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Learning how to build Lotus Seven replicas...together!
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PostPosted: September 5, 2023, 11:56 am 
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Joined: September 4, 2023, 9:53 pm
Posts: 1
Location: Upstate South Carolina, USA
I am starting design work on a pseudo-replica Morgan three wheeler. I built a cyclekart-like one with electric drive a few years ago: https://www.cyclekartclub.com/forum/cus ... #msg-46622. That one was 5kw/48v electric and about 3/4 scale — sized for me and a dog. Body was primarily wood and it would do a (scary) 35mph. I never weighed it, but I could pick up the front end myself to adjust its parking orientation.

Now I want to make a new one, to real road-worthy spec and able to carry two real adults. Bike-engined, but with the engine under the front hood rather than a v-twin hanging off the nose — may or may not even be a v-twin, depends on what I find when I get to that point. I’m aiming for cheap, not going to be looking for an engine pretty enough to use as an art piece. I’d rather not try to replicate what Morgan did for engine and transmission, and instead just go for the engine + transmission BEC all-in-one package. Attempting to attach an image (initial design in SketchUp because it's so easy, but I'm moving to OnShape for more accuracy etc)

I’m trying to determine what my drive train will be, given that the motor will be in the front and the drive wheel in the rear:

1. Simplest: Chain from the engine sprocket to a sprocket on the rear wheel. The distance is about 7.5 feet, and the tunnel would need to be slightly offset (passenger needs to be skinny

2. Simple: Chain from the engine sprocket to a jackshaft on the rear swingarm, then another shorter chain/belt from the jackshaft to the wheel. This allows the tunnel to be central and the jackshaft gets power around to the side of the wheel. Also allows easy swapping out sprockets of different sizes if necessary, but has both a loooong chain run (6 ft) and a short chain run (1.5 ft)

3. Complex: shaft drive from engine to a bevel gear box, driving a sprocket with a short chain/belt to rear wheel. This is basically what Morgan does I believe. I can probably find a shaft drive motorcycle engine, e.g. a Honda VT1300.

4. Wildcard: front wheel drive

I’m hoping for a two cylinder engine somewhere between 50 to 100 HP and 45 to 75 ft-lb of torque.
Priorities overall will be:
1. Light weight
2. Low cost
Oh, and I guess “safe” too, though I would consider this more of a parade car than a track car :-).

Option 3, the Morgan setup, seems like it would be bad for my priorities — sprockets and chains are dead simple and there are more chain driven engines available…. and drive shaft, universal joints, bevel gears, etc etc all add weight and cost. I’d feel silly spending more on the bevel gear than the second-hand engine and transmission, while adding 80 lbs. Car parts are heavy.

I’ve found a few posts suggesting a long chain isn't a good idea, but it's not entirely clear why or if it's just "I'd never do any chain" sentiment from the poster, e.g.:
- viewtopic.php?p=50391#p50391:
- “I'm not sure how practical or safe it would be to run a long chain from a front motor to a rear differential”
- "I wouldn't even consider a long chain."
- viewtopic.php?p=33336#p33336
- "Unless you are building a mid-engined car I think a long chain would be impractical"

This guy has a pretty long chain: https://www.youtube.com/watch?v=KbCXo6fo0-Y&t=376s. It certainly seems to bounce around a lot in his video but I'm not exactly planning on doing what he's doing! And with about 1/3rd less HP to boot!

My questions:
- My co-builder (college son does better welding than me) thinks having a long chain under your elbow is a safety issue. If/when it breaks, it’ll whip around and kill/injure you while it is getting pulled through the tunnel. A shaft can have enclosing loops for safety and is just less likely to break. Is this a valid concern with a chain? Is a chain guard robust enough that hard to make?
- Is it feasible to have a 6 ft span driven by a long chain, and would it need some sort of idler sprockets to keep it from bouncing around too much? Possibly doubling as tensioners? Other concerns? Any examples you could point me to that aren’t from 1920’s cyclecars?
- If we went for a shaft drive, any pointers for getting around to the side of the sprung wheel once at the rear end? E.g. suitable bevel gear box sourcing, or examples of how to use the actual bevel gear from a scrapped shaft drive motorcycle on something custom?
- Should I consider front-wheel drive harder? I haven’t looked into it hard, I’m already worried enough about steering/suspension geometry.

Thanks!


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PostPosted: September 16, 2023, 11:58 pm 
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Joined: September 30, 2020, 11:44 am
Posts: 87
Location: Eastern Oregon
Something like a Goldwing could be as pretty good candidate as a donor. I don't know how hard a driveshaft extension would be. A yamaha V-Star could be a good option as well. their drive shafts are dry (and exposed) so that might be a little easier to extend.

Do you have any aversion to using the rear wheel from a shaft driven motorcycle? There is no (great) reason you couldn't mount a car tire. (see Darksiders on various motorcycle forums.)

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PostPosted: September 17, 2023, 1:02 am 
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Joined: July 29, 2006, 9:10 pm
Posts: 3151
Location: Oregon, usually
Were I at this stage of consideration, I would indeed look at front wheel drive harder. The Morgan three-wheeler was indeed a sporting steed in its heyday, and raced well against four wheeled cars, but that was mostly due to the primitive state of tire technology. Rollover is a big issue for trikes with sticky tires, and I don't just mean the slicks with red trim in Formula 1, I mean any modern automobile tire designed for on-road use in the summertime. You need (okay, I'm not the burning bush, but I've thought about it a lot) roughly 70% of your weight on the double-wheel end of the vehicle to keep it from lifting the inside wheel in a brisk turn (and continuing over unless the driver does something about it).

Which leaves 30% for the rear wheel, and if that's the drive wheel, it may not be enough for the performance you want.

If you want to make a replica, or an inspired-by period piece, google <BSA three wheeler> and see what you think. You get the advantage of rarity, so you can make styling goofs and nobody will know.

But on the rear wheel chain drive safety question, yes, a sheet metal chain cover can be made to keep you and your passenger from getting fatally flailed in the event of a broken chain.

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