Board logo

BEC - solving the reverse gear conundrum
Ivan - 21/8/08 at 12:19 PM

Just a thought for what it's worth - not sure if it's been done before

Dump the alternator, install a suitable old style generator, belt driven off the prop-shaft with a switched lever arm potentiometer.

Put car in neutral, reverse current flow to generator through suitable electronis mechanisms and hey presto a variable speed reverse.

Only disadvantage - a tad more weight and no charging at idle. But then a generator generally doesn't charge at idle speeds anyway.

I know this is a bit simplistic but would be interested in the reasons it won't work.

sickbag - 21/8/08 at 12:29 PM

Or . . . .

. . . Use the starter motor from a suitable bike and bolt a pulley onto that, with a pulley bolted to the UJ flange, then select neutral - pull lever - and backwards you will go.

That's the method I'm going to use on mine when/if I race it.

Ivan - 21/8/08 at 01:03 PM

Only problem with that is you are carrying the weight of the alternator as well.

eznfrank - 21/8/08 at 01:03 PM

What about a broomstick with a shoe on the end to just punt reverse everywhere??

God I hope I pass before April!!!

loggyboy - 21/8/08 at 01:08 PM

Surely for all the extra designing, manufacturing (both costs and effort) it would be easier to fork out for a reverse box.

alanr - 21/8/08 at 01:15 PM

a reverse box will reduce power and slow the car down

motorcycle_mayhem - 21/8/08 at 02:07 PM

On a theme.... alanr has brought up a point that I have thought about....
Has anyone (alanr?) run a rolling road with and without a reverse box.
I currently consume Westfield chocolate reverse boxes, an expensive practice.
I intend to fit a AB electrickery job and a centre bearing propshaft this winter if the car survives the 2008 season (only 3 events to go, touching timber).
How much of a transmission gain (at the wheels) I get will be quite interesting - anyone know some figures??

alanr - 21/8/08 at 02:11 PM

It was AB over at Bury St Edmunds that told me a qualfe reverse can reduce power by up to 12% (well on my Blade) and as every bhp is important, for the sake of occasionally pushing the car backwards -- don't fit one

worX - 21/8/08 at 02:45 PM

I reckon 10% would be about right!
But then I've not done your test, just going from what others have said (inc Andy).
Horrible to thought that you are losing 10 - 12% AND ADDING weight, just to reverse out of Tescos...

Steve

alanr - 21/8/08 at 02:46 PM

Quite agree -- exactly my point

[Edited on 21/8/08 by alanr]

kreb - 21/8/08 at 03:00 PM

It makes me wonder how much power is lost to spinning a drive shaft? It would seem that one of the benefits of a middie BEC would be less power loss through a chain drive.

[Edited on 21/8/08 by kreb]

02GF74 - 21/8/08 at 03:25 PM

quote:
Originally posted by Ivan
Dump the alternator, install a suitable old style generator, belt driven off the prop-shaft with a switched lever arm potentiometer.

I doubt it whether a generator will be particulalry efficient as a motor; likewise a motor if asked to behave as a generator; they will have been designed to be optimum doing one job.

You wouldn'tbe using a potentiometer as it would be wasteful of power and something that can dissipate the heat would be £££. More likely PWM.

quote:
Originally posted by Ivan
Put car in neutral, reverse current flow to generator through suitable electronis mechanisms and hey presto a variable speed reverse.


Not sure it is as easy as that. alternator run at about 2x engine speed, you would be running of prop so about 4 x engine speed engine need to gear it down.

I would expect you would also need to gear it down when the generator is driving the prop (look at starter motor pin ion and fleywheel ring) so that conflict the above.

So apart from the electronics,I would expect you would need to have some way of selecting gears between generator and motor.

alanr - 21/8/08 at 03:25 PM

Well that's what Ive seen on a R1 powered Mini -- as you say, it's got to be a lot less power loss than a prop

motorcycle_mayhem - 21/8/08 at 07:31 PM

Well, hopefully I'll be able to give a definitive answer this winter.
I've heard the 10% loss thing, but that doesn't really sit well with my ignorant perceptions, i.e. that my car currently has 140bhp at the rear wheel. If I've lost 10% through the chocolate box then I'm looking at at 10KW heater, I guess that's why the chocolate melts. I'm happier with 1-2%, but then I'm deluded.
When the scrutineer asks me to go backwards, the car goes backwards. If I take the car out on road, I'll doughnut my way out of Tescos.

sickbag - 24/8/08 at 06:22 AM

quote:
Originally posted by loggyboy
Surely for all the extra designing, manufacturing (both costs and effort) it would be easier to fork out for a reverse box.


But not really in the spirit of locost though is it.

sickbag - 24/8/08 at 06:32 AM

Manufacturers of bikes, and actual power tests, of bikes have found you lose between 10 - 12% of engine power through transmission losses. That means the primary drive, the wet clutch, the gearbox, and the final drive chain.

Although I haven't sen the internals of one of these 'boxes I would assume that 'forward' would be straight through the 'box - with no other gears being turned. Only when in reverse would the idler (or any other gear) being turned.

So I'd be very suprised if just a single gear cluster would lose the same amount.