How do bolts hold a car's subframe in place?

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In summary, the conversation discusses the concerns surrounding the strength of bolts and rubber bushings holding a car's subframe together, especially when aftermarket bushings are used. The expert explains that it is not just the strength of the bolts, but also the friction force generated by the tension that keeps the two pieces together. They also mention the use of high-strength bolts in these situations.
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TheTallJS
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It boggles my mind how a car's subframe is only held on by a few bolts and some rubber bushings. How is it that the bolts don't just shear off? It would seem that the forces of driving, sometimes erratic, should easily overcome the strength of a bit of metal.

People have been putting in aftermarket polyurethane bushings and even solid aluminum bushings into their subframes, and I can't help but wonder how long before those bolts snap. Or if those bolts are of sufficient strength, that they tear through the car's body where the bolts are bolted in to.

Actually, solid bushings seem very appealing. Maybe reducing sprung/unsprung weight would reduce forces that would otherwise cause damage on an unmodified car.
 
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TheTallJS said:
How is it that the bolts don't just shear off?
It is not the bolt shearing strength that holds 2 pieces together, it is the friction force caused by the compression generated by the bolt's tension.

A low-strength steel bolt can withstand 55 000 psi of stress in tension before permanent deformation occurs (i.e. tension when correctly torqued). For a ½" bolt, that is 10 800 lb of force (##= 55 000 \times \frac{\pi}{4}0.5^2##). With a friction coefficient of 0.6, it means the friction force is 6500 lb between the 2 sandwiched plates. Even with a friction coefficient of 0.2, you still have 2100 lb of friction force.

Even if friction is not present, the shear strength is usually 58% of the tension strength, so 32 000 psi for a low-strength steel bolt. In single shear, a ½" bolt can still support 6200 lb of force.

It is usual to use high-strength bolts in those situations, which can withstand twice as much.
 
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1. How do I know if my car's subframe is in danger of falling off?

There are a few warning signs that your car's subframe may be at risk of falling off. These include unusual noises or vibrations while driving, uneven tire wear, and visible damage or rust on the subframe. It's important to have a professional mechanic inspect your car if you notice any of these warning signs.

2. What causes a car's subframe to become loose or fall off?

The most common cause of a loose or detached subframe is rust and corrosion. Over time, exposure to moisture and road salt can weaken the metal and cause it to deteriorate. In some cases, accidents or collisions can also damage the subframe and compromise its structural integrity.

3. Can a loose subframe be repaired, or does it need to be replaced?

This depends on the extent of the damage and the age of the car. In some cases, a loose subframe can be repaired by welding or reinforcing the affected area. However, if the damage is too severe or if the car is older, replacing the subframe may be the safer and more cost-effective option.

4. How often should I have my car's subframe inspected?

It's recommended to have your car's subframe inspected at least once a year or every 10,000-15,000 miles. However, if you live in an area with harsh weather conditions or frequently drive on rough roads, it's a good idea to have it checked more frequently.

5. Is a loose subframe a serious safety concern?

Yes, a loose or detached subframe can compromise the stability and handling of your car, making it unsafe to drive. It's important to address any issues with the subframe as soon as possible to avoid further damage and ensure the safety of yourself and others on the road.

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