first build,bmx chainsaw,chain driven.

ok i was reading through this forum yesterday, gaining knowledge
and getting ideas. alot of clever people on here and some wicked
builds. so i thought i would sign up and share my first one.

my next door neighbour bought one of them 80cc conversions and it
goes pretty good so i thought i would make one. i am using a small
(25.4cc) chainsaw i bought new. pulled all the eccess rubbish off
and eventally got it running. had automatic choke and other various
hoses and plumbing for the chain oil thingo and actually woldnt start
out of the box anyway it runs now and ive just got to get a sprocket
on the cultch.

i am copying my neighbours bike with the sproket on the spokes idea.
this wont be for taking off at all really. i will pedal up to speed then
nail the throttle for the most part and i should be off. its not for a
top speed race either and if i hit hills i will pedal.

this is mainly for transport and medium speeds and for me love of tinkering.
heres some pics of what im working with. i need to space my rear sproket
out from the spokes a bit as the chain wold hit the tyre atm.

its very early stages, i need to mount a rack (might cheat and get a s/h one)
a idler arm/ tensioner arm, and organinse the sprokets. notice the sproket is
on the same side as the original chain on the bike? thats the direction of the saw, i will
make it work. havnt seen many like that.

ive got the bug! comments welcome. cheers.


ok looks like theres no pictures. what a shame. dont know
what happened there. this was the intitial post

welcome to the site and sorry about no picture. we have had a terrible problem with spam so the forum is set up that no links can be posted till your third post. glad to have you with us and look forward to hearing more from you

thanks for the welcome. ile try and get my post count
up so yo can see what im working with. i pulled the front
sprocket off my mountain bike and have that on the rear spokes
about 40t i think.

i can get the sprocket on the rear wheel to run straight but the
sproket still moves up and down a bit at high rpms. does this have
to be spot on? will an idler arm/pulley fix this or will it rattle itself
to pieces?

thanks in advance.

this might be subject for another thread, not sure, let me know
if it is im new here. ta.

sounds like a nice build
have you checked out dave's weedeater bike on youtube he make a great one

Maybe you didn't know this, but the HT kits ( like your buddy has ) have big time internal gear reduction on the output shaft. You will need a jack shaft deal to slow down the chain before it gets to the rear wheel.
As for the up and down sproket, I would try to eliminate it. Maybe elongate holes and punch closed the other side (or a tab of weld). Leaving it as it is does load the chain more and will likely make some vibration.
I made a jackshaft out of a rear single speed freewheel hub. This way you can coast as soon as you release the throttle. Rather than having to wait for the engine clutch to slow down and disengage.

cheers for the help so far im now thinking about how
im going to get a sproket on the clutch of the motor.
im thinking of just getting a mate to weld on some pipe
around the outside of the clutch with some smaller pipe
and flat steel welded to that for the sproket.

im not even sure if the material of the clutch bell is steel
or can be welded, looks like stainless or something.

try and get these pics going...

the bike-
http://i735.photobucket.com/albums/w...n/DSCF0072.jpg

the donk-


i know i might not have the gear reduction and some problems but
its a first build so im going with what ive got. thanks for the input.

When professor points out the HTs gear reduction, it is because the overall gear ratio is essential for making a small motor power a bike. A clutch can't make up for improper gear ratio.

Lets say, for instance, a HappyTime motor has a built-in gearbox with 4:1 reduction. When the motor is spinning 4000 rpm, the motor sprocket is at 1000 rpm. With a 10-tooth motor sprocket driving a 50-tooth rear wheel sprocket, that's another 5:1 reduction. So the overall reduction will be 20:1 (4 times 5). So the motor at 4000 rpm turns the rear wheel at 200 rpm. With a 26-inch rear wheel, that's about 15 mph @ 4000 rpm. (I'm not going to bother doing metric conversions for you, you sound smart enough for that!)

20:1 is a pretty reasonable reduction. Higher-torque motors can get away with a little less, but a 25cc motor really needs the proper ratio.

My bike uses a 25cc motor, friction drive with a roller about 1.25 inches. That gives me about exactly the same, 15 mph @ 4000 rpm.

With an 8-tooth on the motor and a 40-tooth on the rear wheel, you will only have 5:1 reduction. It will not pull, the motor will bog, and you will have to constantly ride that little clutch to try to keep the engine from bogging. The clutch will burn up in a mile.

I'm not trying to discourage you from building. That's a fine motor for driving a bike. You seem to have reasonable expectations:

But you can't copy his bike's sprocket sizes if he has an internal gear reduction. Professor suggested a jackshaft. That's common on homemade chain-drive setups.

What about using the gear reduction that came with the chain saw. Maybe it could be modified to use with a bike chain instead of a saw blade. My first build was with a weed eater. Trust them about the jackshaft. You need some type of reduction between the motor and the tire if you aren't doing a friction drive.

ok first off thanks for the help and ideas.
iam slightly discouraged to be honest but you get that
and im sure thats what defines alot of these "projects" into
something i can acutally fill up and got to the shop on.
although i would really, really like a chain drive but my heart
isnt completely set on it.

i have now installed a rack on the rear and it it strong as
an ox. friction drive hey, jackshaft hey , i have little to no
money at the moment and am currently buidling this out of
what i got.

on the friction drive idea (i cant believe im going back on my chain drive)
can the cultch bell be put straight on the tyre? or do i have to fab a spindle?
if i can just get the clutch on the tyre i might go that.
thanks alot again guys.

It would be nice if you could just run the clutch bell right on the tire. I see three problems with that.

The smooth metal is too slippery. That could be easily solved by gluing on some grit with epoxy.

The bearing probably wasn't made to handle that much sideways load. A bronze bushing would last a little while, anyways. Ball bearings would last a lot longer.

Most important is the diameter of the bell. Roller diameter on a friction drive is just like gear ratio on a chain drive. For your motor, 1.25 inches is about right. (That gives 22mph at 6000 rpm.) If it's much bigger than that, torque will be very poor.

Civilized mentions the gear reduction that came with the saw. Is there one? Does the clutch spin directly off the motor? I'm not familiar with the innards of chain saws.

If that clutch bell is less than 2 inches diameter, I would try it. That motor might have enough torque.