Engine design

[Uniflow]

What about a reverse rotating crankshaft? how does it affect handling on a dirt bike? GP bikes do it. Does the counter rotating balance shafts on the four strokes help with making the bike fell lighter, help cancel out crank gyroscopic effect?

[cnrcpla]

I have heard it affects handling on road bikes, like the heavier it is the more you feel it, I guess it makes the bike feel heavier. And I would assume the same goes for motocross bikes. Also, I do think it helps to take some vibrations away while the engine is running, but don't quote me on that last part.

[factoryX]

Counter balancer/Heavy flywheels soaks up the vibrations by slowing down the rpm climb.  Modern 4t's have both due to large pistons and short strokes.

[Jeram]

Correct me if Im wrong, but as I understand through visalising the physics of it all in my head, the crank rotation direction will only effect handling when under braking and acceleration, that is the corner entry and the corner exit.
The speed at which it tips in is not effected as the amount of gyroscopic energy is not altered, its just going the other way, it just simply makes the bike more stable when tipping in

in my opinion:
under acceleration would I be correct to visualize that the reverse crankshafts acceleration would cause the front end to squat ever so slightly? (ei: would help keep the bike balanced exiting turns)

while under deceleration the decelerating crank would make the front end lift slightly which would make turn-in a little more stable

That theory makes sense when you picture a BMW boxer engine being revved in the driveway, it tips to one side!



As for balance shafts, I dont know why there not used on more enduro two strokes!
The last counterbalanced 2t single cylinder motors that I can recall were the TRX250 and CRM250 hondas, both of which are very smooth with the exception of a slightly tingle in the footpeg which is felt at 10,000 rpm/100mph on the CRM250 on asphalt

[Uniflow]

I only ask because I've got the opportunity to run this new engine reverse with a balance shaft. Keeps the exhaust away from damage. Makes sense, it allows me to stuff the cylinder up against the forward frame down tube ( YZF 250 frame ). Rotary valve is side induction, no carb ( EFI with flat slide throttle ), so no reeds or float bowl to get in the way.
At the moment it's going to be a 350, reverse crank and cylinder, balance shaft, variable rotary valve, water cooled, power valve exhaust, five speed KX 500 gearbox and clutch, run on ethanol   ( E90), running on ethanol you could almost make it air cooled, at least lose half the cooling system and go back to one radiator. What else have I missed?
Nothing has been made yet, just thinking.

[TMKIWI]

The only thing I can think of is the crank inertia regardless of how small will have an effect on turn in.( deceleration)
Instead of squat on the front the bike will squat on the rear.
That may be an issue for a rider to get used to.

[scotty dog]

i thought it would make the bike go backwards.......that would be interesting

[Jeram]

yeah itl be fine,

your biggest issue will be the backwards facing pipe!

will be hard to package a pipe of that length and size into a MX chassis.

just an estimation, the optimum pipe will need to be 1.2m long and around 100-120mm in diameter at the belly!

so you'd probably need a custom subframe so that you could send it directly out the back, then have it turn 180 degrees at the stinger so the silencer faces forwards like on scooters.

A perfectly straight pipe will earn you more than a few ponies on the dyno in comparison to an MX style curved pipe

[SachsGS]

Due to their chain primary drives early Maicos had reverse rotation engines and are considered among the finest handling dirt bikes ever. I own a few of the early Maicos and they have a light, effortless quality about them that you just don't get with a modern bike. The 78 thru. 82 Maicos are chain primary with a 3rd jack shaft to allow conventional rotation of the motor.

The early Gas Gas enduros had balance shaft motors.

[Uniflow]

Back in the 70's I had a friend with a KX 450. The key on the crankshaft was partial shawn so timing was around TDC, it would backfire at idle sometimes and run backwards ( piston port ). One day at a large MX event with the KX entered it was surprising to see at the start everyone shoot forward with one KX 450 shooting out of the start gate backwards! I couldn't believe my eyes,we didn't know any better.

[Uniflow]

 One radiator will be gone, this will help a little for space. Perhaps the airbox could fit there and help leave room out the back, a little bit YZF 450 style.

[Uniflow]

I imagine balance shafts are not used as they do soak a little bit of power ( added bearings etc ) and add a little weight. Most 2T enduro bikes seem to be MX based ( no balance shaft ). I don't think a small power soak to drive the balance shaft on this 350 will be a problem. It will also help with fatigue on the alloy frame over time. I wasn't going to fit one originally but I've been working on another project that has ended up needing a balance shaft to stop excessive vibration, a parallel 700 twin. It's so much nicer to operate when it's smooth, better on personal fatigue.

[SachsGS]

Balance shafts allow for lighter frames so I think , weight wise, it's probably a draw in the end. That balance shaft would draw power though.

 As a kid it was always exiting to ride the old piston port trials bikes. Imagine stalling at the top of a very steep section and, as you are scrambling to find your balance, the bike would slip backwards, fire up in reverse and back down the hill you go!

[Uniflow]

What about taking the stinger off the mid section of the pipe?

[Uniflow]

Sachs, I think you are right. Opposite turning crank through a balance shaft will be best. I've had a bit of a measure up for the exhaust, yes it will be tight. Yamaha has put the shock in the wrong place, what were they thinking?