Designing for efficiency
 

Just as a design exercise, I would like to design a single-seater commuter vehicle. I would like to keep it relatively simple, so it would be in the realm of the home builder. In essence a 'green' version of the Locost home-built sportscars. (Because I am new here, I can not yet post links to websites. If you are curious, search for Locost Builders in Google) Obviously, the design would be geared entirely towards fuel efficiency rather than performance.

What I had in mind was a single seater, three wheeled vehicle. Two wheels at the front, one at the rear. The easiest way to build it would be by using the engine and swingarm from a light motorcycle. The body of the machine would be teardrop shapedfiberglass, with a clear canopy.

I would really like some input on a few design choises I have to make.

Starting with the engine. The whole vehicle would be as light as possible, aiming for under 300kgs (660lbs) including fuel, excluding the driver. Since it will be designed as a commuter vehicle, and part of my commute includes a highway I can not avoid, it has to be able to cruise at 100kph or 60mph. For the engines, I am looking at these options: 125cc, 4spd semi-auto / 250cc, 5spd manual clutch / 250cc CVT -power output for all these engines would be in the 10 to 20Hp range, with the smaller displacement engine being on the lower side of that figure. Of these engines, which would be preferable? I'm thinking the 125 could reach the best fuel economy, but I am worried the weight might be a little too much for the little motor to pull.

Next are the tires. Most low-rolling resistance tires are for 14 or 15 inch wheels. Would smaller wheels be even more efficient? For example, the classic Mini uses 145/80R10, pretty much the smallest tires a street-legal vehicle could run. Would these be preferable over the larger LRR tires I wonder?

There is much more, but I will try not to bore you guys with an enormous wall of text and instead take this design process step by step.

Any input would be greatly appreciated!.

Your vision sounds a lot like the HyperRocket that's featured on the front page of this website.

HyperRocket: 125 mpg, 100+ mph 3-wheel motorcycle | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com

Welcome to ecomodder, noorderbuur ;)

I'd use 4 wheels.
While 3 wheels have less drag and weight, the vehicle will also be less stable.

I'd use a manual transmission.
They're far easier to re-gear using different sprockets, than adapting a CVT.
When adding a streamlined fairing, you'll need to re-gear the drivetrain to lower rpm @ your most used speeds.

i dont think a 125cc engine will be enough...

shoot for a 250cc engine from a Kawi EX250.

By design exercise, do you mean you are not going to actually build a car?

I'm not 100% sure, but think the taller the tire the less rolling resistance (all else being equal).

I'd love to, at some point. Right now it's a matter of time and money though. It'll be a while before I'm in a position to tackle such a project. In the meantime, I can sketch, doodle and think, before moving on to a CAD design. Hopefully by the time I am ready to build it, I'll have a full set of plans to work with.

Stability shouldn't be much of an issue - I won't be racing it, and the center of gravity would be pretty low. Besides, there's something else a 4th wheel adds: complexity. Driving a rear axle means I'd also need to use a differential, adding even more weight and design issues. I'd like to use the entire drive section from a light motorcycle. it keeps things light and simple, plus it would get me closer to a real teardrop shape.

Excellent point on the gearing. Looks like a 250cc motorcycle engine will be the ticket then.. anyone know of particular models that get good fuel economy?

Similar in concept, yes. Though I hope to take more design cues from the Peraves Monotracer - only in three wheeled form. A tilting trike would be ideal, but needlessly complex I'm afraid..

Valid points.

Be sure to keep an eye on the CoG and the weight distribution.
Shifting weight aft will give more traction, but can result in the CoG going outside the triangular wheelbase more easily - thus rolling the vehicle.
It happens on velomobiles.


Search the database at Advanced search - Spritmonitor.de
Use very few search factors - say just vehicle type : two-wheeler - gets you a list of vehicles with low fuel consumption.

One of the types that keeps popping up frequently in that list is the Honda Innova / ANF 125i engine.
It should put you solidly below 2.0 L/100km / above 115 MPG.

Of beter dan 1 op 50 , zoals jullie het in Nederland graag uitdrukken ;)

If you keep the weight down, a 125cc shouldn't struggle too hard when the vehicle is really well streamlined.
I wouldn't rule out the 125cc.

See what's possible with a Honda 125i :
Home
1L/100km or 235 mpg


You might want to replace the heavy fiberglass with Zote foam
The Recumbent Bicycle and Human Powered Vehicle Information Center

Excellent info, Euromodder, bedankt! Looks like I have some reading up to do now.. :)

Wow, threads sure do move to the second page quickly around here. But that's a good sign, right? An active community, I like that.

I have given the whole thing a little more thought and I have decided against the smaller engine from the Honda Innova. It's the semi-auto transmission that made me decide against it - to coast or glide means having to shift to neutral first, rather than just holding the clutch. I'm not even sure engine-off coasting is even possible. I'm looking at the manual-clutch 125cc machines now, such as the Honda CBR125.

I hope to be able to make a choice soon (ish) so I can move forward.

FWIW, re-gearing or tuning the shifting behavior of a CVT isn't all that hard, if your CVT is designed similar to my Yamaha Majesty (400cc maxi-scooter). The shift points are primarily controlled by the Variator Weights (some metal & graphite roller weights wedged in grooves behind the sliding front pulley and a cover behind it) and the rear sheave spring, which governs the amount of force that attempts to keep the rear pulleys pressed together. Most tuning on the Yamaha Majesty's shift points are done by replacing the weights with different-mass weights or changing the variator to a different design (and changing the # of weights installed).

Sounds much easier than swapping gears on a manual transmission IMO. But on the flipside, a belt-driven CVT needs to have its belt swapped every so often (every 12K miles on the Majesty..... at ~$110/belt, that can be a bit expensive, and USUALLY the variator weights themselves are worn down by then so they need replacement too, that's about $60 a set).

The problem with rubber belt CVTs as I see it, is that I've read they MAX OUT at 80% efficiency. Compare that to up to 98% for gearsets, flat rubber belts, and chains.

regarding rubber belt CVTs, I wonder if that 80% figure isn't an average over the course of operation, including during "upshifting" and "downshifting" when the belt is being heated by friction from tension changes...

Unless they're saying that the belt is slipping alot, which I could understand the 80% figure then, however, if that's the case, why not use Kevlar or something? Lasts longer, doesn't slip as much, and has less deflection under load, so that should increase the efficiency of the unit as a whole, should it not?

What about single CVT vs. Dual CVT (two ranged pulleys vs. a fixed pulley and a ranged pulley.)

I haven't seen any data suggesting a difference, though there obviously should be.

hm, interesting about the 80% figure, although I have a little trouble believing it. The CVT in my scooter is only air cooled. Though who knows, that might not mean anything. FWIW I am using a kevlar belt right now, meaning rubber with kevlar reinforcement bands inside vs. whatever they use in the stock belt (which I'll admit I'm not entirely sure... I just bought a new stock belt for my 24K mile service and it has some kind of reinforcement bands built in)

Not sure what you mean by the single vs dual CVT? I guess by that definition mine is a dual CVT -- front sheave assembly is 1 fixed pulley and 1 ranged pulley (called the Variator) and the rear sheave assembly is 1 fixed pulley (opposite side of the belt from the front) and 1 ranged pulley with a strong spring pushing it towards the fixed pulley.

The belt becomes stretched with more tension as more torque is applied from the engine so it helps prevent slippage. This is accomplished with clever design of the sliding mechanism between the rear sheave ranged pulley as it slides against the collar of the fixed pulley (the channels it slides on are curved so that greater torque application results in the rear pulleys wanting to come together to downshift, despite the fact that the front variator pulley's weights want to push the front pulleys together to upshift with increasing force as engine RPM's increase--result is that when you are twisting the throttle back hard and producing your greatest torque, it downshifts yet the tension on the belt is phenomenally higher than the tension on the belt during normal coasting, so the increased torque throughput is met with a belt that's so tense it won't slip).

Really a brilliantly clever balancing act done by purely passive mechanical means. I grin every time I think about it.

Well then again, that belt is pretty thick and stiff and each segment has to frequently change shape, so that could be the efficiency loss. It's a lot like a tire in that regard.
Chains have lubrication providing an advantage over that, and a flat (thinner) belt has less material to change shape at any point on the pulley.

^You got it.

If anyone has better info on CVT efficiency, bring it! Believe me, I've looked for it.

Do you want a reverse gear? or is there a way to work around that with a small motorcycle engine.

I have a chinese verson of the yamaha scooter drive train in my 4 wheeler. It has a fan built into the front pulley and an air vent into and out of the belt cover. There must be some waste heat generated with this set-up.

there are some motorcycle-powered cars that use an electric motor for reverse. Others just get out and push.. ;D

As a car this light is classified and tested as a motorcycle in most areas, a reverse gear is not compulsory.

Direct drive with driveshafts to the front wheels, would allow for regenerative braking, as well as high efficiency.

FWD is out, as I want to keep the vehicle as narrow as possible, also, as far as I know there are no single or twin-cylinder engine/transmission combos available for a FWD application - I might be mistaken in that regard though.

Taking the entire driveline from a light motorcycle seems the easiest way to go forward. it has pretty much everything I would need, minus perhaps a reverse gear.

You can make a fwd setup that satisfies your needs fairly easily.

Run chain from the output of the bike trans to a cog with a bearing on each side of the cog, and a flange outside of each bearing, such that the cog drives each flange, and the bearings support the shaft.

Build some cv axles that bolt to the flanges, and drive the wheel hubs.

Done.

The advantage is that the front track width needn't include room for the engine, allowing it to be more narrow than traditional fwd. layouts.

And skip the differential altogether? That doesn't seem like a good idea at all..

An extremely thin track width means that the actual turns difference in normal operation wil be limited any way, so a diff probably isn't totally necessary.

If you really wanted one, get a rear fixed gear or multi speed trans axle from a riding mower. I've got about 15 multi speed ones... Want one?

A fwd 3 wheeler with a track narrow enough to not need a diff, would be a rolling coffin. Just sayin'.

I think you should look at an enclosed two wheeler along the lines of the monotracer. Much simpler, safer and fun.

You could use the engine out of a quad, that gets you reverse. Polaris made a diesel, it has a CVT, but the range was impressive compared to gas. For a simple differential just put a overrunning clutch at each wheel.

This has been around for years now. Anyone ever here of the Tri-Magnum; they suggest using a Honda Gold Wing motor, but you can use whatever you like. I think this is the basis for the HyperRocket . I've been thinking of building one of these myself using a 400cc bike; if I ever get caught up with my other projects.

Tri-Magnum: Excitement on Three Wheels

There goes your reverse.

Where can I buy one of these simple overrunning clutches?

Would you trust you life to a set of these going 70MPH?

Neato! How do I attach that to my wheels?

Beats me. I'm not about to waste time trying to invent around a differential. But if somebody really wants to build something using overrunning clutches, they are available.

I figured I'd get a comment on that! I would hope a professional automotive engineer would not have a problem solving a small detail like that? Hilliard Corp. has bi-directional, industrial overrunning clutches. Building a drive setup with these will also produce a locking diff. action, much like a Detroit Locker. If my drawings were not lost off my previous computer, I'd show you in detail how to mount them too.

Splicing it into the CV shafts probably isn't too big a deal.

I do like the one-way overrunning clutch though- imagine the nice effortless glides! IIRC the old 2-stroker Saabs had them.

Are these essentially ratchets; like in bicycles? Would this eliminate the need for the differential, and how would the efficiency compare?

Plymouth had freewheeling in the 1930s. Caused most of the archaic laws about illegal coasting in neutral passed late in the 1930s.

The problem was you had to be stopped to disengage the freewheeling and the period braking systems were pitiful compared to modern discs.

regards
Mech

They use rollers which ride up ramps to lock the inner drive member to the outer driven member. This one works in either direction. Efficiency should be better than a normal right angle differential using gears.

Are you referring to normal driving or coasting?

I ask because the gears inside the differential don't move that much, and consequently can't be a source of significant losses. During coasting however, you'd have fewer parts moving with freewheeling clutches far out in the drive line.