Traction Arm Length for bump steer

[Ben26]

Hi guys, this has been covered only a hand full of times as far as I can see and none have definitive answers on actual lengths, only the theory behind changing traction arm length. 

So, today Christian and I from Sideways motorsport in Brisbane tested a number of different traction arm lengths in a r34 gtt and measured toe through from full droop to full compression to measure bump steer (toe changes with changes in bump).

Firstly, we removed the rear coilover, and removed the wheel.

Christian then made a little jig to fit to the hub of the vehicle out of stainless plate that we bolted on to the hub using the wheel nuts, then setup a string line parallel with the hub and took a measurement of the distance from the string line to the hub from the front and back of the stainless jig / plate. 

We then jacked the hub up to full compression and measured the change in distance from the front and rear of the plate. We then had the measurement in mm for the toe change through the wheel travel. We tried a number of different traction arm lengths, from fuly extended to as short as possible. The least amount of toe change we found was with the traction arm set to 7mm longer than standard from centre of bolt hole to centre of bolt hole. The ride height is around 330mm and camber -2 in the rear. 

My question is what lengths have other people tested and found that a certain length minmises bump, im asking because the jig we made is relatively crude and my guess is a little bit of human error would certainly be involved in the measurement process. 

Any insights on this matter are most welcome !!

[GTSBoy]

Everyone else's traction arm length will be specific to;

1. Which car - R32 different to R33/4, etc,
2. Upper control arm length,
3. Static toe setting.

So there's no point in trying to compare too many apples with oranges.

You have likely found a setting which is far better than what you had before.  If your jig is a bit rough, then maybe it's not the best setting possible, but still better.

If you want to consider an alternative way of doing it, strap a mirror onto the hub or disc face.  Place a board upright, parallel to the car about 2-3m out from the side of the car.  Clip some paper onto the board.  Shoot a laser point at the mirror from a spot close to the edge of the board/paper so that it reflects onto the paper.  Keep the total angle as small as you can.  Jack the hub up and down and mark the reflection points on the paper.  You will get a line that goes up and down as the hub swings through its arc.  Any forward/backward motion of the reflected dot is bump steer.  You can change the arm length and generate another curve on the same piece of paper.  Do this and get worse bump steer?  Make the opposite change.  Then just increase or decrease arm length until the line is as close to vertical as possible.  It has the beauty of not needing to work in the same place to adjust the arm as your jig/dial gauge/whatever measurements are being taken.  It's less quantitative as to how big the toe change is in mm unless you measure the change on the paper and do some trig to work out the swing at the wheel, but that hardly matters, because all we're looking for is the smallest amount of bump steer.

[Ben26]

Hi Gts boy, thanks for your feedback, ok I can understand why the upper control arm length and different cars are going to affect the relationship with the traction arm, but why does the static toe influence toe under bump ? I guess my thinking is setting static toe doesnt really affect toe under bump, thats the traction rods job, whether you set you toe at 2mm out or 2mm in for the rear, doesnt it just matter than its not changing under bump ? Or am I looking at this the wrong way ? 

Yep ok thats another neat method of measuring toe change, as you say and Carroll smith says in his books, we dont care how much bump steer we have quantitatively, only that we eliminate it !! 

Edited July 12, 2018 by Ben26

[GTSBoy]

The static toe is set by the tie rod length (or toe control arm with no HICAS), which, for the purposes of the geometry of the swinging of the wheel, is a 3rd link (actually a 4h link) that swings through a different arc than the lower control arm that it runs near.  Different locations for the inner and out ends.  Causes a small tweak to the path of the other arms.

[Ben26]

Ah ok, yep Im aware that you change toe using the tie rod but to me it looked as if adjusting that arm had little to no effect on the traction arm or upper camber arm. I will remember that for next time, thanks for your advice

[hardsteppa]

OP are you using stock upper control/camber arms or adjustable? i think an exact length isn't commonly posted because as GTSboy says, it's relevant to the individual setup. It's prob 2 years back since i did mine - numbers are written down somewhere in the garage but to quote numbers through a foggy haze of memory, i think i measured my traction rod at 218mm stock length bolt to bolt, and ended up at maybe 223mm? 

The point was though, the 'best' length in my testing, was changing it by the same ratio as you change the camber arm to reduce camber once the vehicle is lowered (my rear camber is -1.75 deg). I would imagine Nissan spent a fair amount of time to minimize bump steer when calculating the suspension geometry so it would be no surprise that maintaining the same relative lengths on the arms would give the best results, to approximately maintain Nissan's geometry.

[Ben26]

Hi Andy I have adjustable camber arms as well, so I set the camber roughly where I wanted it to be to begin with, then started playing around with the traction arm length. I measured my stock ones at 210mm middle of bolt hole to middle, and I ended up with a traction arm length of 217mm so it seems we have a similar result. Yea Im aware that the combination of the two upper arms has an effect on the toe curve of the rear suspension, But I was just trying to get a ball park figure of what people were getting. 

Yea that makes sense, so you think that if you get a ratio between the length of the standard traction arm and camber arm, and maintain that ratio, you would achieve a good result. I might have to test that, sounds like it might be a good starting point for future cars / set ups. 

Thanks for your insight my friend.

Edited July 13, 2018 by Ben26

[Komdotkom]

If you are doing this type of thing on a regular basis I'd suggest that you invest in a couple of dial indicators and make a jig like this, it will give you the most accurate and repeatable results. Cheap dial indicators are available for as little as $35 these days so for a total investment of ~$100 and a couple of hours you will be able to create a measurement device that you will doubtless use on countless occasions in future.

 

[Ben26]

Ah yes I have also found decent bump steer measuring devices online for a few hundred dollars so I think for next time I will certainly either build a better jig with dial indicators or purchase an already made rig. 

[Dale FZ1]

I just used the same stringline setup as used for my static alignments.  Very functional.

[hardsteppa]

mine is pretty much same as this. You need a dial indicator, but they're a handy thing to have around anyway, and then $25 at Bunnings for some board and hinges and you're done.

 

 

[Ben26]

Nice, all great ideas !!! Thanks for everyone's input !!

[GTSBoy]

For completeness, I had my traction arms out the other day.  They have adjustable bushes in them, so the stock length is a guesstimate.  But I'd say that at zero adjustment the length is 210mm.  At full adjust (as they were on my car) they are 213-214.

[Ben26]

Yep, on my car I also measured them at 210mm and my length I set at 217mm, the sweet point for me was between 215mm and 217mm so it seems as if we are on the same page even if setups are a little different. Thanks again gts boy.