Talk:Roll center
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The "roll center" can be calculated for each end of the vehicle, and a line connecting them is called the "roll axis". The position of the roll axis has no effect on total weight transfer. Weight transfer is determined by mass, lateral acceleration, track width, and center of gravity height. The distribution of "weights" on the tires is affected by the springs and bars and geometry, so although the total transfer is independent of these factors, the transfer at one end can be increased and necessarily the transfer at the other end is reduced by the same amount.
I think the concept of "roll center" is misleading. First, it's a calculated position that's the result of the car's behaviour. It's very difficult to predict where it will be and the effort is pretty much a waste of time because you can get the answers you want by much easier means. Second, the car doesn't actually roll about it. It's based on pure link geometry while the vehicle behaviour is based on force balance which is assymetrical side to side so you really should be scaling the geometry calculations as the side forces vary. In a dynamic system that contains some unstable elements this is pretty difficult. It's better to deal with the instant center because it's the important part and it's easier to understand and work with.
Addition to my previous comment: When I say instant center I'm speaking of the instant center of one side of the suspension relative to the ground; wheel, upright, and links. When the chassis moves, this geometry changes. The definition of roll center includes both suspension sides relative to the ground, so movement of the chassis up and down and in roll moves the roll center around. The pure geometric analysis ignores some very compliant parts such as the tires. Doug Milliken has had good success using static geometry analysis to predict dynamic behaviour but he definitely includes all the compliances in the analysis. Moving the chassis moves the instant center and so it affects the way tire loads are fed into the chassis. Suspension should be designed so that the instant center "behaves" in a manner that suits what you want the chassis to see. It's possible to get the jacking forces to increase with bump or with droop but there are camber consequences. Since the jacking force is proportional to lateral force developed by the tire, it's best to keep the jacking force fairly low or the car will react to changes in grip levels. Drivers don't like cars to change their balance when grip levels change. Many high speed aero car (F1, Indy, Champ Cars) designs pay no attention at all to instant center movement because they require such stiff springs that they don't move enough to make much change in the jacking force. Or else the designers don't know or don't care. Aero drag and stiffness considerations appear to be more important. I would love to do a test with one though, to change just the geometry and see if it was worth anything. I suspect that for the most part the driver would say he liked some of what he felt but he didn't like being 1 mph slower. Likewise the concept of a camber curve is completely ignored. They place the instant center near the ground at the front and up a bit at the back and that's it. If you try to run the car at a different ride height it really doesn't like it. Alec Mar 28 05
I agree with most of what you say, none the less, people do use it and worry about it (I do, in fact for steering feel). Howver, just because a concept is practically useless doesn't mean that it shouldn't be here. I've also added the SAE definition.
Greglocock 05:52, 8 February 2006 (UTC)
[edit] FBRC
the sae definition is for a height only, it does not recognise (and with good reason) the lateral location. I'd also change my comment above, RCH has a very strong effect on the roll stiffness of the car and is not 'practically useless' at all, it is just that it means many things to many people, ie it is poorly defined and poorly named, rather than being useless. Greglocock 00:44, 26 July 2007 (UTC)
[edit] Wikification
I added some wiki links and categories, changed 'tyre' to 'tire', and added an 'unreferenced' tag, since there are no sources cited either inline or in the reference section. DLPanther 05:20, 31 July 2007 (UTC)
- Note that changing tyre to tire is contrary to WP:ENGVAR and has now been corrected. --Athol Mullen 10:52, 31 July 2007 (UTC)
- My mistake, sorry about that. Shouldn't 'roll center' be 'roll centre' then? If so, I'll let you change it as I don't speak that particular variety of English and don't know what's proper and what isn'tDLPanther 15:36, 31 July 2007 (UTC)
