Crafting a kr250a1 motor into a dirtbike frame ???

[Azza1980]

Ok so I have the engine just looking for a suitable frame

[Jeram]

to make your life easier use a steel frame, ally will be too much fuking about.

RM250s are ledgendary handlers

KTM two and four strokes too. you could pick up a blown up 250SXF cheap and start from there!

[Azza1980]

Steel frame is what I was thinking to I have my eye on a 2005 yz250 frame at my local wrecker there is a lot that needs reworked but its a good platform

[JonW]

LOL no such thing as a cheap KTM, but a fun project never the less...

[2TInstitute]

KR 250 won't make a very good dirt bike engine

[Jeram]

would make a fun supermoto though

[Azza1980]

I think the tall gearing will take some expermenting with sprockets to make it dirt frendly as for the power delivery I think the pipes will have a lot to do with how and when it peaks i will be makeing them myself tig welding ist my job it may take a while to get them right

[Azza1980]

Just did some reserch on the kr,s they were geard to do about 120mph so I will be looking for the biggest rear sprocket I can find and a small front to sort out gearing as useing it for supermoto I have a wr426 sumo I could borrow the wheels from if I want to try that

[2TInstitute]

There simply won't be enough room to fit rear cylinder pipe on in a dirt bike frame. A standard KR has about 40HP which will be less than the 250 engine your intending to replace.

[Azza1980]

Funny you say that I also had my doubts but it fits I have tryed a 2005 yz250f frame the kr250 made 45hp but with bigger carbs and pipes they make mid 50s and there is a 310 big bore kit and porting that can be done but even stock it makes way more torq than a yz250 ever will its fireing twice as often.the rear pipe will go under the seat like a ossa mx bike does and the air box under the tank as lots of new bike do

[JonW]

I did start building a KTM frame with an RZ350 motor with a view that I would tune the engine. It was hassle. Older multi cylinder engines wont fit the space a dirt bike single came from. You can do it, but it will be hard work. In the end I stalled my KTMaha project, as whilst it was possible for a sprint or motard it would have been a poor off road bike, and I couldnt register it etc.

I wish you luck with your journey, please do keep us posted, you might get me interested to continue my project, it would have been killer!

[Azza1980]

There is a rd350 yz on you tube very cool.my engine is a 84 kr250a1 its a tandam twinn a verry compact unlike the gen 2 kr,s I would love to see some pic of your one

[2TInstitute]

Kawasaki's quoted HP figure is at the crank which would be about 35HP at the wheel.
A YZ250 has about 30ft/lbs torque something the KR is going to struggle to do.

[JonW]

There have been few decent road engined off roaders, mostly terrible ones built badly. Just fitting off road pipes and suspension to an RZ was one that sticks in my mind. Truly awful LOL Its on youtube i think? One guy did a DT125 with a 350 motor in the uk and it rocked the mags (i put the scans up on norbos forum i think a few years back, should still be there). Justin did one here and couldnt register it so parted it out. Ive seen a KR1S (i think?) motor in a KDX I think chassis that looked ok and there has been one decent YZ and a guy put a Banshee with 100bhp in a Honda CR500 frame in one of the UK mags about a year back - it was for hillclimbs.

The RZRD500 forum has a few threads on motard projects by the way, worth a look there.

Re the frame. Most modern-ish off road bikes have the swinger thru the rear of the motor, most older road bikes do not. Most single cyl dirt bikes have a massive downtube spar where the exhausts need to be on a twin cyl. Most brake pedal setups will become inboard on the wider twin and need moving out, frame spars too perhaps for strength. IMHO Parallel twins are really too wide for a slim bike. So, I see why an inline would be better, but the issue is length of the motor as the shock and airbox would be where the rear cyl would be, and the front would be too far forwards unless Im mistaken as you seem to have found a good fit, do you have pics? Im keen to see how youre doing. The other issue I found is pipes, they would have to be custom and hence cost a fortune to have made I reckon.
All that said I do still love the idea of a 70ish bhp 115 (or so) kg slim bike with sticky tyres for city use, If only I could get the one in my mind registered here I would build it, I have all the parts to do so. Maybe I should do it and just use it for track days, but I'm too busy and never have time to go to them... so maybe not. LOL

Here is where I got to, I could still do this, I still have the parts or ones that would work... who knows...
http://2smoked.com/KTMaha_-_Half_KTM_25 ... rmoto.html

[Azza1980]

Yes the inline twinn fits in the yz frame but the down tube needs to be modded the engine isent any longer than a single supriseingly be the rear port doesent come close to the shock because the motor is made to lean forward and the sprocket lines up where i t should be for the pipes I will be makeing custom ones I supose all this would seen like a epic amount of work and would cost $$$$$$$ but enginering is my job so I onley pay for materiels I havent made esxpantion chambers before but I have done a load of sheet metal forming and tig welding