The geometry at the rear of a motorbike is slightly less complex than that of cars due to one main linkage connecting the rear wheel to the chassis in the form of the rear swing arm. The main aspect of rear geometry that affects the handling, response and feedback of the bike is anti-squat geometry. Squat is the amount that the rear suspension compresses vertically under acceleration conditions. The more powerful a bike is, the more that the effects of anti-squat have on the handling due to the increased levels of acceleration provided by the more powerful bike.
Anti-squat geometry battles the need to squat when power is being applied by trying to extend the rear swing arm under acceleration. A way to visually represent this is if you put the front wheel of a bike against a wall and began to bring the clutch up (engaging the rear wheel) the rear will try to lift upwards. Therefore, this counters the weight transfer that would move towards the rear under normal accelerating conditions. The anti-squat force is generated by two main forces.
The first is created by the driving force of the rear tyre which tries to extend the rear suspension if the swing arm slopes uphill. This force is therefore controlled by the angle of the swing arm. The second force comes from the chain. The force from the chain pulling on the wheel which usually acts parallel to the top chain run creates anti-squat forces. The magnitude of the contribution of the chain force depends on the angle of the top chain run relative to the swing arm. It also depends upon the distance between the chain run centre point and the front swing arm pivot point.
Anti-squat is expressed as a percentage to say how much weight transfer is offset by the anti-squat geometry. For example 75% anti squat offsets 75% of the weight transfer which will allow 25% of the mass to still act upon the rear suspension and compress it. If the bike had 100% anti-squat geometry it will offset all of the weight transfer. Therefore, no squat will occur due to load transfer. It is possible to have over 100% anti-squat. In this case the geometry will try to extend the rear suspension and would actually raise the suspension under acceleration as oppose to the more common response of compression.
How Does Anti-Squat Affect Handling?
Anti-squat is most prominent in corner exit situations. This is because as the rider begins to see the exit of the corner, the throttle is applied gradually until full throttle is achieved when the bike is back on the straight line after the corner. As the throttle is being applied, the perfect balance of anti-squat needs to be set in the rear end. The ideal amount will allow the bike to slightly compress on throttle, loading the tyre and gaining traction. However, too much squat will begin to unload the front tyre as the rear compresses which can lead to understeer on corner exit causing the rider to back off the throttle to provide grip on the front wheel again to exit safely. If the rider has to back off the throttle then the lap time is affected negatively. If there is not enough anti squat, the front end will have enough grip but the rear wheel might begin to spin causing loss of traction and oversteer. This set up will also create slower lap times as the only solutions would be to apply less throttle (meaning the bike isn’t travelling as fast as it could be) or to back off the throttle slightly when it begins to wheelspin or oversteer which will also slow the bike down.
Fast riders on track tend to set the bike up so that when applying throttle on corner exit, the rear will oversteer very slightly allowing throttle to be maintained as the bike straightens for corner exit, causing the tyre to regain grip in the straight line and send the bike at full pace down the straight. Slight oversteer is more controllable than understeer on corner exit for most racing drivers be it on bikes or in cars as it can be controlled with throttle and steering inputs whereas understeer usually results in the throttle being backed off greatly.
Most bikes have a static anti squat of over 100%. This is because the anti-squat geometry changes when the bike becomes loaded by the rider. Therefore, with the bike loaded, the anti-squat geometry moves to a position below 100% creating the ideal position when out on circuit. When the bike is active on circuit in braking and accelerating conditions, the level of anti-squat will always be changing as it is altered by the different angles of the swing arm achieved during these situations. Therefore, the amount of anti-squat can be slightly different on fast and slow circuits even if it is set the same statically due to the different loadings experienced on the two types of layout.
How to Adjust Anti-Squat
Anti-squat can be adjusted by three main ways:
- Raising or lowering the rear ride height in order to alter the swing arm angle from horizontal. This is the most common technique as it applies to all bikes including road bikes and is simple to apply. However, the downside to this technique is that adjusting rear ride height also affects the trail at the front of the bike which can have negative impacts if done in the wrong direction.
- Another technique used is to alter the angle of the chain in relation to the swing arm by changing the gearing of the bike or raising or lowering the engine. Installing longer or shorter gearing on the bike will alter the chain angle from the swing arm. This will affect the anti-squat geometry and doesn’t require any ride height changes, therefore not affecting the front geometry. However, it will affect the acceleration and top speed of the bike so should be done with serious thought because if the gearing is changed in the wrong direction, the benefits of the anti-squat change might not be enough to outweigh the negative gearing change. On the other hand, if the gearing change would be beneficial then this is a great way to kill two birds with one stone. Raising or lowering the engine will alter the chain in relation to the swing arm which in turn will affect the anti-squat.
- The third way that is used, particularly in racing, is to adjust the swing arm pivot point location. This lifts or raises the pivot point, altering the angle of the swing arm without affecting ride height or gear ratios. This is why it is beneficial for racing teams who have the ideal gear ratios and front geometry set up for a particular track, allowing them to alter the anti-squat independently.
Anti-squat geometry can be increased by altering the above listed options in the following directions:
- Ride Height – Raise the rear ride height
- Chain Angle – Install shorter gear ratios or lower engine
- Swing Arm Pivot Location – Raise the pivot location for the ground