From dirt donk, to motard, to pure GP...

[Kurlon]

It's not a proper midbike, but I figured some would be interested in the next steps of my pit bike's evolution.

This is what I started with a little over 3 years ago:
http://www.x386.net/TTR/media/CRF70F-Factory1.jpg

A bone stock 2005 CRF70 in all it's gutless, undersprung glory.

By the end of it's first season of racing, it had evolved into a screamer 72cc machine, with flat tracker influences.

http://www.x386.net/TTR/media/PR/Bike-on-lawn-with-leathers.jpg

I also ran it in the dirt in the exact same config, other than swapping knobbies on. Flat drag bars, suspension setup for road racing, the whole bit. Unfortunately I don't have any pics of my first session out, in my full race leathers. I ended up buying the dirt gear in this pic at the track to finish the night out. This was my first time racing dirt. :P

http://www.x386.net/TTR/media/trapani-9-30-3.jpg

[Kurlon]

Year #2, I got silly. Very silly. I lucked into a deal for a set of NSR50 wheels, and went nuts getting parts to do a proper conversion. The swingarm is a custom unit that leaves the wheel centered on the bike. Normally G-Craft builds them to maintain the sprocket alignment, pushing the wheel to the right by 7mm. To keep the chain straight now I have to use a homemade offset front sprocket carrier. The motor was re-rigged for the 111cc class that never happened thanks to the series dying. The rear shock is a one off Works, reduced travel, extra length, sprung and valved for road racing. The forks are reworked NSR50 units in SPi clamps. New springs and emulators, plus external preload adjustment.

http://www.x386.net/TTR/media/06Rebuild/06-15Bike/HPIM0409a.jpg

Last summer the bike stayed pretty much as is, save for some power plant swaps. The final motor is a 151cc Jialing 125 build putting 15hp to the rear wheel. This is conf I took to Loudon for a track day.

http://www.x386.net/TTR/media/fishtail/_K5C5820s.jpg
http://www.x386.net/TTR/media/fishtail/_MG_5884s.jpg

The mess all over the number plate in the first shot is a cheapie digital camcorder. Unfortunately the vibration was too much for it, no vid from that session.

Despite declaring the bike 'pavement only' it's still very much a motard setup. The bars are on risers, dirt peg position, etc.

I've got a 'B' bike now that'll handle dirt and mixed mode racing, along with a Pitster X4 Motard that I'm still evaluating to settle on it's final application. This little 70 needs to finally finish it's evolution into a GP machine.

To that end, I've settled on a new front suspension that will be sturdy enough to handle speeds in excess of 140mph. Clipons will be ordered shortly. I'm investigating a few motor options, with a minimum target number of 20hp, ideally I'd like to shoot for mid 30s to possibly 40hp, but I'm going to need access to some very skilled machinists, and a winning lottery ticket to pull that build off. Oh, and I just dug some spare parts out of my JDM collection that will be refurbished and fit shortly...

Time to give midbikes something to have nightmares about when they sleep! :P

[Blitz$M.Inc.$]

thats pretty cool, anything you spend that much time on is your baby
for 30 or 40 hp yer gonna need a different style motor than horizontal

[Kurlon]

I actually have one motor option that keeps the bike 'pitbike legal', ie horizontal single, air cooled. Problem is it's a good 6" longer at least, and weights twice as much as my existing power plants. That said, they'll make 40+ hp all day long, and are proven road racers.

My alternate idea is a home built motor, but I need to track down some machinists to get it done, 'cause I'm not going to be able to pull it off with a dremel and hammer. : )

[Blitz$M.Inc.$]

dont underestimate the power of the dremilbluelaugh
i thought i saw a 250cc horizontal in a quad one time

[The Nutty Professor]

Kurlon I think the Mid's will sleep just fine moon1 I can see you've done a lot of development on your steed and you're ahead on the testing front even-so can you feel that hot air on your neck? That's us breathing down your neck bluelaughbluelaughbluelaugh and coming up fast.

[Crazyj15692]

Wow. looks great I love mini motards!!!

[Blitz$M.Inc.$]

kurlon, about your motor, do you mind giving out the specs, like bore and stroke, the kind of cam and the specs on it and the carb specs like jet numbers:)
im just curious to see how close i am to you, if you dont i understand
and nothing is breathing down my back...cuz im IN the back!!!bluelaugh

[Kurlon]

Blitz, it's actually a pretty mild build. 58mm bore, stock 57.5mm stroke. I picked up a resleeved stock Jialing 125 cylinder that was initially bored to 57mm. Unlike the cylinders TB sells, the spigot hadn't been turned down to fit the cases. I sent it, along with a Lifan Monster 120 head to Kendall of Cat5 Motorsports to assemble on a Thumpstar sourced Jialing 125 he had, as at the time I was in a time and space crunch. (On of the downsides of my apartment, no place to work on bikes, I have to sneak time in at the local motorcycle shop when they're slow, so I can go months without being able to turn a wrench.) He bored the cylinder to 58mm, then turned the spigot down just enough to fit the cases, resulting in a no case bore 58mm setup. Piston is a TB KLX unit. Cam is the defacto standard, TB300a.

Kendall installed a set of Akunar 28/24 valves I provided, as I had bent the stock 27/23 set when my Lifan 120's crank let go. The head was ported, intake for the 26mm Oko it currently breathes through was port matched, and that's about it.

It's a lower compression motor, with a very relaxed, lazy character compared to my prior machines. It's almost like a stock 70 motor, except you end up going a lot faster. : ) Short shift it like a Harley, and it's in it's happy place, not a huge amount of over-rev.

I need to find 2 or 3 more HP minimum before the Mini Nationals in Wisconsin (You're all going, right?). I'll be facing 72cc two strokes in F3. If I can get the big motor assembled by then, I'll jump it into the SuperMoto Unlimited class. Current plans are to find someone who can cut me titanium valves, a better valve spring setup, porting tweaking, skimming the cylinder to set the squish better, combustion chamber tuning, and then I'm going to play with some thermal barrier and heat dispersant coatings to improve the thermal efficiency of the motor. If I absolutely have to, I'll bump the compression.

I'm also working on dialing in a Kiehn PE28 to see if the motor will respond to a bigger carb. Seat of the pants dyno says hell yes, haven't spun it to confirm.

[Blitz$M.Inc.$]

thanks for the insight kurlon
it seems your head breathes better than mine and its slightly bigger
i think youve found the near reliable limit on that 125 block, have you thought about the 150 gpx? there is 20 easy there potentially
im gonna guess that 28 carb will help, it will match your intake valve
wisconsin lol way too far for me, gl to you tho

[Kurlon]

I've got a YX150 to play with in my Pitster Pro X4, not sure what I'm going to do to it yet. That said, I'd really like to get into a bottom end with more potential, as the YX150 tops out about where KLX110s do, including nothing reliable past a 4 speed gearbox. With a good 5 or 6 speed setup, I can really start working a motor for a wider usable speed range.

[Blitz$M.Inc.$]

man you want it allrock2
i know tak makes a 6 speed for something, so its possible

[Kurlon]

Tak's 6 speed for both the KLX and 50 bottom end is a grenade waiting to go off. Typically when you're kicking the motor over, that's when it'll let go. Missed/bad downshifts are also likely to make it rain metal. The 5 speeds aren't all that much better on 50s, KLX110s just blow output shafts when dancing at the 20hp mark.

I'm thinking a reworked Honda XR200/250 bottom end will do the job... :P