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?

8 thoughts on “The case for ATV drivetrain”

  1. As an aside, I’ve always wondered what drove the team designing the 3 wheel Aptera to change from a chair driven rear wheel to a front wheel drive configuration. This seemed like a very big reversal and I’m not sure of the fall out in engine location, car balance, and overall performance. I do know they had trouble passing the panic swerve maneuver in the Xprize competition qualifying.

  2. Both drivetrains are liquid cooled with a radiator at the front no matter what, but yeah, this would mean that the engine would be tucked in behind the driver’s seat. Thing is though, that’s the same thing that’s happening on the scooter drivetrain, the engine just happens to be built into the rear swing arm. The actual suspension pivot ends up being aft of the engine itself and the weight of the engine cantilevers the weight of the transmission and rear wheel — which is what you’re actually seeing on most scooters coming off the rear wheel.

    I’m not sure why the Aptera team went to front-wheel drive. I wasn’t aware they did that. It might be something they’re just doing for X-prize. Although, if they’re just using hub motors anyway, why would it really matter. FWD would have advantages, especially in cold weather. But I suppose having two motors instead of one isn’t a real gain in terms of weight and efficiency.

    Weight distribution is actually a known issue on my project. With the engine behind the driver, and the driver well behind the front wheels, the front-to-back balance of this thing is going to be pretty ass-heavy. There will be the front suspension components themselves, but they’re not that heavy. I know I’ll have to hang the radiator(s) out in front, but I’m thinking I’ll mount a big beefy car battery up there as well. Hopefully the thing won’t be too darty up front. Shouldn’t be a big issue with the CG as low as it will end up being. Although I’ve got to be careful there as well. If the CG is too low, then there won’t be enough weight leaning toward the center of the turn and the vehicle won’t be stable. So many variables.

  3. Not all ATVs are 4×4 (unless I missed something and you’re talking about a specific model). I haven’t ridden one in probably 15 years, but I recall many of the racing quads having a chain-driven final drive (and often a proper clutch). Something like the Yamaha Banshee from the late ’80s/early ’90s. Probably still a thumper, but scads of horsepowers.

  4. It’s true, blalor, they’re not all 4×4. I’ve only just started looking into them. Seems like most of the 400+cc models that I’ve been looking into were all 4×4 and shaft driven. What I need to do is just head to some local powersports dealers and get the low down on what’s out there.

    One other benefit of the ATV approach is that most of those transmissions have neutral in addition to reverse. Having the high/low could also be pretty cool. Use the low setting for stop and go ’round town and keep my final drive sprocket nice and small. Then when it’s time for a long highway haul, switch into high and really pump up my mpg.

  5. Hey Nathaniel,

    Sorry for the long absence. I was on vacation and just got back. Always interesting to read through your well thought thru posts, and I admit I still have a few to go. So if these topics are covered later I apologize in advance. But here goes for now…

    Possible solution to both your length and weight distribution issues to be seen in the Gurney Gator motorcycle (Google). Simply said… put the engine ahead of the driver, between his legs. Your weight moves forward, your length shortens because you can then move the swingarm pivot up, and you also gain the side benefit of getting to use a shaft drive engine if you want. Just put the bevel drive from the ATV driveshaft right at the swingarm pivot and put a sprocket on it so you still have a chain final drive.

    And yeah… lots of 2 wheel drive atvs out there to be had, especially if you look at the sport quads which are almost all chain drive too.

    1. Hi Aaron!

      Thing is, I don’t really look at length as an issue. It’s not any longer than a small car and that doesn’t bother me. Length will mean stability and given that the Monotracer is significantly longer than what I’ve spec’d here, I’m not worried at all. As for moving the engine up front, that has several issues. First, the front suspension box is literally under my feet and there’s no way to stick the motor into the middle of that somehow. So putting the engine in front of me means pushing the front suspension forward, which just makes the vehicle longer, so no real gain there. Second, if I were to put the motor more under me — say under my legs — then I may have shortened up, but I’ve also gotten taller. Height means more frontal area, which means less aerodynamic efficiency. Thirdly, having the engine essentially in the cockpit with me opens up a lot of issues (heat, exhaust leaks into the cabin, etc.) that I’d really just rather avoid. Heat especially. With the engine behind the cabin, I can build a thorough firewall insulating me from both heat and vapor.

  6. Yup, after catching up with the rest of your thoughts I agree, since length isn’t a concern for you, it probably isn’t worth moving the engine. If you were to add a passenger seat though I might reconsider that.

    To most of your other concerns (suspension, height, heat, etc.) I’d say to have a closer look at the Gurney Gator. seems he solved most of those problems by having the cylinder head between the driver’s legs and only the tail of the tranny sticking down under the seat. His seat height is only 18-20″ if I recall, and you really don’t want to get much lower than that for visibility reasons anyway. But he doesn’t have an enclosed cabin to deal with either, so… there are always compromises and what you have now looks as good as any.

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