Thoughts/help for a new boy....

Have you made or bought a converted vehicle if so this is for you
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EVguru
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Postby EVguru » Thu Sep 11, 2008 3:39 pm

Jeremy wrote:Ideally a simple two speed gearbox would do the job, as few road going EVs need more than two gears, often quite widely spaced.

Three gears; reverse is quite useful too!

I know it's easy to do electrically, but you get it for free with a standard gearbox and you can remove the unused gears to reduce drag if you want to go to the effort.

I've wondered about using an external overdrive unit instead of a gearbox. The main snag with some of these would be whether the internal clutch could take the torque. I managed to break a couple of Laycock overdrive units years ago (fitted to a 3 litre Scimitar), in both the cone engagement clutch failed.


It's been a common discussion on the EV list over the years. The ratio change is often relatively small, just designed to to drop a thousand rpm or so at cruise in top gear. There are drag race units that work on a similar principal, but they tend to be quite heavy (and expensive).
Paul

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Jeremy
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Postby Jeremy » Thu Sep 11, 2008 4:11 pm

The nice thing would be to get a lightweight 2 speed box (accepting that electric reversing is easy). Standard car gearboxes are pretty heavy, but then I'm obsessed with making things as light as possible (it comes from having a background in aircraft design!).

I agree about the ratio thing, but seem to recall that some of these units had a ratio change of about 35%. This might just be OK. If the low ratio was OK from a standstill to around 50 to 55mph, then the high ratio would allow top speed to be around the legal limit. Has anyone looked at using the internals from a 4x4 transfer box? These often have ratios of around 2:1.

Having said this, it's interesting that Tesla have now dropped the two speed box from their EV and gone for a single speed system.

Jeremy

Wilfried
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Postby Wilfried » Thu Sep 11, 2008 11:03 pm

Jeremy wrote:Standard car gearboxes are pretty heavy

Would a (modified) gearbox from a motorcycle (Harley?) do the trick ?

Wilfried

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qdos
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Postby qdos » Fri Sep 12, 2008 7:00 am

OK I've mentioned this before elsewhere.

I have a 350 kg quadricycle ACREA Zest (I'm actually the UK importer but this isn't meant to be a plug) which is petrol driven currently at 15kw (20 hp) It uses a CVT box which has forwards and reverse and I've had it to just over 120kmh after that the rev limiter cuts in and acts as a break (3600 rpm)

Now I know CVTs are not the most efficient transmissions in the world but they are very light weight and easy to come by and play with, particularly compared with a regular car transmission.

For a non sports machine the Zest is a great fun car and is so easy to drive. A lot is down to the transmission. The power is a bit frustrating at times and it would be nice to have another 10hp on tap but it's plenty to cruise along motorways with and it's wonderful on the smaller winding country roads. The only things that really bug you at all are long hills and fast overtakes. Basically with 20 hp and 350 kg you can cruise along pretty much with the performance of a luxury coach.

To me this set up makes an ideal platform for a EV and it's fun playing with the aerodynamics it's amazing what a few very minor tweaks can give you on top speed I've managed to get an additional 10kmh very easily and think I could manage another 10 to 15 kmh without doing anything to the engine or transmission.

The question is really do you want a comfortable leisurely cruiser or a bit of a sports car? If you want performance then you need to spend more money otherwise you could get by with a Lynch (maybe 2) and the CVT

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Jeremy
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Postby Jeremy » Fri Sep 12, 2008 9:12 am

Another option is to look at using two PM motors driving a common shaft, with one geared lower than the other. The lower geared motor would be coupled to the output shaft via a one-way clutch bearing, to prevent it being over-driven.

As PM motors can be pretty light, this might well be a reasonable solution to broaden the useful power band. The trick would be to find relative ratios that would allow both motors to share the load reasonably equally in the high mid-range, which would mean careful matching between motor characteristics and reduction ratios.

A chap on the ES forum has effectively done this with a bike. He's fitted a high power, high speed, hub motor to the rear wheel for cruising and augmented this with another motor driving the rear wheel via a chain (with an over-run clutch, I believe). It reportedly works well.

Jeremy


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