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Coil-over geometry question for the suspension experts

 
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Old 11-27-2010, 08:42 AM
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Coil-over geometry question for the suspension experts

So I've put a 4-link on the back of the '55 and I have two options for coil-over mounting. I can either put them on relatively vertical like in the picture or at an angle with the lower mounts moved about 3 inches outboard on each side. I can achieve the same approximate ride height with either set-up so that isn't a concern. What are the pros and cons of either arrangement? If I angle them, I have to notch the lower frame flange about a 1/2 inch for spring clearance.
 
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Old 11-27-2010, 09:46 AM
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With the super late model race car design we always kept them as verticle as possible . Do you have any concerns on the severe angle of the track bar ? Again we always tried to keep it as parallel to the rear end housing as possible .
 
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Old 11-27-2010, 10:29 AM
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My understanding is it's always best to keep your coilovers as vertical and far to the outside as possible. The more you angle a spring, the less ability it has to dampen movement. If you have to do extra work to give it an undesirable angle, I don't see the benefit.
 
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Old 11-27-2010, 10:48 AM
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IMHO for street use on a solid rear axle you do want the top of the coilover to tip in towards the center of the vehicle ~ 10-15*. This put the spring in the line of movement when one wheel moves up and down relative to the other wheel such as over a pothole or when cornering. the axle rotates around the planted wheel, rather than moves up and down. Slightly more angle (to a max of 20-25*) can be used if needed and would be preferable to less angle. In racing applications (circle or drag) the track is relatively flat and without turns (circle track cars have little body lean when turning, it is adjusted out with alignment and biasing) compared to a highway the movement is more straight up and down so the coilovers are set straight up. Hope the description makes sense, if not ask again and I'll do some diagrams.
Can't say I like the angle on the panhard bar either, the axle will dance around unpredictably with that setup, it should be as parallel to the axle as possible.
 
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Old 11-27-2010, 11:05 AM
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Originally Posted by AXracer View Post
IMHO for street use on a solid rear axle you do want the top of the coilover to tip in towards the center of the vehicle ~ 10-15*. This put the spring in the line of movement when one wheel moves up and down relative to the other wheel such as over a pothole or when cornering. the axle rotates around the planted wheel, rather than moves up and down. Slightly more angle (to a max of 20-25*) can be used if needed and would be preferable to less angle. In racing applications (circle or drag) the track is relatively flat and without turns (circle track cars have little body lean when turning, it is adjusted out with alignment and biasing) compared to a highway the movement is more straight up and down so the coilovers are set straight up. Hope the description makes sense, if not ask again and I'll do some diagrams.
Can't say I like the angle on the panhard bar either, the axle will dance around unpredictably with that setup, it should be as parallel to the axle as possible.
First, this is a kit supplied by TCI, I'm not garage engineering it. I don't know their engineering credentials but they seem to have a good reputation. In the "vertical" installation, there is a small amount of angle(haven't measured it, yet). The panhard bar is attached to the lower frame mounted trailing arm bolt and the opposite lower axle mounted trailing arm bolt. It triangulates the whole assembly. Why would you think it would dance around? What I can see, in the vertical installation, is all forces are in the "Y" axis and as such, should cause higher apparent spring rate. If angled, you would have both an "X" and "Y" component and would lead to a softer ride but add a component to side loading. On a road course, this might have some affect.
 
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Old 11-27-2010, 01:42 PM
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An angled panhard bar will change effective length as it goes thru it's range of motion, pulling or pushing the end of the axle it attaches to. It's possible that your bar is long enough and the arc small enough that the change is minimal. All the suspension design books and experts I'm familiar with, including the SAE engineering course text "Race Car Vehicle Dynamics" by Milliken and Milliken say that a panhard bar should be as long as possible and installed parallel to both the axle and ground. If you don't think the geometry of the panhard bar makes much difference, yo have never watched a NASCAR race and heard the commentator say "they just went up (or down) on the bar...". When the pitcrew member sticks a wrench thru a hole in the rear window and turns it he is moving the chassis end of the panhard bar up or down in fractions of an inch to change the handling.
If you are satisfied with TCI's engineering, why are asking for our opinion then reject it when we give it? A road course is not significantly different from street driving as far as handling, the same dynamics are in effect. I can justify my opnion, but it would require a lot of writing and you don't seem to want to hear it any how.
 
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Old 11-27-2010, 03:03 PM
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Originally Posted by AXracer View Post
An angled panhard bar will change effective length as it goes thru it's range of motion, pulling or pushing the end of the axle it attaches to. It's possible that your bar is long enough and the arc small enough that the change is minimal. All the suspension design books and experts I'm familiar with, including the SAE engineering course text "Race Car Vehicle Dynamics" by Milliken and Milliken say that a panhard bar should be as long as possible and installed parallel to both the axle and ground. If you don't think the geometry of the panhard bar makes much difference, yo have never watched a NASCAR race and heard the commentator say "they just went up (or down) on the bar...". When the pitcrew member sticks a wrench thru a hole in the rear window and turns it he is moving the chassis end of the panhard bar up or down in fractions of an inch to change the handling.
If you are satisfied with TCI's engineering, why are asking for our opinion then reject it when we give it? A road course is not significantly different from street driving as far as handling, the same dynamics are in effect.
Don't get all out of joint. My specific question was about Coil-over installations, not a panhard bar. I am perfectly happy with the panhard bar design because of its length and the way it's attached. I measured it through its full travel from full droop to full bump and there was at most 1/16" change in the wheel to frame dimension.
Back to the original topic, I measured the "vertical" coil-over installation and they are at 10* off of vertical. The alternative mounting design puts tham at almost a 45* angle which is what I'm questioning. On my race car, the coil-overs are just off of vertical but that's an independent rear suspension. I don't have any experience with a solid axle, hence my question. I am interested in what you, or anyone else has to say about the rear coil-over design and a straight axle set up.
 
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Old 11-28-2010, 11:54 AM
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Looking at the rear axle parallel to the ground plane, ie. the axle from the front or rear:
When one end of the axle moves up or down independent of the other end i.e. one wheel rides over a bump or drops into a pothole or the body rolls in a corner, the moving end of the axle is in an arc with the rotational center at the "fixed" wheel contact point with the ground. Ideally you want the coilovers to parallel this vector. You could diagram this arc or calculate it, but it's been done many times and most vehicles have similar track widths, plus it's not that critical to be exact since it's a compromise based on average of rotational movement and pure vertical movement anyway, so 10-15* is the accepted angle.
If yor coilovers are set at 10* now, I'd go with it. 45* is way too much, the coilover would tend to rotate like a solid link rather than compress, 30* is about the functional limit.
 
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Old 11-28-2010, 12:20 PM
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My 4-bar is Chris Alston's Chassis Works, I installed it as directed with the shock tops tilted 10 degrees inboard.

I 've used and had experience with TCI products and my opinon of them is quite good, I'd install it as directed. I agree w/AXracer and besides the designing Engineers usually know what they're talking about. JMHO

 
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Old 11-28-2010, 12:40 PM
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Originally Posted by KidShalene View Post
My 4-bar is Chris Alston's Chassis Works, I installed it as directed with the shock tops tilted 10 degrees inboard.

I 've used and had experience with TCI products and my opinon of them is quite good, I'd install it as directed. I agree w/AXracer and besides the designing Engineers usually know what they're talking about. JMHO

Is that a drag racing setup or for very wide tires? It's a pretty narrow installation for the street IMHO the coilovers should be as close to the ends of the axle as possible.
 
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Old 11-28-2010, 01:05 PM
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Originally Posted by AXracer View Post
Is that a drag racing setup or for very wide tires? It's a pretty narrow installation for the street IMHO the coilovers should be as close to the ends of the axle as possible.

No, that'd be the Battle Cruiser, this is the Pro Street setup.

Those are not the final wheels/tires, just mock up. I'll be running M/T 31X16/15 Street Radials, so there'll be little room to mount one w/the four bars outside the frame.

Yes, I'm building this truck mostly to go fast in a straight line. It is a Trauck afterall. I don't see it being a good autocrosser, I do that in my Corvette

YouTube - Khana Cross Lap - TMS - Lone Star Corvette Clasic
 
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Old 11-28-2010, 01:38 PM
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I ask primarily so that others don't use your setup as an example for their street truck without understanding the comprimises you made and why this is not the best street setup.
examples: With such a narrow CO install 10* inclination may not be enough, since it sortens up the radius of the rotational arc, the CO may actually tip outwards with body lean.
Moving the CO inboard increases the leverage on them, making them act like a much softer unit. That's why you often see 4 coilovers used with inboard mounting setups.

Nice video, excellent color choice. Do any SCCA events? Here I am on the cover of this year's national championships program:
 
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Old 11-28-2010, 02:58 PM
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Originally Posted by AXracer View Post
I ask primarily so that others don't use your setup as an example for their street truck without understanding the comprimises you made and why this is not the best street setup.
examples: With such a narrow CO install 10* inclination may not be enough, since it sortens up the radius of the rotational arc, the CO may actually tip outwards with body lean.
Moving the CO inboard increases the leverage on them, making them act like a much softer unit. That's why you often see 4 coilovers used with inboard mounting setups.
You could be right, but with a fully boxed and gusseted frame, tubular Mustang II CO IFS w/2" drop spindles and a 1.3125" sway bar and
this rear setup w/a TCI thru-frame sway bar that leaves the bottom of the running boards about 4-5 inches off the pavement,
I just don't see enough body roll for it to go over center.

I could be wrong but I believe once I've finished fabrication and remounted all the body work (fiberglass rear fenders) and get true
weight for the truck in final form and ensure I've got accurately rated springs, it'll minimize most body roll.

If not I've noted your 4 shock method and will do whatever to make it work, that's my story and I'm stickin to it.

Originally Posted by AXracer View Post
Nice video, excellent color choice. Do any SCCA events? Here I am on the cover of this year's national championships program:
Thanks, I didn't think I'd like a yellow car till I saw it, now I'm even thinking about painting my F1 Millennium Yellow. It's an absolutely beautiful color.

My job keeps me way too close to home to do SCCA anywhere other than here in Tx, so I've never attended, afraid I'd like it
too much and have to make some more compromises. Too many irons in my fire as it is.
Attending the Corvette Classic once a year (check out the track video below) and a couple other local club events keeps me satisfied.

YouTube - C5 at Texas Motor Speedway - Hot Laps
 
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Old 11-28-2010, 03:05 PM
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Originally Posted by AXracer View Post
Looking at the rear axle parallel to the ground plane, ie. the axle from the front or rear:
When one end of the axle moves up or down independent of the other end i.e. one wheel rides over a bump or drops into a pothole or the body rolls in a corner, the moving end of the axle is in an arc with the rotational center at the "fixed" wheel contact point with the ground. Ideally you want the coilovers to parallel this vector. You could diagram this arc or calculate it, but it's been done many times and most vehicles have similar track widths, plus it's not that critical to be exact since it's a compromise based on average of rotational movement and pure vertical movement anyway, so 10-15* is the accepted angle.
If yor coilovers are set at 10* now, I'd go with it. 45* is way too much, the coilover would tend to rotate like a solid link rather than compress, 30* is about the functional limit.
That's pretty much what I'm thinking, too. thanks
 
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Old 11-28-2010, 03:13 PM
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Originally Posted by KidShalene View Post
No, that'd be the Battle Cruiser, this is the Pro Street setup.

Those are not the final wheels/tires, just mock up. I'll be running M/T 31X16/15 Street Radials, so there'll be little room to mount one w/the four bars outside the frame.

Yes, I'm building this truck mostly to go fast in a straight line. It is a Trauck afterall. I don't see it being a good autocrosser, I do that in my Corvette
Hey Kidshalene...good looking Vette. I run a C-4 in NCCC group 3. Did coil-overs from Rippie and a full heim jointed rear with adjustable upper control arms up front. now they want to move me to RP.
 

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