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Frosty

Guide to wishbone bushes

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Plenty of posts come up about which bushes are best for certain applications etc. so I decided to write a little article to walk through the various options available.

1.0 What is a wishbone bush and what is it used for?

The primary function of a wishbone bush is to permit the wishbone to pivot. The outer part of the wishbone is connected to the wheel upright, and the inner part is bolted to the chassis. The bush is fitted at the chassis wishbone mounting point, so that when the wheel raises or lowers, the bush creates a pivot point, allowing the wishbone to describe an arc. See image.

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The secondary function of wishbone bushes is to add a level of vibration removal from the road that would otherwise be transferred into the chassis. This ranges from a good amount of reduction in the case of road car bushes, to none at all in the case of more race orientated bushes.

2.0 What types of bushes are available?

The common types of wishbone bushes fitted to road and track cars are described below. None of the options are really right or wrong - it's more a case of picking the correct bush for your application.

2.1 Rubber bushes (metalastic)

The most commonly used wishbone bush is made of rubber. These types of bushes are used as standard on road cars throughout the word. They are relatively cheap to manufacture, and work pretty well. Metalastic bushes are a single unit, but consist of 3 parts. An inner metal sleeve, an outer metal sleeve, and the rubber which fills the void between the two. The inner rubber is bonded to the two parts of metal. The inner sleeve is a crush tube which a bolt passes through to secure the bush and wishbone to the chassis, and this crush tube is the widest part of the bush.

rubber.jpg

The overall diameter of the bush is actually slightly too big for the wishbone bush housing, but this is deliberate. It must be pressed in with a great deal of force, so that the outside of the bush is held super-tight by the by the wishbone and cannot move. It's almost as if the bush is welded into the wishbone, and if bushes didn't wear out over time, it probably would be.

So the outside of the bush is fixed into the wishbone, and then a bolt passes through the inside of the inner crush tube, and is bolted tight to the wishbone mounting bracket on the chassis. This also becomes solid, so the only moving part left is the rubber between the two metal sleeves. When force is placed on the wishbone and it is required to pivot up and down, the rubber between the sleeves is what permits this movement.

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2.1.1 Pros

Metalastic bushes are a great starting place for the majority of cars. Because the rubber has to deform to allow movement, the bushes effectively add rate to the suspension. The rate increase is consistent though, since the rubber is of the same specification in each bush. Once the bush is fitted, you don't have to measure of check anything, because the movement comes from the rubber. Just torque them up and away you go. The rubber also helps dampen a lot of the road vibrations from entering the car. Although increased rate is not always desirable (as we’ll see later), we can guarantee that the rate will be consistent in each of the bushes due to the way they are designed, and the fact that stiction is impossible.

2.1.2 Cons

They are difficult to fit. However, their biggest drawback comes as no surprise. Because the rubber permits rotational movement, it's perfectly logical that it will permit all kinds of other movements too. Although the majority of movement is torsional as it should be, the rubber will also permit a small amount of forwards and backwards movement (wobble). The longer the wishbones are, the bigger a problem this will be, since the movement is multiplied at the wheel. So the geometry of the car will all be moving around when the wishbone sees any kind of load. Not really a problem for a road car, but could be for a track car.

2.1.3 Fitting

As described in the overview, metalastic bushes require a huge amount of force to fit, so they are not really a job for the DIY enthusiast. Worse still, they often take even more force to remove if they have been in there a while. Lots of heat, swear words, and often a hacksaw!!! Typically you will need to remove the wishbone from the car, take it to a garage press, and then put around 10 tonnes of force into it to get it in or out.

It's also very important to remember that metalastic bushes work by the rubber twisting, but because the rubber is bonded to the metal, it will only move so far before running out of movement (going solid). For this reason, it is imperative that these bushes are not torque tightened until the wheels are under their own weight. At this point you tighten the bolts which then locks the bushes to the wishbone brackets, and then they have the maximum amount of travel in both bump and droop directions before the rubber locks-out.

2.2 Nylon bushes

A much more performance-orientated wishbone bush is nylon. Unlike the metalastic bush, they come in 3 parts as opposed to 1. Each bush is made up of a steel crush tube (just like what is at the centre of a metalastic bush), and two nylon pieces which are shaped like a top hat with a hole through the middle. The way in which nylon bushes work is different though. Instead of rubber being used to deform and permit movement, the movement is provided by the crush tube being allowed to rotate within the nylon. The crush tube is a precision fit for the nylon, and a bearing is effectively formed. The crush tube remains fixed, while the nylon (and wishbone) rotates around it.

nylon.jpg

The two nylon top-hats are pressed into the wishbone bush housing at either end, and then the crush tube passes through the entire assembly. The crush tube is marginally longer than the combined length of the nylon pieces when pressed into the wishbone. As a result, when fitted the crush tube is bolted tightly against the wishbone mounting bracket which locks the crush tube into place. The wishbone and its nylon inserts are then free to rotate around the crush tube. The faces of the top hats will be very lightly in contact with the face of the wishbone brackets, but this is to prevent the bush twisting in a way it shouldn't.

The end result is an almost frictionless wishbone motion, with virtually no deformation or undesirable geometry movement due to the nylon being solid. There is also no increase in rate, unlike metalastic bushes. This makes it a very desirable choice for track cars.

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2.2.1 Pros

Providing the bushes have been manufactured well, they are relatively very easy to fit (although be prepared to spend some time checking to make sure they are a perfect fit). Geometry is held almost perfectly since no deformation occurs, and no rate is added to the suspension during wishbone movement. They can also be torqued up with the car in the air since the bearing is effectively 360 degree.

2.2.2 Cons

Because of the nylon, most of the road noise and vibration is transferred into the chassis. This could have undesirable effects long-term if the car is used on the road. Also, nylon bushes are a precision fit, and so you must be prepared to make sure that everything is perfect which can take time. If you don't get them to fit well, you could end up with different rates being added at all 4 corners of the car due to stiction.

2.2.3 Fitting

The first job is to ensure that the top hats fit into the wishbone housing perfectly. This means removing all traces of the old bush from your wishbone bush carriers, or ideally running a reamer through them to get them to the exact required diameter to an ultra-accurate tolerance. If the top-hats do not fit snugly (not too tight, not too loose), they may well deform slightly, which means the centre hole will become oval instead of round, and introduce friction into the rotation of the crush tube.

Also be sure that the nylon inserts are perfectly seated. Just check that the welds on the housing do not affect the seating of the nylon insert. If it's not perfectly flat, you'll get friction. The Dremmel is a brilliant little tool here.

Now insert the crush tube through the nylon. The crush tube should rotate freely in the bush without catching. It should also be a snug fit without being able to wobble. If it wobbles, this movement will be translated into the wishbone, multiplied by the length of the wishbone, and then translated into the wheel - very bad! If the crush tube does not rotate, double-check the nylon inserts are a good fit.

Lastly, check that the length of the crush tube is marginally longer than the nylon when fitted to the bush. You are looking for between a 0.1mm and 0.2mm difference. So if your bush measures 40mm across, your crush tube should be a minimum of 40.1mm and a maximum of 40.2mm. If the tube is too short you'll get friction which will add to the rate of the suspension. Take the bush down slightly and then re-measure.

Once you're happy with everything, grease the crush tube, and also the nylon which touches the wishbone bracket. Lith moly grease is ideal for this. Now you can torque your wishbone up. A good fitting bush should fall under its own weight, but with no lateral movement at all.

2.3 Poly bushes

A very popular wishbone bush in the performance and tuning world is the poly bush (or polyurethane bush). Polyurethane is a more advanced type of rubber-like plastic, that is said to work absolute wonders for all cars, due to it being a much better substance than rubber, and the only reason that manufacturers don't use it as standard is because it costs too much for them to mass-produce, which is why it's left to the people who are happy to pay more for aftermarket suspension excellent. This is only what they claim though, and almost the mirror opposite to the reality most of the time in my experience.

This is not down to the poly material itself - it's due to the poor fitment. You often hear of people fitting poly bushes, and then reporting their car is now "handling like it's on rails", and "feeling so much tighter". What in fact they are actually feeling is their suspension not working properly. The reason for this is because poly bushes come with a whole world of problems a lot of the time.

I have seen two different types of poly bush. There are those that fit like nylon bushes, but work like rubber bushes (where the rubber twists to give movement), and there are those that fit more like rubber bushes, but work like nylon bushes, where the poly rotates around the crush tube.

The trouble with the former type is that they are not a precision component, and the poly often ends up gripping the crush tube, or being to long for the crush tube once fitted.

poly.jpg

Depending on the exact bush, the fitting will vary a bit. Most of the poly bushes I have seen have crush tubes which are too short for the bush, so that when you tighten the wishbone, the poly bush crushes against the wishbone bracket and causes the joint to go super-stiff. This stiction pretty much stops the suspension from working, transfers the load directly into the wishbone (thus bending it), and yet these products continue to be the most popular after-market bush choice, simply due to the fact they are dead easy to fit, and come in really cool colours.

This diagram illustrates how poly bushes are fitted. The point at which the red polyurethane is compressed into the wishbone bracket is where things go wrong.

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The type of poly bushes that work like nylon bushes look a lot better though. I have not seen them fitted to a car as yet, but they certainly look to address the stiction problems typically associated with a poly bush. They work like nylons, but still offer a bit of complience. They are ideally suited to fast road and occasional track use. A complete track car would be better off on nylon though since there is no geometry change at all due to being solid.

2.3.1 Pros

They last a very long time due to their construction, and they are very easy to fit. As long as you can get the old bushes out, you probably won't need to remove the wishbone to get the poly bushes in. Due to the fact they are extra-firm, they are not as prone the undesired geometry alterations that rubber bushes suffer during load. But...

2.3.2 Cons

Because they are much firmer than rubber, they resist the current wishbone movement too if they are the type which are designed to work like rubber bushes. They introduce a whole load of extra rate into your suspension. If they are the type that seem to work more like nylon bushes, because they are not a precision fit, the crush tubes are nearly always super-tight in the poly bushes for multiple reasons, which massively limits their rotation, which again adds loads of friction into the movement.

From what I have seen, if you are using a poly bush you will experience a whole load of unwanted friction in the joint which is always undesirable. I am yet to see a poly bush that has fitted well and given good results. IMO they are a quick fix product which actually causes more harm than good.

2.3.3 Fitting

The fitting of poly bushes is actually very crude. They are not precision items, which means each one could give a different rate to each wishbone, a little bit like having your dampers set at different clicks in each corner! In any case, the poly bush fits in a very similar way to that of nylon bushes. The difference is that you often have to really wedge the poly bush in. This in turn deforms the crush tube hole, which means the crush tube needs wedging in too and has masses of friction from that point.

Often once fitted, the crush tube is not the right length for the bush, so when you tighten the wishbone to the chassis, the faces of the poly bushes are crushed into the wishbone mounting bracket and then even more friction is added to the joint. After a few hundred miles, this friction will present itself and you will hear the rubber squeaking. Normally this is when the assembly grease has gone.

I have seen a few cases of wishbones snapping on kit cars, and although the cause is never confirmed, in each case poly bushes have been fitted.

2.3.4 Conclusion

Poly bushes can be made to work, if they are properly sized to the application, but even then, expect to spend a very long time getting everything perfect, and still suffer some geometry change.

Personally I think that poly bushes are not worth looking at. That is until they come in the form of metalastic bushes, whereby the rubber is replaced with polyurethane, but the twin sleeve construction is retained so that stiction cannot be a problem. At this point, I think they could be a very good product.

3.0 Rubber and motorsport

To avoid complicating the rubber section too much, I thought I would mention this here instead. One of the problems with using rubber in motorsport is the rate that it adds to the suspension. As mentioned in the metalastic section, the twisting of the rubber permits the wishbone to describe an arc, therefore allowing the wheel to move up and down. For this reason, it’s very important to torque the wishbones up when the car is on the floor at normal ride height. Sometimes, you may even want to do this with the driver weight on board to so the bushes truly start with zero load on them.

The rubber operates just like a spring, in that it has it’s neutral setting (e.g. spring at full length), and when you move it, it will always resist in an effort to get back to its natural setting. The more you compress a spring, the more it fights you. A 200lb/in spring will push back with 200lb if you compress it 1 inch, and 400lb if you compress it with 2 inches etc.

A rubber bush does exactly the same – the more twist you put into it, the more it pushes back to try and untwist. So this means as your wishbone travels up, the overall rate of that wheel is increasing more and more. Furthermore, when coming out of the corner, that bush unloads its force back at the wheel in rebound.

Depending on your exact setup, and the firmness of the rubber, this may or may not be a problem, but it’s worth pointing out. It means the bump and rebound damping will have a different level of effectiveness depending on where the wishbone is in it’s arc.

Even if poly bushes worked perfectly and were not full of flaws, the extra-firm polyurethane would further pronounce this problem, since the bush would take more load to move, and unload even harder.

The great thing about nylon bushes is that they do not suffer this problem at all because nothing deforms to allow movement. The loading of the rubber may not sound like an issue with all that suspension leverage, but personally speaking, I had to add between 1 and 2 clicks to each damper on my car to get the car to feel the same during bump after switching from metalastic to nylon, so it’s definitely something to think about for those who are looking for ultimate control.

I hope this helps people out a little when looking into which bushes are going to work best for them. If I have made any obvious mistakes or anyone has any suggested improvements then please reply to the topic.

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Very nice work there, makes things clear to anyone considering their options, well done.  I'm sure people will get pointed to this quite often. :t-up:

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Top job,thanks alot! was about to get the nylon from CAT,but no more available.....any gen where i can get them from?
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Top Job Frosty  :t-up:

(Just add Rose Joints  :D )

I thought about it, but I figured anyone going for rose joints would probably know enough about the whole bush thing anyway to have reached that decision :)

Perhaps I'll add them on the next rainy weekend. It will of course mean putting my Microsoft Word Autoshapes skills to the test again lol  :D .

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Top job,thanks alot! was about to get the nylon from CAT,but no more available.....any gen where i can get them from?

Westfield factory supply them as an optional upgrade, I believe. They fitted them on my car.

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Yep, I changed to factory supplied nylon bushes when I went wide track. I've got reasonably compliant spring rates for road use, so I was pleasantly surprised that they weren't too harsh after the Metalastics I started with. In fact from a NVH point of view, I forgot about the change very quickly.

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I am working on a rebuild and plan to go wide track so this is all good info.

Sounds like the poly bush option is out so down to a choice of two.

As the nylon bush is in two halves would it be better to ream the bores to the final finished size after pressing them into the wishbones?

Is there a particular grade of nylon used as I may call in a favour and get a set made.

Thanks for taking the time to put the info up.

:t-up:

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As the nylon bush is in two halves would it be better to ream the bores to the final finished size after pressing them into the wishbones?

If I remember right, this is what the factory suggested.

Alternatively, a few of us have done it the other way round, and precisely sized the outer diameter of the bushes to the wishbone eyelets. - Three or four years later, that approach is still working well. Though if I'd had access to the correct sized reamer, I would probably have done it the factory way.

Sorry, no idea what grade nylon is used. Not particularly difficult for a good machinist to make up though.  :t-up:

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I am working on a rebuild and plan to go wide track so this is all good info.

Sounds like the poly bush option is out so down to a choice of two.

As the nylon bush is in two halves would it be better to ream the bores to the final finished size after pressing them into the wishbones?

Is there a particular grade of nylon used as I may call in a favour and get a set made.

Thanks for taking the time to put the info up.

:t-up:

Glad you enjoyed reading it :)

The wishbone bore size is crucial, so I sent all of my wishbones off to Westfield and I think the all-in cost was £160 to have the metalastic bushes pressed out, the bores reamed, and then the nylons fitted.

Me being me though, I removed them all and double-checked everything. There were a few welds which needed taking down etc. which I wouldn't expect the factory to do since it's very fussy work. I then turned each nylon bush down to give a perfect fit to the crush tube, but sand paper will also do the job too.

You can fit the nylons have have them 90% there straight away, but I wanted to do a really accurate fit, so that is why I checked each one.

The end result is superb though - zero stiction on each wishbone.

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Thank guys.

I just need to get on and do it.

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Excellent article.  Many thanks.  :t-up:

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Great article. Would you mind if I posted a link on the Yahoo Groups Westfield XI site? Some of the contributors are having a hard time understanding the theory and practice of bushes and your article should make things much clearer for them.

Brian

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Great article. Would you mind if I posted a link on the Yahoo Groups Westfield XI site? Some of the contributors are having a hard time understanding the theory and practice of bushes and your article should make things much clearer for them.

Brian

Fine by me. Please let me know if others find it useful :)

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