Engine balance

[Chris Walby]

Something Neil Tidey mentioned about engine balance and what motorcycle speedway guys did a few years back.

 

I understand that the crankshaft/flywheel is balanced with the weight of the piston and conrod and get the bit Neil mentioned about unbalance when the crank pin is at the 3 and 9 o'clock positions. 

 

The nagging thought, when the motorcycle speedway boys moved from vertical mounted engines to inclined to about 30 degrees from horizontal I am sure it was commented that the engines needed to be rebalanced. 

 

Now the question, does it make any difference which orientation an IC engine is mounted (vertical, inverted or side on) to the overall balance/vibration and does that translate to V engine inverted or even on its side?

 

Could always expand this into firing order and crank pin positions if you want and does it really make any difference to the prop/airflow.

[Nigel R]

Presumably the speedway case also involves balancing torque effects on the chassis to maximise grip (or perhaps minimise it, IIRC the big bang moto gp designs - sort of - lowered grip using firing order/timing such that the breakaway was more gentle).

 

Not sure we'd see much effect on airflow for a single being oriented differently.

 

Firing position for twins, minor effects maybe given the different spread of torque pulses?

Edited April 7, 2022 by Nigel R

[J D 8]

 On a speedway bike the drive is set at 90 degrees to the direction of travel, crank shaft across the frame to take the dive via the chain to the wheel.  On a model the plane the crank shaft is in line with the direction of travel and that is where the difference is. On the model the point of unbalance would be some where in the prop arc no matter which way up the engine is fitted.  On the motorbike there may be some advantage of having the 30 degree angle due to the way the drive is taken off but this is just a guess.

[Paul De Tourtoulon]

If you want the answer ride a 4 cylinder inline motobike then the Vee Guzzi,,,,

[Kim Taylor]

I would think that the advantage of having a 'laid down' cylinder on the 'bike would be getting the c of g lower, can't think of anything else that would make a difference.

The difference between an across the frame crank and a fore and aft one (a la Guzzi) in a bike is very noticeable when standing still and revving the motor, not so much in (normal speed) riding. I do seem to recall that the inline four racers used a crank which rotated the 'wrong' way, to use the gyro effect of the crank to lessen the tendency to wheelie, although I don't know if this was an experiment that failed, or if it stuck.

Kim (currently a Moto Guzzi owner).

p.s. none of the above has any bearing on the OP, so sorry for that.

[Chris Walby]

Thanks guys and from a engine balance perspective it seems to make no difference for engine orientation on a model. No impact on prop or airflow so its just down to vibration (live with a single) and overall sound.

 

IMO radials always sound nice, but the multi cylinders are a more practical/affordable route

 

PS on that point, come of Ron a couple of engine tests and flights with the Tigercat please?

 

PPS Kim I think you must be close with the C of G thing as they have controlled tire use so getting mass as low as possible would help, although from experience accelerating flat out towards a fence never became comfortable with me!