Engines: Torque Lost To Friction

[maico760]

I have been searching for explanations and formulas on this subject but have been hard pressed to find what I am looking for unfortunately. I would be appreciative if anyone could either provide some answers or any external resources they could point me to which has detailed information on this subject. I do not have any background in Physics but I do have some Math background.

Let me setup the stage first before I ask my question. This is just background so no time is lost in any misunderstanding of what I am asking.

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The small combustion engine generates a force on the piston forcing it to a downward motion. The piston is connected to a rod known as a "connecting rod" and this is attached to a particular location on the crank. The force will cause the crank to rotate and this generates varying levels of torque (torque = length x force) at the crank with varying speeds (RPM).

Generally the engine will transfer both the speed and torque to the rear wheel through three levels. First is the primary drive, which will consist of a gear at the end of the crank and a large gear attached to a clutch. The clutch allows the engine to vary the speed/torque transfer at any RPM through the transmission, however in this case let's assume it's never engaged.

The gear ratios are used to do two things which is to transfer both torque and speed to the rear wheel. Small engines generate low torque at high speeds so we need to lower the speed and raise the torque. This is where the gear ratios come into play. The primary drive will consist generally of a smaller gear on the crank and a larger gear on the clutch. Torque will be magnified and the speed will be lowered based on this ratio.

A transmission does the same except it has several rows of gears to adjust what action it takes as it depends on the current gear. The torque can be raised and speed lowered in lower gears while it can raise speed and lower torque in higher gears.

The final or secondary drive is what attaches to the end of the motor and to the rear wheel. Depending on the type of motor and the output of torque from the engine the ratio will vary. Generally, the ratio is to lower speed and raise torque to the rear wheel.

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Now, here is my question. The gear ratios which multiply the speed of the engine are fixed. I.E. for any RPM I can know the speed of the output shaft. The same is NOT true for torque, correct? Torque is lost through friction and this I know can be very complicated depending on the environment / lubercats, etc.

I am however looking for general information or formulas for torque and have a few specific questions.

1. When gears change direction, is torque lost? Any general idea of how much?

2. Given the type of setup between gears, what is the relationship to torque loss?

i.e. A primary drive with a Chain Setup vs. a Gear Setup. The chain setup will rotate the same direction. I have heard that chains transfer torque better than gears, why is this, is it due to the direction or is it the contact of the gears making less friction than the contact of teeth?

Is there any generic method of torque loss (i.e. ~2% for chain and ~5% for straight cut gears)

3. Does the weight of the gears affect torque transfer? Is there a formula?

NOTE: I realize some will likely be lost to friction of the bearings in the engine. I am more interested in Chain vs. Straight Cut Gear setup, especially as the primary drive setup (which would be a short chain than used on a secondary drive setup).

Any information or resources are helpful.

 

[Ranger Mike]

1. torque is not " lost" when changing to reverse gear..it is the same " amount" just in reverse direction and it depends upon reverse gear ratio in gear box..
2. torque is lost due to parasitic drag or friction when trandsmitting torque to final drive...gears are notroriuosly torque sucking. just compare the surface area of gears versus that of a true roller chain.
3. gears due weigh a lot more than chain and this is rotatin weight, and there is a lot of this..transmission gears are stacked onto a gear cluster...chanis will " stretch" and eventually break...a chain is only as strong as its weakest link..

 

[maico760]

So the answer in the Chain vs. Gears is surface area is more with gears than with the chain, so that is why a chain is better transfer of torque than gears?


Chain Driven Primary Drive
[PLAIN]http://www.maicowerk.com/Images/Maico/1982490/Legacy/clutch_large.jpg

Gear Driven Primary Drive
[PLAIN]http://www.maicowerk.com/Images/KTM/1984495/Legacy/clutch3.jpg

 

[john.phillip]

I never saw a motorcycle engine with a chain driven primary. From which bike is the first picture ? What is the maximum RPM ?

 

[maico760]

The first picture is my 1982 Maico 490. The second is my 1984 KTM 495. Maicos prior to 1983 had primary chains. Older bultacos also had primary chains.

They require more maintaince. (see video below)



This is why I am asking the question on the advantage of it, I know the disadvantage as you can see above :)

 

[john.phillip]

Usually transmission chains should not be applied where the chain link will achive speeds beyond 25m/s (56mph), so for motorcycle engines capable of +6000rpm a geared primary is more common.

If the sprocket is 80mm in diameter and the engine achieves 6000rpm the chain speed will be close to: v = w x r = (6000 x 2 x pi / 60) x (80 / 2 / 1000) = 25.1m/s.

Even if you work with a smaller sprocket, you can reduce the chain speed but that will influence the primary and secondary shafts center distance, the number of links simultaneoulsy in contact with the sprocket teeth, demanding a double chain, and the size of the secondary crown to keep the primary ratio constant, making the design more dificult than simply moving to a gear mesh.

KTM ? That explains the straight-cut gears.

 

[john.phillip]

Another interesting point is the engine rotation direction that will be reversed depending on whether you are using a chain driven primary or a gear mesh.

 

[maico760]

Yes, the Maico will achieve 7000 possibly 8000 max RPM. The bike can do upwards of 80mph.

These bikes were built for racing. The early 70s had 3 rows of primary chains while the later models, like mine, have 2 rows of chains.

That is right that the mainshaft and the crankshaft will be rotating in the same direction (one reason I asked about the torque loss in direction change, I found someone online making that statement so wanted to verify).

The transmission of the Maico has an extra shaft to reverse the rotation.

[PLAIN]http://www.maicowerk.com/Images/Maico/1982490/Legacy/removegeartwo_large.jpg

The maico as you can see has a small ratio setup for the primary, the usually have larger rear sprockets than Japanese bikes use. Even my 1983 which has straight cut gear primary drive is still geared high, I put a 58 tooth rear sprocket and 12 tooth front sprocket to slow the bike down for tight trails and the bike will still do 60mph top speed.

Is there an equation for torque loss to friction for gears vs. chains? I've only ever heard that chains loose less torque but never seen any equations or figures or reason behind this statement.

 

[john.phillip]

The speed I'm talking about is the primary chain linear speed, not the final bike speed, just to make it clear in case there is any misunderstanding. The chain maximum linear speed is related to the engine maximum rpm and primary sprocket size. Above 6000rpm is recommended to swap the transmission chain by a gear mesh.

Another picture and it becomes more interesting, I can see that is a steel plate above the gear selector fork, with three grooves in it instead of the traditional drum. I would like to see how it works.

It seems the gear mesh is not synchronised, is it ?

I don't know why there are three transmission shafts beside the crankshaft, they could have made the engine crankshaft to rotate in the opposite direction. By adding an extra shaft the efficiency of the transmission will be lower than the traditional 2 shafts + crank, maybe this is the statement you read online.

An extra shaft could provide a better speed reduction giving more torque to the bike, or compensate the rather small primary ratio due to the chain configuration. Dirty bikes usually have primary ratios between 2.5:1 and 4:1.

You may try calculating gear mesh efficiency directly by the method described on the following link, but keep in mind that the presented method ignores other effects related to the gear material behavior under stress, heat, lubrication efficiency and mesh speed. You can take it as an ideal scenario.

http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html

At this link you can peek how this subject can get more complex:

http://www.geartechnology.com/pa/members/sepoct05/section4.pdf

Efficiency of roller chains suffer from the same complexity, under ideal conditions they can be as high as 98% to 99%. So, it is hard to point out which solution will give the best efficiency as gear mesh also can get as high as 99% under ideal conditions.

 

[maico760]

Yes, the extra gear on the drive shaft can be used to further reduce and I believe this gear is different between Enduro and Motocross so they do use it to tune.

The one reason I have thought of why they may not have gone in the reverse direction though is that the engine rotating backwards the force of the engine would be going in the opposite direction of where the bike would be going, possibly causing some strange handling on the bike .These bikes were known to handle well (the bikes are known to be able to turn corners very easily at high speed) and one of the myths is that part of the reason is due to the engine / clutch spinning in the same direciton, but that's another story.

You can click 1982 Maico 490 GS button to see the setup.

http://www.maicowerk.com/Tips/SpeedCalculator/


The primary drive ratio on this machine is 39 Tooth / 21 Tooth.


The gear mesh works the same as the drum in principal. The shifting forks sit in the plate and the pattern on the plate moves the gears together or apart so the gears move with the same / free spinning on the shaft are connected or disconnected.

The setup to actually move the plate though is done by a gear though.

[PLAIN]http://www.maicowerk.com/Images/Maico/1982490/Legacy/shiftingcam2_large.jpg

[PLAIN]http://www.maicowerk.com/Images/Maico/1982490/Legacy/shiftingcam3_large.jpg

[PLAIN]http://www.maicowerk.com/Images/Maico/1982490/Legacy/shiftingcam_large.jpg