First build

[ballistic]

Hello yall. I would like some advice on my first attempt at building a go kart. This will be a build done by myself and my 8 year old son. He aint gonna be doing much besides getting dirty, but it's father son time. Okay, what we have at the moment is a 3hp Briggs & Stratton horizontal shaft engine, it looks like an old model but it's in very good condition. I also have a a centrifugal clutch with a 11 tooth sprocket attached to it and a chain to go with it. So before we start the actual build I would like to get all my goodies in order. What I would like to know is what sprocket should I use get the best acceleration? Would like a bit of speed as well, not to much as my son is only 8. I myself would like to take it for a spin

 

[Doc Sprocket]

Welcome to the forum!

You're gonna want to keep the rear tires small, and the sprocket as big as clearance will allow. Even so, this may well move faster than an 8-year-old is prepared to handle. You should consider compound gear reduction using a jackshaft.

Keep the kart as small, light, and low as you can manage. This will help keep you son safe, and get the most out of that little 3hp.

 

[DaiSan76]

This is more than you would need, but it should give you an idea.

 

[OzFab]

My 8 year old daughter did more than just get dirty, she was a great help (sometimes). If you involve him as much as possible he'll appreciate the finished product a lot more.

Gearing is #1 priority. You want it so it's slow enough now as to not scare him off; somewhere between 9:1 & 13:1 will be ok. You also want to keep the setup simple so it can be changed along the way to make it quicker.

Without the jackshaft, aim for somewhere around 6:1 so, axle sprocket should be somewhere around 70teeth. That way, when he's old enough & confident enough, you can remove the jackshaft completely.

One more thing: If you build the kart in a way that you & he can drive it with a few simple adjustments (double pedal setup, adjustable seat, adjustable height steering) He'll still be enjoying it into his teenage years

 

[ballistic]

Toystory, may I enquire about your avatar pic? The kart looks fairly simple and easy enough for us to do. LOL, I have to still teach myself how to weld and not get arc eyes. Buhahahaha.

 

[Doc Sprocket]

You may inquire. I might even answer.

That's the one non-homebrew frame I have. I believe it's an early 80's Clarke that I am restomodding. I'm not done yet, but it is driveable. Thread here.

 

[landuse]

Like toystory said, you are going to have to get the biggest axle sprocket that your clearance is going to allow. You will want to gear this kart low because of the small engine you have. If it anything like my 3 HP Briggs, you will love it. If you cannot gear low enough (which I don't think you will be able to), you are going to have to use a jackshaft which allows you to gear lower, but use smaller sprockets.

 

[papistinklburgr]

Is that a PREDATOR I see? lol nice build

 

[ballistic]

Okay guys, I have been scrounging around scrap yards and the like and got some goodies. Picked up an old china quad bike frame with the steering, rear axle and brakes still attached. Got a secondhand sprocket from a motorbike mechanic as well, as finding a sprocket for that axle is almost impossible. Gonna be bolting that sprocket straight to the sprocket currently on the axle. Seems like it will clear the road when it's done. Now, I checked the sprocket and it's a 38 tooth sprocket. So what I would like to know is how will my performance be if I use that 38 tooth sprocket and a 11 tooth on the clutch. I believe a smaller ratio will give me a better acc and a higher ratio better top speed. So will this work?

Cheers

 

[landuse]

Okay guys, I have been scrounging around scrap yards and the like and got some goodies. Picked up an old china quad bike frame with the steering, rear axle and brakes still attached. Got a secondhand sprocket from a motorbike mechanic as well, as finding a sprocket for that axle is almost impossible. Gonna be bolting that sprocket straight to the sprocket currently on the axle. Seems like it will clear the road when it's done. Now, I checked the sprocket and it's a 38 tooth sprocket. So what I would like to know is how will my performance be if I use that 38 tooth sprocket and a 11 tooth on the clutch. I believe a smaller ratio will give me a better acc and a higher ratio better top speed. So will this work?

Cheers

Don't even try it!! You will have a gear ratio of 3.45:1. This is waaaaaaaay too high. You will not even get moving and your clutch will be fried in literally 1 second. Theoretically you will have a high top speed, but with zero torque. You clutch will be slipping so badly that it will never engage and just burn itself out. Centrifugal clutches are not cheap here in South Africa, so you don't want to go ruining the one you got.

The problem with getting a bike sprocket is also that it is more than likely not the same pitch as the clutch that you have. You are going to have to work out what pitch your clutch is, and then get the same pitch sprocket for your axle. A motorbike sprocket will generally not have the same pitch as a cent clutch. HERE is a good article on determining the pitch of your sprockets, and then what size chain you will need

You are going to have gearing of at LEAST 6:1 for your clutch to last with your 3HP engine. This means having an axle sprocket of 72T. You start running into ground clearance issues after this, so you might have to use a jackshaft to use smaller gears. A jackshaft uses 4 sprockets, the one on the clutch, 2 intermediate sprockets of varying sizes, and then the axle sprocket. You can get compound gear reduction this way using smaller sprockets.

With gearing, a lower ratio (higher number) gives you better torque/acceleration. A high gear ratio is opposite to this.

Let me know if you need some more help. You could even give me a call regarding ratios if you like

I have added some pics of the jackshaft I used on my minibike to give you some idea of what it will look like. My jackshaft sprockets were on the wrong way in these pics, so ignore that. I had to swap them to get my correct gearing

Oh, and HERE is a great link for all things you might ever want to know.

 

[ballistic]

Hi Landuse, the pitch on the axle sprocket is fine. I seem to be in a poop storm with regards to the ratios here. I thought a 1-4 or 1-5 would be okay for acc. The 1 being the clutch sprocket and the 4 or 5 being the axle sprocket. Maybe I'm mixing up the sequence or something. I took a look at the link provided

http://www.csgnetwork.com/gearratcalc.html

entered my 3600 rpm limit, 8" wheels, estimated sped of 25mph which is app 40km/h and got a ratio of 3.4285714285714284-1 or 1-3.4285714285714284

 

[landuse]

Hi Landuse, the pitch on the axle sprocket is fine. I seem to be in a poop storm with regards to the ratios here. I thought a 1-4 or 1-5 would be okay for acc. The 1 being the clutch sprocket and the 4 or 5 being the axle sprocket. Maybe I'm mixing up the sequence or something. I took a look at the link provided

http://www.csgnetwork.com/gearratcalc.html

entered my 3600 rpm limit, 8" wheels, estimated sped of 25mph which is app 40km/h and got a ratio of 3.4285714285714284-1 or 1-3.4285714285714284

Yeah, the general rule of thumb for go karts is 6:1 ratio. Any higher than that you are getting into clutch smoking territory. You acceleration will also suck BIG time. Tyre size also comes into play here. Large tyres have the same affect as gearing higher, so you don't want tyres that are too large

 

[landuse]

I just saw you added the link to the gear ratio calculator. The gear ratio calculator you were using is unfortunately just a theoretical, or perfect world scenario for gear ratios. It does not mean that you will get moving with that ratio though

I would start from 6:1 if I were you. You only have a 3HP engine, so you are going to have to gear as low as you can go

Like I said over the phone, a jackshaft is probably the best way to go. The calculation for this is (JS in/clutch)*(axle/JS out). In the case of my minibike, I wanted to gear real low, so what I did was use a 16T clutch, a 38T JS in, a 12T JS out and a 57T axle sprocket. This gave me (38/16)*(57/14) = 11.28:1

There are several combinations that can be used depending on what you have. With your 11T clutch and 38T sprocket, I would probably go for something like this:

Clutch - 11T
JS in - 38T
JS out - 14T
Axle - 30/32/36T (one of those, depending on what you can get.

This will give you (38/16)*(36/14) = 8.8:1

 

[ballistic]

AAAAAAARHH! thanx Landuse. will see what i can do as far as getting those sprockets.

 

[landuse]

Post some pictures of the quad frame that you got, especially the rear end. Explain what you are trying to do as well so that people can give you some more help than I can.

Balistic is using the rear of a quad for part of kis kart. I have said that he will need a jackshaft to reach the gearing that he needs. Maybe one of you guys can give some suggestions on mounting a jackshaft on the back of a quad.

 

[ballistic]

Okay, so I spoke to a guy that does engineering at his home and he might be able to make a sprocket for the axle. Will be taking the clutch sprocket and chain to him tomorrow to check the pitch on the sprockets. So I need some advice on whether to go for a 6:1 or 7:1 ratio. I need good acceleration and some top speed. Seeing that I have a 11 tooth sprocket on the clutch I think I would have to go for a 66 tooth on the axle, or maybe a 72.

I think this is gonna be more for me than for the kids now as the Mrs doesn't think it's okay for them to ride it. Will post some pics of what I have in due time and maybe a short description of how I plan to go about it.

 

[Doc Sprocket]

With a 3hp engine, go low with the gearing. 7:1 at least. Keep tire size down, and mind the weight.

 

[ballistic]

Okay toystory will keep that in mind. But pls help me to understand something. Is it the no of teeth, or the total diameter of the sprockets that makes it either acc faster or have a higher top end. Surely the chain only has to do with the pitch.

 

[landuse]

Okay toystory will keep that in mind. But pls help me to understand something. Is it the no of teeth, or the total diameter of the sprockets that makes it either acc faster or have a higher top end. Surely the chain only has to do with the pitch.

Whatever pitch sprocket you have, it is always the tooth count that is going to matter. It is only when you start using pulleys that you work with the pulley diameter.

 

[OzFab]

But pls help me to understand something. Is it the no of teeth, or the total diameter of the sprockets that makes it either acc faster or have a higher top end. Surely the chain only has to do with the pitch.

The chain is only the linking mechanism between the sprockets; if you were using pulleys, you'd use a belt. We use sprockets & chain because there's no chance of slippage &, therefore, no loss of power.

It's the relationship between the sprockets that determines the ratio: The bigger the axle sprocket, the lower the ratio &, therefore, the lower the top speed. The smaller the axle sprocket, the higher the ratio &, therefore, the higher the top speed.

Remember:
High number = low ratio = low speed
Low numbers = high ratio = high speed

Whatever pitch sprocket you have, it is always the tooth count that is going to matter. It is only when you start using pulleys that you work with the pulley diameter.

You've got it the wrong way around, Paul: It's always about the diameter of the sprockets/pulleys/whatever. The fact that sprockets happen to have teeth just makes it easier to figure out the ratios.

Think about it this way:
If you have a #35 axle sprocket & a #40 axle sprocket of the same diameter, they will have different tooth counts, correct?

Now, get a #35 & a #40 drive sprockets the same diameter as each other. When matched to the axle sprockets, you end up with the same ratio but, the tooth count will be different because the #35 set will have more teeth &, when divided down, will give you the same resulting ratio