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 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
coolkidtn4 months ago

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!

coolkidtn4 months ago

Definitely one of the coolest 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

luckyfatman134 months ago

what did you use for the back to hold the tire and weld the pegs on?

ekstremsykkel6 months ago


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!1 year 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.
14, 9:25 PM.jpg14, 9:25 PM.jpg14, 9:25 PM.jpg

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?

Bowchain1 year 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.

chiporty1 year ago
What would be the measurements for a kids gbike?

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?
Stevechummy2 years 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??
Relly good
The Cloak4 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 Cloak4 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.
Kittudas3 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_h3 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_h3 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 fat3 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.
chevy794 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)  chevy794 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!
evindrews4 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 :)
bcoolbelal4 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.
bertus52x114 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).
donai124 years ago
About how much did this cost???
yokozuna4 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?
bcoolbelal4 years ago
was there any like design flaws?? or anything you would have done diffrent in the design??
Harlan Whitman (author)  bcoolbelal4 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.
dmitri114 years ago
is it strange/uncomfortable having your head off to the left of the head tube
Harlan Whitman (author)  dmitri114 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.
chicopluma4 years ago
wow cool bike
bcoolbelal4 years ago
Overall how long did it take to build
Harlan Whitman (author)  bcoolbelal4 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
getbusy214 years ago
I likely! Good information internet!
cool drift
*plays Tron Legacy theme song*
itri454 years ago
Very nice project, im 5'6 could i use 16 inch wheels
Harlan Whitman (author)  itri454 years ago
Yes, you can use 16" wheels. However, you can also just shorten the distance between axles or if you don't want to do that you can change the positioning of the handlebars/ kneelers to better the fit. Also, if you use 16" wheels, it will potentially bar you from an IGSA race if there ever is one.

We did build a 16" racer though and it is certainly a fast and interesting ride.

i will not be racing naytime soon. i may not even have the money to build a bike so right now im just planning.
thanks anyways
Spade1154 years ago
Love the look of the frame. Mean Looking :D
Spade1154 years ago
I really dig this,I would love to do a 16 120 - 21-100 set up for just cruising. lol Would be intresting :D
holy crap that looks freakin' scary! And then you have to get a ride BACK UP the hill? Ehlch.
Torokun4 years ago
This is so awesome! Can you add some videos?
Harlan Whitman (author)  Torokun4 years ago
Added a video on the last step!
Are you guys in Portland? Awesome vid. I would love to try out such bike along the CA coast here...
Harlan Whitman (author)  Torokun4 years ago
Yep we are in Portland. Your welcome to come ride with us anytime. The Cali coast sounds like a lovely place to bomb some hills!
Harlan Whitman (author)  Torokun4 years ago
Thanks! Out of town till the 25th, but by the end of May is our goal for some good footage :)
randofo4 years ago
Do you have a video demonstrating how it works?
Harlan Whitman (author)  randofo4 years ago
Harlan Whitman (author)  randofo4 years ago
This is another Gbike built by S.I.N. Cycles http://www.youtube.com/watch?v=u-DdSdi2LOs We will have some nifty videos of the bike in this build posted by months end:)
srk randofo4 years ago
foobear4 years ago
wow - agog.

video please.

also, how do they dismount at the end of the hill? or do they just wipe out? video also please.

Harlan Whitman (author)  foobear4 years ago
Video added to the last step!
i know that it defeats the purpose but putting a small engine such as a chainsaw or weed eater would be awesome on this thing. XD
Harlan Whitman (author)  golddigger15594 years ago
One of our goals is to build some kind of motorized contraption by the end of the summer. We're thinking 150cc to 250cc or so will be just right. =)
this sound like the beginning of a new class at the Bonneville salt flats. A few years ago a guy went 130mph on a 50cc motorcycle! :)
Harlan Whitman (author)  just mike4 years ago
That is screaming fast for a 50cc love it! Bonneville is a dream of ours just gotta make up our minds on what to build.
I'm thinkin' super modified moped. or a turbo charged bath tub. Do they have a class for that at speed week? :)
clax12274 years ago
But what's happen's when you crash when you reach speed's of over 80mph like you mentioned?
the flux capacitor kicks in
Harlan Whitman (author)  clax12274 years ago
Well not so pleasant things. Brakes, leathers, fullface helmet, spine gaurd, gloves all help with preventing serious injury, but you can still be seriously injured. As long as you don't hit a solid object you normally will slide and roll as you are already very close to the ground. Just like being t-boned casually driving through an intersection. Or wrecking your car or motorcycle driving down the highway. To many variables to say how or if you will walk away from an accident.
just mike4 years ago
Nice work, the bike looks great. I just lost my g-bike in a garage fire. it was just an upside down bmx frame with a banana seat and inverted handle bars. so much fun!
not a lot of people doing this it seems. i heard Portland is a good place for downhillers of all breeds.(something about zoo bombers?) if you're ever in the SLC area shoot me a message and i can point you to some good hills.
Awesome. Simple, and awesome.
eulaliaaaa!4 years ago
It looks like something from the movie TRON.
add a motor and some lights, and you got a light bike!
Simba84004 years ago
Where would I be able to buy those materials?
Harlan Whitman (author)  Simba84004 years ago
Two online sources that will carry most if not all of the raw material metal, nuts, and bolts. Mcmaster.com nuts and bolts, and Onlinemetals.com for metal stock. Seeking out local is better, funner, cheaper and way funner.
Harlan Whitman (author)  Simba84004 years ago
You're local hardware store, metal supplier, bike shop, and scrap heap should have everything you need. On this build, everything but the top/bottom tube, shaft collars, and paint were scrap or from one of our parts bikes.
tinker2344 years ago
nice i like the final product how fast does it go
Harlan Whitman (author)  tinker2344 years ago
We've gotten it to somewhere in the mid to upper 50s so far. We are going to head out to some of the speedier hills and see what we can do soon. We'll make sure to get some video.
cool nice and fast
TorBoy94 years ago
This is an awesome Instructable!

Just a note that Barge is a brand name of glue. The generic name is contact cement, though I believe Barge is superior.
Reminds me of the tron lightcycle
finnrambo4 years ago
summer skeleton racing here we come :D nice instructable the only thing you could improve would be aerodynamics :)
rockerape4 years ago
Any survivors?
nice landscape btw!
eulaliaaaa!4 years ago
Flippin' beautiful!
this is soo good, 5/5
do you think that you could use arc welder to weld the old bike frame to steel?
if so normal rods and normal current?
Harlan Whitman (author)  thelastonekills4 years ago
Thanks! I would imagine so. Don't take my word for it for I have never tried. Your best bet is to get some similar material to pratice on and then do some stress tests. Put one end in a clamp and then a big pry bar on the other and give it some muscle.
Good Luck
sonofstone4 years ago
for the next one, i reccomend that you use schwalbe marathon plus, it has an anti leak layer of around half an inch thick
Harlan Whitman (author)  sonofstone4 years ago
Actually that tire in the second pic is the tire you speak of :)
hate to break it with you, that's not the marathon plus, thats just the marathon, i build bikes for a living, and my company only uses schwalbe, so i recognize them from miles away
Harlan Whitman (author)  sonofstone4 years ago
ohhhhh the "plus" gotcha :) either way they proved themselves. we are really moving towards soley Animal anywho. Thanks for cathching that.
Harlan Whitman (author)  sonofstone4 years ago
Right on! We do love those for that reason. Have no problems with them thus far. We also like any additional weight out there to help carry us through the flats in the road. Well thats the concept at least.
DiamondBack4 years ago
Simple fun... more people should do this. Including me. I think I might build me one next month. Thanks for the inspiration!
Harlan Whitman (author)  DiamondBack4 years ago
:-) Thank you for being inspired, and good building to you!
m0ntu4 years ago
Just wondering if thats mary hill, in the last pic of you wrecking?....lol. It Just looks really familiar to a place where A buddy of mine did a long-board race.
Harlan Whitman (author)  m0ntu4 years ago
Yep Maryhill Loops Rd. also that is Justen Ortiz a pro longboarder who was having a blast on our GBikes during the Maryhill Spring Freeride. Some awesome footage of that event on Youtube and Vimeo. 120 Longboarders and three GBikes. I barely got to ride our machines ;)

Yes look at the image notes on the first step
JBZG4 years ago
Hey, Great project!

Also, just saw pic 3, Be carefull with your shoelaces :)
zack2474 years ago
really neat!
honestly ive never heard of gravity bikes before, but it isnt a surprise since im from the prairies :P
Gravity Bikes been around as far back as 1980.
:P im not saying theyre new, im just saying ive never seen one around where i live, which is the flat, wide prairies.
Yeah, I know just putting it out there.
Honus4 years ago
Awesome! I did gravity bikes a long time ago- it's super fun. I remember in the old rules you weren't allowed to run a 16" front wheel on any course faster than 55mph. Does Dan Hanebrink still holds the high speed record?

Building a suspension version would be really cool. I've got a couple of old pull shocks out in the garage- high pivot, rearward axle path with a low leverage ratio and a short linkage fork.... I might have to draw that up!

Now if they only did GPV sidecars...THAT would be crazy!
Harlan Whitman (author)  Honus4 years ago
Not sure who is currently holding the record. Oh there are some nifty GPV sidecars super rare. We really should put that on the list for this summer.
There's a GPV sidecar in this video- pretty slick:

newtonsz4 years ago
Marvelous, I love the idea, I just felt the adrenaline, I had already reached 100 KMH (about 62MPH) on a 26" Mountain Bike with special tires. After did it I though an accident would been fatal as we, me and my friend, were not using special clothing or even a helmet.
I will keep this idea to, who knows, any day build one. Another project I intend to do is the Mono Wheel Bike: http://cms.obviousmag.org/mt-search.cgi?IncludeBlogs=2&limit=28&search=mono+wheel
nickivan4 years ago
I love the idea, and especially the way you have explained everything Harlan. Thought about it myself a coupla times. Problem is - Im a dreamer not a doer. I havent seen anything like this here in the Czech Republic. Although, years ago, they had gravity go-kart races for kids.
very cool, but i think i would try and figure out a way to put a little more braking power on the ride...especially if your getting up to 80mph.. those brakes just wouldnt cut it for my 280lbs
Harlan Whitman (author)  autoglassguy4 years ago
Agreed, 8inch disc up front and caliper in the rear is our ideal set up. The caliper brakes will do the job. The disc does do it better and requires less force and is much easier to feather.
cowgurhl4 years ago
I am curious as to why not use disc brakes ? Wouldn't the ideal set up be using downhill or freeride hydro disc brakes like hopes or avids.
Harlan Whitman (author)  cowgurhl4 years ago
These brakes work well enough, but disc brakes really are the way to go. This bike will get disc on the front soon:) We are in the proccess of finding the ideal/affordable disc hub that can lace into a 20inch wheel.
I'm guessing the spirit of the build was "using what we have around"... most people don't just have hydro disc brakes laying around.
wobbler4 years ago
Great looking bike!
I use good old spit on the inside of grips. You don't need too much and you have to be careful for a couple of days until it dries out fully, but I usually don't have any problem finding spit.

It also works a treat for getting old grips off. Pull them out a bit and force some spit in the gap, then slowly work the grip back and to, forcing the spit down and the grip will just fly off.. It works a treat.
Harlan Whitman (author)  wobbler4 years ago
Power to the spit really does work great. Sadly I have been banned from using spit! hahaha
This IS a cool sport. I have only one thing to say Haleakala!! It is a 10,000 foot mountain on Maui, Hawaii. The ride to sea level is 35 miles with only 100 yards that is not down hill. 35 miles is the tourist route. There is a much shorter route with longer runs and less turns and towns to go through. This bike is made for it! Maybe you should set up a race in Maui!
studleylee4 years ago
Very cool design!!! Me Like!!! Stole Picture for ideas...
Nice fabrication!!!!
Great... I didn't know such a sport existed!
Has my vote..
Incrxtc4 years ago
Love it! Great design all around.
ElvenChild4 years ago
Marvelously brilliantly awesomy.-what- it's a word.
BtheBike4 years ago
Sweet fab. wut u do in Unalaska ? weld crab pots ?
estructor4 years ago
Nicely done. Wonder if they'd ever allow this down the Pikes Peak Highway....
D00M994 years ago
As soon as I saw the title picture the first words to pop into my head was "TRON Lightcycles"!

Add some EL panels and you have the envy of the neighborhood. :)
Harlan Whitman (author)  D00M994 years ago
Thanks, Considering we bomb at night often that would be very fitting. hmmmm...
rimar20004 years ago
What a good work!

It's a pity that the tallest hill near my home is the highway bridge...
alexsolex4 years ago
children in tahiti (french polynesia) spend most of their time doing this kind of bike riding. They climb up to the top of the road and ride like mad down the see. I kept this as a very good souvenir from there.

I didn't know it was known as a sport somewhere else in the world.
lebowski4 years ago
That is very cool! Nice work.