V11 Sport reaction rod: compression or tension?

[mistercristo]

Hello all,

 

I have what I hope is a simple question: under acceleration, is the rear end rotating towards the front of the bike, compressing the reaction rod, or is it rotating towards the rear, stretching the reaction rod?

 

My semi-familiarity with suspension engineering tells me that since shaft-driven bikes want to raise the rear of the body under acceleration, the pinion is trying to "climb" the ring gear, forcing the swing arm down while rotating the rear end housing forward, which would put the reaction rod into compression.

 

If that is indeed the case, would the stretching (tensile) forces be marginal under braking as I suspect?

 

I would greatly appreciate any and all input.

 

Thanks.

 

P.S. I'm sure most of you know that the drive shaft turns CCW, but I just want to add that.

[68C]

I do know the older simple swinging arm models (T3 and early LeMans etc) lift the rear of the bike as you accelerate as the swinging arm is pushed down forcing the tyre onto the road. This is one of their nicer characteristics adding to that sense of 'oomph' as you pull away.  Of course the down side is they tend to lift the rear wheel if you roll the throttle off quickly, a bit dodgy if you do it mid corner - probably the reason for the various parallelogram systems fitted to the later Guzzi and BMW bikes. It also simply added the ability to have floating rear brakes although that is really only vital on off road bikes.with their long travel rear suspension.

 

As a younger chap I remember sitting at traffic lights and making my T3 bounce up and down by holding the front brake on and half feeding in the clutch, releasing it and then feeding in again repeatedly.  "Only I can control this savage beast!"

[helicopterjim R.I.P.]

How about this .... the wheel is turning clockwise when looking from the right hand side. Therefore the housing is trying to rotate in the opposite direction (counter-clockwise) to impart a clockwise rotation upon the wheel so the reaction rod would be enduring an extension force in order to hold it steady. Braking forces would then be imparting a compression force upon the reaction rod.

 

You can sleep soundly tonight knowing you are not stretching your reaction rod every time you operate the rear brake.

[mistercristo]

How about this .... the wheel is turning clockwise when looking from the right hand side. Therefore the housing is trying to rotate in the opposite direction (counter-clockwise) to impart a clockwise rotation upon the wheel so the reaction rod would be enduring an extension force in order to hold it steady. Braking forces would then be imparting a compression force upon the reaction rod.

That was my alternate theory.  You're saying that the ring gear is trying to "climb" the pinion?  That seems like the tail wagging the dog.  Keep in mind that the driveshaft is turning CCW (as seen from the rear) with the ring gear on the left of the pinion, which would seem - to me, at least - to impart a "downward" (forward-rotating) force.  Any empirical data, or is that also a thought experiment?

 

You can sleep soundly tonight knowing you are not stretching your reaction rod every time you operate the rear brake.

Passing up an easy joke, I really have no concern about what the reaction rod does under braking, as the forces involved are relatively low, so no sleep lost there.  What is keeping me awake at night is not knowing what happens to the bevel box under acceleration, which was my fundamental question.  Also, I wouldn't be concerned with any of this if I was using a factory reaction rod.

[GuzziMoto]

For every action there is an equal and opposite reaction. When the rear bevel drive box is try to accelerate the rear wheel that principle says that the opposing force is trying to rotate the bevel box in the other direction equal to the torque being applied to the wheel to rotate it forward. The whole deal about the pinion climbing the ring gear is a separate force and that force is not as much a factor as it is not a rotational force. When Guzzi added the floating bevel drive with the second U-joint it mostly took that force out of the equation. That force causes the rear to extend under acceleration when you do not have the floating rear drive and second u-joint as the swing arm sorta acts as a lever. It does not do that when the drive shat has the second u-joint.

[andy york]

Me Thinks the reaction rod's main purpose is to keep the orientation of the bevel box in relation to suspension travel. 

I don't think they want any change in the angle of the drive shaft going into the box other than at the u-joint.

The old BMW's and Guzzi"s tend to rise in the rear on acceleration, then when you got to the corner and slammed the throttle shut 

you ran out of ground clearance.

And I'm sure also that it is a lot more involved than above    lol

[luhbo]

Guzzi uses 2 U-joints ever since they brought the V2 to the market. The difference between now and then is that on the former bikes the reaction moment went straight into the swing arm (wheel accelerates right -> swing arm turns left), nowadays the counter moment or reaction force is balanced at the frame (and it's tension, as correctly stated already by HeliJim).

The different kinematics of course demand different locations of the U-joints, independendly of any moments or reaction forces.

 

Hubert

[docc]

. . . (and it's tension, as correctly stated already by HeliJim).

Learn something here every day!