Hello everyone I’m new to the forum and still kinda a rookie on mini bikes lol, I have some knowledge just not too much. I put a doodlebug db30RR with a stock cylinder head 212cc predator together and it runs perfect it has a vm22 Mikuni clone carb and it has arc billet aluminum fly wheel and also has arc connecting rod and a cam I don’t know specifically the size of it but for it having all those parts it’s not going past 45 as that was the top speed I reached on my weight. Mini bike also has 12t clutch with a 70t rear axle sprocket and has 145/70-6 street tires, front forks were swapped out to mega moto suspension forks. And idea what size clutch and sprocket I should use to get more speed out of it. Thank you I’m advance
Gear Ratio
- Thread starter shiesty_Qfiddy
- Start date
- Tags 212 cc clutch doodle bug gokart mini bike predator predator 212 sprocket
At a 12/72t ratio that is 6.0:1 and to hit 45 you were spinning 6500 rpm, according to my gear ratio calculation using a ~14" tall tire that a 145/70-6 tire would come out to. Without knowing what strength valve springs you installed, I'd say at 6500 you're nailing the limits of the valve springs and/or torque output of the hp/engine. Anyway, the current question you run into is how fast you want to go and how fast you want to get there.
To give an idea of some different top ends, if you kept the 12t clutch and swapped to a 60t rear you would have a top end at 54 mph at 6500 rpm. 14/60 would go to 63 mph at 6500 rpm. You could make up some of the loss of acceleration by going to a higher gear ratio by using a torque converter instead of a clutch. You're probably going to want to find a kevlar belt if you try to use a 30 series converter instead of the stock belt it comes with though.
Anyway, at the end of the day, more teeth on the clutch and fewer teeth on the wheel will make more top speed. The big problem with more top speed, especially on a clutch, is that it will take longer to get there. You would likely not be terribly happy with the acceleration compared to current and would be looking into a torque convertor or a clutch that stalls over 3200 rpm to get a good kick when you get going.
To give an idea how slow it can get using a high gear ratio on a clutch. On my Fox with a 4.2:1 ratio setup, getting to 30 mph with a mild-built Briggs model 13 flathead takes ~10 seconds on a clutch that has a 2400 rpm stall. There is other considerations that go into clutch stall speed as well such as intended cruising speed, if the bike is being built for that.
And, as others have noted, rider weight can play into this as well.
Edit: To toss even more variables on there. The clutch itself can add to this. Some random Max Torque is going to have entirely different abilities in comparison to something like a Hilliard extreme duty or a Bully racing clutch. Not to say the Max Torque isn't perfectly fine for 99% of people, including myself, but it's going to have more slip compared to the $130 Hilliard or $300 racing clutch.
To give an idea of some different top ends, if you kept the 12t clutch and swapped to a 60t rear you would have a top end at 54 mph at 6500 rpm. 14/60 would go to 63 mph at 6500 rpm. You could make up some of the loss of acceleration by going to a higher gear ratio by using a torque converter instead of a clutch. You're probably going to want to find a kevlar belt if you try to use a 30 series converter instead of the stock belt it comes with though.
Anyway, at the end of the day, more teeth on the clutch and fewer teeth on the wheel will make more top speed. The big problem with more top speed, especially on a clutch, is that it will take longer to get there. You would likely not be terribly happy with the acceleration compared to current and would be looking into a torque convertor or a clutch that stalls over 3200 rpm to get a good kick when you get going.
To give an idea how slow it can get using a high gear ratio on a clutch. On my Fox with a 4.2:1 ratio setup, getting to 30 mph with a mild-built Briggs model 13 flathead takes ~10 seconds on a clutch that has a 2400 rpm stall. There is other considerations that go into clutch stall speed as well such as intended cruising speed, if the bike is being built for that.
And, as others have noted, rider weight can play into this as well.
Edit: To toss even more variables on there. The clutch itself can add to this. Some random Max Torque is going to have entirely different abilities in comparison to something like a Hilliard extreme duty or a Bully racing clutch. Not to say the Max Torque isn't perfectly fine for 99% of people, including myself, but it's going to have more slip compared to the $130 Hilliard or $300 racing clutch.
Last edited:
The camshaft controls how far and how long the valves open and close. This effectively controls how much gas is let in and exhaust is let out. At different RPM ranges this can effect how the engine behaves and thus a stock cam is designed to provide the best HP and torque within the high end RPM range of a governed engine, typically topping out around 2800-3200 RPM. An aftermarket cam can push the peak power up into the 6000 RPM and even higher range. This is an incredibly simplified explanation of how this effects the engine. Other factors of how the engine is built factor into engine behavior as well. This includes intake and exhaust setup, rotating assembly weight (this includes everything driven by the crankshaft), compression ratio of the engine combustion chamber, what fuel is being used, forced induction (turbo/super charged/naturally aspirated), and on and on. You can't just buy $800 in parts and throw them in an engine and expect to go faster and in fact may create a poorly running basket case that provides little or no gains over a stock engine
What am I saying here? Yes, you can hit 60. There are many factors that will make that bike hit 60 and it turns into questioning how fast you want to get to 60 and how much you are willing to spend to get there. I built a Fox Street Scamp that can consistently go 55 even on 5 degree sloped hills with an aluminum bore Briggs 5hp, on a stock cam no less. I sacrificed acceleration a lot to do so along the way but I cruised 10 miles at 55-60 without heat issues. But, what do I mean by sacrificed a ton of acceleration? It gets to 30 in about 14 seconds. It gets to 55ish in about 35 seconds, it is effectively Toyota Prius levels of acceleration. The acceleration doesn't bother me personally since I didn't want to go fast very quickly. However, a lot of people I've chatted with want to be running 55-60 in the time it takes my bike to get to 30. I've almost always told them to build a GX390 or Predator 440 at that point as it would actually be cheaper than to try to build up a GX200 or clone to do that.
My final thoughts in this rambling post, in the world of engine building there are 2 effective truths regardless of if it's a single cylinder small engine or a V8. 1) There is no replacement for displacement. 2) Square inches = square dollars. Do research on what others have done that was successful and why it was a success. Knowing what was successful is good to be able to replicate that success. Knowing why it was successful sets you up to understand how everything is working together and better troubleshoot when things aren't working well or at all.
What am I saying here? Yes, you can hit 60. There are many factors that will make that bike hit 60 and it turns into questioning how fast you want to get to 60 and how much you are willing to spend to get there. I built a Fox Street Scamp that can consistently go 55 even on 5 degree sloped hills with an aluminum bore Briggs 5hp, on a stock cam no less. I sacrificed acceleration a lot to do so along the way but I cruised 10 miles at 55-60 without heat issues. But, what do I mean by sacrificed a ton of acceleration? It gets to 30 in about 14 seconds. It gets to 55ish in about 35 seconds, it is effectively Toyota Prius levels of acceleration. The acceleration doesn't bother me personally since I didn't want to go fast very quickly. However, a lot of people I've chatted with want to be running 55-60 in the time it takes my bike to get to 30. I've almost always told them to build a GX390 or Predator 440 at that point as it would actually be cheaper than to try to build up a GX200 or clone to do that.
My final thoughts in this rambling post, in the world of engine building there are 2 effective truths regardless of if it's a single cylinder small engine or a V8. 1) There is no replacement for displacement. 2) Square inches = square dollars. Do research on what others have done that was successful and why it was a success. Knowing what was successful is good to be able to replicate that success. Knowing why it was successful sets you up to understand how everything is working together and better troubleshoot when things aren't working well or at all.