The magic of Ducati’s trellis frame
WWhile talking with former Ducati engineer Corrado Cecchinelli at Laguna Seca Raceway one year, I asked him what special property had made his company’s trellis steel-tube chassis so effective in racing.
“It is because it was designed by a genius,” he replied simply.
That genius was Massimo Tamburini, who was associated with Ducati through Cagiva’s temporary ownership of that company. But genius is a description, not an explanation.
When I observed to Colin Edwards that Ducati Superbikes visibly wallowed in turns, he said: “Yeah, they wallow. But they dig in and go around the corner.”
An engineer would call that wallow by its proper name: weave. Weave is the side-to-side caster oscillation of the rear of a motorcycle, at a typical frequency of two to three cycles per second.
Edwards as rider had just been on point through Honda’s struggle to match Ducati’s corner grip with its RC51 Superbike. Upon buying and testing a Ducati, Honda engineers discovered it was half as stiff as the new Honda. A program of stiffness reduction was put in place. Each time side-to-side stiffness was reduced a bit, Edwards said, “I could push a little harder before it would start to chatter.”
Livio Lodi, director of Ducati’s museum and a bottomless well of lore, tells the Tamburini story differently. According to him, the Tamburini family business was plumbing, so when a motorcycle belonging to one of the family was wrecked, it was repaired using materials and techniques at hand: pipe.
Ducati’s trellis frame employs steel tubes to join the steering head to four widely separated points on the company’s 90-degree V-twin engine. Two are deep in the V between the two cylinders, and the other pair is either high on the gearbox behind the rear cylinder, or extending downward on both sides to grasp the swingarm pivot. Seen from either side, the trellis is strongly triangulated (as is a bridge) against longitudinal bending. But seen from above, there is hardly any diagonal bracing to prevent lateral—side-to-side—motions of the steering head.
There are two possible reasons for omitting such diagonal bracing. One is strictly practical: The engine’s intake system occupies
the lower part of this volume, and an airbox of the largest possible volume must fill the upper part, and neither can be pierced by diagonal braces. The other reason is to intentionally provide lateral “give.”
Was the unique ability of the trellis frame to hook up in corners just an accident of necessity? Or had Tamburini found through his own experiments that the stiffer the front frame is made, the less feel, or warning of grip loss, the chassis can provide? If he did know, he was the only one in the company who did. When in 2009
“Yeah, they wallow. But they dig in and go around the corner.”
Ducati’s Motogp team replaced the flexy trellis with the extreme stiffness of a carbon-fiber front chassis, the result was frequent loss of the front by 2007 champion Casey Stoner. Filippo Preziosi, then-chief engineer of Ducati racing, would not knowingly have planned such a disaster.
Another factor contributed to Tamburini’s result: On most Japanese four-strokes, it was tempting to rigidly brace the steering head to the cylinder head directly under it, but in the case of the Ducati, the almost-horizontal front cylinder head was too low to be used that way. Thus, Japanese bikes tended to have stiffly mounted steering heads, while Ducati’s steering heads were laterally flexible.
Honda, proving to itself the value of chassis lateral flex in the Rc51-versus-ducati Superbike struggle, next questioned the practice of rigidly bolting the forward part of their Motogp chassis to a cylinder head. Instead, they extended their forward engine hangers downward to cylinder or crankcase level. The result was a laterally flexible steering head like Ducati’s.
Why was the Tamburini trellis abandoned in Motogp but continued on most production Ducatis? The likely explanation is that ever-increasing slick tire grip in Motogp had finally overpowered the directional stability of the steel-tube frame. As Casey Stoner put it at the time, “On that thing,” pointing to the trellis-framed Motogp bike, “you can’t hit the same point two laps running.”
All this tells us is that accidentally or intentionally, Massimo Tamburini triggered the modern practice of providing lateral chassis flexibility to give front-end feel and grip, but it was Honda that understood its value and systematized the practice.
From above, it’s apparent how little lateral bracing Ducati’s trellis frames offered. Massimo Tamburini’s design proved that the stiffest frame isn’t always the fastest.
A pipe dream: NCR’S titanium trellis frames can be tuned to offer flex characteristics to suit the customer. Describe riding style, skill level, and more, get a personalized level of frame performance.