Topic: Connecting Rods

[scondon]

 It's easy enough to start throwing a lot of money for "performance this" and "performance that" when you build an overbore engine. Headwork,pistons,boring,crank and rod bearings,primary chains,cam chain,cam,maybe some better carbs and pods to increase the top end, etc... Before you know it you're at $2500 and that extra grand for some killer rods seems like a good deal to protect all the money invested so far.

Tell me about it. The $$$ signs add up really quickly. But my MRieck engine will be well worth it. The way I look at it, I could be spending the money on useless stuff or a new $10K bike. I figure for just under $6K I’ll have a killer motor and a hell of a good time.

Sparty

   I'm with ya,Sparty It took me a year to buy all the engine parts and I had to make a decision whether to save up for another 4 months to buy rods or put together what I've got and be riding by spring. It will be going in another street bike that I would like to take to the track at some point so I decided to get the best stock rods I could and call it good.

   I've got a 900cc motor next on the block and all the time I want to put it together. If it doesn't already have strong rods(don't know what they had back in '72) it will by the time I'm done

I think I spent more time assembling my engine the all the rest of the bike.

   Funny how that works. I always get excited when the engine is finally finished 'cause I know I'm almost done with the bike. Even if the bike is a bare frame with a couple wheels

[DaveInTexas]

educate me; why would rods grow in length with hard use - isn't most of the force they experience compressive rather than tensile?

[MRieck]

educate me; why would rods grow in length with hard use - isn't most of the force they experience compressive rather than tensile?

The greatest force experienced is when the rod is yanked down on the exhaust stroke. The upstroke is cushioned by combustion .

[DaveInTexas]

educate me; why would rods grow in length with hard use - isn't most of the force they experience compressive rather than tensile?

The greatest force experienced is when the rod is yanked down on the exhaust stroke. The upstroke is cushioned by combustion .

hmm I might need to be hand-held through the otto cycle, sorry:

Assuming 4stroke, and neglecting valve overlap:

Intake: crank pulls piston down, against little resistance as a valve is open.
Compression: crank pushes piston up, against considerable resistance as combustion chamber is sealed.
Power: Expanding gasses in the cylinder force the piston down against the huge resistance of the vehicle moving forward plus whatever the requirements are in adjacent cylinders (ie another piston needs to move down to provide an intake stroke).
Exhaust: Crank pushes piston up against little resistance, as a valve is open.
Help!   ??

[kghost]

Then spin the MoFo at about 8,000 rpm 

Centrifugal force? 

[Jinxracing]

I'm no engineer, but isn't rod stress more a direct function of piston accelleration/decelleration forces rather than pure power output? Longer strokes cause higher rates of accel/decel at a given RPM, so longer strokes require longer rods as piston speeds increase...

Okay, train of thought just derailed. Perhaps one of the more educated members can expand?

[Dragman836]

educate me; why would rods grow in length with hard use - isn't most of the force they experience compressive rather than tensile?

The greatest force experienced is when the rod is yanked down on the exhaust stroke. The upstroke is cushioned by combustion .

So does a turbocharged bike have any less or more affect on this force

[MRieck]

educate me; why would rods grow in length with hard use - isn't most of the force they experience compressive rather than tensile?

The greatest force experienced is when the rod is yanked down on the exhaust stroke. The upstroke is cushioned by combustion .

hmm I might need to be hand-held through the otto cycle, sorry:

Assuming 4stroke, and neglecting valve overlap:

Intake: crank pulls piston down, against little resistance as a valve is open.
Compression: crank pushes piston up, against considerable resistance as combustion chamber is sealed.
Power: Expanding gasses in the cylinder force the piston down against the huge resistance of the vehicle moving forward plus whatever the requirements are in adjacent cylinders (ie another piston needs to move down to provide an intake stroke).
Exhaust: Crank pushes piston up against little resistance, as a valve is open.
Help!   ??

Just after the exhaust stroke/beginning of intake stroke (overlap) the rod is subjected to stretch(as it is yanked down) which creates the greatest strain on the rod and bolts. It literally wants to pull the rod and bolts apart. Sometimes it succeeds. I'm sure some rods/bolts have failed when put into compression but when you think about it the bolts aren't really stressed as the rod is acting upon the crank journal. You would have to begin to bend the rod in half for it to fail. A decent rod will not bend unless the piston hits something hard....like valves.

[bryanj]

Or in the case of Suzuki 2 strokes with vacuum tap--a cylinder full of fuel!!