Jump to content
HybridZ

Techno Toy Tuning - GTX2 Control arm and T/C Rods


Recommended Posts

Your suspension is very colorful!

 

Thank you very much and for the measurements as well, seems like they shrunk down the nut on the bottom as well. This bodes very well as I have a stack of 15 inch wheels and if you have 25mm of clearance with the 16 inch wheels and a 1 inch smaller diameter wheel would be 1/2 an inch smaller in the radius so hopefully enough space there.

Link to comment
Share on other sites

  • Replies 46
  • Created
  • Last Reply

Top Posters In This Topic

No choice in colors with the suspension bits, save for the strut towers. :)

 

You should be fine but you will need to use the bump steer spacer that comes with the GTX2 kit.  The spacer is shipped mounted to the bottom of the spindle.  You'll need to unfasten everything, remove the spindle, put the spacer above the arm, and re-fasten everything.

Link to comment
Share on other sites

Here is a picture of the early style ball joint/pin

IMG 5261

 

 

Here is a picture of the later style ball joint/pin.

IMG 5350

The nut on the bottom of the pin hit the wheel weight. and was very close to contacting the rim.  I removed the RCAs due to this.  btw, that is a 15x7" et10 rim with no spacer.

 

Works ok, but my LCAs are almost parallel with the ground.  Have considered just milling down the RCAs to the max size needed to fit my rim  AND/OR I could drill the crossmember LCA mounting holes up higher to raise the inside mounting point and thus increase the angle of the LCA.

Edited by Samurai7one
Link to comment
Share on other sites

 I asked about using GTX2 arms with RCAs, this is what Gabriel said:

 

Q) I am getting a set of GTX2 LCAs and I want to know if I need roll center adjusters if I plan to lower the car?  Or do the GTX2s negate the need?

 

A) The GTX2 do not lower the steering outer tie rod, so RCA's or outer tie rods that can be spaced down are suggested if the car is still experiencing bump steer 

:ph34r:

Link to comment
Share on other sites

Everyone with an S30 has bumpsteer.  Some bump understeer is good, but bump oversteer can be very bad.  Some people notice bump understeer more than others. 

Everyone will notice bump oversteer because it makes the car difficult or even dangerous to drive.

 

From the factory, the S30 has bump understeer.  The front wheels toe out during bump and toe in during droop.  This is desirable for a street car because it is stable on the highway. 

The factory bump steer is desirable because the following happens when you hit a bump with the factory bump understeer:  Tire hits bump, suspension on that side compresses,

tire toes out steering toward side which hits bump, car leans away from side which hits bump causing suspension to droop and toe to return toward original heading.  Depending on severity of bump, the car may overshoot the original heading, but the steering will want to return to center.

 

Now, lets say that you have lowered the car, raised the LCA attach points, added roll center spacers, and added caster without changing the rack height.  It is highly likely that you will end up with bump oversteer.

Bump oversteer happens when the tires toe in during bump and toe out during droop.  This is a highly undesirable condition because of the following:  Tire hits bump, suspension on that side compresses, tire toes in steering away from side which hits bump, car leans toward side which hits bump

causing suspension to compress further and toe in further.  This condition requires the driver to compensate by steering the wheel to keep the car pointed straight.  Unfortunately, there is a critical speed above

which the driver cannot react fast enough to correct.  Bump oversteer is VERY bad for cars that are driven at high speed.

 

For a race car, bump steer should be minimized (It is impossible to completely eliminate).  The bump steer that can't be eliminated should be in the form of bump understeer.

Edited by 74_5.0L_Z
Link to comment
Share on other sites

So, when I'm measuring and adjusting bump steer in my garage, I was planning to set toe in to zero at ride height, then lower the car 3 inches and check toe again, then raise the car two inches above static ride height, then measure toe again and compare the changes. Is this acceptable? If not, please advise. Do I also need to check it at some steering angle? Also wonder how to check it with only one tire in bump...about to do some reading on the good old Internet. Only advise if I'm not annoying you with my ignorance.

Link to comment
Share on other sites

OP-kick me off if I'm thread jacking. Looks like Longacre is getting more money from me. Anybody wanna recommend a better bump steer gauge than the Longacre? Looks like I need to check all 4 wheels individually and do tons of trial and error to get minimum bump steer. Then, maybe use droop limiters and bump stops to limit suspension if I can't get bump steer clean. Remember, my goal is straight line stability over acceleration thru braking in the measured standing mile. I'm meeting guys on landracing.com who have essentially fixed their suspension within very narrow bump and rebound travel limits in an effort to keep the car pointing straight down the track. Cars set up to turn left and right may not be able to cheat that way. When I get the suspension assembled and the gauges all set up, I will start my own thread on the process so we can learn together.

post-5903-0-63386500-1447907342_thumb.jpg

post-5903-0-75713500-1447907382_thumb.jpg

Edited by RebekahsZ
Link to comment
Share on other sites

I have the InterComp Bump Steer Gage.  I had to buy the four hole plate separately.

 

When you bump steer the car, you need to do a few things to get accurate repeatable results.

 

1. Get the car on a nice level surface.

2. Center the steering.  For this I have made two equal length (2.39") spacers that fit over the rack shaft under the boots that force the rack to its center position.

3. Find ride height on the strut shaft.  For this I push the bump stop against the strut housing, lower the car, roll it back and forth, and then raise the car and measure the displacement of the bump stop.

4. Remove the springs and disconnect the sway bay.

5. Follow the instructions in the bump steer gage.  Take measurements at 1/2" increments of bump and droop starting from ride height.

 

You want to perform the bump steer measurement on both sides of the car.  If the measurements from the two sides are significantly different, then the cross member may be shifted left or right, or the rack may not be square with the car.   Attached below are two different rounds of bump steer measurement for my car.  The top set was a baseline after I raised by LCA attach points 1/2".  For that initial measurement, I used a stack of spacers to raise the rack .39 inches.  From the curves, I had not raised the rack enough because bump was causing toe in.  So, I made thicker spacers that raised the rack an additional .050".  The second set of data shows that I shifted the bump steer curve slightly to the right and up.  So, for at least the first 1/2" of bump, the tires bump out.  My car has 500 lbf/in front springs, so I do not expect to get very much deflection over typical road bumps.  If I raised the rack further, I would shift the curve farther to the right and farther up.

 

As an aside, I am not really happy with these results.  My next action will be to verify that a shorter rack will help the bump steer.  My calculations indicate that a rack about 2.25" shorter center to center will virtually eliminate bump steer.  To test this I am going to steer the current rack 1.125 inches and lock it in place.  Then I will make a longer tie rod assembly to return the toe to its original setting (on one side).  Then I will repeat the measurements above to verify that the shorter rack will fix the bump steer.

 

Figure 10 of the following link explains how to interpret bump steer curves.

 

http://www.woodwardsteering.com/PDF/tech%20section%20guide.pdf

post-30-0-26475200-1447929675_thumb.png

Edited by 74_5.0L_Z
Link to comment
Share on other sites

I am not sure how you would measure bump steer with just two dial indicators.

 

What are they attached to for a stable reference, and how do you track wheel height displacement from nominal ride height?

Attached to something heavy like jack stand - dial indicators have magnetic base. Measure height with ruler. Get static ride height from LCA angle then jack up and down with spring removed.

Edited by heavy85
Link to comment
Share on other sites

I think 10 or more years ago I took a racecar engineering class from ICP.  It was two days in a shop for 12 to 13 hours each day.  The topic of bumpsteer came up and the shop owner had one of the gauges above.  Steve Lathrop said they were fine but there was a much cheaper setup that was faster to boot.  His rig was a welded frame with two roller bearings that ride along the plate.  He did warn it's best to measure near the actual radius of the tire so not to have error get out of hand.

 

To use this setup you run the wheel up and down and watch the gap on the rollers.  Make a change and note if things are better or worse.  Repeat until the gap is very fine 1/16 of an inch was good enough for them.  Done this way it should go much faster.  I made an even cheaper version of this using 2x2s so I could get this done between races.  This is more about process and less about tools.  The longacre system is nice but unless you need to do this a lot I'd try a lower cost set up.

 

Cary

Link to comment
Share on other sites

  • 4 weeks later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...