Frames, donor bikes and weight.

[AMB2012]

Hi,

And so it begins - I'm searching for that perfect donor bike to fit a 10hp engine to.

My first thoughts were to go for something small and light. Reading around the forum and it sounds like the frame on a 125 or 250 may not be physically strong enough to hold a heavier engine and put up with that diesel-single rhythm.

So the question is - how much of a difference is there in the weight of the frame between a 125-250 and a 500cc bike? I can find plenty of figures for the total overall weight, but how much of that is engine/transmission?

Being able to use a bigger frame would (hopefully) mean no major welding making the paperwork easier and the range of potential donors is much larger (but that's no good if your top speed is 30mph down hill with a following wind).

AMB

[old clunker]

From peoples experiences on this website at building single cylinder diesel engined bikes, the best size of donor frame seems to be around a 350cc to 500cc petrol engine frame. They seem to be strong enough, without being too heavy performance wise, allowing for the extra weight of a diesel engine.
A single cylinder L100 (406/418cc) type diesel will never be quick, but will have about the same performance as a petrol commuter bike, but with vastly greater torque and fuel economy, longer engine life and hopefully much less engine maintenance!
That's one of the reasons why Royal Enfields are so popular for diesel conversions, where the frame is just enough to do the job and cope with the vibrations, relatively simple to convert, and spare parts are plentiful.
Old style British motorbikes have the advantage of already having a separate gearbox that can be used.
Maybe a 250cc trail style bike frame might be just about up to it, since they have to be quite robustly engineered to cope with going off road, and maybe with a bit more extra strengthening around the engine mounting parts of the frame, could possibly be up to the job. The suspension might have to be upgraded as well.
Two stroke motorbikes around 350cc and above - if they can be obtained for a very cheap price, are also viable.
Then there's the choice of a CVT or pre unit gearbox to be fitted in place of the original combined engine/gearbox, or chop and weld the original gearbox - if your aluminium welding skills are good enough!
You can fit a pre-unit 3 or 4 speed Burman, Albion or Villiers gearbox, if you can get a cheap working second hand one.
The choices and different permutations are yours to decide - it's just down to how cheaply to build and how reliable you want the bike project to end up.

[AMB2012]

Performance on a par with a 125 is what I'm aiming for.

I was trying to find some figures on the engine weight vs. the frame weight without much success. About the only thing I did find was that the 250cc version of the Hornet weighs about 25kg less than the 600cc version (150kg vs. 175kg).

What did catch my eye about the Hornet frame was the lack of a bottom rail under the engine - there is a short section from the top frame that bolts onto the front of the engine, with the back of the engine connect to the frame. It leaves a nice big space into which you can bolt your own engine/sub-frame, hopefully avoiding having to get it tested by the DVLA (no "radical alteration" of the frame).

I like the idea of a CVT transmission for it's simplicity, but also like the idea of having a proper gear box. I suspect the CVT will win out simply down to lack of space (I'd feel bad about using an Enfield on my first attempt and ending up mangling it).

AMB

[old clunker]

You should be able to achieve the performance of a commuter 125cc petrol motorbike, with a 10HP diesel engine. That's what I have experienced with my motorbike, though don't be too surprised with the eventual weight! Mine is approx. 420lbs with the diesel engine. It was originally about 365lbs with the petrol engine.

Usually it's best to start a first project with a cheap dead or dying engine donor motorbike, and then 'transplant' a diesel engine into it! that should keep costs down, as the diesel engine is a large part of the cost of the project - Chinese clones cost around £450 new, and the prices seem to keep steadily going up every year.
Only used L100 Yanmars are really viable, (can be found on eBay sometimes) as new ones are just too expensive - similar with Hatz and Lombardini diesel engines. Though you take your chances with used engines, and how much useful life they still have left in them, and what parts might need replacing.

As for fitting a CVT, I don't know much about them since they aren't readily available in the UK, and people don't appear to use them much over here. Though there seems to be positive experiences with people using them, on this forum.
Expect to lose a bit of power output from the engine when using a CVT, and a bit less fuel economy.

What ever you decide, hope you have fun with your project.

[coachgeo]

.....
As for fitting a CVT, I don't know much about them since they aren't readily available in the UK....

hmmmmm.... really.... like don't yall have the 3cyl micro cars around there that run CVT's? Thought they were available in UK where their unheard of on our side of the pond.

[AMB2012]

I don't know where these diesel micro-cars are, but they aren't over here

Diesel Smart cars are still relatively new so there aren't that many available used/scrapped etc. There's the Aixam cars but they seem even rarer (at least in my part of the world).

Small four cylinder engines are more common, but these come from front wheel drive cars so are large and heavy and there are issues fitting a gearbox etc. (which is doable but a bit ambitious for a first timer like myself).

AMB

[mark_in_manchester]

Aixam diesel micro-cars crop up here from time to time - Kubota 500cc (?) 2cyl diesel, through a CVT. A guy from the bike shop I use for MOTs uses one to visit family in Scotland...

[skoleskibe]

A cheap and easy solution can be seen on Youtube, search MZ Yanmar
ITs my bike, i've been using it for more than a year now, and its absolutely reliable.

Its a MZ TS250 Frame, (no downtubes) Engine hung in rubber mountings, ie. not many wibes.
I 'd build it for about 1000 quid (10.000dKr.)

Also spareparts readily available in germany at really fair prices, through webshops (in english) such as:

http://www.akf-shop.de/shop/

http://www.ost2rad.com/

Happy hunting

[Eddy Wane]

Has anyone tabulated which frames will suit which engines. I could trawl this site to see who has done what but if anyone reads this they (you) can list what was used on the build.
It only needs to be a short list i.e.

Frame - Enfield 350 Bullet
Engine - Hatz ABC123
Transmission - Enfield gearbox.

I do not know if this would be helpful to anyone but if such a database was available it would help builders new and old. There is nothing worse than to source the wrong kit.