Building a minibkie for kids. 3hp briggs, what gear ratio, 15" wheel???

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PeterA

New Member
#1
#1
Hi,

I have started making a frame for a minibike. The rear wheel is a 15" diameter (395mm), has a 70 tooth sprocket on it #428 pitch and size chain.

The engine I have is a 3hp briggs.

What gear ratio would I need for hill climbing and going slower for the kids?

I can get a 12 tooth #428 centrifugal clutch that will match my 70 rear wheel, which will give me. 5.83 ratio. I feel this is to tall and the clutch will get burned out quickly.

So what to do?? I could make a jackshaft setup.

I have got a 9 tooth sprocket that matches the 70 tooth rear sprocket and a 1" shaft and bearings. These parts came off a ride on mower.

So 70T rear into the 9T jackshaft output = 7.77 to 1.

Then I can purchase a clutch, chain and an input sprocket for the jackshaft. These can be any pitch of chain as it does not matter as long as they are the same.

But my question here is, what overall gear ratio do I need to aim for at the rear sprocket with the 3hp engine?



Also, would a torque converter work properly with a 3hp engine?


Peter
 
minikenny

minikenny

Member
#2
#2
7.8:1 sounds perfect for 15" tall tires and low horsepower. Torque converter should work perfect as long as you have a way to mount it to the engine. :thumbsup:
 
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PeterA

New Member
#3
#3
So are you saying 7.8:1 without the torque converter? Because I am not sure what bolt holes I have to use and if they even suit a torque converter.
 
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control4userguy

New Member
#4
#4
First off, if you or any other adult will be riding this, a 3hp plant will disappoint. The other thing I noticed is the 25mm counter shaft you are planning on using. Huge. 428 chain?? What kind of bike is this?

As far as the TC is concerned, it's more expense, maintenance and some adjustment may be needed. For a youth bike, IMO, not worth it. I don't know if 7.8:1 will be what you are looking for- many other factors go into the selection of gearing. With that said, a jack shaft may be the ticket as cog swapping is cheap to accomplish.
 
creia

creia

#5
#5
Peter,
"For Kids" is too general, and their is no one answer that will fit all kids. What is their age, weight, height, etc.?
Michael
 
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control4userguy

New Member
#6
#6
Kids/Hills/Low Top Speed/3HP, that's enough info to swag it. Assuming some fiddling with sprockets, I'd shoot for 8 or 9 to one on a CC to start with unless those 15" diameter tires are 9" wide...

Shorter gearing is better in your case. The CC needs to hook-up and stay engaged. I'm guessing a 12T 428 CC is all you can come up with?? Again, strange choice.
 
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control4userguy

New Member
#8
#8
^Therein lies the support to my 1st post. He installs that counter shaft and has an assortment of sprockets on-hand and the mini grows with the rider(s). The only other variable could be what "hill climbing" means. Backyard dips or what? TC would be a better choice for continuous grade tractability.
 
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PeterA

New Member
#9
#9
Kids have never ridden motorbikes at all, they are all under the age of 12.

The 428 chain is what I have salvaged off the mower. Smallest #428 clutch I can source is 12 tooth. So that is why #428 as I have it on hand.

Again that jackshaft off the mower has a 9 tooth #428 sprocket with about 16mm ID that fits onto the end of the 25mm OD jackshaft. The shaft also came off the mower is 25mm, and I got two matching bearings from it too.

So I have a fair amount of parts already from the mower.

I can get a 12t and 85t rear in #35 I think it was, that would give me 7:1 but that rear 85t sprocket diameter is smaller than the #428 rear sprocket. Therefore there will be a loss of torque at the back wheel.

Leading to a burnt clutch etc.

Hills are like through creek beds and hilly countryside. From memory my old deltek decks on had a 3hp and it would struggle with the terrain I am looking to drive on now. So I know I want lower gearing than that.
 
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control4userguy

New Member
#10
#10
So, taking inventory of what you have on-hand or can obtain, the driveline should look like-

12T/#428>9T/#428>12T/#35>85T#35
-or-
12T/#428>70T/#428

Is this correct? I'm not sure how a 16mm ID sprocket fits on a 25mm OD shaft but I trust it's there and mated together properly (?)

It also is starting to sound like a torque converter might better serve you long-term...
 
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PeterA

New Member
#12
#12
I have tried to post a diagram of the setup. I think it needs a moderator to check it first.

You can go to the PHOTOS link at the top of this page in the menu bar, then on the photos page you can search for PeterA in the box on the right. It will show my diagram that I drew up of my gearing etc.
 
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control4userguy

New Member
#13
#13
OK, acting on the fact you can lay hands only on a 12T clutch, do you have room on the left side of the JS for a 15T/#428? This would net you a little under 10:1. 10T/#41 clutch would tend to be my first choice...

Still unknown is final weight of machine/rider and width of tire.
 
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PeterA

New Member
#14
#14
Yes there would be room on the left side of the JS for a 15T/#428. But, it may be hard for me to find that part to purchase in Australia, and I want it ASAP.

Can not seem to get a 10T in #428 clutch in Australia. (Just looking in eBay.com.au)

Rear tyre is 16x6.5-8 about 160-165mm width.
Front tyre is 13x5.0-6 about 125mm width

Weight if bike would be engine + wheels + frame, I guess around the weight of a normal minibike or similar design.
Weight of riders, smallest 35kg, biggest (me) 100kg.......

I am looking at getting these parts:
clutch #35 12T
sprocket #35 18T
chain #35

So, 18T / 12T = 1.5:1
onto the rear which is 70T / 9T = 7.78

Multiply together 7.78 x 1.5 = 11.67 : 1 Final reduction

The place I will get the clutch from has only 18T,13T, or 12T in th #35.

So that can give me 1.5, 1.38 or 1 ratios from engine to JS.
Multiplied by 7.78 rear wheel ratios.

11.67:1
10.73:1
or7.78:1

How does the 10.73:1 ratio sound then? As this might be the one I go for.
that will be 13T clutch-18T jack, then 9T-70T mower parts....
 
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Havasu Dave

Moderator
#15
#15
Peter,
Your numbers jive with what I come up with. 11.7:1 final ratio with a 12t clutch, an 18t jackshaft input, a 9t output, on a 70t drive sprocket.

The thing is, with that 15 inches of outer diameter on the rear tire at that ratio, you are looking at 14 MPH (22.5 KPH) at maximum governed RPM of 3600 RPM. You wont run it that fast for long. Essentially this ratio would result in a bike that cruised along a bit faster than you could walk but it would climb trees.

I had no problem with 2.5 HP on a vintage mini bike carrying me along at something around 7:1 and I weigh 105 kg. Pavement, no hills and I was going a lot faster than 20 KPH.

Your best bet might be to forget about that 3HP and utilize the latest bit of Chinese OHV Wizardry on your bike. (I don't allow them on my property, but I have a large selection of American engines to chose from)

Here is a link to a jack shaft calculator for both ratios, tires size, and speed. I've used it for years. Either use it online or down load it.

http://api.viglink.com/api/click?fo... download my jackshaft calculator spreadsheet

IF you haven't done so already, look into the vintage Tote Gote designs. That type of bike appears to be what you are shooting for.
 
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Havasu Dave

Moderator
#16
#16
Peter, I know shipping and customs aren't kind to you, but this job would be a lot more straight forward with mini bike parts. My wife's friend sends her stuff all the time from OZ, so I know it's not that big of an issue.

OldMiniBikes Warehouse, (site owners) do ship world wide. (Link at top of page)

In the US, we're using 5/8 inch jack shafts, (15.5 mm) with properly bored gearing, bearings, spacers and the like, but your set up looks to be the standard design.

 
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