23-11-09, 15:27
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#1
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Honda Driver
  Stage 5
Join Date: Mar 2005
Posts: 2,101
Member of:
[4Tuning International]
[Honda]
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Despre suspensie (bucsi)
Este un subiect despre care nu prea am vorbit aici si ma gandesc ca fiecare dintre noi se intreaba care e diferenta intre cele de cauciuc, poly sau cele sferice. Am sa postez aici un material luat de pe un al forum, sper sa fie util 
Quote:
Spherical bearings do offer the ultimate in terms of geometry control, but with tradeoffs in initial and running costs. (Note: "rose joints" are a brand name - like Hoover for vacuum cleaners. I try to use spherical bearings as I generally used NMB and Aurora bearings rather the Rose!).
Whether or not that tradeoff makes sense varies for different uses - both for the car and the bush/bearing.
To make an informed decision as to which is most appropriate for what you want you needs to understand how the different options work and therefore what the tradeoffs are. Please ignore the rear trailing arm bush for this discussion (search instead). That's has a very unusual range of jobs to do which require special consideration.
Standard bushes follow a design called "metallastic" bushes. You have an outer metal shell, an inner shell and a rubber compound joining them. The outer metal housing is usually a press fit in the suspension arm. The metal inner is clamped to the other arm by the bolt. Then the rubber provides the movement - the metal parts stay fixed to their respective suspension parts. The rubber part has two conflicting jobs to do. It has to locate the metal inner relative to the outer - control the geometry. It also has to support the movement. So on the one hand it wants to be as hard as possible to keep the geometry perfect. But on the other hand it can't be that hard or the suspension won't move. Also the harder the rubber the harsher the ride - the more vibration etc that's transmitted. The Mugen bushes use a harder grade of rubber as they're less concerned with vibration and more concerned with maintaining proper geometry. However even the Mugen bushes aren't really that hard as they still have to support the movement of the arm. Metallastic bushes are popular OEM equipment since with no moving parts they need no maintaining, just occasional replacement plus by tuning the hardness they allow the harshness/sportiness to be easily tweaked.
Poly bushes work differently. With a poly bush the metal inner is still clamped to one arm. However rather than being fixed, the metal inner rotates in the poly bush itself. On initial examination this looks a lot better than a metallastic bush. The poly can be a lot harder as it's not having to twist as the suspension moves so in should be able to maintain better geometry. However there are two catches. Firstly since the poly is acting as a bearing surface over time the poly and to a lesser extent the metal inner wear. Secondly most of the suspension joints on the average car are not simple hinge joints. They support twisting as well as turning. To support twist the poly has to be soft enough to compress (or the suspension starts to bind as the energy trying to move the suspension has to compress the poly). The wear issue is often ignored - unless you take a suspension joint apart you often cannot see the wear so many people swear their poly bushes never wore at all. The twist is the more serious problem. Most of the bushes on cars need to support twist to a greater or lesser degree so when replaced with poly you have to compromise with the hardness of the poly. So as a metallastic bush replacement poly can only give it's best results on suspension where the joints are pure hinge joints so no twist support is needed. Where twist is needed (most of the Honda bushes...) it's still somewhat compromised. The wear issue isn't a huge one as poly bushes are relative cheap and the easiest to change for maintainenc. (The initial replacement with removal of the original metallastic bush being the hard part).
Spherical bearings are a metal ball with a hole that acts as the inner, with an outer housing. The metal ball is supported directly in the outer housing (sometimes using a PTFE or other liner for self lubrication). The inner ball can both rotate and twist freely while the centre of the ball stays in the same place relative to the outer housing. Since the inner ball isn't moving, rather it's just rotating, it offers perfect geometry control. That's generally what you want for handling (sometimes you use complicance to your advantage, but I digress....). So that's a huge plus. There are however, downsides. Firstly in common with poly bushes the parts are moving relative to each other and hence they wear. Secondly, spherical bearings need adapting to replace the original bearings - usually a housing to fit in the wishbone, plus side spacers to adapt the ball to fit where the bush inner went. Thirdly due to their metal parts and exposed nature they're vulnerable to corrosion. You can seal the housings to varying degrees to reduce that, but it's something to be aware of on a car that does road miles. As to the housings, there are two normal ways of retaining the bearings. The cheaper, race way is to stake the bearing in place. With staking once the bearing is in place a press is used to fold part of the housing over to retain the bearing. Once done, you can't change the bearing without replacing the housing. The other method is to use a circlip to retain the bearing - then you can remove the circlip and replace the bearing easily when needed. Adding circlip groves adds to the machining cost though - so expect such housings to cost more.
So which is best?
For racing spherical bearings. They're actually banned in a lot of categories for cost grounds, but where they're legal most top cars will use them. The handling benefits from being able to maintain designed geometry far outweight the costs of checking and replacing the bearings, plus the harshness isn't an issue.
For serious track day use it's not quite so clear cut. There are strong advantages to replacing a few of the bushes with spherical bearings. Replacing the right couple of bushes gives most of the handling gains of replacing them all without adding drastically to the maintenance or the harshness of the car. For the other bushes either harder metallastic or poly bushes can work. Metallastic bushes require a press and suitable tooling to change the bushes initially, but require changing far less often (they do go eventually). So higher initial costs. Poly is much easier for the DIY mechanic to do and maintain plus tends to be a bit cheaper. So for the DIY inclined it's worth considering. If you understand what job each bush is doing it's very possible to mix and match - exploiting the properties of poly where it doesn't have to twist as much so can be very hard without problems and metallastic where the poly bush would have to be relative soft and therefore poor at geometry control.
Personally I have two cars. One uses spherical bearings almost everywhere (actually used a couple of metallastic bushes still). It's setup to be able to use slick tyres properly though and not to do a lot of road miles.
My second car which while very focussed is a road car that's also capable of track days uses metallastic bushes.
I also look after a car that's a fun road car and track car - that uses a mixture of metallastic and poly bushes. Plus a range of race cars - ever one of them which can use spherical bearings for it's series uses at least some.
For the original poster my suggestion if you're planning on sticking to a mixture of road and track use with such a focused car then I'd change to a mixture of spherical bearings where they have the most benefit, with metallastic or poly elsewhere (the choice on which mainly depending on whether you want the lower workload of keeping the metallastic bushes or not). If the important spherical bearings currently use staked housings I'd change to circlip ones for easier bearing changes.
Final note on costs. There's something of a myth that spherical bearings cost a fortune as they're expensive to replace. If you're using friendly housings the bearings can be replaced easily. As an example bearing cost, the two largest ones I use on any of the cars I work on are Com14 size - load rated at 31tons (when you're dealing with shock loadings and slick tyres you get to the point where you need that!). A pair of them as needed for the car costs £42 for NMB bearings and last for a seasons racing (so more than a years track days for most people).
I'm not going to post which bushes to replace with spherical bearings for the best cost/benefit - I do earn some of my living designing suspension setups and setting up cars so have to drawn the line on what knowledge I give away for free. But I will say with the description I've posted on the pros/cons of different approaches there's enough information to figure it out. I have also posted a little on what works where on the rear previously (search!).
Finally, I've said it before but the trailing arm bush is a very special case. I've posted on the tradeoffs with that before.
-Brian.
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