17 replies to this topic

[MattPete]

Just how do these things work?

I take it there is a pivot point at the bottom where the lower arm attaches, but is there also a second pivot point at the top of the strut where it attaches to the strut tower?

If that's the case, shouldn't McPherson strut suspensions have the same camber charachteristics (good and bad) as swing-arm suspensions?


Thanks in advance,
Matt

[Ray Bell]

The top is a fixed point, so camber changes are totally dependent on the length and angle of the lower arm. On most cars they decline at rest, so body roll will see the outside wheel get a little extra negative camber, the inside gain positive, keeping the wheels somewhat closer to vertical with the road than if there was no camber change.

In practice, usually the spring sits solidly against the top of the tower, the strut shaft is mounted in a rubber block to get its flexibility. Turning mechanisms vary a little, but most have some kind of bearing that allows the whole lot to spin, or they allow the rubber to absorb the twist. Peugeot, with the 404, went to great pains to avoid wear on the shaft (seeing the Ford Consul and Zephyr problems, the only cars of any size with struts to the date of their design) and fitted a bearing at the bottom of the spring so that the shaft remained in one position and the strut body turned around it... so spreading the wear point.

[MattPete]

Originally posted by Ray Bell
The top is a fixed point, so camber changes are totally dependent on the length and angle of the lower arm. On most cars they decline at rest, so body roll will see the outside wheel get a little extra negative camber, the inside gain positive, keeping the wheels somewhat closer to vertical with the road than if there was no camber change.

So, struts act exactly like an old swing axle, as far as camber change goes? That is, the degree of camber is completely dependent and follows the radius of the lower control arm? If that's the case, then the strut must also move inward and outword as the lower arm goes through it's range of motion. Why doesn't the strut snap off?

In practice, usually the spring sits solidly against the top of the tower, the strut shaft is mounted in a rubber block to get its flexibility...

Does the rubber block act as a top pivot point and therefore prevent the rod from snapping as the lower arm goes through it's range of motion?

[Ray Bell]

That's the idea... but because the strut itself remains constantly located at the top the camber change is nothing like a swing axle... the bottom link is on a ball joint.

[PDA]

It would only have swing axle characteristics if the lower control arms attached both inner and outer, in the same plane as the drive shaft, and the dtrut also attached to the same plane. In fact, the lower control arms attach to the bottom of the up right, and below the plane of the drive shaft at the inner end. The strut attaches to the upper part of the upright, giving double wishbone type geometry, rather than swing axle geometry. Camber changes ar thus reduced cf. swing axles. I would expect the geometry to be arranged so that the strut lower pivot has minimal inward and outward movement and that the amount of movement allowed can be accomodated by the rubber top mount.

[AS110]

The McPherson strut has very widely spaced mounting points,this reduces the stress in these areas,making for lighter construction.The top mount is also in the line of it's movement?,so in the full travel of the suspn,it hardly moves.Steering movements are greater,and a bearing is provided for this.Some designs also have caster and camber adjustment at the top mount,camber - move it in,caster - move it back.
Later designs can have seperate spring and strut rod mounts,relieving the stress on the rod.Some tricky lower arms (BMW) give camber change when cornering.

[AS110]

Oops the BMW lower control arm gives TOE changes during cornering,and going backwards! Maybe to counter camber changes and bump steer.
BMW also leans the strut back,like a motorcycle fork.

[Ben]

Leaning the strut back would be to give a caster angle.

See the other thread for the effects of caster angle.

Ben

[AS110]

Yes,leaning the strut back would give caster,but there are other ways to acheive this.
I singled out BMW as they one of the few who do it this way.I think it is more to do with the active/passive suspn.As the wheel moves up on bump,it also moves back,depending on where the steering arms are located(can't remember) this would give toe in or out,each wheel being affected seperatly.

Giving a motorcycle rake to the forks is just an easy way to get the geometry required.The suspn works better vertical.Tony Foale once modified a BMW motorcycle to have vertical forks,while retaining the original geometry.no diference was noticed in handling,but the vertical forks worked better over bumps,but gave more judder under braking.

[Ben]

The only way to get caster on a Macpherson strut is to angle it backwards.

Ben

[AS110]

Check out a supermarket trundler(but please return it!)
Caster,but no rake.But cars require only small amounts of caster,so the rake of the strut is only a degree or two.It's just that the BMW has so much rake,with that much caster it would be a pig to drive.It's how the hub is mounted that would change the caster.

Each car is different,I just find it interesting how they all go about achieving the same result in different ways.

[AS110]

Back this morning with a better explanation to clear things up.
To find caster we run a line through the steering pivot points.That is through the king pin of a beam axle,or with our strut suspn,through the top mount and the lower outer ball joint to the road surface.Next we take a vertical line down from the axle centre to the road.We can measure the angle and call it the caster angle,or measure the distance and call it trail.

Ok,these points and angles are now fixed,so now we can do anything we like with the strut,and it will NOT affect the caster.The spring and dampening unit is separate from steering geometry.

The Mcpherson strut is like telescopic forks on a motorcycle,just an easy and cheap way to do it.Thats why we don't see it in F1.

What's the effect of caster?,bugger all I think.I once used to ride a motorcycle with 8 INCHES of NEGATIVE trail.I admit it wasn't a great handler (a classic "chopper"of the early 70s) but it was ridable and stable at 100mph.It was the most powerfull bike I had ever ridden,and was more worried about power delivery than handling.Just another case of theory and practice not agreaing.

Yes,I did follow the "castor" thread.Check my post for my thoughts on "castor".

[imaginesix]

Originally posted by MattPete
Just how do these things work?

I take it there is a pivot point at the bottom where the lower arm attaches, but is there also a second pivot point at the top of the strut where it attaches to the strut tower?

If that's the case, shouldn't McPherson strut suspensions have the same camber charachteristics (good and bad) as swing-arm suspensions?

Thanks in advance,
Matt

Thanks for asking, I've been overdue to read up on struts for a while now.

First of all, the strut is mounted at the top (at the strut tower) in such a way that it can pivot in any direction. The wheel assembly is mounted solidly to it, so that any camber change of the wheel must come from the tilting of the strut. This is how it differs from swing-arm suspensions.

The strut will tilt when the bottom pivot is forced to move in any direction other than pure compression/extension. The lower control arm (LCA) is what locates the bottom of the strut, so we can rephrase that to say that the LCA will cause camber change whenever it is at an angle other than perpendicular to the strut axis. So the further the LCA and strut are from 90 degrees to each other, the more camber change will exist.

Due to packaging constraints, it is very difficult to acheive a satisfactory angle between the LCA and the strut. The LCA itself must be very close to level, with it's outer joint deep inside the wheel, to give a solid foundation for proper handling. That leaves only the angle of the strut as the variable to acheive proper camber change, but it too is restricted in angle. It's top mounting point (to the chassis) also sets the kingpin angle, so it cannot be moved very far inboard, while the strut body must clear the tire, so it cannot be moved very far outboard.

The end result is that the LCA and strut end up perpendicular to each other in the compression range of the suspension movement (where it counts most), and thus no camber change exists. Past that point, the wheel will start to lose camber as it compresses further!

Other disadvantages are the greater unprung weight, greater friction, and the expansive overall size of the system.

[imaginesix]

Originally posted by AS110
Check out a supermarket trundler(but please return it!)
Caster,but no rake.

Despite the name, casters don't have any caster. They only have trail.

Caster and trail are not two different ways of measuring the same thing. You can have no caster and lots of trail, or lots of caster and no trail, or any combination in between.

Caster allows camber change with steering input, and can compensate for the undesireable camber effects of kingpin inclination. It can also be used as a self-centering mechanism when the caster axis crosses the wheel's vertical centerline below the ground.

That is what you experienced on your chopper. I bet that thing was a monster to turn in low speed corners. That is because you were using the weight of the bike to keep the wheel pointed straight, so when you went to turn, you in fact had to put enough effort into the steering to raise the front of the bike.

[Rainer Nyberg]

Ben is right....

I believe the man who's is named after this strut was called 'MacPherson' and not 'McPherson'.

[AS110]

Yes,comparing furniture,motorcycles and cars is not really possible,it's just that with a motorcycle the geometry is visable to the naked eye.It's a bit like using the water anology to explain electicity,it doesn't always work,but can help you with the basics.

The chopper was certainly a pig at low speeds,but I sure thought I looked real cool! Even if it had 8in of positive trail,I would of still been lifting the weight of the bike on low speed manouvers,not as bad as negative trail,but the other side of zero.That must be why it was not totaly unridable,the weight working through 8in of leverage kept the wheel pointing straight.

[AS110]

His name was Earle S MacPherson,and worked for GM in the 40s ( so did Donald McPherson)
He designed a small car for Chevrolet with a lot of radical thinking,but it was too much for GM and they dropped it.So MacPherson went to Ford,and designed the Mk 1 Zephyr,using his struts,and suspended pedals,another of his inovations.
His original design used the sway bar as a radius rod to locate the lower control arm,and Fords used this until the last of the Escorts (it may still be Fords system,but we don't get them anymore in NZ,I have a 87 Escort van)

[Croaky]

As far as I know, MacPherson struts are widely used 'cos they're compact, especially in FWD cars where the engine compartment is already crowded with a transverse engine and transmission, leaving very limited space for the suspension. As everyone's said, it's a compromised system compared to fancy double wishbones but the space saving makes it attractive for mass produced cars.