Crankshaft rotation theory

[ray_ray]

In theory, would a dirt bike engine with the crankshaft rotating in the opposite direction of 99% of the engines today, help or hinder the handling of the bike of the 450 cc size?

Edited October 16, 2010 by ray_ray

[KJ790]

It would have little to no effect on handling at all.

[ray_ray]

This 'gyroscopic effect' that everyone is so concerned about would not be effected?

By no effect, you mean it will not change or at least have no ill effects on the bikes handling from an engine that has the cranking spinning in the traditional manner?

[TDW]

I think all engines have a torque characteristic, some much more than others. Some multi cylinder engines are balanced well enough that alot of the torque movement is minimized or lessened.

Most engines if not all, torque movement is opposite the direction of crankshaft rotation.

While I dont know for sure the torque movement of a single cyl thumper-I think it would have a tendency to lift the front.

That being the case-reversing the eng rotation should affect the handling characteristics of a dirt bike negatively IMO.

Edited October 16, 2010 by TDW

[highmarker]

If you read the press release on the bmw motor it has a very low and forward center of gravity. Geometry changes of a new chassis can only get you so far. So you'll have take the upsides and downsides of that weight placement. Reversing the crank rotation was a packaging necessity with the intermediate shaft running right off the clutch mounted crank, doubt it's the cause of these supposed handling effects? and hopefully they remembered to reverse the clutch crank end bolt threads!

[ray_ray]

If you read the press release on the bmw motor it has a very low and forward center of gravity. Geometry changes of a new chassis can only get you so far. So you'll have take the upsides and downsides of that weight placement. Reversing the crank rotation was a packaging necessity with the intermediate shaft running right off the clutch mounted crank, doubt it's the cause of these supposed handling effects? and hopefully they remembered to reverse the clutch crank end bolt threads!

Why would placing the clutch plates on the crank require the crank to rotate backwards with respect to a normal designed engine?

[KJ790]

This 'gyroscopic effect' that everyone is so concerned about would not be effected?

By no effect, you mean it will not change or at least have no ill effects on the bikes handling from an engine that has the cranking spinning in the traditional manner?

Nope, if all you did was reversed the direction that the crank rotated, the gyroscopic effect would remain the same, as that affects movement perpendicular to the crank's rotational axis. The only thing that would change is the torque effect induced by the engine's acceleration, which is so minimal on these small engines it wouldn't be noticed while riding it. On large car and truck engines this torque effect can be rather large however.

[TDW]

Alot of this conversation will have to be speculation because of no factual information or measurements about the torque effect produced from a 450 thumper.

But the fact still remains that there is a torque movement produced and that combined with a high revving engine and the lightweight, narrow frame of a motorcycle that spends time flying thru the air makes its effect an important consideration.

[ray_ray]

True and that would be a different case as apposed to turning on the ground and there has been remarks about the air time of such a configuration also .....

[KJ790]

Alot of this conversation will have to be speculation because of no factual information or measurements about the torque effect produced from a 450 thumper.

But the fact still remains that there is a torque movement produced and that combined with a high revving engine and the lightweight, narrow frame of a motorcycle that spends time flying thru the air makes its effect an important consideration.

Yes, in the air is where the most noticeable difference would occur. On the ground it wouldn't make a huge difference since the torquing of the engine is front to rear, just putting more force to one wheel of the the other depending on which way the engine is spinning. Add to that the fact that when under load on the ground the engine does not accelerate fast enough to cause a torque effect, unloaded in the air would be where it could be noticed. However, this would be coupled with the torque effect from the rear wheel (which is why panic reving works in the first place), and would likely be over shadowed in most cases by the wheel's effect.

[TDW]

Just a guess here but you might see bikes cartwheeling thru the whoops more frequently with a reverse rotation motor:foul:

[The Spanky]

This conjecture about the direction of the crank rotation seems pointless to me. The rotation is still perpendicular to the chassis, so it doesn't change the gyroscopic forces transmitted to the chassis. In addition, you have other shafts (balance, tranny) that are rotating opposite of the crank, partially if not completely cancelling the effect.

The "slow handling" is chassis related.

[grayracer513]

The gyroscopic effect would not be noticeably altered in any way, since the direction of rotation is not significant, only the axis, speed, and mass. What would be altered is the direction of the torque reaction of the engine assembly.

To address one question first, there is no reason whatever that the use of a crank mounted clutch constrains the direction of the engine in any way. If three geared shafts are use as is typical, the crank will necessarily rotate forward. If the primary drive is a chain running to a transmission consisting of two parallel shafts, the engine must rotate in reverse. Likewise if for some reason it would have a 4th gear driven shaft. It's just a matter of layout. The old Yamaha YD/YDS3 series had a crankshaft clutch and a three shaft layout rotating the engine forward.

Any time a rotating assembly is accelerated by a torque force, the component being accelerated resists being accelerated, and a force equal to that resistance is transferred to the rest of the vehicle. This can be calculated quite accurately because Newton has already established that the reaction will be equal to but opposite of the active force. So a typical 450 with the forward rotating engine producing 30 ft/lb of torque on a dyno will apply a reactive force of 30 ft/lb against the frame, attempting to rotate it backward.

Now, consider the chassis and in particular the driving wheel. When the total machine attempts to rotate the wheel forward against any load, the wheel resists it due to its own inertia and the load presented by the weight of the bike. The reaction then is for the entire bike to try to rotate around the axle.

In the air, the bike becomes a free body. If the throttle is suddenly opened form a low engine speed in gear, there will be two separate torque reactions, one a reaction to the engine against the frame, and the other a reaction to accelerating the mass of the rear wheel against the chassis as a whole. IMO, the more significant of these is the reaction of the rear wheel because it has a greater inertia mass.

Remember that the torque reaction will only be as great as the applied torque, which in turn can only be as great as the resistance offered by the rotating mass. In the air, there is no driving load other than the inertial resistance of the rotating assemblies. If both the rear wheel and the rotating assembly of the engine were equal in that regard, the use of a backward rotating crank would cancel the inertia of the rear wheel, and the rear of the bike would not drop in the air when the throttle was opened. However, as I said, I believe it will come out that the inertia mass of the rear wheel is greater than that of the engine, owing to the much larger diameter, and that the drop of the rear end will still happen.

[TDW]

In theory, would a dirt bike engine with the crankshaft rotating in the opposite direction of 99% of the engines today, help or hinder the handling of the bike of the 450 cc size?

I saw the other thread and some of that discussion has been brought over to this one-because one is related to the other to some degree.

I was mostly responding to the simpler question of does reverse rotation of the crank effect the handling of the bike.

So I think we all agree-for an expample-that if a 450 engine was say hanging by a cable, level by its own weight and running and then accelerated it would tilt front to back.

If thats true then I dont think it would be a good choice to use reverse rotation motor set up. It doesnt have any positive attributes in this aplication.

If it was as good or better in any way the factories would and could easily change the rotation of the engines.

[The Spanky]

I saw the other thread and some of that discussion has been brought over to this one-because one is related to the other to some degree.

I was mostly responding to the simpler question of does reverse rotation of the crank effect the handling of the bike.

So I think we all agree-for an expample-that if a 450 engine was say hanging by a cable, level by its own weight and running and then accelerated it would tilt front to back.

If thats true then I dont think it would be a good choice to use reverse rotation motor set up. It doesnt have any positive attributes in this aplication.

I still don't agree with your synopsis. In the dangling-bike scenario you mention, the mass of the spinning wheel will have a much greater effect and will completely mask any effects from engine rotation. I don't believe the direction of crank rotation has any effect on the handling that you can feel. If the effect was strong enough to change the handling characteristics of the bike, you would be able to sit still and watch the forks compress when you rev the engine and decompress when you let off the throttle. Obviously that doesn't happen.

[TDW]

Its a dangling-engine scenario, that would illustrate plainly the torque reaction of the engine alone when it is accelerated.

If changed the two would counter act each other, in normal rotation the spinning rear wheel causes the front end to want to stay up, and the (even slight) torque reaction from the engine helps cause the front to want to go up or stay up also. So reverse rotation would cause alittle more force trying to push the front wheel down, as slight as it might be, it would make a difference in the wrong direction.

[Pguard]

The gyroscopic effect would not be noticeably altered in any way, since the direction of rotation is not significant, only the axis, speed, and mass.

A centrifugating washer without stabilizers but with 2 lbs of clothes turning to the left wouldnt tend to move to the left?

[The Spanky]

Its a dangling-engine scenario, that would illustrate plainly the torque reaction of the engine alone when it is accelerated.

If changed the two would counter act each other, in normal rotation the spinning rear wheel causes the front end to want to stay up, and the (even slight) torque reaction from the engine helps cause the front to want to go up or stay up also. So reverse rotation would cause alittle more force trying to push the front wheel down, as slight as it might be, it would make a difference in the wrong direction.

I am not denying that there is some slight amount of torque input into the frame from engine rotation. I just don't believe it's enough to be of any significance. I don't believe it's even enough to be felt by the rider.

[The Spanky]

If engine torque could affect the chassis attitude in any significant way, why don't BMW boxer twins or Gold Wings try to lean you over sideways when gassing them while riding? Because the tiny effect of the engine is completely masked by the much greater effects of things like spinning wheels.

[Pguard]

If engine torque could affect the chassis attitude in any significant way, why don't BMW boxer twins or Gold Wings try to lean you over sideways when gassing them while riding? Because the tiny effect of the engine is completely masked by the much greater effects of things like spinning wheels.

but spinning wheels can't mask the gyroscopic effect of the new Husaberg engine