IJ Scores a Mini Bike

[Iron John]

So while we are kinda sorta on the topic of brakes, does anyone have experience with this drum brake:



It worked really well on the Chariot, and while the total weight was quite high, the speed was only a few mph so this one worked fine.

 

[chancer]

Kelly Wood maybe?

 

[Iron John]

Dagnabbit, I did way too much work on this today. Things started simply enough; I cleaned the rear wheel/tire/stub axle/sprocket/brake assembly as I had sprayed it down with white lithium to prevent things from rusting years ago when I tore the Chariot apart.

With everything bright and shiny, of course I wanted to take a closer look at how everything laid out. Natural, right?

I started by clamping a long piece of aluminum channel to my work table:



(Sorry for the sideways photos but the Forum software flips portrait photos because it is SO SMART [not].)

I then blocked the minibike in place at my desired ride height:



Next, the rear wheel was placed in place and I determined that the wheelbase would be right around 50":





Now the stock swing arm was in the way so I decided to pull it. I fully expected it to be a hillary to get apart, but surprisingly instead of the rocker shaft being rusted in place, it slid right out by hand.

Cool. Now let's see which motor I would use. I have a 5 horse Briggs kart engine and a 6.5 hp Honda. My first choice was to use the new Comet 20 I have on hand as opposed to building a jackshaft setup.

The Honda motor, with its goofy half-a-big-twin canted cylinder, was disqualified immediately because the Comet 20 would not mount due to the canted cylinder.

The Briggs also had a problem. The gas tank hangs off the carb; the tank needed to go. I have no problem with that as I intend to mount a tank up above, but the Briggs fuel tank is married to the carb. What a goofy design - small motors are supposed to be maintainable by anyone out in the field; why do the manufacturers hide fasteners and otherwise make the unit impossible to easily maintain?

I pulled the carb and fuel tank as an assembly and I will have to address the ramifications of replacing the carb down the road. Could be worse.

Mounting the Comet 20 on the engine was next. It really wanted to be clocked and the motor raised 3/4". No way. That's slim shady. To run the Comet clocked horizontally and the motor directly on the motor plate, I had to cut the frame.



To be continued...

 

[chancer]

Hah I love it. "A Hillary to get apart"
That is nicest thing I have heard her Called today! I can only guess the stock swing arm was CROOKED! lol

BTW that thing is looking good. and since you have a Honda and a Briggs and a Carb issue.
Search the "Bronda" by Kartfab. Interesting idea if nothing else.

 

[Iron John]

Sigh, today was just one of those in-for-a-penny, in-for-a-pound situations. At each step of the way I just could not resist seeing what would come along next.

To pick up where we left off, the Comet driven sheave interfered with the stock left side jackshaft/swing arm mount.

I could have futzed around with offsetting the motor to the left and moving it forward, but that would be slim shady. I want the motor centered left to right and front to rear on the motor plate.

Remember I swore that I would not cut the stock frame? I cut the stock frame.

I only needed to trim the front of the stock jackshaft/swing arm mount. That piece was redundant anyway since the Comet 20 does not require an external jackshaft. Furthermore, if some day down the road I switch to a jackshaft, well, the stock jackshaft situation would not work with this width tire, anyway.

The whizz wheel came out and made quick work of the haircut:



Now the motor sits where it should with plenty of clearance for the driven sheave:



Next, can I fit the chain between the frame and tire? (Again, sorry for the sideways pictures, but the Forum software flips portrait photos to landscape because it was programmed by Geniuses.)


Yes, it seems I can.

I then slid the sprocket in and used the nasty, rusty original chain as a straightedge to take a more accurate look see:



We are definitely in the ballpark. If needed, I can widen the frame rails at the rear of the seat to give more chain clearance, but I don't think that will be necessary. That additional frame mod is in my pocket in case I need it.

As a result of today's work, I have the following working hypothesis:

- 5 horse Briggs.

- Will need to add an external fuel tank and change the stock carb/manifold.

- Comet 20.

- 84 tooth sprocket.

- Left side drive.

- Motor can sit flat and centered on the motor plate left to right and front to rear.

Things I don't yet know:

- Will the brake be mounted left or right?

- If the brake is mounted on the left, can I fit it between the wheel and sprocket?

- Rigid or swing arm? My preference is to keep the swing arm, but the arc of the swing arm and the chain will be slightly different. Enough to matter? We'll see.

- Will the brake pictured do the job? Anyone work with one of these before?

 

[Iron John]

Hah I love it. "A Hillary to get apart"
That is nicest thing I have heard her Called today! I can only guess the stock swing arm was CROOKED! lol

BTW that thing is looking good. and since you have a Honda and a Briggs and a Carb issue.
Search the "Bronda" by Kartfab. Interesting idea if nothing else.

LOL, as a matter of fact, the stock swing arm was crooked. Someone must have stomped on it as it is bent inwards. But luckily, it was not a hillary to get apart.

 

[chancer]

Bronda
http://www.diygokarts.com/vb/showthread.php?t=25246&highlight=Bronda

 

[Iron John]

BTW that thing is looking good. and since you have a Honda and a Briggs and a Carb issue.
Search the "Bronda" by Kartfab. Interesting idea if nothing else.

Funny you should mention that (thanks for the lead; I had not seen that before).

I was thinking along those exact lines as i have boxes of various carbs - clones and Tecumseh and who-knows-what - and it is easy to make an intake.

I was wondering if making the intake longer would not give me a little extra low-end grunt.

 

[chancer]

I dunno but there is also this how to for a manifold, May help on your mini. So the carb is not up against the tire.
IIRC the briggs intake faces rearwards,
https://www.affordablegokarts.com/curved-manifold.php

 

[Iron John]

I dunno but there is also this how to for a manifold, May help on your mini. So the carb is not up against the tire.
IIRC the briggs intake faces rearwards,
https://www.affordablegokarts.com/curved-manifold.php

That's an interesting approach.

Yes, the Briggs carb points straight back and I was thinking I'd angle it out towards the rear.

I do have a pantload of space underneath, though. Lookee:





I can put that space to good use for something. Maybe a battery and a tool box.

Is there a way to retrofit the Briggs flywheel so it can charge a battery?

Here's one oddball carb I have. It is already set up for a cable throttle. One of you dudes will know if it is potentially suitable.

 

[Iron John]

I am also wondering about that long chain run. Looks like I might experience chain whip.

I guess I can add one or two idlers to keep things under control.

Again, looking for insight from you all.

 

[mckutzy]

Nice looking start... You will need to build a fairly strong swing arm, as these tires are "Biased" in their weight balance(They need that to fit around the caliper/drum in the vehicle there for).
Equally speaking what are you doing for the reinforcement of the frame for the new swingarm(if thats your rout)
(Im a hardtail frame fan myself.... so inclined to go that way)

I used the jackshaft to mitigate the the long chain on my bike....

On my bike I used the opposite of the above like in carb relocation. On my bike I just relocated the filter, as I had the gov in still thus needed the linkage. I did however need to rejet as I was leaning out.
As you have seen in previous threads, I can say that a mechaincal brake is a good option( with the right type of disc) and better would be a hydraulic setup. Drums can work, bu not so much in this aplication.

 

[Iron John]

Nice looking start... You will need to build a fairly strong swing arm, as these tires are "Biased" in their weight balance(They need that to fit around the caliper/drum in the vehicle there for).
Equally speaking what are you doing for the reinforcement of the frame for the new swingarm(if thats your rout)
(Im a hardtail frame fan myself.... so inclined to go that way)

Do you mean they tend to carry more weight on the 'outside' of the rim, so the axle supports need to be beefy enough to carry much of the load on one side??

I used the jackshaft to mitigate the the long chain on my bike....

The jackshaft on the Comet 20 only rides an inch or two ahead of where the jackshaft would be anyway. I think the Comet 20 is set up as a compact package (it completely mounts to the motor) and my wheelbase is a good 8 or 9 inches more than stock. What's the guideline for center-to-center distance for #35 chain? I can see idler sprockets in my future. I am also going to need to be able to adjust the rear axle separately side-to-side so that I can align that huge sprocket precisely to the motor/TC.

On my bike I used the opposite of the above like in carb relocation. On my bike I just relocated the filter, as I had the gov in still thus needed the linkage. I did however need to rejet as I was leaning out.
As you have seen in previous threads, I can say that a mechaincal brake is a good option( with the right type of disc) and better would be a hydraulic setup. Drums can work, bu not so much in this aplication.

This particular drum has dual internal expanding shoes like a conventional automotive drum brake. It *might* be OK. But I agree that a hydraulic disk would be the best.

Once I have an idea of how I am going to run the rear subframe (whether sprung or rigid) I will be able to see what I have to work with in terms of component placement. I can always just use this drum for mockup purposes.

 

[mckutzy]

Sorry I meant that the tires are more or less biased with a bit of the weight on the one side but also the width... That can offset the amount of room to deal with for brakes and sprocket...

I don't think there is a max point to point distance fo a given chain type... Typically any situation with a chain drive like this... A 1/2" deflection from taut is about the norm.

 

[Iron John]

Sorry I meant that the tires are more or less biased with a bit of the weight on the one side but also the width... That can offset the amount of room to deal with for brakes and sprocket...

I don't think there is a max point to point distance fo a given chain type... Typically any situation with a chain drive like this... A 1/2" deflection from taut is about the norm.

Gotcha. I currently have the sprocket on the inner side and there might be room behind it to fit the brake inside the wheel.

I have to figure out a simple way of adjusting the rear axle for chain length and square with the bearings I have. I can also slide the engine some for chain length and wiggle it a little for square. If I need more I can always Chicago the engine mounting bolts.

 

[Iron John]

Today I spent a few minutes contemplating a new swing arm. Right off the bat, there are two ways I can approach the design.

Option 1 is for the swing arm to originate on the pivot inside the frame rails, like stock. The swing arms would splay out at an angle, running between the frame and the tire, and once outside angle straight backwards for the axle bearing mounts.

Option 2 is to run straight swing arms that mount on the outside of the pivot.

Option 2 is easier but makes for a very wide setup - probably perfectly functional but might look slim shady.

Anyway, thoughts? Pros and cons of either approach?

 

[ezcome-ezgo]

Option 1 FTW.

 

[mckutzy]

That's how stock motorcycles attach a swingarm for larger rear tires.
It bell's out to width needed.

 

[Iron John]

That's how stock motorcycles attach a swingarm for larger rear tires.
It bell's out to width needed.

Bell out - those are the words I was looking for.

OK, the bells will be ringing on this one.

 

[Poboy kartman]

Bell out - those are the words I was looking for.

OK, the bells will be ringing on this one.

****'S BELLS!!!!! Uh huck....