Baja DoodleBug DB30 Jackshaft or Torque Converter?

[Nimdy]

This is my first post here, so please be patient if I'm asking something that has already been asked a thousand times.

So I'm a junior high school student, and in my state, we have to complete an EA project (Extended Application) to graduate. The project is supposed to be based on a potential future job. As I wanted to become a Mechanical Engineer and design engines for automotive and industrial purposes, I made my project to modify a Predator 212 and see how much performance I could squeeze out of it. Long story short, I now have a Baja DoodleBug bike that I will swap a Predator 212 on it, and I need to do some before and after measurements of how it performs on the bike. For this purpose, I need to be able to adjust the gear ratios to keep the top speed around 20-25 mph. Since between my initial and final tests, the engine's top rpm will range from 3600-6000, I need a lot of adjustability, and just swapping the rear sprocket isn't going to give me that.

I will be mostly testing the bike on dirt and gravel trails, so at first, I thought a torque converter because the gear reduction would allow me to use a much wider range of rear sprockets. Or I could use a jack-shaft and have a lot of adjustability in the ratio just by changing the sprockets; that sounds much cheaper and more reliable. What do you guys think is best for this project? I don't know enough about how a jack-shaft or torque converter responds to conditions like this.

 

[sparkwizard]

A jack shaft would give you a constant gear ratio. It would also give you multiple options when changing ratios, because you can change 4 different sprockets to come up with a huge selection of ratios.
Torque converters are versatile because they do give a low starting ratio and they automatically adjust to driving conditions by "downshifting" as RPMs drop.
If I understand your project correctly, you would need a constant ratio, so I think a jackshaft would work best.
Welcome to the crowd, and let us know how it's coming along. OH... and you be careful with a Predator on a DB30.

 

[massacre]

3600-6000 rpm? Is the governor deleted?

 

[sparkwizard]

3600-6000 rpm? Is the governor deleted?

I missed that part.


Hey Nimby, why not modify the original engine? Honestly, quite a few people think a Predator on a DB30 is a handful in stock condition.

 

[Nimdy]

A jack shaft would give you a constant gear ratio. It would also give you multiple options when changing ratios, because you can change 4 different sprockets to come up with a huge selection of ratios.
Torque converters are versatile because they do give a low starting ratio and they automatically adjust to driving conditions by "downshifting" as RPMs drop.
If I understand your project correctly, you would need a constant ratio, so I think a jackshaft would work best.
Welcome to the crowd, and let us know how it's coming along. OH... and you be careful with a Predator on a DB30.

Sorry for taking so long to reply after I made this post I went into crunch mode for the end of school.
Thanks I had a feeling I was going to have to go with the jackshaft but im glad that others agree

 

[Nimdy]

3600-6000 rpm? Is the governor deleted?

Yes I will be deleting the governor I'm trying to push the engine as far as I can in terms of performance and I'm going to need the higher rpm to get there. I'm not going to just remove the governer I plan on installing a Honda rev limited ignition coil as some safety so I can get up to 6500 and make sure I don't go over.

 

[Nimdy]

I missed that part.


Hey Nimby, why not modify the original engine? Honestly, quite a few people think a Predator on a DB30 is a handful in stock condition.

I was going to use the original engine that came with the bike at first but after riding it and tearing it down I found an insane amount of wear and a lot of junky fixes done by the previous owner. So it was either I buy another DB30 engine and use that or I could buy the Predator 212 which I know I can get for cheap and it has a lot of aftermarket parts. Plus I have worked on the 212 a bit in the past because we had a generator with one in it so I have a little more experience with the engine.

 

[sparkwizard]

In your quest for more power and RPM, you do plan to install a billet connecting rod and flywheel, correct?
If you want to see 6500, those parts will need to be aftermarket billet aluminum parts , not only for safety, but because the stock parts will come apart before they spin that fast.

 

[Nimdy]

In your quest for more power and RPM, you do plan to install a billet connecting rod and flywheel, correct?
If you want to see 6500, those parts will need to be aftermarket billet aluminum parts , not only for safety, but because the stock parts will come apart before they spin that fast.

Yes I already have plans for that and have it written into the budget

 

[sparkwizard]

Glad to hear it.

 

[Strigoi]

Yes I already have plans for that and have it written into the budget

What kind of budget? If you're going all out you'll be thousands into that engine and pushing 20+hp.

I attached a pic of an engine that I'm $2,500 into, including the $400 clutch to handle the power. It's in the 20+hp range and not for beginners at all. That's for building the engine or riding whatever it's in.