Bore & Stroke

[kstevo]

Excuse the noob question but what does the size of the bore and stroke effect... like is there a general rule that bigger first number = better/faster or a bigger/smaller ratio is better/stronger

thanks all

 

[Cactus Jack]

The bore and stroke affects the cylinder capacity ......

A big bore x short stroke can rev a lot higher than a small bore x big stroke .

A bigger stroke gives more lugging power and is better where a wide ratio tranny is used .

A short stroke works best with close ratio gears and more of them .

A short stroke engine can achieve a higher cylinder filling capacity due to lower piston speed and the resultant increased induction time ....

Here's an excerpt from a link :

[There are two ways of increasing the amount of torque generated by an engine - either increase the capacity (or, more correctly, capacity times volumetric efficiency), or increase the length of the lever arm (or stroke).

For increased power, you can increase either (or both) the torque, or the revs at which that torque is generated. Increasing stroke will increase torque, so theoretically it would be good to have very long stroke engines. The problem is, if the stroke is too long, the volumetric efficiency decreases, particularly as the engine speed increases (which is why long stroke engines don't like high engine speeds, apart from the rotating friction and harmonics).

Now this decrease in engine speed more than compensates for the torque increase, which is why very high power output engines tend to have very short strokes ]

 

[kstevo]

So does that mean that a bike with a bore x stroke of 56.5mm x 59mm would have heaps of torque but not a very high top speed and a bore x stroke of 60x53mm would be the opposite, a high top speed but lower amount of torque? but a what point does the stroke become too high, or is there no point which it becomes to high as it would not have an overall porformance drop just increase either one? and the bore and stroke is determined by the manufacturer and to increase either on would be to 'bore it out' or 'stroke it(?)'?

bore x stroke basically means top speed x torque yeah?

 

[Cactus Jack]

So does that mean that a bike with a bore x stroke of 56.5mm x 59mm would have heaps of torque but not a very high top speed and a bore x stroke of 60x53mm would be the opposite, a high top speed but lower amount of torque? but a what point does the stroke become too high, or is there no point which it becomes to high as it would not have an overall porformance drop just increase either one? and the bore and stroke is determined by the manufacturer and to increase either on would be to 'bore it out' or 'stroke it(?)'?

bore x stroke basically means top speed x torque yeah?

Yes the bore x stroke is determined by the manufacturer so you'd have to fork out for a bore kit and stroked crank to alter either ....

There is a certain conrod length to stroke ratio that's optimal for max power coupled with reliablity ... but basically .... the longer the stroke ..... the longer the rod and taller the cylinder you'd want to use to decrease rod angularity and piston thrust on the cylinder wall ..... aka power robbing friction .....

The 56.5 x 59 will be heaps snappier from off idle to just past midrange , then it'll drop off rapidly .... 8,000 rpm ??? The 60 x 53 will feel weaker down the bottom , get stronger in the midrange and hold it right thru to higher revs ....... 9,500 rpm ??? But the head is reported to let them down ......

CRF150R's would come on strong in the mid range and go off up top where the others sign off because they have a 66 mm bore x 43.7 stroke ...... = 14,300 rpm ..... and 21 HP .....

The main thing that governs top speed is horsepower coupled with the right gearing .... torque equals acceleration ...... so where the strongest torque is in the power curve is where the bike will accelerate the fastest ......

 

[Cactus Jack]

Ahhhh !!! You've got a Lifan 150 ???? In a pit bike on a dirt track with the right rider on board , they'll probably be quicker than a YX150 ....... which is what I've got as well as 140's ..... but I want to buy a LF150 ......

A long stroke motor simply can't rev as high as a short stroke motor can due to inertia and mechanical strain of the reciprocating parts .... an easy way to prove that to yourself is to move your fist back and forth a short distance in rapid succession as fast as you can then try it again moving your fist twice the distance while trying to match the speed ...... your fist will lose speed and your arm will feel the stress ....

Higher rpm = higher speed in each gear .... you'd have to go bigger on the front sprocket and make the long stroke 150 pull a taller final drive ratio to get the same top speed as a higher revving short stroke motor .....

CRF150R's are short stroke 150 cc engines , the same cc's as a Lifan 150 ...... if you get the chance ... try to race one ....

Now it depends on how good the YX 150's are tuned as stock as to how good they go ..... the Lifan has a far superior flowing head .... Over in the states they fit TB-V2 and Takegawa heads which bring the YX 150's to life ...... a short stroke engine has no advantage IF it can't breath enough air to allow it to rev .......

Either way ..... the most important thing is to be happy with what you've got ..... A good rider can win on a half decent bike on most dirt tracks ...... road racing with long straights is a different story ........

This link explains it all in excellent detail .... be prepared for a good read ....

Atlas F1 Bulletin Board - Short stroke vs. long stroke

 

[kstevo]

Thanks mate I'm begining to get a good understanding or at least some kind of understanding. Well I don't have either actually but am trying to decide what to get. I'm tossing up between an MSO 140 Pro, PitPro 150XR and an Atomik Nitrous 150 or maybe even a Pimpit Gangsta 150 (only one with a Lifan 150).

Do you reckon the YX 150s can't breath well? I'm lookin for a bike with a nice amount of power, I weigh about 85kg and keen on getting a 150 but am kinda picky so I wanna know everything I can before I make my decision. What do ya reckon of the 150 over the 140? Is there a BIG difference as some say? What type of bikes do ya have (PitPro, MSO, etc)?

The bike'll be used a bit in paddocks, a bit on the track and a little bit sneakin around the back roads So I'm kinda lookin the a bike with a bore/stroke probably quite balanced as I would like some top end but also want some good torque.

If I got a YX 150 is there anything I could do to allow it to breath better or would I have to get a different head that fits?

Thanks for the info and replys mate!

 

[cass38a]

Now with cactus short punching senario get a mate around and do this to his head. You need plenty of short sharp punches to have a knockout but you only need one big one to have the same effect. Depending on the type of riding you do both can be good and you always need to compromise revs for torque or vise versa.

 

[Cactus Jack]

Another scenario is to picture two adults riding dinkies ..... one has long crank pedal arms and the other has short crank arms ........

The guy with the long pedals takes off quicker and reaches his max pedals per minute (ppm) and top speed quickly but the short arm dinky soon catches and passes him ...... so the guy riding the long arm dinky tries to pedal faster ...... but his knees start banging into his jaw which eventually knocks him out causing him to lose control .... barrel roll and flip end for end to a stop .....

That's basically what eventually happens to the piston and rod in long stroke motors that people rev the hell out of like they're a formula 1 motor ....... and that's exactly why formula 1 and most MX 4 strokes are short stroke motors ........

 

[kstevo]

lol





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[I_THUMP]

Another scenario is to picture two adults riding dinkies ..... one has long crank pedal arms and the other has short crank arms ........

The guy with the long pedals takes off quicker and reaches his max pedals per minute (ppm) and top speed quickly but the short arm dinky soon catches and passes him ...... so the guy riding the long arm dinky tries to pedal faster ...... but his knees start banging into his jaw which eventually knocks him out causing him to lose control .... barrel roll and flip end for end to a stop .....

That's basically what eventually happens to the piston and rod in long stroke motors that people rev the hell out of like they're a formula 1 motor ....... and that's exactly why formula 1 and most MX 4 strokes are short stroke motors ........

Short arm pedals would have a low topend speed due to the pedal having to go around at a faster speed than long arm pedals when they are both going at the same speed. Is that right? Please correct me if im wrong...

 

[Cactus Jack]

Have you ever ridden a dinky ???? ....

Remember .... dinkys are like Penny Farthing bikes and have the pedal arms attached directly to the axle and the wheel is also attached directly to the axle ...

So when a wheel is rotating ..... the further you move out from the centreline ..... the faster in feet per minute the pedal moves to turn the axle at the same rpm as a pedal that's attached closer to the axle centreline (via a shorter crank arm) ....

That's why putting a smaller sprocket on the rear makes a bike go faster ........ the teeth are where the pedal would be .......

 

[I_THUMP]

oh i see, I didnt know what a dinky was.