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?

Existential crisis of criteria?

Things have been quiet here at Project Streetliner for a while. Progress is paused on my small-scale prototype while I wrench on a pair of old motorcycles. Meanwhile I’ve had a lot of ideas rolling around in my head. Thing is though, these haven’t been ideas of progress, but rather ideas on how this project might drastically change.

Right now, the vehicle is designed as a tilting, petrol-powered, highway capable commuter vehicle. Thing is though, my commute now is mostly city streets. Do I really need to do 70 mph? If I’m just zipping around surface streets, is the leaning really needed? Is there actually an opportunity to do a full EV or an EV + generator hybrid vehicle? That has me wondering if I shouldn’t re-spec this project for something closer to my actual use scenarios. Should I shift to more of a velomobile-style bike path runner?

For example, one idea I keep thinking about is basically taking apart a golf cart f0r its motor and controller, welding up a custom solo frame, then maybe adding Honda’s smallest generator for onboard charging. If it’d just be a bike path / lane vehicle, then there’s no real need for a full cage. I’d still be wrapping the whole thing in some sort of aerodynamic body. That’d be pretty sweet.

The other idea I keep tossing around is a non-tilting version of the Streetliner — something perhaps a tad lower and a bit wider up front. This concept would still be scooter or motorcycle based and highway capable, but having a flat trike setup means a slightly simpler chassis and suspension setup. I’m really ambivalent about this one, simply because it isn’t that much of a complexity save. It would mean a control change that could be nice — that is, I could use conventional car controls rather than a handlebar and motorcycle controls.

Speaking of motorcycle drivetrain, that’s been on my mind as well. I understand those mechanicals a LOT better than I used to. An older, air-cooled japanese motorcycle engine is simple, reliable, powerful, and even reasonably efficient. Parts are still abundant, and whole, running motorcycles are available for less than $1000. Changing sprocket sizes for different gear ratios is really easy if I go the motorcycle route. That coupled with the aerodynamic gains from the body shape should make my 60 mpg goals pretty easy to reach. The disadvantage there could be weight. But if my efficiency goals can be met, then does an extra 100 lbs really matter?

The last conceptual quandary I’m trying to figure out is broader purpose. That has a couple of connotations. Do I optimize the vehicle as an in-town commuter with highway capability for getting around town on the bypasses? Do I optimize for surface streets only — so a top speed of 55 mph or less? Or do I optimize the Streetliner as a long-distance touring vehicle? A coast-to-coast road trip at 60+ mpg would be pretty cheap to do. But it’s not just me these days. For a long-distance tourer, I’d want to be able to have my wife with me and we’d need at least a little luggage space, even if we’re traveling light. That’s a pretty big change to design criteria. I’m open to it, but like any of these questions, it’s a significant shift from the current concept. Decisions, decisions.

Any thoughts?

Small scale prototype progress

I’ve had these videos for a few months but have had a bitch of a time getting them off of my iPhone in their entirety. Finally DropBox came to my rescue and so here they are. The quality isn’t great, but you can see some of my progress toward turning an old R/C helicopter into the first small-scale prototype for the Streetliner. Since shooting these, I’ve actually reworked the rear swing arm in brass and have abandoned the profile-style chassis. Even still, this is a good look into where I started on the SSP. Enjoy!


Sidetracked, but not inactive

I got an email yesterday morning asking essentially “where the hell have you been?” and it’s a very valid question. Things have been quiet here at Project Streetliner for the past six weeks or so — not for lack of activity, but for other priorities. The Mrs and I bought a pair of three-decades-old Honda motorcycles for a laughably small sum of money about a month ago. We wanted to be able to wander the tri-state area the way we can wander the Twin Cities on our scooters, and now we can. Or at least, we’ll be able too after I do a little more wrenching. So work on the Streetliner R/C prototype has paused until I get these machines sorted out, but that should only be a few weeks. The flip side of this, however, is that these being my first interaction with the mechanicals of proper motorcycles, I now have a much better understanding of this sort of machine. This can only help me make better decisions on the Streetliner. As I’ve gotten more familiar with the simplicity of these machines (especially these early ’80s Honda bikes we’ve got), I’m considering a lot more motorcycle components in my end vehicle. I’ve even toyed with the idea of changing from scooter to motorcycle drivetrain. The cost savings alone are pretty compelling. But that’s a bit rash at this point. We’ll see.

Sorry for the long silence, progress is indeed being made. In the mean time, please enjoy some snapshots of my R/C prototype in-progress. I’ve since re-made the rear swing arm almost entirely out of brass and abandoned the profile body, but these first efforts were very enlightening.

Any R/C experts out there?

I’m trying to answer a radio transmitter programming question. I’m familiar with R/C systems from flying R/C aircraft when I was a kid, but we never really flew anything that was complicated enough to need a programmable transmitter. For the Streetliner R/C prototype I’m working on right now, I have sort of a unique control challenge I’m not sure how to solve.

I plan to include a tilt lock system on the R/C prototype (RCP) similar in function to the tilt lock mechanism I have planned for the full size vehicle. That is, an on-demand way to lock the front suspension tilt and keep the body from being able to lean. This for low-speed maneuvering, stop lights, and parking. The R/C mechanics of this are simple enough — just dedicate a servo to push in a pin or some other mechanical stop. The problem is, because this is a free-tilting vehicle that will turn via counter-steering, once I lock the tilt, my left/right control will actually be reversed (since you turn left to lean right on a motorcycle or similar single-track vehicle like this). By locking the tilt, I’ve returned to conventional steering.

So what I’m trying to figure out is if that’s something I can program. Can I set up my radio transmitter such that when I flip a switch, the tilt lock servo actuates whatever it actuates AND the steering input reverses? In a way, I suppose it’s similar to the pitch control reversal on the “inverted mode” switch for an R/C helicopter (which I’ve just realized that I have one of those just laying around! Insert maniacal laugh! Bwahahaha). Trouble is, that’s on the pitch axis, not the roll axis and that inversion does not also push a servo to do anything. It also reverses the blade pitch, which isn’t quite what I’m after either. Or would that work?


Any ideas, internet? Anybody program a radio lately?

Small-scale prototype coming along

Here’s how things are coming along so far. I’d originally imagined that I’d create the whole chassis setup out of soldered brass tubing in an effort to simulate as much of the full-scale structure as possible. I’ll probably still do that, but for this prototype I realized that I still need to evaluate a lot of the big-picture stuff. I need to verify that counter-steer leaning actually works. I need to see what effect rake angle has on the handling and stability. I also need to build these trickier components — like the steering knuckles — so that I better understand them. It isn’t that the full-size pieces will be exactly the same, but by building them at all, I’ll understand them better.

In the past, I’ve also written about the concept of a treadmill prototype. The idea was to go dead simple and just knock something up to demonstrate the front suspension. That would then have some sort of cable-driven steering input and the idea was to tether that model to a treadmill and see how it behaved. Now that I’ve got it knocked up to an extent, I’ve realized that I won’t be able to do that to the level of precision I’d need to in order to have something useful. So I’m going to do it as a radio controlled prototype so that I’ll have adjustability and precision inputs. I’ll be able to actually drive the thing around. My construction style for now is mostly 1/4″ ply, brass tubing, #6-32 hardware, and then miscellaneous pieces of brass hardware. The body will be in profile and should allow me to mount all the servos, batteries, and other gear needed to turn this into a real test platform.

The most fun I’ve been having with this is not even in the actual build, but in the days on end spent thinking through how to execute some of the precision mechanical pieces needed to do this. What I’ve arrived at is a combination of brass tubing, brass hardware and then soldering everything together with as much precision as I can. Brass is soft enough that I can sand it to precision, and the soldering lets me build surprisingly complex joints that are tough enough to stay together without being sloppy like wood or plastic. In time, I probably will do a true miniature Streetliner made out of brass tubing and even with a scale body in place. But first things first — have to prove that it works first.