AW: Minibike from Scrap

[DennisB]

Finished the (biggest part of) my Bike today
The engine sounds awesome
Still need to build a chain tensioner.
6, 5 hp Honda clone (removed governor, 18lb springs, high flow air filter and breather filter, jetted)
I know that I don't have much welding skill, but I jumped as hard as possible on the bike and nothing broke
To Do:
- chain tensioner
- paintjob
- some kind of brake
- cutting useless edges if steel from the frame off
- fenders

 

[mckutzy]

This looks cool.

Some things to think on.
Your going to need a more stable triple tree. Any braking forces are going to stress and most likely break the front end off. Bracing top and bottom of the head tube bearings.
The muffler looks like it is hard mounted to the frame, if so, it is usually a bad idea cause it will put alot of stress on the motor header bolts and potentially cause them to break off.

 

[the..gokart..god]

nice job

 

[Mac]

A nice chrome exhaust pipe heat shield would be nice.

 

[landuse]

I agree with the heat shield idea. You don't want to be burning the inside of your thigh. I would also make a kickstand

Other than that, it looks like a killer bike!!

 

[DennisB]

Thanks for your comments and improvement ideas. The exhaust is not mounted to the frame yet. I want to mount it to the frame with a brace around the exhaust.

 

[mckutzy]

You need to have a rubber or sprung hanger for it to flex.

 

[rocketkart]

i would go get some flex pipe exhaust.

 

[robbie]

What ratio do you have in that gearbox?

Regarding the brace on your exhaust pipe, you actually want it to be hard mounted at the end. You can put a flexible joint in it and mount it on a flexible mount, which would be OK. But with a rigidly mounted engine, a rigid pipe, and a rigid frame, the best way to mount the end is with a rigid mount. The whole point is to keep the end of it from bouncing up and down because any motion at the end is transferred to the engine with amplified leverage, like a crowbar, which will loosen the attachment at the engine. So if you have hard mounts and a rigid pipe, and then you put a flexible mount at the end, all you're doing is PARTIALLY eliminating the risk of loosening the joint at the engine.

 

[OzFab]

WHERE DID YOU GET YOUR AIR FILTER ADAPTOR? IT LOOK SIMILAR TO MY CUSTOM ADAPTOR...

IIWM, I'd run the exhaust under the seat...

Here's another job to add to the to do list: BILLET CONROD! With no governor & 18lb valve springs, you have nothing to stop the stock rod from smashing its way through the block

 

[DennisB]

I bought the air filter adaptor here. But I think you don't want to order it from Germany
There is a 2:1 ratio in the oilbath-clutch and the chain - part has ratio of 50:11. In conclusion about 10:1. I think I have to disassemble the whole engine/ bike if I want to do the billet conrod job, right?
Tested the bike today :Youtube
This was at my grandma's place btw.
I have also machined a chain tensioner on the lathe, but I wasn't sure how to mount it correctly. I will post a picture of that later.

 

[landuse]

That is a cool bike. You will definitely need that tensioner though

 

[DennisB]

Would that be a useful position for a chain tensioner ? (I would attach a spring from the tensioner to the rear wheel axle and there where my thumb is will it be bolted to the frame)

 

[rocketkart]

i'd do it the other way around and go up with it.

 

[landuse]

Yup. I would have it push the chain in instead of pushing it out like you have it there

 

[machinist@large]

Would that be a useful position for a chain tensioner ? (I would attach a spring from the tensioner to the rear wheel axle and there where my thumb is will it be bolted to the frame)

That would be a good spot for it, but rocketkart and Paul both are giving you the straight dope of how it's supposed to work.

i'd do it the other way around and go up with it.

Yup. I would have it push the chain in instead of pushing it out like you have it there

You'll also have to build it stought; when moving forward there would be very little load on the unloaded (bottom) side of the chain, but when you decelerate using compression braking from the engine, you could have almost as much force loading the tensioner as you would if it were on the top side of the chain.

All the advice the rest have given you about the exhaust mounts is good info too. The fork bearing tube most definitely needs to be fully boxed in for strength as well; for reference I would look at almost any sport bike or full dress touring rig to get an idea of what you need to counter the mechanical leverage this joint needs to withstand when braking....

Pat

 

[itsid]

I looove that thing!

Okay I'd be scared of that fork that looks just like the suspensioned fork I broke on a bicycle once; other than that. perfectly nice Mopped.

So it is...

'sid

 

[DennisB]

After a hour of trying to mount the chain tensioner with the spring I've just welded it to the frame (I think this won't be a problem with a unsuspensioned rear). Works quite well. But I definatly need some brakes. Nearly crashed in my garage today

And a heat shield or heat exhaust wrap tape would be a good way to prevent burning my leg

 

[rocketkart]

looks good but i would have takin some more links out of the chain it looks offley close to each other.

 

[itsid]

looks good but i would have takin some more links out of the chain it looks offley close to each other.

I agree, shorter chain and a spring loaded chain tensioner to comensate strech would be a good idea I think

'sid