[ordy]

Hi



Quick note do all 968 suffer from bump steer, mine always has had some, which makes driving a tiring experiance, cant see anything worn, changed struts, track rod ends, still the same, strut tops still the same, standard 17 inch wheels, still the same.



I have read 944 had the same problem but they had manual racks,



Any ideas, or is it just live with the experiance on cars that feel alive



Thanks



Ordy

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[bombfactory]

I've heard the term, but I'm a little embarrassed that I don't know what bump steer is. Can you describe when it happens and what it feels like?

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[94SilverCab]

http://en.wikipedia.org/wiki/Bump_steer



Bump steer or roll steer is the term for the tendency of the wheel of a car to steer as it moves upwards. It is typically measured in degrees of steer per metre of upwards motion or degrees per foot.



Seems like another term for understeer...



I have experienced both under and over-steer on my 968 in various configurations. This has all been sorted at this point, bu it took some serious doing (Sways, drop links, caster blocks, strut braces, tires, alignment, etc., etc. , etc.).



Jay

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[tamathumper]

Can't say as I experience that, but I have every brace known to man and God installed at this point. But, to be honest, I didn't have bump steer when I had no braces on the car, either.

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[Rap]

Excuze the question, does the wheel turn into the turn?

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[bombfactory]

If bump steer produces understeer, then the outside wheel will turn away from the direction you want to turn. That doesn't sound good. How can you tell if you have it - or do all 968s have it to one degree or another? Can lowering the car or going to 18" wheels make it worse?

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[mbardeen]

Bump steer in the Jeep world comes from changing the geometry of the suspension. When the suspension compresses, the angles change and the wheels will turn one way or the other. This is because it has a solid front axle. It has a pitman arm from the steering box, then a draglink running down at an angle to one side of the suspension and a tie-rod to the opposite side.









This situation does not happen with our cars. We have a rack and pinion setup, with equal length tie-rods to both front wheels, so bumps shouldn't affect steering very much.



Edit: Realized my understanding of bumpsteer was fairly rudimentary.



See stuff like:http://www.circletrack.com/chassistech/c...ewall.html for a better explanation than I could give.



Ordy: Have you checked your wheel bearings? If they are not tight, that might account for what you describe as "bump steer". Another thing to check would be the rubber bushings around the steering rack. If they're worn, the could allow the rack to move around.

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[flash]

we do suffer from bump steer, especially when you start lowering the car, or add heavier wheels.



the chassis itself flexes quite a bit, the rubber bushings have a lot of give, and the geometry is less than optimal. as things move, it can easily get into a "perfect storm" situation and this allows for bump steer to present itself.

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[danderson]

My understanding of bump steer is that the steering links(tierods)have a different radius from the a-arms. Therefore as the wheel moves vertically, the arc swept by the tierod is different fromt the arc swept by the a-arm. These different arcs cause the wheel to turn in or out depending on the car. On an MG we would shim the steering rack to reduce the effect.

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[flash]

lol - yup - i lifted mine about an inch on my Bs.

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[ordy]

Sorry for the late input, the problem i have is that the road shocks are transmitted through the steering , the bumper the road the more the steering wheel flicks, so inturn on a very rough road its hard to keep it in a straight line, not good.

It may be that we have just crap roads in the uk, which have had no money spent on them in 20 years.



A similiar problem happens with early 911 and 944 however they have manual steering racks, i thought the power steering would have dampened the effect however its not good.



I have read that lowing the track rod ends can help with 944 etc, will it help with 968?



Thanks

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[flash]

to what ride height have you set the car? is it stock, lowered, what?

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[Langley968]

[quote name='ordy' timestamp='1349870463' post='133614']

road shocks are transmitted through the steering , the bumper the road the more the steering wheel flicks, so inturn on a very rough road its hard to keep it in a straight line, not good.

It may be that we have just crap roads in the uk, which have had no money spent on them in 20 years.

[/quote]



I am NO expert in these matters, but could this discription not also be some degree of "tramlining" ? http://en.wikipedia.org/wiki/Tramlining

When I picked up my car three years ago, I was puzzled by how hard the car was to steer when there were imperfections in the pavement. As soon as I changed the tires, it was completely GONE! It was so bad I was hesitant to take one hand off the steering wheel. I even hesitated to let my wife drive the car at that time. Now I can drive the car with one hand without being afraid of it suddenly lurching to one side or the other.

Just a thought...

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[tamathumper]

My tramlining, although not severe, was solved by getting new tires as well. I've read that wider tires in the front can also contribute to it.

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[Cloud9...68]

+1 on the tramling related to tires - my problem went away when I bought new tires as well. But what the OP is describing sounds like something different.

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[JWahlsten]

What are your alignment settings? I'd like to know what your caster and toe settings are. Cars with zero toe-in to toe-out can feel like they wander when straight and be tiring to drive in highway conditions.



As the suspension on these cars droop the wheels tend toward toe-in, but under compression you get toe-out. If you're really low, this can get rather severe possibly causing the issue you're concerned about.

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[flash]

which is exactly why i asked how low the car was. i went through this when i was playing around with ride height, and found there was definitely a point that was "too low" for both handling and bump steer.



as to tramlining, tire pattern and compound play a large role here. tires that have longitudinal grooves, which are better at shedding water, also create more tramlining issues. block pattern tires do much the same. so do harder compounds, though those last longer.



it's all a compromise when it comes to tires.

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[ordy]

Hi, the suspension is set up with normal clubsport height, no not to low, as of the tracking etc, i got the settings off the forum, and set it up for the most stable in a front line, i think it was your setting flash.



However the tyres may be the next option, what is the best tyre pressures for road use and what tyres would you recommend? i have the normal 17inch cups that come with a club sport.



I have bridgestones on at the moment however something with a softer tyre wall to absord road bumps may be the route?

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[pinball1008]

I have S02's all round on 17" wheels and I get terrible tramlining.

The previous tyres were a size smaller on the front (215's I think) and I had no problems.

I will probably go with Falkens the next time and with the smaller size on the front.

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[Langley968]

[quote name='ordy' timestamp='1350033375' post='133714']

I have bridgestones on at the moment however something with a softer tyre wall to absord road bumps may be the route?

[/quote]



When I bought my 968 there were Bridgstones on it and the tramlining produced "white knuckles" at times. The tires had hard sidewalls, so in a spirited drive that was OK going around corners. My current Sumitomo tires may not be the best for cornering (they are good - just not great!) but they are a delight for going out for a cruise...

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