# Why aren’t drive chains completely taut?

I don’t understand why roller drive chains in motorcycles have some degree of slack (around 1 inch movement) in them? What’s wrong with having a completely taut chain with no slack?

Also, does the amount of slack in the chain affect transmission (power delivered) in any way?

• Less strain on all of the driveline, and it smooths power delivery (to an extent). Under drive, the losses should be completely negligible. They only time the slack will really be noticed is under transient load changes. Commented Nov 10, 2017 at 18:06
• How does slack smoothens power delivery? Commented Nov 10, 2017 at 18:12

The main reason slack is needed is the rear suspension of the bike. In a "perfect" setup, the axis of the rear swing-arm and the final drive of the engine would be one and the same. This is not normally possible so a compromise must be made. In this diagram, you can see the two different axis.

Because of this, on a typical street bike, the chain has more tension when you are sitting on the bike and the rear suspension is slightly compressed. A common mistake is to make the chain tight when it's on a stand and then ending up with a chain that is way too tight once you're riding.

Chain and suspension geometry is a pretty complex topic. The diagram above is from this site which also has a lot more technical information.

https://www.sportrider.com/more-fun-geometry#page-3

Another reason for the slack is the physics problem of why you can never pull a rope across two points to make it perfectly straight. Gravity is going to make the chain sag and appear to have some slack. The amount of force needed to make the chain appear to be tight and slack free would be detrimental to the wheel and final drive bearings. There's no benefit to tightening out all of the slack - only increased wear.

• Remember, the drive side will be under tension when the vehicle is accelerating.. Commented Nov 11, 2017 at 3:13
• Additionally supporting this reasoning, is on single-speed bicycles, where there is no suspension, the chains are tight. Commented Nov 11, 2017 at 5:20
• @whatsisname unless the chain is worn and haven't been tightened for a while. Commented Dec 18, 2017 at 17:50

its a balance between responsiveness, stress/strain, and efficiency.

very slight slack can act as a kind of dampener in the power delivery kinetic chain.

too much slack can cause too much impact and hurts responsiveness.
can result in things coming off from being too loose.

too little slack and the parts will be too constrained, causing lots of stresses/strain and excessive friction.
too much stress can also cause deflection which can cause misalignment as well.
too much stress or friction accelerates wear and deflection/misalignment can increase possibility of something coming off.

thats just off the top of my head so probably missed some points.

• During the drive, I think only the upper half of the chain is in tension as it is the one that’s doing all the pulling of the rear wheel’s sprocket. The lower half, However plays no role in transmission ever so it can be as slack as long as it doesn’t fall off. The amount of slackness then shouldn’t affect the responsiveness of the transmission as the upper half would always be in tension during the drive. Commented Nov 10, 2017 at 19:10
• You say that the chain being taut introduces more friction into the scene. How does that work ? Commented Nov 10, 2017 at 19:12
• The force required to make the chain taut has to come from tension provided by the structure that holds the gears. The extra tension will automatically increase friction. If it doesn't increase friction, then you're getting tension out of nowhere. Commented Nov 10, 2017 at 21:58
• @user115962 - You also need to consider engine braking, as during this time, the other side of the chain will be in tension. When the engine goes from power to braking, transition occurs as to where the slack is found. Commented Nov 11, 2017 at 0:12
• most of the effects I glossed over are mainly apparent during initial application of power, change in direction of movement, or other significant changes in velocity/acceleration. Basically: times when the chain loses tension or starts off with no tension (aside from the normal chain tension) and comes under tension (again). During the drive and at near constant speed or acceleration, the tension will stay more or less the same and that type of behavior will not readily show itself. More than likely, you will not stay at constant speed or acceleration for very long in the real world. Commented Nov 18, 2017 at 0:21

If the chain is too tight then it loads the bearings : and like any bearing subjected to load , excessive loading can actually cause the bearing to lock or seize.

Not much to contribute here, but nobody has said it. If chain is too tight. it will eat out the sprockets very quickly and of course the chain itself as well. Gotta always have a little slack in any chain drive application.