Want to build a rock-climbing Phatty

#1
I have the urge to build a fat-tire mini-bike for climbing the very hilly woods around my house. Want to use a 6.5 Predator (of course) . Must need to be VERY torqy-not worried
about speed. Something capable of 20-25 MPH will suffice. Probably will need a Comet 30 and jackshaft arrangement? What kind of gearing would be good for this. I want to use 22" tires so that will have to be factored into the gear ratio. Anybody give me a good sprocket combo so I could avoid trial-and-error? Thanks
 
#2
72t rear sprocket with a 10 tooth on the tav should get you pretty much perfect. Get a 30 series tav. Are you going to build a frame or start out with something?
 
#6
I'm looking high and low for a rolling three wheeler frame near me in Atlanta. I'd like to chop one up into a minibike. If I can't find one by the time I'm ready to build I'm going to get a three wheeler front fork assy off of eBay and go from there. I have a Baja frame that I think I will use by widening the rear to accept the 22x11-8 tires. Thanks for the gearing calculator. I would love to keep it simple without having to use a jackshaft but I think I'm going to have to go that route for the low end torque I'd need.
 
#7
I found an app that factors in the Comet 30 torque converter with an option for with or without a jackshaft. The guy who made the app did a heck of a job. Since it's made for a go cart do you think the numbers will be off for a two wheeled minibike? Or is the weight difference the only real factor.

It only comes up as an app. The web site is not up.

http://app.kartcalc.net/#home
 

chrisr

Active Member
#8
I you do not want to go with the additional jackshaft, if you go with a 60 tooth rear, a 8 tooth on the Comet 30 driven yields 7.5:1 or a 72 tooth rear with a 8 tooth yields a 9:1. The 8 tooth with 5/8 ID are not real common but can be found, as well as the 72 tooth rear sprocket.
 

delray

Well-Known Member
#9
if you elect not to use a jackshaft you may want to consider using a 7 rear driven with the 72 rear sprocket and if you use a 30 series you have the option of getting a bando belt and they work better then anything out there(real comet belt is still ok). you kind of achieve two different things with the 7inch,lower gearing and less slippage with more belt contact on the driven unit. makes the transmission into a torque monster with all the correct parts installed. I personally don't like to go to small on the sprocket behind the torque converter(driven). to small tends wear out quickly because of all the torque being applied on just couple teeth at a time when spinning. going with a 40 chain will slow down the wear and tear a lot. go with a 40 and with a 40 rear sprocket you will have a less change of damaging it when you are off road in wood trails. clone motors work good right out of the box with just a few mods done to them. jet kit/header/air cleaner and set the governor just below 4000 rpm's. you don't need to go crazy with a billet rod,flywheel,carb, cam...….etc
when you start to get into aftermarket stuff/cams then you need to have more gearing and stall speed to achieve performance the cam has and start to loss your bottom end performance that you need for your type of riding you are looking for. not building a race bike,just want to have fun off road with it.
 
#10
if you elect not to use a jackshaft you may want to consider using a 7 rear driven with the 72 rear sprocket and if you use a 30 series you have the option of getting a bando belt and they work better then anything out there(real comet belt is still ok). you kind of achieve two different things with the 7inch,lower gearing and less slippage with more belt contact on the driven unit. makes the transmission into a torque monster with all the correct parts installed. I personally don't like to go to small on the sprocket behind the torque converter(driven). to small tends wear out quickly because of all the torque being applied on just couple teeth at a time when spinning. going with a 40 chain will slow down the wear and tear a lot. go with a 40 and with a 40 rear sprocket you will have a less change of damaging it when you are off road in wood trails. clone motors work good right out of the box with just a few mods done to them. jet kit/header/air cleaner and set the governor just below 4000 rpm's. you don't need to go crazy with a billet rod,flywheel,carb, cam...….etc
when you start to get into aftermarket stuff/cams then you need to have more gearing and stall speed to achieve performance the cam has and start to loss your bottom end performance that you need for your type of riding you are looking for. not building a race bike,just want to have fun off road with it.
So you're saying 7" Comet 30 with a Bando belt and a 7 tooth gear to a 40 tooth sprocket and I'd be good? I have 6" clone Comet 30 with a Comet brand belt right now. I was thinking of a 9 tooth gear driving an 84 tooth sprocket. It's 8-1/2" diameter would give me about 6-7" clearance from the bottom of the sprocket to the ground. I'm debating whether or not to have a jackshaft or not. With a J/S I can reduce the diameter of the rear sprocket and increase ground clearance but have to worry about more "cogs" breaking out in the middle of nowhere. The simpler torque converter to rear sprocket is more appealing but the reduced ground clearance might be an issue. What say you guys?
 

delray

Well-Known Member
#11
no I didn't say anything about using a 7 tooth and a 40 tooth? the 7 inch is the rear driven pulley for the torque converter. much bigger then a 6 inch pulley. this will allow to lower the gearing. so a 9 or 10 tooth sprocket to the rear sprocket(72tt) should give you good amount of low end and still make the bike go 20-25 mph.....

the 40 I mention earlier is the chain size. does that make more sense.
 

chrisr

Active Member
#12
The simpler torque converter to rear sprocket is more appealing but the reduced ground clearance might be an issue. What say you guys?
You can do this with a 8t on driven (tav2 kit) and either a 60t or 72t rear; what Delray is referring to, is using the bigger driven 7" vs. 6", which the 7" will give better torque multiplication vs. 6". The 7" driven are typically used on go karts.
 

chrisr

Active Member
#13
I am not sure if you can use a 7" on the tav2 kit, maybe someone can chime in on that, I have only seen the 7" on go karts that have the driven as part of the engine plate.
 
#14

delray

Well-Known Member
#15
I am not sure if you can use a 7" on the tav2 kit, maybe someone can chime in on that, I have only seen the 7" on go karts that have the driven as part of the engine plate.
yes it can be done on the old style units that use the steel backing plate. unless you don't have the fabing skills it may not be for everyone to try.
the two things that need to be fab are the mounting tabs and the cover to make it all work.
posted this thread before,but if you haven't seen it yet. here it is again.
https://www.oldminibikes.com/forum/index.php?threads/thunder-bolt.115101/page-3
 
#16
yes it can be done on the old style units that use the steel backing plate. unless you don't have the fabing skills it may not be for everyone to try.
the two things that need to be fab are the mounting tabs and the cover to make it all work.
posted this thread before,but if you haven't seen it yet. here it is again.
https://www.oldminibikes.com/forum/index.php?threads/thunder-bolt.115101/page-3
Thanks for the info on that. Sounds like the 7" will be the way to go. That 72t sprocket looks "yuuuuge"!!
 

delray

Well-Known Member
#17
you had mentioned earlier about running a 22" tire. here is how it would look with a 21" tire with a 60 tooth 40/chain

so a 72tt 40/chain should still give you good clearance on a 22" tire.
 
#18
Thanks for that pic. It looks like I won't need the jackshaft after crunching the numbers on a couple of the online ratio calculator s and info from you guys. Thanks all
 
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