I received an email that has some questions that other people may find relevant....
Hi Wes,
I've been playing with your rack and pinion spreadsheet but have a few questions.
1) I updated the info with my specifics - 205/50-16 tire size, plus inner tie-rod joint width of the Flaming river R&P unit (603.7mm). Niether of these is very far off of your original dimensions.
It looks like placement of the rack - both up and down and forward and back is extremely sensitive! Even slight movement really skews the bump-steer characteristics. Your original settings for rack position look to be about the best overall - I'm just wondering how one would implement them in a real-world installation. Seems like once your main attachment points are fabricated and installed - both front-back and up-down shimming/adjustments would be necessary to really get it dialed in, in small increments of movement. Especially when you're talking about dimensions of less than 0.1mm making a fairly large change!
On the lower section it states: "Table below calculates tie rod lengths, enter angle car normally sits at"
Where is this angle entered? Is this the overall chassis angle of attack or trailing arm angle?
Referring to trailing arm angle, I know you're working this up for an off-road application but I'm dealing with on-road, with most likely much less suspension travel - probably only around 130mm travel total. How can I plug this value into the spreadsheet, probably with a minimal top torsion arm angle (maybe about 4 degrees down?) as the normal static attitude of the upper arm? How does this affect the angle of the lower arm (or is this automatically compensating?)
Very neat work on the spreadsheet though - fun to play with!
Cheers,
Jeff
Hi,
A lot of this stuff has been covered on the
www.aussieveedubbers.com forum. I have attached the latest version of the spreadsheet so you can play with it.
Yes it is very sensitive to position of the rack. In order to address this you need to be able to shim it up and down. So yes you would trial fit it, then measure the bump steer. Using the excel sheet and comparing to the bump steer pattern you measured you should be able to accurately shim the rack to the correct position (by figuring out how far out, and in what direction it's out).
A center mount rack would be less prone to the problem. You would get half the error because your tie-rods would be twice as long (roughly speaking)
The funny thing is the stock steering box set-up suffers the same sensitivity to set-up accuracy. The stock short tierod will be very similar to a rack design. From my measurements so far, the stock tie rod end centers on the steering box appear to be located at;
drivers - 255, 36, 119
passengers - 215, 41, 104
You can plug those figures into the rack location in the excel sheet and see what the stock set-up is like. The problem the stock set-up has is that it is not adjustable or shimmable. Whereever it ends up is where is stays.
The angle the steering is set-up at is referring to the angle of the top trailing arm. -4degrees would be slightly up, +4 would be slightly down (lowering relatively speaking). You can see this just moves the bump steer curve left or right essentially.
If you don't want to use the full suspension travel then just ignore the upper and lower measurements. The big calculation table that is now grey in colour shows the upper trailing arm angle, and for each angle you can read across and read the "y" value which is essentially the suspension height in mm above or below a top arm angle or "0 degrees" (horizontal). And yes it calculates what the lower arm angle is for any upper arm angle.
Wes
Wes -
www.offroadvw.net - 200HP Quad Cam V6 in a VW Baja - with climate control...
🙂
www.taylorcycles.com.au - My DH MTB racing brothers shop.
