Howdy, We are S.I.N. Cycles builders of gravity fueled machines. This is our most recent build, hope you enjoy! Just want to add that if you are interested in some of our other machines check out http://sincycles.blogspot.com/ 

A Gravity Bike? It is what it sounds like, a bicycle that goes down hill and otherwise has not much use. These machines are a joy to ride almost soothing; just the sound of the wind and road. Speed varies depending on the hill.  We regularly get up to 50mph but speeds up to and beyond 80mph can be accomplished. While the speeds may be questionably legal, these machines are legal on all public roads where bicycles are allowed. 

If you ever care to race in an organized race. Conforming to the IGSA standards might be a good idea. Listed below are the three rules we build loosely around and have not found the need or desire to go out side of those bounds.  The rulebook can be found at http://www.igsaworldcup.com/rulebooks/2009_igsa_rulebook_final.pdf  

The basic standards are:
Wheel size - 51cm / 20” max
Bike weight - 34kg / 75lbs max
Axle to axle length - 127cm / 50” max
Many more rules can be found in the rule book. But where we race, none apply :)

Tools needed or have access to:
Metal cutting tools of some kind - Angle grinder and cut off wheels, metal band saw, hacksaw, hole saw.
Assortment of wrenches
Drill press
The more the merrier!

We dig through metal scrap bins for most of our metal material and then let the material inspire.
We did buy 8ft of  .75" x 1.5" x .125" mild steel for this project.
Rim Brakes
Brake Levers 
Brake cable/housing
L200 foam
Wheel cover
x2 20 inch wheels
Old bmx bike to pluck any tid bits from
Shaft collars

We truly hope you enjoy this inside look!
S.I.N. (Sir Isaac Newton) Cycles

Step 1: Prototype

This is a critical step to create a comfortable ride. Everyone has there own body length and preference of how something should feel. So play around. We joke sometimes saying every bike is our prototype for the next bike we build.

As you can see in the photo our prototype is super high tech utilizing a chunk of wood, rope, zipties, a fork, and some wheels. It was just together enough that we could carefully straddle it. Pretending where the handle bars, kneelers, and rear pegs might be. Card board boxes milk crates and other random items come in handy at this stage.

Be sure to take more measurements than you think you need. Also plenty of photos sure don't hurt. Documentation is a wonderful thing.

Sometimes we will draw over the photo to play with the overall aesthetics and frame layout. It is cheaper to learn things on paper than steel.
<p>Amazing bike and great write-up. I'd like to put together a rig optimized for (1) handling/stability, and (2) ability to bail easily, with no concern for aerodynamics. The use would be on a tight 1.5ish lane road with lots of &quot;first gear&quot; corners.<br><br>I picture the best setup being something like a scooter, standing on your feet, toes forward, offset on either side of a downtube/brace. (That would allow a low center of gravity and good clearance when leaned over, is my rationale.) You mention a 23* head tube angle--are there any other general parameters that work best?</p><p>Cheers!</p>
<p>Hey guys! Test your builds out with Global Speed Index to get your speed, and check other Gravity Bike Gear and riders from around the world. Global Speed Index is an App for Gravity Bikes and Action Sports. Check it out @ http://www.GlobalSpeedIndex.com - one you register, get a free month subscription to PRO Services, just send us a message via Contact Us in settings screen with GBIKE in the body. Happy building, and happy riding!</p>
Made one out of an old bmx I spent too much money on and never used. Quick question, how shallow of a head tube angle do you think you can get before the wheel bites too much?The bmx fork does have good rake so I'm joking that will compensate. I've used a mtb stem and bars and placed them under the head tube to reduce angle to ideally stabilise it but it's pretty flat. Just wondering what your opinion is
Hey guys. Just wondering how you would make a gravity bike from a regular BMX frame. I know that you turn it upside down and obviously adjust a couple of things so that the handle bars arent on the ground ahaha. But how would you make the kneeboard for it??
Howzit mate, I've literally just dome that today and it works great. It helps if you have a full set of pegs, take the seat mount that exists and there should be an Easy way to bolt on pegs. The best way to build something is build it then iron out the problems<br>
<p>i'm planing to build the gravity bike, and i'm a 12 year old. i'm planning to do it over the summer, and the weekends.</p>
Go for it.<br>Make sure that you consider safety when designing and building.
That is just so awesome it's not funny. I saw this and immediately decided to build one. I have never been so inspired by an instructable! It's all i've been thinking about for the last few days! Went to the local junk yard and picked up a couple of (darn good) bikes for $20 and some steel for $10. Should give me about 95% of the stuff i need to make it. Also, i found some very cheap 160mm discs/calipers for US$26 express shipped from china and i am going to see what i can do to machine a hub to mount the disc to a *standard* hub. Fingers crossed it all works out and they are not total crap. Cant wait to start building it!
Well, as i said, i just HAD to do it. This is my effort so far. <br> <br>VBrake rear, 160mm Disk front, fully suspended. Total build cost, about AUD$100 but that uncludes 3 donor bikes (from my local scrap yard). The bottom bar is full of lead. The only thing really left to do is put a seat on it. Rides well, spring rates are perfect. Now i just have to find some hills to run it on (and make some brackets to strap it to the roofracks on my car). Still trying to source some wheel covers at a reasonable price but with my feet further forward, they tend to stay out of the spokes anyway. <br> <br>Thanks Harlan for such an awesome inspirational instructable!
I know I've responded late, but that's awesome. Nice job :)
Instead of filling the tube along the bottom of the frame could you just use a solid bar instead of the hollow?
Yes. Although somewhat impractical and difficult to work with.
Does riding this hurt your back at all? Also, what happens when you come across an uphill? Do you just have to walk it up??
Pretty much. You move in accordance with gravity- hence, gravity racer. Going uphill with this racer would mean that you're defying gravity.
<p>Definitely one of the<em> coolest </em>instructables ever. After I read this I favorited it immediately. I hope to make this. Quick question though. How would you wire it to make it electric? Thanks</p>
<p>Depends what motor you use, and how it is set up.</p><p>On the rear wheel hub axle, there should be a sprocket if you're using an old wheel set-up from a bicycle. I assume that if you use a mounted motor which will drive an independent sprocket, you connect the two with a chain, like a bicycle. You'd want a full coverage guard on that though. You'd then fit controls onto the handlebars and wire accordingly. Wiring can vary from motor to motor. </p>
<p>Yeah! I am creating this work of beauty. I am making an ible about it. (CREDIT GOES TO YOU ;) ) It's going to be made out of an old BMX bike. hope to post it soon!</p>
<p>what did you use for the back to hold the tire and weld the pegs on?</p>
<p>Hi!</p><p>We are a team of 15 people trying to make a bike quite like yours. We would like to make a bike that gives the same feeling of speed as yours, though we would like it to have pedals so you can also bike uphill. I added a picture of an early sketch and appreciate all sorts of comments and help! </p><p>What do you think? Is it possible and what is important to think about during this process? :-)</p>
Just finished making this as my project for my senior engineering class in high school along with 2 friends. Added a 36v hub motor using 3 12v deep cycle lead acid batteries works great. Thank you for the instructable. Used an ammo box to hold the batteries.
<p>This is awesome! I want to do the same thing except 48-60v and using lithium batteries (to reduce weight). How well did it preform? And does it turn well?</p>
Works great. I would recommend building an aluminum frame. It will help with the weight issue. As mine is now it can go up some inclines but with my heavy frame you can severely notice the reduced speed. The bike however turns great. We are currently working to paint the bike to prevent rust as well. Hope your build brings you as much fun as mine did.
<p>What is your top speed on flat land?</p>
I would say about 15-20mph on flat but have not actually done calculations though I plan to in the coming week or so I will post the definite speed then.
<p>It's been a few months. Any updates on your bike?</p>
Painted product
<p>Came out nice! You should post up a youtube video of it in action!</p>
<p><iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/vJSmPAGhf6A" width="500"></iframe></p><p>Here you go hope you like it. Just tell me if you want to see more.</p>
<p>Woa man that is sick! I'm going to have to make one with the leftover tubing after I finish my dirtsurfer.</p>
<p>Thanks it's so worth it. And good luck with your dirtsurfer. </p>
What would be the measurements for a kids gbike?
<p>my two top full length bars were 54in across so based off of my hight take what ever your child's height is in inches and divide by 1.33 and that should give you a good length for the laying down portion the rest is up to preference on positioning the knee pads and handle bars as well as how low to the ground you want it. </p>
<p>this bike is awesome<br>I would like to make one but with a 50cc pocket bike engine and mountain bike gears at the back<br>has anyone got any tips or advice?<br>thanks</p>
<p>Depends if you already have the 50cc pocket engine I hope all goes for the best in you build. However if you are looking to buy a motor i would recommend using a hub motor there is no worries about how it will fit or be integrated and you can use the batteries that power it as a weight for going down hill.</p>
<p>cool, thanks :)</p>
Relly good
How hard do you think it would be to put an electric motor on one of these?
You could add some hub motors, then you could even use couple sealed lead acid batteries as more ballast.
Bingo!!! my thoughts exactly
The frame would have to be modified to accommodate a chain, but other than that, it'd work. If you were going to do so and use it to cruise around town, I'd suggest putting a large / tall flag on the back like some recumbent riders do. This way people have a better chance of seeing you considering it is so close to the ground.
HEY this is really great i want to build for my self. i think this is first Gbike gona build in India. <br>Just need little help plese guys i want to iknow the measurement of the frame full frame. <br>what is the size of frame and lower frame. plese <br>
I have never heard of G-Bikes but thanks to you I have now seen an amazing addition to the bicycle world! This looks like on heck of a lot of fun and building it yourself just makes it more fun! I was a cyclist for many years and have been 50+ mph a few times but the thought of it being so close to the ground sounds wild! Well done good luck racing your fantastic Instructable.
Your Rear wheel covers are easy to find, look at the Thrift Shops for the Stationary Bikes that have the covers on the wheel. They have been kept inside and never have been crashed. Nice job on the machine. To bad the guys here play polo and nobody does any bombing here in Idaho.
i bomb quite frequently in your state sir.
Good call never thought of that!
im building a bike for this but im starting with a frame for a stingray chopper.. i may post a build for it<br>
what length is the kneeler all way through??
how do you stop them like once you want to stop
Brakes we have never built a downhill machine without them!
Is there A possibility this could be made of wood? If so, would there need to be some modifications considering this?

About This Instructable




Bio: The word I hear most explaining me is whimsical. I'm an artist designer crafter person. By day I work for Michael Curry Design making ... More »
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