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Old 06-02-2009, 05:14 PM   #89
Any bike can go anywere
Andy-Gadget's Avatar
Joined: Jul 2006
Location: Hobart, Tasmania
Oddometer: 1,363
I have been doing some research into the whole “roll centre” – “centre of gravity” thing, and have come up with the following thoughts, which I now share with you for discussion/character assassination:

All the information I can find about roll centres has to do with cars, which have suspensions completely different to the suspensions seen on almost all sidecars.

By different suspensions, I refer to the arc that the suspension allows the wheel to travel through.
On cars, the movement is up and inwards, necessary to ensure that the flat car tyre tread remains flat to the road.
Even very simple car suspension, the model T type solid front axle, does this.

Motorcycle suspension works on a different plane, as it needs to, being after all a single-track vehicle, combined with rounded profile motorcycle tyres it isn’t needed to move in any other plane.
That plane is purely vertical with no lateral movement relative to the chassis at all, or at least this is what the designer had in mind, the “hinge in the middle” Japanese chassis of the 70’s being an example of result not matching design intention.

I am forced to interpret this major difference to mean that the volumes of work done on car suspensions is of little use when applied to sidecars, made even less relevant if flat treaded sidecar tyres are used, as the contact patch area is influenced by the angle of the wheel to the road surface.

Some of the points made with regard to car suspensions hold true, such as the use of “roll bars” to ensure the correct tyre contact patch relative to the road surface, but is done for different reasons.

On a typical car system, the virtual centre of pivot of the essentially side acting suspension system ensures that the tyres remain correctly oriented to the road surface, and so the “roll bar” is just that, it controls body roll, which at small lateral force situations is a comfort driven system.

The typical motorcycle system, being longitudinally based, means that the body roll of a sidecar also causes the tyre contact patch to roll relative to the road surface, less of a problem with sidecars still fitted with motorcycle tyres.
But reducing the contact patch area, and hence its ability to resist the side forces being applied to them, on sidecars fitted with flat treaded tyres.
In this situation, the “roll bar” is essential from a road holding point of view, not just comfort.

If you take my point that there is no “roll centre” with sidecars, due to the suspension orientation, then the argument that it is the “roll centre” to “Centre of Gravity” distance that is driving the cornering dynamics of a sidecar is no longer valid.
It is driven more by the distance from the road surface contact with the tyres at the three extremes to the “centre of Gravity” that is important.

Roll bars are useful to maximise the tyre contact and so resistance to lateral forces.

Off road sidecars have a higher “centre of gravity” than road sidecars due to the greater under body clearances required and will be more susceptible to “tipping” than a low slung road outfit, and there is little that can be done to help the situation other than clever loading to move the C of G to its most advantageous position between the three tyre contact patches.

EDIT, just found this site here,
"If a man empties his purse into his head, no man can take it away from him.
An investment in knowledge always pays the best interest"
Benjamin Franklin.
'95 Sport 1100 Guzzi,'84 Cali 2 Guzzi, '95 Guzzi mille, '84 R65LS, '84 R100, '82 R100RS, '05 DL650, '72 Bultaco Sherpa T 350, '78 Montessa 348, "00 XR650R

Andy-Gadget screwed with this post 06-02-2009 at 11:31 PM
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