Rack placement change for additional Ackerman

69427

69427

The Artist formerly known as Turbo84
Joined
Mar 30, 2008
Messages
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Clinging to my guns and religion in KCMO.
I'm having a mental block at the moment, and unless someone can straighten me out soon I'm going to have to revert to a bunch of geometry calculations to make this issue make sense (I can do the math faster than I can build a model of the steering linkage to solve this). I'm interested in seeing if I can change the steering rack location so that when cornering the lightly loaded inside tire will steer/rotate a larger angle than the heavily loaded outside tire. I'm trying to figure out if the increased angle of the inner tire will allow the it to contribute more to the cornering effort if it is made to run at a higher slip angle. Two questions:

1) Am I full of crap?
2) Do I move the rack forward or backward ('96 C4 suspension with front steer rack)?

Thanks for any help,
Mike
 
Mike,

Moving the rack forward or aft changes the Ackermann. Back increases Ackermann. The Woodward Steering tech section has a good discussion. Have you read Ron Sutton's handling discussions on "another forum". Lots of detail. His solution regarding Ackermann, in a nutshell -- a little for autocross, less for road racing -- used as a tuning tool. Late Corvettes have almost zero Ackermann. It's all a matter of establishing the right slip angle for the inside and outside tires, considering tire loading. It is a little more complex than that, but along those lines.

Try this link for the Ackermann discussion. It's on page 72. Remember that this is mostly a circle track discussion, and they like a lot more Ackermann, especially on the dirt. http://woodwardsteering.eu/images/basic rack and pinion tech.pdf

Pappy
 
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mfain said:
Mike,

Moving the rack forward or aft changes the Ackermann. Back increases Ackermann. The Woodward Steering tech section has a good discussion. Have you read Ron Sutton's handling discussions on "another forum". Lots of detail. His solution regarding Ackermann, in a nutshell -- a little for autocross, less for road racing -- used as a tuning tool. Late Corvettes have almost zero Ackermann. It's all a matter of establishing the right slip angle for the inside and outside tires, considering tire loading. It is a little more complex than that, but along those lines.

Try this link for the Ackermann discussion. It's on page 72. Remember that this is mostly a circle track discussion, and they like a lot more Ackermann, especially on the dirt. http://woodwardsteering.eu/images/basic rack and pinion tech.pdf

Pappy
Click to expand...

Hey Pappy, I appreciate the input.
I'll check out the Woodward link you provided. I'm not familiar with Ron Sutton but I'll try to track down his writing.
Thanks again. :thumbs:
 
Pappy, been reading Ron Sutton's posts. Very interesting fellow. He's got me thinking about a few additional alignment/geometry tweaks I can do over the winter to try to improve the front end grip. Thanks for directing me to his write ups. :thumbs:
 
Steering rack relocation

Yesterday I had the first track day opportunity to see if the steering improved (less mid corner understeer) after changing the rack position. I moved the rack rearward by 5/8" (limited by the placement of a frame stiffening crossmember). I don't have any steady state skid pad data, but the car definitely felt better through the two tight mid-speed corners at Topeka. Over the winter I'll cut out and reposition the frame brace and experiment around to see if any additional rack placement adjustment will further improve the front grip.
 
69427 said:
Yesterday I had the first track day opportunity to see if the steering improved (less mid corner understeer) after changing the rack position. I moved the rack rearward by 5/8" (limited by the placement of a frame stiffening crossmember). I don't have any steady state skid pad data, but the car definitely felt better through the two tight mid-speed corners at Topeka. Over the winter I'll cut out and reposition the frame brace and experiment around to see if any additional rack placement adjustment will further improve the front grip.
Click to expand...

Sounds like it is going well. Ackerman is one contributor to dynamic toe out (which is what you are trying to gain to load the inside tire --increased slip angle). The other contributor is bump steer with turn and compression. Zero bump steer with the wheels straight ahead does not necessarily mean zero bump with the wheels turned. The somewhat scientific way to see where you stand is to set the front ride height at the compressed height you experience under braking during turn-in (have to remove the front springs) and check the toe out with the wheels turned 20 degrees. The real-world check is tire temperature across the tread of both front tires to see how much you are working each tire.
 
mfain said:
69427 said:
Yesterday I had the first track day opportunity to see if the steering improved (less mid corner understeer) after changing the rack position. I moved the rack rearward by 5/8" (limited by the placement of a frame stiffening crossmember). I don't have any steady state skid pad data, but the car definitely felt better through the two tight mid-speed corners at Topeka. Over the winter I'll cut out and reposition the frame brace and experiment around to see if any additional rack placement adjustment will further improve the front grip.
Click to expand...

Sounds like it is going well. Ackerman is one contributor to dynamic toe out (which is what you are trying to gain to load the inside tire --increased slip angle). The other contributor is bump steer with turn and compression. Zero bump steer with the wheels straight ahead does not necessarily mean zero bump with the wheels turned. The somewhat scientific way to see where you stand is to set the front ride height at the compressed height you experience under braking during turn-in (have to remove the front springs) and check the toe out with the wheels turned 20 degrees. The real-world check is tire temperature across the tread of both front tires to see how much you are working each tire.
Click to expand...

Fabricating a new front suspension crossmember to (hopefully) improve the geometry and lose a couple pounds. I'm re-reading Sutton's handling thread, and am trying to get comfortable enough with the caster/dynamic camber tie-in so that I can make a little more progress (ie: additional welding) on the LCA mounts. The concept makes sense, but I'm still trying to verify the late C4 KPI, and just doing some math to figure out how much LCA pivot inclination may be beneficial. (Once I weld everything up I'm kinda stuck with it for a while, so I'm trying to minimize my mistakes/errors if I can.)
 
I put "slugs" at each mounting point for the LCAs, which gave me 1 inch of vertical adjustment (+/- 1/2 inch from center). My "center" position makes the rear of the LCA 1/2 higher than the front. The other thing to consider as that the vertical height of the LCA mount helps define the roll center, so that needs to be considered. The picture below is not great, but it does show the slug in a LCA mount. The second picture is something that may interest you. I bought some CNC machined ends that adapt C5/6/7 hubs and C6 brakes to a straight axle (to create a floating rear axle). I am looking at adapting the flange end to a C-4 upright so I can use a ZR1 rear bearing/hub instead of the weaker C-4 unit. The C7 rear hub also fits the ends, but both the Zr1 and C7 use 33 splines, which is different (I think) from the C4 outer axle. One other thing - if you look at Ron Sutton's commercial site, you will find he has some very reasonable prices for designing complete or portions of your steering geometry (like LCA mounting points).

Suspension 4.jpg

Radial Mt. Brake 2.jpg
 
Pappy- Ive been trying to figure out a way to build some adjustment at my LCA... do your arms have a cross shaft? or are they independent (like C5/6) I ask because Im wondering how or if you can change the height of one pivot (to change anti-dive for example) without changing its relation to the other control arm bushing pivot axis. Or if that just does not matter. I know I probably worded this horribly but I cant think of a better way to describe this. In short, can you (do you?) have two different pivot axis for the LCA?
 
vette427sbc said:
Pappy- Ive been trying to figure out a way to build some adjustment at my LCA... do your arms have a cross shaft? or are they independent (like C5/6) I ask because Im wondering how or if you can change the height of one pivot (to change anti-dive for example) without changing its relation to the other control arm bushing pivot axis. Or if that just does not matter. I know I probably worded this horribly but I cant think of a better way to describe this. In short, can you (do you?) have two different pivot axis for the LCA?
Click to expand...

My LCAs have individual pivots, but I use mono ball ends (from Howe) in each pivot, so minor misalignment is not an issue. The same would be true if you used a big rod end at each pivot. If you are using more fixed-type bushings (stock, poly, Derlin), then, when you make an adjustment to the LCA angle you would have to use an assortment of slugs (1/16", 1/8", 1/4", etc.) to align the bolts through the two independent pivots in the same plane of motion - i.e. a straight line. When I built the brackets to accept the inboard ends of the LCA, I aligned them 90 degrees to the LCA angle that I felt was correct for my needs and set them up for 0-offset slugs. Changes from that should be minor and alignment of the pivot bolts could be maintained by using the various, correct offset slugs at each end of the bolts. The alignment would be a little cumbersome (math in public required), but it's not something you should have to do often. If you have a clear shot through the pick-up points from front to back of the LCA (I don't), you could slip an alignment bar through the holes and pick the correct slug offsets to make the line straight. The mono-balls make it easier, but they are metal-to-metal and not very smooth on the street. Hope this helps.

Pappy
 
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Howe monoball for corvette

mfain said:
vette427sbc said:
Pappy- Ive been trying to figure out a way to build some adjustment at my LCA... do your arms have a cross shaft? or are they independent (like C5/6) I ask because Im wondering how or if you can change the height of one pivot (to change anti-dive for example) without changing its relation to the other control arm bushing pivot axis. Or if that just does not matter. I know I probably worded this horribly but I cant think of a better way to describe this. In short, can you (do you?) have two different pivot axis for the LCA?
Click to expand...

My LCAs have individual pivots, but I use mono ball ends (from Howe) in each pivot, so minor misalignment is not an issue. The same would be true if you used a big rod end at each pivot. If you are using more fixed-type bushings (stock, poly, Derlin), then, when you make an adjustment to the LCA angle you would have to use an assortment of slugs (1/16", 1/8", 1/4", etc.) to align the bolts through the two independent pivots in the same plane of motion - i.e. a straight line. When I built the brackets to accept the inboard ends of the LCA, I aligned them 90 degrees to the LCA angle that I felt was correct for my needs and set them up for 0-offset slugs. Changes from that should be minor and alignment of the pivot bolts could be maintained by using the various, correct offset slugs at each end of the bolts. The alignment would be a little cumbersome (math in public required), but it's not something you should have to do often. If you have a clear shot through the pick-up points from front to back of the LCA (I don't), you could slip an alignment bar through the holes and pick the correct slug offsets to make the line straight. The mono-balls make it easier, but they are metal-to-metal and not very smooth on the street. Hope this helps.

Pappy
Click to expand...

Hello Pappy,

Does Howe offer monoball bearings for C-3 lower control arms?, if they do, do you have a PN, thanks
 
[/QUOTE]

Hello Pappy,

Does Howe offer monoball bearings for C-3 lower control arms?, if they do, do you have a PN, thanks[/QUOTE]

I don't believe they make the inner pivot mono-balls specifically for a C-3 LCA, but I am using part number 22915, which they list for a Camaro. The outside diameter of the part is 1.65 inch and the length of the outer case is 2.05 inch. The length of the center part (mono-ball pivot) is 2.4 inches. One end comes with a sleeve that the inner slip-fits into (kind of a spacer) that has an outside diameter of 1.86 inch. The inside diameter of the mono-ball (for the bolt) is 9/16 inch. I built my own LCAs, so I just machined a tube that the bearings slide into. I haven't measured the outside diameter of the C-3 bushings, but it may be close to the Camaro. Or, you might be able to sleeve the end of the control arm to accept the Camaro part. Below is a picture of one bearing (mono-ball in its housing) and the spacer sleeve. There is a guy here in Phoenix who is having a similar set-up machined that is shorter and uses a 1/2 inch bolt, but I don't know the outer diameter of the housing. I suspect he could "build to spec".

Howe LCA Mono-ball.jpg

Hope this helps.
 
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Monoball

Hello Pappy,

Does Howe offer monoball bearings for C-3 lower control arms?, if they do, do you have a PN, thanks[/QUOTE]

I don't believe they make the inner pivot mono-balls specifically for a C-3 LCA, but I am using part number 22915, which they list for a Camaro. The outside diameter of the part is 1.65 inch and the length of the outer case is 2.05 inch. The length of the center part (mono-ball pivot) is 2.4 inches. One end comes with a sleeve that the inner slip-fits into (kind of a spacer) that has an outside diameter of 1.86 inch. The inside diameter of the mono-ball (for the bolt) is 9/16 inch. I built my own LCAs, so I just machined a tube that the bearings slide into. I haven't measured the outside diameter of the C-3 bushings, but it may be close to the Camaro. Or, you might be able to sleeve the end of the control arm to accept the Camaro part. Below is a picture of one bearing (mono-ball in its housing) and the spacer sleeve. There is a guy here in Phoenix who is having a similar set-up machined that is shorter and uses a 1/2 inch bolt, but I don't know the outer diameter of the housing. I suspect he could "build to spec".

View attachment 1823

Hope this helps.[/QUOTE]

Thank you for the info, will see what what I can come up with.
 
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