Torque Arm vs 4-Link

[dhutton]

I know what the math says and I really don't care what the math says.

In the past 20 years I have a handful of real world instances on street cars, not race cars, where the bolt in a single shear lower shock mount failed. When those bolts bent or broke they did not care that mathematically they weren't supposed to. Most of those were 5/8" in diameter as well in a street rod application. We can point to a number of factors for the failure but the fact remains that they failed.


My personal ridetech equipped 71 Camaro still has the factory bump stops on the rear frame rails above the axle.

So what is the design solution for these failures? It seems they are all done this way.



Don

[stab6902]

So what is the design solution for these failures? It seems they are all done this way.

Don

I totally believe what Donny says regarding even 5/8" bolts failing. We didn't even talk about fatigue yet... lots of smaller hits can do the same damage over time.

The best solution is to run the factory style bump stops between the axle and frame rail like Donny said. Cycle the suspension without springs, and make sure the bump stop is really what limits travel.

[Sleeper68]

So what is the design solution for these failures? It seems they are all done this way.

Don

The solution? Double shear mounting. What hasn't been brought up yet is what makes single shear mounting a real killer: Moment loading. What is a moment? Essentially a torque. Yes the fastener will see a shear load, but it will also see a torque. Fasteners are not designed to be loaded in moment and shear (also known as complex loading). The moment has a tendency to bend the bolt, sometimes it is called a bending moment because of this.

Here's an excerpt from an airplane design guide:

Single Bolts in Single Shear:

There are some joints in airplanes which must be held together with a single bolt. Some examples of these are strut-end attach points and bracing-wire attach points.

There are two types of bolted or pinned joints. The first is a single-shear joint, where the bolt goes through each of the parts to be joined only once and is loaded in shear at one point along its length. The bolt is tightened to clamp up against the two parts being joined.

The second type of bolted joint is a double-shear joint, where one part forks around the other and the bolt goes completely through both the fork and the part within the fork. In this joint, the bolt is loaded in shear at two points. The bolt in a double-shear joint may be clamped up, but it is not necessary for the stability of the joint.

While both types of joint can be used safely in some applications, there are some significant problems with single-shear joints; they should be avoided in highly-loaded and flight-critical areas wherever possible.

The first problem with single-shear bolt installations is that the bolt carries all of the load at the single-shear plane. If the same bolt were installed in double shear, it would be able to take twice the load because the load is shared between the two shear planes in the double-shear joint. This, in and of itself, is not dangerous as long as the bolt is properly sized to take the loads it must carry.

The second problem is more severe, and is the primary reason single-shear joints should be avoided in critical areas. A double-shear joint is stable. If the nut loosens, and the bolt moves in the hole, a double-shear joint remains in place, and the internal loads in the joint do not change. The joined parts do not move relative to each other because one is captured by the fork in the other. As long as the bolt still goes all the way through the fork, the strength of the joint is not compromised and nothing shifts undesirably.


A single-shear joint, on the other hand, is dependent on the clamp-up of the nut and bolt head for stability. If the nut loosens, the two bolted-together parts can move apart. If a gap forms between the two parts, the load carried by the bolt is no longer carried in pure shear. The gap provides a lever arm for the forces on the bolt, which then exert bending moments on the bolt and the bolted parts. These moments can bend both the bolt and the bolted parts. The bending causes the bolt to upset and tend to align itself with the forces on the joint. This can cause the gap to open further, and thus increase the moments. This situation is unstable, and will often lead to large distortion, or failure of the joint. Both are highly undesirable, and potentially dangerous.

Single-shear attachment of bracing wires seems to be a relatively common error, particularly among the designers of ultralight and low-performance light airplanes. It is very simple to attach a wire to a tang and simply bolt it to the hard structure. The airplane I mentioned last month as having multiple potential single-point failures in its tail had the tail bracing wires attached this way. Even more frightening was the fact that the tail was not only braced by these wires, but held onto the airplane by tensioning them. A small loosening of the bolt in the single-shear tail-wire attach joint on this airplane could cause the bracing wires to loosen and lead to the departure of the tail.

The proper way to attach bracing wires and control cables is to put a fork on the end of the wire and attach the fork to a tang on the structure or the control horn with a properly-safetied bolt or clevis pin in double shear. Strut-attach and wing-attach fittings should always be designed so they bolt together with the bolt in double shear.

In the case of the coilover mount, the load is always applied at distance from where the bolt is fixed to the mount. This distance imparts the greater load (moment load) in addition to the already doubled shear load.

"there are some significant problems with single-shear joints; they should be avoided in highly-loaded and flight-critical areas wherever possible." - Take this to mean: avoid single shear fastening in suspension components whenever possible as it is ride and safety critical

To summarize: Not only does double shear reduce the shear stress to half of that of single shear, it also reduces the bending moment because each end of the bolt shares (reacts) the moment.

It is a simple task to change the lower coilover mount to double shear. The cost is minimal, the safety factor increase is immense. I think the choice is clear.

Attached is a picture of an easy way to change to double shear. FYI, DSE uses fasteners in double shear for their upper and lower coilover mounting on the quadralink.

[Rod]

What hasn't been brought up yet is what makes single shear mounting a real killer: Moment loading. What is a moment? Essentially a torque. Yes the fastener will see a shear load, but it will also see a torque. Fasteners are not designed to be loaded in moment and shear (also known as complex loading). The moment has a tendency to bend the bolt, sometimes it is called a bending moment because of this.

ohhhh we are getting all booky and stuff.. fun, let me get a beer and sit down

[Mark@lateral-dynamics]

fun, let me get a beer and sit down

Can you please save one for me too! Or three?

[Rod]

Can you please save one for me too! Or three?

ha ha ha ha,,, beer that man