Solid suspension bushings

[Yves40]

Hello,

I would like some advice on the subject of solid suspension bushings. For the car i'm restoring there are basically 3 options : rubber as per stock, poly's or solids.

I would like better handling car, since this particular car doesn't handle well from the factory, so that leaves me only the option of either poly's or solids.

I do not favor the poly's because they are a kind of sticky plastic and during rotation they tend to stick to the axle of the arms and start to squeek.
So i was leaning towards installing solid bushings, but people warn me against premature wear on these in operation. Now I do remember that in a lot of applications bushings if greased regularly work fine. Due to the fact that apparently there is also a factor of misalignment between the arm and shaft or in the shaft itsself to be considered i'm reasoning that lot of the premature wear they talk about comes from the misalignment.

Does anyone have any input on this

Thanks in advance.

 

[Tmoose]

Make, model, year?

 

[Yves40]

Chevrolet Corvette 1979, small block.

 

[GregLocock]

My old truck had greased bearing shells in its front suspension. Every 1500 miles these had to be regreased. Every 3000 miles they had to be replaced. What a stupid system.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Yves40]

3000 miles ? Are you sure that you didn't have an alignment problem at the shafts that rotate in them ? If the shafts are not straight or the bushing not aligned with the shaft, it will not rotate freely and start to eat in either bushing or shaft.
I don't think that metal on metal greased every 1500 miles would wear out that fast.

 

[LionelHutz]

You need the rubber in the rear or else change to a jonny joint. There really are no other suitable options.

I'm pretty sure there are aluminum over steel bushings available for the front arms. If not Corvette specific then I bet you can find some other model part that fits. They don't use bushings though, just a aluminum replacement bushing piece that rotates over a steel sleeve. I suspect they will wear quickly and be as noisy an squeeking poly bushings.

There is a third option. Trade it for a car that handles better.

 

[NormPeterson]

Polyurethane is a poor choice when there is multiaxis rotation at the bushing. Compliant bushings accommodate this by their own deformation, generating nominal end moments in the links and contributing very slightly to the roll resistance at that end of the car As the bushing stiffness goes up, so too do the moments and that end of the car's roll resistance (which can shift the handling balance by perhaps more than you'd guess).

Poly's squeaking mostly doesn't come from the sliding surface at the inner sleeve.

It is possible to modify at least some of the more common poly bushings to reduce off-axis stiffness and the squeaking, at some likely cost in durability. Then again, poly bushings in multiaxis motion applications should be considered "wear parts" anyway.


Norm

 

[Tmoose]

What about that Corvette's "handling" don't you like?

http://www.corvettefaq.com/c3/vip.pdf

This article from the 80s by John Greenwood includes some testimonials from folks who were quite pleased with the results after making some of the modifications he suggested.
A lot of time was spent on getting the steering box adjusted correctly, and installing an idler arm stout enough to keep the RF wheel under a semblance of control.

He had nothing but praise for the stock rubber bushings in the lower front control arms, but said the rear lower control arm pivots need to be lowered.


Dan T

 

[GregLocock]

These aftermarket bushing parts look nice enough on the shelf, but they are designed to fit in the same housing as the original rubber part, and are subjected to the same forces, which they can handle, and gross deflections due to arm rotation, which they often cannot. Rubber is very very stretchy. PU is not. Therefore a PU bush has to be very carefully designed. OEMs are very good at getting the process right so that the rubber sticks to the metal housing where it should. Aftermarket suppliers, well who knows?

It may also amaze the aftermarket modifier to learn that some bushes are designed to be soft to get a particular handling characteristic.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Yves40]

The front of the vette in question is quite heavy, so the suspension gets loaded with a lot of forces,contributing to deflection in the bushings. Plus the original upper arms allow for very little caster. I went with the VBP offset arms, that have more "built in" caster. These only come equiped with poly's. There were a couple of problems with these as 1. The eyes were not aligned properly, loading the bushing, 2. there is a lot of play between the sleeve and the shaft, allowing for even more movement.

As far as the rear goes, the axle serves 2 purposes, transmitting power and serving as a suspension member. The workings of this all depends of the play the inner axles have in the diff.
Then you have the trailing arm link that creates inconsistent toe in, toe out on bumps and such.
I intend to install a custom 5 link in the rear. For the rear at least i can go with sphericals. These allow for misalignment. For the front no such option exists.

Norm,

I do not understand what you mean with end forces on the bushings and rollresistance.

 

[NormPeterson]

Greg - the concept of needing or wanting softness in some directions and stiffness in others is pretty familiar here. You don't have to be an OE to make voided bushings as long as you have a fair idea what you're trying to do.


Yves - The ideal link is a rod that is loaded only in tension or compression. For this to happen, the end connections theoretically must be frictionless ball joints of some sort, so that no moments are developed there.

Most OE rubber bushings in real suspension arms are a fair approximation of a more or less spherical joint at least to the extent that they don't develop much resistance when asked to rotate about an axis that is not coincident with the axis of the bolt that holds everything together.

PU offers less resistance about the axis that it can pivot about, but much more resistance otherwise because you are then requiring the much stiffer material to deform. In out-of-the-aftermarket-parts-box condition, most PU bushings are pretty much held captive inside a little box comprised of the bushing shells and the chassis brackets. There isn't even much place for the material to "squish" off to.

The best illustration I can think of is a trailing arm from any solid axle link suspension, where if you imagine the link to be an upper case "I" shape with the bushing pins at the top and bottom. Grab the top pin and try to twist the bottom one - PU will offer quite a bit of resistance to this sort of thing. And that's basically what multi-axis rotation is doing, just not as pure torsion.

Anyway, once you are developing end moments on things that shouldn't be developing them, the force to make that happen has to come from somewhere. Here, it's the motion of the suspension, and as soon as you develop a force as a result of applied displacement you've got a stiffness that is working to oppose that motion. This is another "elastic" suspension stiffness, and is additive to that which comes from the springs, the sta-bar (if present), and dynamically from the dampers.

There was a small study conducted by one of the aftermarket businesses attempting to quantify what people had been calling "roll bind". PU in both ends of just the lower trailing arms created something like 60 lb/in in resisting an applied roll - which was roughly comparable to the stiffness of the factory rear stabilizer bar in the same car. I will tell you that this was not a Corvette, but the concept of large forces being induced where PU must undergo multi-axis motion remains valid.

Heck, I'll even tell you that some creative (destructive?) drilling and belt-sanding of PU bushings in one of my own cars made a clear difference in the one-rear-wheel bump mode. The reduction in lateral head-toss was that obvious. I figure I'd eliminated over 80% of the stiffness added by the as-bought PU bushings.

You have to think in 3-D.


Norm

 

[Yves40]

Norm,

I think i know what you are telling.

As far as the A-arms go, the mayor forces come from bumps and holes in the road, these make the suspension go up and down, held under control by both spring and shock. These cause a rotational movement around the bushing points (which in case of a corvette rotate around a shaft that is mounted with bolts to the chassis).

The only forward and backward movement that these shafts undergo are the roll resistance of the wheels and braking force. These are the forces that will work in an other direction than the rotation that the bushing does allow.

But isn't that the whole idea of a suspension, to keep it in its place and only allow for the movement it is intended to make, meaning up and down in this case ? I'm guessing but i would think that braking and rollresistance are relative not as big forces compared to the up and down forces that a suspension undergoes...Please correct me if I'm wrong.

I read a post on a vette forum yesterday in which someone had installed the same type of solid (guldstrand) bushings) but without aligning shaft and without regular greasing. Considering these things, he still got 10000 miles out of them, which i considered not to bad, all in all.

The uppers shaft hole is bored out of center to provide some additional camber gain. This poses another problem, since it make aligning the bushings much more difficult. You actually have to degree the bushing in relation to the arm the get them installed in the right position, in which the shaft rotates free in the bushing.

For the lower arms, i found out that the factory A-arms holes are not completly squared up, plus the quality of the lower shafts is not good (most are bent out of the box).
But i can understand the factory here, since normal rubber bushings allow for some deflection taking up any misalignment that might exist.

Furthermore I remember reading about the Ferrari Challenge cars in which they replace factory rubber bushings with solids. These cars ate their bushings. But i'm guessing this all comes from a misalignment error at the bushing points, which for a car designed for rubber bushings is not that important.

 

[NormPeterson]

As far as the A-arms go, the mayor forces come from bumps and holes in the road, these make the suspension go up and down, held under control by both spring and shock. These cause a rotational movement around the bushing points (which in case of a corvette rotate around a shaft that is mounted with bolts to the chassis).

The only forward and backward movement that these shafts undergo are the roll resistance of the wheels and braking force. These are the forces that will work in an other direction than the rotation that the bushing does allow.

It isn't just the forces coming from bumps that matter. It's the forces internal to the suspension that happen because of geometric conflicts. Your C3 rear suspension design has the hub carrier trying to rotate in front/rear view about an axis that's essentially parallel to the vehicle centerline as defined by the half shafts and the lower lateral links. It is also trying to rotate about whatever axis that the chassis side pivot pin in the trailing arm defines. Something has to give, else the suspension won't move at all or something will bend (you cannot simultaneously pivot the trailing arms or the lateral links about two skewed axes - satisfying the arc in front/rear view would necessarily mean that the trailing arm rotation is being yanked out of its plane). So you make it all work either with the bushings via their compliance or by the full rotational freedom provided by spherical joints.

Once you get involved with compliances in order to allow this motion, the problem stops being purely kinematic and you can't use a purely geometric approach or consider only externally applied forces to fully understand it.


Norm

 

[Yves40]

Well Norm, I have a solution for the rear that involved spherical bushings.

The problem is tightening the front.

Due to the fact I don't seem to be able to get some good straight cross shafts for the A-arms, I doubt I could ever get the solids to work properly.

Anyway, the aftermarket poly's have little give in them, all the give comes mainly from play in between the shaft and center metal bushing, so I consider this not to be a good option. This plus the squeeking of poly's makes me lean towards the stock rubber and accept the fact that front suspension will never be tight.

 

[Tmoose]

Yves said "........accept the fact that front suspension will never be tight."

======================

http://www.corvettefaq.com/c3/vip.pdf

In 1984 John Greenwood said " your new Corvette lost that “tight” feeling after the first year or so. Many of you have spent a lot of time, money and hours of frustration trying to put that tightness back only to find that many of
the products touted for that purpose just don’t do the job! I’ve also had those same experiences. Some ideas sounded so good I tried them twice only to get same result. That’s when I decided that the only way to cut through all the unproven theories would be to test my own ideas on the race track, apply the results to street and/or track driven cars and share the results with .. Corvette owners.."

His very first recommendations are to adjust the steering box with a very particular procedure (to control the left wheel) and install a special idler arm (to regain control of the right F wheel.)

"Don’t rush to change to solid A-arm bushings or sway bar frame mounts that may need replacement. I’ve tried both with
poor results.. The lower front A-arms are particularly sensitive to the design of the solid bushings.

Loads are great enough on the control arm shafts so that anything short of top quality spherical bearings will prove inadequate. I’ve seen many autocross and race cars whose nylon, steel or bronze bushings in the lower control arms don’t allow free travel. (They bind under load and continually change the roll stiffness, or they become
hammered out and loose.)
I’ve experimented with solid versus rubber A-arm bushings in
race competition and the changes were so subtle that few
professional drivers could tell the difference. The problems caused by camber deflection inherent with the rubber bushings are overstated, in my experience, and the cure is most often worse than the problem. So stay
with the rubber bushings for now."

Do you have a rear sway bar? If not, adding a modest one can dramatically reduce the "front heavy" feeling. (Those small block corvettes were around 50/50 F/R wight distribution).
Mr Greenwood suggested a good starting point would be 5/8” to 11/16” diameter rear, and 1" front bar.

 

[ChrisDuncan]

""I do not favor the poly's because they are a kind of sticky plastic and during rotation they tend to stick to the axle of the arms and start to squeek.""

Dow 111 works to lube poly bushings to stop squeek. Some of the better bushing kits are supplied with lube.

 

[NormPeterson]

I don't think sticking to the inner sleeve is where the poly squeaking usually comes from. If you consider polyurethane suspension bushings to be "wear parts" - which you probably should anyway - there may be better solutions than simply throwing more grease at them.


Norm