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<  Suspension, Steering, Wheels, Tires  ~  Ackerman Angle

PostPosted: Sat Apr 01, 2017 6:43 pm
VerdeVerdeJoined: Sun Aug 19, 2007 9:15 pmPosts: 522Location: South Australia
When trying to figure out Ackerman angle, the diagram below is often the sort of thing that is used to figure it out.
But the diagram only looks at the top balljoint or stut top and doesn't allow for the Kin Pin Inclination angle.
Using I modeled the front suspension of my 75 with its long top balljoints. With a static camber of -2*, the KPI is a bit over 6*.

Incidendally, with 16x8 rims, with a +30 offset and 215/45 tyres, the steering has a scrub radius of 88mm! :shock:

Anyway, the question is: Where along the line of the of the KPI should the line be that converges with the rear axle centre line???
With the bumpsteer correction that I've done to work the long top balljoints, the centre of the outer tierod end is about 250mm from the ground.
So should the intersection line that goes to the centre of the rear axle actually be looking at the KPI angle 250mm above the ground?
When looking at the typical Ackerman diagram, this would put the front axles intersection point further out than the top balljoint would and further in than the bottom balljoint would.
Does this sound correct?
VSusp_default_values_-_2017-04-02_11.14.55.png [ 100.7 KiB | Viewed 1366 times ]

Ackermann.jpg [ 26.18 KiB | Viewed 1366 times ]

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PostPosted: Fri Apr 07, 2017 10:47 pm
GoldGoldJoined: Sun Mar 20, 2016 2:46 pmPosts: 175
Hey Duk , I'm also waiting for some answers on this . How must higher have you gone on top balljoint - 25mm ? How much did you bend the steering arms on hub or modify the pickup point ?

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PostPosted: Tue Apr 11, 2017 12:31 am
VerdeVerdeJoined: Sun Aug 19, 2007 9:15 pmPosts: 522Location: South Australia
Hey KevinR,
I don't have a standard balljoint to compare the long balljoint with, but I think it was close 25mm.
The bump steer corection was done by using spherical rod ends. Their pivot point is higher than a tie rod end, so with a 5mm spacer I was able to get rid of almost all of the bump steer.
It's actually a slightly modified bump steer correction kit for Nissan Skylines and Silvia/180/200SX's with 14mm in rods.

There is a bigger badder plan in the head and this uestion needs to be answered.
1456721_10200864137900999_881513307_n.jpg [ 49.22 KiB | Viewed 1303 times ]

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PostPosted: Tue Apr 11, 2017 2:14 am
GoldGoldJoined: Fri Oct 23, 2015 4:15 amPosts: 50
As noted the upper and lower ball joints aren't in a direct vertical line, to determine Ackerman you use the imaginary line between the upper and lower ball joint. Level horizontally with the steering ball joint.
Just curious, what sort of Ackerman figure are you aiming for?

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PostPosted: Fri Apr 14, 2017 1:35 pm
GoldGoldJoined: Sun Mar 20, 2016 2:46 pmPosts: 175
Are you running toe in or out ? Do you want a true Ackerman or more ?

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PostPosted: Fri Apr 14, 2017 5:11 pm
VerdeVerdeJoined: Sun Aug 19, 2007 9:15 pmPosts: 522Location: South Australia
The idea is to have zero toe, as close to zero bump steer and simply run true Ackerman.

The reason why I ask this question is because I want to increase the track width substantially. But I will be using +50 offset RX7 wheels and RX7 wheel hubs. These have a more central mounting face than the Alfa's wheel hub.
My thinking here is to substantially reduce the scrub radius of the steering and use longer control arms to help give more consistant roll centre location.
With the uprights further apart, the angle of the steering arms will need to change to make sure the Ackerman angle is correct for the wider track width.
I am seriously tempted to create upside down bottom balljoint uprights/spindles, but slightly raise the bottom balljoint for wheel clearance. This would require the outer tierod end to be lower. That shortens the steering arms significantly and that will help quicken the steering.

So wider track width for more stability and turn in response for a given wheel/tyre/spring rate (actully, natural frequency) and antiroll bar combination.
Longer control arms and different offset wheels and wheel hubs to reduce the scrub radius. This means there is less room required inside the wheel well to allow for steering, so wider wheels and tyres can be used. There should also be less stress on the stub axles.
Revised geometry to raise the front roll centre height and substantially improve the camber curve. But the camber curve won't need to be too dramatic........
Bent steering arms to correct the bump steer from the revised geometry and correct the Ackerman angle (which I have read isn't correct in the standard configuration, but have never checked). Steering arms would be bent while maintaining minimum bumpsteer and having 6+ degrees of positive caster. Extra caster is used to make sure that the caster angle is greater than the King Pin Inclination angle. I've not got my head around why, but I have read that the caster angle should be greater than the KPI angle to help reduce mid corner understeer.

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