[dmcurry22]

I want to split the tube front axle in the middle and build an coil over 4-bar IFS (Fatman IFS kit) on the 32 ford roadster I'm building. It was my hope of eliminating the tie rod, drag ling and panhard and replacing all that with a mustang manual rack and pinion unit. A tech at Fatmans said I couldn't do that. I think he is wrong! That rack will not know whether its steering an Mustang A arm front or my split tube axle design. Is fatman wrong or am I a loonatick? Thanks for any help, dmcurry22

 

[Frisco]

You have an interesting idea. I assume you are talking about splitting the straight axle in the middle and having it pivot from that point. There is such a unit available from an aftermarket company.

Except for the newly introduced Unisteer rack & pinion , the only way a rack & pinion could previously be utilized on a straight axle and function somewhat correctly was to mount the rack directly to the axle. This looked terrible and suffers from some severe "Bump steer". This is because of the way a straight axle moves when going over any irregularities in the road.

With your idea you could mount the rack to the frame if it is centered. I am not convinced that you would be satisfied with the results though because of the severe camber changes that would occur on normal road irregularities.

For more info about camber and what it does go to the following link and scroll down.

Toe, caster & camber

If you do try your idea out, please post photos and your results.

 

[[email protected]]

The guy at Fatman is correct. If you try to use a MustII type unit. The front end will suffer from extreme toe in/out and bump steer in operation because the pivot points are all wrong.

I tried this back in the early seventies. The closest thing that will almost work is an old Cavalier centerpiont rack. Or possibly an Opel short rack.

Tho I havent tried the Cavalier on a straight axle setup like you are suggesting, it works great on my 48 Dodge business coupe. The center pivot linkage comes closest to the pivot (center point) of the axle.

 

[cboy]

I am not convinced that you would be satisfied with the results though because of the severe camber changes that would occur on normal road irregularities.

Hey Frisco,

Are the camber changes you mention a result of the rack steering or just due to the short radius the axle must move in? If they are caused by or worsened by the rack, could you give a short explanation why.

Dewey

 

[Frisco]

Hey Frisco,

Are the camber changes you mention a result of the rack steering or just due to the short radius the axle must move in? If they are caused by or worsened by the rack, could you give a short explanation why.

Dewey

The camber changes I mention are due to axle movement and not the rack & pinion.

Camber on a straight axle is fixed. On a full straight axle as one wheel pivots up the camber as related to the road surface changes. However since the axle is one piece the opposite wheel and the corresponding camber (in relation to the road surface) also changes a similar amount.

By splitting the straight axle and pivoting the two halves in the middle, as one wheel moves up and down due to road irregularities the axle end and wheel will swing in an arc while the opposite wheel will be doing the same and not always in any correlation to the other wheel. This would make for some very interesting handling. (Think where's the toilet paper.)

I am editing this to add that the above condition would occur with a straight axle that is split in the middle. Ford trucks that have a split axle have the ends of the axles mounted to the opposite frame rail (they cross over each axle half) and although the same principal would apply the overall effect seems to be decreased. A limited travel up & down may be a solution of sorts.

 

[cboy]

Ford trucks that have a split axle have the ends of the axles mounted to the opposite frame rail (they cross over each axle half) and although the same principal would apply the overall effect seems to be decreased. A limited travel up & down may be a solution of sorts.

That was going to be my next question. Why do Twin I-Beam Fords work and not what dmcurry is talking about. My '32 pickup has Twin I-Beams and the rat rod I am building also has twin I-Beams. (You can see how both are set up in my journal) The '32, at least, works great. In fact, I like it so much that's why I'm putting it on the rat.

But back to the "why" question. Indeed, the axles on the F-150s are longer - but they are not necessarily all the way to the opposite frame rail. And the other interesting thing - they are not even of equal length - at least on some years. In fact, the passenger side axle I am using from an '81 F-150 is just a couple inches longer than the half way point on the frame - so almost like sawing a tube axle in half like dmcurry wants to do.

One definitely has to deal with camber issues when using F-150 or other "split" front axles, but I think it can be done and I think you can obtain pretty good ride characteristics.

 

[Frisco]

That was going to be my next question. Why do Twin I-Beam Fords work and not what dmcurry is talking about. My '32 pickup has Twin I-Beams and the rat rod I am building also has twin I-Beams. (You can see how both are set up in my journal) The '32, at least, works great. In fact, I like it so much that's why I'm putting it on the rat.

But back to the "why" question. Indeed, the axles on the F-150s are longer - but they are not necessarily all the way to the opposite frame rail. And the other interesting thing - they are not even of equal length - at least on some years. In fact, the passenger side axle I am using from an '81 F-150 is just a couple inches longer than the half way point on the frame - so almost like sawing a tube axle in half like dmcurry wants to do.

One definitely has to deal with camber issues when using F-150 or other "split" front axles, but I think it can be done and I think you can obtain pretty good ride characteristics.

I'm not saying that a split front axle will not work. I do think that the Ford "Twin I Beam" style is more functional than splitting the axle in the middle. To the best of my knowlege though, the split axles do not use Rack & pinion steering (Not sure about the later model years). As I said in my first post, the idea is interesting and I would like to see the results. Innovation is always good. I am following your build in your journal. Nice work and good photos. :thumbup:

 

[dmcurry22]

Hi Frisco & Bobcrman:

Thanks for jumping on this. I'm new to the sit and had trouble getting into the discussion. I was ready to pull the trigger on this design yesterday until I spoke with Fatmans. In my mind I still cant visualize the difference in the wheel reaction of my split axle and mustang A arm setup. One wheel can go one way the the other another. Frisco, do you feel compfortabe with Bobcrman's idea of centerpoin cavalier or opel. I still like the idea but i don't want to kill myself!

Thanks guys

dennis

 

[dmcurry22]

Hi guys, I'm back from an appointment. After your input I think I see the light, as bobcrman pointed out the angles are all wrong. I hope what he is saying the axle and the R&P steering rod must move in parallel to eliminate the camber change problems. For example, if the 1/2 axle moves on a 24 inch radius and the steering moves on a 12 inch radius (mustang R&P) I've got problems. Since the split axles on my car will have a center pivot, I'm going to check out the center pivot opel and cavalier R&P units to see which will looks best. I feel confindent in this design now and am ready to go! Thanks a million. Dennis

 

[Frisco]

what he is saying the axle and the R&P steering rod must move in parallel to eliminate the camber change problems. For example, if the 1/2 axle moves on a 24 inch radius and the steering moves on a 12 inch radius (mustang R&P) I've got problems.

That is where the problem is. I am not familiar with the Cavalier Rack & Pinion, but it is worth looking into. Let us know what you come up with.

 

[cboy]

In my mind I still cant visualize the difference in the wheel reaction of my split axle and mustang A arm setup.

The difference is an A arm setup works something like a 4-bar suspension. The spindle is held by an articulated joint or joints on the top and another on the bottom. As the spindle travels up (or down) the kingpin remains vertical and the spindle remains horizontal, they just move up and down in the same plane.

On a "split axle" - there is only one point of articulation - at the pivot point where the axle is attached to the frame. As the axle travels up or down the kingpin and the tip of the spindle move in an arch - and thus the camber is constantly changing. To make the point even clearer, if the split axle did not have a bump stop it would swing upward until it was vertical rather than horizontal - and the kingpin would now be horizontal rather than vertical. The result would be 90 degrees of camber. This just points out that in a split axle the camber is always changing while in an A-arm it remains almost the same.

Dewey

 

[dmcurry22]

Thanks Dewey for enlightening me on the geometry of the independent front. You brought up another point that I would like addressed before I start buying. I know (I think I do) where the pivot point will be if I split the axle and anchor in the center of the front crossmember, it will be right there. Where is the pivot point on a solid axle. Is it the opposite wheel or the center of the transverse spring or some other point. My concern is the spring weight for the split axle configuration. If the pivot on a solid axle is in the middle of the transverse spring I will be OK in sizing my coil overs, but if the pivot point is the opposite wheel I think I am going to have to go with a much lighter spring weight to get the same ride. Any thoughts on this guys?

thanks again,

Dennis

 

[ScottROD34]

Good Question

After watching a Show on, extream musclecar, Building the pro touring Scca Fast back the upper and lower arms inter sect the ends of the rack and pinion. I have no Idea why the unisteer works Any Ideas?

 

[Triaged]

1) On twin I-beam vehicles the beams should not be the same length. This is because one is mounted in front of the other. If you run a line from the radius arm bushing to the beam bushing the angle off the centerline of the vehicle should be the same for both sides of the suspension.

2) Sometimes it is hard to search for something if you don't know what words to plug into the search engine. Of the topics being discussed here a search for the following (on here or google) would provide much useful info.
-Bump Steer
-Instant Center
-Jacking Forces
-Roll Center
-Swing Arm (early VW & Corvair rear suspension but basically the same as the suspension you want to use with the same up-sides and down-sides)
Here is a site I just found that has some drawings of different suspensions
http://www.autospeed.com/cms/article.html?&A=2934

3) A good reference book if you want to learn some more would be Chassis Engineering by Herb Adams
http://www.amazon.com/Chassis-Engin...9604600?ie=UTF8&s=books&qid=1188628774&sr=8-3


4) If you want to do a split beam up front the Cavalier steering rack would be a good choice because of the center mount steering. Here is a picture of one from www.napaonline.com

Power steering ones are much more common then manual however.

 

[Chris Kemp]

Ford did the same thing on their Twin I Beam front suspension. But the axles were longer and pivoted from the opposite frame rail instead of in the center. The left axle was curved forward and the right axle was curved backward (or maybe visa versa) so that the wheels were in line with each other. This lengthened the pivot radius which helped to reduce bump steer and the associated camber problems. To get a good tire life out of this suspension you still had to rotate the tires more often and if you didn't the outside and inside edges of the front tires would become more rounded. I don't see why you couldn't do something similar. Maybe use the axles out of an early ranger with a rack of your choosing. I also like the Caviler center rack. I remember seeing something like this on a street rod at a local show about fifteen years ago. It had conventional hair pins with a single transverse leaf spring and at first glance I thought it was the standard model A/B single I-Beam suspension. The longer pivot radius seemed to work just fine for ford and I think the reason production was ended was because of the cost of forging the axles as compared to stamping. I will probably stir up some controversy in saying this but of all of the pick-up trucks that I have owned the early Ford Twin I Beam had the best ride. I had a 1965 F-100 and after a few years of driving it I replaced the worn out 352 and three speed column shift transmission with a chevy 350 and turbo 350 trans. It then turned out to be the best tuck I ever owned and I wish I hadn't let it get away.

I have a post up here about the use of a rack and pinion on a straight axle. Please read it http://www.hotrodders.com/forum/rack-pinion-straight-axle-151016.html#post1064101 I can use some help.

Thanks!
Chris

 

[aosborn]

Where is the pivot point on a solid axle. Is it the opposite wheel or the center of the transverse spring or some other point. My concern is the spring weight for the split axle configuration. If the pivot on a solid axle is in the middle of the transverse spring I will be OK in sizing my coil overs, but if the pivot point is the opposite wheel I think I am going to have to go with a much lighter spring weight to get the same ride. Any thoughts on this guys?

Typically on a solid axle system, there is a tie-rod connecting the right and left steering arms, so there is no bump steer induced there. Where the problem lies is the drag link that connects the steering gear (typically on the left frame rail) to the steering arm on the right. It is the length of the drag link that will determine how much bump steer will be introduced when the suspension moves into bump or droop. The longer the drag link the better typically.

As to spring rate selection, the critical dimension is the distance from the tire centerline to the lower shock mount on the axle. The longer the distance, the more leverage on the spring, (from the tire/wheel assembly) the stiffer the spring needs to be. The distance from the lower shock mount to the inner split axle pivot will not be a factor.

Overall, the longer the split axle, the less camber change you will get. With the small amount (relatively speaking) of suspension travel that is typical for a low hot rod, it should be manageable, even with a pivot in the middle. When a vehicle is turning, the outboard suspension compresses causing the tire/wheel assembly to go toward negative camber. That is a good thing. The inboard going positive is not so bad as it is much less loaded. Ford had it right by designing the beams as long as possible. Keep in mind that the inboard pivot point height location is critical so when you are at final ride height, the base camber setting is correct. Any ride height adjustments will affect your camber setting, and it isn't adjustable other than by ride height changes unless you make the inner pivot location adjustable or bend the axle.

Andy