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Discussion in 'Other Vehicles' started by Dr Obnxs, Oct 5, 2010.
This really explains a lot. It's worth looking at.
On MF a poster (Ryan) mentions the difference in steering geometries between the first and second generation MINIs as a possible reason it's more pronounced on the later model. A few things I've observed on the second generation: increased caster, inner ball joint position relative to the pivot axis, control arm angle to the outer ball joint, angle of the control arm pivot axis, and king pin axis & offset. Interesting the only similar things mentioned as contributing factors in the Ford presentation are the king pin and caster relationships, nothing about the control arm path.
Ryan may chime in here...
but I think the control arm geometry couples into torque steer more indirectly via roll center and camber gain as the suspension moves.
I actually thought that this would generate more discussion! Some of the graphics in the PDF show how to change camber and offset decrease scrub radius and hence torque steer.
GMâ€™s HiPer Strut Rethinks MacPherson Suspension
I really like the Revo knuckle design. Reminds me of my old RC race car setups from when I was a kid.
I'd be interested in seeing what roll center adjustments could do for the R56 torque steer.
A couple of things...
Good roll center managment will help in that the contact patch won't change as much under cornering cause the car will lean less than a car with poor roll center managment. But it still will require doing other things (offset/camber etc) to minimize the scrub radius.
Cool article on the GM HiPer strut.... Same idea as the Revo-Knuckle. Ford didn't quote the wieght penalty, but the GM article says 20+ lbs! Yikes! Did you notice that the system would be great in small FWD cars with big wheels? Sound familiar? Anyone want to design one for the MINI?
The Ford and GM solutions look like clever thinking.... until you look at the essentially similar system that Renault have had in production for about 5 years on their hottest FWD models:
Renault's PerfoHub Pictures | evo
Renault's PerfoHub Pictures | evo
I think the Ford version is probably cleverer, but then with 5 years to think about it, it ought to be!
Both articles are very interesting. The new designs that have two ball joints and a bushing in the position of the ball joint on a strut suspension will have a big impact on how large a tire can be installed. The spring does not have to be in the King Pin axis, and the king pin offset is reduced. As the strut axis changes from the king pin axis the strut gets more side load. GM talks about a better camber curve. I can see how it is slightly better, but certainly not as good a double swing arms. One thing I havenâ€™t figured out is how to calculate the swing arm length. I think it is determined the same way a strut system is determined. In that case the swing arm length would be longer, and the roll center would not change as much as the car rolls. The GM design seems to allow for camber adjustment, Fordâ€™s design doesnâ€™t unless the upper ball joint is in a eccentric.
I didn't know about the Renault set up...
they are all variations on the very same theme. Basically doing something to change the pivot axis to be different than the upper pivot of the strut and the lower ball joint. I guess this cat has just been skinned by three very similar designs!
Camber can still be changed by working with the upper strut mount, as that tilts the whole assembly. I haven't really thought about it enough to know how that's gonna screw the pooch in other ways, if it does at all...
And I think the swing arm lenght is still based on the lower control arm pivot points like the traditional Mac Strut design.
Anyway, there are now examples of three car companies using this basic design to reduce torque steer in more powerfull FWD cars... Maybe there's something to it!
I do not think moving the top of the strut in to adjust the camber would screw this system up more than a strut suspension. When you move the top of the strut in you are changing the king pin angle and the swing arm length gets shorter. In most cases that isnâ€™t a big deal, but as the struts get shorter, and the angle get more severe, it can be a big deal. The GM design seems to use an eccentric between what I would describe as the regular strut, and the top of the new spindle. Hope everyone understands my names for parts no one has defined.
The swing arm length on a strut suspension is determined from the instant center. And the instant center is the interception of a line drawn through the lower control arm pivots in the side view, and a line horizontal to the king pin angle at the top of the strut pivot. The new design offsets the kingpin axis, and reduces the angle. Thinking out loud I think the upper line is drawn horizontal to the king pin axis, but the height is determined by the top of the strut.
The Renault design has a link to prevent the strut from rotating because they use a ball joint at the bottom of the strut. GM and Ford use a bushing and bolt. I am not sure if this makes a difference, but it would be very interesting to have a computer kinematic model to understand how this system moves. There is a lot going on here.