I’ve been pondering this for awhile. Mountain biking is one of my favorite activities and what’s better than doing your favorite activity with a friend?
So what I’m talking about is a 2-seater, side-by side quadricycle that is optimized to go off-road. This means big, fat knobby tires, wide range of gears, good turning and braking capabilities, and mostly importantly, shocks on all 4 wheels.
Do you think I could build this out of spare mountain bike parts and frames? Could I make it light enough to climb some hills (I know I wouldn’t be climbing anything steep but you’ve got to go up to go down so it would need some reasonable climbing capabilities), and yet strong enough to hold 2 people and not bust up halfway down a hill?
How would the gearing work, with two people pedaling (I assume however a tandem bike works but I’ve never ridden one)?
There’s one on Wikipedia but it’s a 1-seater model and out of production. I did find a page that had some pretty positive comments about it, but apart from being out of production, it was over $7,000.
I’m not looking to start a business selling these or anything, I just think it’d be fun to have and tool around with on some doubletrack whenever I have friends over.
AFAIK, these guys are still in business, and while its not an off-road design, you could use it for some ideas. And while there’s no quadracycle listing on Instructables, there’s plenty of tips on how to build your own bicycle.
If I were going to make this, I’d probably build the frame out of 7075 or similar aluminum tubing. It’ll just be easier to work with stock for the frame, so you don’t have to fitz with trying to modifying somebody else’s. I would think you would have no problem using mountain bike parts for most parts, although since there’s going to be more stress, you’re going to want some heavy duty components. I’d probably give each rider their own set of gears, so that two riders of different strength could pedal without worrying about what the other is doing.
The suspension is going to be the hardest part. In a normal bike, when you turn, the bike leans over, so that the tire is still taking the brunt of the force radially. When the CG of the quad is two or more feet from the wheel, though, the vector being introduced is going to be pretty tough to deal with. You’re going to have to figure out how to keep the wheel from snapping off at the axle. Cambering will help a lot, but you can only camber so much.
Steering should be pretty simple, just connect the linkages where the handle bars would normally go.
These people have built a 2-person 4-wheel off-road vehicle not unlike what you describe. It’s designed for a (planned) expedition through central Asia. Interestingly it doesn’t have rear suspension, just front. I think it uses a conventional bicycle drivetrain, with each rider powering one rear wheel. So if only one rider is pedaling, it becomes 1-wheel drive, but that should be OK. No need for a fancy differential gear system. It was actually custom-built by Greenspeed but I can’t find any info on their web site.
The position of the CG shouldn’t matter. The stress on the axle is determined by the lateral force on the tire (i.e. the centripetal force from the tire’s contact patch) and the radius of the wheel. Think of the ground as grabbing at the bottom of the wheel and trying to pull it sideways.
Anyway, I think standard MTB wheels should work fine. MTB wheels are designed to cope with quite a bit of lateral force. Recumbent trikes can’t lean into turns, and some use standard 26-inch rear wheels (example). But if you’re paranoid about it, you could use 150mm through-axle hubs (example) used on some downhill bikes. Or use smaller rear wheels.
Agreed, though a few minutes spent learning about Ackermann steering geometry would be well worth it. It should be simple to implement. And of course one rider needs to be in total control of the steering; the other rider needs a dummy handlebar to hold on to, which isn’t connected to the steering.
p.s. Other examples of 2-person quads: Couchbike ZEM Surrey