EVguru wrote:It's very tempring to think that getting rid of the standard gearbox MUST be more efficient, but that's not always true.
To be honest, Paul, I cannot assess the relative efficiencies of the various systems so have to take your word for this. I note what you say about GM and their single motor decision, also.
My approach is simply to try to avoid any complexities which can be eliminated; the approach of the one-off builder rather than large manufacturer. I know we've been talking generally and probably looking for optimum solutions, so I accept that GM's solution may well be the best way to go.
Originally I asked about power/weight ratios with the objective of creating a single speed clutchless design, based on my approach to building; ie, as simple and light as possible. For an EV with it's need for a large load of batteries I believe this is quite relevant.
I feel this is worth exploring as an idea because others attempting one-off conversions might also find it useful. Malcolm, for example; right now working on a twin motor set-up. Losing the clutch, gearbox AND differential and avoiding the expense of a large, new motor seems very good sense in simplification terms, even if electrically it may not prove so efficient.
Finding a final drive and differential unit of appropriate weight/strength and with exactly the right ratio, to avoid the losses, complexities and extra weight of a two-stage reduction will be no problem for GM, but for us is not that easy.
Efficient design is, as you've said about OVERALL efficiency and for any designer this also includes system cost, in both time and money.