Avoiding Impact Loading

[Hugh Janus]

Chassis flex can reduce sharp peak forces transmitted to a rider’s body, increasing performance, comfort, and stamina. (Brown Dog Wilson/)

We all know that where people work on their feet, everyone is more comfortable and productive if there is cork or rubber flooring. I have spongy rubber floor mats at my porting bench. What do they do? They protect our joints from sharp impacts that, totaled over days and weeks, add up to soreness.

Motocrossers speak of chassis “harshness” as setting the upper limit on what they can survive through a full moto. If chassis flex clips the sharp peaks off the forces that it transmits to the human body, that body works better and delivers greater stamina.

As I’ve noted before on this site, roller chains couldn’t take over the motorcycle powertrain job until there was a flexible element between them and the engine’s hard combustion thumps. Rollers split and flew off, and joints tightened until some form of spring drive was added for their protection.

Come to think of it, I strongly suspect the present long lives of drive chains are not entirely the result of O-ring construction, which seals in and retains lubricant in every chain joint. Also important is the rubber cush drive in the rear wheel which, like chassis flex for MXers, clips the sharp peak forces that would otherwise have hurtful results—in this case more rapid fatigue and stretch of drive chains.

Why do modern motorcycle engines with digital engine control so often have cylinder-head-mounted accelerometers? They are there to detect the sharp peak combustion pressures caused by detonation—an abnormal form of combustion. Those peak pressures are known to shock rod and crank bearings into failure while they are doing the more obvious damage of blasting bits of aluminum out of our piston crowns. When detonation is detected, the ECU instantly pulls back ignition timing to stop the knock-knock-knocking before it can damage parts.

When Keith Duckworth’s master work, the DFV Formula One engine, went into service in 1967, it would from time to time break a tooth or so off one of its cam drive gears. Calculations had been made to estimate how much torque this drive had to transmit, but the actual forces in the running engine turned out to be 10 times greater. The reason was that both the crankshaft and the cams have their own dynamics, flexing constantly in twist as cylinders fire and valves are heaved up off their seats and set back down again at 1,500 Gs. When one set of parts is trying to go north when another is unwinding to the south, the resulting momentary forces can be tremendous. Since the engine was already in production it would have been very expensive to redesign the whole cam drive, so Duckworth came up with something he could build into one of the gears, containing a circle of tiny torsion bars, each with a little crank at one end. This added just enough flexibility to the drive to clip off the tall force peaks that had been making gear teeth old before their time.

Other makers, faced with similar problems, have found solutions in such things as flywheels or dynamic “Napier dampers” (Tootsie Rolls in holes). Or they have tried driving the cams from the flywheel end of the crankshaft, or from its center, hoping for “a quieter life.” There’s a lot more to engine development than ordering in 50 grand’s worth of super-rad parts from the usual suspects and bolting them together.

This Honda six crankshaft drives the cams from the center because that location has zero torsional motion. This reduces vibration and impact loading. (John Owens/)

When an aspiring World Supersport team’s rider kept reporting something weird in the bike’s rear suspension they hired a crew chief with experience (as opposed to just a youthful willingness to work 24 hours straight and then drive the transporter to Czecho). Removing the spring from the rear suspension unit, he then cycled the assembly through its full travel. Or tried to. There was a glitch, a momentary resistance at one point. That, added to what the rear tire was already being asked to do, was sometimes too much. It was kicking the back end loose without warning. Gotta watch those peak loads.

Aircraft structures are “lifed” for a specific number of total flight hours. Such calculations are based upon years of experience with gust load intensity encountered at various altitudes, based upon the aircraft’s mission requirements. What puts an end to the life of an aerostructure? Fatigue or stress corrosion cracking, as a result of X spectrum of stresses. At present a new technique has been brought to bear on this problem: gust relief. What if, as a gust suddenly begins to accelerate a wing upward, the aileron on that wing were quickly deflected to reduce the severity of the gust load? It works.

I also recall reading that the long-serving B-52 bomber’s airframe was lifed for operation mainly at high altitude where low atmospheric density just can’t muster as many bumps and thumps. When those aircraft were brought down to lower altitudes for use in the Southeast Asia war their airframes fatigued considerably faster owing to rougher air.

Rougher air at low altitudes fatigued the B-52’s airframe faster than at high altitudes where the low atmospheric density had less impact. (U.S. Air Force photo by Senior Airman Juan Torres/)

This begins to sound a lot like what Ducati has done with its Evo2 anti-wheelie system. Instead of waiting for the wheel to rise a certain distance before intervening (by reducing engine torque a bit), Ducati’s system looks for a rapid initial upward acceleration rather than a specific upward displacement.

And now helmets. A famous movie star regarded a motorcycle helmet as a serious infringement of his personal freedom, so when he had the misfortune to tip over, in the resulting tumble he hit his head on something hard with nothing between to clip the sharp peak off of the impact force. That peak load made his hospital stay a lengthy one.

Same with auto-inflating “air bag” riding suits. That collarbone may not break this time if, as your shoulder approaches ground zero, a suddenly inflated air bag clips the sharp spike off of the impact.

I mustn’t leave out the ultimate impact-reducing system, essential to every vehicle, operator, and passenger: the brakes.

Source

[Tym]

Julian would have a field day with this chit!