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Was: How much Caster is too much? Now: Caster and Ackerman


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Yeah, I'm planning on going to a custom cross member. Also, I am going to do a raised steering rack with rod ends. I will be using high offset spacers on the top side of the steering knuckle. I'm moving the motor back behind the cross member and steering rack. Its a lot of little changes that I hope add up to something decent ;)

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On a similar note,

 

I have been playing around with some S4/S5 RX7 stub axles - when you look at the RX7 crossmember, steering rack and stub axles (which have the steering knuckle in them) compared the the stock Z stuff there is a weird difference that you guys might be able to explain for me?

 

The S4/S5 RX7 Lower Control Arms are the same length as the Stock Z LCA's, the rack's both sit about 50mm in front of the crossmember measured from the rack ends back to the outta LCA pivot hole,

 

BUT..........the RX7 steering knuckle distance from the ball-joint is HUGE, like 135mm, from memory, a good 40-50mm longer than the Z's ball-joint to tie-rod end distance on the steering knuckle!

 

Is that factory design'd in Ackerman? or Am I not getting this concept?

 

Another odd thing is the position of the LCA pivot relative to the rack end, its easily, 30-40mm lower, also the tie-rod end position on the RX7 stub axles is a good 20mm higher than the ball-joint location - factory built in bump-steer I guess?

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BUT..........the RX7 steering knuckle distance from the ball-joint is HUGE, like 135mm, from memory, a good 40-50mm longer than the Z's ball-joint to tie-rod end distance on the steering knuckle!

 

Is that factory design'd in Ackerman? or Am I not getting this concept?

 

Another odd thing is the position of the LCA pivot relative to the rack end, its easily, 30-40mm lower, also the tie-rod end position on the RX7 stub axles is a good 20mm higher than the ball-joint location - factory built in bump-steer I guess?

Not really familiar with the RX7 per se, but this all sounds correct. The problem with using longer steer knuckles on a Z is the rack ration isn't right for it. If you could find a different rack that might make a longer knuckle a viable option to get more Ackerman.

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It is a Macpherson strut design, but the strut detaches from the stub-axles via 2 bolts, the hub, stub axle and steering knuckle are all one piece (the hub does detach though) and the strut is another!

 

I will post a pic so you can see the difference in knuckles!

 

But it is basically just a Mac strut design,

 

The problem with using longer steer knuckles on a Z is the rack ration isn't right for it. If you could find a different rack that might make a longer knuckle a viable option to get more Ackerman.

 

I was going to use the Subaru Rack - how can I find out if this rack will be suitable for the rather long RX7 steering knuckles? I would use the RX7 rack, but its just way to wide to be a practical choice!!!

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There is plenty of ackerman to be had in a front mounted rack layout. You just need to move the steering rack back. I can model it up for you in CAD later today.

 

Justin

 

Yeh sure but not with the stock rack location. The more I think about it the more a custom front X member seems to be a practical alternative that can be designed to address several issues, including sump clearance when installing a front sump motor.

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l_fb89e22714de41528fa316f8b3c28d83.jpg

 

I suppose my own adapter solved the problems. High caster but low caster-trail distance, 50% accerman, quick ratio, EPS and bump-steer correction.

Edited by KAZU
spell correction
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Yeh sure but not with the stock rack location. The more I think about it the more a custom front X member seems to be a practical alternative that can be designed to address several issues, including sump clearance when installing a front sump motor.

 

I think you are correct. a custom x member that moves the LCA inner mounts forward and inch, while leaving the steering rack in its factory position front to back would help a lot. It would also increase the wheelbase a little so you'd have to watch out for tire interference.

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I think you are correct. a custom x member that moves the LCA inner mounts forward and inch, while leaving the steering rack in its factory position front to back would help a lot. It would also increase the wheelbase a little so you'd have to watch out for tire interference.

 

Moving the inboard pick-up, on its own, has no effect on Ackerman. As stated earlier, moving the outboard pick-up (forward) does, providing the steering knuckle follows it (or moves even farther forward).

 

What you're looking for is a change in angle between the steering knuckle pivot points and the tie rod pivot points. If you make a change that does not effect that angular relationship, it's not doing anything for Ackerman. Change that angle and you change Ackerman.

 

 

Ackerman.jpg

Edited by RTz
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I was refering to moving the inner mount forward while keeping the LCA perpendicular to the centerline of the car. This would move the whole strut assembly forward too.

 

How do you calculate the percentage ackerman? I can figure out the dynamic toe change in cad, but I don't know what people are refering to regarding the percentage ackerman.

 

Justin

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100% Ackerman means that the wheels follow the arc of the turn exactly, vs parallel steer where they both turn at the same angle. I'm sure there is a way to calculate it without measuring, but since I suck at math and trigonometry I was just going to measure it and then compare to 100% Ackerman which you can find in books, and figure it that way.

 

I think you're right about the control arm angle. I made the spacer in front of the LCA as short as possible so that the control arm could stay as perpendicular to the frame rail, and then moved the rack back to get the Ackerman. It's the angle of the tie rod that matters, not the angle of the control arm. I modified Ron's jpg to show the difference. In my suspension the TC rod pivot and control arm pivot are also in line front to back.

 

Ackerman3.jpg

 

I'm very interested in what KAZU did. I know tube80z has talked about that before, and I've always been a little confused by the idea. It seems to me that by putting the strut off line with the ball joint, when you turn the wheel you're actually moving the struts side to side in relation to the centerline of the car. Maybe that doesn't matter in actual practice, but it seems like a really weird side effect of offsetting the strut and ball joint. Do I have that wrong? I feel like I don't have the full story there...

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I couldn't figure out the effects of strut offsetting, but on several cars such as air-cooled Porcshe 911s or Ford Focus WR cars you'll find such offset. I am a big fan of the steer feel of BMW 3 series, well known as high caster angle car and, as you can see, the strut and spindle of the car are obviously at offset position from the king pin axis. Thus, even though strut offsetting appears to be weird, I guess there is no practical side effect.

600x450-2008061700094.jpg

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