96+ K member with 94 95 spindles? - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 8 Old 09-13-2010, 11:21 PM Thread Starter
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96+ K member with 94 95 spindles?

For you suspension gurus... I'm not sure if I have this right

MM says to not use a 96+ spindle because it will increase bump steer. From what I understand; the lower tie rod location makes for a sharper angle of the tie rod. WHen you hit a bump, the arc of the angle will pull the toe in more than if the tie rod was closer to level.

So my question is, if I were to use a 96+ k member in a fox, the rack will be lower than stock. If I use a 95 spindle, the tie rod ear will be higher. Will the cause the same bump steer problem just with the tie rods angling up instead of down?

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post #2 of 8 Old 09-14-2010, 09:32 AM
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Do you have it in reverse?
The 96+ are better (not worse) for bumpsteer. That is why my 1989 Fox got them.
Search this forum, this has been answered at length before...

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post #3 of 8 Old 09-15-2010, 12:03 AM Thread Starter
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Quote:
Originally Posted by drudis View Post
Do you have it in reverse?
The 96+ are better (not worse) for bumpsteer. That is why my 1989 Fox got them.
Search this forum, this has been answered at length before...
No...

On MM's page it says 96+ are worse. I was previously under the understanding that the ideal tie rod angle for the least bumpsteer was to make the tie rod parallel to the ground... which would make sense that the 96+ would be better. The rack is lower because of the lowering springs, so lower the steering arm with the 96+ spindles and the tie rod would seem to be closer to factory.

BUT...... after reading Jack's explanation, the ideal bumpsteer setup the pivot points between the A arm bolt and the ball joint should be parallel with the tie rod pivot poings. Search didn't tell me much till I read this.....

Quote:
Originally Posted by Jack Hidley View Post
To answer post #12. The exact proper geometry to minimize bumpsteer is to have the tie rod axis pointing directly at the instant center of the front suspension in roll, when the car is at ride height. If the tie rod and lower control arm are parallel, with a strut suspension, the tie rod will be approximately pointing at the instant center.

There are two main concerns with bumpsteer.

1) During pure cornering, with no bumps in the road. If you turn the car to the right, entering a corner, the body is going to roll to the left. This puts the left front suspension in the direction of bump travel (compression). There is NO bump encountered by the car, but the direction of suspension travel is called bump. If the bumpsteer curve is such that under bump the toe goes more negative (toe out), that is just like someone turning the steering back to the left a little bit as the car rolls into a right hand corner. This is called roll understeer. The car is going to turn a larger radius corner the more it rolls. This is a stable condition as the car tends to want to go straighter.

If the bumpsteer curve is such that under bump, the toe goes more positive (toe in), that is like someone turning the steering more to the right as the body is rolling to the left in a right hand corner. This is called roll oversteer. The car is going to try to turn a smaller and smaller radius as it rolls. Notice that this situation has positive feedback. Once the bumpsteer starts to turn the wheel to the right more, this decreases the radius of the corner. Since the cars velocity is constant and the radius has decreased, the g force has gone up. The increased g force causes more body roll, which causes the front tire to turn more to the right which causes more g force......until the car spins.

For this reason, automakers design cars so that the bumpsteer curve always goes toe out under bump, to some degree. This keeps the car from becoming unstable in a corner and spinning. If you install 96+ spindles on a Fox Mustang with a stock k-member, the bumpsteer curve will have a lot more toe out under bump than it had with the stock of 94-95 spindles. From this standpoint it is "safe".

2) During a one wheel bump and no cornering. Under these conditions, it doesn't make much difference whether the wheel goes toe in or toe out under bump. The car is going to dart, when it hits a bump. If it has a lot of toe change for a given amount of bump travel, it is going to dart a lot. Using 96+ spindles on a Fox car with a stock k-member is going to cause a lot of dartiness compared to the stock or 94-95 spindle. From this standpoint, it's not "safe".

I've just given two very simple examples here. There are obvious other combinations of vehicle motions to consider such as bump and roll, bump and braking, braking and roll, etc. The less bumpsteer the car has, the better behaved it will be under all of them.

Sometime in the near future I'll be posting a lot more information on this subject.
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post #4 of 8 Old 09-15-2010, 12:05 AM Thread Starter
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So I gather the 96 K member with the 96 spindles will be best.

Slightly off topic... I just did the 5 lug conversion with 94 95 spindles in my fox with fox stock K member. I am going to lower it a little more before I get it aligned. If I add more caster when I get it aligned; will that make more bumpsteer because the caster will raise the steering arm?
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post #5 of 8 Old 09-15-2010, 01:30 AM
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Metalman,

Just typed a detailed response and accidentally nuked it, so this will be short.

Yes, 1996+ spindles will work best with a 1996+ k-member. You can use the 1994-95 spindles, but their use would make the bumpsteer stack 1" taller than with the 1996+ spindles. This will make the steering less precise because there will be more torque placed on the steering arm.

As soon as you start moving suspension and steering system pivots on the car with extended balljoints, offset rack bushings, aftermarket k-members, etc, you need to look at all of the changes to determine which model of spindles will work better. Since nearly everyone is trying to lower their Mustang (not raise it) and many of the aftermarket suspension parts are aimed at fixing the resulting FCA angle this causes, to get a good bumpsteer curve you almost always need to lower the outer tie rod. The 1996+ spindles almost always work better in these situations since their steering arm is 1" lower, which results in a shorter bumpsteer spacer stack height.

Yes, as you add caster, this will increase bumpsteer. Fortunately, this is very easy to correct for. For every degree of caster added, just increase your bumpsteer stack height by 0.068". This will put the outer tie rod pivot back in the precaster adjusted location.

In the post quote above, I said something to the effect of the tie rod and FCA needed to be approximately parallel to each other for minimum bumpsteer. Here is a sketch of the exact solution. Lines drawn through the tie rod and FCA need to both converge at the instant center of the front suspension. Both of these lines also need to converge at the IC with a line drawn through the UCA (if we were dealing with a double A-arm suspension) or on this case a line drawn through the upper strut mount pivot at right angles to the centerline of the strut axis. In the case of a strut suspension, this line represents the virtual upper control arm.

http://www.longacreracing.com/articl.../bsdrawing.jpg

Jack Hidley
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post #6 of 8 Old 09-15-2010, 02:01 AM Thread Starter
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Quote:
Originally Posted by Jack Hidley View Post

As soon as you start moving suspension and steering system pivots on the car with extended balljoints, offset rack bushings, aftermarket k-members, etc, you need to look at all of the changes to determine which model of spindles will work better. Since nearly everyone is trying to lower their Mustang (not raise it) and many of the aftermarket suspension parts are aimed at fixing the resulting FCA angle this causes, to get a good bumpsteer curve you almost always need to lower the outer tie rod. The 1996+ spindles almost always work better in these situations since their steering arm is 1" lower, which results in a shorter bumpsteer spacer stack height.
I am still confused by all of this.

If your only change is lowering the car with lowering springs, are the 96 spindles still advantageous because of the smaller stack required?

If the above statement is correct, it would seem that the advice on the MM site to NOT use the 96 spindles with a stock K would only apply to cars still at stock ride height???
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post #7 of 8 Old 09-16-2010, 06:34 AM
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Metalman,

This entire subject is quite difficult to explain in words, without visualization assistance.

Here is another post I made on the subject, which has animation videos in them of good and bad bumpsteer behavior.

https://forums.corral.net/forums/show...6&postcount=36

Look at the bumpsteer curve below:

http://home.comcast.net/~jhidley/Bumpsteer_graph.JPG

If we had a car with "perfect" bumpsteer behavior, ie none, the graph would be a vertical straight line lying on top of the Y axis. This means that no matter how far the front tire went up or down (+3" to -4") the toe would always remain at 0 (no change). Now if you take this car and lower it, what have you changed about the bumpsteer geometry? Absolutely nothing! All you have done is moved the zero point in the suspension travel curve maybe 1.5" upwards along the graph, which is a perfectly straight vertical line.

In reality, we can't build a suspension with perfect bumpsteer behavior. If you did, it would compromise other aspects of the suspensions performance too much. With any suspension, the bumpsteer curve is going to have an area of the curve where it is flattest. You want to adjust the location of the various joints in the steering system to put this flat area of the curve in the range from ride height to about 2" of bump travel. See this link for a good example of this:

http://img6.imageshack.us/img6/3955/s14bumpsteer.jpg

Each of the graphs has three curves. I assume that the person who made them adjusted the height of one of the steering system pivots to three different locations to get the three different graphs.

Once you lower a Mustang, you have shifted the flat portion of that curve to a slightly different area of the graph. It only takes a small adjustment of the height of the outer tie rod to get the curve back to where it was. If you were to lower the car and then reset the caster back to the setting it had at stock ride height, the bumpsteer curve would then be virtually identical to the original!

The point is, lowering the car does NOT create some massive change in bumpsteer behavior on a Mustang. So if all you are doing is lowering the car, there is absolutely no reason to change the spindles to fix a bumpsteer problem. It is guaranteed to make it much, much worse. The only time you will need to change spindles or install offset steering rack bushings is when you have made some large relative change to the vertical location of any of the following four pivots: Inner tie rod pivot, outer tie rod pivot, balljoint, FCA inner pivot.

Sometimes changes to the location of these pivots are obvious. When you buy a k-member that advertises that it raises the FCA pivots 2", you know about it. Sometimes they are not. When you add caster to the front suspension by moving the strut top back 1/2", you are raising the outer tie rod pivot as a result.

Jack Hidley
Maximum Motorsports Tech Support
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post #8 of 8 Old 09-16-2010, 08:33 AM
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Mods, can we make this a sticky? ...it seems to get asked a lot and I'm sure Jack's fingers a tired from typing that over and over ;-)
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