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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Bowchain7 days 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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shteev4 months 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??
mbell216 months ago
Instead of filling the tube along the bottom of the frame could you just use a solid bar instead of the hollow?
Stevechummy9 months ago
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??
guffe jensen11 months ago
Relly good
The Cloak2 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 Cloak2 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.
Kittudas1 year 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!
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.
chevy792 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)  chevy792 years ago
Good call never thought of that!
fallcrash2 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
bcoolbelal2 years ago
what length is the kneeler all way through??
carlsands2 years ago
how do you stop them like once you want to stop
Harlan Whitman (author)  carlsands2 years ago
Brakes we have never built a downhill machine without them!
evindrews2 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)  evindrews2 years ago
Totally possible! It would take some tinkering no doubt.
archipeep2 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)  archipeep2 years ago
RAD! Bet that was a memorable experience! Yeah the puppeteers come first if they are not happy no one will be :)
bcoolbelal2 years ago
Could someone please tell me the measurements and sizes of the metal frame he used, as i dont understand what he means by 8ft of .75 x 1.5 x .125 mild steel. thanks
What I believe he means is that the square tube he bought was eight feet long, three quarters of an inch wide, an inch and a half high, with an eighth of an inch thickness.
bertus52x112 years ago
This is very, very cool. If I could weld, I would build one!
I especially like the "bomb" between your legs. You should add a fuse on top to complete the picture...
Have you ever considered using solid wheels? (for weight but for a very strong gyroscopic effect and thus stability as well).
donai122 years ago
About how much did this cost???
yokozuna2 years ago
Not sure that it's for me, but nevertheless, a fantastic instructable with excellent supplemental documentation. Five stars for you.
A g bike has officially been added to my to do list ( just gotta finish the chair, guitar, 70s skateboards, and the scraper bike haha.)Just a few questions though. 1) Whats the ride height? 2) How much dose the bike weigh? And 3) How tall is it?
bcoolbelal2 years ago
was there any like design flaws?? or anything you would have done diffrent in the design??
Harlan Whitman (author)  bcoolbelal2 years ago
The next build will involve more clearance between the kneelers and ground. A frame design that allows for more clearance between the helmet and frame. Also thinner wall top tubes to keep more weight lower on the frame.
dmitri112 years ago
is it strange/uncomfortable having your head off to the left of the head tube
Harlan Whitman (author)  dmitri112 years ago
Not so much don't even think about it really. I just move around until I feel that sweet spot in the wind wherever she may be. The problem is that the chin guard on the helmet hits the frame in the position I want to be in. Next build should fix that.
chicopluma2 years ago
wow cool bike
bcoolbelal2 years ago
Overall how long did it take to build
Harlan Whitman (author)  bcoolbelal2 years ago
From start to finish it look a little under two weeks. We had to have it finished so we could run it at Maryhill.
Wow you guys are legends, did u work on it like day and night to finish it
getbusy212 years ago
I likely! Good information internet!
cool drift

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