SIGNS OF CHANGE
TDC
Change is constant— and always instructive.
KEVIN CAMERON
Every now and then I’ve had moments of revelation in which, looking at a bike or its photos from the perspective of passing time, I’ve realized just how much change has taken place.
I spent 10 years (1974-84) with Yamaha’s “Great Equalizer,” the two-stroke TZ750. Because it was widely available in that period, anyone who learned to ride it could challenge factory riders. In that sense it continued the tradition of Colt’s .44-40 Frontier Six Shooter.
Yet one day, looking at an example of the TZ in my shop, I realized that in light of present knowledge it had become a sad expression of mistaken thinking. There was its quite powerful engine (maybe 120 hp at 10,500 rpm) all the way back against the rear tire, a style dictated by the narrow, hard tires still dominant during its design, circa 1972. When the bike burst on the scene at the 1974 Daytona 200, much grippier slick tires had just appeared, pioneered by Dunlop and Goodyear.
What was the predictable result? Too-easy wheelies that interrupted so many drives off corners (which is why engines today have moved to the other extreme—all the way forward). The skinny steel tubes in the TZ’s conventional twin-loop chassis were way too flexible for the grip of the new slicks. Insinuating myself into the photo area at the Daytona chicane during 1978 practice, I saw Kenny Roberts brake, turn in left, then make the quick direction change to flop his TZ (in factory form called OW-31) over to the right, straight into hard chatter. Kenny even looked irritable in that moment, and would later tell me he’d never liked the 750 “because it had too much of everything.”
Just yesterday I was looking at an English magazine, featuring on a corner of its cover a ride on Barry Sheene’s restored 1976 500GPchampionship-winning Suzuki RG500, factory nomenclature XR14. Instead of the TZ750’s four frame tubes joining the steering head, the Suzuki has six. Above the gearbox each of the two “tank tubes” (supporting the fuel tank) splits into two—one pair going to the top of the head tube and two to the bottom, greatly increasing its ability to resist braking force. On previous designs it was only the tubes’ bending strength that resisted braking force, but with this triangulated design the tubes are stressed mainly in tension or compression.
Now I looked at the front wheel, whose little discs (290mm or 11.4 inches) can now only be called “cute.” Compare to present-day production bikes, with 300-320mm discs. Real racebikes have moved even further; Brembo brings round a choice of 320, 340, or 355mm carbon-carbon discs to every MotoGR What has made this huge increase of braking power possible? Tires are fundamental, and braking force transmits as pure bending through the fork tubes. That’s the reason forks bind if not given a clearance of 0.005 inch.
With the XR14, Suzuki was just starting to increase fork-tube diameter to equal the gains it was making in chassis stiffness. Sheene’s ’76 bike had 35mm tubes, increasing to 37mm in 1978, and 40mm in 1980.
The big change came in 1987, as racing sought to overcome front chatter by putting the larger-diameter outer tubes at the top, creating the socalled “inverted fork.” Why wasn’t this done in the 1930s, when telescopies were being developed by BMW? Probably because 1) stiffness was not yet an issue (BMW’s first teles had 28mm tubes!), and 2) oil seals capable of containing fork damping oil still needed help from gravity. Today, inverted forks are the norm.
Further inspection of the photos shows obvious concern for the twisting stiffness of the fork assembly. When there’s not enough stiffness, the result is steering delay as all the parts wind up under the rider’s effort to steer, and may even slip where they are clamped. Increasing the clamping issue are dual pinch bolts at the lower fork crown and pinch bolts on both sides to clamp the fork sliders to the front axle.
Also in these photos it’s clear that axles, like fork tubes, have started to grow modestly in diameter and greatly in stiffness, contributing further to the right-now steering modern bikes offer, the steering delay designed right out of them.
Only the year before, Suzuki had started using brake discs lighter than the 7 pounds (nearly 3.2 kilograms) of the early 7mm-thick production discs. Honda early on gave its racing prototypes 5.5mm stainless discs, and in 1975 both Suzuki and Kawasaki had a go at plasma-coated aluminum discs. Alas, brake heat was too much for them, causing the soft aluminum to ooze out from between the hard surfacing. Today makers have mastered the metallurgy and heat treatment necessary to keep stainless discs flat and crack-free under heavy heat flux.
Invisible in the photos is the fact that Sheene still rode on bias-ply tires. For another eight years racers would struggle with the extra heat generated by the “scissoring” of bias plies as round tires flexed continuously through their flat footprints. By 1981 Marco Lucchinelli, riding the XR35 descendant of Sheene’s ’76 bike, could base his championship-winning strategy on the “heat crisis of bias tires.” Knowing the top men would waste their tires’ peak performance in battling each other, he hung back, riding smoothly until the tires of the front-runners suffered irreversible chemical change. Then, with his carefully conserved tire performance, he eased past helpless rivals to five wins and the title. The radialply revolution came in 1984, and the application of silica-reinforced wet weather tread compounds in 1992, giving today’s production tires capabilities undreamed of in 1976.
Sheene’s bike has a simple, unbraced swingarm very like the first rectangular-section swingarms introduced to US racing by gifted men without engineering degrees: Harry Hunt and Don Vesco (of streamliner fame). Yet even here progress lurked: That swingarm was aluminum rather than steel, a cautious feeling of the way forward. All-aluminum chassis would arrive in racing during 1980 and roll into showrooms seven years later.
Don’t we wish we could look at today’s bikes, which we so smugly (and rightly) admire, and see the future that is taking shape in their features?
In a year we’ll know more, and in 10, a lot more.
