Factory Five Racing Forum banner
1 - 20 of 138 Posts

·
Going nowhere fast.
Joined
·
19,728 Posts
Discussion Starter · #1 ·
dv/dt recently said (somewhat fatalistically):
The problem with the set up we use from the Mustang is that the tie-rod is too long/rack is too short. You could fix this by moving the steering arm on the spindle inboard but that would mess with the Ackerman. The other choice is to lengthen the rack. I lieu off doing this we will always be working with a compromise.
I was thinking about what it would take to fab some rack extenders when I saw that they are commercially available (to use a Mustang II rack in a pickup truck). ~$80 will get you a set of 2" or 4" extenders from flaming river.

With a longer rack, we could probably get our tie rods to point to the instant center with less radical changes in rack location, leaving a lot more of the stock Ackerman. What do people think?

Mike

[ January 08, 2006, 07:34 PM: Message edited by: Michael Stora ]
 

·
Registered
Joined
·
2,624 Posts
Michael

The quote is some what correct. You can move the steering arm inboard with brackets and not change and or improve ackerman. It depends on how you make the brackets....

The longer rack issue is that you need a rack that has the inner pivot point at exactly the same pivot point of the lower A-arm inner pivot. The mustang rack is to short.

Once you have this rack, then you can worry about the tie rod arm length....
 

·
Premium Member
Joined
·
16,561 Posts
Michael,
Dang man I searched the net for those and found nothing. Was starting to look at options for fabbing a set as well. But I checked their site and can't find anything on them. Do you have a link to any online info?

Cheers, Rod
 

·
Premium Member
Joined
·
9,698 Posts
The longer rack issue is that you need a rack that has the inner pivot point at exactly the same pivot point of the lower A-arm inner pivot.
I'm using the Flaming River manual 15 to 1 rack, and the inner pivot is exactly at the A arm inner pivot. Infact I had some concern that the A arm bolt head might wear on the rack boot. BTW, I'm using Levy's bump steer mod.

John
 

·
Registered
Joined
·
2,624 Posts
J Persons

That is funny. I have the same rack and it is about 3/4" to 1" short per side.... I guess it all depends on where you are measuring the inner pivot point.... The pivot point is the center of the ball not the end where the tie rod end exits the pivot...

I am familiar with the Levy setup. The inner tie rod pivot is what keeps that setup from being perfect...
 

·
Going nowhere fast.
Joined
·
19,728 Posts
Discussion Starter · #6 ·

·
Premium Member
Joined
·
9,698 Posts
Originally posted by Don Vaught.:
J Persons

That is funny. I have the same rack and it is about 3/4" to 1" short per side.... I guess it all depends on where you are measuring the inner pivot point.... The pivot point is the center of the ball not the end where the tie rod end exits the pivot...

I am familiar with the Levy setup. The inner tie rod pivot is what keeps that setup from being perfect...
Well, unfortunately I can't verify it just now. I'm in Dallas and won't be home for a couple of weeks. But I remember pulling the boot back so I could verify the inner pivot before welding in the rack mounts. John
 

·
Premium Member
Joined
·
3,886 Posts
Interesting! Probably the 2" extensions would be just about right...hmmmm.
Am I correct in assuming that's 1" one each side? Or is it 2" each side?
I have pindrive arms, and my tierods don't have enough thread left to be able to shorten them 1", so I don't think I could do it without shortening the threaded sleeves of my Baer bumpsteer kit. That's a big step that I'd hate to take unless I was sure this mod would work.

I'd sure like for someone to try this and tell us how it workes.

Forrest

[ January 08, 2006, 06:31 PM: Message edited by: forrest1 ]
 

·
Senior Member
Joined
·
5,559 Posts
I assume those extenders are to lengthen the inner sockets on the rack - correct? I have the 15:1 rack that FFR supplies (I think Unisteer), but it looks just like the FR. Does anyone know if these joints will interchange to it? That seems like a good way to improve the geometry cheaply, or at least correct one more dificency anyway.

Brian
 

·
Premium Member
Joined
·
16,561 Posts
What you really want is shown in the following diagram. We are the upper case:



We actually want the pivot point of the inner tie-rod end to be on a line drawn between teh upper and lower control arm pivots. We also want the outer tie-rod pivot to fall on a line drawn between the upper and lower ball joints. In the case of an FFR using the stock rack and Fox spindles we have neither. In these pics my stock Mustang rack is centered and I used an aluminum straight edge to show the line between the upper and lower pivots and ball joints.





So what we have is shown in blue.



So it's off by about 3/4" on the inboard tie-rod and an 1-1/4" on the outer. So you need a longer rack AND also to move the steering arm/outer tie-rod pivot point inboard and forward. The forward part is credited to Don V who pointed out that doing so minimizes the change in Ackerman which results when you move the steering arm pivot inboard.

The other choice is to maintain the length determined by the first drawing above but move it outboard leaving the steering arm point in the same location.



This should get rid of bump steer and increase Ackerman slightly. This is what the extensions would do and 2" does come pretty close.

Any comments?

Cheers, Rod
 

·
Going nowhere fast.
Joined
·
19,728 Posts
Discussion Starter · #11 ·
Thanks for the pictures Rod. How do SN95 spindals differ in this geometry?

Are you going to be the guinea pig?

Mike
 

·
Premium Member
Joined
·
3,886 Posts
Rod,

A very nice illustration of the problem and solution!

I'd order a set right now except for the pindrive arm problem--does anyone know if shorter tierods are available? I've cut mine off shorter to fit the pindrive arms and my bumpsteer kit, and I've just about run out of threads on the rod, so I can't cut it any shorter very easily.

Forrest
 

·
Premium Member
Joined
·
16,561 Posts
Not sure about the SN95 but I don't think the final setup is much different. The question is do they fit the stock Mustang rack?? Also, I haven't found anything saying if it is $80 for one 2" extender or for 2. I get the feeling it is one. I may still see if I can get something machined locally. Still a lot of things/options to consider.

Cheers, Rod
 

·
Premium Member
Joined
·
16,561 Posts
Hi Don,
Here is one of your mod that moves the steering arm to outer tie-rod end pivot point inboard and forward.



This actually represents both sides of the car since it shows the results of 2" of inboard and outboard rack movement. I took quick measurements off the car so they are not 100% accurate but do accurately represent the results. In this case the steering arm lug was moved inboard 1.123" and forward 0.549".



The info in blue represents the original location and in red is with the pivot moved. A couple of interesting points are as follows:

Total steering angle is reduced which would be expected. This isn't necessarily a bad thing since many of us need rack restrictors so we don't rub the F-panels.

The amount of Ackerman is reduced (shown here as a ratio of inboard tire angle to outboard tire angle):

Original = 25.2857/22.0326 = 1.148
New = 21.6563/19.7306 = 1.098

Note that outboard rack movement results in inboard tire angle.

Can't say if this is bad until I get a chance to model the whole car and see what the ideal ratio would be. And yes, if you move it further forward you will increase the Ackerman but at the expense of steering travel so you have to compromise.

Great fun this!!

Cheers, Rod
 

·
Senior Member
Joined
·
1,672 Posts
Rod you are clearly an engineeer !!!

Only a engineer would think analysis is fun.

It wold be intereting to compare the Levy bump stop.

Charlie
 

·
Premium Member
Joined
·
16,561 Posts
Charlie,
:D :D

Here's an approximation of Gordon's kit. Based on photos I estimated the rack is about 3" in front of the steering arm to outer tie rod pivot point. So here are the results:



In this case the outboard tire turns in slightly more than the inboard which should result in some scrubbing in tight turns.

Cheers, Rod
 

·
Banned
Joined
·
5,303 Posts
that was my impression looking at gordon's kit: that there would be a considerable negative ackermann effect. this might increase high speed stability at the expense of lower speed turning. not good for autocross.

i prefer the solution of a wider rack. just have to find one.

james
 

·
The Never-Ending Builder
Joined
·
5,329 Posts
The actual measurement from the center line of the steering rack to the center line of the tie rod hole on the steering arm is 3.5 inches. I know everyone here is trying to help everyone in some way or another, but if you're analyzing someones product, either for praise or criticism, you need to have the correct facts at hand. A half inch of measurement can have a considerable difference in the performance of many items. DV/DT, what does your software say about this number? I can't speak to the street system that others have, as I'm using the Race System (with SN95 spindles), but what I have is infinitely adjustable, for several parameters.

Hank
 
1 - 20 of 138 Posts
Top