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  #1 (permalink)  
Old 09-09-2008, 07:19 PM
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Default Re-Design of front suspension

A friend (Dean Lampe) convinced me I should check the bump steer of my build project. Like most of our projects one thing has led to another and I find my front geometry is severely at fault. I am showing 1/2" bump-out and 1/2" Droop-In at 2" movement in each direction. Dean suggested I correct it, that it would not make for a good handling car.

As I analyze the frame/design, I learn the steering rack is put in about 1" too high. The design of the frame make it almost impossible to move, at least easily. All of a sudden, a simple bumpsteer correction project is turning into a complete redesign of the front end.

On the existing design, the roll center calculates to about -1.2" (1 1/4" below ground level). What is the preferred range for a roll center for a FE powered car? I understand the roll center is really a dynamic point but is there a preferred range/point I should be shooting for?

As I look at the various options to correct the geometry, most of them are affecting the roll center. Can anyone point me in a correct direction. Is there an preferred starting position?

Thanks

Paul
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Old 09-09-2008, 11:08 PM
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Since the rack cannot go down and your roll center is below ground level it follows that if you raise the inboard pickup points for both top/bottom w/bones by 1" you would be killing two birds with one stone- fix both the bumpsteer & raise the roll center at the same time- sounds like its too low anyway. Before doing anything you should check that your wishbone pickup points are in a position that will give good camber settings in roll etc as well. If you are going to shift stuff around it pays to consider all factors-saves doing it twice.
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Old 09-09-2008, 11:35 PM
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Paul, Welcome to the intricate world of suspension geometry!! When I began building cars I knew nothing about it. I had bought a kit that handled terribly. In my search to find out why I studied suspension geometry. I am no expert but I have learned alot and the cars I build are getting alot better.
I have a couple of thoughts about your numbers. Obviously your bump steer numbers are terrible. But, so many aspects of the geometry has an effect on each other that the height of the steering rack may not be the sole culprit. You may know the things that I will point out, forgive me if you do.
The control arm angles will have a profound affect, as will the inner tie rod location of your steering rack. The upper aframe angle will effect the camber loss or gain through the range of travel. If it is wrong it will make it almost impossible to eliminate bump steer. The correct angle depends on the components used, ride height, ect. The angle, forward or back, of the spindles tie rod location in relation to the steering rack will have a huge effect on Ackerman and bump steer as well. The inner tie rod of the steering rack needs to be in the exact same plane as the upper and lower aframes. If you were to draw a line from the upper aframe inner pivot point and the lower aframe inner pivot point, the inner tie rod should hit the center of that line. If its not you will not correct bump steer with the steering rack height.
A roll center below the pavement is good. The greater the length from the center of the frame this is the better the car will handle. Essentially the greater the length the smoother the camber transitions will be.
Another crucial aspect of steering is the scrub radius. Essentially if you were to draw a line through the upper and lower ball joint the point where it hits the floor should be the center of your tire contact. This would be a zero scrub radius. Usually on Cobras the center of the tire contact point is several inches out from where the ball joint line hits the pavement. This difference in length acts as a lever on your suspension components and steering components. Scrub radius is aggrevated by the use of deep dish wheels. The effect is high steering effort and alot of kickback through the steering wheel. With a large scrub radius the car will feel twitchy. A car with a zero scrub radius require very little effort to steer, even with out power steering.
I have cut off and redone the front clip on several cars to correct these types of problems. Sometimes I wondered if it was worth it. But, when you drive a car that has both the front and rear suspension nailed it is exhilirating and worth it. Have fun! John
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Old 09-10-2008, 05:52 PM
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Default More Info/Help

Thanks for your help and support, guys.

It appears I have two different avenues.

1) Lowering the rack to the optimal position solves the bumpsteer problem but the roll center is still below ground level. Some sources say the optimum position would be 2-5" above ground. Yet John prefers a position below ground.

2) it looks like changing to a drop spindle with the same frame ride height may also be a solution. Doing this, in essence, lowers the balljoints, in effect, doing the same as repositioning the inner ends. This raises the roll center into the above ranges. I would have to reposition the outer steering knuckle to above the arm to make the geometry proper. I haven't studied this far enough to find if there are associated problems, but it does look promising.

Any comments on the Roll Center location? I understand if the roll center is too low, it then requires a stiffer spring rate to control the roll of the car. A higher position affords a little better ride as lower rates can be used. Where do you draw a happy medium?

Paul
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Old 09-10-2008, 08:09 PM
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Just a couple of points on Johns explanations.

Quote:
Originally Posted by john chesnut View Post
Paul, Welcome to the intricate world of suspension geometry!! When I began building cars I knew nothing about it. I had bought a kit that handled terribly. In my search to find out why I studied suspension geometry. I am no expert but I have learned alot and the cars I build are getting alot better.
I have a couple of thoughts about your numbers. Obviously your bump steer numbers are terrible. But, so many aspects of the geometry has an effect on each other that the height of the steering rack may not be the sole culprit. You may know the things that I will point out, forgive me if you do.
The inner tie rod of the steering rack needs to be in the exact same plane as the upper and lower aframes. If you were to draw a line from the upper aframe inner pivot point and the lower aframe inner pivot point, the inner tie rod should hit the center of that line. If its not you will not correct bump steer with the steering rack height.

This is basicly true but you have to allow for the ackerman if any, which is usually built into the steering arm. Any distance this is offset from a line drawn thru the top/bottom ball joints or pivots on the upright when viewed from front/rear must be added or subtracted from the rack length on the 'line' you mention drawn between the chassis mount points for the a-arms

A roll center below the pavement is good. The greater the length from the center of the frame this is the better the car will handle. Essentially the greater the length the smoother the camber transitions will be.

Not sure what you mean by this length from chassis center, unless you mean the instantaneous center of the upper/lower a-arms which is used to calculate the roll center, Paul, a pic of the car now with the angles of the top /bottom A-Arms etc would help us advise further
John
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Old 09-10-2008, 08:32 PM
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There is a couple of recent issues of Kit Car where Dean has some good info about setting up his 40. You must have read those. I believe Paved Track Stock Car Technology says the roll center should be between 2 and 5" as you said. Although you might think a lower RC is better I think that below ground level has a gyroscopic effect on the car. One of the keys to roll center height is getting the lower control arms level at ride height isn't it? Heidts makes MII suspension components and has a pretty good explanation and diagrams on their website of how they should be set up and why it works. They also have awards for their 2" drop spindle you mentioned. See http://www.heidts.com/pdf/Understanding_IFS.pdf
The pictures suggestion would sure help, maybe there's something obvious like the rack is mounted on the wrong side of its brackets or something silly like that. I don't know where your manufacturer is at right now but I'm sure they would have some technical support/advice available for this dilemma.
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Last edited by mickmate; 09-10-2008 at 08:37 PM.. Reason: Getting old and forgetting stuff!
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Old 09-11-2008, 12:46 AM
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How about RC difference between front and rear in Cobra?

I just wondering that was RC location same in both end in origin suspension desing.

I have one book (Allan Staniforth 978-0854299843) and there is a lot of usefull hints that how to desing these suspension changes and how to correct these bumpsteer problems. There is also some words about RC differences between fron/rear and what that may cause (or what you may do with that if you wish...).

Sorry I'm just in start point with suspension parts so at the moment I have only guestions for this topic...
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Old 09-11-2008, 05:44 PM
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Default More Info

My build is a Hurricane - who is currently in flux right now. I will see the previous owners in a couple weeks, they may give me some history. I spoke to the new owners but they didn't have any input or suggestions to help.

More details on the suspension:
The rack is a Mustang II, bolted to the end of the frame rails. But the design of frame has a frame extension running forward to hold the cowl frame and quick jacks. The boss on the drivers side of the rack bottoms out on this extension, keeping it from going any lower.

The tie rod angle is about 2 degrees up. The LCA pitches up about 6 degrees, it is not horizontal.

Trying to move the outer tie rod end to correct for bump steer, it needs to go up about 7/8". But that puts it right in the middle of the steering arm. Moving the tie rod joint above the steering arm goes too far, worsening bump steer.

Laying out the suspension, the roll center ends up about 1/2" below ground. The more I read, this might not be all bad, books are saying -1" to +3". But then there are others that say +2" to +5"

If change the spindles, that flattens out the pitch of the LCA to almost horizontal but it moves the roll center to about 4 1/2". (I am doing layouts on this now to determine the exact amount and the degree of change)

The real dilemma is there are about 50 Hurricanes presently on the road and no one is really complaining about their handling. The old owners claim "we drastically reduce bump steer by ensuring the tie rod ends are as close as possible to level when the car is at ride height" As I understand it, level is good only if the LCA is also almost level. In this case, it isn't.

Dean owned and built a Hurricane. He said he never measured it on that car. He feels most manufacturers do the best they can to achieve a good look and never really get into optimizing the geometry or design. Kind of "close is good enough". He said it handled "OK" but realizes now, it could have been much better.

He said he was amazed at how much better his GT40 handled after it was corrected - And he thought it was OK before he started!

I've attached a picture of the front end

thanks for the help and dialog

Paul
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Old 09-11-2008, 05:52 PM
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Default Balljoints?

Mickmate,
In another thread, you indicated you knew of a source of ball joints that extended the pivot point?

Steeda makes their X2 and X5 but they are press in types. Do you know anyone who makes a screw-in (small chrysler, Moog K772) that has a longer stud?

Paul
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Old 09-11-2008, 09:13 PM
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Hey Paul, you've been paying attention! The set up we have used on race cars converts from a ball joint to a heim. It has a longer stud through the heim with spacers that can be changed above or below to raise and lower the steering pivot point. If you switched to a heim it would get that pivot point closer to the arm and the LCA and TR more parallel.
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Old 09-11-2008, 09:17 PM
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Have you tried simply fitting the tie rod end from the top even as a temporary check to see how that helps. Bit of angle distortion in pic, but it might be close if it was able too be fitted like that.
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Old 09-12-2008, 09:09 AM
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Default Been There -- Done That

Jac,
I did attempt that but by the time you mirror the ball joint to the topside of the steering arm, it is off just about the same amount in the other direction.

It appears the optimum point is almost the middle of the steering arm. I need a clevis

Paul
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Old 09-12-2008, 09:12 AM
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Default Ball Joints

Mic and others
I have found that Howe Racing makes a K727 style balljoint that you can get a 1/2" extended stud. Not as much as I need but a step in the correct direction.

Has anyone used these and your experience?

If I use one of these and also shim the taper joint with .012 shim stock, that moves the LCA outer joint down 5/8"

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Old 09-12-2008, 10:40 AM
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It seems you could go to the Heim joint with stud arrangement with the heim on top of the steering arm, them raise the mounting point of the rack by the corresponding amount. If that is a Mustang rack, I believe eccentric bushings are available to make up this adjustment.
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Old 09-12-2008, 08:12 PM
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Hey Bob, there's a good idea offset rack bushings. If they aren't available make them. If you know that rack is the wrong height why wouldn't you cut that whole front projection off, try locating the front near bumper mounts and change the height of the back for correct rack height. You know the originals used spacers under the rack to correct location? Move it to where you can shim it to fine tune it? The taper fits a little funny from the other side doesn't it..........I'm not sure how or where you're talking about shimming Paul.
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Old 09-12-2008, 08:37 PM
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Default Offset bushings

The most you can offset a MII rack is about .330 inches. That leaves about .050 wall thickness at the edge of the bushing. Any more and it will break thru.

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Old 09-13-2008, 07:05 PM
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Any idea how much you need Paul?
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Old 09-13-2008, 08:18 PM
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Default Calculated Distance

MickMate,
Using the standard MustangII tie rod ball joint, I needed about .96 inches - moving the rack down. The tie rod and LCA would have been parallel then.

I installed a Steeda bump steer kit as the heim joint mounts closer to the steering arm.

I "blueprinted" my lower ball joint connection, adding a .012 shim to the taper part of the shaft. Cut a flat pattern that formed into a cone and fits into the joint.

During production, they ran the taper reamer into the part a little too far, resulting the ball joint sucked up .156 higher. By correcting this, every little gain helps.

I just got done laying these changes into my layout and now I need to move the rack .426 down.

Being a perfectionist, I probably will move it 1/2" down, that way I have some allowance to dial-in the bumpsteer.

I am now investigating a way to support the Mustang II rack in some manner that will allow me to cut off the mounting boss that is normally used. With that out of the way, I can move it down.

On another note, have you ever seen a site or catalog that shows mounting and tie rod pivot dimensions on steering racks. When TRW was in the rack business, their X-4000 catalog had this info. But I can't find one anymore and can't find a site that offers the info. I called Flaming River and they offered very little help. I sure would like to find a rack that was 23 1/2" between pivots - this would also help my situation.

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Old 09-14-2008, 05:05 AM
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Maval/Unisteer can make a rack to your specs. I do think they have one without the mounting bosses also. www.unisteer.comhttp://www.unisteer.com/help/index.p...5&pcid=0&nav=0
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Old 09-14-2008, 06:51 AM
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They give the inner joint distance for the Cobra rack on their site http://www.flamingriver.com/index.cf...rod/prd239.htm
Poke around and see if you can find that dimension.
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