I am going crazy with pics now that I can post them!
I have participated in threads discussing the rack conversions (steeroids Vs ETO, Bumpsteer, etc) and I wanted to finally show mine off.
I had a mirror image conversion using a F*rd steering box out of an Australian GT with a new RHD drag link. It handled as well as could be expected for a 40 year old design but the problem was I had vibes in the drivetrain because I had to move the motor over so far to fit it in. As time went by I learnt a lot and discovered I had bent the drivetrain too much and this was causing the vibes. As I was planning a body off I started work on moving the motor back to straighten the driveline. This involved redesigning the steering conversion and I figured I could get a rack in instead of the box and drag link design.
Looking at the racks available in Australia I settled on one out of a 85 Commodore and set about getting the rest of the design right.
My set up uses completely original front end apart from the steering arms which have been shortened and lowered.They were shortened because the rack did not have enough travel to give full lock. The lowering in association with the angle on the tie rods combines to give 0.00 bump steer over 95% of the suspension travel and the last 5% is when the wheels are at full droop or the car is airborne!
Another feature is that it has theoretically perfect acherman angles which means both front tires are tracking perfectly in a corner. The bad side of this is the turning circle is larger.
The entire set up is bolt in and could be replaced with the original stuff at a whim, not that I would ever give up my rack now!
Some of the pics are out of focus and I apologise as they were taken with an analog camera and scanned.
Question time begins!





:cheers:

 

James,
Looks great, I haven't seen it from that angle B4.

Regards from Down Under :cheers:

aussiejohn

 

What, no Questions?
I thought Norval or TT would jump in by now.

 

I completely missed this topic. That looks good. How wide is that rack and how did you get 0 bump steer??

 

Looks good. Need bigger pics tho.

But I agree with TT, Zero bump steer? Comon, tell us the real numbers. even if it's below .001" it's still not zero :skep:

:jester

 

The bumpsteer measurements were made at the tyre edge with a plumbob.
I mocked up the steering arms with mild steel and adjusted them and the rack height to give the best reading. There was a bit of luck involved as I didn't have to move things around too much.
I then had the steering arms modded to the same specs and heat treated and xrayed. There was about 3mm bumpsteer at full droop(last 20mm of travel) but I did not have the shocks or springs in so It was sitting on the bumpstops. The front end would almost be airborne to get to that position while driving. The rest of the travel had less than 1mm of deviation by my measurements.
The rack is 615mm centre to centre of the tierod balljoints. Rack travel is only 125mm. It would be easier to work out the conversion with a shorter rack with more travel :D

 

The reason I'm so amazed at your figure of 0 bump steer is that when I look at the pics the rack seems way too wide. The pivots are nowhere near the ideal plane and intersecting with the IC (jsut add some lines to that pic and you will see). Of course that intersetion is not the only point where you can get close but it is the only point where you can get to or very close to 0 for the entire length of travel. I would have suspected a lot more bump steer through the entire travel.

Being an ETO rack and the pivots actually too far outboard the angle change and resulting horizontal tie rod component varying much more with travel I would have suspected it to be even worse than with a CTO rack. With the CTO rack a lot of the geometric errors in it are cancelled by the long tie rods. The longer the tie rods and the shallower tha angle (the hing point being as far in the center as possible) the less the horizontal tie rod component will change.





[Modified by Twin_Turbo, 9:21 AM 2/10/2004]

 

Gotta go to work, will explain later today.
James

 

Keith,
When you say bigger pics, you mean higher res?
I have shrunk the res down to make it easier to load.
Do you want to blow them up to look at them closer or are your old man eyes failing? :D

 

[QUOTE]The reason I'm so amazed at your figure of 0 bump steer is that when I look at the pics the rack seems way too wide.

Yeah, the rack is too wide if you have straight tierods ( in the horizontal plane) but because of having to mount the rack behind the lower wishbones and the shortened steering arms(because of the limited rack travel) my tierods are angled forward. This has the effect of making the outer tierod end travel in a shallower arc than if they were straight. It is acting as if the tierods were longer. The limiting factor is the angular travel of the rack end tierod ball joints. They must be able to move freely through the entire suspension travel and steering lock and I have already put an initial angle of 22 degrees in there.
Demonstration:
Take a 30cm ruler, pen and paper. Fix one end of the ruler to the surface you are working on and scribe an arc on the paper with the pen on the other end of the ruler. Now raise the fixed end of the ruler about 100mm above the surface and fix it there. Scribe another arc on the surface with the moving end of the ruler on the surface. It will be a shallower arc and imitates a longer ruler laid flat on the surface. The greater the angle, the shallower the arc, the longer the tierod it is imitating.

I just happened to luck out with my design in that the arc was pretty near perfect and only the adjustment was the outer tierod end height to bring it line with the arc the steering arm described.

The plate from the crossmember and the two arms up to the chassis rails locate the rack well enough that the extra forces of the angled tierods don't seem to affect the handling. The tierod ends on the rack may wear out earlier than straight ones but I am keeping an eye on them for slop.
The Rack mounting will also help stop any flexing in the Chassis. I do tend to over engineer something when I do it.
I have driven the car for nearly 2 years now and inspect it regularly, not one loose bolt or signs of early wear.

My bumpsteer measurements were taken at the tyre's edge but with a straight edge bolted to the hub and a plumbob to the floor. As the hub moved up and down through its full travel, the lines on the floor stayed parallel.
Come to think of it, the straight edge was probably longer than the diameter of a wheel.



[Modified by RHD '68 L89, 8:44 PM 2/11/2004]

 

Yes it's my old man eyes that need higher res pics. :rolleyes:

But really you don't need to change them, I was just whining...

 

Looks cool!

Did you check the bump steer through the suspension movement? :cheers:

 

I like it!! :thumbs:

 

Nice rack! :D

 

Just one question - when can you make me a kit?? I'll pick it up at the Convention at Easter. ;)

 

Yo Chris!
Good to hear from you.
I am thinking of looking for a shorter rack to do the sums on. it needs at least 150mm of travel and to be around 550mm long from centre to centre of the tierod balljoints. Then I would be able to use the stock steering arms although they would probably have to be bent to adjust for the bumpsteer.
If you can do the search on the rack I can do the sums.
:cheers:

 

Well it is good to see this done and posted but there is another one i will try and get pics of it.This guy has also got 0 bumpsteer. A true reading has been done through the suspension and all works out. But he also says it was mostly luck. But great job. I hope to have pics shortly :D :D