slate blue

Hi all,

When I was under the car recently I noticed that the front lower rear bushings were getting chewed out. Not surprisingly really when you think that the rubber is over 30 years old and had how many cycles?

So I have never been that enamoured with their design and these bushings are the reason for suspension binding after the car is let down off a jack.

These are the first bushings I have seen made in a multi-piece design. They make a lot of sense to me. After they are fitted I will check the movement of the arms with out the damper fitted to get a sense of the hopefully reduced binding.

The other up shot is the better control they will afford in terms of less deflection. The purple bushings are for the street and are said to have the same NVH as standard rubbing bushings. There is a black/black option for race cars also.

So I bought the whole suite that was available for the front suspension but I have requested the company to make steering rack bushes and rear suspension bushings. They do do some rear bushes but it's not a complete.
Front arm rear lower bush



Front arm lower front bush



Top arm bushes



Sway bar bushes

Dave928S

Neat ! I also don't understand the reason for the grab and lack of free range movement in the standard bushings, unless they were wanting the bushes to have a damping effect beyond (up or down) static load height.

There are quite a few things I'd like to ask the original Porsche design team.

Lizard928

I do not personally like poly bushings, the bonded rubber ones are designed to grab on both sides and twist. This prevents any and all rubbing.
With a poly bushing, it has to be able to rotate, this creates wear.
If you get any dirt/dust/debri in where the pieces rotate, it will cause very rapid wear.
The other problem is that this movement can also cause unwanted noise requiring one to constantly be under there greasing the joints to prevent the noise.

Dave928S

^^ Mmmm .... totally valid points.

slate blue

Firstly sorry for the upside down pictures, I don't know how to edit the pictures without deleting them first. If the mods want to fix that it is o.k by me.

The durability issue was initially a concern for myself and I have discussed this with Powerflex but they stand by their products and offer a lifetime warranty.

https://www.powerflex.co.uk/lifetime_warranty.php

I also asked about fitting grease nipples, again the answer "they are not recommended and very little maintenance if any is required for these bushes.

For the upper arm bushes they are sealed up pretty good, the lower arms could get some sort of dust cover. I suspect that would be worthwhile. I did think about Superfinishing the stainless sleeve for less friction. I may do that when I get my machine.

slate blue

When I started the thread it was via the phone as such I was a bit limited regarding the information I should have added.

https://www.powerflex.co.uk/road-series/why-pu

The above link details the benefits of these PU bushes. When I get onto my desktop computer I will add the McLaren study of the Porsche 928 suspension movements and how they deviate under loadings.

Remember to when you read the suspension deviation numbers, that would have been with stock tyres and wheels. Not the stickier rubber and add in more power and better brakes the modern modified 928 may have. The deviations may be massive and some of that movement is what I put my axle hop down too.

Alignment of the suspension is such a critical area when the car is on the limit. Not just for the high level of adhesion but the stability of the feel.

This link is also worth a visit https://www.powerflex.co.uk/road-series/faq

terry gt

I got the rebuilt lower arms and the upper arms and the front suspension still squeaks and feels mushy and soft . And lots of rear wheel hop in first and second gear.
I have been driving my buddy's new Ferrari 488 , even auto crossed it , and noticed a huge difference in steering response . So crisp and responsive . both car were on R compound tires .

FredR

Greg,

Hope you will be able to post some feedback soon. I have also been talking to Poweflex as I plan my suspension project.

I have some spare lower arms and recently pulled the driver side unit off the car and replaced it with its brother that had been sat in my garage for some 10 years. This arm was from an earlier model to take advantage of the longer throw for camber purposes and probably dates back to around 2004 if my memory serves me correctly.

Either way I removed the bushes for inspection. The rear bush I carefully removed using my Dremel kit to cut through the inner metal sleeve and for the front one I used a bench press down at my local friendly machine shop. The rubber stuff that came off the car looked in appalling condition leaving me wondering how it worked at all. I know my upper A arm bushes can be moved up and down relatively easily so to my mind they have also gone. I did not test the lower arm for movement with the suspension unit disengaged as yet but will be doing so shortly all being well.

I suspect my suspension gets more affected by our extreme heat although possibly not that much more than you chaps experience down under. Given they offer this "lifetime warranty" and they articulate that they are utilising latest technology materials it seems a fair challenge to me. How these things work is not clear to me. My expectation is that as with the original design the movement is taken up by the material flexure. If the stuff is moving with the suspension it would not last 5 minutes surely?

It will be interesting to see what drops out of this.

Rgds

Fred

slate blue

As promised the McLaren F1 development story, it discusses such issues as what Terry Gt mentioned albeit that Ferrari will be being electronically tweaked on the run. So we have the numbers that the suspension deviates from where it should be and the stiffer lower arm bushings should really help that caster wind off at the front. I hope to measure up the dimensions of the rear bushes and maybe they will have something that will work. I won't need the sleeves as we can make them here. I am going to change the washers to alloy to save some weight.

Reproduced from
F1 – McLaren’s Road Car An Autocar & Motor Book
free with the 2 March 1994 issue
 Haymarket Magazines Ltd 1994 _________________________________

SUSPENSION

It had been decided that the F1 would be a refined road car. A harsh, noisy ride was out of the question but so too was the compromised wheel control that results from road car rubber- bushed suspension. Steve Randle, the car’s dynamicist, was therefore charged with creating a stable suspension which did not incur the NVH (noise, vibration, harshness) penalty of a rose- jointed race set-up.
Suspension design does not begin, though, when the chassis engineer starts to sketch wishbones and spring/damper units. In racing circles the first requirement is to arrange the car’s principal masses correctly, a discipline which Gordon Murray imposed on the F1’s design from day one.

Instant steering response needs a low polar moment of inertia in yaw, which means a wheel at each corner and the main masses — engine, fuel, occupants — close to the centre of gravity. In most road cars this is compromised by packaging limits, but Murray was having nothing of that. The F1’s weight distribution (42/58 per cent front/rear) changes by less than one per cent from a full to empty fuel load, and even luggage is carried close to the centre of gravity.

Having achieved the right distribution in plan view the same must be done in side elevation. Starting with undesirable weight transfer under cornering and then correcting it with anti-roll bars is a compromise Murray could not accept, so the distances between the suspension roll centre and body mass centroid had to be the same front and rear. Since the roll centres must be low to avoid jacking effects, this meant the engine had to be as low as possible in the body. Dry sump engine lubrication also reduced engine height by valuable inches.

Only when these basics were correct could design of the suspension itself begin. Adaptive damping and ride height control were ruled out on weight grounds. Progressive rate springing was omitted too but for different reasons. First, the only way to achieve a stepless increase in spring rate is either by using complex pushrod linkages or costly taper-ground springs. Second, too much progression can suddenly increase weight transfer when a wheel hits a mid-corner bump,
making handling unpredictable. What small amount of wheel rate progression there is in the F1 is an inherent feature of the suspension linkages themselves, supplemented by carefully optimised bump rubbers.

Wheel travel front and rear was set at a generous 90mm (3.5in) in bump and 80mm (3.1in) in rebound and the target unladen bounce frequencies at 86 cycles per minute (1.43Hz) at the front, 108cpm (1.80Hz) at the rear. With the finalised car slightly over target weight, the actual ride frequencies have fallen slightly to 84.5 and 105cpm. Although these frequencies are higher than those of everyday road cars, they are still low for a sports car of this performance potential.

It was the wheel rates and wheel travel which determined the downforce generated by the underbody. Too much downforce would simply have squashed the car on to its bump stops, making the handling dangerously unpredictable at high speeds.

Describing the suspension as double wishbone sells it ludicrously short. Its cleverness lies in how longitudinal wheel compliance has been engineered in without loss of wheel control. It is this compliance which allows the wheel to move backwards when it hits a bump, endowing the F1 with its remarkable ride.
Murray didn’t know how much longitudinal wheel compliance to provide. In racing cars every effort is made to eliminate compliance to maximise wheel control. So McLaren bought a Honda NSX and put it on the electro-hydraulic kinematics and compliance rig at Anthony Best Dynamics. A Porsche 928S and Jaguar XJ6 were also measured.

Different methods of achieving the required compliance are used front and rear in the F1 because the suspension pickup points, the forces acting on the wheels and the required geometrical constraints are different at either end of the car.
At the front wheels the priority was to prevent castor wind-off under braking, which compromises stability. Here, where braking and cornering forces are reacted through the tyre contact patch, a solution was adopted which McLaren calls Ground Plane Shear Centre.

Subframes on either side carry the wishbones on rigid plane bearings but are mounted to the body by four compliant bushes, each 25 times stiffer radially than axially. These are aligned at tangents to circles which have the middle of the tyre contact patch as their centre.

The castor control of this arrangement is outstanding. Castor wind-off has been measured at 1.02 degrees per g of braking deceleration, whereas the NSX, 928 S and XJ6 measured 2.91, 3.60 and 4.30 deg/g. Toe change under braking and camber change under lateral force are also very small.

At the rear, where cornering and braking forces are again reacted through the contact patch but tractive forces through the wheel hub, a different configuration is used called Inclined Shear Axis. Complicated by the lower wishbone mounting on the gearbox, which is itself compliantly attached to the body, the suspension and engine mounts were designed as an integrated system.

Wheel control is again exceptional, the priority this time being to control toe changes under braking and traction. Measured values are 0.04 deg/g toe-in under braking, 0.08 deg/g toe-out under traction, both of which are negligible.

Equivalent figures for the 928 S were 0.30 and 0.35 deg/g, both toe-in.
Otherwise the steering and suspension broadly conforms with road car practice. The castor angle and king pin inclination, for example, are both relatively low at 46 and 8 degrees respectively. However, the ground level offset (the distance between the centre-line of the tyre and where the steering axis meets the ground) is 25mm compared with the sub-10mm values typical today.

Aside from longitudinal wheel compliance, one of the critical determinants of a car’s ride quality and its ability to maintain consistent tyre contact on bumpy roads is the ratio of its sprung to unsprung masses. In a light car it is therefore essential to have light suspension — easier said than done in a vehicle which needs tyres and a braking system commensurate with a top speed
of over 230mph.

slate blue

What worries me Fred is the temperature (hot temperatures) and how that will effect the grease and whether it will run out of the bushings? My cars will never do big milage and I won't drive them in anything but good conditions. I used use the car as a daily driver but the effort to get the car ready with all this work and money invested I am only doing it once so I make no apologies for being fussy about leaving it at home if its pouring rain etc. So lets see what happens. My issue with this restoration is that when I put the pieces back I really need to address the coatings, (platings) this adds so much time as there is nobody near me that can do this. Just another delay

FredR

Some interesting reading and perhaps suggests that going with the stiffer bushes for the lower arms might be appropriate given that caster phenomena mentioned.

Also interesting were the figures for the 928S rear suspension- in case anone did not notice that seems proof positive that the Weissach arm actually does what it is supposed to do or at least that is how I read it.

Although at face value the numbers they measured might not seem good, one must remember what is being compared to what and the 928 probably puts up a pretty good account for itself all things considered.

It also goes to show that you really do get something for your money with the McLaren.

I have often wondered just how much those bits and pieces hanging off the A arms float around under load- at least I now have some idea. Putting modern wide sticky rubber on the things does nothing for longevity as I can attest to. Now I have to clean up the results as it were.

Franky by now I suspect most all 928's that have ben used half decently likely have knackered suspension if the owners did but know it.

Rgds

Fred

Speedtoys

At least you've found that last inch of 'go fast' you had in you, before putting questionable upgrades into the chassis.

ALKada

SwayBar

+928

Speedtoys

Not sure it mentioned here..but the stock bushes are a GREAT deal of spring rate in the suspension.

Lose them, drop spring rate, and everything will operate differently than it did before, not on the good side.