You’ve probably heard of people fitting poly bushes to their car to improve handling. But what are poly-bushes and what does replacing your standard bushes actually achieve?
What are bushes and what is their purpose?
Bushes are rubber inserts that mount in the suspension arms and subframes where they connect to the hub, chassis and subframe. They mount between these parts and offer 2 main purposes:
- They allow flex between the components that they join and absorb vibrations from the wheels and stop the vibration being transmitted into the chassis and o the driver.
- They act as a pivot point on the suspension where they join to the subframe, chassis or upright allowing the suspension to move and operate about the mounting point.
Why replace a suspension bush?
To understand why replacing suspension bushes is important, its best to start with what happens when you turn your steering wheel out on track to change the direction of your car. This is something that is often taken for granted, but have you ever thought about the chain of events that takes place through various components?
First the driver moves the steering wheel, this rotates the upright and produces a slip angle at the contact patch of the tyre, the force then pushes through the tyre, flexing it and transmitting the force into the wheel. This force is then directly transmitted into the hub. Next the hub tries to pull itself away from the body on the inside wheel, pulling the bush which pulls the suspension arm that it is attached to. This force then pulls and flexes the bush where it mounts to the chassis or subframe and then finally the subframe is pulled as the subframe bushes flex and then transmit the force into the chassis, which results in the chassis changing direction.
This chain of events takes time as each component and bush needs to be fully flexed before it completely transmits the force into the next step, finally ending up with the chassis changing direction with the wheels. Although the time is minimal, it can be felt in a track environment where direction changes are fast and frequent and can cause the car to lack in responsiveness to steering inputs and can make the chassis feel un-planted in the corners.
Therefore, uprating the suspension bushes can improve driver feedback and the handling of the car. If the amount of flex in the suspension bushes can be reduced then the time taken for the chassis to respond can be reduced, creating a more nimble, responsive car.
Also, if your bushes are deteriorating then this can produce a strange floating sensation when cornering where the car can feel like the chassis is moving towards the outside of a corner when cornering hard. This is the outcome of the worn bushes generating too much flex allowing the chassis to move noticeably when experiencing centrifugal force from cornering.
What are the options?
There are two main options to replace your suspension bushes with, polybushes or solid bushes. Each option has its own advantages and disadvantages each have their own suitable application.
Polybushes are a polyurethane replacement for the standard rubber bush. Its properties are stiffer then rubber and allow less flex. They are also less susceptible to heat, meaning that the bush is mounted near the engine, exhaust or brakes that they will stay firm rather than becoming soft like a standard rubber bush.
Polybushes are the easier option to install. Once the standard rubber bush has been pressed out, the polybush is then inserted in two halves from either side of the component with an insert through the centre for the bolt to contact with.
The stiffer compound of the polybush reduces the amount of flex at each connection and therefore, decreases the response time of the chassis, resulting in the car changing direction faster and making the driver have more feel through the steering, connecting them to the car and circuit better.
A negative aspect of polybushes is that they increase the amount of vibration through the car being transmitted by the wheels, leading to a more uncomfortable ride. However, the benefits of handling often outweigh the reduction of comfort.
Solid bushes are the hardcore option and are often only used on pure racing car applications. Solid bushes are usually made from spherical bearings and replace the standard bush by press fitting into the original arm or by having the arms machined with circlip grooves to lock the bearing sin.
The subframe bushes can also be replaced with solid bushes in the form of aluminium blocks with holes in for the blots that press into the subframe where the original bushes once were. The solid bushes remove all of the flex meaning that after the tyre has flexed, the force is almost instantly transmitted to the chassis making the steering response much faster and making the car more nimble and planted through the corners.
The downside to solid bushes is that all vibrations and bumps are felt in the chassis too making for a very uncomfortable ride to an almost unbearable point if the car is used on the streets. Therefore solid bushes are only best used in race applications where the car never sees use on the roads.