Poly vs Standard bushes

[Alfamania]

I just wanted to ask your opinion on whether I should change my bushes to Polyurethane or standard?  I am due for replacing the rear watts linkage bushes including the centre watts bush as well so I thought I'd ask? Does anyone know if this is the central watts linkage bush here: https://www.centerlinealfa.com/suspension/1831


Thanks guys

[shiny_car]

Referring to the ePer schematic:



Firstly, it's a trade-off:
1. OE-spec/rubber: probably cheapest; softest; most comfortable ride; most amount of 'give' and thus least amount of control; typically last the shortest
2. PU (polyurethane): firmer; transmits more harshness, but usually very acceptable; less give for better 'directness'; usually last much longer than rubber and should last the lifetime of the car
3. spherical bearing or similar: somewhat solid; harshest, and some people will dislike this for a street car; no give, so very 'direct'; should last the lifetime of car, but if they get dirty/gritty, will wear; owners of track cars may favour these

The bush you show is for the front of the DeDion triangle, and marked #8 in the diagram. For this bushing, most enthusiasts will recommend PU, and meant to be a good compromise between price, longevity, control, and comfort.

The spherical bearing placed there is also meant to be very good, but I've read about noise/clunkiness. I haven't decided what I'm going to fit to my own car, but I am favouring the spherical bearing at this stage, particularly as I won't drive the car much, so don't have to put up with any harshness very often.

[Alfamania]

Thanks  It seems that standard bushes locally are same price as Poly ones from UK or US. The spare place have a good price on standard ones but dont have them in stock 

[Alfamania]

How about the Superpro rear watts kit?

[rowan_bris]

Richard Anderson at Avanti Spares in Brisbane has the poly ones - I have just got two sets!

[shiny_car]

Some people will use PU bushes through the whole car, and I think it's a good idea.

PU Watts bushes is not meant to contribute significantly to harshness, but should improve control and longevity. Nothing to lose IMO.

[Alfamania]

Sounds Good! Im getting them 

[Storm_X]

I bought some from super pro awhile ago the most expensive was the watts but I want to do solid bearing bushes with adjustable bars.

[Alfamania]

Thanks for the reply, could I ask what do you mean by: BTW the rear feels a little my stable with the Polyurethane bushes

[LukeC]

I have full poly through my entire car (not counting the sway bar bushes). With 5.5 degrees caster and the faster ratio P-S rack the car has very precise steering with exceptional highspeed stability. It is only harsh on some of the roads in my suburb that are really overdue a resurface.

Fulcrum Suspension in Queensland have been making poly bushes since God was a boy. Go to their website, do a search on what you need, find a local dealer and order them. You will get bushes overnight. Make sure you install with heaps of grease... when they are dry, they squeak like you would not believe.

[Duk]

A couple of things about the supposed extra longevity of PU bushes.

I think most of the claims about longer life come from marketing drivel.
A PU bush needs to twist/slide or compress/distort or a combination of the 2, when the suspension component works through its travel. If it has to slide (and even in compression/distortion) application, the bush (obviously) needs to be lubricated. If and when the lubrication depletes or gets dirt or rust in it, then the bush will start to wear.
The other thing to consider is that PU bushes aren't very tolerant of offset loads. Loads like the combined distortion and twisting/sliding.
I've seen this plenty of times with misaligned PU bushed drive coupling at work (plenty of PU bushed drive couplings in heavy industry), where the bushes get hammered to death despite their apparent flexibility and toughness.
I've never heard of or read of a car manufacturer using PU bushes, even for their much more sporty variants, they'll use stiffer rubber bushings. Hardcore things like the Porsche 911 GT3RS jump to spherical bearings. I'd say that Ferrari do too, with their hardcore track oriented cars (The F50, probably the F360 Challenge Stradale and maybe the F430 Scuderia). And obviously Alfa Romeo did too, with the SZ/RZ cars.

Rubber bushes are much more tolerant of the above loads because even in a purely rotating nature, there is no sliding, the rubber just distorts. There is no requirement for lubrication and because of the bonding, no chance of moisture or dirt getting into the bushed pivot to cause any problems.
Where rubber will deteriorate faster (years!) is because it is effected by ozone and heat much more than PU is. Even if a brand new car was never used, its bushes would deteriorate with age.

I am a fan of using PU bushes, but prefer to use them in mainly twisting/sliding pivot points that have minimal distortion angle and minimal compression of the bush when the pivot articulates. The Watt's linkage bushes are good examples of that.
A good example of a bad use of PU bushes is the top control arm of the front suspension when you have a lot of positive caster wound into the wheel alignment. This is something I have done and it will be interesting to see how long the bushes last.
Another good example of where not to uses them, is as top (at the front) and top and bottom (at the rear) shock absorber mounts. The bushes are the pivot points and because these bushes need to compress to allow pivoting of the shock absorber (even though it isn't much) the extra resistance to pivoting puts more side loads on the shock absorber's internal bushes, seals and pistons, wearing them out faster.

And finally, as mentioned, PU bushes will suffer rapid wear if rust gets into the grease. Because the pivot point needs to be clean and smooth, lots of times bushes get pressed into bare metal holes with a smear of lithium grease applied. And while that is probably fine, to me the idea of leaving the metal, which 'should' be protected by the grease, bare, is a bad 1. So before I put PU bushes into cleaned pivot points, I coat the pivot point with CRC Urethane Coat. This stuff is typically used inside electric motors, but I've found it a very durable coating for pivot points. It also has great ability to get around bends, with fantastic self leveling abilities, so I use it inside steel intercooler plumbing too!   

[Alfamania]

Thanks, sounds promising. Do you still add grease all over after the CRC Urethane coat?

[aggie57]

Duk - well said.  A classic example of improper use was the front mount of Sud front suspension lower control arms.  More than one ripped itself away from the body after being replaced by inflexible poly or similar bushes.  Given the suspension travel in a Sud this is an extreme example but it demonstrates what can happen.

[colcol]

Aggie, i saw a Sud have the front control arm pull the mount out of the body at Phillip Island, while waiting to go out on a run, the front suspension collapsed in the dummy grid while waiting, instead of the rubber bush doing all the twisting, it was rigid and all the suspension movement was taking place in the soon to be departing mount, very lucky driver that day, suspension collapse thru turn 1 - no thanks, Colin.

[Duk]

Thanks, sounds promising. Do you still add grease all over after the CRC Urethane coat?

Yep.