Virtually *all* syncromesh and non syncromesh gearboxes have all the forward gears in mesh *all* the time. Reverse alone is usually on a slider. Half the gears are fixed to shafts, the other half are on bearings (in fixed and free pairs). Syncromesh or not. SAAB, VW, Webster (non-syncro), Borg-Warner T-10, they all do it.
Secondly any gearbox with a reduction of more than 5:1 will likely have more than one stage. Partly to keep gear size under control. A 10:1 reduction with a 2.25" pinion then has to have a 22.5" mating gear. which means a gear case more than 24" in diameter once you include bosses. That's 3X the diameter of a typical FWD car clutch and probably twice the diameter of the bellhousing. A two stage reduction can get you about as far as 25:1 before it becomes less practical. I have seen crownwheel and pinion setups that go to 7.17:1, but they are not common. The crownwheel is in excess of 15 inches in diameter. It goes in the rear axle of a sixteen ton bus. Further reduction at the axle is done with an epicyclic ahead of the pinion to make it a two speed rear axle. Again, the exception, not the norm. Look at the Brusa AT 1200 for example. (www.brusa.li) from the measurements you can see that it has to be at least a two stage reduction to get the ratios they claim (7:1 to 14:1) and fit in the shape with the center to center distance. That is an EV AC drive fixed ratio gearbox. As for inefficiencies that are the same regardless of single or multispeed, there would be bearing losses in input, intermediate and output shafts, shaft oil seals, losses in the differential. Windage losses from fewer rotating parts would be less, but many sources of loss remain. So taking out a few gears would gain a bit, but there are may losses that are not related to whether it is multispeed or not. I was using the VW golf as an example of a FWD car, they sold about 20 million of them in gas and diesel models and the tire size and engine speed is pretty common for that sort of car. I also happened to have the factory workshop manual handy. Final drives vary from 3.67 to 4.25 and first gear is about 3.45, second is between 1.8 and 2.2. Depending on the engine and model. *My point* was that AC wasn't a cure-all and that although very flexible, they still leave something to be desired as far as the torque-speed envelope goes. Both mechainically and electrically. Seth Peter VanDerWal wrote: > > > > > > >First, what rubbing surfaces in a syncromesh? Syncromesh is engaged or > >not, with a transition of sliding lasting perhaps a few hundred > >milliseconds. > > > <Sigh> I didn't say the sliding was in the syncro's . Metal sliding on > metal is an inherent property of syncromesh transmissions. > You said it yourself;" all forward gears in mesh all the time"....think > about it. > > > > >Second, in a traction AC drive you are limited in how low you can gear. > >At 13:1, a likely first gear (VW golfs came with 12.7 to 14.5 for first > >gear depending on engine), the motor has to hit about 13,000 rpm for > >highyway speeds, assuming a 22.8" wheel, > > > Ok so it's not exactly the same as first gear on your vehicle, call it > first and half (more than first, less then second) > > -snip- > > > > >All my texts say 98% efficiency for helically cut gears, 99% for spur > >cut. Which isn't that much for loss. If you picked up 2% by shuttling > >around on the efficiency map for the AC motor (which is very doable) > >then you could at least break even. And as a bonus, multispeed manual > >gearboxes are quite common, unlike single speed. > > > Well thanks for proving my point. According to you a single reduction > transmission would be about 98% efficient, whereas MEASURED efficiency > on multispeed manual transmissions is around 90% give or take a percent > or two. > > Actually 98% for helically cut gears assumes a ratio of 3:1 or less. > Higher ratios tend to have slightly more loss. -- vze3v25q@verizondotnet
