Author: Michael McCoy

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The Science Behind Bolts

Materials can deform both elastically (returning to its original shape and structure) and plastically (permanently changed in shape, structure, or both). Every material under any sort of load (lets consider a fastener in tension) is deformed elastically, even if it is such a small amount that it’s impossible to notice. The strain (deformation, elongation in this case) in the sample is proportional to the stress on the sample (force over area) times Young’s Modulus which is a property of the particular material.

Stress vs strain: E, the slope of the curve in the elastic range, is Young’s Modulus. This is an indicator of the stiffness of the material.

When the stress on a test sample reaches the elastic limit (aka the yield point) of the material in question, the deformation becomes permanent. For example, a head bolt or stud will elongate slightly (and temporarily) as it’s tightened up to its yield point, then it will begin to elongate more per unit of stress applied to it, and the deformation will be permanent. The deformation at this point is generally uniform, that is to say it occurs evenly along the length of the bolt, until the ultimate tensile strength of the fastener is exceeded and it fails (no longer provides the correct tension). At this point, a noticeable “necked down” section appears If more stress is applied, and the bolt will eventually fracture (break). Some fasteners, such as BMW head, main bearing, and rod bolts, are designed to be torqued past their yield point and deformed plastically when installed. This leads to a joint that is very resistant to fatigue in the fastener, because the tension in the bolt cannot be released and reapplied. The bolts also strain harden as they are tightened, increasing their ability to hold further loads applied after they are installed.

What does this mean to me?

Never reuse a bolt that requires angle torque, or that BMW requires be replaced. BMW specifies torque angles for fasteners that are torqued to yield because they are a more consistent way to specify the installed strain in a fastener than a torque value that can vary with the specifics of the application, such as lubrication on the threads.

Also, whenever a torque value is specified with a particular lubricant on the threads, ensure that it is always used. Failing to do so can lead to different amounts of stress in the fastener (and thus different clamping loads) at the same applied torque.

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Suspension 101: Swaybars & Roll

IMG_8127Good suspension is meant to keep tires in maximum grip as much as possible. Maximum grip occurs when all 4 tires are in their “happy zone” temp and load wise. When cornering in a high-performance vehicle, load transfers from the inside tires to the outside tires due to the the lateral acceleration of the vehicle. Load transfer is a function of track width and CG height and it will happen no matter what suspension is installed. This transfer is what we aim to minimize with good suspension set-ups.

We want cg height as low as possible because for each unit of load we add to a tire, we have to take it off another tire. For each unit of load added, we gain less overall lateral grip than we lost from the removal of that same unit of load on the other tire. Body roll is only important in its effects on suspension geometry. We can control this roll with springs without changing the lateral load transfer. If we use sway bars to control this roll though, we are using what are essentially torsion springs to move load from the inside tire to the outside tire through the chassis. (the chassis is essentially *pushing down* on the outside wheels to try to level itself out, and its getting the force to push down on the outside tire by stealing it from the inside tire.) The other side effect of this form of roll control is limiting rebound travel of the inside wheels.

IMG_8128

Remember, we want to minimize the load transfer during cornering. This is why we try to get the CG (center of gravity) as low as possible. Change in spring rate = no change in lateral load transfer.    Sway bars = change in lateral load transfer. That is a fairly simplistic view of it, but that should hopefully give you a better idea of the basic functions.

As Milliken & Milliken from Race Car Vehicle Dynamics put it: “Anti-Roll Bars — these are usually in the form of a torsion bar spring which connects the vertical motions of the left and right wheels. No twist of the torsion bar takes place if the wheels move up and down together (ride), but in roll the bar is twisted as one wheel moves down and the other up from some initial position. Twisting of the bar adds load to one wheel and revoves it equally from the other. Anti-roll bars change the distribution of the lateral load transfer between the front and rear tracks, and also reduce the body roll angle and add to the one-wheel bump rate of the suspension.”

“Load transfer is independent of spring rate. If the wheels are truly independent, then the LLT is only a function of track width, CG height, and cornering force. When you add sway bars to the mix, they wheels are not fully independent anymore, and that is where the added load transfer comes from.”