The Steering Head

[Admin]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Any book or learned paper on motorcycle stability will likely use the age-old and accurate description of the motorcycle as “two casters, one short and the other long, joined at a common pivot.”

That common pivot is the steering head, a pair of rolling-element bearings separated by about 6 inches. The short caster is the bike’s front end or fork, and its “length” in this context is the trail, typically 4 inches or slightly less. The long caster is the rest of the motorcycle, and its length is more or less the wheelbase.

At present, a MotoGP bike’s maximum normal upright braking produces a deceleration rate of roughly 1.45 G. As the acceleration due to gravity (1 G) is 32 feet per second per second, that means the bike is slowing at the rate of 1.45 x 32 = 46 feet per second, each second.

At present, bikes are prevented from decelerating any faster by the combination of their high center of mass (around 22 inches) and short wheelbase. The bike’s need to camber, or lean over in turns, dictates the high center of mass. The need for quick steering dictates the short wheelbase, typically around 56 inches for a GP machine. When you countersteer the front wheel to roll your bike over for a turn, its sideways motion steers the bike by acting through the lever arm of wheelbase—the longer the wheelbase, the less steering you get from each inch the front end moves sideways, and the slower the steering response becomes.

Brake too hard and the rear tire comes up off the pavement, and the higher it rises, the easier it becomes to lift it higher yet. So for maximum braking, the rider “pulls up to a hover” with the rear tire barely clear of the pavement, and holds it there. In his years of dominance, Marc Márquez routinely approached corners this way.

Related: Motorcycle Wobble And Weave—Making Sense Of “Death Wobble” Talk

MotoGP racers routinely lift the rear tire off the pavement during braking; here Fabio Quartararo demonstrates at Germany’s Sachsenring. (MotoGP/)

Calculating the Forces

If the bike weighs 357 pounds, the rider and equipment weigh abitger 175 pounds, and half-fuel is 16 pounds, the total is roughly 550 pounds. The braking force, mainly generated by the front wheel, is the mass of bike, rider, and fuel multiplied times the deceleration rate, or 1.45 G. In our example, that equates to 797 pounds.

From the middle of the steering head to the pavement, we have a distance of about 29 inches when the front end is mostly compressed by hard braking. The tire’s braking force acts on this 2.42-foot lever to try to wrench the steering head out of the chassis. That 2.42 feet, multiplied times the force of 797 pounds we’ve calculated, results in 1,928 pound-feet of torque on the steering head. No wonder today’s vintage riders on their classic Manx Nortons report poor braking stability, even though their old-time drum brakes can’t achieve as rapid a deceleration rate as can the carbon-carbon brakes in MotoGP. I recall riders of the 1960s referring to the front end “wandering” during hard braking.

Front-End Hop

And there’s worse. One late-1960s day at the Harewood Acres track in southern Ontario (now an oil refinery), a gentleman with a 500 Triumph asked during lunch break if anyone would object if he did a couple of laps in the reverse direction. His proposal was accepted. With his tires warmed up, he braked for a nearby turn and his bike promptly began a crazy front-end hop at several cycles per second—ruck-a-ruck-a-ruck. The bike did this every lap.

On another occasion at another race track it was an early ‘70s Kawasaki triple doing the ruck-ruck. As the rider initially applied the brake, the bike dove as the suspension compressed, adding dynamic load to the tire’s footprint and generating a very large force that flexed the fork tubes back until the tire began to slip, allowing the fork tubes to snap back and the tire to take another bite of the pavement in the rapid and violent cycle we were seeing. Since both the Triumph and the Kaw had chassis made from slender steel tubes, you can be sure that their flexible steering heads contributed to this hop.

Every time tire grip and the forces generated by acceleration and deceleration are increased, designers need to consider greater chassis stiffness.

Fork Tubes Get Bigger

No surprise, then, that racebike fork tubes (and after some delay, the tubes on production bikes as well) have steadily increased in diameter. In the early 1930s, BMW put a lot of development into telescopic-fork design, eventually fitting a tele with 28mm fork tubes on a production bike. In the 1940s Gilera decided 32mm was right, and in 1965 when the Italian-made Ceriani fork was imported into the US, it had 35mm tubes—and they were heat-treated as well.

That’s about where the numbers were when disc brakes arrived in 1969-72, and slick tires appeared on racebikes in 1974. That combination quickly developed higher and higher braking force, so fork-tube diameter jumped to 36, 37, then 39mm, and onward until we have the even larger tubes of the present day. When the assembly was inverted and the larger-diameter outer fork tubes were placed on top and clamped in the steering yokes, flex resistance was very greatly increased—the so-called “upside-down” forks of the present era.

Let’s estimate how much force hard braking exerts on the steering-head bearings. If the steering head is about 6 inches high and the lever arm of the braking force is 30 inches long, the force applied to the bottom head bearing will be roughly five times (30 divided by 6) the braking force, or 5 x 797 = 3,985 pounds. This tells us why steering heads have rolling bearings—so we have some chance of steering while such large force is being transmitted across those bearings.

In my 1965 Yamaha TD1-B the head bearings are 19 cute little 3/16-inch (4.76mm) steel balls in each of the pairs of bearing races, top and bottom. Ball bearings are not very stiff, the reason being that under load, the ideal point contact between ball and race deforms into an oval as the ball flattens against the race. To increase stiffness, tapered roller bearings are the choice.

Bracing the Steering Head

Up until the 1970s many a new motorcycle chassis was inspired by the twin-loop Manx Norton frame of 20 years earlier, but the Norton’s excessive steering-head flexure under increased loads (better brakes, better tires) soon saw chassis tubes extending to both bottom and top of the steering head from just in front of the rider’s seat, greatly increasing the steering head’s resistance braking-induced deflection. The triangles thus formed would evolve into today’s two large chassis beams, while frame members that traditionally looped under the engine atrophied away, leaving the engine itself as a stressed chassis member.

Often the engine is used as a stressed chassis member in modern motorcycles. (Ducati/)

Slick tires also sped up the process of corner entry. As we have seen, muscular pro riders in a hurry have been known to bend bars or clip-ons permanently as they applied large steering forces to achieve rapid rollover or direction change. This meant that steering heads had to be stiffened to better resist right/left twist as well. Such construction reduces steering delay (the time taken for the chassis to twist under steering forces), and has made modern chassis much quicker responding to our inputs. It feels good.

You may object that street riders are neither as strong as racers, nor are they in as great a hurry. True! But since technology has so greatly increased braking force and tire grip, it’s important to make their benefits available to the everyday rider too. Yes, we can perfectly well enjoy a ride on a 50-year-old classic flexi-flier, but we should know that such antiques, ridden hard, are capable of brake hop, flex steer, and other unproductive behaviors.

Also, have a look at photos of early Superbikes of the 1,000cc sit-up era, 1975–1982, and you will often see a slick tire on the back and a patterned-tread tire on the front. The reason? The extra grip of a front slick upset the more flexible chassis of the time and resulted in chatter, but the lesser grip and greater flexibility of a patterned tire often banished that chatter.

Cast twin-beam frames now incorporate the steering head, increasing rigidity. (Yamaha/)

Second-generation Superbikes such as Honda’s 750 Interceptor (1983) were intentionally given much stiffer chassis that made it possible to run slicks at both ends.

Chassis evolved rapidly in the decade after 1980, leading to the two dominant types of the present—the twin-aluminum beam chassis pioneered by Antonio Cobas, and the straight-tube “trellis” chassis that has evolved out of Massimo Tamburini’s 916 Ducati.

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[fastbob]

Colin Colin Colin .. none of us are getting any younger mate . You really can't afford to waste good riding time pondering stuff to this degree . Give it a rest old chap . Motorcycles were just a brief glimmer of beauty in the headlong march to  oblivion . Just tell us what biscuits you like ..

Edited July 2, 2022 by fastbob

[Guest]

age-old and accurate description of the motorcycle as “two casters, one short and the other long, joined at a common pivot".

 

I've lost count of the number of people who have said this to me. At the petrol station, down the shop, at a cafe, even queuing at traffic lights. Yes it's two casters, one short, one long, stop going on about it.

 

 

[Bender]

4 minutes ago, goat said:

age-old and accurate description of the motorcycle as “two casters, one short and the other long, joined at a common pivot".

 

I've lost count of the number of people who have said this to me. At the petrol station, down the shop, at a cafe, even queuing at traffic lights. Yes it's two casters, one short, one long, stop going on about it.

 

 

It's magic I tell ya

[billy sugger]

I always though it was my hands doing the steering

[fastbob]

54 minutes ago, billy sugger said:

I always though it was my hands doing the steering

On my first GSXR 1100 It was definitely the bike doing the steering . I was just a passenger on that evil thing .