I've had this thought on my mind for years now, I've also been lurking these forums for more than a decade joined up a year ago to ask this question or something similar but never got around to it once my account was approved etc. Anyways I've always been a huge fan of the 2 stroke GP bikes since I was a kid when you could have your pick of TZ or RG or RS etc street or cup bike for like $5k with some spares. Now they are getting to be way too much for a normal person to own/ put a decent amount of miles on on a budget IMO. I want to build something I can enjoy on an extreme budget that I could possibly compare to a 2 stroke GP bike in some aspect and just be happy with/ enjoy a lot. Of course finding a 2 cylinder space age 125 or 250cc engine isn't happening on a budget. Banshee engines used to be somewhat affordable but now they are easily 5k+ for a decent build. Im also not going to find a GP grade chassis on the budget I want. But anything I buy in my state with a title can be made street legal with the 2t engine. So im thinking within a budget of like 3-5k max I could do a 250 dirtbike engine (most likely yz250 these have been the same for 10+ years so plenty of engines and parts out there) have the cylinder ported to make it top end biased and pick up as much power as possible, Have a pipe made once the engine is mounted and leave it at that as far as powertrain. Something like the 90 watt lighting coil already available, some 18650 battery's made into a 12v cell to be super light weight, run lights and be budget freindly. As far as chassis (this is where I really don't know what would be even remotely close) I really just want something light, with decent handling and brakes. Like I said I'm not going to out do a factory race team and in my opinion if I tried things would start to cost as much as buying a real race bike. But something that could come in around 250lb or less, have decent suspension and brakes for a 50hp engine. Dirtbike based chassis: I've seen guys attempt this on this forum and it seems it became too much and was never finished bike was eventually sold for parts. Seems expensive and alot of making custom parts to do Supersport type front and rear end on a aluminum dirtbike frame/ rake would be all wrong too? Is there an aluminum dirtbike chassis thats easily adapted to road going wheels/forks/swingarm? Ninja 250 chassis: pre 2008 face-lift I can find these complete all day for like 1k or so, get some money back from parts I won't need, cut off the rear subframe and engine cradle to replace with custom parts hopefully save some weight and remove all unused brackets probably wouldn't be close to 250 lb even super stripped down, suspension isn't great neither are brakes but they do seem to have some aftermarket support. Plus no inverted forks even on the 08+ models Yamaha r3: inverted forks, seems to be a better frame and swingarm than the ninjas but these would be a chore imo to find in the budget I want, would be all over copart and Facebook looking for a lightly wrecked or neglected and blown up one to cannabilize within my budget. Are there any donor bikes I'm maybe not thinking of? I'm really serious about this i have decent fabrication experience, a welder and good selection of other tools I've never tried anything custom with a bike though it's all been cars so far would like to start or complete this project this summer. I realize I may get some hate for this and that's fine. Sorry for the wall of text btw I tried to keep it somewhat organized.
https://www.sflminigp.com/ https://www.apriliaforum.com/forums/showthread.php?360172-AG%92s-RS-KX-Track-Rat-!
Aprilia rs250 hybrid is about as close as your gonna get to a TZ with off the shelf parts. I'm putting one together now.
NOLA, didn’t I see a build thread on another forum of a hybrid? Or am I mistaking you for someone else?
Nah...not me. There are a few threads on the aprilia fourm about builds. 50joe has one on his build from years ago.
I think what you are considering is a great idea. There have been quite a few 250 single powered builds and the yz250 is a great engine. You may consider what class it would fit in. In some organizations it would not be fast enough for the designated class unless you were good at using the light weight flickability and were good at dragging elbows. Here in the 2 stroke section I have some info on my cheap Frankenstein build under "500cc two stroke single test/build". Inserting the engine in an existing bike would be sweet but early ex250s had 16 inch wheels which you would probably want to swap. Also I think the newer 17 inch wheel ex250 is pretty heavy. I wonder if you could use a cbr250r with a two stroke swapped in. It would be cool if you could keep the aluminum yz frame but swap on front and rear from something else. That may a very light bike. I have swapped around wheels and suspension on several bikes and it may not be as hard as you think.
This was done in the UK in the late 70's/early 80's using a YZ250, there were quit a few of them around. Here's a link to a thread about them on a British forum from around 12-13 years ago, including a magazine article and spec. sheet. https://forums.autosport.com/topic/151779-barr-yamaha-250/ I also saw a couple of MT125 Hondas with a YZ engine fitted during that period too.
I have this vision of finding a way to get a KTM 350 setup on a GP bike or a supermono setup. Find the tallest gearing you can get (maybe use some 500xcf gears?) and let it destroy the 300ssp/sbk class. You’d probably get a good NF4 chassis for cheap (I have a full NF4 bike I’d sell if you’re interested (in Boston)) and use that as a donor given their ergos and such are setup for road course riding. The NX4 is great too but motors are easier to come by and should be kept together in my mind.
The gearing is straightforward. If you can get tall enough gearing with just off the shelf bigger fronts and smaller back sprockets, great. If not you can put a smaller one on the back by buying a rear sprocket with the teeth count you want that is for a different bike but same chain size. Have a machine shop drill it for the different center hole size and different bolt hole pattern. I successfully did this on my husky dirt bike myself by being very careful. This got me almost where I needed in rear sprocket size but could not go any smaller because the dirt bike hub large diameter would not let me go smaller. I next bought a cbr250 rear wheel and brake to swap into the Husky. This allows me to go even smaller on the rear.
Rebel Gears will just cut you the sprocket you want, with limits of course due to bolt pattern. Most dirtbikes you can go down to 37T.
Old running dirty bike. Shorten springs. Rebore and replate and a new top end. Rewire rims for 17s. All stuff you can do to save $. It won't beat a purpose built machine but it will scoot and will handle like a two stroke. Lighter and peakier.
