Gearing dilemma...need advice!

S

slash pine

New Member
#1
#1
I need lower gearing...!

I recently installed a Preditor 212 on an mmb80 which has a 70-72 rear sprocket with 13.5"(+/-) wheels...

I was going to try a 10 or 11 tooth front sprocket/clutch but all I could find in 3/4" bore with a 35 pitch sprocket is a 12 tooth...

My issue is (in a word) HILLS...! with just the Preditor upgrade and the 12T front sprocket the bike is still sluggish trying to start off on a slope...I have to really give it the throttle (just about to the wheelie point) and it seems like it will wear out the clutch in no time...secondly to navigate up and down our road (out to the mail box etc.) it requires speed/momentum and there are several blind curves on hills that make it dangerous to have to go so fast to maintain momentum...

I need to reduce the gearing so I can "putt" up the hills...I don't care about top end speed...

I understand a TC is a good option but first I don't fully understand the difference between a TC and a jack shaft...and secondly I want to keep the cost to a minimum right now (have tractor parts to buy) nor do I have the time for any frame modifications etc...

I am considering a short jack shaft starting with a 10 or 11 tooth clutch w/#40 pitch chain back to the shaft and another 10 or 11 tooth (#40) sprocket...with the secondary sprocket a #35 pitch 11 tooth back to the OEM rear...(I can fab this up for less than $50 and a couple of hours)

Will this get me what I need? any suggestions...any other quick, cheap options?
 
S

slash pine

New Member
#3
#3
bluefall said:
go with a tooth clutch i run 10 72 #41 on a mini i got and it goes up hill ez
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To find a 10 tooth clutch I would have to change the rear sprocket (it's #35 pitch)...I can't find a simple replacement for the rear sprocket...

I can't find a 10 or 11 tooth 3/4 clutch in #35 pitch...

I did find one 11T clutch but it is $120.00
 
Motra

Motra

Active Member
#7
#7
A 60 tooth driven is going the wrong way if you want it geared lower, that's higher.

If you already have a #35 clutch why would you get a #40? And the gearing you're proposing is such a small change you'd hardly even notice a difference. With 12T to 70T you've got a 5.833:1 reduction which gives you about 87 lbs. of thrust and 28 MPH. Going to an 11T primary you'd have 6.364:1 gearing with 94 lbs. of thrust and 26 MPH. Not much difference.

If you use your #35 12T clutch and go to a 20T sprocket on a jackshaft, then a 12T to your existing 70T driven you'd have a 9.722:1 gear reduction with 145 lbs. of thrust and 17 MPH. Once you get into the 150 to 200 lbs. of thrust range you have plenty of power.
 
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Motra

Motra

Active Member
#8
#8
The difference between a jackshaft and a torque converter is that the jackshaft is a single speed and the torque converter is an automatic transmission. A single speed is like driving with your car in one gear; it's very limited. With a TC your bike would have 233 lbs. of thrust and a top speed of 31 MPH. Huge difference.
 
trinik7597

trinik7597

Active Member
#9
#9
Motra said:
A 60 tooth driven is going the wrong way if you want it geared lower, that's higher.

If you already have a #35 clutch why would you get a #40? And the gearing you're proposing is such a small change you'd hardly even notice a difference. With 12T to 70T you've got a 5.833:1 reduction which gives you about 87 lbs. of thrust and 28 MPH. Going to an 11T primary you'd have 6.364:1 gearing with 94 lbs. of thrust and 26 MPH. Not much difference.

If you use your #35 12T clutch and go to a 20T sprocket on a jackshaft, then a 12T to your existing 70T driven you'd have a 9.722:1 gear reduction with 145 lbs. of thrust and 17 MPH. Once you get into the 150 to 200 lbs. of thrust range you have plenty of power.
Click to expand...
this is some great info motra :thumbsup: what rpm range are you figuring for these results
 
S

slash pine

New Member
#10
#10
Motra said:
A 60 tooth driven is going the wrong way if you want it geared lower, that's higher.

If you already have a #35 clutch why would you get a #40? And the gearing you're proposing is such a small change you'd hardly even notice a difference. With 12T to 70T you've got a 5.833:1 reduction which gives you about 87 lbs. of thrust and 28 MPH. Going to an 11T primary you'd have 6.364:1 gearing with 94 lbs. of thrust and 26 MPH. Not much difference.

If you use your #35 12T clutch and go to a 20T sprocket on a jackshaft, then a 12T to your existing 70T driven you'd have a 9.722:1 gear reduction with 145 lbs. of thrust and 17 MPH. Once you get into the 150 to 200 lbs. of thrust range you have plenty of power.
Click to expand...
Thanks...this is exactly the type of information I was seeking...

The only reason I was going to get a #40 clutch is because I could not find an 11T-#35 with 3/4" bore...so to keep the OEM #35 rear sprocket I started thinking about a jack shaft...

...as soon as I can get the bike where I can do some measuring etc. I will see if I have the room for the setup you suggest...looking at your figures it seems like a good place to start...any idea what the dia. of a 20T sprocket is? (can't seem to find that info from suppliers)

Can you supply the math formula for determining the thrust ?

Also Thanks for the primer on TC/JS differences...I'm quite interested in learning more about the the TC kits (future second bike)...
 
cxbra

cxbra

New Member
#13
#13
I dont understand how that bike isnt pulling like a mule with that 6:1 ratio! (12x72). Im running the same ratio on my El Tigre and the front wheel comes up on command from 10mph-25mph, its a BEAST. It tops out at about 35mph and will pull me up any paved hill. It has a lot of torque off of idle. Ive run a 5:1 ratio on a previous bike (BC1100 with modified 5hp flathead) and it had a very "peaky" powerband. It made no power in the low-mid range but it really pulled hard up top. That bike was hard to get going up a hill, but it was because of the ratio.

I would suggest checking into your engine. Is it BONE stock, untouched? If so, there is some secret hp and throttle response tricks to get you going. When I first got my Predator it was nice, but running extremely lean and it contributed to twitchy throttle response. It would struggle at full throttle and accelerate as I was letting off the gas, typical lean situation. First thing I did was
1) Enlarge the jet. That made a world of a difference. It allowed the engine to run how it was supposed to, and it is still a little on the lean side after almost doubling the size of the jet bore!
2) Enlarge the stock intake box air hole. Stock, you will notice that it is choked off from the factory. Bore out the bottom of the intake box to match the bore on the bottom of the intake. Very simple, huge seat of the pants hp increase over the bone stock intake.
 
S

slash pine

New Member
#14
#14
cxbra said:
I dont understand how that bike isnt pulling like a mule with that 6:1 ratio! (12x72). Im running the same ratio on my El Tigre and the front wheel comes up on command from 10mph-25mph, its a BEAST. It tops out at about 35mph and will pull me up any paved hill. It has a lot of torque off of idle. Ive run a 5:1 ratio on a previous bike (BC1100 with modified 5hp flathead) and it had a very "peaky" powerband. It made no power in the low-mid range but it really pulled hard up top. That bike was hard to get going up a hill, but it was because of the ratio.

I would suggest checking into your engine. Is it BONE stock, untouched? If so, there is some secret hp and throttle response tricks to get you going. When I first got my Predator it was nice, but running extremely lean and it contributed to twitchy throttle response. It would struggle at full throttle and accelerate as I was letting off the gas, typical lean situation. First thing I did was
1) Enlarge the jet. That made a world of a difference. It allowed the engine to run how it was supposed to, and it is still a little on the lean side after almost doubling the size of the jet bore!
2) Enlarge the stock intake box air hole. Stock, you will notice that it is choked off from the factory. Bore out the bottom of the intake box to match the bore on the bottom of the intake. Very simple, huge seat of the pants hp increase over the bone stock intake.
Click to expand...
It's not the bike/engine...it's the hills...! (I'm in the mountains)

The bike has plenty of power it hauls @$$ ! but the clutch slips starting off on a hill etc...otherwise it's a "wheelie bike"...!

I need the bike to just putt up the hills at a bit more than walking speed...I don't care about top end performance...around here if you're not going up hill you're going down...!
 
cxbra

cxbra

New Member
#15
#15
slash pine said:
It's not the bike/engine...it's the hills...! (I'm in the mountains)

The bike has plenty of power it hauls @$$ ! but the clutch slips starting off on a hill etc...otherwise it's a "wheelie bike"...!

I need the bike to just putt up the hills at a bit more than walking speed...I don't care about top end performance...around here if you're not going up hill you're going down...!
Click to expand...
Where are you riding, on the moon? Onroad or offroad? It sounds like there is a very simple solution for you called a torque converter. That way, you could make it up a hill, but not top out at 18mph.
 
S

slash pine

New Member
#16
#16
cxbra said:
Where are you riding, on the moon? Onroad or offroad? It sounds like there is a very simple solution for you called a torque converter. That way, you could make it up a hill, but not top out at 18mph.
Click to expand...
I appreciate all input...

I'm in the lower end of the 'Blue Ridge' mountains...it's steep in these parts...gravel roads...

With this bike I am trying to keep things to a minimum on costs...(and get some experience)...I do plan on a second bike and a TC is a likely option if it will work on the grades here...by starting with a jack shaft (that I can experiment with different sprockets) I will be able to relate different gear ratios to actual situations...

I'm not sure typical TC's are going to be geared low enough initially...to start off on some of the grades here... It's not so much "making it up" a hill...it's starting from a dead stop on a (steep) hill...18 mph is plenty fast enough for the needs...
 
Motra

Motra

Active Member
#18
#18
Here's one I built with the basic jackshaft kit and an engine mounting plate, both from OldMiniBikes. This was a 12T clutch to an 18T, then 12T to 70T. It worked great.



 
Motra

Motra

Active Member
#19
#19
slash pine said:
Can you supply the math formula for determining the thrust ?
Click to expand...
Here's something I put together years ago. I also made Excel spreadsheets with all the calculations built in so it's a simple "fill in the blanks" sort of thing. It gives the drive wheel torque, max thrust, speed at which max thrust takes place and max speed. I'm interested in the thrust because I'm into Tote-Gotes that will climb anything. They make about 300 lbs. of thrust at about 4 MPH, that's what it takes to climb mountains with a heavy load. A small minibike that makes 200 lbs. of thrust will jump right out from under you if you're not careful. If anybody wants the spreadsheets I'd be happy to email them to you.



 
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S

slash pine

New Member
#20
#20
Thanks for posting all the data and information...
I ordered a jack shaft kit from OMBW ...I also ordered an array of different sprockets (not all from OMBW) to see what configuration works best for this terrain...then I will relate that with the data provided to put theory numbers to hands on testing etc...

Once I have the shaft mounted I may consider a two speed setup if there is room...
 
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