Optimal Gearing For a Torque Converter Bike?

[Mrturbo458]

So I've been working on a mini bike for my girlfriend (she has named it the Blitz Krieg) and we've done lots of mods to it already, it has a stage 3.5 ish engine, 22mm mikuni, head shaved .040" mild port job, mod 2 cam, etc. And I converted the rear brake to a hydraulic setup, because the cable operated one was junk and dangerous.

After trashing the clutch, we decided on a torque converter, and went with a 30 series off of Amazon (really happy with the quality for the price). We got the converter installed and it cannot be ridden without pulling massive wheelies! Even if you barely touch the throttle. And I'm not a light/small person, so it would be horrible for my girlfriend (she's smaller).

My question is, what is a good rear sprocket size to pair the (I think) 12T front sprocket with to help make better use of the bike? It currently has a 70T rear sprocket and about 11" tall tires.

Just looking for what you all have experienced, and a good starting point, so I don't have to buy a bunch of sprockets I'll never use. I was thinking like a 50-55T maybe? Obviously wheelie bars would help, however I feel like it would never come off of them, so that's why I'm asking about the gearing.


Any help is appreciated! And I hope you like the pics of the build so far!

-MrTurbo

 

[J.S.@SMS]

Just by looking at it I would say that that rear sprocket is too big for a bike with that engine, that gear ratio and those tires.
The build looks great. One question, is that a 212 or a 196.

 

[Mrturbo458]

Yeah, that's the problem. I'm just wondering where to start as far as which size rear sprocket to get.

I know there isn't a magic answer, but I'm just trying to get a feel for what could be a good starting point, based off of what you are all running out there. I think I'm going to get some split sprockets for it.

And thanks! It's a predator 212. It definitely rips, before the torque converter it flat out got with it.

 

[landuse]

I would go with a 60T sprocket to start out with. This will give you a 5:1 ratio which should decrease the wheelie torque you are talking about. It might be trial and error though looking for the correct size, but I think a 60T is a good place to start

 

[Mrturbo458]

That sounds like a good plan!

 

[KartFab]

5:1 ratio is ideal for small mini bikes with small tires. Like landuse said. I still think you are going to pull wheelies though, haha! Also the smaller the rear axle sprocket, the higher chance you have of throwing the chain at start or under acceleration with that cheap 35 chain. IIRC gopowersports sells those 6 bolt aluminum sprockets, so you can switch over to your 10jackshaft sprocket and use a 50t 6 bolt aluminum sprocket for the same gear ratio, just less headache if you end up throwing chains. You can use high quality 420 chain. Just my 2 cents.

 

[Mrturbo458]

I would guess 5:1 might be a little wheelie-ish still, especially for my lighter girlfriend. I was thinking about the #35 chain, and I think I will at least run it (if it works) for now, due to the cheaper cost of the sprockets and chain to get it dialed in. Then we can upgrade to a larger chain if we kill it/if it gives us problems. I'd rather not run a jackshaft, but if I have to I will.....

Part of this is to see what the limits are for some of these parts, I know it doesn't make sense.

Thanks for the input!

 

[bob58o]

5:1 ratio is ideal for small mini bikes with small tires.

I'd say this is true when using a regular clutch, not so much with the TC -especially if you don't like wheelies. 5:1 with a regular clutch is 5:1. 5:1 with a TC is 13.4 :1.

Keep the 70 tooth and get this 15T #35 5/8" bore C-type Jackshaft sprocket.
http://azusaparts.com/product/sproc...ain-58-bore-316-keyway-no-set-screw-15-tooth/
You'll lose 20% of the wheelie power and it only costs $10 plus shipping.

But with the MOD2, which might rev over 7k RPM - top speed will probably be over 55mph, which is scary fast. But since you hotrodded the motor, I guess you wanted fast.

OR... With 11" tires on a small minibike with a TC, you could probably get away with a 48 tooth sprocket to go with the 12T. Will probably get you close to 60mph and you'll lose ~30% of your wheelie power.

OR... just go back to using a regular clutch, oil it regularly, and don't putt around at low speeds too much.

OR... just make use of the long seat. Sit on the front of it and learn to gently ease into the throttle.

 

[Mrturbo458]

Thanks for the input! Seems like swapping the front sprocket out might be a good option to start with.... And it's cheap.

Fast is good, that's kinda the whole point of modding it, after all!

I tried sitting up on the "long" seat, basically on the gas tank and it still wheelies with a tad bit of throttle.


I'll try some gearing changes and go from there!

Thanks!

 

[J.S.@SMS]

I would very much appreciate it if you could post a pic of the throttle setup for the GC160. I have had one for a while now and I have been wanting to do something with it, however it has the sticky throttle (stays where you leave it/doesn't return) from its days as a pressure washer engine.

 

[Mrturbo458]

The gc160 that used to be on it? I'll see if the throttle setup is still on the engine.

 

[Mrturbo458]

JS:

Here are some pics of the throttle setup on the old gc160.

 

[J.S.@SMS]

Thanks. I will see about making one based off of the pictures, but will likely end up buying one online if I can find one.

 

[Tpdingo]

I wonder if you might want to get a speed limiter driver for the minibikes so less experienced and lighter riders could do so with a high gearing.

Then it won't accelerate or go TOO fast....probably just crazy fast instead of lunacy fast!

 

[Mrturbo458]

Well, I finally got around to getting a split sprocket setup. Yeah it's #35 chain, but it'll be fine for now. Ordered a GPS split sprocket adapter and a 53 tooth and a 60 tooth rear sprocket. That way I have 2 options, if the 53 is too tall. I have a feeling it's going to be insane still, but it should be much better. All I can do is try I suppose....

Had to modify the sprocket adapter and the split sprocket is inboard of the adapter, due to my chain alignment, but it's still changeable without pulling the wheel.

Didn't get a test ride in unfortunately, hopefully Monday.

 

[Mrturbo458]

Installed the 53T rear sprocket, and it has much less wheelie power. Yes, it still needs wheelie bars, but with careful throttle control it can be driven around without madness ensuing. I really like the bike now with the TC on it.

 

[Joe-405]

On my dragbike I had a 40 something on the rear and still wheelies out to the 60’. It was a mild 420cc but still didn’t hurt anything on the tav. You can get away with a 4:?? To 1 ratio on regula roads. To **** with bottom end torque and gain some more top speed.

I miss my mini like yours. I extended it a few inches and put 12x6x6 rear turf tire and taller front then somebody wanted it before I could enjoy it. There is a threa on here somewhere with a lot of pics. It was super sweet with those other tires and slammed.

Guess we deleted the thread can’t find it. I did find one picture tho.............

 

[Mrturbo458]

That's pretty sweet! Yeah I was thinking more 40T but I am not sure of the direction this bike will take yet. For a little 212 this thing rocks!