tomek9210

Wheels geometry questions - ackermann steering and kingpin inclination

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Hello,

I was watching Nicjasno's videos and some questions arosed.

Let's say I have front SLA double wishbone suspension, no drive, lower arm is 6 studs long, upper arm is 5 studs long, there is a kingpin inclination and caster angle. I want to have ackermann steering and avoid bump steer.

Where should I put the steering arm and how long should it be for steering rack ahead of front axle or behind it? Should it be 6 studs long when it is closer to the lower arm and 5 studs long when closer to the upper one?

 

And another situation - let's say I have multilink suspension - two angled upper links and two angled bottom links, bottom ones are longer, there is also a kingpin inclination and caster angle, no drive. The question remains the same :wink:

 

I will appreciate your advice :classic:!

Thank you in advance.

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On either suspension variant, the best place for the steering links and rack would be behind the axle. If it is in front, you will generally end up with reverse Ackermann geometry.

Regarding the steering arms... That is more difficult. It really depends on the supporting structure built around the whole axle and the space in the chassis itself. From my experience, the steering arm needs to be parallel to the lower suspension arm, and either even with, or one stud above the center of the wheel. One stud below is generally quite difficult, and I have not done that before, so I cannot say what effect that might have. Hope that helps!

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The steering arms need to be paralel to either the upper or lower suspension arms. It depends which arm they are closest to. I'd advise against placing them in the middle . If you have unequal length wishbones, it's basically impossible to avoid bump steer in this case with lego.

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Thank youfor your answer. But let's look at this picture.

maxresdefault.jpg

The steering arm is one stud closer to the wheel centre. And when we draw the imaginary line through steering arm pivot point and lower arm pivot point in the direction toward the rear axle, we should get the same, more or less, situation when the gear rack would be placed behind front axle and steering arm placed one stud closer to the centre of the vehicle than steering arm pivot point. That results in ackermann geometry, am I right?

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You are talking about this i assume:
Ackermann2.jpg

But one can also achive ackerman the way i did (and dodge) in the challenger suspension, where the steering arms are angled backwards. The e30 also has them angled slightly backwards to achive this:
ackerman.jpg

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I came across this website some time back, which explains bump steer quite clearly. This figure (taken from that website) illustrates the various possible positions and lengths of tie rod (aka steering arm) that can be used in order to have zero bump steer.

 

bsdrawing.jpg

And for ackermann, you just have to position (i.e. translate horizontally) the steering arm closer to the vehicle centre (if behind front axle), or closer to the wheel centre (if ahead of front axle).

Edited by PorkyMonster

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8 hours ago, nicjasno said:

You are talking about this i assume:

But one can also achive ackerman the way i did (and dodge) in the challenger suspension, where the steering arms are angled backwards. The e30 also has them angled slightly backwards to achive this:
 

Yes, that is what I am talking about. But your dodge is quite different, because there are two bottom arms and the steering link is angled the same as the spring carrying one.

https://www.dropbox.com/sh/psueac221sge516/AAAV1yhC8uRI2Lf2TPKAk98la/comparison front.jpg?dl=0

In e30 the bottom arm itself is angled.

https://www.dropbox.com/sh/j834hctryh55g32/AADUZGqowKsCEvmKnYmf20Fta/Details 1?dl=0&preview=2016-06-16+23.35.18.jpg

 

2 hours ago, PorkyMonster said:

I came across this website some time back, which explains bump steer quite clearly. This figure (taken from that website) illustrates the various possible positions and lengths of tie rod (aka steering arm) that can be used in order to have zero bump steer.

And for ackermann, you just have to position (i.e. translate horizontally) the steering arm closer to the vehicle centre (if behind front axle), or closer to the wheel centre (if ahead of front axle).

That is what I was suspecting :classic:

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The angle of the Main wishbone/load carrying arm is irrelevant. The main thing is that if you don't have the luxury to position the steering arm pivot closer to the wheel center (in the case of the steering rack being in front of the axle), you can also achive ackerman by having the steering links angled back.

That said, in lego, you don't have the luxury of adjustable steering links, like in real life. So in the e30 i had to angle the wishbones back, to maintain the same length of wishbone and steering link in order to avoid bump steer. In the challenger, the whole design of the real life suspension came my way :)

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