Kit Car Urethane Bushes are often mis-understood

Kit Car Urethane Bushes and how they work is often misunderstood. Even by those selling the parts.

It’s been a few weeks since I’ve been in the garage, so I feel a little bad about asking for a refund from a well established yet obviously ignorant parts supplier.

In case you’re wondering why I haven’t blogged, I’ve been camping and white water rafting for a few weeks in France. The hospital has confirmed that I only have bruises and not fractures, plus the burns won’t be permanent. What a fantastic Holiday! Just how I like them. Now where are the pain killers? As the nice lady in the hospital said, “Oh, Man Up!”


I have just taken the urethanes, crush tubes and outer sleeves from their packaging only to find that they are total junk. The urethane are obviously cheaply molded and each one is different. The outer sleeves are not seamless (not a major bummer, but still annoying) and the crush tubes are the wrong length and steel that already shows signs of corrosion. In other words, they are fit for setting up the chassis but nothing else.

How Do Urethane Bushes work on a Kit Car?

The first thing to understand about Urethane Bushes; despite their appearance; is that they do not work the same way as a rubber bush.

The Urethane Bush has more in common with the needle roller bearing than it does the rubber bush.

Roller Bearing

The Urethane of the joint does not twist like rubber, it slides over the inner crush tube. A hard bushing material such as urethane should be treated as a bearing material like brass, hence the use of slippery plastic surfaces.

With the type of rubber joint typically found on production vehicles, the rubber part of the joint is bonded to the inner and outer tubes.

Rubber Joint Rotation

The curved red lines in the above drawing show the rubber part of the joint stretching and twisting as the joint rotates.

With a Urethane Joint the two urethane inserts slide and rotate over the central tube known as a ‘crush tube’. This crush tube should be made of polished stainless steel and lubricated so that friction is reduced. Urethane does not have the twisting properties of rubber and is in fact non-elastic. Rotation is potentially 360°

Urethane Joint Rotation

Urethanes with Inner Crush Tubes

Urethane Joint Lubrication

Urethane joint lubrication has always been an issue and they are never going to last as long as a rubber joint.

A common mistake people make is, to tighten the whole assembly up so that the side walls of the two urethane inserts clamps up and grips on the surfaces of the mounting bracket. The only part of the joint that should not rotate is the central crush tube. Ideally the crush tube should be fractionally longer than the assembled joint; long enough to allow the urethanes bearing faces to be greased but not so long excessive sideways movement is present. Sideways movement will always happen and when it does the grease will be ejected. Hence the need for regular maintenance and low life expectancy.

Ideally and grease nipple should be added so that a copper based grease can be injected between the two urethane inserts, but this may not always be a retrospective addition due to the length of the joint.

Urethanes Joint Cross Section

As previously mentioned, the urethanes I purchased were all different. Once pressed into the outer sleeves; in this case Front A-Arms; they deformed to such an extent that getting the internal crush tubes to fit required a lot of body weight, bouncing up and down on the vice. Each assembled joint was a random length and every crush tube ended up shorter than the assembled joint. They didn’t rotate as they should and I could stand on one A Arm, without it moving!

Front Upper A Arm

My crappy steel crush tubes were 12.7mm I.D. and 19mm O.D. As a test, I pressed a fresh urethane into an outer sleeve and measured the resulting internal hole. At its’ smallest point it was a mere 16.2mm. This explained the tightness and lack of movement. 19 into 16.2 is going to be tight! When the joints are this tight and compressed I might as well use a rose joint as there sure as hell isn’t going to be much noise and vibration suppression. Plus, I’ll need to get some high quality stainless bar for crush tubes and better quality urethanes.

Time to start again, maybe this time I’ll use nylon bearing sleeves and needle roller bearings. What I have will get the car rolling, but thats about it.

Can you tell I’m not too impressed with what I have and urethane joints in general?

My Options

Rubber Bushes

Rubber BushBefore the time of rubber bushes people used brass on steel or steel on steel bearings.

With the advent of rubber bushes, designers had the ability to control various aspects of suspension design by changing the hardness or durometer of the rubber. Rotation movement is offered by the rotational sheer of the rubber itself. Since there is no friction caused by the rubber wearing against the steel there is no need to allow for lubrication and they can be molded with no critical tolerances.

Rubber bushes offered:

  • Cheaper and quicker build times
  • Better noise and vibration isolation
  • Long Life typically 100,000 miles
  • No lubrication
  • Lower tolerance manufacture

The tubes are made of fairly low grade steel. The rubber twists within its’ confines to give a limited amount of movement. typically ± 15°.

For production cars under normal driving conditions they are a great and obvious choice.

So why use urethane bushes or rose joints if rubber bushes offer all of this?

The answer is the same properties that absorb vibration also allow deflection under cornering loads.

Rubber Bush Deflection

When a vehicle enters a corner the bushes allow the A Arms to move in relation to the frame. This results in positive camber and a smaller tyre contact patch. A tyre can only give its’ maximum cornering power when the wheel is perpendicular to the road. For this reason many cars have static negative camber.

There is also the problem of Deflection steer’ The bushes compress but the steering rack doesn’t. This will result in a change of steering angle. If the steering rack is in front of the arms then there will be ‘Deflection under-steer’ and if behind, ‘Deflection over-steer’. Deflection under-steer is often unnoticeable and easy to correct. Deflection over-steer can be unpredictable and unwanted. Hence why most modern cars have the rack in front..

The rear suspension can also suffer from similar effects, (toe and camber changes) and they can often be much more dramatic due to the torque produced by the engine. The resulting torque steer can cause the car to over-steer during braking and under-steer under power old or soft suspension.bushes can be the culprit.

Rose Joints and Roller Bearings

Rose joints and roller bearings will eliminate deflection but will require regular cleaning, greasing and checking for wear. Only top grade joints should be used as all vibration and suspension forces can damage cheaper joints. All noise and vibration is transmitted to the driver, steering wheel and passengers. This can be acceptable on race cars but very tiring on road cars.