Category: Drivetrain

More ATV thinking

So the seed planted by Aaron to use ATV drivetrain is growing into something cool. On twitter, @blalor suggested Honda’s line of TRX sport ATVs. They’re 2WD, chain-driven, EFI, and possibly perfect. I keep eyeing the 400 and 700 models. Both are single-cylinder “thumper” motors with a wide bore and thereby tons of available low-end torque — which will come in handy once I gear it up. I wish Honda were more forthcoming with horsepower numbers, but I’m not finding any. That’s but a detail at this point though.

This ATV setup lets me do a couple things. It lets me run a larger motorcycle wheel and tire on the rear. That gives me higher gearing than both the scooter drivetrain and the original ATV setup by default. Rear sprockets are very easy to change, and this gives me lots of latitude in adjusting the final gear ratio of the drivetrain without touching the actual transmission. Reading up on the TRX specifically, there are aftermarket solutions for high performance exhausts and apparently it’s pretty straightforward to re-map the EFI. That should let me tune a “butter zone” where the motor is at it’s most powerful in its high ranges and it’s most efficient in its low ranges. That way under acceleration, I’ve got plenty of power, but while cruising, it should just bump over and sip fuel. What could be even sweeter is if I could set up a switcher box of some sort that would let me pick between two EFI maps on the fly. One for power, the other for efficiency.

One thing I was very curious about was how the chassis would need to change in order to incorporate a separate engine and rear swing arm. One of the simple beauties of using a scooter drivetrain is that the engine, transmission and rear suspension are all basically one big piece. This is simple, but it isn’t automatically light and there’s no reverse gear option. So what happens when I swap out for an ATV/motorcycle rear end?

I found some reference images of the TRX700xx powerplant and rendered up a quick cross-section. This image compares the original concept Suzuki Bergman powerplant with the TRX. Everything is approximate, of course. The other key shift in this rendering is that I dropped the seat height basically to the floor. This to decrease the forward cross-section that much more. The trade-off is adding about 10″ to the wheelbase between the two changes. This puts the length at about 3″ longer than the wheelbase of BMW 3-series coupe. Not bad, considering it’ll present about 1/3rd the frontal area.

The next question was obviously what does this do to the body shape? Making the hood line lower and the tail longer did some neat things for the side shape of the body. I especially like how that big wheel looks in the rear. The overall effect is really slick. I can’t wait to model this in foam or pine and see what it looks like in 3D. More to come.

The case for ATV drivetrain

Oft contributor and friend of Project Streetliner, Aaron, has more than once made the case for using the drivetrain off an ATV (or “4-wheeler” as we southern kids grew up calling them). At first blush, I’ve always hated the idea because I’m a scooter snob. I don’t know ATVs and I don’t care about ATVs. But there’s one big factor that makes Aaron’s case for the ATV drivetrain impossible to ignore: reverse. From his previous comment:

IMHO… I’d say to go with a small ATV engine for several reasons.

1) They are virtually identical to many motorcycle engines, except their gearbox has reverse, which will save you a lot of hassle later.

2) Many of the “manual shift” variety have an automatic clutch, which will gain you most of a scooter’s “twist-and-go” convenience while still keeping the ability to gear up simply by changing sprockets (something you can’t do easily with a scooter – see Craig Vetter’s woes with his Freedom Machine). Even if you decide to go with a CVT ATV instead of manual, you can still run a sprocket/chain off the bevel drive that normally drives the rear axle.

3) ATV engines are commonly larger bore single cylinders which are setup to generate a lot of low end torque, which will be useful after being geared up to your desired top speed. Many small displacement motorcycles don’t have a lot of torque and may have to be geared down to move a heavier vehicle then they were designed for as quickly as you’d like (especially if you decide to make it a 2 passenger trike).

4) Depending on what you decide to do about the front suspension, you may get many of the parts you need right off the engine donor.

5) If your EV desires grow, you can always retrofit it with one of the Motorcycle EV Conversion kits coming out lately. That option won’t be as easy if you start with a scooter engine/swing-arm/wheel.

6) And last, I don’t think the hp difference will be enough to limit you to surface street speeds with a gearbox to choose your final drive ratio. With a CVT or EV you have the problem of being “wound out” at your peak RPM and not able to accelerate further. Craig Vetter had this problem as well. With a gearbox almost any engine that will move you 55 with “power to spare” (as you dictated) will also get you to 70 for highway runs.

I really can’t argue with any of this. Especially with many of the “sport” ATVs on the market today, horsepower won’t be in issue. I wonder a little about just how these engines are tuned, given that mpg isn’t your primary concern off road. They might be set a lot richer to allow for higher performance. If their carbureted, that’s not so hard to fix, but if they’re EFI, which most are these days, then getting that re-mapped could be quite tricky. I really only have two concerns: weight, and form factor.

Weight is an obvious concern, as the less weight the Streetliner has to haul off the line, the more efficient it will be. Some of this can be overcome with gearing and aerodynamics, but the lighter the better is a good rule of thumb. I’m concerned about weight in the ATV primarily because it’s a 4WD vehicle. I only need to drive one wheel, so I’d effectively have a second output shaft and differential just getting lugged around with no purpose. The most mad scientist thing I can think to do with that second shaft would be to run an air conditioner compressor off of it or perhaps a second alternator. That has interesting connotations, but I’m still not sure.

The more troubling concern is that of form factor. What I’ve designed so far has a wheelbase about 4″ longer than my MINI. Granted, the MINI is not a big car by any measure, but getting much longer than that worries me in terms of low-speed maneuverability. I don’t want the thing to be so long it can’t easily park. The guys who build those impossibly long chopper motorcycles run into this all the time. Their stretched-out bike can barely turn around coming out of a parking lot without doing a 12-point turn. I know that’s hyperbole, but I still don’t want to go down that road if I can help it. The scooter drivetrain has not just an economy of engineering simplicity and bolt-on convenience, it has an economy of space. Without knowing more about the ATV powerplant, I’m not sure if I can cram it in behind the seat on the Streetliner the way I’d want to. I doubt it’s impossible, I just need more information.

So it’s certainly on the table. I need to get some measurements and figure out which ATVs would be good candidates. Looking around online, the tricky thing is finding one that isn’t shaft drive. For this to work, I really want to have the final drive be chain and sprocket so that I can adjust the end gearing as easily as possible. I guess we’ll see.

Any thoughts?

Some thoughts on reverse

This vehicle exists in sort of a strange place in-between being a car and a motorcycle. It will lean like a motorcycle and utilize big scooter mechanicals, but it will also be enclosed like a car. There will be no need to put my feet down at intersections, because a simple on-demand locking system on the tilting suspension will allow the vehicle to stand on its own three wheels (which will also remove the need for a side or center stand). Being enclosed will mean both comfort, aerodynamic efficiency, and actually a large amount of safety. There really is a lot of elegant convergence here. The locking tilt actually enables me to never need to put my feet down, which makes enclosing the vehicle easier with no need for holes or “bomb bay doors” to pass my feet through. What I have yet to figure out, however, is how to back up.

You simply can’t not have reverse. Sure, most motorcycles / scooters don’t have a reverse gear, but you still have to “walk” them backwards to get out of parking spaces and other common driving situations. Some large motorcycles such as the Honda Goldwing do have actual reverse mechanisms, as do electric bikes like the Vectrix that use their hub motor for regenerative braking. But so far, I haven’t been able to locate a scooter in the 400-600cc range that includes a reverse gear. This year’s Honda Silverwing info says the following:

The V-Matic means no shifting, ever—not even into Neutral or Reverse.

This would seem promising, but a phone call to my local Honda dealer confirms that this is in fact just really, really bad marketing copy. So I’m at a bit of an impasse. Reverse is a must, but I can’t find it built-in to any of the powerplants I’m considering for this project. So I’ve got to figure this out.

Electric option #1: Hub motors in the front wheels
There are a couple of ways that I could utilize electric motors for reverse. One route would be to use hub motors in the front wheels and essentially make an electric hybrid. Piaggio has a hybrid version of the MP3 that does exactly that. Front hub motors would make regenerative braking available, and with that, reverse. The major, and in my opinion, irrevocable barrier to this option is its complexity and its expense. I’d likely have to purchase those hub motors directly from Piaggio and they wouldn’t come cheap. Then there’s the batteries, which add significant weight, and the added complexity of all the speed controllers and power management systems it would take to link the motors to the throttle and balance them against the IC engine. As cool as a hybrid would be, it’s an awful long way to go just for reverse.

Electric option #2: Drive the rear wheel on either direct friction, a sprocket, or a clutch
There are a number of electric and IC engine kits out there for motorizing bicycles. Many of these involve a friction roller that contacts the rear tire. Although probably not the most elegant solution, something similar could work very well. A small, high-torque motor like a wheel chair motor or a even an ATV winch motor could be suspended from a subframe that could be lowered against the rear wheel to turn it backwards. I’m picturing something like a hand-brake lever with a trigger button on it to run the motor. The Lightstar Pulse used a similar system, except that they used an aluminum cone pushed against the wheel rim instead of a rubber wheel on the tire. Online owners report that it’s adequate, although very slow and apparently a massive battery drain. I was already planning to run a significantly larger battery than would be standard in a scooter in order to support a handful of ancillary electronics, so perhaps that would be enough, presuming the bike’s stock charging system can replenish it.

One related idea I had in this vein would be to use a sprocket on the wheel that the motor could engage with its own toothed gear. Perhaps a starter motor/solenoid system would work. Apparently the Honda Goldwing uses a reversible starter motor for its reversing functions, albeit at the flywheel. The one thing I wonder about would be the wear and tear of that kind of engagement. As for the sprocket, I was thinking that I could just have teeth put on the rear brake rotor. It’d mean a bit of precision bracketry, but may indeed be the ideal arrangement.

The third variation would be to use a belt or chain drive between a motor and a sprocket/pulley on the rear wheel and be able to engage a clutch on that mechanism. Depending on how it was set up, it could even double as a low-speed charging system to recoup some of the energy used in backing out of a parking space.

Manual option #1: The Fred Flintstone method
Most velomobile trikes have a pair of holes in the bottom of them to allow the rider to back up under foot power.  This could work for the Streetliner, presuming I have enough leverage to scoot the weight from a seated position. Having a pair of permanent holes in the underside of the vehicle does not appeal to me at all. Nor does having some sort of door/hatch system. This thing is complicated enough. Although, I have to admit, there is an elegance to just hoofing it.

Manual option #2: Some sort of crank to the rear wheel
Perhaps some of the electric methods described above could be similarly executed with a hand or foot crank in the cockpit. I’m not looking for fast reverse, just enough to get in and out of parking spaces and perhaps back up far enough to then pull forward around an obstacle like a stalled vehicle. There’s something deliciously old-school about that, but it’s probably not the best solution.

Any ideas? I haven’t come to a solution I like yet. What haven’t I thought of?

Donor bike

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So at some point in this process, I’m going to have to decide on a power plant forProject Streetliner. My plan is to purchase either a used scooter, or one or more wrecked scooters to scavenge for parts. The question is, which one? The thing is, as I’ve researched this project more in-depth, I’ve realized that there’s more to be had from the right donor bike than just the rear end. Part of this project is to minimize the amount of custom engineering I’d have to do. Whatever I wind up with, I also want to stick to that bike (or at least that brand) as much as possible so that I can minimize the number of spare parts sources I have to manage. In a lot of ways, this ceases to be a scratch vehicle, and becomes an elaborately custom scooter. I like the sound of that! Components should really include:

Brakes
Obviously the rear brake disk and caliper are already part of a scooter rear end, so that’s handy. But what could and really should also be harvested from a donor scooter are the front brake components (calipers, disks, wheels, lines) and both brake master cylinders as well. Ideally, I’d like to use a bike that has either two front wheels (such as the MP3) or dual disk front brakes on the single front wheel (such as the Aprilia Sportcity, or the Suzuki Burgman). That would ensure that the brakes are already balanced and matched to the wheels and that the master cylinder is already beefy enough. Some maxi scooters also include a parking brake, which would definitely come in handy on a 3-wheeler.

Wiring, switches, controls, telemetry
Harvesting the wiring harness, gauges, and other goodies out of the bike would save a ton of work. It’d be pretty dumb not to utilize the gauges, the hand controls, and other switches and such that have already been engineered to work together on the bike. Why wire up a custom circuit for turn indicators when there’s one already built into the donor bike?

Wheels and tires
There really isn’t much interchangeability to scooter wheels and tires. There’s not the aftermarket for rims and such like there is for cars. So I’m likely to be stuck with the wheels and tires native to that scooter. One exception I know of is the Piaggio QUASAR engine line that is shared between Piaggio, Vespa, and Aprilia bikes. The 12″ wheel of the Piaggio MP3 could be plausibly swapped for the 16″ wheels of the Aprilia. The main advantage there is that swapping to the larger size effectively makes the end gear ratio higher (and therefore adds to fuel efficiency).

So when it’s all said and done, the only parts of the donor bike I wouldn’t be utilizing are the front fork, lights, the handle bars themselves, the majority of the chassis structure, and the seat. That’s it! The last lingering question for me was engine size.

I’d originally envisioned this project to utilize a 250cc scooter engine such as what’s shared between the Vespa GTs 250, Piaggio MP3 250 and Aprilia SportCity 300i. That, or a similar 250cc engine from Honda, SYM, or Kymco. A 250cc motor is kind of the sweet spot between performance, weight and fuel economy in the scooter world. The MP3 seems an ideal choice for the obvious reason that it’s already a leaning trike. The MP3 also comes in 400cc and 500cc versions should the need to up-size arrive.

Speaking of which, I’m pretty sure at this point that up-sizing is inevitable. This is mostly because of the likely weight of Project Streetliner. My design criteria is to stay below 600 lbs curb weight. Scooters in the 250-650cc engine classes tend to weigh in between 340-550 lbs. Thing is, it’s not speed I’m concerned about, it’s economy. You see, a small engine having to work really hard will usually get worse fuel economy than a slightly larger engine that isn’t having to work so hard. (For example, on a racetrack a BMW M3 gets better fuel economy than a Toyota Prius when both are driven at the Toyota’s quickest speed.) So if my vehicle does indeed weigh 600 lbs, a 400cc engine will likely get better mileage (and conveniently give more power) than a 250cc.

So that brings me to my latest bit of hands-on research. I stopped by the local Suzuki dealership to look at a Burgman 400 — a bike considered by the folks who like such machines as the premier maxi scooter on the market. It seems perfect! The Burgman is the front runner right now for a couple key reasons. It already gets the mileage I’m shooting for, weighs in at 474 lbs, has dual front brakes, and a great reputation. Looking it over, it’s now the definite front runner in my search for a donor bike. What’s more, Suzuki has sold a boat load of them so there ought to be plenty around in the secondary market. That certainly can’t be said for the MP3, I’d wager. Anyboy have any other suggestions?