Swing Arms
Your anti-squat linkage
By: James R. Davis
It amazes me how misinformation and folklore spreads and soon becomes 'fact'. For example, no doubt you have heard that if your rear wheel is driven with a chain then when you accelerate the rear-end of the bike will squat while if the rear wheel is shaft driven it will rise. You may also have heard that you cannot do either a 'wheelie' or a 'stoppie' if your bike is shaft driven. WRONG!
Observe the diagram above. It shows a rear wheel and its swing arm. The swing arm, you will note, is joined to the bike frame via a pivot which is higher than the hub of the wheel. (This configuration is normal. There are a few bikes which do not even have a swing arm and fewer still that have the swing arm connected to the frame at a point lower than the wheel hub.) Wherever the swing arm attaches to the frame, it is behind the bike's Center of Gravity.
Whether the motorcycle is chain driven or uses a shaft, the connection of the rear wheel to the frame of your bike is as described. Your rear suspension system connects the frame of the bike to the rear wheel by attaching one end of your shocks to the swing arm. (Let me make it clear that I will be talking primarily about shaft driven motorcycles but chain drives are VERY similar. I will describe the subtle difference at the end of this article.)
So let's look at what happens when you accelerate. This diagram shows that as the wheel turns (clockwise) it moves to the right. In doing so it exerts a force to the frame through the swing arm. Since that arm is angled upwards, the force exerted is divided between a forward component and an upward component - it is NOT simply pushing the motorcycle forward. As a result, the motorcycle moves forward and, because the upward force is behind the bike's CG, the rear-end of the bike moves UP. You will also observe if you look closely that as the angle of the swing arm increases, the wheelbase decreases. THAT is the significance of an upward pointing swing arm.
Now a mental exercise ... what would happen if you tried to accelerate and the tire was not allowed to rotate? Clearly the only thing that could happen is that the swing arm (and thus the bike's frame) would try to rotate counterclockwise. This, of course, is true even if the tire does rotate. That is, traction at the contact patch would push back against the tire with exactly the same force as the tire pushes against the road surface from acceleration, regardless of whether the swing arm is pointing up or not. For any given length of swing arm, the higher the frame pivot point is above the contact patch, the greater will be the lifting of the rear end from acceleration. In other words, whether that pivot point is above or below the wheel hub, acceleration must result in the raising of the rear-end of the motorcycle.
On the other hand, weight transfer attempts, in all cases, to try to lower the rear-end during acceleration. Weight transfer is a function of how high the center of gravity is compared to the length of the wheelbase. The ratio of height of swing arm pivot point to how far forward it is of the rear wheel hub (sometimes called the chain angle) is always greater than the weight transfer ratio and THAT is why the rear end of the bike lifts during acceleration.
As mentioned earlier, chain driven motorcycles have almost exactly the same layout as shaft drives, but there is a slight difference in rear-end-lift that results.
The top of the chain and the swing arm point to an imaginary 'pivot' that is slightly forward of where the swing arm actually pivots. Thus, the chain angle (c2) on a chain driven motorcycle is slightly smaller than the chain angle (c1) on a shaft driven motorcycle and, thus, has a somewhat smaller tendency to lift the rear end during acceleration. The was particularly true with early shaft drive designs and led to some observers making the ridiculous claim that it was because shaft drives tried to 'ride up' on their pinion gears.
The top of the engine, particularly on chain driven bikes, tends to torque towards the rear of the bike when accelerating. This, at once, tends to compress the rear shocks and extend the front ones. This is rarely sufficient to offset the effect of the swing arm described. That is, the decompression of the rear shock caused by the vectored force derived from the 'chain angle', if it is greater than the weight transfer angle (almost always true), is greater than the compression of those shocks by the weight transfer or engine torque. This means, if you are paying attention, that the ENTIRE bike will rise as a result of acceleration, not just the front-end. And that means that the CG rises as well.
So why should anyone care?
Because just as it is almost always true that the rear-end will rise when you accelerate, the rear-end will squat when you use the rear brake. This, because the chain angle remains greater than the weight transfer angle. And, on normal swing arms, when the rear brake is used the swing arm is pulled level with the road (i.e., front end of it will move down.) This lowers the Center of Gravity and lengthens the wheelbase at the same time. In other words, it makes the bike easier to control. It becomes more stable.
You want to make the job of riding slowly easier? Use the rear brake.
As to 'wheelies' and 'stoppies', both are accomplished as a function of how high the CG is relative to the bike's wheelbase (which defines the weight transfer angle), not whether the bike is chain or shaft driven. [I assure you that the GoldWing below is shaft-driven.]
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)
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