BUMP STEER what to look for?

[JoshCole1122]

I checked all suspension and tie rods and everything seems tight. The car is lowered a little bit (PO). Should i consider the rack spacer kit? Any measurements etc. I can take to see what is the cause?

thanks!

[Driven97]

Well, what symptoms do you actually have that makes you think you have a bump steer problem? Is it suddenly worse? Did you make any recent changes to the car? How much lower than stock are you? Are you on factory wheels or an aftermarket setup that may have different offsets?

[JoshCole1122]

When I'm on curvy roads and hit a bump the wheel will jerk in my hands a decent amount, this could be normal i suppose on these cars. I have only owned the car less than a year and put maybe 500 miles on yet since.

The car is lowered about an inch up front i would guess, i can measure. I'm on factory 16" wheels no spacers etc up front.

I need to check for alignment and measure rack levelness etc. still.

[faapgar]

What is the model & mileage?

[Driven97]

The rack spacers only raise the rack a very small amount - maybe 1/4"? So while they certainly don't hurt, I can't see they would make a huge difference.

The steering on 911s does talk back quite a bit, but it shouldn't be distracting. They're pretty sensitive to height - lower looks cool but the cars work best at designed height. Most people are fine with the tradeoff.

If you want to try something quick and free, throw a wrench on the TB adjusters and turn them both tighter the same measured amount - say exactly two full turns. That'll raise the front evenly. Car will look weird, but whatever. Then go for a short drive and see if the bump steer symptoms are better or not. Since you've counted your turns, after you can easily return the car back to the exact way it was before you goofed around.

[JoshCole1122]

Originally Posted by faapgar

What is the model & mileage?

carrera, 120k miles

[Bill Verburg]

Originally Posted by JoshCole1122

I checked all suspension and tie rods and everything seems tight. The car is lowered a little bit (PO). Should i consider the rack spacer kit? Any measurements etc. I can take to see what is the cause?

thanks!

Bump steer is always going to be a fact of life w/ a 911, you can minimize it but never eliminate it.

As Driven mentioned the direct nature of the 911 steering makes for a wheel that is always alive, writhing left & right w/ the road irregularities.

It does get worse as the car is lowered and as wheel ET pushes the tire outboard

A rack spacer kit will be required at some point of lowering but where is up for debate

I've been ok w/ just a rack spacer down to ~150mm front height( stock is 113mm min)

[JoshCole1122]

Originally Posted by Bill Verburg

Bump steer is always going to be a fact of life w/ a 911, you can minimize it but never eliminate it.



As Driven mentioned the direct nature of the 911 steering makes for a wheel that is always alive, writhing left & right w/ the road irregularities.



It does get worse as the car is lowered and as wheel ET pushes the tire outboard



A rack spacer kit will be required at some point of lowering but where is up for debate



I've been ok w/ just a rack spacer down to ~150mm front height( stock is 113mm min)

What do you mean in your last statement?


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[Bill Verburg]

Originally Posted by JoshCole1122

What do you mean in your last statement?


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the front ride height of a 911 is determined by measuring a & b then, subtracting b from a(a - b = front height)


spec street height(except for the artificially high '78-81 US SC) is 108mm+/-5mm(113mm is minimum spec height)

Race cars like RSR can be down in the 160s

I run mine @~150mm w/ rack spacers

[JoshCole1122]

Awesome. I’ll measure to see where I’m at. I can make some custom spacers to achieve the desired 150. That might help!


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[faapgar]

Carrera 120k miles.Feedback through the steering wheel.Very few shops understand that at 50-60K miles the front torsion bars are toast.Many cars came to my shop after the dealer replaced the front stuts with alignment for just under $2,000.I would have them come to the shop and for $100 per side replace the front torsion bars in one hour while they waited.Send them on a test drive and they return happy but mad at the shop that did not fix the problem.You can chase this an adjust but it will not go away.Kudos to Bill Verburg for a clear explanation.He must have known my shop friend Grady.Happy Thanksgiving to all.Ciao Fred

[faapgar]

Hi,to be more like Bill I will clarify.Torsion bar is your spring.When you get the feedback every component is stressed if you feel it in your hands.When I went to Porsche school in the early 70,s they stressed to change the foam rubber o-ring in front of the adjusting cover and coat the whole bar in grease to prevent corrosion.At any point where a rust pit occurs that is where it will break.Happy Thanksgiving to all.Ciao from C.R.Fred

[Trackrash]

The rack spacers on a lowered car will probably help and will be all you need. However the bump steer that 911s have inherently actually will make the car feel stable in turns.

If you raise the spindles and don't properly adjust for the resulting bump steer you will notice the car is twitchy and darty. If your spindles are not raised, you don't have anything to worry about IMO.

So if you don't have a problem with the car feeling twitchy, unstable, and darty, you are OK as far as bump steer goes.

If you are really concerned, you could measure your bump steer and adjust accordingly.

[JoshCole1122]

I love the way the Porsche is so reactive and organic feeling with the road. It is an observation and something I have noticed when enjoying driving it on windy curvy roads. It isn’t terrible and I never feel like it’s darty or unstable. Just Jerks the wheel.


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[manbridge 74]

Almost every old 911 that I get on my lift has a giant “stair step” when looking at rear of front lower control arms where the bushing has lost concentricity. Replacing my 45 year old bushings made a huge difference in stability in curves or in windy conditions.

I never considered the bars to lose spring rate, just sagging with age. I guess if bushings got worn enough to cause the bar to lose some diameter from contact then bar loses spring rate.

[Craig_D]

You can lower your front end properly (as Porsche did with the IROC RSRs 19mm) and I did 30mm, by raising the spindles on your struts. Elephant Racing does this mod.

This raises the front axles (spindles), leaving you with a lowered front end but with proper steering and suspension geometry, even with a lowered car.

I still have a very lively steering wheel...that's why I love driving an air-cooled 911, but I have no real bump steer issues at all.

Also, I have no spacers on my steering rack (because my steering geometry is neutral).

Cheers,

- Craig

[WP0ZZZ]

Originally Posted by faapgar

Very few shops understand that at 50-60K miles the front torsion bars are toast.

What do you mean exactly by "toast"? External corrosion, plastic deformation, loss of stiffness...?

[chrismorse]

Having read and re-read Carroll Smith's iconic "Tune..." series, I recall him stating that we want the car's response to driver input to be as linear, direct and predictable as possible. We do not want the car doing things on its own.

In particular, we do not want the the cornering car to make the laden wheels turn in on their own when going over a bump or dip. This has an upsetting effect on the weight transfer and confuses the driver. (Pardon the sketchy paraphrasing).

IIRC, the typical toe change graph shows the car going into toe in on bump, increasingly so, with greater bump, conversely, moving from stock design ride height into droop, shows the toe going into toe out, again, at a faster rate with increasing droop.

The most gentle, gradual change is shown close to stock height, with the tie rod right at horizontal.
When the car is lowered a bit, you can get the tie rod back to that desired "horizontal" position by using rack spacers.
If a greater lowering is desired, for less fore/aft, & lateral weight transfer, or even cosmetics, either the spindle must be raised on the strut tube, or height adjustability must be accomplished at the tie rod connection to the steering arm.

It is not often appreciated that the rear also has different toe change response to height change.
Slamming the car, going for"the stance" is quite likely to get the f/r toe change curves mixed up and result in a squirrelly handling car.

As my old boss used to say, when he saw me hacking on my hot rod 914, "stock is best".
Or, as one of the other pundits advised, "before you start to modify your car, first figure out how you are going to improve on the factory engineering".
Obviously, this's ignores the compromises that must be made to mass produce a sports car.
Chris

[famoroso]

Originally Posted by chrismorse

"...or height adjustability must be accomplished at the tie rod connection to the steering arm."

Chris

Jae Lee's (of Mirage International fame) handiwork...

Bump steer / kickback events are no longer the cringeworthy affairs they once were. Although, I still flinch in anticipation as I drive over bumps... only to be pleasantly surprised by the drastic reduction (not elimination) of steering wheel kickback. Hopefully, I can erase that flinch programming with additional seat time. 👍🏼

[Trackrash]

Originally Posted by chrismorse

The most gentle, gradual change is shown close to stock height, with the tie rod right at horizontal.
When the car is lowered a bit, you can get the tie rod back to that desired "horizontal" position by using rack spacers.

This is only true IF a line drawn from the centerline of the ball joint to the centerline of the torsion bar is also horizontal. I only bring this up because this, being the conventional thinking, caused me to waste much time trying to fix the bump steer problem on my car.
I don't believe you need a tierod bumpsteer kit UNLESS your spindles have been raised. At least that has been my experience.