20.1 Centre Of Gravity.
Date: Wed, 20 Mar 1996 20:51:13 -0500 (EST)
From: Joe Schofield.
As ever (well, often) from this contributor, here's two pennyworth on the centre of gravity debate inspired by a book I once read (and still posses).
Vic Willoughby (veteran, respected UK motorcycler - not biker! - journalist) and Tony Foale (well known 70's and 80's UK frame builder and innovator) wrote a book together (published in 1984) on chassis design called 'Motorcycle Chassis Design - the Theory and the Practice. I've been studying it in the light of the recent thread on Centre of Gravity, and I've extracted the following lines which may be of help:
"... Just as important as the amount of the machine's weight are its distribution and, as the following considerations show, the location of the mass centre.
Balance. Low weight and a low centre of gravity both facilitate good balance. Fig 2.22 (nice illustration of low c of g and high c of g and proportional unbalancing effects) shows how, for a given angle of lean, the unbalancing couple is directly proportional to the weight and the height of the centre of gravity.
Angle of lean. The angle of lean necessary when cornering is slightly affected by the centre of gravity height.
Weight transfer. Under braking, weight is transferred from the rear wheel to the front; under acceleration the transfer is in the opposite direction. Lengthening the wheelbase decreases the weight transfer, as does lowering the mass centre height and reducing mass."
And so on. The bottom line of all of this - and there's a lot more - is that the closer the centre of gravity is to the roll axis, i.e. the line joining the two tyre contact areas, the more desirable it is from the point of view of a rapid and effortless change in banking angle.
What's interesting here is that we knew all this all along, and so did the chassis designers at Norton, Triumph, BSA, etc. That's why the cases of Brit twins are snugged down as low as possible between the frame rails, and why nothing contemporary, or possibly built much later, will change direction quite as quickly as any low-slung Brit Iron dinosaur. I'll cite the Norton Dominator here of which I have direct (650SS, once-upon-a-time) experience, but you fill in your own preference on the dotted line.
I don't wish to cast aspersions at Japanese designers, but I will state categorically my own belief that they have a different definition of the word "design" than classical Brit Iron designers did. Japanese designers gave us anti-dive systems, balance shafts, on-demand extra choke carbs and EXUP exhaust valving. These things are clever stuff by any standards but invariably treat the symptom rather than the cause. "Get yourself out of a design problem by designing something else that is both clever and complex, for this in itself is a goal," might have been the reigning orthodoxy at Japanese design schools. It's smart, expensive and trick... and it works, sort of. British designers - and others - marched to the beat of a different drum: (forgive me from quoting from William Bushnell Stout, designer of the 1920's Ford Tri-Motor air transport) "Simplicate and add more lightness".
One of these concepts is a purist design philosophy, the other simply a means to an end. When, barely slackening pace, I gracefully left-right-left at high speed through a series of tight bends, feeling for the road on either side with my toes, I'm grateful that the people who designed my bike's chassis thought it wise to get the weight down and put it as close to the road as they could.
Joe Schofield.
