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U-Bolt Technical Information


Types of U-Bolts
Three basic types of bends are used on U-bolts depending on the suspension design and the shape of the mating parts: Round Bend, Semi- Round Bend and Square Bend.  Additionally, each of these types of bends may used forged material.  This is used primarily where additional clearance is required between the U-bolts and, for example, the frame. 

Round Bend

Semi-Round Bend

Square Bend

Forged Top
Semi-Round Bend
Technical Infomation:
U-Bolt Grades
The U-bolt grade is a measure of the material's strength.  Grade 5 and 8 are the most commonly used with grade 8 material offering an increase in yield strength of over 40% better than that of grade 5 material.  This allows for higher torque levels, and thereby, improved spring clamping.
Available in 5/8" through 1 1/4" diameters, grade 8 is the recommended choice for all applications except in four spring trailer suspensions where grade 5 has been determined to provide adequate strength.  If the original grade or the application is unknown, grade 8 material is always the safest choice.
U-Bolt Torque and Clamping Force
The primary function of the U-bolt is to provide the clamping force required to rigidly clamp the spring to the axle assembly.  From a practical standpoint it is not possible to directly measure the clamping force when installing a U-bolt.  Therefore, torque specifications are relied on as an indirect measure of the clamping force.
The formula for the relationship between torque and clamping force is:
Clamping Force =
K  x  Diameter
The importance of this formula is how the clamping force is influenced by "K" which is a measure of the friction between the nut and the U-bolt threads and washer.  The higher the friction, the less clamping force that is developed with the same torque.
Example: (Assuming 1" rod and 500 ft-lbs of Torque)
K= 0.20     (New U-bolt and nut with lubricated threads) Clamping force = 30,000 lbs
K= 0.45     (Reused U-bolt with dry and damaged threads) Clamping force = 13,300 lbs
Clamping force is reduced by over 55% even though the reading on the torque wrench is identical.  The importance of using new U-bolts and nuts with lubricated threads is essential to maintain required clamping forces.
How to measure a U-bolt
Regardless of the bend type, all U-bolts are measured in the same manner as show here.  The U-bolt size is given as Diameter (A) x Distance Between Legs (B) x Leg Length (C).  Additionally the bend type must also be specified; either round, semi-round or square. 
Forged top U-bolts are best ordered by application and leg length since the type of forging is usually unique to the particular application and must be maintained to provide proper fit and clearance. 
Semi-Round U-bolt Fit
An often overlooked cause of failure to maintain proper U-bolt clamping force is the potential for mismatch between the shape of the semi-round U-bolt and its mating part, the top plate.  Unlike either square or round bend U-bolts where the shapes from one manufacture to another usually do not vary significantly, semi-round bend shapes are usually unique to a particular vehicle or suspension manufacture.  The following three figures show what can happen when using semi-round U-bolts.
In figure 1 the U-bolt has been formed with a shape that causes all the pressure to be concentrated at the center of the top plate.  As the spring flexes under normal use the U-bolt will tend to work into the top plate which will lead to loose U-bolts and possibly premature spring failure.
In figure 2 the shape mismatch is causing the pressure to be concentrated at the corners of the top plate.  Again, deformation of the top plate will occur, clamping force will be lost and premature spring failure could result.
In figure 3 the proper U-bolt fit has been achieved.  Note how the shape of the U-bolt closely matches that of the top plate thereby ensuring that the pressure from the U-bolt is evenly distributed along the width of the top plate.  With the load evely distributed, the possibility of the U-bolt working loose is greatly reduced.
When working on an application using semi-round U-bolts, visually inspect for proper match between the U-bolt and top plate before attempting installation.  By ordering semi-round U-bolts by application rather than just by the general description of "semi-round" this type of problem can be avoided.
By follwing the service tips outlined below, many of the common problems associated with U-bolts can be avoided
Do Not Reuse U-bolts
  • Used U-bolts will have rusted and damaged threads from the previous installation
  • A previously torqued U-bolt will suffer from distored threads from the engagement of the deep nut.  Deep nuts should be tighened once and retorqued, never loosened and retightened.
  • A used U-bolt may have suffered from fatigue as well excessive stress since achieving accurate torque with commonly used impact wrenches is very difficult.
Removed U-Bolts should NEVER be placed back onto the vehicle, they should be thrown away.  Suspension U-Bolts are manufactured with a smooth rolled thread, while the mating Hi-Nuts are manufactured with sharp cut threads.  When a U-Bolt is tightened to it's recommended torque level, the U-Bolt threads stretch as they mate with the Hi-Nuts.  Although, not always visible to the naked eye, this damages the threads.  Removing the Hi-Nuts from the U-Bolt will cause a cross-threading that will not allow the U-Bolt to be adequately re-torqued.  A common practice in most maintenance facilities is to use a impact wrench to tighten U-Bolts.  Consistent, accurate torque is next to impossible to obtain with a impact wrench, and in most cases an over-torqued fastener is the result.  We recommend using a torque wrench when installing U-Bolts.  New U-Bolts should be torqued after 2 weeks of usage.
As previously explained the primary function of the U-bolt is to maintain the clamping force on the spring assembly and related parts.  Since clamping force is a function of the condition of the U-bolt, the damaged threads of the used U-bolt will make reaching this force very difficult.  Considering that reusing U-bolts can lead to premature spring failure and other suspension damage it is clear that U-bolts should never be reused.
Inspect for signs of loose U-bolts
Virtually all leaf spring failures through the center hole are caused by inadequate U-bolt clamping.  Also look for signs of movement within the area between the U-bolts.  Worn or polished surfaces on axle seats or top plates are sure indicators that unwanted movement has occured.  Closely inspect and if necessary replace each damaged component.
Retorque the U-bolts
Retighten U-bolts after the first 500 to 1000 miles.  As new or repaired springs wear in, some settling of the spring stack will occur.  Even a minor amount of this settling can cause a dramatic reduction in U-bolt clamping force.  If possible, retorquing of U-bolts should be done with the vehicle under load.
Use recommended torque specifications
Compared to other fasteners, recommended torque levels for U-bolts are significantly lower.  This is due mainly to the stress concentration effect of the U-bolt bend itself, the dynamic loading that the spring transmits to the U-bolt and the difficulty of acheiving even stress on both legs of a U-bolt.
Recommended torque levels from the vehicle or suspension manufacture should always be used where possible.  If these specifications are unknown, the following chart can be used as a guide



(Dia x Thread)


(Ft. Lbs)

3/8 x 24



7/16 x 20



1/2 x 20



9/16 x 18



5/8 x 18



3/4 x 16



7/8 x 14



1 x 14



1-1/8 x 14



1-1/4 x 14



Torque U-bolts Evenly
Follow these guidelines when installing U-bolts

  • Lubricate U-bolts and washers with oil or anti-seize compond to reduce nut friction
  • Tighten all U-bolts until they are sung only.
  • Tighten in the sequence shown to approximately 1/3 of recommended torque.
  • Repeat, using the same sequence, gradually increasing the torque through a second and third stage until the recommended final torque is attained.
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