Overview of 2016 racing season
So, one more race to go for this season with Cascadia Supermoto here in WA State!
I thought I would share my experience riding the mini in hopes to help a few out there avoid pitfalls I have run into. For reference, the Mini class runs Asphalt-Only, so my setup does not work well in the dirt. It carves the corners like a knife, though.
My bike: 2008 Pitster X4M (motard)
I am 200 lbs naked. Not a light rider.
Front: I am using 630mm AKX01RC forks, bone stock internals. I fabricated my own air service port that threads into the bleed screw port to allow for air pressure adjustability. They seem to work real well for me at 15psi of air pressure in the forks.
Rear: Elka shock. Stock length setup made for the X4 frame. Internals stock, 1000lbs spring. I fabricated a bolt-on relocated upper shock mount that lowers the rear of the bike a bit and stands the shock up for a more progressive spring and shock rate. This made the rear much more like a sport bike rather than a dirt bike.
Frame: When installing the 630mm forks on the X4 frame and using 12" wheels, the rake ends up at around 18 degrees (really really twitchy). I also got feedback from other riders that my bike appeared to be flopping through the corners like the frame was twisting, so I modded the frame half-way through the season. I cut it several inches behind the steering head, raked the front out to 24 degrees, and gusseted/plated the frame on both sides with 3/16" plating for reinforcement. This completely stabilized the bike, added a little wheelbase, and eliminated the squirmy feeling deep in the corners. I also discovered through this process that the original frame assembly left the front and rear wheels misaligned: when the rear wheel was 90 degrees to the floor and aligned with the front wheel, the front wheel was not 90 degrees off the floor from the side. If you looked close at the steering head on the frame, you could see it was never welded on square from the side.
Tires: Running a 2.15 front rim and 2.5 rear. Recently switched from Dunlop TT93GP's to PMT Med slicks. The TT93's were really good for DOT tires, but the PMT's are definitely and improvement.
Engine: I started with a YX150 block, Daytona style clutch cover, V2 head, and 64mm bore. The V2 head was running a TB S35 cam, upgraded valve springs, and Ti retainers. The clutch is the YX 6-disk version, using two stock springs and two heavy duty springs. Here are the problems I encountered with the engine, listed in the order that they happened:
-cam chain tightener insufficient. This was due to the cylinder being decked for a 0.030" squish clearance. I had so much material taken off the cylinder that there was too much chain slack for the tensioner to take up even at full extension. I added material to the tensioner and it worked perfect. This can be avoided if using standard-height cylinders.
-cam chain wear. This engine would literally eat the top cam sprocket, stretch the chain, and throw the cam timing retarded every day it raced. This problem went away after the most recent replacement of the cam chain and sprockets. The latest upper cam sprocket from T Bolt was evidently hardened and shows zero signs of wear after several days out.
-upper rod end galling. Not sure what caused this, but I noticed some apparent galling of the top rod end on the wrist pin when tearing the engine down for cam chain replacement. Junked the 150 crank and installed a 160 crank for a new rod end and new lower cam chain drive sprocket.
-blew a hole in the piston and cracked it. Not sure if the crack happened first, or the hole. Caused by the TB28mm carb having brass shavings stuck in the pilot jet port in the carb, causing it to run real lean just off idle. The cracked piston may have been caused by too much cylinder wear, but I didn't mic the cylinder to measure the wear. The piston measured to the same spec as the brand-new piston, but the cylinder had wear marks you could feel in it. Odd for a steel lined cylinder to wear faster than an aluminum piston. Replaced the steel cylinder with a plated cylinder of the same bore, and it is maintaining good performance.
-cam wear. Noticed the engine slowly loosing power over several sessions out, and the top of the engine making growling sounds like a bad bearing somewhere. Pulled the TB cam, found what looks like the hardened surface wearing through at the ramp-up of the exhaust lobe, and the bearings feel gritty. Scrapped that cam, installed a Takagawa decamp S35 cam, all is much happier (especially the kick-starting). I wish I had run this cam from the very beginning.
-clutch failure. Noticed clutch slipping slightly after then engine was heated up several laps in. Pulled the clutch out and found: outer compression plate cracked and separating right where they molded together the aluminum and steel (that sits under the retaining nut). I also found the kick-start gear engagement very loose and sloppy. Replaced the whole assembly with the same YX 6-clutch spare on hand, all is happy again. For some reason, I can't seem to find those fancy billet quality clutches available that used to be on the market several years ago.
-last and not least, 3rd gear failure. I was running hard through the s-turns and didn't get it engaged into 3rd hard enough, pops out under load like a bent shift-fork. Parts on order. Supposedly the new sets are improved. We'll see when they arrive...
Having to fix this thing after nearly every session is killing me!
is looking to get backing it in
to that mini motard fun again
Nice write up
Now you make me want to try the PMT sicks in soft
Piranha Street Sumo Lucas Oil
1 1/8 Tag Metals Alius Bars Stewart Bend blue
Mobster Supermoto Mag Wheels 12x2.15F 12x2.50R white
Dunlop TT93's 100/90-12F 120/80-12R
Piranha Race Sumo Geico
1 1/8 Tag Metals Alius Bars Stewart Bend black
Workz Supermoto Mag Wheels 12x2.15F 12x2.50R red
Dunlop TT93's 100/90-12F 120/80-12RHidden Content
I run PMT softs you will love them!
Its been in the high 90's here for the last two race days. The mediums are sticking noticeably better than the Dunlops were, and are even wearing slower then the Dunlops. I imagine the softs will be fantastic in colder weather.
Tire pressures seem to heat the tires up well at 18f-19r pounds hot (immediately after pulling off the track).
The other thing I forgot to mention was the front brakes, but I had another thread on those issues. That was all ironed out before the first race.
Hope someone who has the power reads this: Want to make better power out of this engine without too much R&D? According to my measurements, if you run the YX150 connecting rod (longer rod) on a YX160 crank (58mm stroke) and use a stock height TB Parts 64mm cylinder, the squish clearance should come in nearly perfect at around 0.030". You will need to remove the high compression dome on the TB piston to keep the engine running on pump fuel, but the current TB piston valve reliefs clear perfectly fine on a V2 head with stock V2 valve sizes at this squish clearance. This combination should net a great squish clearance, decent compression ratio for pump fuel, keep the cam chain tension happy, and allow for a slightly better rod ratio with the longer stroke, all with current off-the-shelf pieces (except the lower compression flat-top piston). Running the current high compression domed TB Parts 64mm would make for an even more ripping engine on race gas.
Last edited by Wrench; 09-15-2016 at 11:11 PM.
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