Lizard928

Try again,

That is $175 for the pair.......... so less than half of the price of your kit, and I doubt the modifications were hard or time consuming. Though I personally would have changed out the heims in that kit to males and used the stock inners....

GregBBRD

Thank you. Looking at one "instant" point in a constantly changing (dynamic) situation never gives the complete equation.

While it is simple to measure "toe change" on a vehicle in a static setting by raising and lowering the front of the vehicle as a whole, not so simple to figure out what is happening in a corner, with one tire is in droop and one tire in compression....in a constantly changing situation. (Keep in mind that the tires move through an entire "range" of motion from corner entrance to corner exit....and "toe" is constantly changing.)

Mark Anderson was chasing a particular problem. Regardless of what he did with camber, he was wearing the inside of the tires. Note that this may have not have been happening in a corner, but may have been simply an issue doing down the front straightaway.....

Let me explain:

Like Mark Kibort mentions above, The "effect" of the toe change, at the apex of a corner, is somewhat moot....one tire is doing 90% of the work and the other is pretty much just along for the ride....sometimes barely toughing the ground.

However, when going down a straightaway, the actual ride height of a vehicle can radically change, due to down forces (or lift) depending on your own particular aerodynamics. If you have a radical "toe change" with suspension movement, these ride height changes can only result in "scrubbing" of the tires...sometimes severe.

In short, "bump steering" a vehicle is virtually always going to help.....if the only benefit is to reduce "scrub" while going in a straight line. (Obviously this is going to reduce rolling resistance and thus increase potential speed.) Where the toe actually ends up being set and how much toe change you build into the vehicle (in a static setting) is going to depend on what dynamically happens in a corner (leading up to the apex, where only one tire is doing most of the work).

While I'm not allowed to say if the pieces Carl is offering are well engineered or not (verboten by Hacker), "adjustability" is generally a good thing.

Mark Anderson

http://www.stableenergies.com/ERP/products/272/

it does appear that is is $175 per side

blau928

Hi Carl,

I missed the note of the 180mm WSM spec on the chart posted..

I was trying to figure out what was the deviation point from that of where you originally measured. If you just measured the toe after droping the car 25.4 mm without adjusting the camber etc again, then I understand now what is happening when you did your measurements..

I personally think that the alignment specs would need to be reset after lowering to get a baseline against which to compare, as there would be more than just the toe measurement that will be off after the car is lowered 25.4mm. As such, the toe change by itself would not allow a complete picture of what is happening with the suspension after the vehicle was lowered.

Then again, I am no suspension expert, but I found that on my car, the measurements changed after lowering the car. I know the car has to settle after touching the suspension or lifting it off the ground.. The car was put on the rack for a spec check before and after lowering. The after check was done after lowering and allowing the suspension to settle by driving the car gently several miles around town before putting it back on the alignment machnine and doing a rolling compensation (no lifting) to calibrate the machine sensors...

I don't remember how much what changed as it was a while ago that the car was set to 152mm ride height all around, but it was not the same as when the baseline was taken before lowering.. Same tire pressures, wheels, shocks, springs etc. with all done on the same day..

Interesting you mention the busings.. Did you measure the flex of the bushings to see what the difference was compared to the pre and post lowering..? I am guessing no, as that would require some really sophisticated equipment and or strain gauges etc?

Thanks

Bill Ball

Hi Carl;

Sure looks like your kit reduces toe change, in this case on a car set up at 155 mm ride height.

To help me understand your data I would need to know just a bit more. Was this toe change measured in inches with toe plates? If so, how wide apart were the front and rear tape measures? If this total toe change (both wheels)?

Was compression/decompression on both wheels or just one wheel?

Carl Fausett

Richard, no, you do not adjust the camber with each inch of travel during a bump-steer measurement.

Again, if you touch the alignment settings during the bump-steer measurement, you will loose your delta between measurements. You are trying to measure what the change in toe is through 6 inches of suspension travel (3 up, 3 down) as you simulate driving through a corner or over bumps.

Carl Fausett

Bill,

all measurements in my charts reflect one wheel. I haven't even started to talk about Accumulated bump-steer, but you can see that mid-corner, one tire is gaining toe-out while the other is gaining toe-in. You can see how scuffing and scrubbing would be caused, not to mention a lack of pointing-up by the car.

The bump-steer was measured with the jig below. One of several designs you can buy or build. In this design, instead of having to read two dial indicators (one on each side) a solid pointer is used on one side so all the change is recorded on just one indicator making it easier to read with less error.

The indicators are located at about the same distance away from the center of the spindle as the distance from the center of the spindle to the contact patch with the pavement.

Having said that - the distance the indicators are spaced away from each other is not that important so long as that distance is constant throughout the tests - what we are looking for is the relative change from one position to another.

These picture I am sure will help. The first was the bump steer test jig on a car with early (long) a-arms. You can see the floor jack being used to create "bump". The spring has been disconnected.

The second picture is on a late-model car with short a-arms. The tubes on the floor are filled with lead for our Bonneville ballast, here we are ratcheting the suspension down using them as weight to measure change during droop. The spring was also disconnected.

In addition to disconnecting or removing the springs for these tests, the sway bars were also disconnected.

The horizontal lines on the indicator board are 1" apart.

Carl Fausett

Here is the entire 6" range of travel.

A piece of white tape was applied to the tie rod arm so it would photograph better.

Pictures in order:

3 inch bump
2 inch bump
1 inch bump
0 position (level)
1 inch droop
2 inch droop
3 inch droop

blau928

Hi Carl,

Point taken on your measurement parameters. I understand what you are saying, comparing just one variable, in this case, toe, the rate of change as you measured it is just that, just the change in toe measurement. In addition, with the bump-steer kit you are offering, the toe correction with your part based on your measurement will negate the toe delta as you have measured it. (I get that bit..)

However, based on my understanding of suspension geometry and the dynamic effect on the wheel in motion, there are several things that still remain unclear to me.

With the suspension being dynamic, and the effect of toe when cornering on the loaded wheel being minimal in the 928 case, due to the design, then are you in fact saying that the change in toe measurement alone can give the full picture of the dynamic effect on wheel movement in relation to the suspension setting and the effect on steering, as it would take into account the variance in contact patch of the tire as a result of the dynamic changes other than toe.?

My understanding is that this will happen only if the suspension is truly a dual equal length swingarm with 0 kingpin angle... (928 is not this, as I am sure you can confirm...)

Again, my point is that the suspension geometry is a dynamic arrangement, and various other parameters will change when one variable is changed. If only the toe is measured as an exclusive variable, then what happens to the effect of the dynamic relationships in all the other suspension connection points that affect wheel movement and the ability to steer in a corner or the amount of tire contact patch the kit will enable one to have on the ground.?

This is the part I am trying to understand when using your bump-steer kit..

I hope that is clearer than what I first wrote..

Thanks,

Carl Fausett

Richard,

I see what you are saying but I do not have the data to answer that question.

You know that all adjustments to the suspension effect toe (you always adjust toe last when doing an alignment because changes to caster and camber will change toe) but in effect, what you are asking is what other suspension angles will be changed if toe is adjusted first?

To my practical knowledge - none. The caster and camber are held by the eccentrics in place and they are not moving. Changing toe does not change caster or camber.

As to the effect it might have on toe throughout the range of travel - that's what we measured and I have reported.

As to the effect other than caster, camber, or toe that might be brought about by the installation of a bump-steer kit - I think none, as the upper and lower swing arms and bushings are unchanged. There is no reason for it to move. But, my suspension knowledge is practical and experience-based, and you might want a suspension engineers response to that.

Perhaps consult my bibliography? (Post 55)

Mike Simard

The measurement setup is nice. Good work.

I didn't see that much change when checking with a similar setup and I'm surprised at those numbers. I believe them but wonder why.
Variation of the position of the inner tie rod would cause bump steer change, and Porsche seems to have made different length racks, there are alot of factors.
I do remember doing bump steer on another car and was able to see toe changes on the gauges with as little as .015" raising/lowering of the rack.

The reason the extended tie rod thing bothers me so much is that I understand as a machinist how they could be made in a poor way and also an experience with my own car.
On it's first test drive on track, after heavy braking, the inner tie rod collapsed and left the wheel flopping around with alot of play. There's a plastic liner inside that collapsed from the braking force of slicks and good brakes. Normally the tie rod sees little force but under braking it can be alot more, and then there's the accumulated effect of vibration on a spindly part like that, I really can imagine failure as slightly possible.

I'm sorry for being hard on you Carl, it's just that things like that are important to me and since it's safety related and done blindly with everyone else without understanding, I feel it needs to be dealt with.

I would still never do that to adjust bump steer, I'd move steering racks, shorten or space inner tie rods, anything else. But that's just my opinion

FredR

Good to see this interesting thread going where it should - lots of impressive technical discussion-and analysis- well done!

Not ranting today but cheering on!

Regards

Fred

Carl Fausett

It's big of you to apologize, Mike. Thank you. At least you can see I did my homework. For my part I should have published my data with the initial post rather than two days later.

I agree with you it would be nice to correct the angularity by relocating the steering rack - but of course that is not a simple thing on a 928. I can just imagine telling people how to lengthen their steering column because their rack has been moved further down. Ugh! It left this as the only practical option for a bolt-in solution.

blau928

Hi Carl,

No problem about not having the answer.

Yes, I was curious as the toe is adjusted last as you mentioned, and as I noticed on my car, the other specs changed after lowering it..

I also believe that the camber will change when the suspension is lowered in an unequal length A arm suspension with 0 kingpin angle due to the geometry, as the movement in the arcs of the upper and lower arms is unequal. (This is without considering the other forces such as roll, yaw and pitch inthe dynamic movement of the vehicle..) My dad has he Smith book and a few others, maybe I'll revisit it, and also ask an engineer buddy. (My dad keeps telling me to rip out the spindles and replace the suspension with equal length A arms and make a real race car out of the 928 like his old GT2 car...)

Didn't mean to overcomplicate the thread with technicalities..

Thanks again,

BC

As a data point, I had an altercation in the can-opener-car and the right front wheel was... asked to turn right when I was actually turning left. Meaning something put massive force on it at the rear of the wheel to push in on the tie rod. The tie rod pin stayed put, and was undamaged. The inner tie rod arm became... a letter S.