Gearing ratio question……

[DirtyDan]

Hi everyone…….
I have a RZR170 clone that I’m repowering with a Honda GX390 and need a bit of help with the gearing ratio.
I’ve seen calculators for this, but I’m not sure if they take into account the CVT ratio and gearbox ratio.
This is my planned setup:
Comet 44 magnum 1” bore driver
Comet 44 8.5” driven 3/4” bore
Honda GX390 1” output
Backing plate with a 3/4” jackshaft built in
Blast LED 40 series reverse gearbox
The buggy is a fairly heavy (600#+)mini side by side with aggressive 25” tires.
It will be used almost exclusively off road.
So I’m kinda wondering where I should start with the gearing to avoid burning up belts and clutches???
I’m not too concerned about top speed as I’d rather have reliability over speed.
I have a 520 series 42 tooth sprocket on the axle already and there isn’t much room for a larger sprocket without sacrificing ground clearance.
Any ideas where to start with the sprockets????
Thanks in advance!!!!

 

[karl]

The torque converter backplate is reduntant, you can mount the driven clutch straight
to the reverse gearbox. So do you plan on running a chain and sprockets between the backplate
and reverse box?

Also I am a bit skeptical about that Blast LED unit, something tells me its
the same 7hp rated box sold everywhere else, just with a 3/4 diameter, longer input shaft.
Would be curious to crack it open, and see if still only has a little #41 drive chain transmitting all
the power in foward gear.

I found blast LED's ebay seller page, and they do not list the "40 series" reverse unit in their store,
only the standard 30 series reverse unit. But they sell the 40 clutch setups and backplates.
Just something to think about, if they really are the only ones in the market with
a stronger reverse box, why would they not sell it on ebay, only their site where you cannot leave feedback
or return it.

 

[DirtyDan]

Not sure at this point if I’m even gonna use a reverse gearbox because of clearance issues. (I haven’t ordered the gearbox yet)
I’ll have to wait until I start mocking up the new engine and torque converter and see if there is room.
It looks like I can run the shaft of the reverse gearbox straight through the backing plate hole of the driven pulley and just run a chain from the gearbox to the axle sprocket.
I have no idea of the quality of the reverse gearbox but it’s literally the only small reverse gearbox that I can find that’s still available.
It sucks that the Comet reverse box was discontinued.

 

[Denny]

You could go to Gokart Supply and use their gear ratio calculator. I think you can plug in not only tire diameter but clutch size. But as long as you get final drive ratio that is the most important. Depending on hills and terrain you’ll want to shoot for some where in between 6:1 and 9:1 just a rough guess. You may have to experiment.

 

[madprofessor]

Don't we know the full speed ratio of most torque converters? My understanding is that while it may be 3:1 at a standstill, it's only about 1:1.1 (a small amount of overdrive) at full speed.
If that's reasonably accurate, then all you have to do is divide the number of teeth on your axle sprocket (42) by 1.1 (= 38.2) to get the number of teeth you will plug into the ratio calculator as being your axle sprocket, instead of what the sprocket really is (42-tooth). Then you can do a straightforward calculation with just the basic sizes, like tires, motor sprocket, axle sprocket, horsepower.
Example: (42 / 1.1 = 38.2-tooth axle sprocket) That should be more than good enough to predict to some wide degree of accuracy what the top speed would be, as those calculators are far from being precise. Every kart is going to have its own pluses and minuses in performance, so you could never be caliper-precise if you had the simplest drive there is. Weight matters, like how much of that big chocolate cake you ate yesterday, and whether or not you hit the head this morning. My own weight fluctuates 3#-4# every day. Plan for slowdowns with passengers, and who's big ole beer-body is driving.

 

[DirtyDan]

Don't we know the full speed ratio of most torque converters? My understanding is that while it may be 3:1 at a standstill, it's only about 1:1.1 (a small amount of overdrive) at full speed.
If that's reasonably accurate, then all you have to do is divide the number of teeth on your axle sprocket (42) by 1.1 (= 38.2) to get the number of teeth you will plug into the ratio calculator as being your axle sprocket, instead of what the sprocket really is (42-tooth). Then you can do a straightforward calculation with just the basic sizes, like tires, motor sprocket, axle sprocket, horsepower.
Example: (42 / 1.1 = 38.2-tooth axle sprocket) That should be more than good enough to predict to some wide degree of accuracy what the top speed would be, as those calculators are far from being precise. Every kart is going to have its own pluses and minuses in performance, so you could never be caliper-precise if you had the simplest drive there is. Weight matters, like how much of that big chocolate cake you ate yesterday, and whether or not you hit the head this morning. My own weight fluctuates 3#-4# every day. Plan for slowdowns with passengers, and who's big ole beer-body is driving.

Thank you…… this is exactly the info I was looking for!
I was unaware that the CVT produces an actual 1:1 ratio (approximately).
I was under the impression that it was a much higher ratio.
I read that the BlastLed 40 series reverse gearbox is advertised as having a 1:2 forward ratio.
Does that mean that it creates a 50% rpm reduction, or doubles the rpm???
If it actually creates a gear reduction, that would work in my favor as I wouldn’t have to buy a larger axle sprocket. (Which would be a huge pain in the *** because of ground clearance)

 

[madprofessor]

I believe what I'm seeing in the ad you posted is what's often advertised as FNR gearbox, for Forward, Neutral, Reverse. Most of the buggys I've seen advertised like at GoKartsUSA.com say they have a CVT w/reverse............. Go Karts | Buggy | over 450 Models | GoKarts USA®
The Comet 44 Magnum driver and 8.5" driven is a torque converter (TC) a.k.a. a Constant Variable Transmission (CVT)................. COMET 44 MAGNUM | Gopowersports That's what we generally think of when discussing gokart drives. That BlastLED gearbox is nothing I know about, so if it has any kind of gear reduction, well I just don't know about them.
Here's the TC on their site................40 SERIES GO KART TORQUE CONVERTER 1" DRIVER and DRIVEN COMBO Replaces COMET (blastled.com) Here's the blastLED 40 series gearbox I found on their site, but it doesn't look like much, just a higher rated FNR gearbox..............40-SERIES-GO-KART-FORWARD-REVERSE-GEAR-BOX (blastled.com).................They say its forward ratio is 1:2, and reverse is 1:2:6, and I can't tell what they mean, so out of curiosity I called them, she didn't know offhand, but will call me back.

 

[DirtyDan]

I believe what I'm seeing in the ad you posted is what's often advertised as FNR gearbox, for Forward, Neutral, Reverse. Most of the buggys I've seen advertised like at GoKartsUSA.com say they have a CVT w/reverse............. Go Karts | Buggy | over 450 Models | GoKarts USA®
The Comet 44 Magnum driver and 8.5" driven is a torque converter (TC) a.k.a. a Constant Variable Transmission (CVT)................. COMET 44 MAGNUM | Gopowersports That's what we generally think of when discussing gokart drives. That BlastLED gearbox is nothing I know about, so if it has any kind of gear reduction, well I just don't know about them.
Here's the TC on their site................40 SERIES GO KART TORQUE CONVERTER 1" DRIVER and DRIVEN COMBO Replaces COMET (blastled.com) Here's the blastLED 40 series gearbox I found on their site, but it doesn't look like much, just a higher rated FNR gearbox..............40-SERIES-GO-KART-FORWARD-REVERSE-GEAR-BOX (blastled.com).................They say its forward ratio is 1:2, and reverse is 1:2:6, and I can't tell what they mean, so out of curiosity I called them, she didn't know offhand, but will call me back.

Yeah…… the advertisement is confusing.
I think it might be a language barrier mishap.
I think they switched the ratio numbers around backwards because reverse is usually geared lower to slow the travel speed.
I’m not fond of the idea of using a Chinese reverse gearbox, but I have no other option except to start praying to find a used Comet FNR box.
But the good news is some goodies showed up today………
This project actually looks quite feasible now with only minor nip and tuck required.
Time to fab up a engine mounting plate.

 

[Denny]

If you ignore any overdrive (I do) and just gear for 1:1 it will not only get you in the ball park but on the field. I believe most 40 series are slightly underdriven.

 

[Oldguy1951]

Hi everyone…….
I have a RZR170 clone that I’m repowering with a Honda GX390 and need a bit of help with the gearing ratio.
I’ve seen calculators for this, but I’m not sure if they take into account the CVT ratio and gearbox ratio.
This is my planned setup:
Comet 44 magnum 1” bore driver
Comet 44 8.5” driven 3/4” bore
Honda GX390 1” output
Backing plate with a 3/4” jackshaft built in
Blast LED 40 series reverse gearbox
The buggy is a fairly heavy (600#+)mini side by side with aggressive 25” tires.
It will be used almost exclusively off road.
So I’m kinda wondering where I should start with the gearing to avoid burning up belts and clutches???
I’m not too concerned about top speed as I’d rather have reliability over speed.
I have a 520 series 42 tooth sprocket on the axle already and there isn’t much room for a larger sprocket without sacrificing ground clearance.
Any ideas where to start with the sprockets????
Thanks in advance!!!!

I am running a setup very similar to what you describe (440cc engine/Comet 44 magnum torque converter/FNR gearbox). The gearbox gives approximately a 40% reduction in gearing in the real world. I have a 10t sprocket on the gearbox going to a 23t sprocket on the jackshaft, then a 16t sprocket on the other end of the jackshaft to a 60t axle sprocket. My Buggy weights around 600lb empty. With my Grandson and I, this setup gives stump pulling power on the low end with a top speed of 35mph on the top end at 5000rpm (the engine is modified to live at 5000rpm all day - billet rod/etc). The tires are 26" in diameter. Hope this helps!

 

[DirtyDan]

I am running a setup very similar to what you describe (440cc engine/Comet 44 magnum torque converter/FNR gearbox). The gearbox gives approximately a 40% reduction in gearing in the real world. I have a 10t sprocket on the gearbox going to a 23t sprocket on the jackshaft, then a 16t sprocket on the other end of the jackshaft to a 60t axle sprocket. My Buggy weights around 600lb empty. With my Grandson and I, this setup gives stump pulling power on the low end with a top speed of 35mph on the top end at 5000rpm (the engine is modified to live at 5000rpm all day - billet rod/etc). The tires are 26" in diameter. Hope this helps!

What brand FNR gearbox are you using???
Is it the 30 series or 40 series???
My setup will be very similar to yours only I’ll be running the final drive from the gearbox sprocket directly to the axle sprocket if space permits.
I still haven’t received my Comet 44 driver clutch to start the mock up yet.

 

[Oldguy1951]

What brand FNR gearbox are you using???
Is it the 30 series or 40 series???
My setup will be very similar to yours only I’ll be running the final drive from the gearbox sprocket directly to the axle sprocket if space permits.
I still haven’t received my Comet 44 driver clutch to start the mock up yet.

Just your standard ebay/amazon one. Been using for 2 seasons, holding up great! The key is to not ram it back and forth from forward to reverse under power while stuck (doesn't happen much with my big mud tires)! I have a winch both front and back to handle any such situations!

 

[DirtyDan]

Just your standard ebay/amazon one. Been using for 2 seasons, holding up great! The key is to not ram it back and forth from forward to reverse under power while stuck (doesn't happen much with my big mud tires)! I have a winch both front and back to handle any such situations!

Thank you very much!
I was weary of the gearboxes but this makes me feel better.

 

[DirtyDan]

Making a bit of progress on the engine/gearbox mount after finally receiving the 40 series reverse gearbox from BlastLed.
The gearbox seems of good quality and is definitely bigger and more rugged than the 30 series.