Jonny's mini rat tractor

Hi fellow mower fettlers! Let me introduce my entry for the 2015 build off. I do have a mega project turning over in the back of my mind, but I don't have the bits needed for it yet and I am also getting married in August, so don't have the time to build it either! This is a little project just so I can enter this year.

Here's my pile of bits.


So my plan is to build a teeny weeny approximately half scale lawn tractor. In the same vein as a mini moto/minibike. So far all I have done is get the tiny Briggs (2 or 2.5hp) horizontal shaft rebuilt and going. Pleased as I payed peanuts for it on the bay and was described as 'suitable for spares only'.

The rear wheels for the mini tractor are from the front of a Murray 8/30 . The front wheels are solid tyres from a push mower. I'm thinking of just having power/brake to one rear wheel to save diff hassle. I have bought roller bearings for all the wheels to improve it's rolling. Brake is a disc and caliper off of a mini moto.

Chassis will be made from 1" box section steel (old school table frame you can still see some gum stuck to it! I'm a teacher and hate to see them go in the bin)

Seat and steering wheel are off of a murray something or other.

I'm hoping to get started over the easter hols. Can't wait!

Where's the hood and rest of the tractor?

Not been made yet! Remember this is a half scale tractor, so wouldn't find one small enough anyway. It'll be scratch built, probably made from the sheet steel 'rescued' from a filing cabinet, again from the school bins. Nearly everything at school is paperless now and I've used many filing cabinets from school for shed storage and for sheet steel.

Half scale should be sweet. Like true half scale? Measure and cut to half? That would be wicked if you can get it to look like stock like that.

Hmm, I'd not thought of doing it that way. I might try that and see where it gets me. I'm far more a make it up as I go along kind of bloke. Obviously seat would need to be full size. I weight 260 pounds and don't fancy trying to get my back-side into a half size seat! Although saying that, I'm not sure yet if the teeny Briggs will have enough power to move me yet!?

Mock up!



So... This is the sort of size it would be if it were half the scale of my MTD 548 lawnflite. 22" wheelbase. Looks about right to me. I looks like everything'll just about fit width and lengthways. I'll see if I can get a day or two of working on it over the easter hols. I'm well excited about this. I got an air filter and throttle on the Briggs this afternoon.
Pumped!

I don't know about what body pannels you got, but loos really rat rod right now! Awesome.

I thought of doing something like that. My idea was to make it as small as possible while still being driveable. I wanted to do a minibike too. Figured I would call it the mosquito. Should be neat when this thing is done. Not sure what you have in mind for a drive line, but centrifugal clutch might be easiest to deal with on something this small.

TBH, I was going to use a belt/tensioner/peddle set up, but now you mention it, a centrifugal clutch would make all types of sense! I guess I'm too used to fiddling about with belts and pulleys.

Maybe a small mtd fnr trans. Shaft drive w/centrifugal clutch or chain to the brake rotor shaft.

Not really a development, but I have had my transmission bits delivered. Not had any spare time, been busy on other projects!
I went for a 10 tooth on the clutch and rear sprocket is 41 tooth. Nice and low geared for my teeny weeny engine!

reminds me of my mini rat rod mower i built good luck!

Do you have any photos of it?

I think your ratio is a little steep TBH. Coming from a Go-Kart background where centrifugal clutches are commonly used, we would normally use a 60 tooth axle sprocket and for that little Briggs I'd suggest maybe even a 72. Just for an example, On my Go Kart, I have a lightly modded Honda GX160 (no governor, performance exhaust etc) and a 10 tooth clutch sprocket and a 48 tooth axle sprocket. Although she goes fast, it takes for ever to move from a standstill to 10mph. So unless your using a jackshaft setup, then your 41 tooth axle sprocket won't work.

Thanks for the heads up. Maybs I'll go for a jack shaft/compound gear set up

jonnya wrote:

Thanks for the heads up. Maybs I'll go for a jack shaft/compound gear set up

Yeah, or just buy a bigger sprocket which would be easier for that amount of space. What size are your rear tyres?

The rear wheels are only about 250mm diameter (10") so not much room for a daft big sprocket.

Right okay, I though they'd be bigger than that. Maybe a 60 tooth sprocket would do then, but if you've only got a 3hp? engine then you may want bigger. But that 41 tooth won't work unless you have a jackshaft, or an opposed twin laying around

Yah, 4 to 1 is about 50 mph with a slow start off the line unless you got a gearbox.

Hmm, 50mph on my teeny contraption is not what I had in mind, more like 5mph! I shall get the calculator before going much further. Guys, I really appreciate your help on this. You have probably saved me about 2 days worth of getting it wrong, then messing about putting it right. Thanks heaps.

jonnya wrote:

Hmm, 50mph on my teeny contraption is not what I had in mind, more like 5mph! I shall get the calculator before going much further. Guys, I really appreciate your help on this. You have probably saved me about 2 days worth of getting it wrong, then messing about putting it right. Thanks heaps.

Yeah no problem! I'd say go for a 7 to 1 ratio or an 8 to 1. Also, check out here this will help determine what speed your ratio is going to get you. If your rear tyres are 10", and you're running a 7 to 1 ratio, this will give you a top speed of 15mph. Hope this helps!

Now then, I have done some sums and looked on ebay for bits which I could use. I could buy another 10 and 41 tooth sprocket and create a compound gear on a jackshaft. This would be possible but tricky to fit in the available space.
My other idea is to use bits from a mini moto. I have a pair of mini motos one in bits, one almost compete. These have a tiny clutch sprocket and large rear sprocket and would give me a good engine to axle ratio (possibly!?).
Now the possible problem with trying to use a mini moto clutch would be the engagement speed. Surely a mini moto's two stroke engine runs at much higher rpm. Anyone had experience with adjusting/swapping the springs on a mini moto to make the clutch engage at a lower rpm.

i dont have any pictures but i do have a play list of my build and some running videos on my youtube channel https://www.youtube.com/playlist?list=PLDforydpVKU321vJ1triqkGUm6Q4wRG7M

so, I haven't really done anything much to the mini tractor and probably won't do much until August now. Wedding plans hotting up! I'm planning to have my stag weekend at a friend's dad's farm with some tractor fun! Get as many mowers running as poss. put 12hp in the little Mountfields (Murrays over there) take the original ratish rod. Then basically hoon round a field for the day with ales in the evening! Yay! Maybe I'll enter them in the build off as 'quickest builds'? I want to do weekend turn arounds, optimistic! Or maybe I could win the prize for 'most entries' in the build off!?

So I bought this off a friend for half the price of a new one. It'll help with the tractors no end. It has a milling head too which I will put on later, I'm only really interested in the lathe at the mo..
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Here's a picture of a mini moto brake disc/carrier I made using my old lathe. Pretty quick to make and dead cheap too. Never fitted to a machine but this one will probably end up on one of my stag do machines.

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I plan on making similar things to mount the brake disc & sprocket to the hubs on the mini rat tractor. I'll need to get some 1/4" plate to make the sprocket carrier. This'll then be turned down to fit in the centre hole of the hub and welded. I think!? I have roller bearings to fit in here too. Not quite worked out yet. As you can see my steel bar is 'pre ratted!'/rusty!
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So guys, prob gonna have to put this project on hold until next years build off. There's just too much work to do before the deadline. I'm gonna concentrate on project number 3 as I recon this is a bit more realistic to get done.