Bump steer kit

[Department Of Boost]

Right after you measure how much bumpsteer you have and determine that it needs correction.

FACT^^^^^^^^^^^^^^

 

[claudermilk]

Not sure why that happened either. The OP was asking if a bump steer kit is required just because you are lowered and the answer is no.

Because S197Forum. It's what we do.

 

[Pentalab]

It's a pet peeve of mine... the BULK of the issues with a Panhard (it's a guy's name) bar is in the bushings. Minimize the deflection from those bushings, and it tightens right the hell up. Stock pencil-eraser to poly is a night and day difference, and my preferred bushing material, steel (rod ends!!), is night and day again.

In the end, both a PHB and a Watts do the same job, that of lateral axle location. Yes, the Watts does it better over a wide range of motion, but if you do the math, a rod-ended PHB only allows something like .080" of lateral deflection with a nominal 1" bump...

I will try and dig the video up, an autocross fellow mounted a go-pro camera just aft of his rear driver's side wheel well, pointed down at an angle.
(think it was a 2008 stang). He had replaced the oem PHB /PHB brace with a BMR adjustable PHB + BMR brace. (Poly bushings). Every time he turned right, the left rear tire would vanish inside the wheel well. Every time he turned left, you could see the top of the tire tread. Way more than .080". More like 1". I don't remember if the rest of the suspension was stock or not.

 

[dontlifttoshift]

Stop right now. Tell me where there is 1" of deflection in even a poly bushed PHB and then you can continue your argument.

Let's say we have a car that is shaped just like a box and this box fits over the tires and touches the ground. Now stand at the back of the boxcar and have a friend lift up the passenger side of the box while the drivers side of the box stays on the ground......now you have more clearance between the box and the drivers side tire but oddly enough you have less clearance on the passenger side. That's why the video of the PHB looks the way it does.

Now your homework is to find a similar video of a car with a watts link and show me how it is different.

Did you measure your bumpsteer yet?

 

[csamsh]

It's me with the video. It's not due to bushing deflection. It's due to the arc through which the panhard bar travels, the roll of the car, and the deformation of the tire at the contact patch. All of those variables working in concert...purpose of video was to show what happens to the rear axle during a run, not say "panhard sucks!" (But a Watt's is better...much better for autox).

IIRC I was running quite a lot of roll steer at the time too.

http://vorshlag.smugmug.com/Racing-...72813/30799281_2z8QJb#!i=2664718105&k=Mz2bTZt

 

[Norm Peterson]

Don't watch the tire tread - there's going to be some sidewall deflection going on that could account for much of what you're seeing. You can see this relative to the wheel, too bad its almost impossible to track the wheel against the sheetmetal.

I would expect the contributions from polyurethane deflection and pure geometry to be similar - around 0.03" at a 1.5" change in PHB chassis side height and 1.5 lateral g. Don't forget to double that for the full range of movement between R and L turns.


FWIW, I've got one of Sam's early kits, the one with the 22 mm tubular rear bar. Seems to be working well enough with the rest of the combination and definitely not too much bar. Only 4.6L may have something to do with that, though.


Norm

 

[SoundGuyDave]

Time to break out Socrates...

Given: We can prove, mathematically, that the actual motion of the axle relative to the chassis is only on the order of 0.060" total on a hard slalom.
Given: Tire carcass deflection will be an order of magnitude (or more!) greater than 0.060."
Therefore, I think it's safe to assume that Panhard bar arc and poly bushing deflection are essentially "noise" within tire carcass shift, and possibly even wheel deformation at those kinds of loads.

So, if we assume that drivers CAN feel the difference between a Watts and a Panhard in a slalom, what exactly is the difference? Is it in the bushings? Nope. Is it in the actual degree of lateral location of the axle relative to the chassis? I highly doubt it. So, what else is left? I'll submit, that even if all other variables remain constant (tires, bar rate, spring rate, dampers, AS%) that there isn't a Watts kit on the planet that puts the roll center in the same spot as the stock Panhard bar location. Is it possible that roll center migration is the real "culprit" here? Let's assume a Panhard bar that is parallel to the ground at static ride-height. In a right-turn, the roll center on a PHB moves up and to the right. Slightly. On a left-turn, the roll center on a PHB moves down and to the right. Slightly. Is it possible that this migration of the roll center in an arc is what the drivers are feeling when the say the PHB makes the car darty compared to a Watts? The vertical change alone could easily account for the difference in "feel" between rights and lefts. Bud doesn't that same thing happen with the different styles of Watts (chassis vs. axle mounted)? But is there anything in that minute lateral motion of the roll center that that Watts constrains, but the PHB doesn't? Or is it perhaps the gross overall effect of a (relatively) massive relocation of the roll center that makes the Watts car feel "planted?" Hmmmm....

 

[2013DIBGT]

Bold for emphasis.

With a taller ball joint, you will need the bumpsteer kit. You changed the geometry, you need to correct it.

Not sure how my post alluded to being unaware of this. The Banging of the head was to signify the PITA involved with installing all the pieces involved.

Did you measure bumpsteer without the taller ball joints?

No, too much work to measure the before Bumpsteer but it was present in small doses. My main reason for going with the 302S Arms was not due to Bumpsteer though. It was a costly approach but I already needed to replace the stock FCA bushings anyway (Again) and doing so was getting old. I also had ZERO deisre to burn out the old bushings with a flame thrower. The more durable Howe ball joint was an added bonus. Being able to repalce the Ball on its own in a reliable manor if it ever fails is nice also due to the threaded body. I figured if I was going thru the pain anyway I might as well do it right and be done with it.

Can't say I'm a fan of the boots on the Howe Ball joint though. Weak sauce!

In the end it was all worth it though. Car feels great over all road surfaces now.

 

[dontlifttoshift]

I figured you had a grasp on in, others reading might not see the correlation between the tall ball joints and the need for bumpsteer correction at that point, that was all.

 

[tigercrazy718]

I figured you had a grasp on in, others reading might not see the correlation between the tall ball joints and the need for bumpsteer correction at that point, that was all.

No thanks for that clarification, that was something I was not aware of before, and I was planning on installing Boss S arms.

 

[Norm Peterson]

Time to break out Socrates...

Given: We can prove, mathematically, that the actual motion of the axle relative to the chassis is only on the order of 0.060" total on a hard slalom.
Given: Tire carcass deflection will be an order of magnitude (or more!) greater than 0.060."
Therefore, I think it's safe to assume that Panhard bar arc and poly bushing deflection are essentially "noise" within tire carcass shift, and possibly even wheel deformation at those kinds of loads.

So, if we assume that drivers CAN feel the difference between a Watts and a Panhard in a slalom, what exactly is the difference? Is it in the bushings? Nope. Is it in the actual degree of lateral location of the axle relative to the chassis? I highly doubt it. So, what else is left? I'll submit, that even if all other variables remain constant (tires, bar rate, spring rate, dampers, AS%) that there isn't a Watts kit on the planet that puts the roll center in the same spot as the stock Panhard bar location. Is it possible that roll center migration is the real "culprit" here? Let's assume a Panhard bar that is parallel to the ground at static ride-height. In a right-turn, the roll center on a PHB moves up and to the right. Slightly. On a left-turn, the roll center on a PHB moves down and to the right. Slightly. Is it possible that this migration of the roll center in an arc is what the drivers are feeling when the say the PHB makes the car darty compared to a Watts? The vertical change alone could easily account for the difference in "feel" between rights and lefts. Bud doesn't that same thing happen with the different styles of Watts (chassis vs. axle mounted)? But is there anything in that minute lateral motion of the roll center that that Watts constrains, but the PHB doesn't? Or is it perhaps the gross overall effect of a (relatively) massive relocation of the roll center that makes the Watts car feel "planted?" Hmmmm....

I think we have to look at all of the forces that might be involved as you're driving the car.

A PHB that's axle-mounted on the driver side will always affect the LR tire more than the RR due to its proximity to the LR, and the difference in PHB angularity in left turns vs rights generates a less upward/more downward PHB vertical force in left turns than in rights. Basically, this arrangement is inherently skewed toward improving left turn behavior at the expense of right turns.

You also have the driveshaft torque loading, which always loads the LR and unloads the RR under power (arguably the more important consideration), vice-versa on trailing throttle.

I'm going to suggest that the way the PHB and driveshaft effects combine can't help but be asymmetric except under some very specific and probably rarely/briefly encountered conditions. We went through much of this over on corner-carvers years ago, and the general consensus was that a PHB should be chassis-mounted on the driver side for more symmetrical behavior.

If we're really lucky this weekend, the Sears Point* NASCAR cup coverage might even mention this, because I'm pretty sure that the road course cars mount the "track bar" this way, and like the way the OE S197 bar is mounted for all the rest of the races where right turn behavior can be sacrificed in the name of improved left turn performance & tuning.

Because the driveshaft effect is potentially quite different between autocross and big track driving - 2nd gear vs 3rd or 4th, I don't think I'd expect PHB "feel" to be the same at both venues.


*that track will always be Sears Point in my mind no matter how many more times it gets renamed. Is it Infineon or Sonoma these days? Or has it changed yet again and Google/Bing/etc. not had time to catch up?


Norm

 

[csamsh]

I think we might be paralyzing by analyzing.

I just need two sentences....I put a Watt's on the car, and then it slalomed better. I don't really care how or why it worked.

 

[claudermilk]

I think we might be paralyzing by analyzing.

I just need two sentences....I put a Watt's on the car, and then it slalomed better. I don't really care how or why it worked.

Didn't Whiskey say just that?

 

[SoundGuyDave]

I think we might be paralyzing by analyzing.

I just need two sentences....I put a Watt's on the car, and then ran it on a track, and it was no better. I guess it isn't worth much.

There. Fixed it for you...


And that's why "why" is so important.

 

[csamsh]

There. Fixed it for you...


And that's why "why" is so important.

Agreed.


Application...application...application

 

[tigercrazy718]

Meh in my opinion the car felt much more stable after I put it on and did a track day with it. It was the only change made. Your results may vary.

 

[Norm Peterson]

"More stable" tends to suggest slightly more understeerish as well as more "settled". Can you elaborate a bit, particularly with respect to left turns vs rights?


My opinion - just unpacking the boxes that the Watts link ships in and installing the contents where a PHB once lived sounds a lot like when in days past you swapped a 4-barrel Holley carb and an aluminum "high-rise" manifold on your 2-barrel carb'ed motor and called it better. Yup, it likely was, but in all probability not as good as it could have been with some tuning (which required a little understanding of how it all worked or access to somebody who did). For optimized performance it was either that or you were usually lucky with tolerances and the way those things were generically assembled. And I actually like the idea of a Watts . . .


Norm

 

[Pentalab]

I tried to draw this all out on paper, and still can't see how tire deflection at the contact patch results in the huge swings in and out..at the top of the wheel well.....as depicted in the video. What would be interesting is to repeat the go-pro camera video.... with the camera mounted in the same position, but with a WL watts link installed /fays /steeda etc.

I believe the PHB video was using an adjustable BMR PHB + mating BMR phb brace. I had the same setup (which had greasable poly ends) on my 2010 for 2 yrs then switched to the WL-watts link. However, the WL-watts link was installed the same day as the Eaton tru-trac. The effects I'm seeing are a combo of the watts link + tru-trac. That makes it difficult to evaluate, since both were installed simultaneously. The net effect however, imo, is a big improvement over the adj BMR PHB + brace + oem ford traction lock differential.

 

[Norm Peterson]

I tried to draw this all out on paper, and still can't see how tire deflection at the contact patch results in the huge swings in and out..at the top of the wheel well.....as depicted in the video. What would be interesting is to repeat the go-pro camera video.... with the camera mounted in the same position, but with a WL watts link installed /fays /steeda etc.

Tire deflection at the top is going to be a function of sidewall stiffness in the lateral direction and I think pretty heavily dependent on wheel width vs tire section. Basically, if you've got a significant amount of sidewall 'bulge' out past the wheel flanges (like you get with tires on rims near, at, or below min-recommended width), the entire tire tread is going to try to migrate laterally a whole lot more than if it's on a max-recommended width wheel.

I can tell you that the lateral tire tread deflection at the top of 285/35-18 PSS is held to something under about 0.1" when that size is mounted on 18x11's. Static strut to sidewall clearance at the almost nonexistent "bulge" is barely 0.043", and there's no rubbing. I don't have any video showing lateral deflections either at the top or down at the contact patch, but it'd be interesting.


Norm

 

[Whiskey11]

I tried to draw this all out on paper, and still can't see how tire deflection at the contact patch results in the huge swings in and out..at the top of the wheel well.....as depicted in the video. What would be interesting is to repeat the go-pro camera video.... with the camera mounted in the same position, but with a WL watts link installed /fays /steeda etc.

I believe the PHB video was using an adjustable BMR PHB + mating BMR phb brace. I had the same setup (which had greasable poly ends) on my 2010 for 2 yrs then switched to the WL-watts link. However, the WL-watts link was installed the same day as the Eaton tru-trac. The effects I'm seeing are a combo of the watts link + tru-trac. That makes it difficult to evaluate, since both were installed simultaneously. The net effect however, imo, is a big improvement over the adj BMR PHB + brace + oem ford traction lock differential.

This video exists in multiple forms already:



That's the best one I have. Fays2 Watts link with 285/35/18 RS3V2's on SVE Drifts in 18x10 ET43, stock 20mm rear GT bar and 275lbs/in rear springs. Rear RC is in the highest position.