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

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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

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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.

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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.

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Edited by RebekahsZ

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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

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Edited by 74_5.0L_Z

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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

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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

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I finally unpackaged mine.

 

I am planning on doing the same as you did with moving the spacer on the bottom. For stacking the spacers, is that the correct orientation? Or does it not really matter since the nut on the bottom hits the wall before the spacer could come into play.

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Those TTT control arms look just like the set I recently fabricated. I got the monoballs and pins from UB Machine.

 

My theory on bump steer is I dont worry about measuring it.....just try to get it as close to 0 as possible. No expensive tool needed. Just 2 pcs of plywood, piano hinge, 2 small screws that extend out to touch rim, and a weight to hold on ground firm. Remove spring and jack car through range of motion. Adjust until screws ride on wheel rim throughout range.

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I finally unpackaged mine.

 

I am planning on doing the same as you did with moving the spacer on the bottom. For stacking the spacers, is that the correct orientation? Or does it not really matter since the nut on the bottom hits the wall before the spacer could come into play.

 

 

I just saw the questions.  I do not think that the spacer orientation matters.  I tried to match up ends on the spacer though.

 

I'm glad this post showed up. I'm thinking about getting these. Ordering 1, does it include both sides of the front suspension or do I have to order two? Arizona sells the sides separately, not sure how ttt packages their products

 

An order includes both sides plus the T/C rods.

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I've been searching around trying to determine what to use for my front suspension. I was thinking of going the stock style T3 control arm and t/c rod, but this set does come out a bit cheaper. I would like to track my car a little at some point, but primarily would just be street driving. Would there be any reason to go for the stock style ones over the gtx2 set?

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You might have a bit more compliance with a regular ball joint over a race ball joint. You could also run a standard style tension arm. I'm not sure how much the race ball joint will cost to replace, or if they need to be replaced, but that could be a factor. 

 

Wheel size can also be a concern. If you want to run stock or around there, this one is going to be real close depending on what you are running in regards to bumpsteer spacing. I have about 1/2 an inch from the wheel on mine with 15 inch wheels, the standard style ones may be able to accomodate a smaller rim just based on the fact it doesn't have material right under the ball joint.

 

Frankly it seems they may have addressed some concerns that may have made the regular style arm more appealing over the gtx-2 arms, that is what pushed me over to buying them after all.

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The only reason to move away from a stock arm is to be able to run a bunch of caster. Which often involves cutting your fenders and airdam for tire clearance. So, stock LCAs offer your best option over either aftermarket arm. Spend your money on things that will make you faster, like a good seat and seatbelts. Or tires.

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