Picture of Gravity Bike
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 

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  

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
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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!

What do you think? Is it possible and what is important to think about during this process? :-)

Bowchain made it!11 months ago
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.
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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?

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.

What is your top speed on flat land?

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.

It's been a few months. Any updates on your bike?

Bowchain10 months ago
Painted product
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Came out nice! You should post up a youtube video of it in action!

Here you go hope you like it. Just tell me if you want to see more.

Woa man that is sick! I'm going to have to make one with the leftover tubing after I finish my dirtsurfer.

Thanks it's so worth it. And good luck with your dirtsurfer.

chiporty10 months ago
What would be the measurements for a kids gbike?
Bowchain chiporty7 months ago

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.

this bike is awesome
I would like to make one but with a 50cc pocket bike engine and mountain bike gears at the back
has anyone got any tips or advice?

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.

cool, thanks :)

shteev1 year ago
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??
mbell211 year ago
Instead of filling the tube along the bottom of the frame could you just use a solid bar instead of the hollow?
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??
Relly good
The Cloak3 years ago
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
Harlan Whitman (author)  The Cloak3 years ago
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.
Kittudas2 years ago
HEY this is really great i want to build for my self. i think this is first Gbike gona build in India.
Just need little help plese guys i want to iknow the measurement of the frame full frame.
what is the size of frame and lower frame. plese
andrew_h2 years ago
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!
andrew_h made it! andrew_h2 years ago
Well, as i said, i just HAD to do it. This is my effort so far.

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.

Thanks Harlan for such an awesome inspirational instructable!
mr fat2 years ago
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.
chevy793 years ago
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.
Harlan Whitman (author)  chevy793 years ago
Good call never thought of that!
fallcrash3 years ago
im building a bike for this but im starting with a frame for a stingray chopper.. i may post a build for it
bcoolbelal3 years ago
what length is the kneeler all way through??
carlsands3 years ago
how do you stop them like once you want to stop
Harlan Whitman (author)  carlsands3 years ago
Brakes we have never built a downhill machine without them!
evindrews3 years ago
Is there A possibility this could be made of wood? If so, would there need to be some modifications considering this?
Harlan Whitman (author)  evindrews3 years ago
Totally possible! It would take some tinkering no doubt.
archipeep3 years ago
Very coo!

On another note, I recall carrying a 200lb., two person, elephant costume down Main St. that your company designed for the Lion King Celebration back in 94. I remember meeting Michael a couple of times. Seemed like a nice guy. Was attentive to our puppeteers.
Harlan Whitman (author)  archipeep3 years ago
RAD! Bet that was a memorable experience! Yeah the puppeteers come first if they are not happy no one will be :)
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