Rear Wishbone Mounting - why do vertical bushes get used when it seems "wrong"?

[Overlap77]

Can anyone tell me why a vertical mounting is used on front wishbone rear bush?
It seems "wrong". The only benefit I can see is deflection in the event of kerbing preventing damage.

e.g Golf Mk6 - nice design - horizontal bush allowed easy movement

Golf Mk7 - vertical plane mounted bush seems to restrict and interfere with free movement of the arm and cause geometry change

thanks for any input available

 

[Dave K]

Strength, at least on race cars, since the load is fore-aft, it's better to put it in double shear in that direction.

-Dave

NX 9, Teamcenter 10

 

[BUGGAR]

And look closely at the bushing and how it's detailed for directionality of motion. This is how the bushing absorbs motion in the "wrong" direction.

Jimmie Johnson taught me this about the bushings holding the Camaro differential in place. And now he owes me another ride for this shameless plug.

 

[Overlap77]

I've never seen a lower arm rear bush of that design on anything designed with racing in mind e.g Caterham, Radical etc.
I can see how the void permits the up and down movement but surely on limit the bush will cause more geometry change?

 

[SwinnyGG]

You're not going to find a lot of similarities between the suspension of a Golf and the suspension of an SR8.

A lot more went into the design of that lower control arm than what was best from the standpoint of engineering perfect suspension geometry. Cost, packaging that suspension system into a relatively small area, manufacturability of both the control arm and the way it attaches to the car, tolerance for abuse (potholes, curbs, etc), longevity/warranty costs, you get the idea.

You're not going to be able to answer this question with one simple statement as to why that design is better- the decision was made as a result of all of the factors above.

 

[Overlap77]

Anyone who designs saloon car suspension for the manufacturers care to comment? thanks

 

[MikeHalloran]

I am not a suspension designer or NVH engineer, so feel free to dispute or ignore me.

Do note that the bush, or any such rubber bush, has a non-infinite, and likely known and controlled, stiffness, in every possible direction including rotation.

In this particular case, it may have cost more, in money or weight, to erect appropriate structure to make the hinge axis of the joint horizontal, than to just flex it in an unobvious way.

Mike Halloran
Pembroke Pines, FL, USA

 

[BrianE22]

Not quite sure I see the problem. The lower bushing in the top photo above is a cruciform structure. A structure like that would be soft torsionally and stiff in the radial directions which is what you want, correct?

 

[BrianPetersen]

If you look at the way this arm is oriented in the car and you study the different directions that lateral cornering loads are applied and the way fore-aft bump impact loads are applied, you will see that the bushing that is the "wrong" way (towards the front of the car) primarily gets loaded on bump impacts, it doesn't play much of a role in pure cornering loads. You will find that the bushing that is the "right" way is pretty stiff and the bushing that is the "wrong" way is pretty soft in the direction that the arm would "give" in response to a bump impact. You will also find that the steering tie-rod is pretty close to parallel with a line drawn between the ball joint and the stiff (rear) bushing so that fore/aft compliance of the suspension assembly in response to a bump impact doesn't affect the steering much. (The whole hub is allowed to deflect backwards and the motion is close to a parallelogram with the steering tie-rod.)

The objective is to have nice tight accurate steering AND a nice compliant quiet ride on choppy pavement.

Race cars aren't designed with consideration to NVH at all, but road cars are.

The bushing that is bolted together the "wrong" way is sufficiently soft and compliant that it really doesn't matter which way the bolt goes through, and for whatever reason it's probably easier for that bolt to go the other way when the subframe is going down the line.

 

[BrianPetersen]

I should add that it is quite apparent that the Golf Mk6 A-arm is still bolted vertically in the subframe via that cast-aluminum bracket that also contains the bushing using 3 bolts. The Golf Mk7 arrangement allows that extra part to be eliminated and now it's just one bolt instead of three.

 

[GregLocock]

It's an L arm not a wishbone. So the NVH bush is designed to be soft in vehicle Y and in RX. Ideally it would be stiff in Z and X, but it isn't vital. RY and RZ don't matter. Often thst bush is a hydrobush.

Cheers

Greg Locock


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

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[Overlap77]

thanks Greg. COuld you possibly help a layman understand the Axis involved by way of a drawing. The only thing I could find online pertained to robotics and I couldn't reference it to a car.

 

[NormPeterson]

The axis convention is more related to aeronautics and flight dynamics - I think William Milliken may have been the first to "borrow" it for automotive use.

Under "Reference Frames", scroll down to the "Body Frame" for a description in words. X is roll, y is pitch, z is yaw

https://en.wikipedia.org/wiki/Flight_dynamics_(fixed-wing_aircraft)

The only 'disadvantage' I can see with the "wrong" bushing orientation for a road car is if the bushing material and its shape are insufficiently compliant and start adding significant amounts of wheel rate, or themselves become overstressed as a consequence of suspension travel and deteriorate.


Norm

 

[BrianPetersen]

I don't have a way of putting a drawing here but here's a photo of how it is installed in the car.

http://www.tyrolsport.com/files/forumpics/GolfR/Front Suspension.JPG

The "stiff" bushing - the "handling" bushing - is the one that is left/bottom in the photo. Draw an imaginary line from the ball joint to the center of that bushing. Note that this imaginary line will be very close to parallel with the tie-rod (in three dimensional space).

The "soft" bushing - the "ride" or "NVH" bushing - is just outside the top of the photo. This is towards the rear of the car.

I had my earlier post backwards because I was using my own car for reference, which isn't a VW, and it has the "ride" bushing towards the front of the car and it's evident from this photo that on the VW the "ride" bushing is towards the rear of the car. But, it makes no difference, it works the same. Imagine what happens if the car goes through a pot-hole and there is a whack coming from the front of the tire contact patch. It wants to push the whole wheel and hub backwards in the car. Now imagine what happens as the "ride" bushing flexes and the "handling" bushing does not. The whole wheel and hub move back in the car a little, against the compliance of the "ride" bushing. But because the "handling" bushing is stiff, the whole arm pivots around that bushing a little bit. The leg of the L that leads to the lower ball joint moves back as the leg of the L that leads to the "ride" bushing moves inward because the whole deal is pivoting around the stiff "handling" bushing. But because the tie rod forms a parallelogram with the line between the ball joint and the "handling" bushing, the steering is not affected. The hub moves back, absorbing the hit and helping to keep it quiet inside the car, but the car isn't sent off course and there isn't a big whack to the steering.

Now imagine that you are driving down a smooth road and you turn the steering wheel. Since the ball joint and the "handling" bushing are almost in line in the cross-car direction, the cornering force predominantly goes through the "handling" bushing and very little of it goes through the other branch of the "L" through the "ride" bushing - so the steering is accurate.

Old cars that didn't have the separate "ride" and "handling" bushings either had a soft ride and mushy, vague steering (because both A-arm bushings were soft), or they had accurate steering and a lot of NVH (because the A-arm bushings were stiff). Design details like the one under discussion here are how new cars can have both accurate steering AND a quiet ride.

I'm sure there will be plenty of aftermarket "solutions" for the people who think any compliance in the suspension at all is a bad thing that has to be eliminated at all costs.

 

[Overlap77]

thanks Brian. that explains it very well. I've noticed on an old 205 GTI i had what an AMAZING difference stiff poly bushes made (both "correct" bushes) but huge NVH increase. Conversely my daily driver is a Fiat which has the rear bush like the Golf and even though the bush has wear it still drives very well around the corners but very little NVH.

Feedback from people seems to suggest the Mk7 golf has better ride over the bumps than the Mk6. What is unusual is I suppose there is conflict internally as to which they use as eg Mk3,Mk3,Mk4 all used NVH rear bush in "wrong plane" then Mk5&6 used "correct" handling bush then 7 went back to the way they did before. Or perhaps as the Mk4 was criticised as being woeful dynanamically (mainly due to too much front roll stiffness) perhaps they set about making the Mk5 a highpoint which it was in comparision.

It'd be great to see some action film of both designs at speed !

 

[BrianPetersen]

The Mk6 Golf still had the separate "ride" and "handling" bushings on the L-shaped arm, the only difference is how the "ride" bushing was mounted. But that bushing is so soft that it doesn't make any difference which way the bolt goes through it.

Remember that on ALL modern cars, the bushings don't actually "pivot". The arm pivots because the rubber deforms. Nothing actually moves - that's why cars don't need chassis grease jobs any more.

A lot of aftermarket urethane bushings do require the bushing to move - with the result that they have to be frequently lubricated, and they squeak, and they wear out. Been there ...

 

[Overlap77]

Yes i see what you mean. The more sporting models would have the voids removed in the "ride" bush. Fiat Coupe/Alfa 155/156 etc have an arm design where you need some force to install so the bush is twisted at ride height and under no twist at full droop.

 

[BrianE22]

Hmm, I thought z was vertical force, y was lateral force and x was for/aft force (at tire patch).

 

[GregLocock]

X longitudinal y lateral z vertical rx roll ry pitch rz yaw. That is not universal but all 4 car companies I have worked for use it. It is more or less enforced by software these days.

Cheers

Greg Locock


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[SwinnyGG]

Hmm, I thought z was vertical force, y was lateral force and x was for/aft force (at tire patch).

It is.

The other thing to think about is that in both scenarios, what you are calling 'wrong' and 'correct' bushing orientations, the bushing will react the same way to bump forces. In both cases, the axis of the bolt through the bushing is in the x-z plane, and the force acting on the bushing during bump is along the y axis. Whether the bolt is parallel with the z axis or the x-axis does not really matter.