection will be 1/2 inch.

Place the plunger head of a belt tension gauge against the middle part of the longest span of the belt. Set the dial indicator on the gauge at "O." Push the plunger on the gauge against the belt and read the deflection (in inches) on the gauge tension scale. If over or under the correct foot-pounds, use a socket and wrench to adjust the belt to the proper tension. Remember that other belts will have shorter or longer spans. Use the formula: 1/64 times the span in inches.

Things You'll Need

  • Owner's repair manual
  • Socket set
  • Ratchet wrench
  • Ruler
  • Tape measure
  • Belt tension gauge
  • Pen and paper
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.