Why the LCA bushing has the hexagon instead of round hole?

[sapote]

To match the shape of the control arm end, but why the designer didn't have it with the round end instead of hex end?
I think with the round end inside a round hole, the end can rotate in the bushing instead of twisting the rubber which causes the tear along the red circle.

[digger]

It is designed to not rotate and would probably help with orientation . Earlier 3 series models used a round pin and bore

[jayjaya29]

The twisting of the rubber is the intended design of the bushing. E36/E30 front control arm pins are round but they do not rotate within the rubber bushing, it also is fixed to the rubber via a tight press-fit.

[sapote]

It is designed to not rotate and would probably help with orientation . Earlier 3 series models used a round pin and bore

If it's a round shaft and round hole, no orientation needed to mount. Functionally nothing wrong to allow the shaft to rotate in the bushing, but customers might complain about the noise of a "cheap suspension". Porsche rear spring plates rotate in the round bushings.

[sapote]

The twisting of the rubber is the intended design of the bushing. .

why? Twisting cut the bushing life short.

[digger]

If it's a round shaft and round hole, no orientation needed to mount. Functionally nothing wrong to allow the shaft to rotate in the bushing, but customers might complain about the noise of a "cheap suspension". Porsche rear spring plates rotate in the round bushings.

Since the CA pin insertion into the bushing is designed to NOT slip the orientation matters as you want to minimise pre stress of the rubber when the bushing is installed at ride height to maximise life of the rubber. You cant just install willy nilly without affecting the life of the bush. Where as with a hex there are limited positions to get orientation wrong

You could add a bearing/ball joint or bushing of suitable material but this adds adds cost for the OEM that doesnt bring them any benefit but risk of squeaks as the bushes have a reasonable service life already

The aftermarket poly bushes for the E36/30 work this way they often have a bushing that is pressed over the control arm round spigot and slips into the poly bushing with "clearance fit" that is inside the lollipop and it is basically a rotating pin joint. Some just have CA pin into the bush for the uber cheap versions usually harder poly etc. Powerflex do this for e46

https://powerflexaustralia.com.au/car-make/bmw/3-series/e46-1999-2006/e46-m3-inc-csl/e46-m3-inc-csl-road-series/pff5-4601m3/

[cobra]

If you spin the control arm in the rubber, it will eventually wear away the rubber and become a loose fit.
By keying it and allowing the bushing to flex instead, the life is longer.

[sapote]

If you spin the control arm in the rubber, it will eventually wear away the rubber and become a loose fit.
By keying it and allowing the bushing to flex instead, the life is longer.

This bushing handles much less load than the Porsche rear Spring Plate bushing, and so I would grease it up to let the shaft rotate easier. I still think the engineers wanted to zero out any potential creaking noise by locking the shaft to the bush, and not about functional reasons. I think flexing the big donut breaking it faster than rotate inside the hole. I might try to smooth out the hex on the shafts and let them rotate inside the rubber. I also think why they didn't use a ball joint here instead?

[sapote]

Since the CA pin insertion into the bushing is designed to NOT slip the orientation matters as you want to minimise pre stress of the rubber when the bushing is installed at ride height to maximise life of the rubber. You cant just install willy nilly without affecting the life of the bush. Where as with a hex there are limited positions to get orientation wrong.

Now I think I understand your concern about preloading the rubber bushing, like the ones on the rear suspension. The front LCA bushings carry much less load as it has a relative longer shaft from the bushing to the ball joint near the wheel, so force by the hex shaft on the bushing is less. It has much less effect on the wheel alignment compared to the rubber bushing on the rear control arm which also needs to handle all of the force when accelerating the car. This is why the rear bushing is installed very tight, not the front LCA bushing though.

[digger]

Now I think I understand your concern about preloading the rubber bushing, like the ones on the rear suspension. The front LCA bushings carry much less load as it has a relative longer shaft from the bushing to the ball joint near the wheel, so force by the hex shaft on the bushing is less. It has much less effect on the wheel alignment compared to the rubber bushing on the rear control arm which also needs to handle all of the force when accelerating the car. This is why the rear bushing is installed very tight, not the front LCA bushing though.

its the torsion pre-strain that is of concern is rubber has finite strain capability. Normal bump/droop travel say +/-25mm might be an angle of +/-5 degrees so not much, but if you install 30 degrees offset the bush is working 25-35 degrees and proportionally much higher strain level. Rubber or anything does not likes "high" strain it causes fatigue under cyclic loads and even overload on a single cycle if high enough

As far as loads go CA bush takes the majority of the longitudinal wheel loads which include from braking and longitudinal impacts due to bumps (bumps cause a vertical component but also longitudinal component due to the component of the vector) these get transferred to a lateral load in the CA rear bush radial direction when you do a a sum of moments about the subframe ball joint. There is also some lateral cornering and steering load transfer but should be smaller in magnitude as the control arm take these more direct laterally to the subframe and through tierod. Not much vertical load is borne by the bush compared to the rear of some suspension types due to the motion ratio difference

[heinzboehmer]

I also think why they didn't use a ball joint here instead?

NVH for sure.

Control arm pivots about the ball joint that attaches to the front subframe, so any front/back loads at the tire get converted to left/right loads at the bushing.

Ball joint there means more NVH transfer into the cabin when hitting bumps and similar.

[sapote]

NVH for sure.

Control arm pivots about the ball joint that attaches to the front subframe, so any front/back loads at the tire get converted to left/right loads at the bushing.

Ball joint there means more NVH transfer into the cabin when hitting bumps and similar.

noise can be an issue, but the inner ball joint is bolted to the subframe and it's also a noise source similar to the proposed rear ball joint altough this one is closer to the cabin. I also think the ball joints with zero plays would not cause a lot of noise.

My Citroen SM has zero rubber joints on all the suspension parts which are either ball joints or roller bearings, and no noticable more road noise than other cars. It's a flying carpet.

[Tbonem3]

We have a follow up question - why did God design Sapote this way?

[Pklauser]

Manufacturers typically take into account the rubber preload when calculating spring rates as well. The force exerted by the pushing acts as a spring. This one maybe not so much given its distance and the angles, but worth considering.

[sapote]

Today I filed the hex end into a smooth surface for it to rotate inside the bush, lubed the shaft and bushing with grease for low friction. Will see how long it will last. Done on a 325iT car.

Stock shaft:

Filed shaft: