Larger sprocket...

[Bowling]

I'm looking for a larger sprocket for better hill-climbing, but the problem is the stock rear sprocket is very close to the ground and I'm worried a larger one will be a little too close for comfort. I don't want the chain running into rocks and such and end up breaking. So I was wondering, would it be possible to change the little sprocket on the clutch to a larger one?

EDIT- And this is for my Baja Doodlebug, so if anyone has experience with one then your help would be greatly appreciated; thanks.

 

[minidragbike]

You will need a smaller gear in the front, not larger one.

 

[Bowling]

Alright thanks... Glad you told me that before I bought anything lol. So a smaller sprocket on the clutch will make a difference?

 

[minidragbike]

It changes the ratio. It's numerically higher, but it's called lower gearing. Like first gear in a car vs third gear. Check this out.

http://www.gokartracing.org/Calculate.htm

 

[Bowling]

I must be doing something wrong... I put in my gearing as I have it now and it shows up as 15 mph... Maybe I'm doing the RPM wrong.

 

[minidragbike]

You have to measure you rear tire with a measuring tape. The kind that you get your clothes fitted with. Normally a 12" tire has a 34-35" tire circumference depending on rim width and tire compound.

What RPM are you running?
Gearing?
Rear tire?

I can help you out.

 

[Yellowhand]

It's actually kind of amazing how much clearance we really need around the circumference of the rear tire in relation to sprocket size...

We lose a little clearance if the tire isn't fully inflated...we lose a little more from the weight of the rider compressing the rear tire...when we hit a bump, the tire compresses quite a bit, but unfortunately the sprocket doesn't...

Just thinking out loud...sometimes I build stuff that 'looks' like it will be okay while it's just sitting on the bench...and I'm not sitting on the bike.

 

[minidragbike]

Good thinking!
I know what your talking about. That works both ways too.

The most you would loose is maybe 2-3mph if your moving slow with heavy load with less then average tire pressure. As your gaining more speed the tire begins to get taller then your stock measurement. Which can also vary and have you gain 2-3 mph more. Tires usually want to get tall as they rotate.

Most of us minibikers deal with scrub breaks. You will notice the distance between the scrub break, and tire. You can't have it to close because the tire grows and will rub the break causing friction and lost of speed.

That's why measuring the tire at it's regular inflated rate is best for the medium estimate of speed.

 

[Bowling]

Can anyone tell me about jackshafts? I'm wondering if this would be a possibility. How would I find out what kind I need and how hard are they to install? I've decided to just give up the top speed in favor of hill-climbing since the hills here are bad and I never get to take it over 30 much on the trails anyways.

 

[minidragbike]

Don't give up on fun.
I had a kid bugging me every weekend about my old TAV setup. I personally know it's way to dangerous for this kid. But he just doesn't care about safety. When you want to have fun nothing else matters.

 

[Bowling]

I'm not worried about safety, I just want it to climb a hill without spending alot of money since I'll eventually sell it for a larger one.

 

[Yellowhand]

Here's an example of a jackshaft on a bike that wouldn't work nearly as well without the jackshaft...

The jackshaft is used to adjust two main problem areas...gear ratio and sprocket alignment.

This bike has a 12 tooth front sprocket and a 40 tooth rear sprocket, both #35 pitch...without the jackshaft, this bike would have a top speed much faster than desireable, and wouldn't be able to handle any hills because low-speed operation is way out of the bottom of the powerband.

The 40 tooth driven sprocket on the jackshaft, combined with the 13 tooth drive sprocket on the other side basically halves the final gear ratio, allowing the bike to perform better while starting off from a dead stop and climbing hills, but top speed is also cut in half.

Years ago, I had a 3hp minibike (similiar to your 2.8hp) that used a jackshaft setup that allowed it to climb any hill I could find, but top speed was about 8mph, which sounds really slow, but I never took it out onto the street, and it worked like a champ out in the trails and woods, which is what I wanted.

A complete Azusa 5/8" jackshaft kit with 13t and 18t sprockets is about $30 at Northern Tools, and you would also need the $10 chain kit. This kit would need to be welded onto your frame, would make your bike about 1/3 slower, but better able to climb hills and take off from a stop.

 

[Bowling]

Besides the welding, would I need to do any other metal-work to get it on the frame? If not, then I should be able to do it... BUT... I'd have to find a new brake set-up. It came with a disc brake that I could just find a way to put on the other side but the brakes really suck so I was thinking about finding one of those disc brakes that use the sprocket as the rotor since they look easy enough... either that or a drum-brake if it would be easy enough.

EDIT - I also noticed this jackshaft on northern tool is 10" long and my frame is roughly 9" wide...

 

[Yellowhand]

The Yellowjacket has the bearing hangers up on extensions because I knew I was probably going to need to use the larger circumference 40 tooth sprocket to get the final drive ratio I wanted, but the extensions aren't needed when the 18 tooth sprocket is used...the 18t doesn't extend out past the diameter of the bearing hanger, so clearance from the frame isn't an issue.

Here's a couple photos of this jackshaft kit on other bikes...I've never needed a jackshaft longer than 10", but replacement shafts are available in almost any length you could want. They function more reliably when kept as short as possible, and both of these have been shortened down from the original 10" length. Both of these bikes use a 12t clutch and a 60t rear sprocket, with the 18t jackshaft sprocket set up as the driven unit, and the 13t driving the rear wheel. This is an excellent final ratio for all-around use, in my opinion. Top speed is limited, but the bike has good take off and climbing ability with stock, relatively low horsepower engines in the 3-5hp range.

How does the brake caliper mount on your bike?

The caliper mounts I use slip over the axle instead of being tabbed to the frame, so the rear wheel assembly can easily be flipped around so the sprocket is on the other side...only the caliper stay is mounted to the frame, and it doesn't need to be welded...a clamp works fine.

 

[Yellowhand]

Can someone post some photos of a Baja Doodlebug?

It would make it easier to imagine possible changes if we could see how this bike looks in it's stock configuration.

The website for Baja Motorsports wasn't heavy on tech info or photos...

 

[delray]

hey yellowhand, i can try to post some pictures later today or tonight(late) I have the same mini bike? minibike has the same disc brake set as yours.
bowling noted that his brake sucks? maybe it need little adjustment? my disc brake works great. better then any drum set up. other problem with this bike is it only has 2.8hp . this only can push a kid around?
i will post some info that i am doing to this bike to make it go little better.
..........................:thumbsup:

 

[Yellowhand]

Thank you, sir.

Yeah, I think the jackshaft is going to be critical to get some performance from a 2.8hp engine, but, back in the day (60's) there were a lot of 3hp minibikes...my little Sensation was similiar to the doodlebug, and my brothers and friends put hundreds of off-road miles on that thing. We used to move the cows from pasture with it...little minibike cowboys...

I get decent performance from those cheapie disks and calipers, but I spend a lot of time disassembling and adjusting them, and I sometimes send the rotors out to be coated, which ruins the 'cheap' aspect to them...the titanium nitride coating isn't needed, it just looks cool, and I'm getting other tools coated at the same time, so I just toss them in the batch.

 

[Bowling]

Turns out my brake rotor was crooked... Fixed with a hammer lol. I might try your brake set up since it'd be easier than re-welding the bracket back on. Is there a kit for that? If not, how hard would it be to get a drum brake on? Thanks for all the info... I'll try to get a few pics tomorrow.

 

[foreveryoung]

Can someone post some photos of a Baja Doodlebug?

It would make it easier to imagine possible changes if we could see how this bike looks in it's stock configuration.

The website for Baja Motorsports wasn't heavy on tech info or photos...

HI everyone!!! I am new to this forum and mew to the mini bike world I,ve been lurking for a few weeks and learned a lot from members of this forum there is a lot of useful information and great pics

 

[delray]

hey guys, i purchase baja doodle bug for only $202. I installed a 80tt sprocket and built a header tube and exhaust out of stainless steel and ported the exhaust little.
here are some pictures of my baja doodle bug

http://new.photos.yahoo.com/brian_c_53711/album/576460762401134267

chain tensioner roller that came on the mini bike looks to be made out of the same stuff as roller skates wheels.
now with the header tube and 80tt sprocket the mini bike moves me pretty good for a small engine. mini bike doesn't feel like it really slow down in speed. it takes off better too. I think the con-rod is going to come un-glued.
motor is called a lifan 2.8 this info is located on the pull start cover,but on the back side cover it as 98cc power 2.5hp 3600rpm
my question , is there a aftermarket con-rod for this engine? is this a clone honda engine? if i mic the con-rod for dem's. could i go to web site that sells performance rods and match one up?
i don't mind purchasing new rod. i only have $14 in the sprocket. if i gear it down any more with a bolt on or a weld jackshaft the bike will perform real good up small hills,but top end speed will blow up the engine for sure with out the governor.
......................